# Thread: Discussion of Special Relativity Primer

1. I hope you don't mind a couple of comments regarding your nice description?
Originally Posted by Janus
1. Inertial frames of reference

Essentially, this is any frame of reference which is not accelerating
Accelerating with respect to what? I don’t believe that this is a valid definition of an inertial frame of reference. It makes one question what one is accelerating or not accelerating with respect to. For example: Consider a frame of reference in free-fall in the Earth’s gravitational field (or better yet, a uniform gravitational field). Such a frame is considered to be an inertial frame of reference. Most people would say that this frame of reference is accelerating. It’s accelerating with respect to the Earth.

An inertial frame of reference is defined as a frame of reference in which the law of inertia holds good. That is to say frames in which F = ma (really dp/dt)

Basically what this means is that a frame of reference in which Newton’s first law, a.k.a. the law of inertia, is satisfied.

If you used this definition then when the reader goes on to learn general relativity he'll be prepared, i.e. he won't have to unlearn anything. This is true since free-fall frames are considered to be inertial frames of reference in GR. And in GR a free-fall frame is accelerating with respect to the earth's surface.

Originally Posted by Janus
One point about reference frames is that they are not the same thing as "point of view". A point of view is attached to an object or observer, a frame of reference is not. If A accelerates, his "point of view" goes with him,
I disagree. An observer is commonly definition as just another term for “frame of reference.” You can find this definition in such textbooks as “Introducing Einstein’s Relativity” by Ray D’Inverno as well as “Exploring Black Holes” by Taylor and Wheeler. Eg from “Exploring Black Holes” Glossary, page GL-5

Observer – Collection of rods and recording clocks associated with a given frame of reference.

Thus when an observer accelerates his system of clocks and rods, which is attached to him, accelerates with him.

Originally Posted by Janus
2. The postulates of Relativity

SR is built on two postulates:

1. The laws of physics is the same for all inertial reference frames.
2. The speed of light is a constant for all inertial frame.
Personal Opinion - The second postulate should read “ The speed of light has the same value in all inertial frames of reference.” Or you could say that the speed of light is invariant, but that’s mathematical. I’d go with this one here since “constant” might be confusing for some layman.

Note that accelerating observers measure different speeds of light.

2. Correction. My edit button isn't working properly, otherwise I'd edit my previous post. The definition I gave of observer was a bit off. Here is the correct definition.

An observer is commonly defined as a system of clocks and measuring rods. Sometimes the term "observer" is defined to be synonymous with "frame of reference" (inertial depending on context). In such a case the difference between frame and observer differ only with respect to the coordinate axes. In this case two observers may be in the same frame of reference with the only difference being their coordinate axes rotated or shifted. I've never seen anything but Cartesian coordinates used so I guess that's assumed.

3. Accelerating with respect to what? I don’t believe that this is a valid definition of an inertial frame of reference. It makes one question what one is accelerating or not accelerating with respect to
Acceleration isn't relative.

4. Originally Posted by AlexG
Acceleration isn't relative.
Sure it is. Let S, S', S'', and S''' be defined as follows:

S') Let S' be an inertial frame of reference in flat spacetime.

S') Let S' be a frame of frame of reference which is uniformly accelerating at rate g relative to S in the +z direction.

S'') Let S'' be at rest in a uniform gravitational field which is in the -z direction of magnitude g

S''') Let S''' be a frame of reference in free-fall in a uniform gravitational field which is in the -z direction of magnitude g

No experiment done within S' and S'' can distinguish the difference between them.

No experiment done within S and S''' can distinguish the difference between them.

S' is at rest with respect to an inertial frame of reference. S'' is at rest with respect to the source that is generating the field, i.e. is not accelerating with respect to the source. If the source is a spherical body with uniform mass density with a cavity cut out of it whose center is not colocated with the center of the sphere then the gravitational field inside the cavity will be uniform.

In the absense of all other bodys, let S'''' be at frame of reference at rest far removed from the sphere. Then S'''' is an inertial frame of reference which is accelerating with respect to S''' which is also an inertial frame of reference.

In all cases above the frames of reference required a reference in order to specify with respect to what it was accelerating. Therefore is relative.

I really didn't need to give such an elaborate example. It's common knowledge and trivial to understand that when you have a frame of reference it is always at rest or accerating with respect to something.

Let S be an inertial frame of reference. Let S' be a rotating frame of reference with respect to S' Its a known fact that S' is rotating with respect to the "distant stars."

Therefore acceleration is relative. If you believe otherwise then please post a proof.

In the future I would like to request that you provide a proof when you make an assertion which is contrary to someting someone else says. E.g. a mere assertion that Acceleration isn't relative is insufficient to prove and/or explain why you believe that acceleration isn't relative and what exactly you mean by it.

Thanks!

5. It still seems to me that you're using acceleration and relative motion interchangeably.

It's common knowledge and trivial to understand that when you have a frame of reference it is always at rest or accerating with respect to something
When you have a frame of reference, it is always at rest or in relative motion with respect to something else. Relative motion is not the same as acceleration.

6. Acceleration is absolute. You are only accelerating if you can feel it.

Whilst in inertial motion, you cannot feel it.

Originally Posted by Popper
Note that accelerating observers measure different speeds of light.
Not in their own frame of reference, they don't. The speed of light, locally, is always c, even in an accelerating frame. The coordinate speed of light however, can differ from c (which is the cause of what is known as a Rindler horizon).

7. Originally Posted by Janus
After I take a break from the keyboard for a while I might continue
Keep up the good work!
I just wondered if your going to show how it is possible to send information back in time if information can travel with instant speed.
I never felt comfortable about that statement.

8. Originally Posted by sigurdV
I just wondered if your going to show how it is possible to send information back in time if information can travel with instant speed.
It is not possible to send information back in time, because it is not possible for information to travel instantaneously. Propagation of information is limited to the speed of light.

If it were possible to send information instantaneously (which it is not), you could be sending that information into the past of a recipient that was in motion in relation to you, and this is due to (wait for it...) the relativity of simultaneity.

9. MODERATOR NOTE : This is the discussion which ensued after posting the Special Relativity Primer sticky; I have moved it here because this discussion deserves its own dedicated thread.

10. Originally Posted by SpeedFreek
It is not possible to send information back in time, because it is not possible for information to travel instantaneously. Propagation of information is limited to the speed of light.
I say: IF faster then light signaling is possible then information, according to some authorities on the subject,
can be sent backwards in time. It would be of interest to see their proof dont you think?

Its superfluous to inform me that FTL messaging is impossible according to the Theory of Relativity.
You should already have inferred my knowing that from what I wrote!
Originally Posted by SpeedFreek
If it were possible to send information instantaneously (which it is not), you could be sending that information into the past of a recipient that was in motion in relation to you, and this is due to (wait for it...) the relativity of simultaneity.
Well well well...
Why cant you get it out of your system?
That I should believe that :

(a) Instantious signaling is possible.
(b) Sending messages back in time is possible.

What I suspect is that any proof that shows that instantious signalling

makes it possible to send information into the past is invalid.

11. Well well well...
Why cant you get it out of your system?
That I should believe that :

(a) Instantious signaling is possible.
(b) Sending messages back in time is possible.
I don't think that is what SpeedFreek is saying. Quite the opposite, actually.

12. How instantaneous signaling could be used to sent information to the past under the rules SR:

You have two frames AB and DC moving relative to each other. DC is moving to the right according to AB, as shown below.

When A is next to D he sends an instantaneous signal to B when B is next to C.

A___________B
D___________C >

B then hands the signal to C with the understanding that C will get the message back to A.

This is where it gets tricky.

The first diagram shows the positions of A, B, C and D at one instant according the frame AB. However, since CD is moving with respect to AB, the distance C-D is length contracted as measured in the AB frame.

In the CD frame, it is AB that is moving and thus length contracted, so that when B and C are adjacent, things are like this in the CD frame:

F_________________A_____B<
D_________________E_____C

Note that A and D are nowhere near each other, so if C wants to relay a signal to A, he can send an instantaneous signal to E and have E give A the message as they pass each other.

If we then check back with frame AB, upon the reception of the message from E we see that things are like this:

________A___________B
D_______E___C>

Since CD is moving to right, A aligns with E before A aligns with D. If A sends his signal while next to D and receives it back when next to E, then he receives the signal before he sends it.

Ergo, Relativity + instantaneous signaling = causality violations.

13. Originally Posted by sigurdV
I say: IF faster then light signaling is possible then information, according to some authorities on the subject,
can be sent backwards in time. It would be of interest to see their proof dont you think?

Its superfluous to inform me that FTL messaging is impossible according to the Theory of Relativity.
You should already have inferred my knowing that from what I wrote!
All you wrote was:
"I just wondered if your going to show how it is possible to send information back in time if information can travel with instant speed."
But it is not possible to send information back in time because information cannot travel with instant speed.

What you should have asked was:
"I just wondered if you're going to show how it is impossible to send information back in time, because if information can travel with instant speed it would violate causality"
Or something similar. Then I might have understood your intentions.

Originally Posted by sigurdV
Well well well...
Why cant you get it out of your system?
Because, even if you don't understand it, simultaneity is the WHOLE issue with your question.

Originally Posted by sigurdV
That I should believe that :

(a) Instantious signaling is possible.
(b) Sending messages back in time is possible.
Neither is possible.

Originally Posted by sigurdV

What I suspect is that any proof that shows that instantious signalling

makes it possible to send information into the past is invalid.
Well unfortunately your suspicion is wrong. Sorry.

In order to consider what it means to send information instantaneously, first you need to understand how the concept of "now" applies at a distance, and what "now" means for an object in motion in relation to you. If you understand the train and embankment thought experiment, you should already understand how "now" is a relative concept.

http://www.theculture.org/rich/sharp...es/000089.html

14. Originally Posted by AlexG
It still seems to me that you're using acceleration and relative motion interchangeably.
I'm sorry, AlexG, but I don't know why you're saying this. Please explain.

I've never said that you can measure the absolutete transelatory motion of a frame against absolute space, only between inertial frames. I can't undertand how you came to believe otherwise since I never said that an experiment can be done to disinguish the absolute moition of a frame of reference.

The relative motion and acceleration can be down between two frames and it's always detectable buy the definition of a frame of reference. Since you have something which moves and accelerates with that frame of referemce then an accelerating frame is not the same as an inertial frame. Is that what you thoughn?that wha you mean? If so then please explain how you came to such a conclusion. Thanks.

15. Consider two frames, A and B initially at rest with respect to each other. Now imagine frame B accelerates away from A. To B, it "looks" like A is accelerating in the opposite direction, but an observer in A does not measure the effects of any acceleration where as an observer in frame B does.

16. Originally Posted by beefpatty
Consider two frames, A and B initially at rest with respect to each other. Now imagine frame B accelerates away from A. To B, it "looks" like A is accelerating in the opposite direction, but an observer in A does not measure the effects of any acceleration where as an observer in frame B does.
Bear in mind that these frames are not symmetrical, because one involves acceleration. The normal rules of Lorentz transforms do not apply in this case.

17. Originally Posted by Markus Hanke
Bear in mind that these frames are not symmetrical, because one involves acceleration. The normal rules of Lorentz transforms do not apply in this case.
You are correct, sir or madam.

