# Time Dilation and Length Contraction From Different Reference Frames

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• 09-29-2014, 06:29 PM
Scheuerf
Time Dilation and Length Contraction From Different Reference Frames
There are two planets, planet A and planet B, with a person on each one, person A and person B. There is also person C equidistant from the two planets. Person A on Planet A takes a rocket ship to Planet B moving at .87C.

If person C says that it takes 10 hours for person A to reach planet B, how long do Person A and person B think that it takes for person A to reach planet B? Also, how long is the distance between the two planets to person A and B compared to person C?
• 09-29-2014, 06:57 PM
Jilan
I would think that persons B and C will agree with each other on the time and the distance. Person A will disagree.
• 09-29-2014, 11:32 PM
Scheuerf
Okay that's good I was thinking the same thing. If I'm correct, person A will experience a shorter time and travel a shorter distance, but now I'm confused about how much each person ages. I would think that Person B and Person C would each age 10 hours, but from person A's reference frame it seems like person B and person C would age less than ten hours which doesnt seem possible.
• 09-29-2014, 11:35 PM
pzkpfw
Given the setup, B and C stay within their inertial frames, and A will experience acceleration to do the moving from A to B ... so A is in multiple frames, breaking the symmetry.

See: twins paradox.
• 09-29-2014, 11:38 PM
x0x
Quote:

Originally Posted by Scheuerf
There are two planets, planet A and planet B, with a person on each one, person A and person B. There is also person C equidistant from the two planets. Person A on Planet A takes a rocket ship to Planet B moving at .87C.

If person C says that it takes 10 hours for person A to reach planet B, how long do Person A and person B think that it takes for person A to reach planet B? Also, how long is the distance between the two planets to person A and B compared to person C?

Person C=T=10hrs
Person B =T=10hrs
Person A is more complicated:

Distance: Person B=Person C=L
Person A:
• 09-29-2014, 11:40 PM
SpeedFreek
Quote:

Originally Posted by Scheuerf
There are two planets, planet A and planet B, with a person on each one, person A and person B. There is also person C equidistant from the two planets. Person A on Planet A takes a rocket ship to Planet B moving at .87C.

If person C says that it takes 10 hours for person A to reach planet B, how long do Person A and person B think that it takes for person A to reach planet B? Also, how long is the distance between the two planets to person A and B compared to person C?

At .87c, gamma () ~2 because

If the planets are at rest in relation to each other (you gave no details indicating otherwise), and person C is at rest equidistant between the planets , then:

If person C observes that person A, whilst travelling at .87c, took 10 hours to go from A to B:

Person A finds that their journey only took 5 hours, because their own journey time is divided by

Person B observes that the journey of person A took 10 hours, because person B is at rest in relation to person C, and the planets are at rest in relation to each other. Person A is moving at .87c relative to both B and C, so both persons B and C will see the journey take the same amount of time, twice the time that the journey actually takes from the frame of person A.

Persons B and C calculate the distance between the planets to be 8.7 light hours, because it took someone travelling at .87c 10 hours to make the journey.

Person A calculates the distance between the planets to have been 4.35 light hours, because it only took them 5 hours when travelling at .87c, which means they could only have crossed a distance of 4.35 light hours in that time. Or, to put it another way, to person A, planets A and B are both moving in the same direction at .87c in relation to themselves, so the distance between those planets when they are at rest in relation to the observer is length contracted by the factor along the axis of motion.

Hopefully someone else here can put this more succinctly. :)

EDIT: I see x0x already did so, whilst I was composing this. I approximated .87c to have a gamma of 2, when it is actually closer to .866c where gamma is 2.
• 09-30-2014, 05:20 AM
KJW
Quote:

Originally Posted by Scheuerf
Okay that's good I was thinking the same thing. If I'm correct, person A will experience a shorter time and travel a shorter distance, but now I'm confused about how much each person ages. I would think that Person B and Person C would each age 10 hours, but from person A's reference frame it seems like person B and person C would age less than ten hours which doesnt seem possible.

Quote:

Originally Posted by pzkpfw
Given the setup, B and C stay within their inertial frames, and A will experience acceleration to do the moving from A to B ... so A is in multiple frames, breaking the symmetry.

See: twins paradox.

No. One can regard person A as in a single inertial frame that passes planets A and B rather than performing any acceleration at planets A and B. The resolution to Scheuerf's problem lies in the notion of simultaneity, which differs between persons A and B (and C). Thus, persons A and B start the time at planet B at different times, but end the time at the same time when they meet.

It should be noted that everyone agrees on the reading of a clock at any event in space and time. Thus, contrary to the claims of the anti-relativity crackpots, there is no paradox with regards to time dilation.
• 09-30-2014, 05:30 AM
x0x
Quote:

Originally Posted by KJW
Thus, contrary to the claims of the anti-relativity crackpots, there is no paradox with regards to time dilation.

Huh, what "anti-relativistic crackpots"? This is not coherent with the exercise in discussion.
• 09-30-2014, 05:44 AM
KJW
Quote:

Originally Posted by KJW
It should be noted that everyone agrees on the reading of a clock at any event in space and time. Thus, contrary to the claims of the anti-relativity crackpots, there is no paradox with regards to time dilation.

Because everyone agrees on the reading of a clock at any event in space and time, everyone agrees on the interval of time as measured by a given clock between the two events in space and time. Therefore, differences in time intervals can only result because the events in space and time that form the interval differ, not because there are any inconsistencies between the realities of the different observers.
• 09-30-2014, 05:47 AM
KJW
Quote:

Originally Posted by x0x
Huh, what "anti-relativistic crackpots"? This is not coherent with the exercise in discussion.

Sorry. By "anti-relativity crackpots", I wasn't referring to anyone in this discussion.
• 09-30-2014, 10:59 AM
VeeDee
Quote:

Originally Posted by SpeedFreek
At .87c, gamma () ~2 because

If the planets are at rest in relation to each other (you gave no details indicating otherwise), and person C is at rest equidistant between the planets , then:

If person C observes that person A, whilst travelling at .87c, took 10 hours to go from A to B:

Person A finds that their journey only took 5 hours, because their own journey time is divided by

Person B observes that the journey of person A took 10 hours, because person B is at rest in relation to person C, and the planets are at rest in relation to each other. Person A is moving at .87c relative to both B and C, so both persons B and C will see the journey take the same amount of time, twice the time that the journey actually takes from the frame of person A.

Persons B and C calculate the distance between the planets to be 8.7 light hours, because it took someone travelling at .87c 10 hours to make the journey.

