Is Simultaneity "Real"?

[Mike_Fontenot]

Note that, for the OP to think she "ages instantaneously by a large amount"*, he must be talking observation.

No. I'm NOT talking about what he sees with his eyes, or what he sees on a TV screen of the TV pictures she transmits to him. Anything he directly sees of her when he is at the turnaround is out-of-date ... it's OLD information. I'm talking about what he CALCULATES her CURRENT age, "right now" to be. That calculation ADDS her amount of ageing during the transit of the TV image to what her age was when she transmitted the image. And when he does that calculation, he finds that her age increases instantaneously by a large amount at his turnaround (just enough so that they agree at their reunion about their respective ages). He can GET that calculated age several different ways. He can get it by actually doing the addition, as described. OR, he can get it directly from the Lorentz equations. OR, he can get it easier, by using a Minkowski diagram. OR, he can get it (easier still) using the CADO equation.

 

[DaveC426913]

And when he does that calculation, he finds that her age increases instantaneously by a large amount at his turnaround

I think you're being ambiguous about frames of reference, which is giving you problems explaining your ideas, and interpreting the twin scenario correctly.

I suspect you are looking for a solution for a problem that doesn't exist, so I will respectfully bow out of this discussion.

 

[Neddy Bate]

I have quoted the OP describing observed phenomena incorrectly. I am correcting that.

If you want to discuss what he calculates her age to be - you should quote the parts of the OP where he is talking about actual age on Earth when he is away - and then address the OP, rather than me.

My view is that whenever the traveler is not accelerating, he is entitled to use the well-known time-dilation result (which says that he will conclude that the home twin (she) is ageing gamma times slower than he is). So he concludes that she ages gamma times slower than he does, on his ENTIRE outbound leg, and on his ENTIRE inbound leg.

Dave,

You should be able to see from that quote that, for the strictly inertial outbound leg of the journey, Mike is letting the traveling twin (he) calculate the home twin's age (she) using the well-known time dilation result. That is where we all seem to agree that the traveler (he) would say that she ages at half of his own rate, (assuming gamma=2). Are you saying that this is based on what the traveling twin observes with his eyes? Because it isn't. The well-known time dilation result is for what her rate of aging actually is, according to him, not what he sees as her aging rate.

You should also be able to see from that quote that, for the strictly inertial return leg of the journey, Mike is letting the traveling twin (he) calculate the home twin's age (she) using the well-known time dilation result. That is where he and I agree that the traveler (he) would say that she ages at half of his own rate, (assuming gamma=2). But you are saying that, on the return leg, he sees her age faster with his eyes. But that is not what Mike is talking about. Again, the well-known time dilation result is for what her rate of aging actually is, according to him, not what he sees as her aging rate.

But he also concludes that during his instantaneous turnaround, she instantaneously ages by a very large amount,

And finally, you should be able to see from that quote that, at the turnaround, Mike says that the traveling twin (he) calculate the home twin's age (she) to advance forward by a very large amount. That is where he and I agree that it would have to be so, since she must be twice his age at that point, (assuming gamma=2). But you are not addressing that, because you are talking about what the traveler sees with his eyes. Meanwhile, I have shown you the chart from wiki article that shows that the stay-home twin is older than the traveling twin at the turnaround point. But your graphs do not reflect this. Are you correcting the wiki article and Mike at the same time?

But he also concludes that during his instantaneous turnaround, she instantaneously ages by a very large amount, exactly enough so that he finds her to be the older when they are reunited. And that same viewpoint leads to the conclusion that if he accelerates in the direction AWAY FROM her (when they are widely separated), she will rapidly get YOUNGER, according to him.

Since you are having trouble with the part where the traveler (he) concludes she rapidly gets older, I assume you will have even more trouble with the part where he concludes she rapidly gets younger. But we cannot get there if you are only discussing what is seen by eye, because that is not what this is about.

 

[DaveC426913]

Since you are having trouble with the part where the traveler (he) concludes she rapidly gets older, I assume you will have even more trouble with the part where he concludes she rapidly gets younger. But we cannot get there if you are only discussing what is seen by eye, because that is not what this is about.

And this is why I've bowed out.

 

[Neddy Bate]

I agree with you. But a lot of people don't. I got banned from the Physics Forum about 10 years ago for my blasphemous views. The favored flavor of simultaneity over there is the Dolby and Gull "Radar method", in which the current age of the home twin (she), according to the traveler (he), increases at about half of his ageing rate for the first half or so of the outbound leg, then about twice as fast as his ageing for the second half or so of the outbound leg. That rate continues for about half or so of the inbound leg, and then drops to about half of his ageing rate for the last half or so of the inbound leg. So in the radar method, her age starts increasing faster than his ageing well BEFORE he does his turnaround ... her ageing rate depends on how he will choose to accelerate in the future ... it violates causality. And even more pleasing for many people, she never gets younger (according to him) for any scenario in the Radar method.