18. Originally Posted by Popper
The relative motion and acceleration can be down between two frames and it's always detectable buy the definition of a frame of reference. Since you have something which moves and accelerates with that frame of referemce then an accelerating frame is not the same as an inertial frame. Is that what you thoughn?that wha you mean? If so then please explain how you came to such a conclusion.
I'm having real trouble understanding you here.

An accelerating frame is not the same as an inertial frame. In an inertial frame, you can consider yourself at rest as there are no forces acting upon you, but in an accelerating frame you can feel a force acting upon you.

19. Originally Posted by SpeedFreek
I'm having real trouble understanding you here.

An accelerating frame is not the same as an inertial frame.
I agree.

Originally Posted by SpeedFreek
In an inertial frame, you can consider yourself at rest as there are no forces acting upon you, but in an accelerating frame you can feel a force acting upon you.
You can consider yourself at rest in any coordinate system. E.g. define the system S’ as the coordinate system inside the cabin of a spaceship. As you accelerate you can say that you’re at rest in the cabin. The frame of reference defined by the walls and floors of my apartment define another frame of reference. In this case I can say that I’m at rest in my apartment sitting in a chair. Or if you’re standing on the surface of the Earth then you can say that you’re at rest in the Earth’s frame of reference.

20. Popper, it doesn't help that in your post #4 you mislabelled S as S'. It is confusing!

Acceleration in Special Relativity

In special relativity accelerating frames are different from inertial frames. Velocities are relative but acceleration is treated as absolute. In general relativity all motion is relative. To accommodate this change general relativity has to use curved space-time. In special relativity space-time is always flat.
So, in this thread about Special Relativity, what exactly are we discussing? It seems like you are trying to introduce GR into a discussion about SR. This is where the confusion lies.

In Minkowski spacetime, acceleration is not relative, it is absolute. Why are you introducing gravity into a discussion about Special Relativity? I am just trying to understand the point you are making.

21. Popper, this was your statement.

It's common knowledge and trivial to understand that when you have a frame of reference it is always at rest or accerating with respect to something

It is trivial to understand that two frames of reference my be in motion with respect to one another, and yet neither is undergoing any acceleration. You seem to think that if two frames are in motion with respect to one another, there is acceleration involved. You seem to be using the terms 'relative motion' and 'acceleration' interchangeably.

22. Sorry. That should have read
It's common knowledge and trivial to understand that when you have a frame of reference it either an inertial frame of reference at rest or moving uniformly, or accerating with respect to something, perhaps another frame or a rock which can define such a frame.

23. Originally Posted by AlexG
Acceleration isn't relative.
I have to admit, I'm curious as to why you'd make such a statement, and with no proof even? Just curious but do you assume that whomever reads your assertions will accept them without challenge? If not then perhaps hext time can you post either a proof or explanation with the assertion. Thank you kindkly.

24. Originally Posted by SpeedFreek
Not in their own frame of reference, they don't.
Sure they do.

Originally Posted by SpeedFreek
The speed of light, locally, is always c, even in an accelerating frame.
That is only true when the poptential of the particle is zero, i.e. at the same level as the observer. However if I was on the ground and a light beam was bounced between the tops of two skyscrapers then the observer on the ground would measure the speed of light to be faster than when measured at ground level.

Originally Posted by SpeedFreek
The coordinate speed of light however, can differ from c (which is the cause of what is known as a Rindler horizon).
What else did you think I was talking about? Consider a uniformly accelerating frame of reference. The metric for such a frame is

ds2 = c2(1 + gz/c2)2dt2- dx2 - dy2 - dz2

Let dL2 = dx2 + dy2 + dz2

Since light moves on a null trajectory we have ds = 0 so now we are left with

0 = c2(1 + gz/c2)2dt2 - dL2

or

dL2= c2(1 + gz/c2)2

Divide by dt2 to obtain

(dL/dt)2 = c2(1 + gz/c2)2

Let dL/dt = v = speed of light. Then

v2 = c2(1 + gz/c2)2

Now take the square root of each side to obtain

v = c(1 + gz/c2)

Thereore when light moves through this field v = c only when z = 0===> Q.E.D

25. So Popper is pmb?

Originally Posted by pmb
That is only true when the potential of the particle is zero, i.e. at the same level as the observer. However if I was on the ground and a light beam was bounced between the tops of two skyscrapers then the observer on the ground would measure the speed of light to be faster than when measured at ground level.
And how exactly does this potential apply in special relativity?

And I was referring to the proper speed of light, rather than the coordinate speed. As I said, even in an accelerating frame the speed of light is always c, locally. But I'm sure you know what I meant.

I cannot see any problems with Janus' original statements, in the context of Special Relativity. In Special Relativity, acceleration means you are changing frames of reference.

26. Originally Posted by SpeedFreek
And how exactly does this potential apply in special relativity?
Someone merely noted i.e. said "Note" and said that pertained to accelerating observers. That's al. It's no big deal. It's just that some relativists consider accelerating frames in flat spacetetime to be part of special relativit, and not general relativity. So a note was made to that effect. No biggy in my opinion.

Originally Posted by SpeedFreek
And I was referring to the proper speed of light, rather than the coordinate speed. Perhaps I should have said, even in an accelerating frame the speed of light is always c, locally. But I'm sure you know what I meant.
I recommend that when you mean the proper speed then you should state it explicitly. Unfortunately the proper velocity of light is undefined. That's because d(tau) is zero along the path of light.

Originally Posted by SpeedFreek
I cannot see any problems with Janus' original statements, in the context of Special Relativity. In Special Relativity, acceleration means you are changing frames of reference.
I you looked at thos posts you' see that they were merely opinions. Not an major corrections or anything. And I think the part about the definition of an inertial frame actually is wrong, by the way. Everyone defines "Inertial frame" as "a frame of reference in which the law of inertia holds true." Look it up in any basic physics text and it wil tell you the same thing (e.g. Ohanian's basic Physics text #I, Haliday and Resnik, "Classical Mechanics," by Godstein, Safko and Poole). The one he gave is problematic.

By the way - I'm having problems sending a PM to the Admi. I wanted to tell him that Popper and I am the same person. I used popper/pmb to see whether the person who banned me was going to hold that grudge here. Now that I'm comfortable with it I want to tell the amin so that they can delete the pmb account. When you see the admin them please let him know. Thanks.

27. Originally Posted by SpeedFreek
So Popper is pmb?
I had a reason to create two names when I got here. That reason no longer exists but I haven't been able to use PM. The edit area always comes out blank gray

Originally Posted by SpeedFreek
And how exactly does this potential apply in special relativity?
To me? It doesn't. You seem extremely hung up on a mere passing thought that I had regarding something following the word Note and you just don't seem to want to let go of it. Since it's of topic as, my note was, I'll discuss this only in PM and let this die here and now.

28. Originally Posted by pmb
I recommend that when you mean the proper speed then you should state it explicitly. Unfortunately the proper velocity of light is undefined. That's because d(tau) is zero along the path of light.
Well I supposed I shouldn't have used the term "proper" speed of light, as light doesnt have a valid frame of reference anyway, but I really don't understand what the problem is here, as I thought it was obvious what I meant. I meant proper time.

This is what I meant:

Sit in your spaceship with a small light clock in your lap. The light in the clock bounces back and forth or up and down at the same speed, relative to you, whether you are accelerating or not! Do you deny this? Because this is what I am talking about when I say the speed of light is always c, locally, whether you are accelerating or not. Or are you trying to tell us that if you are accelerating you can measure the time-dilation of your own clock?!

See the confusion you are causing?

29. Originally Posted by pmb
To me? It doesn't. You seem extremely hung up on a mere passing thought that I had regarding something following the word Note and you just don't seem to want to let go of it. Since it's of topic as, my note was, I'll discuss this only in PM and let this die here and now.
Well, to me you have undermined what was supposed to be a simple Special Relativity primer, which covers things like the constancy of the speed of light in inertial frames, relative motion between inertial frames, the Lorentz contraction, the relativity of simultaneity or the reason for the asymmetry in the twins paradox. You are immediately launched into discussion about observers at rest or in free fall in uniform gravitational fields (which really has no place in a Special Relativity primer), and immediately questioned whether acceleration is absolute in SR (in which it is usually treated as such) by applying arguments where acceleration takes on a completely different meaning.

Now, I know your intention is to ask whether it is better to treat acceleration more like GR does, in SR, in order to avoid having to redefine acceleration when we finally get to GR, but why are you questioning what AlexG or Janus is saying, when they are only treating acceleration in the usual way for Special Relativity? Surely you understood their meaning?

I think you are jumping too far, too fast, especially for a first reply in a SR primer thread.

30. Originally Posted by pmb
I had a reason to create two names when I got here. That reason no longer exists but I haven't been able to use PM. The edit area always comes out blank gray
.
It does again. You have 8 posts in total here now and the majority is already as part of a scuffle you caused. I am not sure exactly why that is. I hope we don't continue this trend.

31. Originally Posted by KALSTER
It does again. You have 8 posts in total here now and the majority is already as part of a scuffle you caused. I am not sure exactly why that is. I hope we don't continue this trend.
I don't understand why you automatically assumed that it was I who caused a scuffle? Most relativists define special relativity to be physics in flat spacetime and they include accelerating frames of reference to be part of it. So I simply made a note and provided proof that the speed of light in an accelerating frame of reference is not constant and varies with position.

I gotta tell ya Kalster, I'm not all that happy with being accused of something when you're not sure of what happened. That is the kind of think that I hate in discussion forums and that it happens in the first week is mhint to get lost.

Next time please make sure you know the facts before you make accusations. I'm scrambling my password so I can't log back in. Please tell the other forum to ban Popper too.

Good bye

32. Thank you! Exactly what I wanted!

Originally Posted by Janus
How instantaneous signaling could be used to sent information to the past under the rules SR:

You have two frames AB and DC moving relative to each other. DC is moving to the right according to AB, as shown below.

When A is next to D he sends an instantaneous signal to B when B is next to C.

A___________B
D___________C >

B then hands the signal to C with the understanding that C will get the message back to A.

This is where it gets tricky.

The first diagram shows the positions of A, B, C and D at one instant according the frame AB. However, since CD is moving with respect to AB, the distance C-D is length contracted as measured in the AB frame.

In the CD frame, it is AB that is moving and thus length contracted, so that when B and C are adjacent, things are like this in the CD frame:

F_________________A_____B<
D_________________E_____C

Note that A and D are nowhere near each other, so if C wants to relay a signal to A, he can send an instantaneous signal to E and have E give A the message as they pass each other.

If we then check back with frame AB, upon the reception of the message from E we see that things are like this:

________A___________B
D_______E___C>

Since CD is moving to right, A aligns with E before A aligns with D. If A sends his signal while next to D and receives it back when next to E, then he receives the signal before he sends it.

Ergo, Relativity + instantaneous signaling = causality violations.

33. Originally Posted by Popper
I never said that an experiment can be done to distinguish the absolute motion of a frame of reference.
A minor question: Can friction of the interstellar or intergalactic non empty space be used as an indication of direction and speed...
I heard that doubling the speed in interstellar space multiplies friction by five.
So measuring friction might be of some use ..OR?

34. You don't need that, you just look at the CMBR, see CMBR dipole anisotropy. But note that this isn't an absolute reference frame in the relativity sense, because you cannot gauge your motion without "looking outside of your box".