Person A calculates the distance between the planets to have been 4.35 light hours, because it only took them 5 hours when travelling at .87c, which means they could only have crossed a distance of 4.35 light hours in that time. Or, to put it another way, to person A, planets A and B are both moving in the same direction at .87c in relation to themselves, so the distance between those planets when they are at rest in relation to the observer is length contracted by the factor along the axis of motion.

Hopefully someone else here can put this more succinctly. :)

EDIT: I see x0x already did so, whilst I was composing this. I approximated .87c to have a gamma of 2, when it is actually closer to .866c where gamma is 2.

Correct.
I can add this:
If clocks on planet A and B are synchronised and read 00 when the astronaut flies by, then for the astronaut, when he passes planet A, the clocks on planet B already read 7.5
Hence for the astronaut, to reach planet B, his clock ticks 5 hours but planet B's clocks tick from 7.5 to 10 = 2.5
I.o.w. for the astronaut planet B clocks 'tick slower'.

[IMG]http://i102.photobucket.com[IMB]
• 10-01-2014, 12:52 AM
Scheuerf
Okay that makes sense, but I have one last question. The distance between Planets A and B is longer for person C, but is the rocket ship itself contracted at all from the reference frames of persons B and C?
• 10-01-2014, 01:46 AM
x0x
Quote:

Originally Posted by Scheuerf
Okay that makes sense, but I have one last question. The distance between Planets A and B is longer for person C, but is the rocket ship itself contracted at all from the reference frames of persons B and C?

Yes, it is. By the same factor, 0.49
• 10-01-2014, 05:35 AM
SinceYouAsked
Quote:

Originally Posted by VeeDee
Correct.
I can add this:
If clocks on planet A and B are synchronised and read 00 when the astronaut flies by, then for the astronaut, when he passes planet A, the clocks on planet B already read 7.5
Hence for the astronaut, to reach planet B, his clock ticks 5 hours but planet B's clocks tick from 7.5 to 10 = 2.5
I.o.w. for the astronaut planet B clocks 'tick slower'.

http://i102.photobucket.com/albums/m...ps3760718f.jpg

Just a quick question on your Loedel figure. The vertical and horiz axes are for the ficticious stationary observer. However, you show the t axis at >45 degree, and thus is superluminal. Did you really want to do it that way? I suppose it's not really a Loedel figure as drafted, yes?

Thank You,
SinceYouAsked
• 10-01-2014, 11:13 AM
VeeDee
Quote:

Originally Posted by SinceYouAsked
Just a quick question on your Loedel figure. The vertical and horiz axes are for the ficticious stationary observer. However, you show the t axis at >45 degree, and thus is superluminal. Did you really want to do it that way? I suppose it's not really a Loedel figure as drafted, yes?

Thank You,
SinceYouAsked

Wow, what a f**k up! Indeed! It's a common mistake I make. Too much in a hurry...
The diagram is indeed a loedel and correct (my gray lines are the light axes), but I rotated it too much...! A common mistake I make... :-( Actually I draw the diagram with the x axis hoizontal, and after finishing the drawing I rotate the whole lot. But often I rotate too much.
This should be correct:
https://i.imgur.com/0c8k0DA.jpg
I see I have to correct the diagram (about JT's eqn) in the other thread as well. :-(

Thanks for letting me know!
• 10-01-2014, 02:53 PM
x0x
Quote:

Originally Posted by VeeDee
Wow, what a f**k up! Indeed! It's a common mistake I make. Too much in a hurry...
The diagram is indeed a loedel and correct (my gray lines are the light axes), but I rotated it too much...! A common mistake I make... :-( Actually I draw the diagram with the x axis hoizontal, and after finishing the drawing I rotate the whole lot. But often I rotate too much.
This should be correct:
http://i102.photobucket.com/albums/m...ps3b685b27.jpg

I see I have to correct the diagram (about JT's eqn) in the other thread as well. :-(

Thanks for letting me know!

There is a second, smaller mistake in your diagrams. The speed is 0.87 (as per the OP) , your diagrams are drawn for 0.866.
• 10-01-2014, 02:59 PM
VeeDee
Quote:

Originally Posted by x0x
There is a second, smaller mistake in your diagrams. The speed is 0.87 (as per the OP) , your diagrams are drawn for 0.866.

Nitpicking?
Enjoy it.
• 10-01-2014, 03:21 PM
x0x
Quote:

Originally Posted by VeeDee
Nitpicking?
Enjoy it.

I am not nitpicking, I am pointing out another error. Since you were cleaning up your diagrams, I though that you'd want to correct this smaller error as well. You don't have to take it personally, just correct your diagrams.
• 10-02-2014, 02:12 AM
SinceYouAsked
Quote:

Originally Posted by x0x
I am not nitpicking, I am pointing out another error. Since you were cleaning up your diagrams, I though that you'd want to correct this smaller error as well. You don't have to take it personally, just correct your diagrams.

Well, I think it's fair to say that when one states v=0.87c, the intent is v = √(3/4)c where gamma = 2, in which case v ~ 0.866025c which if rounded up becomes v = 0.87c. IOW, they round off. I cannot imagine a good reason to select v=0.87c precisely versus v=√(3/4)c, given that gamma = 2 makes everything so much easier in analyses. Just an opinion though.

Thank You,
SinceYouAsked
• 10-02-2014, 08:10 AM
VeeDee
Quote:

Originally Posted by SinceYouAsked
Well, I think it's fair to say that when one states v=0.87c, the intent is v = √(3/4)c where gamma = 2, in which case v ~ 0.866025c which if rounded up becomes v = 0.87c. IOW, they round off. I cannot imagine a good reason to select v=0.87c precisely versus v=√(3/4)c, given that gamma = 2 makes everything so much easier in analyses. Just an opinion though.

Thank You,
SinceYouAsked

I share your opinion ;-)
• 10-02-2014, 07:29 PM
SpeedFreek
Quote:

Originally Posted by SinceYouAsked
Well, I think it's fair to say that when one states v=0.87c, the intent is v = √(3/4)c where gamma = 2, in which case v ~ 0.866025c which if rounded up becomes v = 0.87c. IOW, they round off. I cannot imagine a good reason to select v=0.87c precisely versus v=√(3/4)c, given that gamma = 2 makes everything so much easier in analyses. Just an opinion though.

Thank You,
SinceYouAsked

I too share your opinion, as you can probably tell from my post above. It seemed pretty obvious to me that the OP was composed such that the speed was approximated with a gamma of 2 in mind.
• 10-02-2014, 08:41 PM
Jilan
For what it's worth, I agree too.
• 10-03-2014, 06:14 AM
SinceYouAsked
Great minds think alike.