I tried to find information about the Dolby and Gull "Radar method" but I can't seem to pin it down. If it really says that the home twin's aging rate changes BEFORE the traveling twin even turns around, then it must be wrong.

If you recall in some other thread, I had posted this spreadsheet screenshot:

For reference, t' is the traveler's age in his own frame, t is what he calculates for the stay-home twin's age, and "traveler's eyes" is what he sees on her clock using his own eyes, (and a telescope presumably).

Even though the traveler calculates that the home-twin's age goes from 10 to 70 during the turnaround, that does not mean that the ages in between never happened. They do happen in the turnaround, which in reality would not be perfectly instantaneous. So there is nothing wrong with the times when the traveler sees (by eye) that the home twin's age is between 10 and 70. Those are delayed signals that came from during the turnaround. I suspect the Dolby and Gull "Radar method" is trying to fudge things so that does not show up, somehow.

 

[Mike_Fontenot]

I tried to find information about the Dolby and Gull "Radar method" but I can't seem to pin it down. If it really says that the home twin's aging rate changes BEFORE the traveling twin even turns around, then it must be wrong.

Here's a reference to their paper:

Dolby, Carl E. and Gull, Stephen F., "On Radar Time and the Twin 'Paradox' ", Am,J.Phys., 69 (2001) pp. 876-880.

I talked to one of them some time after I had read their paper (I don't remember which one I talked to ... it was a long time ago), and when I asked him if he realized that his method was violated causality, he said "Yes, it is blatantly non-causal. We knew that when we wrote the paper." So it wasn't a show-stopper for them. But it certainly is for me. I pointed that out on the Physics Forum before I got banned, and it didn't bother it's proponents there. There are many physicists who find the idea of the home twin getting younger (according to the traveler) to be so repugnant that ANYTHING is preferable to them.

 

[DaveC426913]

Since Mike has invoked his experience on PF several times now, it might be helpful to give those discussions a browse (locked after member banned).

https://www.physicsforums.com/threads/moving-clocks-run-slow.512598/#post-3394813

 

[Neddy Bate]

Here's a reference to their paper:

Dolby, Carl E. and Gull, Stephen F., "On Radar Time and the Twin 'Paradox' ", Am,J.Phys., 69 (2001) pp. 876-880.

I talked to one of them some time after I had read their paper (I don't remember which one I talked to ... it was a long time ago), and when I asked him if he realized that his method was violated causality, he said "Yes, it is blatantly non-causal. We knew that when we wrote the paper." So it wasn't a show-stopper for them. But it certainly is for me. I pointed that out on the Physics Forum before I got banned, and it didn't bother it's proponents there. There are many physicists who find the idea of the home twin getting younger (according to the traveler) to be so repugnant that ANYTHING is preferable to them.

Thank you for the link. I don't understand how they can justify having the traveling twin calculating different aging rates for the stay-home twin based on anticipation of turning around in the future. The "traveler's eyes" column in my spreadsheet (post #25) shows the correct time that the traveler sees on the home-twin's clock using his eyes/telescope, and that is a one-way signal which never had to be sent from the traveler to bounce off of the stay-home twin. So I think it is less convoluted than whatever they are doing and calling it "radar time".

And indeed it does appear that they are trying to 'solve' the 'problem' of the twin getting younger, which is not really a problem to begin with. Consider the quote:

"Although this period of acceleration can indeed fix the gap between G and H, it cannot resolve the
more serious problem (mentioned also in Marder[7] and in Misner et al.[8]) which occurs to Barbara’s left.
Here her hypersurfaces of simultaneity are overlapping, and she assigns three times to every event! Also,
if Barbara’s hypersurfaces of simultaneity at a certain time depend so sensitively on her instantaneous
velocity as these diagrams suggest, then she would be forced to conclude that the distant planets swept
backwards and forwards in time whenever she went dancing!"

The traveling twin (in this case Barbara) only has one hypersurface of simultaneity at any given point in time, so they are not overlapping. Since there is only one, and it is changing its angle, there is no need to assign three times to every event. Perhaps I'm missing something, but it's hard to tell what it might be.

 

[Mike_Fontenot]

Since Mike has invoked his experience on PF several times now, it might be helpful to give those discussions a browse (locked after member banned).

https://www.physicsforums.com/threads/moving-clocks-run-slow.512598/#post-3394813

WOW! Thanks for posting that ... I had never seen that thread before! (Either before or after I was banned). Those dates WERE about the time they banned me. And I noticed that they had UPDATED several of those posts only a couple of years ago ...interesting!

I DO still look at the Physics Forum from time to time, but obviously I can't post there. It used to have a lot of interesting discussions, but it's been pretty dull the last few years. The Physics Forum has done a LOT of banning over the years ... far from a free exchange of ideas.

