1. I'm sure many of you have heard of the twin paradox. Basically, if one person traveled away from earth near the speed of light, and came back years later special relativity would tell us that the person that traveled away from earth and returned would say that he aged more than people on earth, while people on earth would say that they aged more then the person that traveled away from earth and returned. From what I've read online, what happens is that the person traveling away from earth and returning ages less because this person is the one that feels the acceleration and this makes the time dilation asymmetric between the people on earth and the person traveling away from earth and returning. This doesn't explain much. Can somebody explain further why this happens?

2. Originally Posted by Scheuerf
I'm sure many of you have heard of the twin paradox. Basically, if one person traveled away from earth near the speed of light, and came back years later special relativity would tell us that the person that traveled away from earth would say that he aged more than people on earth, while people on earth would say that they aged more then the person that traveled away from earth. From what I've read online, what happens is that the person traveling away from earth ages less because this person is the one that feels the acceleration and this makes the time dilation asymmetric between the people on earth and the person traveling away from earth. This doesn't explain much. Can somebody explain further why this happens?
The explanation is complex and requires a good grasp of calculus. Can you follow this?

3. Originally Posted by Scheuerf
Basically, if one person traveled away from earth near the speed of light, and came back years later special relativity would tell us that the person that traveled away from earth would say that he aged more than people on earth, while people on earth would say that they aged more then the person that traveled away from earth. From what I've read online, what happens is that the person traveling away from earth ages less because this person is the one that feels the acceleration and this makes the time dilation asymmetric between the people on earth and the person traveling away from earth. This doesn't explain much. Can somebody explain further why this happens?
Let's take apart what you said:

In the first part you wrote - if one person traveled away from earth near the speed of light, and came back years later special relativity would tell us that the person that traveled away from earth would say that he aged more than people on earth - Do you see the problem with this? In the first part of this sentence you said [i]traveled away from earth near the speed of light, and came back[i] meaning it was a round trip which requires acceleration but in the second part you said "special relativity would tell us that the person that traveled away from earth would say that he aged more than people on earth" meaning it was a one way trip outwards and thus no acceleration.

In the second part you wrote - the person traveling away from earth ages less because this person is the one that feels the acceleration and this makes the time dilation asymmetric between the people on earth and the person traveling away from earth. I don't know what you're trying to say here because you said that the person traveling away from earth ages less but you made no comment about him turning around and coming back but then you say this person is the one that feels the acceleration If you meant that he turned around and came back then the rest of what you said is true.

With that in mind please write it over again and I'm sure you'll see what's going on clearly this time.

4. Here is a nice diagram which may help.

Special Relativity : Section 15

You need to bear in mind that the turn-around won't be instant. It is the period of acceleration during the turnaround that resolves the paradox.

5. (Sorry, quote button not working for me).

Jilan #4, it's not so much the acceleration that resolves the paradox, but that one twin stays in one inertial frame the whole time, and the other is in multiple inertial frames - and that's what makes the situation non-symmetrical. It gets mixed up of course, because acceleration is normally how one twin ends up in multiple frames during the experiment.

From Wikipedia, under "Role of acceleration":

Although some texts assign a crucial role to the acceleration of the travelling twin at the time of the turnaround, others note that the effect also arises if one imagines separate outward-going and inward-coming travellers, who pass each other and synchronize their clocks at the point corresponding to "turnaround" of a single traveller. In this version, acceleration plays no direct role ...

6. There are two approaches to this I have discovered. Some texts say the paradox can explained with SR. Others that actually you can only resolve it with GR. That wiki quote is food for thought......

7. One can also consider the Doppler shifts of the light from each twin to the other twin. It's true for both twins that when the other twin is seen to be moving away, the light will be Doppler redshifted. And when the other twin is seen to be approaching, the light will be Doppler blueshifted. But consider when each twin sees the other twin turn around from moving away to approaching. Because it is the travelling twin that actually turns around, he sees the redshift become blueshift immediately. By contrast, the stay-at-home twin only sees the redshift become blueshift when light from the travelling twin's turn-around reaches him. Therefore, the stay-at-home twin sees the travelling twin redshifted for a greater proportion of the journey than visa-versa, and this accounts for the travelling twin being unequivocally younger upon return.