35. Originally Posted by Farsight
You don't need that, you just look at the CMBR, see CMBR dipole anisotropy. But note that this isn't an absolute reference frame in the relativity sense, because you cannot gauge your motion without "looking outside of your box".
Your answer is interesting but too brief to be understood...could you enlarge a little?

36. May I raise a point that confuses me?

In Post 2 of the Primer there is a point made about synchronizing clocks. viz:

"Imagine you have two clocks separated by some distance and you want to synchronize them. A good way would be to sent a light speed signal to them from a point halfway between them. Each clock reads zero until the flash reaches them and then starts running (we'll assume identical clocks that will run at the same rate from then on)"

and:

"This works for the frame in which the clocks are at rest, but what happens according to a frame in which the clocks are moving? Then, according to the postulates of Relativity, we get this: ...

... Note that the light hits the left clock first, starting it and then the right clock, starting it. Now even though both clocks run at the same rate after both have started, the rightmost clock will always run behind the left clock.

Or put another way, while in the frame in which the clocks are at rest, it is always the same time at the position of each clock, In the frame where the clocks are moving from left to right, it is always a later time at the position of the right clock than it is at the position of the left clock."

Now if we are discussing the same two clocks; The same two physical clocks; they must either show the same time or they show a different time but the one thing of which I am certain s that one clock cannot shew two times simultaneously.

Again if light follows one path in one frame it cannot follow a different path in another frame.

Now we know that in the frame of the observer on the track, the light is emitted from the two lights simultaneously and reaches the observer simultaneously.

This is the real paths of the light in that frame, so how can the light - physical waves/photons take a different path measured from another frame?

I would ask/investigate how the light in the track frame, following that same path could still be measured to travel at the speed of light from the moving frame.

And find the answer is given by the Lorentz formulae, which are the mathematical description of the hyperbolic rotation described by Minkowski.

I do not write this as a new or critical view but merely following the logic of Einstein's and Minkowski papers.

How do we come to accept that a clock can display different times depending on where it is measured from?
How can the path that light follows vary according to how it is measured?

37. Originally Posted by flopper
May I raise a point that confuses me?
No. Just kidding! :P

It's all a consequence of the speed of light being the the same in all inertial reference frames. For example, consider the following diagram (you'll have to excuse my awful art skills).

Say we have a spaceship indicated by the black box. Inside we have two mirrors with a beam of light bouncing up and down.
Now, say we boost the spaceship to the right.

Inside the spaceship, i.e. the frame at rest, the light is still bouncing up and down. But to an outside observer, the light follows the hypotenuse of a right triangle. From geometry we know that the hypotenuse of a triangle is the longest side, so to an outside observer the light takes longer to go up and down! The light can't possibly have sped up to compensate because we assume that the speed of light is the same no matter what inertial frame you are in. So the only possibility is that the light takes longer to propagate depending on which frame you are in. Thus, the observer inside the spaceship says the light took, say, one second to go up then down. But the outside observer measures the time it took to be longer than one second, so his clock reads different.

38. Right so two observers measure different times for the light to go up and down.
No problem with that.
But all the same we know that the light is physically within the spaceship and goes up and down in a second.
That is a physical reality.
The fact that the outside observer measures the light to take longer could mean that the longer time in that frame has the same duration as one second in the spaceship's frame.

Consider synchronised clocks in the space ship.
The clock at the bottom reads 0 when the light is emitted;
the clock at the top reads 0.5 seconds and
the clock at the bottom reads 1 second when the light arrives back there.

These are physical readings on those clocks.
Yet from the outside the observer measures 0 at the emission, let us say 1 second at the top and 2 seconds at the return.

So how can these events all be true and apparent differences be resolved?

By realising that when the spaceship's clocks read 0.5 seconds 1 second has passed as measured by the outside observer and at 1 second in the spaceship 2 seconds are measured by the outside observer.

That is time is passing faster for the outside observer; and if time is passing faster then the spaceships clocks must be running slower as measured by that outside observer.

That the difference is how the times are measured under different conditions; resting/moving; and how those conditions affect the measurements.

After all the measurements will still be the same; time dilation occurs; relative simultaneity still happens exactly as Einstein described. . .

39. It's important to separate the idea of clocks from what is really going on, as that may give the wrong impression that it's all artificial. Clocks are just a measure of time. Therefore it is time itself that is running slower or faster, not the pocket watch or atomic clock you use to measure with. It seems to me, and correct me if I'm wrong, that you are not confused with the mechanism but rather the consequences i.e. time dilation. These are of course not intuitive and run counter to our everyday perception of time, but the effects are very real and can be experimentally measured.

40. Originally Posted by KALSTER
It does again. You have 8 posts in total here now and the majority is already as part of a scuffle you caused. I am not sure exactly why that is. I hope we don't continue this trend.
Are there any assurances that, in the fugure, I won't again erroneously be accused causing something that wasn't my fault as I was here when it was asserted that it was I who caused a scuffle when in fact it was due to another poster's igorance of the fact that today special relativity is commonly defined so as to include non-inertial frames of references and as such instances where the speed of light is not constant?

41. Originally Posted by SpeedFreek
And how exactly does this potential apply in special relativity?
According to the modern definition of special relativity - SR is the physics applied in flat spacetime. So accelerating frames of reference apears in SR. In accelerating framesof reference there are terms such as g00 = (1 + gz/c[SUP]2/SUP])[SUP]2/SUP]. The term gz is a potential term. So even in SR there are gravitational fields as Einstein defined them

Originally Posted by SpeedFreek
And I was referring to the proper speed of light, rather than the coordinate speed
If that's what you meant then that's what you should have said. But there is no proper speed of light because proper speed is found by dividing my d(tau) but for light d(tau) = 0.

And yes, the speed of light is always locally measured to be c. But the speed of light is not always measured locally. In fact the light may travel a very long distance in spacetime and as such not stay local.

As for why I mentioned it, if you looked close at my post I said it was a note, to be done with by Janus as he sees fit.

I know you think that the sticky was perfect. But we are not all the same people here and we all have different levels of education and understanding of physics. In this case I'm well aware of the definition of an inertial frame of reference and you can find it in nearly all mechanics and relativity text defined as those frames of reference in which the laws of inertia hold true. (Newton's first law, F= dp/dt = ma is known as the law of inertia)

I'm highly educated on these points so when I point something out then you can be certain that its imporant.

So let's all play nice and not have anymore accusations about who's causing what problems, shall we?

42. Originally Posted by beefpatty
It's important to separate the idea of clocks from what is really going on, as that may give the wrong impression that it's all artificial. Clocks are just a measure of time. Therefore it is time itself that is running slower or faster, not the pocket watch or atomic clock you use to measure with. It seems to me, and correct me if I'm wrong, that you are not confused with the mechanism but rather the consequences i.e. time dilation. These are of course not intuitive and run counter to our everyday perception of time, but the effects are very real and can be experimentally measured.
Originally Posted by flopper
By realising that when the spaceship's clocks read 0.5 seconds 1 second has passed as measured by the outside observer and at 1 second in the spaceship 2 seconds are measured by the outside observer.
What is this describing if not time dilation?

The space ship observer reads that his clocks shew 1 second when one second(proper time) has passed.
The outside observer measures 2 seconds to have passed when the (moving) spaceship clocks read 1 second, i.e he is measuring that the spaceships clocks have only measured 1 second when 2 seconds (co-ordinate time) have passed (as he measures time) therefore the Spaceship clocks are 'running slow'.

43. Originally Posted by flopper
What is this describing if not time dilation?
Yep, it's time dilation. Sorry, I think I'm confused as to what you are confused about, haha!

I think I am confused because in one post you said,

How do we come to accept that a clock can display different times depending on where it is measured from?
How can the path that light follows vary according to how it is measured?

44. Hi Beefpatty and Flopper

Time, and AGING in particular, is a difficult concept to grasp:

A basic religiously held belief
is that time passes
with the same speed
within the same frame
and that there
is no universal frame.

The process of hibernation clearly shows
that aging can proceed with different speed
in the same frame!
So something is wrong ...

I will not tell you what. I will not risk thread closure...

45. Originally Posted by sigurdV
The process of hibernation clearly shows
that aging can proceed with different speed
in the same frame!
So something is wrong ...
Provide evidence for that claim.

46. Originally Posted by sigurdV
Hi Beefpatty and Flopper

Time, and AGING in particular, is a difficult concept to grasp:

A basic religiously held belief
is that time passes
with the same speed
within the same frame
and that there
is no universal frame.

The process of hibernation clearly shows
that aging can proceed with different speed
in the same frame!
So something is wrong ...

I will not tell you what. I will not risk thread closure...
While some conditions might effect the effects of aging, they don't alter the actual fact of aging.
If you were able to put a person into a state where he could last 71 years with only 1 year physical change to his body, this does not change the fact that his body still had to exist and was subject to time for 71 years. He would be 71 yrs older, even though his body only showed the effects of 1 yr of aging

However, if a person were to travel relative to the Earth at 99.99% of c, he could return having aged 1 year with 71 years passing on the Earth, and he would only experienced 1 yr of time. Not only would he show the physical effects of 1 yrs aging, he really would be only 1 year older.

47. Originally Posted by flopper
May I raise a point that confuses me?

In Post 2 of the Primer there is a point made about synchronizing clocks. viz:

"Imagine you have two clocks separated by some distance and you want to synchronize them. A good way would be to sent a light speed signal to them from a point halfway between them. Each clock reads zero until the flash reaches them and then starts running (we'll assume identical clocks that will run at the same rate from then on)"

and:

"This works for the frame in which the clocks are at rest, but what happens according to a frame in which the clocks are moving? Then, according to the postulates of Relativity, we get this: ...

... Note that the light hits the left clock first, starting it and then the right clock, starting it. Now even though both clocks run at the same rate after both have started, the rightmost clock will always run behind the left clock.

Or put another way, while in the frame in which the clocks are at rest, it is always the same time at the position of each clock, In the frame where the clocks are moving from left to right, it is always a later time at the position of the right clock than it is at the position of the left clock."

Now if we are discussing the same two clocks; The same two physical clocks; they must either show the same time or they show a different time but the one thing of which I am certain s that one clock cannot shew two times simultaneously.

Again if light follows one path in one frame it cannot follow a different path in another frame.
Imagine you have a frame in which you light source with a string of clocks stretching out from it in opposite directions.

000000L000000

The light turns on and it it reaches each clock, it starts it running. Since the speed of light must be a constant, the light travels at the same speed in both directions and you get this:
000000L000000
000001L100000
000012L210000
000123L321000

The blue color indicates that the light has reached a clock and the numbers indicate the clocks ticking off time after starting.

Clocks at equal distances from the light source show the same time.

Now consider a second frame with its own light and clocks (this time the light is red):

000000L000000

Again, the light flashes and starts the clocks:

000000L000000
000001L100000
000012L210000
000123L321000

We see the same thing, clocks at equal distances from the light read the same time.

Now let's add the first frame and have it moving with respect to the second. Only this time, the blue light doesn't flash, and the clocks in this frame start when hit by the red light. The light flashes when the two lights are opposite each other.