 

[Mike_Fontenot]

[...]

In addition to my not being willing to accept any simultaneity definition that violates causality, it also seemed to me that saying "that her current age at some instant in his life can't be determined until some future time" implies that she actually HAS no current age at that current instant for him. That seems to imply that she doesn't even EXIST at that current instant for the traveler! Much too weird for me ... no thanks.

Also, another objection that I sometimes get for my results that show negative ageing by the home twin is that that means that the CADO reference frame, which gives her current age as a function of his age, isn't reversible (i.e., that it's not a one-to-one function), because she can gave the same age for multiple ages of himself. They then cite the requirement in general relativity that a "chart" must be one-to-one. That's true, because in GR charts are used to patch together a way of describing the entire universe, and that process requires reversibility. But that's a red herring: ALL that is important to the traveling twin in special relativity is that the traveler's reference frame tell him the current time and current position of every object in the (assumed flat) universe, and the CADO reference frame does that. The CADO reference frame has NO NEED to be a chart, because it is a special relativity result, not a general relativity result.

 

[DaveC426913]

The Physics Forum has done a LOT of banning over the years ... far from a free exchange of ideas.

Yes. Their modern format supports standard established physics. Alternate or fringe physics is not conducive to student learning.
I thought their comments on your ideas would be useful here.

 

[Neddy Bate]

In addition to my not being willing to accept any simultaneity definition that violates causality, it also seemed to me that saying "that her current age at some instant in his life can't be determined until some future time" implies that she actually HAS no current age at that current instant for him. That seems to imply that she doesn't even EXIST at that current instant for the traveler! Much too weird for me ... no thanks.

Yeah, I'll pass on that also. As I said in post 2, simultaneity in SR comes from Einstein synchronization of clocks, so it is not some undetermined, undefined, or arbitrary thing.

Also, another objection that I sometimes get for my results that show negative ageing by the home twin is that that means that the CADO reference frame, which gives her current age as a function of his age, isn't reversible (i.e., that it's not a one-to-one function), because she can gave the same age for multiple ages of himself. They then cite the requirement in general relativity that a "chart" must be one-to-one. That's true, because in GR charts are used to patch together a way of describing the entire universe, and that process requires reversibility. But that's a red herring: ALL that is important to the traveling twin in special relativity is that the traveler's reference frame tell him the current time and current position of every object in the (assumed flat) universe, and the CADO reference frame does that. The CADO reference frame has NO NEED to be a chart, because it is a special relativity result, not a general relativity result.

I'm not sure about all that, because I am not familiar with GR charts. However, SR is a special case of GR, and so anything which can happen in SR can also happen in GR. Are you saying that the negative ageing predicted by SR in this type of case would not happen if there were the slightest gravitational field present? I would not guess that you are saying that. So why is a 1-to-1 chart not necessary then?

 

[Neddy Bate]

Regarding 1-to-1 charts, here is another thing to consider in the SR reverse aging scenario:

There will always be a 1-to-1 chart between the stay-home twin and the traveling twin, provided we look at it strictly from the inertial frame of the stay-home twin. In that frame, neither one of the twins ever has reverse aging. So you can always claim a 1-to-1 chart that way, although the traveling twin can disagree all he likes.

 

[Mike_Fontenot]

[...]
As I said in post 2, simultaneity in SR comes from Einstein synchronization of clocks, so it is not some undetermined, undefined, or arbitrary thing.

[\QUOTE]
(quoting isn't working correctly again, so I'll have to indicate that manually)

Mike Fontenot responds:

I agree. But not all physicists agree with that ... many think that there are multiple different simultaneity "conventions", and everyone is free to choose which one to use in any given circumstance.

Quote from Neddy Bate:

I'm not sure about all that, because I am not familiar with GR charts. However, SR is a special case of GR, and so anything which can happen in SR can also happen in GR. Are you saying that the negative ageing predicted by SR in this type of case would not happen if there were the slightest gravitational field present? I would not guess that you are saying that.
[\QUOTE]

Mike Fontenot responds:

No, I'm not saying that at all.

Quote from Neddy Bate:

So why is a 1-to-1 chart not necessary then?
[\QUOTE]

Mike Fontenot responds:

Charts are necessary in general relativity, because when spacetime is curved, no single coordinate system can cover ALL of spacetime. So to cover all of space, multiple charts must be stitched together where they touch each other. The charts have to agree with each other where they touch each other at their mutual boundaries, and to insure that, they have to be invertible. But in special relativity, spacetime is flat everywhere, and so the coordinate system of any given observer (like the inertial frame of a perpetually inertial observer, or the CADO frame of an accelerating observer) covers ALL of spacetime. So there is no need to stitch together any coordinate systems.