8. Originally Posted by Jilan
There are two approaches to this I have discovered. Some texts say the paradox can explained with SR. Others that actually you can only resolve it with GR. That wiki quote is food for thought......
There is absolutely no need for GR in explaining the paradox.

9. Originally Posted by x0x
There is absolutely no need for GR in explaining the paradox.
Well you are agreeing with about 60% from my latest text count. Strangely enough they were having the same debate 100 years ago. Einstein favoured the acceleration type argument.
Twin paradox - Wikipedia, the free encyclopedia
I am still to be convinced either way.

10. John Baez runs down most things, I think: The Twin Paradox

Jilan, the "The Equivalence Principle Analysis" section may be of interest ...

The Equivalence Principle analysis of the twin paradox does not use any real gravity, and so does not use any General Relativity. (General Relativity is the study of real gravitational fields, not pseudo ones, so it has nothing to say about the twin paradox.) Nevertheless, what General Relativity does say about real gravitational fields does hold in a restricted sense for pseudo gravitational fields. The one thing we need here is that time runs slower as you descend into the potential well of a pseudo force field.

11. Thanks pzk, it's the time gap I have difficulties with, that howling void. It just doesn't seem physical to me.

12. Yeah, with "instant acceleration" that'll happen! (Non-physical).

The "time gap" is (I think) best thought of in the scenario with multiple observers who don't ever accelerate but just synchronise clocks on passing. One observer is always on the outbound leg, another is always on the inbound leg.

With that, it's also very clear that it's about multiple frames; or at least, it's non-symmetrical.

13. Originally Posted by Jilan
Well you are agreeing with about 60% from my latest text count. Strangely enough they were having the same debate 100 years ago. Einstein favoured the acceleration type argument.
Twin paradox - Wikipedia, the free encyclopedia
I am still to be convinced either way.
Doesn't matter what you are "convinced".

Hey...How does it feel to so incredibly boorish and obnoxious based on "schoolmarm finger-wagging" and mathematical formulas that explain nothing?

The advance of physics theory is reduced to..."How can you have any pudding if you don't eat your meat?" (courtesy of Syd...I hope he's at peace now)

......

15. Originally Posted by Gerry Nightingale

Hey...How does it feel to so incredibly boorish and obnoxious based on "schoolmarm finger-wagging" and mathematical formulas that explain nothing?

The advance of physics theory is reduced to..."How can you have any pudding if you don't eat your meat?" (courtesy of Syd...I hope he's at peace now)

......

Hi Gerry,

If you don't mind my asking, who are you so upset with, and why?

Hello to Physicist...it ain't you, so no worries...ain't Jilan neither! Give up? (maybe I posted it to myself!) Why? I don't like people who are nasty to someone I like...that's all

the reason I need.

17. Originally Posted by Gerry Nightingale

Hello to Physicist...it ain't you, so no worries...ain't Jilan neither! Give up? (maybe I posted it to myself!) Why? I don't like people who are nasty to someone I like...that's all

the reason I need.
I knew it wasn't me since we get along fine. I had a feeling it was x0x since he seems to be a never ending source of trouble.

18. Originally Posted by Physicist
I knew it wasn't me since we get along fine. I had a feeling it was x0x since he seems to be a never ending source of trouble.
Too bad, you can't see his posts. It said.
"Doesnt matter what you are convinced"

It matters a bit to me though! I was reading some more about this yesterday and the acceleration type argument seems to fall down when you consider a shorter trip. The howling gap is smaller although the accelerations are the same. The overall effect as pzk says is a function of the different inertial frames and the distance travelled. The proper time will always be less for the traveller which has moved a greater distance between spacetime events.

Proper time - Wikipedia, the free encyclopedia

19. Originally Posted by Jilan

It matters a bit to me though! I was reading some more about this yesterday and the acceleration type argument seems to fall down when you consider a shorter trip.
This is false on two levels:

1. The calculation based on the equations of accelerated motion in SR ALWAYS work.

2. Your contention that the accelerated motion not working might necessitate GR, is untrue.

If you do not do the calculations and just babble about them you get to make errors. Just like Farsight.

20. Originally Posted by Jilan
Too bad, you can't see his posts. It said.
"Doesnt matter what you are convinced"
So, not only very rude but poor English to boot, huh?