000000L000000
000000L000000

.000000L000000
000000L000000

..000001L000000
000000L000000

...000012L000000
000001L100000

....001234L0000000
000012L210000

I did not include time dilation as a factor here, not only due to the difficultly of accurately representing it with this format, but because for this demonstration, it does not matter what the relative clock rates between frames are, only that clocks in the same frame run at the same rate. (nor did I include length contraction)

As can be seen, Clocks an equal distance from the red light still read the same time regardless of whether they are moving with respect to the red light or not. Of course, since the blue light changes position with respect to the red light, this means clocks an equal distance from the blue light do not read the same time.

Now let's turn the blue light back on and have it turn on the clocks. In its frame like we originally did, and have the red light turn on its clocks. Both light falsh as they pass each other. We still view things from the red light frame. Again, the speed of light has to be a constant, so the blue and red lights have to travel at the same speed (neck and neck), and the lights must travel at the same speed in both directions, so we have to get this.

000000L000000
000000L000000

.000000L000000
000000L000000

..000001L000000
000000L000000

...000012L000000
000001L100000

....001234L0000000
000012L210000

The results are the same as last time. Clocks an equal distance from the blue light do not read equal times.

If I were to switch so that we we at rest with respect to the blue light, the opposite would be true, Clocks an equal distance from the blue light would read the same but clocks an equal distance from the red light would not.

So according to Einstein, not only is it possible for clocks that read the same times in one frame to read different times in another, there are instances where this must be the case.

48. Originally Posted by Quantime
Provide evidence for that claim.
Im not expert on biology. But "hibernation" exists in nature. By lowering bodily temperature and slowing the metabolical processes animals survive harsh conditions. On cellular levels Im told that theres a sphore condition (if I spell correctly) that stops metabolism completely. So in principle it seems possible to change all cells of the body into sphores and turning them back to normal cells at the end of eternity.

The problem with freezing and reviving the body seems to be that water in the cells freezes to ice and destroys the cells irreparably. Ive heard that some animals have glucose in their cell water and can survive freezing temperatures...But I have no evidence available.

But what state the technique of resurrecting a frozen body is at the moment is not the real issue here!
There ARE bodies (and single heads) in deep freeze right now somewhere in the USA.

THAT raises the question of how the age of the eventually revived persons should be determined.:
IF a twin is deepfrozen and successfully resurrected when his space travelling brother returns from a 100 year trip, making him only a few years older (at the return expecting his home staying brother to be dead and buried) THEN (to his surprise) the twins look to be of the same age. But are they really?

This "hibernating" cannot be done with unliving things like radium so I guess that a close look at the body tissue of the hibernated will show that even if he still has a normal lifespan to live out, his body really is older than it looks from his appearance.

My point is that it seems as if the speed of physical processes within a frame does not uniquely determine for how long a human can exist within that frame. The speed of metabolism can be different at the same speed of physical processes ,that is: in the same frame. The question of how to measure age is not satisfyingly clear. And should be permitted to be discussed in this forum. Without risk of thread closure or banishment

So as to question of "evidence"... why dont ask for it in the biology thread?
I can only point to the already deep freezed bodies waiting for future scientists to revive them.
I have no idea if they ever will be revived or not...

ALL Im saying is that although they may LOOK young at "resurrection day" they ARE obviously really old.
Indicating that there are different ways to determine "age" giving different results.

49. Originally Posted by sigurdV
Im not expert on biology. But "hibernation" exists in nature. By lowering bodily temperature and slowing the metabolical processes animals survive harsh conditions. On cellular levels Im told that theres a sphore condition (if I spell correctly) that stops metabolism completely. So in principle it seems possible to change all cells of the body into sphores and turning them back to normal cells at the end of eternity.

The problem with freezing and reviving the body seems to be that water in the cells freezes to ice and destroys the cells irreparably. Ive heard that some animals have glucose in their cell water and can survive freezing temperatures...But I have no evidence available.

But what state the technique of resurrecting a frozen body is at the moment is not the real issue here!
There ARE bodies (and single heads) in deep freeze right now somewhere in the USA.

THAT raises the question of how the age of the eventually revived persons should be determined.:
IF a twin is deepfrozen and successfully resurrected when his space travelling brother returns from a 100 year trip, making him only a few years older (at the return expecting his home staying brother to be dead and buried) THEN (to his surprise) the twins look to be of the same age. But are they really?

This "hibernating" cannot be done with unliving things like radium so I guess that a close look at the body tissue of the hibernated will show that even if he still has a normal lifespan to live out, his body really is older than it looks from his appearance.

My point is that it seems as if the speed of physical processes within a frame does not uniquely determine for how long a human can exist within that frame. The speed of metabolism can be different at the same speed of physical processes ,that is: in the same frame. The question of how to measure age is not satisfyingly clear. And should be permitted to be discussed in this forum. Without risk of thread closure or banishment

So as to question of "evidence"... why dont ask for it in the biology thread?
I can only point to the already deep freezed bodies waiting for future scientists to revive them.
I have no idea if they ever will be revived or not...

ALL Im saying is that although they may LOOK young at "resurrection day" they ARE obviously really old.
Indicating that there are different ways to determine "age" giving different results.
None of this has anything to do with the topic at hand.

51. Originally Posted by Janus
While some conditions might effect the effects of aging, they don't alter the actual fact of aging.
If you were able to put a person into a state where he could last 71 years with only 1 year physical change to his body, this does not change the fact that his body still had to exist and was subject to time for 71 years. He would be 71 yrs older, even though his body only showed the effects of 1 yr of aging

However, if a person were to travel relative to the Earth at 99.99% of c, he could return having aged 1 year with 71 years passing on the Earth, and he would only experienced 1 yr of time. Not only would he show the physical effects of 1 yrs aging, he really would be only 1 year older.
YES! You are saying exactly the same thing that I say! Congratulations!

But it seems that you dont understand that? Do you think you have contradicted me?
Theres a difference between personal age (telling approximately how long time one can be expected to live)
and age telling how long one has existed.

There are two different aging concepts. You prove that sufficiently. How come you dont accept me proving the same thing?
And what "official name is given to the the different concepts so they can be kept separated, and not be mistaken for each other? I have tried to introduce the terms "official" time or "outside time" for time of existence within the universe or in any frame inside it. Is that Ok with you?

To make sure we see the same facts let us suppose there is a THIRD twin travelling together with his brother , but , hibernating at the same time! That brother would APPEAR zero years older! But his official age is the same as his travel partner. The question I want to be answered is: Has the three brothers been parts of the universe for an equally long time?. From the view of the universe... are they of the same official age or not?

Are all parts of the universe of the same age in the "official" sense? That is: irrespectively of how time dilated they are?
Similarly to the fact that all bodies in the same frame are equally old relative the frame no matter how hibernated they are.

IF you say: "there is no universal frame" THEN you are begging the question!

It seems that the basic problem is that, according to theory, time within a frame is time dilated so there cant be a universal frame giving the same age to contents within all frames!

My tentative solution is that the "Universal Frame" contains and relates only THE FRAMES within the universe,
NOT directly the contents within each frame. Then contradiction perhaps is avoided.

The universal frame is not one of its own frames.

I see the situation as perhaps similar to Bertrand Russells paradox:
Its not really so that there is no "set" of all sets...it is a CLASS!

52. To make sure we see the same facts let us suppose there is a THIRD twin travelling together with his brother , but , hibernating at the same time! That brother would APPEAR zero years older! But his official age is the same as his travel partner.
Sigurd time is relative, period. What you are describing as aging is indeed two aging methods, one of time dilation and the other of entropy in their unique reference frames. There is no third twin, all objects in the same inertial reference frame age temporally the same. Janus has informed you of this numerous times.

Has the three brothers been parts of the universe for an equally long time?. From the view of the universe... are they of the same official age or not?
The universe is not a reference frame, I think you are trying to find a plausibility to Newtonian Time.

If you have a theory that challenges relativity as you are seemingly implying then create a thread about this in the 'Alternate Theories and New Ideas' sub-forum, and expect no lesser degree of people challenging your ideas, you aren't the first and I doubt, the last to challenge relativity with your own theory.

And Kalster has also asked kindly for you to not post in this thread anymore, this is a primer thread that Janus has given us of his own free time and effort and is not open for debate, the fact that a new thread to discuss this was already a polite privilege for scientific discussion, if you want to open a debate and refute established scientific fact, please start it in another thread as requested.

Kind Regards,
QT.

53. Originally Posted by beefpatty
Yep, it's time dilation. Sorry, I think I'm confused as to what you are confused about, haha!

I think I am confused because in one post you said,

I have opened a new thread in the miscellaneous section: the Emperor's New Clothes where I hope to answer your query.

54. Originally Posted by sigurdV
A minor question: Can friction of the interstellar or intergalactic non empty space be used as an indication of direction and speed...
I heard that doubling the speed in interstellar space multiplies friction by five.
So measuring friction might be of some use ..OR?
Note: First off I appolgize for my outburst the other day. I've been unusually ill lately and as a result mean as a rabid. It's not an excuse, just a reason. Those are two very different things. I'm usually not like that but I've been through some real life nightmares in my life lately and it's unfortunately caused PTSD. Unfortunately when I percive myself as being attacked I have reacted with the defense mechanism of a rabid dog. Please forgive me. I'm really a nice man! Illness and horrible experiences can seriously mess you up. All I ask you is that you keep in mind that I never sought these illnesses out.

Second, Farsight is right (smart man that he is). All you have to do is look at the background radiation. But all that tellls you is that you're moving relative to a particular frame of reference. The laws of physics are still the same in all such frames of reference.

55. Hi SpeedFreak,

Whassup? I apologize for the delay. I've been very ill and in such a bad mood as to be like a rabid junkyard dog who'd rip apart anybody who'd step the wrong way. So I thought that it would be best if I took a break from this thread until I was better and tried to understand what was tripping you up so much as to cause you so much confusion.

Originally Posted by SpeedFreek
Well I supposed I shouldn't have used the term "proper" speed of light, as light doesnt have a valid frame of reference anyway, but I really don't understand what the problem is here, as I thought it was obvious what I meant. I meant proper time.
Why should it be obvious what you meant? We all make mistakes and I assumed you merely made an error here. After all the only speed we’ve been talking about here was the speed of light. So when you spoke of the proper speed, which is undefined, I assumed you simply made an error. There is only one speed in this discussion and the only proper time intervals we are considering is the proper time of the light in question. When measuring the speed of light with a clock the time intervals being read off are not proper time intervals, they are coordinate time intervals. What “proper time” intervals are you talking about? By the way, in a previous post you wrote Not in their own frame of reference, they don't..I have to admit that I was confused by this in an earlier post. However if you will recall light does not have its own frame of reference. And please recall my response

That is only true when the poptential of the particle is zero, i.e. at the same level as the observer. However if I was on the ground and a light beam was bounced between the tops of two skyscrapers then the observer on the ground would measure the speed of light to be faster than when measured at ground level.
Do you see what I said here? I said that its true only when the potential of the particle is zero. But if a beam of light is moving through a gravitational field its position will change and with that change comes a change in position and thus a change in potential and thus a change in its speed. Nobody has denied that the local speed of light is anything but 3x10^8 m/s, nobody. So I don’t know why you’re worried about it. I said that the speed of light changes as if moves through a gravitational field, perdiod. If you choose to think only in terms of the local speed of light then I’m sure everybody will agree to let you think that way. I have no objections. But here is a fact

c= c0(1 + Phi(z)/c2)

As the light moves from one value of z to another and the potential changes from one value to another as a result then c will change as a result – Perdiod. Fact! Whenever you hear someone say that the speed of light speeds up or slows down in a non-inertial frame of reference then this is what they mean. It is what Eintein meant too.