(end of Mike Fontenot's post)

 

[Neddy Bate]

I agree. But not all physicists agree with that ... many think that there are multiple different simultaneity "conventions", and everyone is free to choose which one to use in any given circumstance.

Fair enough, but the Lorentz equations of SR already assume the Einstein synchronisation convention.

Charts are necessary in general relativity, because when spacetime is curved, no single coordinate system can cover ALL of spacetime. So to cover all of space, multiple charts must be stitched together where they touch each other. The charts have to agree with each other where they touch each other at their mutual boundaries, and to insure that, they have to be invertible. But in special relativity, spacetime is flat everywhere, and so the coordinate system of any given observer (like the inertial frame of a perpetually inertial observer, or the CADO frame of an accelerating observer) covers ALL of spacetime. So there is no need to stitch together any coordinate systems.

Let's say that someone were stitching together some charts in GR, and there happened to be an area with very little gravity. Within this area we have our stay-home twin (she) and our traveling twin (he) who moves away from his sister for awhile, then changes direction back toward her, then changes his mind and changes direction again, speeding up and moving away from her again even faster. This causes him to say his sister got even younger than she was just before he changed direction the first time.

Are you saying that GR could not stitch that area together with the other surrounding areas, because the traveling twin had two different ages when his sister had the same age (due to her getting even younger when he sped up away from her)? If so, would it save the day to use her inertial data instead of his? Because according to her, neither she nor her brother ever got any younger.

I've never stitched together anything in GR, but I am curious as to why this simple SR thought experiment would "break" GR in that sense. It doesn't seem like it should, so maybe the answer is to use an inertial frame instead of an accelerating one, though it seems that GR shouldn't even have that limitation.

 

[Mike_Fontenot]

Fair enough, but the Lorentz equations of SR already assume the Einstein synchronisation convention.
[\QUOTE]

Yeah, they do. So all the physicists who say there are multiple simultaneity conventions that can be chosen shouldn't ever use the terms "Lorentz equations", or "time-dilation", or "length contraction", or "Minkowski diagrams" ... but they do.

Let's say that someone were stitching together some charts in GR, and there happened to be an area with very little gravity.
[\QUOTE]

Almost all of space would be like that. GR is really only applicable when you are very near large masses, and even then the effect is close to negligible unless you are very near a massive black hole, or maybe a wormhole. In almost all of space, SR works just fine. Worrying about GR when trying to explain the twin "paradox" is just wasted time, and a diversion from what is really important.

[...]
If so, would it save the day to use her inertial data instead of his? Because according to her, neither she nor her brother ever got any younger.
[\QUOTE]

That would dodge the whole issue in the twin paradox, which is "Why is the time-dilation equation not sufficient to determine the traveler's expectations of his twin's age when they are reunited?"

The twin "paradox" is about special relativity. It is applicable to almost all of space. People who bring general relativity into it are just using it as a way to avoid facing the really important issues in the twin "paradox", because they don't like the obvious answers that special relativity gives.

 

[DaveC426913]

Mike:

Your closing quotes should have a forward slash '/', not a backward slash '\'.
To-wit:

[ / QUOTE ]

 

[Mike_Fontenot]

Mike:

Your closing quotes should have a forward slash '/', not a backward slash '\'.
To-wit:

[ / QUOTE ]

 

[Mike_Fontenot]

Thanks. I'm surprised I didn't catch that, but I'm very glad to get rid of the problem ... it's been a pain for me and everyone else.

 

[Mike_Fontenot]

Are you saying that GR could not stitch that area together with the other surrounding areas, because the traveling twin had two different ages when his sister had the same age (due to her getting even younger when he sped up away from her)? If so, would it save the day to use her inertial data instead of his? Because according to her, neither she nor her brother ever got any younger.

I'm aware that I didn't answer your above question, in my previous reply. That's partly because it's been a long time since I studied GR, and I'm rusty now when it comes to charts. (My sources for the chart stuff were Robert Wald, and also Sean Carroll.)

Here's the closest answer I could come up with now to your well-posed question:

I'm pretty sure it WOULDN'T "save the day" to just use her reference frame instead of his reference frame. I don't think that "charts" are defined based on specific observers ... they are supposed to tell you everything you need to know about the region they cover. I THINK what's happening is that charts concern the entity SPACETIME, not the separate entities TIME and SPACE. SPACETIME itself is the same for all observers in the region, whereas the twin "paradox" is concerned with different observers views about SPACE and TIME.

That may sound pretty fuzzy to you, but it's the best I can do. Hopefully someone, who believes that the non-invertibility of the traveler's reference frame invalidates it, will chime in and explain their position better than I can.

I think a lot of "modern" physicists like to think only in terms of spacetime, and avoid thinking about or talking about space and time separately. They are the same physicists who think (maybe secretly) that Einstein was an unsophisticated rube. I don't agree with their view. At any rate, you can't really talk about the twin "paradox" without talking about space and time separately.