Thank You,
SinceYouAsked
• 10-03-2014, 06:02 PM
phyti
For clarity and scale without the clutter, the Minkowski is superior to the Loedel.
The first pic is the C frame which includes A and B, with the ship S (green) moving at .87c. There is an additional light signal from B (4.35, 0) to S. C's description of S with time dilation of .49 would be the same from A and B.
The second pic is the S frame, where the light signal from B serves to locate the event B(t) = 0, in the S frame. The extreme spatial and temporal excursion is a result of the synch convention. S calculates time dilation for B as 10/(15.35+4.93) = .49, which also applies to A and C. .
Since B has arrived at S early, 4.93 vs 10 hr, S concludes space in the x direction has contracted.
Considering this case, I would prefer: time dilation has altered his perception of space.
He has done nothing that would alter the universe in such a dramatic way. Space has not changed but his perception has.
https://app.box.com/s/uhj7plk0rcdv62ss4mcl
https://app.box.com/s/wygek10xw0ojtc3cs6h2
• 10-03-2014, 07:04 PM
x0x
Quote:

Originally Posted by phyti
For clarity and scale without the clutter, the Minkowski is superior to the Loedel.
The first pic is the C frame which includes A and B, with the ship S (green) moving at .87c. There is an additional light signal from B (4.35, 0) to S. C's description of S with time dilation of .49 would be the same from A and B.
The second pic is the S frame, where the light signal from B serves to locate the event B(t) = 0, in the S frame. The extreme spatial and temporal excursion is a result of the synch convention. S calculates time dilation for B as 10/(15.35+4.93) = .49, which also applies to A and C. .
Since B has arrived at S early, 4.93 vs 10 hr, S concludes space in the x direction has contracted.
Considering this case, I would prefer: time dilation has altered his perception of space.
He has done nothing that would alter the universe in such a dramatic way. Space has not changed but his perception has.
https://app.box.com/s/uhj7plk0rcdv62ss4mcl
https://app.box.com/s/wygek10xw0ojtc3cs6h2

Nice, thank you, this is what I asked for.
• 10-03-2014, 07:53 PM
JTyesthatJT
Quote:

Originally Posted by phyti
Since B has arrived at S early, 4.93 vs 10 hr, S concludes space in the x direction has contracted.
Considering this case, I would prefer: time dilation has altered his perception of space.
He has done nothing that would alter the universe in such a dramatic way. Space has not changed but his perception has.

He did not just perceive his clock elapsing 4.93 hours during the journey, that is how much time it really elapsed. Also, he did not just perceive planets A and B moving by him at 0.87c, that is how fast they really were moving. So, surely he did not just perceive the distance as 4.29 light-hours, that must be the real distance. You say you prefer to think of this as a matter of perception, but that does not make sense to me.
• 10-04-2014, 12:35 PM
VeeDee
Quote:

Originally Posted by phyti
He has done nothing that would alter the universe in such a dramatic way. Space has not changed but his perception has.

It has nothing to do with 'perception'.
In Special Relativity there are no preferred reference frames in the universe. Different observers will, based on their respective observations (and kepping c=constant in mind), consider a different set of simultaneous events as his 'present 3D space/world'. Special relativity is about 3D sections though 4D spacetime, not about 'seeing/percieving a preferred (absolute? preferred?) 3D space/world of simultaneous events' in a different way because the observer moves... Special reltaivity is not about optical illusions.
Here I show the '3D section' lines through 4D spacetime:

https://i.imgur.com/0c8k0DA.jpg
• 10-04-2014, 01:00 PM
mayflow
Quote:

Originally Posted by Scheuerf
There are two planets, planet A and planet B, with a person on each one, person A and person B. There is also person C equidistant from the two planets. Person A on Planet A takes a rocket ship to Planet B moving at .87C.

If person C says that it takes 10 hours for person A to reach planet B, how long do Person A and person B think that it takes for person A to reach planet B? Also, how long is the distance between the two planets to person A and B compared to person C?

Well, let's think it out. The ship is traveling around 261 million meters per second, yes?
• 10-05-2014, 04:28 AM
SinceYouAsked
VeeDee,

Next time you post a Loedel, try saving the graphics file as a .PNG file format (portable network graphics), and then post that (versus a .JPG). I have found that the picture quality is superior, and your figures deserve a great graphics quality. Try that, see what you think. Microsoft photoeditor allows .png file format on save.

Thank you,
SinceYouAsked
• 10-05-2014, 09:41 AM
VeeDee
Quote:

Originally Posted by SinceYouAsked
VeeDee,

Next time you post a Loedel, try saving the graphics file as a .PNG file format (portable network graphics), and then post that (versus a .JPG). I have found that the picture quality is superior, and your figures deserve a great graphics quality. Try that, see what you think. Microsoft photoeditor allows .png file format on save.

Thank you,
SinceYouAsked

The problem is not the format. I did a few tests; for same sketch PNG and JPG give same sharpness.

The above sketch is a blown up version of the post #15 sketch. I shouldn't have done that.

The problem is that it seems impossible to post a small size sketch at high resolution.
If I jazz up the resolution pixle/inch, then a small simple sketch will quickly fill up the full width of the screen. I find that quite agressive for a simple sketch. But maybe I should not worry about that.
• 10-05-2014, 06:33 PM
SinceYouAsked
Quote:

Originally Posted by VeeDee
The problem is not the format. I did a few tests; for same sketch PNG and JPG give same sharpness.

The above sketch is a blown up version of the post #15 sketch. I shouldn't have done that.

The problem is that it seems impossible to post a small size sketch at high resolution.
If I jazz up the resolution pixle/inch, then a small simple sketch will quickly fill up the full width of the screen. I find that quite agressive for a simple sketch. But maybe I should not worry about that.

I see. I was just wondering, because I had always found the .PNG files viewed better than the .JPG files when i posted them. Not sure why I experienced that, but you have not. No big deal. Yes, stretching the figures will lower the quality, but sometimes the trade off is worth it. Your figure was plenty viewable enough.