This is what I meant:

Sit in your spaceship with a small light clock in your lap. The light in the clock bounces back and forth or up and down at the same speed, relative to you, whether you are accelerating or not! Do you deny this?
Okay. Sure. But I already agreed to that, didn’t I? As I said in post #24 That is only true when the poptential of the particle is zero. In your (extremely bizzare) example the potential remains zero. I call it bizzare because clocks in relativity are taken to be objects of negligible spatial extent. If you have a pulse of light moving inside the clock then you’re ignoring the meaning of what a clock is. You should have referred to a pulse moving from x to x+dx where Phi = 0.

However the speed of light changes as it moves through the frame of reference. I do as does any physicist who properly understands relativity and the physics related to this scenario. I've even given you a mathematical proof that what I’ve explained is right. Had you followed the derivation in post #24 you’d have seen precisely what it all means. There’d be no question of what is meant by the speed of light changes as it moves through a non-inertial frame of reference. To see the proof again just go to my website on this subject - http://home.comcast.net/~peter.m.bro..._in_gfield.htm

Let me refer you to the original work. See On the Influence of Gravitation on the Propagation of Light by Albert Einstein, Annalen der Physik, 35, 1911 section 3 Equation #3. Einstein writes
If we call the velocity of light at the origin of coordinates c0 then the velocity of light c at a place with the gravitational potential Phi will be given by the relation

(3) c = c0(1 + Phi/c2)

The principle of the constancy of the velocity of light holds good according to this theory in a different form from that which usually underlies the ordinary theory of relativity.
The only thing that’s changed in the last 102 years is the definition of the term “gravitational field..” That’s the reason you thought I was talking about GR in a thread about SR.

What I have said is not something new, not a new theory, not controversial and can be found in many places in GR textbooks. Why GR textbooks when we’re talking about SR you mat ask. Because GR textbooks are the only textbooks which extensively talk about non-inertial frames of reference.

You went on to say
Because this is what I am talking about when I say the speed of light is always c, locally, whether you are accelerating or not. Or are you trying to tell us that if you are accelerating you can measure the time-dilation of your own clock?!
“Because this is what I am talking about ..“? Yeah, but I already told you that I agreed to that. Why come back to it again? Didn’t you read that I said “True…” or perhaps you were confused because I was using my other handle?

In any case, you seem to have jumped to and focused on local measurements of c when that was not what I was talking about and what is not talked about when people talk about the speed of a beam of light moving through a non-inertial frame of reference.

Please don’t take what I state and start thinking that it’s wrong merely because you never use the term “speed” to mean anything but what you measure locally. I can’t imagine why you’d think that way. Its an important point but hardly something to cling to when talking about how fast a beam of light is moving when its not a phi = 0. If the light in your spaceship cabin (which is accelerating uniformly in flat spacetime) bounces from floor to ceiling then the speed of light will change as reckoned by the observer at z = 0.

Let me clarify something for you since you thought I was dragging GR into an SR thread – Definitions changed and I am not to blame. I cannot be blamed for people calling things “gravitational potentials” even when their non-zero in a flat spacetime. Blame someone else, not I and certainly not Einstein. Why? Because of the Einstein’s Equivalence Principle which states
A uniformly accelerating frame of reference in flat spacetime is equivalent to a uniform gravitational field.
I hope that with this principle stated that you can see where the confusion lies. Einstein defined the presence of a gravitational field according to whether the frame of reference was inertial or not whereas modern physicists define the presence of gravitational field according to the presence of spacetime curvature. That means that the subject of light moving in a non-inertial frame of reference in flat spacetime with non-zero “potentials” (whether you want to “call” then gravitational potential or field potentials or whatever) now belongs within the domain of special relativity.

Got it now?

See the confusion you are causing?
I appologize for being the bearer of bad news but it is you who is confused my good man. All I see is how you’ve been confusing this by trying to restrict the speed of light to local measurements and not understanding that something’s that used to belong to the domain of GR now belong to the domain of SR with the unfortunate terminology tagging along with it, i.e. gravitational potential.

Did it never dawn on you that, just perhaps, it was you who was confused due to some misunderstanding of relativity that you were unaware of? Hey. We’re all human and we all make mistakes. But I seriously object to being accused of causing confusion. Just because you were confused doesn’t mean that I caused it. In this case you changed the problem from something I stated (c varies with Phi) to something you have in mind, i.e. c = 3x10^8 m/s when Phi = 0

I understand that some of you will balk at such terms like "gravitational/field potential" or "gravitational force? Therefore please see either the article I wrote which I refer to you below or this web page
Gravitational Force

You can see more GR at My website

I can and will give you or anybody the references to anything and everything that I've mentioned anywhere on the internet.

All the details and references about everything I’ve said are in a paper I wrote. Whoever is truly interested in this subject can read it online at [physics/0204044] Einstein's gravitational field

Now I have a request - Please don't accuse me of things, e.g. like "causing confusion", in the future. In return I will do the same and not accuse you of anything too. Is it a deal?

Okey dokey smokey?

56. Originally Posted by SpeedFreek
Popper, it doesn't help that in your post #4 you mislabelled S as S'. It is confusing!
(sigh) So what do you want me to do? Apologize for being human and making an error with all those primes floating around? Okay. I apologize for being human. Lol!

Originally Posted by SpeedFreek
So, in this thread about Special Relativity, what exactly are we discussing?
Special relativity.

Originally Posted by SpeedFreek
It seems like you are trying to introduce GR into a discussion about SR. This is where the confusion lies.
Then you’ve made a very serious mistake. You're confused perhaps for two reasons (1) I used gravitational fields as examples of inertial frames of reference which are accelerating with rrespect to other inertial frames of reference.

Let me clarify this to make sure you're not confused again - There is such a thing as a "locally inertial frame of reference. Then there are inertial frames of reference which are far removed from gravitational bodies. Suppose I'm in Space Station Freedom. Then I'm in a locally inertil frame of reference. Let's say that you're in a spaceship very far from a gravitational source. Then when your rocket engines are off and you're in an inertial frame then your frame is accelerating with respect to the space station's frame. Yet each frame of reference is loally inertial.

(2) you might be confused by me using gravitational potentials in a thread about SR. Sorry but I'm not in control of definitions. You can have a non-zero gravitational potential in a flat spacetime. If you don't like the name then you can refer to it as a "field potential" or whatever. Don't let names of terms confuse you.

Frames of reference in free-fall in a gravitational field are inertial frames of reference. That’s one of the reasons I objected to his defining inertial frames that are “not accelerating.” I’m sitting in my apartment right now at rest in my chair. I am at rest in my frame of reference. If there were a space capsule in free-fall then, as reckoned by me, that frame of reference is accelerating with respect to me even though according to the observers inside the frame is not accelerating. That’s why I asked “accelerating with respect to whom?” I should have simply stated the more widely known definition of inertial frame as a frame of reference in which the law of inertia holds true and skipped all that unnecessary bickering.

Originally Posted by SpeedFreek
In Minkowski spacetime, acceleration is not relative, it is absolute. Why are you introducing gravity into a discussion about Special Relativity? I am just trying to understand the point you are making.
Who spoke of Minkowski spacetime? Certainly not I. I was mentioning that accelerating is always relative to something. In some cases its relative to a gravitational field. That was my point. When people go to GR they’re not used to free-fall frames as being inertial. That’s all I was cautioning and it doesn’t matter anymore because nobody cares about it. The subject is dead now. I just wanted to make sure that you weren’t confused anymore.

So, as Ace Ventura, Pet Detective, would say "Well alrighty then!"

57. So..... ignoring anything about gravity or the equivalence principle, as neither apply to the examples in the SR primer...

Originally Posted by Popper
Frames of reference in free-fall in a gravitational field are inertial frames of reference. That’s one of the reasons I objected to his defining inertial frames that are “not accelerating.”
Janus did not mention anything about gravitational fields, except that his scenario was set "far away from any gravity source". So, in the spirit of SR, nowhere in the primer are gravitational fields an issue. These Special Relativity thought experiments are essentially set in an empty universe, so the fact that freely falling frames in gravitational fields are inertial frames (which is not strictly correct anyway) is seemingly irrelevant to the discussion, as I have said all along.

In the context of the SR primer, an inertial frame is a frame in uniform motion - it is coasting at a constant speed in an essentially empty universe - it is not accelerating - there are no fictitious forces felt in an inertial frame in SR - it is not subject to any external forces. Is this clear enough?

An inertial frame is a frame which is not accelerating.

Originally Posted by Popper
I’m sitting in my apartment right now at rest in my chair. I am at rest in my frame of reference.
You can feel a fictitious force acting upon you right now. You are in an accelerated frame, because you are at rest in a uniform gravitational field, resisting gravity rather than freely falling. Not that this has anything to do with the SR primer, of course.

Originally Posted by Popper
If there were a space capsule in free-fall then, as reckoned by me, that frame of reference is accelerating with respect to me even though according to the observers inside the frame is not accelerating.
That frame is indeed accelerating in respect to you, but this is simply because you are at rest in an approximately uniform gravitational field, and thus you need to use GR where the principle of inertia has been replaced by the principle of geodesic motion.

Which, once again, has nothing to do with the contents of the Special Relativity primer.

Originally Posted by Popper
That’s why I asked “accelerating with respect to whom?” I should have simply stated the more widely known definition of inertial frame as a frame of reference in which the law of inertia holds true and skipped all that unnecessary bickering.
But Janus defined an inertial frame as a frame that is NOT accelerating, and is far away from any gravity. So it is accelerating with respect to nobody. The law of inertia applies to frames that are not subject to external forces - if there are external forces which cause a change in the velocity of the frame, then the frame is non-inertial.

Originally Posted by Popper
Who spoke of Minkowski spacetime? Certainly not I. I was mentioning that accelerating is always relative to something. In some cases its relative to a gravitational field. That was my point.
And Janus's point was that an inertial frame is NOT accelerating in relation to anything, as there is no gravitational field to consider... it is a frame that is just coasting along at a constant velocity.

Originally Posted by Popper
When people go to GR they’re not used to free-fall frames as being inertial.
Freely-falling frames in GR aren't really inertial. A freely-falling frame in GR might be the closest thing to an inertial frame in SR, but it is not the same as an inertial frame. You might want to consider whether referring to freely-falling frames in GR as inertial might introduce some misconceptions, and just stick to referring to geodesic motion, where it might be more obvious that two freely falling frames in a uniform gravitational field can change their velocity relative to each other, and thus the principle of inertia doesn't actually hold true as both frames are subject to an external force.

Anyway, when are we going to actually discuss the examples given in the SR primer?

58. I have read through this thread hoping to find the answer to something I'm not clear on. It's entirely possible that I missed it, but for clarity could I ask if, in the twins paradox, were they to count the swings of a pendulum in the others frame, would they agree?

59. Originally Posted by wilbo
I have read through this thread hoping to find the answer to something I'm not clear on. It's entirely possible that I missed it, but for clarity could I ask if, in the twins paradox, were they to count the swings of a pendulum in the others frame, would they agree?
No, it doesn't matter how you measure time: mechanical clock (e.g. pendulum), atomic clock, biological age, etc. you will get the same result.

