Twin paradox initial acceleration

[idea2000]

Hi,

I've seen many variations of explanations of the twin paradox using special relativity, but i haven't yet seen an explanation that bothers to take into account the initial acceleration of the traveling twin away from planet Earth. Is it safe to say that this can be neglected? And, also, if the traveling twin were to accelerate away from Earth and then decelerate onto the distant planet and then stay there without coming back, would he still be younger than his twin at home?

Thanks for any help that anyone can provide!

 

[HallsofIvy]

I'm puzzled by your first question because I've never seen an explanation that doesn't take the initial acceleration into account. As for what happens if the traveling twin never returns, there is no "absolute" way for the two twins to compare their ages- the question of whether he would still be "younger" is moot.

 

[ghwellsjr]

Hi,

I've seen many variations of explanations of the twin paradox using special relativity, but i haven't yet seen an explanation that bothers to take into account the initial acceleration of the traveling twin away from planet Earth. Is it safe to say that this can be neglected? And, also, if the traveling twin were to accelerate away from Earth and then decelerate onto the distant planet and then stay there without coming back, would he still be younger than his twin at home?

Thanks for any help that anyone can provide!

There are also many variations of the Twin Paradox, some of which have the same accelerations for both twins and some of which have no accelerations (involving triplets, actually). They all give the same result. It is safe to say that you can neglect the contributions of the relative aging during the accelerations.

There are also many variations on how the twins can start off together and end up apart at rest with respect to each other but it is not possible to make any conclusion about which one is younger while they remain separated. The "answer" depends on what assumptions you make, which, of course, means that there is not an answer.

 

[Meir Achuz]

The effect of acceleration is proportional to the distance between the twins. At the start and end of the trip, this distance is zero.

 

[mathplease]

Explanations I have read resolve the paradox by stating that one of the twins must accelerate in order to get back. This seems to imply that the twin could not return simply by continuing in the same direction with constant velocity. Is this also true in a closed universe?

 

[Mike_Fontenot]

[...]
I've seen many variations of explanations of the twin paradox using special relativity, but i haven't yet seen an explanation that bothers to take into account the initial acceleration of the traveling twin away from planet Earth. Is it safe to say that this can be neglected? And, also, if the traveling twin were to accelerate away from Earth and then decelerate onto the distant planet and then stay there without coming back, would he still be younger than his twin at home?
[...]

Yes, and yes.

Whenever the twins are (either perpetually, or momentarily) co-located, they will agree about their corresponding ages, regardless of their relative velocity.

Whenever the twins have (either perpetually, or momentarily) zero relative velocity, they will agree about their corresponding ages, regardless of their distance apart.

In all other circumstances, they will generally disagree about their corresponding ages.

For more information, see these two postings:

https://www.physicsforums.com/showpost.php?p=2923277&postcount=1

and

https://www.physicsforums.com/showpost.php?p=2957404&postcount=5 .


Mike Fontenot

 

[Ich]

Is this also true in a closed universe?

No, in a closed universe, you can return (and be younger) without accelerating. Closed universes are not covered by special relativity, though.

 

[idea2000]

I'm puzzled by your first question because I've never seen an explanation that doesn't take the initial acceleration into account. As for what happens if the traveling twin never returns, there is no "absolute" way for the two twins to compare their ages- the question of whether he would still be "younger" is moot.

Could the far away twin send a beam of light once he gets to the distant planet? Maybe the twin on Earth could adjust it for the time it took the light to travel back to Earth, or, alternatively, maybe the twin could encode his age into the beam of light and the twin on Earth could adjust for how long it took the light to travel back to Earth? Is this possible? Thanks for any help in advance...

 

[ghwellsjr]

Could the far away twin send a beam of light once he gets to the distant planet? Maybe the twin on Earth could adjust it for the time it took the light to travel back to Earth, or, alternatively, maybe the twin could encode his age into the beam of light and the twin on Earth could adjust for how long it took the light to travel back to Earth? Is this possible? Thanks for any help in advance...

No, No, No, and for extra measure No.