Thank you,
SinceYouAsked
• 10-06-2014, 09:56 AM
Boing3000
Quote:

Originally Posted by SYA
I was just wondering, because I had always found the .PNG files viewed better than the .JPG files when i posted them

You've got sharp eyes, PNG is a lossless format, and will give better result with artificial image with simple schema.
• 10-06-2014, 05:19 PM
phyti
JTyesthat JT #26
Quote:

He did not just perceive his clock elapsing 4.93 hours during the journey, that is how much time it really elapsed. Also, he did not just perceive planets A and B moving by him at 0.87c, that is how fast they really were moving. So, surely he did not just perceive the distance as 4.29 light-hours, that must be the real distance. You say you prefer to think of this as a matter of perception, but that does not make sense to me.
"Perception is the apparent behavior of objects as represented by mental images formed from processing and interpreting sensory input. In summary, perception is reality confined to the mind of the observer."

Time dilation and length contraction affects all compositions of matter, including rulers and clocks, and thus the observer is not exempt. His clock runs slower and his sense of time runs slower, otherwise he would detect a change in his clock. The common data for A would list B as 10 lh (light hr) distant, which equals 10/.87 = 11.5 hr. (which corrects my entry of 10). In an effort to reconcile the time with the distance, without questioning the clock or the speed, and being physically at B, he concludes the distance to B has shrunk. He is the only one who thinks/perceives this, and he has no explanation for it.
All other observers describe length contraction and time dilation of S, and have no need for an explanation of something they didn't experience.

The calculated distance of 4.93 lh is as defined above, 'his reality confined to his mind'.
If we allow S to consult his SR manual, he could expect his frame to run at a reduced rate, and anticipate the time-distance anomaly.
Einstein was focused on the relative aspects of observations, with regard to coordinate transformations between observers. Perception was not part of the criteria for his theory, yet it is contained within it. It also eliminates the magic!
• 10-06-2014, 05:24 PM
phyti
VeeDee #27
Quote:

It has nothing to do with 'perception'.
per•cep•tion [pər sépshən](plural per•cep•tions)
n
1. perceiving: the process of using the senses to acquire information about the surrounding environment or situation the range of human perception
2. result of perceiving: the observation or result of the process of perception
3. impression: an attitude or understanding based on what is observed or thought
4. powers of observation: the ability to notice or discern things that escape the notice of most people
5. psychology neurological process of observation and interpretation: any of the neurological processes of acquiring and mentally interpreting information from the senses
[14th century. Via Old French from the Latin stem perception- , from percipere (see perceive).]
Encarta ® World English Dictionary © & (P) 1998-2004 Microsoft Corporation. All rights reserved.

Quote:

Special relativity is not about optical illusions.
Maybe you missed the class on abstraction. All things mathematical and geometric are abstract or representational. The images you form when observing are not the objects observed. No one knows 'what' is at the location of the object. Points, lines, circles, etc. are mental concepts, to assist in thinking.
In the case of ship vs universe there are two choices.
1. The ship is launched in direction x by expending energy E.
2. Some one expends energy E, while choreographing all the other objects to move instantly in the opposite direction.
Which one seems physically possible, and which one sounds like magic?
• 10-06-2014, 05:49 PM
JTyesthatJT
Quote:

Originally Posted by phyti
In the case of ship vs universe there are two choices.
1. The ship is launched in direction x by expending energy E.
2. Some one expends energy E, while choreographing all the other objects to move instantly in the opposite direction.
Which one seems physically possible, and which one sounds like magic?

Your logic is flawed.

Say planets A and B are stationary relative to each other, but moving together with a velocity of -0.87c through the universe. A spaceship is launched from A toward B with a velocity of +0.87c. Since this velocity happens to be in the opposite direction of the other velocity, the spaceship actually expended energy to stop moving through the universe. By your reasoning above, the spaceship must be moving, because it expended energy.

Furthermore, the spaceship still measures the distance between the planets as length-contracted, even though the spaceship is stationary with the universe, and the planets themselves are moving. It is the exact same result as if the planets were stationary with the universe, and the spaceship were moving. So whatever you are talking about does not make any difference to the outcome.
• 10-06-2014, 07:00 PM
Boing3000
Quote:

Originally Posted by JTyesthatJT
Your logic is flawed

I have seen such logic from Farsight, alias John Duffield, alias movement is defined by hand waving (literally).

It seem that the sockpuppet show is back.
• 10-06-2014, 07:06 PM
x0x
Quote:

Originally Posted by Boing3000
I have seen such logic from Farsight, alias John Duffield, alias movement is defined by hand waving (literally).

It seem that the sockpuppet show is back.

phyti is not a Farsight sockpuppet.
• 10-06-2014, 09:15 PM
Boing3000
Quote:

Originally Posted by x0x
phyti is not a Farsight sockpuppet.

My mistake. I must have gone trough the wrong wormhole then :rolleyes:

>Rollback
• 10-06-2014, 09:49 PM
mayflow
I AM GUILTY of being a sockpuppet of Mayflow, and to free my consciouness, I have to admit that that is not important to me, and does not nullify the fact, that I am now curious about what -.87C is?
• 10-06-2014, 11:20 PM
SinceYouAsked
Quote:

Originally Posted by mayflow
I AM GUILTY of being a sockpuppet of Mayflow, and to free my consciouness, I have to admit that that is not important to me, and does not nullify the fact, that I am now curious about what -.87C is?

Mayflow,

All velocities are strictly per POV. As such, any inertial body may be stationary per itself (or those comoving with it), or that same body is moving per those who move relatively wrt it. The velocity -0.87c simply means that per "some inertial POV", the motion is -0.87c.

All spacetime coordinate systems "by convention" depict increasing values of x with greater separation rightward from the origin, for any horizontally depicted x-axis. Also by convention, if the velocity is positive, eg 0.87c, then the motion is in the direction of increasing x (a rightward motion). Therefore, if the velocity is negative, eg -0.87c, then the motion is in the direction of decreasing x (a leftward motion). Yet, all motion is relative, and as such is strictly per POV.

Thank You,
SinceYouAsked
• 10-07-2014, 12:09 AM
mayflow
Quote:

Originally Posted by SinceYouAsked
Mayflow,

All velocities are strictly per POV. As such, any inertial body may be stationary per itself (or those comoving with it), or that same body is moving per those who move relatively wrt it. The velocity -0.87c simply means that per "some inertial POV", the motion is -0.87c.

All spacetime coordinate systems "by convention" depict increasing values of x with greater separation rightward from the origin, for any horizontally depicted x-axis. Also by convention, if the velocity is positive, eg 0.87c, then the motion is in the direction of increasing x (a rightward motion). Therefore, if the velocity is negative, eg -0.87c, then the motion is in the direction of decreasing x (a leftward motion). Yet, all motion is relative, and as such is strictly per POV.