21. x0x, I am not following you. A shorter trip can involve the same accelerations but the time discrepancy is less. You are getting my comment about the acceleration the wrong way round. The shorter trip argument would suggest that the acceleration periods (hence GR) could not be the answer. This ties in with pzk's post #5.

22. Originally Posted by Jilan
x0x, I am not following you. A shorter trip can involve the same accelerations but the time discrepancy is less. You are getting my comment about the acceleration the wrong way round. The shorter trip argument would suggest that the acceleration periods (hence GR) could not be the answer. This ties in with pzk's post #5.
I see, so you are not arguing against using the equations of accelerated motion in SR anymore. Are you now convinced that the problem can be fully resolved within the realm of SR?

I was reading some more about this yesterday and the acceleration type argument seems to fall down when you consider a shorter trip.
What were you reading? What gives you the idea that "the acceleration type argument seems to fall down when you consider a shorter trip.". Can you post the associated math rather than talking around the subject?

23. x0x, I think I made it clear that I am not arguing for or against anything. I am considering the merits of both arguments and trying to keep an open mind. Here is the site with the diagram for the shorter trip. Its the second diagram from the bottom.

250.Twins

24. Originally Posted by Jilan
I was reading some more about this yesterday and the acceleration type argument seems to fall down when you consider a shorter trip. The howling gap is smaller although the accelerations are the same. The overall effect as pzk says is a function of the different inertial frames and the distance travelled. The proper time will always be less for the traveller which has moved a greater distance between spacetime events.
Yes, that is correct. I'm glad to see that you have realised this. ^

Now hopefully my thread about equal accelerations will make more sense to you. If not, that's okay, just keep thinking about it. What's important is that you are a curious learner, and you are making good progress.

25. Originally Posted by Jilan
x0x, I think I made it clear that I am not arguing for or against anything. I am considering the merits of both arguments and trying to keep an open mind. Here is the site with the diagram for the shorter trip. Its the second diagram from the bottom.

250.Twins
Thank you, nowhere do they claim that "the acceleration type argument seems to fall down when you consider a shorter trip."
Nor do the authors make any reference to using GR in solving the problem.

26. I don't think we are reading the same article. Opposite the relevant diagram it says
"It might be noted that the relative span of each simultaneity gap appears to grow as a function of distance which cannot be linked to the period of acceleration and deceleration"
The next two paragraphs then says that the Lorentz transforms give the correct answers without recourse to GR.

27. Originally Posted by Jilan
The next two paragraphs then says that the Lorentz transforms give the correct answers without recourse to GR.
Good, so you are no longer claiming that GR is necessary, right?

Opposite the relevant diagram it says
"It might be noted that the relative span of each simultaneity gap appears to grow as a function of distance which cannot be linked to the period of acceleration and deceleration"
The conclusion is drawn on the diagrams that show instantaneous turnaround (i.e. infinite acceleration). Not everything he writes in his blog is correct.

SR, over the last 50yrs. has become the "the call of the Syrens" causing Einstein's "The good ship GR" to be rendered and broken by the rocks of "Calculus".

......

SR is just that...Special. The problem is in thinking that "SR" pertains to the reality of the "real" Universe...and it doesn't. All of the possibility factors that can be imagined can, at some

level of mathematical construct, be made "real"...at least on paper.

Anyone w/ a fair amount of skill can posit a "thing" and then demonstrate the reality of the "thing" by the use of equations. (this always "works" as long as you don't lose track of your

"blinders"...like watching telly w/ the "magic glasses)

29. Originally Posted by x0x
Good, so you are no longer claiming that GR is necessary, right?

The conclusion is drawn on the diagrams that show instantaneous turnaround (i.e. infinite acceleration). Not everything he writes in his blog is correct.
For the third time, I am not claiming anything, just reviewing the arguments.

One thing that surprises me is that the effect of the length contraction gets few mentions. The distance between the start point and turnaround point will be less for the traveller than as measured by the one at rest. So if they agree on the separation speed they won't agree on the distance travelled or the time taken by the traveller.

30. Dear Scheuerf,

Welcome to the bear-pit!
Originally Posted by Scheuerf
I'm sure many of you have heard of the twin paradox. Basically, if one person traveled away from earth near the speed of light, and came back years later special relativity would tell us that the person that traveled away from earth and returned would say that he aged more than people on earth, while people on earth would say that they aged more then the person that traveled away from earth and returned. From what I've read online, what happens is that the person traveling away from earth and returning ages less because this person is the one that feels the acceleration and this makes the time dilation asymmetric between the people on earth and the person traveling away from earth and returning. This doesn't explain much. Can somebody explain further why this happens?
The arguments used above in the various replies are multifaceting and can lead away from the truly fundamental question.