60. Originally Posted by Strange
No, it doesn't matter how you measure time: mechanical clock (e.g. pendulum), atomic clock, biological age, etc. you will get the same result.
Thanks Strange. Actually I don't think I made myself clear. What I meant to ask was that if the twins were to count any physical event in the others frame, would they each have counted the same number when they were reunited?

61. Originally Posted by wilbo
Thanks Strange. Actually I don't think I made myself clear. What I meant to ask was that if the twins were to count any physical event in the others frame, would they each have counted the same number when they were reunited?
That is slightly ambiguous. What are they counting?

If twin A counts B's pendulum swings, then he will agree with B. If B counts A's pendulum swings, then he will agree with A.

If they each count their own pendulum swings then they will disagree.

62. Originally Posted by Strange
That is slightly ambiguous. What are they counting?

If twin A counts B's pendulum swings, then he will agree with B. If B counts A's pendulum swings, then he will agree with A.

If they each count their own pendulum swings then they will disagree.
Thank you for your patience. I'm just trying to understand whether once the twins are reunited one of the pendulums has swung more times than the other.

63. Originally Posted by wilbo
Thank you for your patience. I'm just trying to understand whether once the twins are reunited one of the pendulums has swung more times than the other.
In the usual description, where one twin stays where he is whilst the other flys away and then comes back again, the answer to that is yes. One of their clocks will show less elapsed time than the other, when they meet.

64. Originally Posted by SpeedFreek
In the usual description, where one twin stays where he is whilst the other flys away and then comes back again, the answer to that is yes. One of their clocks will show less elapsed time than the other, when they meet.
So does it follow that any physical event you care to count happens less the faster you go?

65. Originally Posted by wilbo
So does it follow that any physical event you care to count happens less the faster you go?
As viewed from another frame of reference, yes.

Note that the twin "paradox" is a bit of a special case as it is not symmetrical. In the more general case of two frames of reference in relative motion with no acceleration then both will see the other's clocks/pendulums/lifetimes running slower.

66. Originally Posted by wilbo
Thank you for your patience. I'm just trying to understand whether once the twins are reunited one of the pendulums has swung more times than the other.
Aha!
Take two clocks made and synchronised at the same time.
Rotate or oscillate one very fast and observe and compare time passing on the clocks...
Then you should be able to observe what you are after. How do you interprete the result?

67. Originally Posted by Strange
As viewed from another frame of reference, yes.

Note that the twin "paradox" is a bit of a special case as it is not symmetrical. In the more general case of two frames of reference in relative motion with no acceleration then both will see the other's clocks/pendulums/lifetimes running slower.
Fair enough. So when the twins are reunited, will the twin that experienced no acceleration have counted fewer pendulum swings in the others frame than the rocket riding twin will count in his?

68. Originally Posted by wilbo
Fair enough. So when the twins are reunited, will the twin that experienced no acceleration have counted fewer pendulum swings in the others frame than the rocket riding twin will count in his?
It is the other way around - the travelling twin is subject to time dilation, so he counts fewer swings than the stationary twin, i.e. less proper time elapses for him. This is why he ages less than the twin left behind on earth.
In general terms, a stationary observer not subject to gravitational fields always experience the most proper time.

69. Originally Posted by Markus Hanke
It is the other way around - the travelling twin is subject to time dilation, so he counts fewer swings than the stationary twin, i.e. less proper time elapses for him. This is why he ages less than the twin left behind on earth.
In general terms, a stationary observer not subject to gravitational fields always experience the most proper time.
Actually, both statements are right (if I have understood wilbo's question).

Earthbound Twin (ET) will count more of his own pendulum swings than Rocket Twin (RT) does his own. But ET will count the same number of RT's pendulum swings as RT does. After all, they must agree at the end about the difference in age (pendulum swings).

70. Originally Posted by sigurdV
Aha!
Take two clocks made and synchronised at the same time.
Rotate or oscillate one very fast and observe and compare time passing on the clocks...
Then you should be able to observe what you are after. How do you interprete the result?
As far as I understand, the rotating or oscillating clock should register a given interval after the clock it is being compared with. I'm not after anything in particular, but one way to interpret this is that the interaction between matter and the dimensions that mark it's location in space and time cause the dimensions to be warped. If that if a fair assessment, is there anywhere I can find out about the mechanism by which matter and dimensions affect each other?

Strange has rephrased my question better than me:

Originally Posted by Strange
Earthbound Twin (ET) will count more of his own pendulum swings than Rocket Twin (RT) does his own. But ET will count the same number of RT's pendulum swings as RT does. After all, they must agree at the end about the difference in age (pendulum swings).
The Hafele-Keating experiment, and as far as I know, every subsequent test of relativity, supports the theory. Has there been any test that demonstrates that time is slowing down rather than that a physical event happens after a comparable event in our frame?

71. Originally Posted by wilbo
If that if a fair assessment, is there anywhere I can find out about the mechanism by which matter and dimensions affect each other?
I'm not sure what you mean by "mechanism"; it appears to be the way that the geometry of the universe works.

The Hafele-Keating experiment, and as far as I know, every subsequent test of relativity, supports the theory. Has there been any test that demonstrates that time is slowing down rather than that a physical event happens after a comparable event in our frame?
And I'm not sure what you mean by "a physical event happens after a comparable event in our frame".

But every experiment and observation either directly (Hafele-Keating, muon lifetimes, GPS operation) or indirectly (the success of QED, the many 10s or 100s of thousands of different tests of Lorentz invariance) confirms time dilation.

72. Originally Posted by Strange
Actually, both statements are right (if I have understood wilbo's question).

Earthbound Twin (ET) will count more of his own pendulum swings than Rocket Twin (RT) does his own. But ET will count the same number of RT's pendulum swings as RT does. After all, they must agree at the end about the difference in age (pendulum swings).
ET does not count the number of RT's pendulum swings and RT does not count the number of ET's pendulum swings. In general relativity there is no meaning to "time here" vs "time there". Time (and space) is a local notion only.

If by some means ET were receiving light from RT's pendulum he would count the same number of swings as would RT, simply because a swing is an event and events are invariant. But due to Doppler shift and the finite speed of light the RT swings would seem very slow on the outbound trip and very fast on the inbound trip. But that is an optical effect, and what one sees with one's eyes or with a camera is quite different from what is actually happening. For more on this sort of thing see the chapter on Einsteinian optics in Rindler's Essential Relativity, Special, General and Cosmological.

73. Originally Posted by wilbo
As far as I understand, the rotating or oscillating clock should register a given interval after the clock it is being compared with.
I agree. So it is "matter" that interests you?
We could test by putting organic tissue from the same source in the clocks.
Matter in the rotating clock will not age as fast as in the stationary clock.

Lets forget about Relativity for a moment:
What is the Thermodynamic description of the situation?

74. Originally Posted by Strange
I'm not sure what you mean by "mechanism"; it appears to be the way that the geometry of the universe works.
The main problem I have, I suppose, is that I am trying to understand a mathematical treatment of the universe and I just don't have the mathematical smarts to do so. I don't know how literally I am supposed to take the 'geometry of the universe'.

Originally Posted by Strange
And I'm not sure what you mean by "a physical event happens after a comparable event in our frame".

But every experiment and observation either directly (Hafele-Keating, muon lifetimes, GPS operation) or indirectly (the success of QED, the many 10s or 100s of thousands of different tests of Lorentz invariance) confirms time dilation.
Well the classic example is the light clock on the train. As I understand it, it shows that the passenger and the bystander will each see the others clock run slower, but that both of them will experience the 'ticking' of their clock as though it were bouncing straight up and down. Is it not the case though that neither clock is stationary and that both beams of light have to follow a more complicated path than someone at rest relative to a clock will perceive? The same must also be true of every particle in that frame, so that every physical event, which ultimately are all the result of particle exchanges, is slowed by exactly the same amount as the light clock. The maths of relativity works, but does it follow that the tools it uses, dimensions, geometry and stuff I don't even know about, exist?

75. Originally Posted by wilbo
The main problem I have, I suppose, is that I am trying to understand a mathematical treatment of the universe and I just don't have the mathematical smarts to do so. I don't know how literally I am supposed to take the 'geometry of the universe'.
Well, the mathematics (of special relativity, at least) is fairly simple: the Lorentz transform. It just describes a factor by which lengths, times are changed relative to another frame of reference (state of motion). One way of visualising this is as a rotation.

Warning: Analogy ahead; this is not to be taken too literally. If an object is stationary (relative to you) then the only "motion" it has is through time (which you can consider to be at 'c' or 1 second per second - neither of which really mean anything). If it starts moving through space (relative to you) then it "swaps" some of its motion through time for its motion through space. As it gets close to c (speed of light) it is hardly moving through time at all.

(I'm sure Dr Rocket will have something pointed to say about that, but if it helps you visualise things ... )

As to how literally to take the geometry of space time? How literally do you take triangles or squares or even numbers? Are they "real"? I don't know: that is philosophy.

Well the classic example is the light clock on the train. As I understand it, it shows that the passenger and the bystander will each see the others clock run slower, but that both of them will experience the 'ticking' of their clock as though it were bouncing straight up and down. Is it not the case though that neither clock is stationary and that both beams of light have to follow a more complicated path than someone at rest relative to a clock will perceive? The same must also be true of every particle in that frame, so that every physical event, which ultimately are all the result of particle exchanges, is slowed by exactly the same amount as the light clock. The maths of relativity works, but does it follow that the tools it uses, dimensions, geometry and stuff I don't even know about, exist?
Sounds a bit like you are looking for some sort of "mechanical" explanation. I'm not sure that works.

76. Has there been any test that demonstrates that time is slowing down rather than that a physical event happens after a comparable event in our frame?
Atmospheric muons.
Muons are particles with a very short lifespan ( 2.2ns ), and there are plenty of them being generated by the interaction of energetic cosmic rays with the upper atmosphere. Classical mechanics tell us that those particles' lifespans are too short for them to reach the earth's surface before they decay, even at speeds close to the speed of light. However, we do indeed detect them here on earth. The resolution is of course that, because they travel at relativistic speeds, they experience time dilation as compared to a stationary observer on earth. In the earth's frame, this in practical terms means that their lifespan is extended, giving them enough time to reach the surface, and this is exactly what we find.
From the point of view of the muon itself nothing special happens on the clock, but the depth of the earth's atmosphere along the direction of the particle's travel is length contracted, leading to the exact same result.

77. Originally Posted by DrRocket
ET does not count the number of RT's pendulum swings and RT does not count the number of ET's pendulum swings. In general relativity there is no meaning to "time here" vs "time there". Time (and space) is a local notion only.

If by some means ET were receiving light from RT's swing pendulum he would count the same number of swings as would RT, simply because a swing is an event and events are invariant.
I think this is the bit that causes, if not the most confusion, then a fair portion of it. To some lay people a clock is a device that counts physical events. What I understand you to be saying is that a clock on RT's rocket will count the same number of swings as one that remains with ET. To me that means that the clocks will read the same, but that in fact more time has passed in ET's frame.
Thank you for the reference to Rindler, I shall look it up.

78. Originally Posted by wilbo
I think this is the bit that causes, if not the most confusion, then a fair portion of it. To some lay people a clock is a device that counts physical events. What I understand you to be saying is that a clock on RT's rocket will count the same number of swings as one that remains with ET.
I said no such thing.