Thank You,
SinceYouAsked

Left and right do not make positive and negative to my thinking. You speak of point of view but as you move your frame of reference, left can become right and it can become something in between, so how can one be considered positive and one negative? I could see it more if we were to say one thing is moving away from me and one thing is coming closer to me, but even then it would make more sense to me to say that something is approaching my position or the something is moving away from it. Even in math, negative numbers are just moving away from zero in a different direction than the positive ones.

Let us apply units. If I have an apple and have another one added, I have two apples. Positive, right? But what if I really don't want any apples (POV)? If I lose that apple (it moves away from me) I have no apples, which is what I really wanted to begin with from my point of view. From my point of view that was a positive. Which is more important, the movements of light or the movements of the mind?
• 10-07-2014, 07:11 AM
Boing3000
Quote:

Originally Posted by mayflow
Left and right do not make positive and negative to my thinking

They absolutely do, because later you wrote
Quote:

Originally Posted by mayflow
Even in math, negative numbers are just moving away from zero in a different direction than the positive ones.

Quote:

Originally Posted by mayflow
I could see it more if we were to say one thing is moving away from me and one thing is coming closer to me

And I see no point in confusing the notion of direction with the notion of distance. Now if the distance is changing, the delta will have a direction.

I understand that as a sock-puppet you don't understand left and right (it's arm-less ;)) so you won't use those symbol but you can use hosewise and anti-hosewise
It is just a convention.
• 10-07-2014, 08:13 PM
phyti
JTyesthat JT;
There is no question about relative motion, since every thing IS relative by definition, except light speed.
Light speed is independent of objects in motion.
The integral universe cannot have motion since there is nothing to serve as a reference. When the ship is considered as slowing down, relative to the remainder of the universe, how or why or from what source, would the universe acquire that speed? Imagine reproducing the perception of the anaut in the ship, by accelerating objects one by one, in the opposite direction, while coordinating their movement, beginning with the ship at rest (to its environment).
Why do physicists accelerate a few particles and not the lab?
If two anauts passed each other in remote space, far from any significant mass, there motion could be divided between them in any portions from 0 to 100%.
If a particle is moving at .95c, it's highly improbable that the universe is in reality, moving at that speed (relative to light, the universal standard) in the opposite direction. Where would the energy come from? In fact you would have to use a portion of it as energy to acheive this. It's another case of a hypothetical observer moving with the particle and imposing the effects of his motion on the rest of the world.

A similar case exists with the skater who spins at 2 rev/sec. Does the object 4 lyr distant, circle her at 24c?
Not really, that's her perception of things. If we introduce a 2nd skater on the opposite side of earth, and thinking in reciprocity mode, the universe would have to be circling two different centers 8000 mi apart simultaneously!
• 10-07-2014, 08:17 PM
SinceYouAsked
Quote:

Originally Posted by mayflow
Left and right do not make positive and negative to my thinking.

It doesn't matter. It's a convention, meaning a man made agreed upon choice. While there can always be arguments against any conventions, their are benefits to having them that outweigh the arguments. Not everyone has to like them. For example, I get rather annoyed when folks tell be I cannot use a negative velocity in the standard LT convention, because they argue that the Inverse LT eqn already has a polarity built in that handles it. Yet, conventions are defined simply to keep everyone on the same playing field, for otherwise everyone forever argues about trivial matters. That's the benefit, like them or not. The LT convention also requires that the unprimed variables represent the POV that beholds of the positive velocity, and this keeps folks from mixing up unprimed/primed systems and positive/negative velocities in analyses ... which happens all the time otherwise, for those trying to learn the theory.

Quote:

Originally Posted by mayflow
You speak of point of view but as you move your frame of reference, left can become right and it can become something in between, so how can one be considered positive and one negative?

You are only saying that if something starts as defined remotely toward your right traveling outward at +v, then you do a U-turn, it then ends up remotely on your left traveling away at -v. This is true, and there is nothing wrong with that. Per SR though, all motion is inertial ... there are no U-turns or rotating POVs. As such, that scenario cannot happen. In navigation and flight, current POV and stationary-space POV are both always maintained. Stationary-space is nothing but some initial current POV (usually calibrated prior to "true-north and earth-center for down") that is carved into stone for all future references ... such as when an inertial gyro is spun up to speed and the inertial navigation system initialized. All motion may then be considered as "relative to that" as a reference. So it's not as though your concerns are not considered or handled in navigation or plots of space and time.

Quote:

Originally Posted by mayflow
I could see it more if we were to say one thing is moving away from me and one thing is coming closer to me, but even then it would make more sense to me to say that something is approaching my position or the something is moving away from it. Even in math, negative numbers are just moving away from zero in a different direction than the positive ones.

All fine, but it doesn't change anything far as the defined agreed upon conventions go. If you define positive velocity as approaching you, another may claim it's digressing from he. Then you have eqns that are supposed to handle all situ, but cannot, because the conventions used by observers are apples vs oranges. Need apples and apples, or oranges and oranges.

Quote:

Originally Posted by mayflow
Let us apply units. If I have an apple and have another one added, I have two apples. Positive, right? But what if I really don't want any apples (POV)? If I lose that apple (it moves away from me) I have no apples, which is what I really wanted to begin with from my point of view. From my point of view that was a positive. Which is more important, the movements of light or the movements of the mind?

If you lose both apples, then all in the cosmos agree you lost two apples. Conventions however, are defined to ensure that one does not say you have 2 apples while another says you have zero apples.

Thank You
SinceYouAsked
• 10-07-2014, 08:40 PM
phyti
Mayflow #41
Velocity as a vector, has 2 components, speed and direction.
Speed has only positive values. You can’t go any slower than 0.
Velocity changes sign only if the direction changes sign.
Left and right do not have values, they are qualifiers from a 2-valued system of logic, eg. hot /cold, up/down, wet/dry, etc.

Boing3000 #36
I can and do think for myself. I will agree with a true statement regardless of who says it. I don’t know the details of Relativity by Farsight, but agree with his interpretation of ‘time’ in general. History shows what it is, its purpose, and its application. SR by Einstein shows time (as we use it) is subjective. Despite 100 yrs later, people still cling to the idea of ‘time’ as some background phenomenon governing events.
Maybe it’s a mass security blanket.
• 10-07-2014, 09:13 PM
mayflow
Quote:

Originally Posted by phyti
Mayflow #41
Velocity as a vector, has 2 components, speed and direction.
Speed has only positive values. You can’t go any slower than 0.
Velocity changes sign only if the direction changes sign.
Left and right do not have values, they are qualifiers from a 2-valued system of logic, eg. hot /cold, up/down, wet/dry, etc.