My advice is always to go back to original articles - because the underlying issues of that era come to the fore, rather than the overlays of later eras & discussion, because these in turn are distracting and can lead to massive confusion.

You must go back to Einstein's original article on this question. I.e. Naturwissenschaften 48 (29th November 1918) 697-702. There is an English translation on the net but it is rather crude: e.g. the word "principally" should be translated "in principle".

Whatever Jilan, xOx, Schrodinger's Cat, Junkipedia or I myself say here about the twin paradox itself is NOT important. It's what Einstein said that is important - though Super-KLW's comment is exceedingly worthy of note as you agree!

What Einstein will show there is that he introduces GR to try to resolve the logical paradox emerging from SR. I cannot comment further on this thread on this issue so if you want to discuss this in greater detail please open a separate thread on the "Personal & Alternatives Forum" at the bottom of the website.

TFOLZO

31. Originally Posted by Jilan
For the third time, I am not claiming anything, just reviewing the arguments.
Actually, you have been claiming. Nonsense stuff.

One thing that surprises me is that the effect of the length contraction gets few mentions. The distance between the start point and turnaround point will be less for the traveller than as measured by the one at rest.
So if they agree on the separation speed they won't agree on the distance travelled or the time taken by the traveller.
Yes, so? The methods that I have presented circumvent this issue by dealing with elapsed time exclusively.

32. In reply to #31, re: SR.

Change the word "circumvent" to the phrase "conveniently ignore" so as make the "solution???" work.

......

Jilan has not "claimed" anything...she is "looking at various interpretations" and weighing their respective merits, if any are to be found. Writing "this is what I read from so-and-so" does

not constitute an endorsement nor a denial, it means she is examining a condition from different perspectives (this IS the proper way to examine any theory or posit)

......

Coming from someone writes rebuking comments to others, you demonstrate a complete inability on your part to recognize there are still "issues yet to be explained" in every single post

you write to others...and then wait for an answer so you can claim "I'm smarter than you!" because "I know the rationales of calculus theory, and you don't" is a completely "non-science"

posture, especially given that you are quoting the works of others, NOT your own! (really smart people create new theory...mediocre people who "think they know" quote theory)

......

33. Thanks Gerry, you are a true gent. I'm beginning to think that English is not the first language of xOx so we should make allowances. I hope life is treating you well.

34. Originally Posted by x0x
Actually, you have been claiming. Nonsense stuff.

Yes, so? The methods that I have presented circumvent this issue by dealing with elapsed time exclusively.
X0X, read me four four times now, I am not claiming anything.

That said I am pretty sure now that time dilation alone does not solve the paradox. Length contraction is required also. You seem to have a compunction that I stick my fork in somewhere so I am obliging you now.

35. Originally Posted by Jilan
x0x, I am not following you. A shorter trip can involve the same accelerations but the time discrepancy is less. You are getting my comment about the acceleration the wrong way round. The shorter trip argument would suggest that the acceleration periods (hence GR) could not be the answer.
If you are going to invoke the gravitational/acceleration equivalence in GR to explain the time difference, you also have to take into account that gravitational time time dilation is related to the difference in gravitational potential and not to differences in local gravitational forces. Thus, for the accelerated observer, the equivalent gravitational field is one that is uniform and extends for infinity. When this acceleration is during turnaround, this places the Earth twin "above" him in this field, and since the gravitational potential differs with "height" in the field, and the shorter trip results in a lower height for the Earth for this observer than the longer trip does, this is why GR is still applicable.

To put it straight, While you can invoke GR when dealing with the twin paradox, it is not necessary to do so to do so. To paraphrase how one site puts it:

Part of the difficultly in explaining the Twin paradox is not the lack of a valid explanation, but rather the multiplicity of valid explanations. If you are looking for the "one correct viewpoint", you are likely to be frustrated. This is not as because all these explanations conflict with each other, as they don't, it is just that there are multiple avenues from which you can approach this scenario.