I said that if ET could literally see (receive light from) RT's pendulum he would see the same number of swings of RT's pendulum that RT sees, because a swing is an event and events are invariant.

However, ET's clock will not register the same passage of time between that point at which RT and ET are together and that point at which they reunite after RT's trip.

79. Originally Posted by DrRocket
I said no such thing.

I said that if ET could literally see (receive light from) RT's pendulum he would see the same number of swings of RT's pendulum that RT sees, because a swing is an event and events are invariant.

However, ET's clock will not register the same passage of time between that point at which RT and ET are together and that point at which they reunite after RT's trip.
That is what I was trying to say in post #69. Maybe not as clearly.

80. Originally Posted by wilbo
I think this is the bit that causes, if not the most confusion, then a fair portion of it. To some lay people a clock is a device that counts physical events. What I understand you to be saying is that a clock on RT's rocket will count the same number of swings as one that remains with ET. To me that means that the clocks will read the same, but that in fact more time has passed in ET's frame.
Thank you for the reference to Rindler, I shall look it up.
W.Rindler Ph.D? No matter.
My point with the two clocks is that you all the time can compare the clocks!
In principle the rocket need not leave earth... just let it rotate fast enough!
Then "Here on Earth" applies to both twins. No problem in distance is involved.
Better laboratory conditions both twins can be observed in the same moment
How Thermodynamics enter the picture is probably a matter for professionals:
But since time is different for both systems, then shouldnt entropy differ as well?

81. My interest in the situation takes another direction:
Its said that observers in constant motion relative each other
will see its other as time retarded ...

What happens if they both start spinning within their ships?
They become as dizzy as sigurdV?

82. Originally Posted by sigurdV
W.Rindler Ph.D? No matter.
My point with the two clocks is that you all the time can compare the clocks!
In principle the rocket need not leave earth... just let it rotate fast enough!
Then "Here on Earth" applies to both twins. No problem in distance is involved.
Better laboratory conditions both twins can be observed in the same moment
How Thermodynamics enter the picture is probably a matter for professionals:
But since time is different for both systems, then shouldnt entropy differ as well?
Rotation has nothing to do with it. In fact general relativity cannot handle intrinsic rotation, and neither can special relativity. They can handle circular motion, but we have not specified any motion of that sort.

I have no idea what you are contemplating in your consideration of entropy. We are dealing here with a theory that is really a theory regarding the nature of space and time and have not at any point specified any thermodynamic system. Thermodynamics is inherently statistical in nature and applies to systems that consist of a large number of particles. While the bodies that are at issue here would in reality be composed of a large number of atoms, and one could discuss the thermodynamics in that sense, we have not begun to address the question at the atomic level So for the purposes at hand thermodynamics is irrelevant.

Moreover, the fact that two clock may be located relatively close to one another does not particularly affect the fact that one cannot compare the readings of those clocks when relativistic effects are important. The clocks are not coincident, and if they are, no matter how you perturb the problem statement, still in motion with respect to one another at some speed sufficiently high to make relativity important. Thus you cannot make that comparision. Were you to consider them as macroscopic bodies of some size and spin one or the other really really fast, then you would have a real mess since various parts of the clock would be in motion relative to the lab at wildly differing speeds.

If you were to literally spin one twin so that the portions of his body most removed from the axis of rotation were moving at any appreciable fraction of the speed of light, then his body would literally fly apart due to the centripetal acceleration.

83. Originally Posted by DrRocket
I said no such thing.
My mistake. In the context I presumed you were contradicting Strange:

"Earthbound Twin (ET) will count more of his own pendulum swings than Rocket Twin (RT) does his own. But ET will count the same number of RT's pendulum swings as RT does. After all, they must agree at the end about the difference in age (pendulum swings)."

As you say, though:
Originally Posted by DrRocket
... ET's clock will not register the same passage of time between that point at which RT and ET are together and that point at which they reunite after RT's trip.
Which I presume means the pendulum in ET's clock swings more times than the one in RT's.

84. Originally Posted by DrRocket
Rotation has nothing to do with it. In fact general relativity cannot handle intrinsic rotation, and neither can special relativity.
Ok if it cant handle spinning objects then its not a complete theory. Sorry to hear that. Im a fan of Albert E.
So move the rotation axis out of the rotating object. Will that Relitivity now be abppliable?
(Its true I didnt specify the experiment...I wanted only principles not actual technical solutions.)

Originally Posted by DrRocket

They can handle circular motion, but we have not specified any motion of that sort.
Hopefully an expert may be imaginative and assisting in interpreting and applying the conditions to reality.
Originally Posted by DrRocket

I have no idea what you are contemplating in your consideration of entropy. We are dealing here with a theory that is really a theory regarding the nature of space and time and have not at any point specified any thermodynamic system. Thermodynamics is inherently statistical in nature and applies to systems that consist of a large number of particles. While the bodies that are at issue here would in reality be composed of a large number of atoms, and one could discuss the thermodynamics in that sense, we have not begun to address the question at the atomic level So for the purposes at hand thermodynamics is irrelevant.
Originally Posted by DrRocket

Moreover, the fact that two clock may be located relatively close to one another does not particularly affect the fact that one cannot compare the readings of those clocks when relativistic effects are important. The clocks are not coincident, and if they are, no matter how you perturb the problem statement, still in motion with respect to one another at some speed sufficiently high to make relativity important. Thus you cannot make that comparision. Were you to consider them as macroscopic bodies of some size and spin one or the other really really fast, then you would have a real mess since various parts of the clock would be in motion relative to the lab at wildly differing speeds.

If you were to literally spin one twin so that the portions of his body most removed from the axis of rotation were moving at any appreciable fraction of the speed of light, then his body would literally fly apart due to the centripetal acceleration.

85. Originally Posted by wilbo
My mistake. In the context I presumed you were contradicting Strange:

"Earthbound Twin (ET) will count more of his own pendulum swings than Rocket Twin (RT) does his own. But ET will count the same number of RT's pendulum swings as RT does. After all, they must agree at the end about the difference in age (pendulum swings)."

As you say, though:

Which I presume means the pendulum in ET's clock swings more times than the one in RT's.
Originally Posted by DrRocket
Rotation has nothing to do with it. In fact general relativity cannot handle intrinsic rotation, and neither can special relativity.
Ok if it cant handle spinning objects then its not a complete theory. Sorry to hear that. Im a fan of Albert E.
So move the rotation axis out of the rotating object. Will that Relitivity now be abppliable?
(Its true I didnt specify the experiment...I wanted only principles not actual technical solutions.)

Originally Posted by DrRocket

They can handle circular motion, but we have not specified any motion of that sort.
Hopefully an expert may be imaginative and assisting in interpreting and applying the conditions to reality.
Originally Posted by DrRocket

I have no idea what you are contemplating in your consideration of entropy. We are dealing here with a theory that is really a theory regarding the nature of space and time and have not at any point specified any thermodynamic system. Thermodynamics is inherently statistical in nature and applies to systems that consist of a large number of particles. While the bodies that are at issue here would in reality be composed of a large number of atoms, and one could discuss the thermodynamics in that sense, we have not begun to address the question at the atomic level So for the purposes at hand thermodynamics is irrelevant.
Yes now when you mention it I never saw thermodynamics referred to in this forum before!
As you very well know there are cases that needs different theories in order to be properly described and explained? (Big bang needs both quantum and relativity theory,they say.)When "observers" are discussed it should perhaps not be overlooked that they are Thermodynamical Systems? Is this lack of communication between different scientific departments some "Babel Tower Effect"? To a layman like me it seems obvious that if TIME slows down for a system then this should change the thermodynamical description of the system in question reflecting the change in physical circumstances, and comparing might give science new insights.

Originally Posted by DrRocket

Moreover, the fact that two clock may be located relatively close to one another does not particularly affect the fact that one cannot compare the readings of those clocks when relativistic effects are important. The clocks are not coincident, and if they are, no matter how you perturb the problem statement, still in motion with respect to one another at some speed sufficiently high to make relativity important. Thus you cannot make that comparision. Were you to consider them as macroscopic bodies of some size and spin one or the other really really fast, then you would have a real mess since various parts of the clock would be in motion relative to the lab at wildly differing speeds.

If you were to literally spin one twin so that the portions of his body most removed from the axis of rotation were moving at any appreciable fraction of the speed of light, then his body would literally fly apart due to the centripetal acceleration.
Tecnical obstacles doesnt concern "thought experiments".
Its the principles we are searcing for,not the tecnical solutions needed for testing the deduced principles!
All we demand is that anything assumed is not IMPOSSIBLE in PRINCIPLE, details (thought experimentators like Einstein) is left for the future. The clocks, to concern about a detail, need not be mechanical in natuer I suppose light reflectin from mirrors on opposite sides within a centrifuge might be used. I dont care how you test the principles later on...lets find the principles first by accepting assumptions:

Mr Einstein there are no elevators in empty space!

In the twin paradox the AGE of the twins is discussed and time itself is not what causes the aging of the twins...I think it is more connected to the thermodynamics of the system...but who knows IF we dont care to look?

Returning to the question of how observers see each other when in constant movement seeing each others times pass slower...my question really is if this is still true if both observers experience strong gravitational effects as if they were on planets moving away from each other with constant speed. I never saw the possibility taken into consideration.

If a strong field makes the observers "see the truth" then why does not the very weak field generated by their bodies have the same effect?

86. Originally Posted by sigurdV
Ok if it cant handle spinning objects then its not a complete theory. Sorry to hear that. Im a fan of Albert E.
So move the rotation axis out of the rotating object. Will that Relitivity now be abppliable?
(Its true I didnt specify the experiment...I wanted only principles not actual technical solutions.)
I did not say that it cannot handle spinning objects. I said that it cannot handle intrinsic spin.

But you did not specific anywhere near enough information to address a clock as a spinning object. Read the rest of what I wrote and you will see that there are serious problems with your scenario.

Originally Posted by sigurdV
Hopefully an expert may be imaginative and assisting in interpreting and applying the conditions to reality.
Yes now when you mention it I never saw thermodynamics referred to in this forum before!
You don't need an expert. You need a clairvoyant who can figure out what you are thinking.

While we can interpret problems and apply those interpretations to reality, it is not possible to convert nonsense into logic. Just specifying that a clock is spinning tells us nothing. What is apparent is that you do not understand special relativity, even though you might think that you do. There are some subtle logical consequences of special relativity, one of which is the non-existence of rigid bodies, and a consequence of that, one cannot spin objects at high rates of revolution, else one could get the outer portions traveling faster than light.

We can handle thermodynamics just fine. But thermodynamics has nothing to do with the problem that was posed. Again, read what I wrote.
As you very well know there are cases that needs different theories in order to be properly described and explained? (Big bang needs both quantum and relativity theory,they say.)When "observers" are discussed it should perhaps not be overlooked that they are Thermodynamical Systems? Is this lack of communication between different scientific departments some "Babel Tower Effect"? To a layman like me it seems obvious that if TIME slows down for a system then this should change the thermodynamical description of the system in question reflecting the change in physical circumstances, and comparing might give science new insights.