I will tell you how I see this differently. Cold simply is. Dark simply is. Dry simply is, etc. Heat and light and humidity are created by certain generations of energy. If these energy sources cease, dark dry and cold remain. Up and down are simply relative to your position of reference and have nothing to do with physics.

In digital electronics like in relays, you have a normalized state. The normalized state of the Universe is in these terms 0. It is not activated. There is not heat, there is not light, there is not moisture. This would probably be called nowdays a singularity. The state before a big bang. The Universe is now in various stages of being turned on by these energies. If these energies cease, there will be a lack of these energies, but it does not mean there is nothing - it is like the kinetic is gone, but the potential remains which was likely the state before the big bang occurred as we think of it.
• 10-07-2014, 10:16 PM
Boing3000
Quote:

Originally Posted by Phyti
SR by Einstein shows time (as we use it) is subjective

I don't think you can put it like that. Time is not subjective at all. It is measured by clock, it is very real. SR and GR show is is relative and how to relate it with other FoR.
Space also is by that same theory.

Quote:

Originally Posted by Phyti
Despite 100 yrs later, people still cling to the idea of ‘time’ as some background phenomenon governing events

Yeah that does still exist. But space-time is this background, has far as we know.

Quote:

Originally Posted by Phyti
The integral universe cannot have motion since there is nothing to serve as a reference

There are a lot of reference possible. My FoR for example. So I can decide if the universe seems to hurl pass me or not.

Quote:

Originally Posted by Phyti
When the ship is considered as slowing down, relative to the remainder of the universe, how or why or from what source, would the universe acquire that speed?

An improbability drive, or a more affordable rocket :D
• 10-08-2014, 11:48 AM
VeeDee
Quote:

Originally Posted by Boing3000
So I can decide if the universe seems to hurl pass me or not.

Why do you say 'seems'? Because 'in reality' or 'in fact' it does not move?
In the spacehips reference frame the universe does move. In another observer's reference frame the universe does not move, Although I wonder whether you might find such an hypothetical observer...depends what you consider 'the universe'.... SR tells us there is no preferred reference frame. All are equally valid. No preferred frame, no ether. Sars and galaxies move relative to each other. That's it. The only 'background' is indeed 4D spacetime.
• 10-08-2014, 11:58 AM
VeeDee
Quote:

Originally Posted by phyti
JTyesthat JT;
There is no question about relative motion, since every thing IS relative by definition, except light speed.
Light speed is independent of objects in motion.
The integral universe cannot have motion since there is nothing to serve as a reference. When the ship is considered as slowing down, relative to the remainder of the universe, how or why or from what source, would the universe acquire that speed?

It 'acquires' that speed because the worldlines of spaceship and what you call 'univese' take off in different directions through 4D spacetime.
Quote:

Imagine reproducing the perception of the anaut in the ship, by accelerating objects one by one, in the opposite direction, while coordinating their movement, beginning with the ship at rest (to its environment).
Why do physicists accelerate a few particles and not the lab?
Takes less energy.
Feel free to move the lab, and study from there the relative moving particles. Let me know, I interested in a free ticket for the ride.
Quote:

If two anauts passed each other in remote space, far from any significant mass, there motion could be divided between them in any portions from 0 to 100%.
If a particle is moving at .95c, it's highly improbable that the universe is in reality, moving at that speed (relative to light, the universal standard) in the opposite direction. Where would the energy come from? In fact you would have to use a portion of it as energy to acheive this. It's another case of a hypothetical observer moving with the particle and imposing the effects of his motion on the rest of the world.

A similar case exists with the skater who spins at 2 rev/sec. Does the object 4 lyr distant, circle her at 24c?
Not really, that's her perception of things. If we introduce a 2nd skater on the opposite side of earth, and thinking in reciprocity mode, the universe would have to be circling two different centers 8000 mi apart simultaneously!
• 10-08-2014, 06:16 PM
Boing3000
Quote:

Originally Posted by VeeDee
Why do you say 'seems'? Because 'in reality' or 'in fact' it does not move?

It was a precaution not to hurt anybody's (FoR) feeling ;)

For the rest I agree 101%
• 10-08-2014, 07:06 PM
SinceYouAsked
Quote:

Originally Posted by VeeDee
Why do you say 'seems'? Because 'in reality' or 'in fact' it does not move?
In the spacehips reference frame the universe does move. In another observer's reference frame the universe does not move, Although I wonder whether you might find such an hypothetical observer...depends what you consider 'the universe'.... SR tells us there is no preferred reference frame. All are equally valid. No preferred frame, no ether. Sars and galaxies move relative to each other. That's it. The only 'background' is indeed 4D spacetime.

True, however regarding the ether, relativity makes no call except this ... any absolute reference for motion is superfluous to the relativity theory, even if the relativistic effects are driven by an existent ether itself. As such, the ether may exist or not, but if it does exist, it must exist in such a way to uphold the LTs (which of course also requires no preferred frames).

Thank You
SinceYouAsked
• 10-08-2014, 09:56 PM
VeeDee
Quote:

Originally Posted by SinceYouAsked
True, however regarding the ether, relativity makes no call except this ... any absolute reference for motion is superfluous to the relativity theory, even if the relativistic effects are driven by an existent ether itself. As such, the ether may exist or not, but if it does exist, it must exist in such a way to uphold the LTs (which of course also requires no preferred frames).

Thank You
SinceYouAsked

I'm not sure I follow you here.
Einstein said in his OEMB the ether is superfluous. SR doesn't need ether.
True, this still doesn't prove the ether does not exist, but if it does exist it won't be to motivate relativistic 'effects'.
• 10-08-2014, 11:07 PM
SinceYouAsked
Quote:

Originally Posted by VeeDee
I'm not sure I follow you here.
Einstein said in his OEMB the ether is superfluous. SR doesn't need ether.
True, this still doesn't prove the ether does not exist, but if it does exist it won't be to motivate relativistic 'effects'.

OEMB: The introduction of a “luminiferous ether” will prove to be superfluous inasmuch as the view here to be developed will not require an “absolutely stationary space” provided with special properties, nor assign a velocity-vector to a point of the empty space in which electromagnetic processes take place.

So, the ether may exist, but is it does exist, it cannot change the LT solns.

Now keep in mind that Lorentz published his LET theory (in 1904) about 6 months before Einstein published his 1905 OEMB paper. LET attained the exact same solns, and it assumed apriori a luminiferous ether that possessed a master stationary frame, the only frame in which light traveled at c. However, the LT solns of LET are apparent (only the measurer at rest in the stationary ether measures what is real), whereas per SR all measurements are real.