Nobody knows the time it takes for light to travel from the remote twin to the Earth twin, or from the Earth twin to the remote twin. All we know is the time it takes for light to make a round trip. Whatever that round trip time is, we can't know if it is equal for both halves or partitioned unequally between the two halves of trip. If we assume that light takes less time to get from the remote twin to the Earth twin, you will come to one conclusion about their relative ages, or the opposite conclusion for the opposite assumption. This is, in effect, what happens when you pick different frames of reference to view the situation in.

This is the reason you cannot compare absolute ages or clock times when they are remotely located and why we have to deal with relativity of simultaneity.

In SR, we arbitrarily define a frame as when the two times are equal for remote clocks that are stationary in that frame and unequal for co-moving clocks.

 

[Mike_Fontenot]

[...]
maybe the twin could encode his age into the beam of light and the twin on Earth could adjust for how long it took the light to travel back to Earth? Is this possible?

Yes, it is. You are asking all the right questions.

To make it easy to keep the twins straight, let the "home" twin be a "she", and the "traveling" twin be a "he".

He can transmit a TV image, holding a sign that gives his current age. When she receives that image, she can compute how much he aged during the image transit, and thus determine how old he is when she receives his image.

And likewise, she can also transmit a similar message, which he can use to determine her age when he receives her image.

Whenever their relative velocity is ZERO, they will agree about the correspondence between their ages.

Whenever their relative velocity is NON-ZERO, they will NOT agree about the correspondence between their ages (unless they are co-located). And even though they disagree, they are BOTH correct, because each of their conclusions is based on their own elementary measurements and calculations. It's weird, but it is an inevitable consequence of the fundamental tenet that the speed of any given light pulse will always be measured, in any and all inertial reference frames, to have exactly the same value in all cases. It's weird, but it's NOT inconsistent.

Mike Fontenot

 

[idea2000]

Hi,

Thanks for your wonderful response, I do have one more question...

I can see how to use the invariance of the spacetime interval to calculate the dx and the dt of what each twin sees from his own perspective during the trip. The math makes sense, however, what i don't understand is how both twins can see their counterpart age slower during the trip and yet still come to the conclusion that one of them is younger than the other at the end of the trip?

For example, the traveling twin counts himself as having 100 heartbeats and counts his Earth twin as having 75 heartbeats during the trip to the distant planet. During the return trip from the distant planet, he again counts himself as having 100 heartbeats and his Earth twin as having 75 heartbeats. When he arrives at Earth, would he conclude that he had 200 total heartbeats while his Earth twin had only 150? And, if so, wouldn't he be the older one? (In other words, how can the traveling twin see the stationary twin's clock as moving slower the whole time and yet end up being than the stationary twin?) If anybody could provide any help, it would be greatly appreciated! Thanks!

 

[ghwellsjr]

Hi,

Thanks for your wonderful response, I do have one more question...

I can see how to use the invariance of the spacetime interval to calculate the dx and the dt of what each twin sees from his own perspective during the trip. The math makes sense, however, what i don't understand is how both twins can see their counterpart age slower during the trip and yet still come to the conclusion that one of them is younger than the other at the end of the trip?

For example, the traveling twin counts himself as having 100 heartbeats and counts his Earth twin as having 75 heartbeats during the trip to the distant planet. During the return trip from the distant planet, he again counts himself as having 100 heartbeats and his Earth twin as having 75 heartbeats. When he arrives at Earth, would he conclude that he had 200 total heartbeats while his Earth twin had only 150? And, if so, wouldn't he be the older one? (In other words, how can the traveling twin see the stationary twin's clock as moving slower the whole time and yet end up being than the stationary twin?)


If anybody could provide any help, it would be greatly appreciated! Thanks!

In your example of the traveling twin counting 100 heartbeats while his Earth twin has 75 is a nebulous statement. First, I'm going to assume that both twins have hearts that beat at the same rate when they are at rest together.

If you really mean that he "sees" his twin's heart beating at 75% of his own during the outbound part of the trip, then he will "see" his twin's heart beating at the reciprocal or 133% of his rate for the inbound trip (assuming he's traveling at the same speed). So the traveling twin will experience 200 heart beats during the trip and the Earth twin will experience 208. So you can see that the traveling twin is indeed younger. You can look up relativistic doppler if you want to see why the reciprocal has to be taken for the inbound trip.