Tecnical obstacles doesnt concern "thought experiments".
Its the principles we are searcing for,not the tecnical solutions needed for testing the deduced principles!
All we demand is that anything assumed is not IMPOSSIBLE in PRINCIPLE, details (thought experimentators like Einstein) is left for the future. The clocks, to concern about a detail, need not be mechanical in natuer I suppose light reflectin from mirrors on opposite sides within a centrifuge might be used. I dont care how you test the principles later on...lets find the principles first by accepting assumptions:

Mr Einstein there are no elevators in empty space!

In the twin paradox the AGE of the twins is discussed and time itself is not what causes the aging of the twins...I think it is more connected to the thermodynamics of the system...but who knows IF we dont care to look?

Returning to the question of how observers see each other when in constant movement seeing each others times pass slower...my question really is if this is still true if both observers experience strong gravitational effects as if they were on planets moving away from each other with constant speed. I never saw the possibility taken into consideration.

If a strong field makes the observers "see the truth" then why does not the very weak field generated by their bodies have the same effect?[/QUOTE]

87. Originally Posted by sigurdV
As you very well know there are cases that needs different theories in order to be properly described and explained? (Big bang needs both quantum and relativity theory,they say.)When "observers" are discussed it should perhaps not be overlooked that they are Thermodynamical Systems? Is this lack of communication between different scientific departments some "Babel Tower Effect"? To a layman like me it seems obvious that if TIME slows down for a system then this should change the thermodynamical description of the system in question reflecting the change in physical circumstances, and comparing might give science new insights.

Tecnical obstacles doesnt concern "thought experiments".
Its the principles we are searcing for,not the tecnical solutions needed for testing the deduced principles!
All we demand is that anything assumed is not IMPOSSIBLE in PRINCIPLE, details (thought experimentators like Einstein) is left for the future. The clocks, to concern about a detail, need not be mechanical in natuer I suppose light reflectin from mirrors on opposite sides within a centrifuge might be used. I dont care how you test the principles later on...lets find the principles first by accepting assumptions:

Mr Einstein there are no elevators in empty space!

In the twin paradox the AGE of the twins is discussed and time itself is not what causes the aging of the twins...I think it is more connected to the thermodynamics of the system...but who knows IF we dont care to look?

You have no idea what you are talking about.

The basics of the big bang require only general relativity. In fact we have no idea how to apply quantum theory to the conditions of the big bang, as there is no quantum theory that is applicable in the presence of extreme curvature of spacetime. Basically the theory of the first few moments following the big bang is a complete mystery. But the big bang itself is a logical consequence of general relativity, the observed expansion of the universe, and the observation of some minimal amount of matter in the universe -- a theorem of Hawking and Penrose.

Thought experiments are very much constrained by basic scientific principles, which you seem to want to call technical details. Your centrifuge has a radius and that is quite important to the problem as you seem to want to pose it. It is linear notion that is important, not angular rate of spin.

You need to take a closer look at your "assumptions" and recognize that those "assumptions" fail to adequately describe the experiment that you seem to have in mind.

Originally Posted by sigurdV
Returning to the question of how observers see each other when in constant movement seeing each others times pass slower...my question really is if this is still true if both observers experience strong gravitational effects as if they were on planets moving away from each other with constant speed. I never saw the possibility taken into consideration.
There is indeed an effect due to gravitation. Google "gravitational time dilation". But be careful this involves coordinate time, which is different from the time that clocks measure, which is the proper time of a segment of the world line of the clock.

The twin paradox is most cleanly explained using general relativity. You can find more on this elsewhere in this forum, but the basic idea is that the non traveling twin has a world line that is nearly a geodesic (one can help by placing the non-traveling twin at one of the poles of the earth) while the traveling, and accelerating, twin's world line is clearly not a geodesic. This the proper time between the two spacetime points at which they are coincident is greater for the non-traveling twin.

If both twins are on planets moving away from each other at constant speed, then they never meet after they part and one cannot compare their ages. In general relativity there is no meaning to "time here" vs "time there"..

Originally Posted by sigurdV
If a strong field makes the observers "see the truth" then why does not the very weak field generated by their bodies have the same effect?
This makes no sense.

88. Originally Posted by DrRocket
You have no idea what you are talking about.
This makes no sense.
Is the meaning of your statements really a function of their parts?
Or is perhaps your intention only to say "Hello and goodbye ****head"?
It is with some misgivings I now begin reading the details of your letter
Originally Posted by DrRocket
Thought experiments are very much constrained by basic scientific principles,
which you seem to want to call technical details.
A random sample
confirms my worst suspicions:

89. Ok if it cant handle spinning objects then its not a complete theory. Sorry to hear that. Im a fan of Albert E.
We have known that this is not a complete theory for the past 100 years - GR cannot describe spin-orbit coupling ( which is, as DrRocket correctly points out, not the same as spinning objects in general ). No news here.

In fact general relativity cannot handle intrinsic rotation,
This is fixable if we consider connections with non-vanishing torsion on our space-time. Conveniently this might also get rid of the singularity problem.
But then of course we are no longer dealing with "classic" GR.

90. Has there been any test that demonstrates that time is slowing down rather than that a physical event happens after a comparable event in our frame?

Originally Posted by Markus Hanke
Atmospheric muons.
Muons are particles with a very short lifespan ( 2.2ns ), and there are plenty of them being generated by the interaction of energetic cosmic rays with the upper atmosphere. Classical mechanics tell us that those particles' lifespans are too short for them to reach the earth's surface before they decay, even at speeds close to the speed of light.
Thank you for this Markus, but it's not clear to me why the decay of particles should not count as a physical event.

91. Originally Posted by wilbo
Thank you for this Markus, but it's not clear to me why the decay of particles should not count as a physical event.
I don't understand what you mean by this. Under classical mechanics the muon should decay on its way through the atmosphere, and never get detected on the surface. In relativistic mechanics the muon doesn't decay on its way, but hits the detector on the earth's surface where it is recorded. They are very distinct outcomes, quite independent of the frame or observer, and independent of the sequence of events. The muon either gets to the earth's surface, or it doesn't. And since the lifespan of a particle is an intrinsic property and does not depend on any external factors, the only possibility is indeed time dilation & length contraction between the two frames.

Note that "time slowing down" is a very misleading term. Time does not slow down locally, because if you were to piggy-back on such a muon you would not notice anything special on your watch. What happens is that the causal relationship between the two frames ( muon and earth ) changes. Time dilation is only defined as a relationship between two frames. Locally however, time always progresses at "1 second per second" ( this is also misleading, but I don't know how else to formulate it ).

92. Originally Posted by wilbo
Has there been any test that demonstrates that time is slowing down rather than that a physical event happens after a comparable event in our frame?
The Hafele-Keating experimenet did just that.

Hafele

93. Originally Posted by Markus Hanke

Locally however, time always progresses at "1 second per second" ( this is also misleading, but I don't know how else to formulate it ).
This is not misleading. It is about the only sensible thing that one can say. Moreover, as one sees from general relativity, time is a local notion. There is no such thing as a global time in the universe as it is presented to us. All global notions of time require approximations that are clearly justified only in idealizations that do not apply to the actual universe with its clumps of matter.

94. Originally Posted by DrRocket
This is not misleading. It is about the only sensible thing that one can say. Moreover, as one sees from general relativity, time is a local notion. There is no such thing as a global time in the universe as it is presented to us. All global notions of time require approximations that are clearly justified only in idealizations that do not apply to the actual universe with its clumps of matter.
Well, it is misleading in the sense that it can be taken to imply that there is more than one kind of "second", and that the two kinds can somehow be compared. Locally though this is never the case. But I think the meaning in this context is clear, namely that an observer in his very own frame will never notice anything special happening on his clock.

95. Originally Posted by Markus Hanke
Locally time always progresses at "1 second per second" .
This sounds as obvious truth!
So I cant help feeling something might be wrong with it!
I am NOT claiming that it IS wrong, I only want to TEST the statement.
So I try to construct a way to test the statement:

1. I compare two clocks in the same "room" ,one is stationary
the other moves. To see that a moving clock is "lagging behind"
you have to wait a long time...so I try to find a way to shorten that time.

I rotate (or oscillate) the clock. How shall it move in order for relativity
theory to be appliable... IS there in principle no way to move the clock
ín the room so relativistc effects are noticeable in my lifetime?

2. I want to compare the clocks during the experiment
so I imagine that the rotating clock reports its time by radio waves...
and in front of me is acomputer screen displayng and comparing
the time registered by both clocks...Is this set up impossible in principle?

3 If the time of the rotating clock is "lagging behind" the stationary clock
then one second of rotating time is less than one stationary second.
And whether this fact is local or not needs to be explained.

4 We are not assuming any particular sizes here so let the picture on the screen
be radioed to an observer in the rotating clock, then he will be able to see
what I see from the outside: that one rotating second is not a stationary second.

96. Originally Posted by sigurdV
This sounds as obvious truth!
So I cant help feeling something might be wrong with it!
I am NOT claiming that it IS wrong, I only want to TEST the statement.
So I try to construct a way to test the statement:

1. I compare two clocks in the same "room" ,one is stationary
the other moves. To see that a moving clock is "lagging behind"
you have to wait a long time...so I try to find a way to shorten that time.

I rotate (or oscillate) the clock. How shall it move in order for relativity
theory to be appliable... IS there in principle no way to move the clock
ín the room so relativistc effects are noticeable in my lifetime?

2. I want to compare the clocks during the experiment
so I imagine that the rotating clock reports its time by radio waves...
and in front of me is acomputer screen displayng and comparing
the time registered by both clocks...Is this set up impossible in principle?

3 If the time of the rotating clock is "lagging behind" the stationary clock
then one second of rotating time is less than one stationary second.
And whether this fact is local or not needs to be explained.

4 We are not assuming any particular sizes here so let the picture on the screen
be radioed to an observer in the rotating clock, then he will be able to see
what I see from the outside: that one rotating second is not a stationary second.
Please provide the theoretical basis for your thought experiment, including the appropriate mathematics.

97. Originally Posted by DrRocket
The Hafele-Keating experimenet did just that.
A clock is a device for counting physical events. Other than mathematical expedience, why is time necessary?

98. Originally Posted by wilbo
A clock is a device for counting physical events. Other than mathematical expedience, why is time necessary?
A clock measures time. Among other things it is necessary so that you don't get fired for being late for work.

99. Time is what keeps things from happening all at once - John Wheeler

100. Originally Posted by DrRocket
A clock measures time. Among other things it is necessary so that you don't get fired for being late for work.
AlexG

Time is what keeps things from happening all at once
- John Wheeler

On the other hand, since not everything happens at once the duration of one event can be compared to the frequency of regular, generally oscillatory or cyclical, events. It is these events that clocks count and for convenience, an agreed number of events are labeled t in mathematics.
In a situation where a group of objects are at rest relative to each other, for example a passenger and their light clock on a train, any such oscillatory events that are compared within that frame will give the same result, regardless of any movement relative to any other frame. In essence, Special Relativity describes what observers see in frames they are not in, for example a bystander on the platform. The time dilation observed by the bystander is described by the length of the path the light has to follow from their point of view. But surely, what is true of the photons in the light clock is true of every particle in that frame (on the train); so that every event is dilated by the same factor as the light clock. The reason being that every event is ultimately the result of particles moving around and every particle in a given frame has to follow the same path as the photons in any light clock in that frame.

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