36. Thanks Janus, I'd kind of reached the conclusion that different interpretations would get to the same place. It's not unlike QM in that respect. I am pretty close to the one that works for me. I have rationalised a simultaneity argument that does not cause a paradox in my my head. That's enough for me, my head hurts now.

37. Originally Posted by Jilan
X0X, read me four four times now, I am not claiming anything.

That said I am pretty sure now that time dilation alone does not solve the paradox.
"You are pretty sure". An you are pretty wrong. The amusing thing is that you keep claiming that you are not claiming anything but you are. And , in all cases, what you are claiming is wrong.

Length contraction is required also.
Incorrect, it isn't. Try learning for a change. Here.

38. In reply to post #37.

I read the "link". All of the equations are correct (yes, I can follow the "bouncing ball) and true...except the conditional predicates are false. Clocks and yardsticks do not serve to define the

relative values of separate "time frames of velocity" with regard to the REAL Universe.

.....

Aw...shucks, mam...twern't nuthin'.

"Stay the course...and carry on".

40. Originally Posted by x0x
"You are pretty sure". An you are pretty wrong. The amusing thing is that you keep claiming that you are not claiming anything but you are. And , in all cases, what you are claiming is wrong.

Incorrect, it isn't. Try learning for a change. Here.
For the second time in this thread we appear to be reading different articles. In the specific example in the wiki link you gave it states:
"The calculation illustrates the usage of the phenomenon of length contraction and the experimentally verified phenomenon of time dilation to describe and calculate consequences and predictions of Einstein's Special Theory of Relativity"

41. Originally Posted by Jilan
For the second time in this thread we appear to be reading different articles. In the specific example in the wiki link you gave it states:
"The calculation illustrates the usage of the phenomenon of length contraction and the experimentally verified phenomenon of time dilation to describe and calculate consequences and predictions of Einstein's Special Theory of Relativity"
Err, not the paragraph that I pointed you out to. It shows how one calculates the elapsed proper time. No length contraction of any sort, as I explained to you. You searched for "length contraction" anywhere in the wiki article, in classical Farsight style.
You need to start learning how to do math, "talking" about physics is not physics at all, it is "Farsight - physics". And you are getting closer and closer to him by the post.

42. I read the whole article.

43. In reply to post #41, re: time dilation.

"Time dilation" is an abstract of "suppositional theory". The "suppositional theory" is based upon a particle or mass approaching relativistic speeds (A.E. NEVER advocated that any physical

thing could achieve such velocities, never mind meet or exceed "c'".)

......

It is a COMPLETE denial of "The General Theory of Relativity" to advocate that that anything of any physical, real nature could be induced to attain relativistic velocity.

......

The "idea" of "bending/warping/stretching" of an abstract such as "time" is moot. All equations that have been written or have yet to be written cannot validate "time" as real thing.

44. Originally Posted by Jilan
...but you selected a quote that suited your misconceptions, rather than trying to understand the specific section that contradicts your misconceptions. You are operating exactly like Farsight. Take care, you are getting there.

45. x0x thank you for your words of encouragement. I am not quote mining though as you may recall in post #29 I said

"One thing that surprises me is that the effect of the length contraction gets few mentions"

So I was quite surprised when you sent me the wiki link that actually did mention it

46. Originally Posted by Jilan
x0x thank you for your words of encouragement. I am not quote mining though as you may recall in post #29 I said

"One thing that surprises me is that the effect of the length contraction gets few mentions"

So I was quite surprised when you sent me the wiki link that actually did mention it
Actually, I sent you a link showing how one solves the paradox WITHOUT using length contraction.
You claimed that "length contraction is REQUIRED ALSO". I proved to you that it ISN'T.
Instead of learning you elected to search for "length contraction" somewhere else, anywhere in the article. Note that the paragraph you have chosen has NOTHING to do with the explanation of twins paradox since it is devoid of the asymmetry that causes the paradox to begin with. You aren't interested in learning, you are simply becoming more and more like Farsight, a quote-miner.

47. In reply to #46 post, re: "length".

Soooo...The "pot" now accuses the "kettle" of being "black". A conditional state of Lorentz factor DOES NOT apply!?!? This is truly "special" relativity, just like Farsight!!!

"You" have "proved" LORENTZ WRONG??? Not necessary? Which part of "Never-Never Land" do such equations pertain to?

......

Ta ra