IMO, it is conceivable that an ether exists, that does not possess the apriori requirements of Lorentz, that supports all SR solns as real. This is just to say that an ether may exist that possesses the Lorentz symmetry in a way "that supports all measurements equally real" ... one reason SR was accepted over LET. In SR, a ruler measures itself and obtains the real measurement, because it is uncontracted per itself.

So, an absolute frame of reference for motion as presumed for the 1905 ether, is superfluous the LT solns. This is just to say that any inertial frame of SR possesses the features of Lorentz's master ether frame, and as such, no SR frame is preferred over another.

Thank You,
SinceYouAsked
• 10-09-2014, 07:29 AM
Boing3000
Quote:

Originally Posted by SYA
So, the ether may exist

I don't follow you either. Is not the ether called a "field" nowadays ? Space-time being the gravitational field, and ether being the EM field ?
• 10-09-2014, 08:44 AM
SinceYouAsked
Quote:

Originally Posted by Boing3000
I don't follow you either. Is not the ether called a "field" nowadays ? Space-time being the gravitational field, and ether being the EM field ?

The ether of the 1800s was presumed to allow for the motion of light. Nowadays, the ether is presumed a medium that allows for and supports the existence and propagation of fields in general, be it a gravitational field, EM field, fields related to the weak or strong force, the Higgs field, etc. If an ether exists, it does not exist in the strict classical sense. Since SR, the term "field" has been used instead of the term "ether", although many (including Einstein) did still presume an ether must exist. Only problem, there seems no way to prove its existence. If it exists, it'll likely be quantum mechanics that defines it, because electrodynamics cannot. I'm not sure where QM stands on the medium (or ether if you prefer), but I have no doubt it refers to fields. So I agree, nowadays, fields are what's used in lieu of ether.

The gravitational field is created by energy-momentum warping fused spacetime, so spacetime-curvature and gravity-field go hand in hand. One may consider them one in the same. However, what is spacetime before it becomes curved? Is there something there apriori, or not? How does something come from nothing? IMO, and that's all it is ... it seems harder to believe that a field exists unto itself occupying what would otherwise be nothing, than that a medium exists for which fields can arise and propagate. A field may be thought of as a variation or configuration of the medium (ether) within itself ... but if fields are required for space and time to have any meaning, then what is the need for an ether anyway, since math can model existence using fields alone? By Occam's Razor, it seems fields is all we need, and no ether required. Yet, it seems rather compelling that fields exist in something versus within themselves, so I'd leave it at that.

Maxwell thought an ether existed, but never mentioned it in his EM theory because he could not prove it. His equations did not require describing it, so as a good physicist should do, he did not mention it in his theory. Einstein thought a medium existed for many years, but he referred to his medium as curved spacetime, because he had no proof of an ether, but the dimensions of space and time exist. No proof of an ether, and your theory works without defining it, then best not to mention it ... whether you think an ether exists or not. I personally presume an ether exists.

Thank You,
SinceYouAsked
• 10-09-2014, 09:31 AM
Boing3000
Very good post SYA and I can tune myself easily to that point of view, especially expressed so clearly.
Except for this bit
Quote:

Originally Posted by SYA
... it seems harder to believe that a field exists unto itself occupying what would otherwise be nothing

I only understand field as potential distributed in something. That something for me is not ether, but space-time or more specifically a topology of events (what we called reality ;-)

I also think that there is a experimental way to see if such ether exist. By triangulation. If EM field is not influenced by gravitational field (it apparently is, because BH have a charge, and as such "emit" a field even so space-time does not allow it), that would mean that there is no basic support for them to interact. Even if both are triggered by the same potential source, namely matter which sole have energy or charge. Although, I understand that field themselves have energy (and are also source of those potential), except that I don't know of any field that have an electric charge.

Anyway, by triangulation I mean that if we observe that a gravitational wave "bend" an EM field, we kind of know that both are linked by something, which I agree to call ether.
• 10-09-2014, 09:38 AM
VeeDee
Quote:

Originally Posted by SinceYouAsked
I personally presume an ether exists.

If you mean 4D spacetime = ether. Einstein used it that way.
But that's not Lorentz's absolute 3D ether I was referring to.
And because this is a thread under Einstein Special and General relativity, I don't want to get involved in Lorentz's ether :rolleyes:
• 10-09-2014, 05:46 PM
phyti
Boing #47

Quote:

I don't think you can put it like that. Time is not subjective at all. It is measured by clock, it is very real. SR and GR show is is relative and how to relate it with other FoR.
Space also is by that same theory.
I just did. Clocks provide a periodic event, that enables a person to put events of interest in order. This provides an historical record that can be used for various purposes. If the traveling twin returns younger, he and his clock must have ticked at a slower rate than his twin. That seems very subjective and relative to me.

Quote:

There are a lot of reference possible. My FoR for example. So I can decide if the universe seems to hurl pass me or not.
Your frame is part of the universe. The point is the universe has zero momentum.
The momentum of the ship with tiny mass and large velocity is mV.
The momentum of the (universe-ship) with huge mass and tiny velocity is approx. Mv.
Conservation of momentum would seem to prohibit (U-s) from moving at V. The problem disappears if the reciprocal motion is determined by the perception of the anaut in the ship, as it does with the skater example. Altered perception does not violate any rules of physics.

Veedee #49
Quote:

It 'acquires' that speed because the worldlines of spaceship and what you call 'univese' take off in different directions through 4D spacetime.
It (U-s) doesn't acquire that speed. The exhaust mass moves the ship away from earth, while impinging on the earth surface. Since the earth is so massive compared to the ship, the exhaust energy is dispersed in the ground as heat, and never imparts uniformly directed momentum to the earth. If the earth does not move in the opposite direction, then neither does (U-s).
Even if the ship was 100% converted to energy, it wouldn't move (U-s) at any significant speed. These examples have omitted the other important issue of instantaneous acceleration of (U-s) required to satisfy the perception of the observer who initiates the motion.
• 10-09-2014, 06:27 PM
JTyesthatJT
Quote:

Originally Posted by phyti
The exhaust mass moves the ship away from earth, while impinging on the earth surface.

That's not the way rocket engines work. They expel mass, but the mass does not have to impinge on anything to propel the rocket forward. Rocket engines are designed to be able to fly in the vacuum of space.

As an analogy, consider the recoil you feel when firing a rifle. A rocket moves in the opposite direction of the expelled mass just as a rifle moves in the opposite direction of the fired ammunition. There is no violation of conservation of momentum of the system. The rocket and the bullet are still correct to say that other things in the universe are moving past them. It is not just their perception, it really is so.
• 10-09-2014, 06:46 PM
Boing3000
Quote:

Originally Posted by phyti
I just did.

That is funny, I just said the same think "You just did". And I still think that is a mistake, in my FoR anyway.
Quote:

Originally Posted by phyti
If the traveling twin returns younger, he and his clock must have ticked at a slower rate than his twin. That seems very subjective and relative to me.

An obvious contradiction. There is nothing subjective in Relativity. It is very objectivable and twins could be precisely related using mathematics that give the same result every time whoever the twins or cousins or best friends or clocks are.

Quote:

The point is the universe has zero momentum.
I don't think so. Momentum, as in , need some vector quantity to be defined for the universe. From any point of view I doubt that one can sum up all other the vectors (velocities) of things and found ZERO. Maybe you mean that ?

Quote:

Originally Posted by phyti
Altered perception does not violate any rules of physics

What is an "altered" perception ? All perceptions are as real as another. They all confirms the rules of physics...

Quote:

Originally Posted by phyti
The exhaust mass moves the ship away from earth, while impinging on the earth surface.

The earth momentum is irrelevant to any rocket. It's gravitational field is. The rocket expelled momentum could as well never hit the earth. Put the rocket on a rail, fire it tangentially to the surface, and sooner or later it would take off anyway.
• 10-09-2014, 06:58 PM
SinceYouAsked
Quote:

Originally Posted by Boing3000
Very good post SYA and I can tune myself easily to that point of view, especially expressed so clearly.
Except for this bit

I only understand field as potential distributed in something. That something for me is not ether, but space-time or more specifically a topology of events (what we called reality ;-)

That's fine. That's only to say that you refer to any existent ether as spacetime, which is curved everywhere, and hence gravity is everywhere. There is presently no need to inject the notion of an ether, far as relativistic prediction go.

Quote:

Originally Posted by Boing3000
I also think that there is a experimental way to see if such ether exist. By triangulation. If EM field is not influenced by gravitational field (it apparently is, because BH have a charge, and as such "emit" a field even so space-time does not allow it), that would mean that there is no basic support for them to interact. Even if both are triggered by the same potential source, namely matter which sole have energy or charge. Although, I understand that field themselves have energy (and are also source of those potential), except that I don't know of any field that have an electric charge.

I am no expert of black hole theory, but an EM field should be influenced by gravity IMO. The photon travels the energy free-est path, and so it travels a curved path in warped space around a star, or even nearby the outside of a galactic perimeter. That's an influence. Add that per a distant inertial POV far from gravity source, light traveling momentarily thru a high gravitational field should seems to slow down "while inside the gravity well as it passes thru", because of the spacetime curvature. Gravity produces motion, and charge in motion produces magnetic fields, that's an influence. There is highly charged matter outside an even horizon, it's motion governed by various factors including the gravity. So, I'm not sure I understand as to why a gravitational field might be presumed to have no influence on an EM field.

Quote:

Originally Posted by Boing3000
Anyway, by triangulation I mean that if we observe that a gravitational wave "bend" an EM field, we kind of know that both are linked by something, which I agree to call ether.

I would agree. If a medium exists, that which one might call the ether, then it warps unto itself to produce all the forms of energy, including mass, gravity, EM, etc. EM would propagate at c along the curved medium, the curvature as defined and quantified by both fields and the dimension of space and time.

Thank You,
SinceYouAsked
• 10-09-2014, 07:37 PM
Boing3000
Quote:

Originally Posted by SYA
So, I'm not sure I understand as to why a gravitational field might be presumed to have no influence on an EM field.

Me neither, but BH are supposed to not have hairs. That means that if I picture a normal field with a nice round porcupine image (whose spike gets thinner by the sqaure law) near a black hole horizon, these spike should be bent in some way (spacetime strong curvature), meaning the square law would fail in 3D-space. For that mater, these should also be measurable on weaker gravitational field. No ?
Another way to put my question, is that EM field are supposed to be defined by exchange of virtual photon (boson force carrier). I totally fail to understand how a photon (virtual or not) could escape(be emitted by) a BH.
Also, I was not speaking of photon, that are "variation" of the EM field, but of the basic static EM "field" (ether like). Fluctuation in field (or things ;-) are indeed piloted by space-time topology.
In short, a gravitational wave "crossing" a EM field should make photon pop-up into existence , or not ? I am just a layman, but I suppose that it is a basic question for specialists...
• 10-09-2014, 08:52 PM
SinceYouAsked
Quote:

Originally Posted by Boing3000
Me neither, but BH are supposed to not have hairs. That means that if I picture a normal field with a nice round porcupine image (whose spike gets thinner by the sqaure law) near a black hole horizon, these spike should be bent in some way (spacetime strong curvature), meaning the square law would fail in 3D-space. For that mater, these should also be measurable on weaker gravitational field. No ?

I'm no expert either, but I would think there to be a density of flux lines (not thinner or thicker), and nearer the gravity source then the higher the density of gravitation field lines (flux). At any single "gravitational potential" surface (in empty space), the flux is orthogonal to that equa-potential surface where it intersects, and said surface is curved.

From a relativity standpoint, nothing can exit an event horizon because once inside, future time proceeds only toward the singularity (directly or not) ... and so all worldlines end there including the photon's.

Quote:

Originally Posted by Boing3000
Another way to put my question, is that EM field are supposed to be defined by exchange of virtual photon (boson force carrier). I totally fail to understand how a photon (virtual or not) could escape (be emitted by) a BH.

Well, no photon can escape the event horizon once it intersects it. However, if a photon were created just outside the event horizon, it must escape the event horizon so long as its velocity direction (when produced) does not intersect the horizon. Given particles are created constantly just outside the event horizon, some particles (Hawking radiation) escapes. For any generated particle of rest mass, it would have to possess escape velocity to do so.

Quote:

Originally Posted by Boing3000
Also, I was not speaking of photon, that are "variation" of the EM field, but of the basic static EM "field" (ether like). Fluctuation in field (or things ;-) are indeed piloted by space-time topology.

In short, a gravitational wave "crossing" a EM field should make photon pop-up into existence , or not ? I am just a layman, but I suppose that it is a basic question for specialists...

I'm no expert, but that's what I would figure. The high gravity field produces the particle in free space just outside the horizon, and high speed collisions of particles just outside the event horizon can also produce various particles including photons as well, given the already existent immense gravity field in that locale.

Thank You,
SinceYouAsked