But if you really mean that the time dilation is such that the traveling twin calculates that his twin's heart beat due to time dilation is 75% of his own during both legs of the trip, then we have more calculations to do.

First we note that a heart beat running slow at 75% yields a gamma of the reciprocal of 75% or 1.333 and from that we can determine that the relative speed between the twins is .661c during both legs of the trip and from that we calculatet that the relativistic doppler for the outgoing trip is 45% and for the incoming trip is 221%. If we assume that the traveling twin counted 100 of his own heart beats for each leg of the trip, he will count 45 for his twin on the outgoing part and 221 for the incoming part for a total of 266 so once again the traveling twin is younger.

 

[Mike_Fontenot]

[...]
... what i don't understand is how both twins can see their counterpart age slower during the trip and yet still come to the conclusion that one of them is younger than the other at the end of the trip?
[...]

The traveler (the one who accelerates at his turnaround) does NOT conclude that his twin ages more slowly than he does during his ENTIRE trip.

He DOES come to that conclusion for each of his two constant-velocity segments (the outbound and inbound legs).

But, during his turnaround, he concludes that his twin suddenly ages by a large amount. The sum of those THREE amounts of ageing by his twin (during his outbound, turnaround, and inbound segments) exactly agrees with the total amount of her ageing that he directly sees when they are reunited.

The amounts of her ageing during the two constant velocity segments are easily obtained from the time-dilation result. The amount of her ageing (according to him) during his turnaround can be easily determined using the equation I gave in this previous post:

https://www.physicsforums.com/showpos...77&postcount=1 .

The home twin doesn't accelerate, and so she concludes that the traveling twin's total ageing is the sum of only TWO terms (his ageing during the two constant-velocity segments). She can easily compute each of those two ageing amounts from the time-dilation result. She concludes that he doesn't age AT ALL during his turnaround. (And she obviously doesn't think anything happens to her own age during his turnaround, in spite of the fact that he concludes that she suddenly ages a lot during his turnaround).

The TOTAL amounts of ageing, as computed by each twin, will exactly agree (as they MUST, since the ages of the two twins are indisputable when they are reunited).

Mike Fontenot

 

[idea2000]

Okay, thanks for all your wonderful responses...

I'd like to clarify for just one second just to make sure that I know what is going on...

So, let's set the information for the example first:

1. Distant planet is 4 years away (one-way) as measured by the Earth twin
2. Distant planet is only 1 years away (one-way) as measured by the traveling twin
3. Total round trip is 8 years as measured by the Earth twin and 2 as measured by
the traveling twin

And let's just say, for examples sake that each person experiences 1000 heartbeats in a year...

So, during the first leg of the trip, both frames are in free float and they both see each other aging slower. This is no problem from the Earth twins perspective as she measures her own heart rate and experiences 4000 of them, while the counting only 1000 for the traveling twin. Everything is fine and dandy. The traveling twin, however, by the time he counts up to 1000 of his own heartbeats, has only gotten to, say, 500 for the Earth twin. So where did all the other 3500 beats go? I'm guessing that all 4000 have already happened on Earth, but they haven't yet happened in the traveling twins frame (because of relativity of simultaneity) until he decelerates and turns around. Is this correct?

Okay, so if the above is correct, then during the deceleration phase, the traveling twin sees the Earth twin experience 3500 heartbeats. Then when he comes to a full stop and he starts to accelerate back towards Earth, that is when he sees the Earth twin experience yet an additional 3500 heartbeats, for a grand total of 7000 during the dec/acceleration phase.

During the last leg of the trip back to Earth, both frames are again in free float and neither is distinguishable from the other. Again, the traveling twin sees himself as experiencing 1000 heartbeats while seeing the Earth twin experience only 500. Meanwhile, the Earth twin sees herself experience 4000 heartbeats during the return trip, while seeing the traveling twin experience 1000 heartbeats.

When they finally meet to exchange information, the traveling twin has seen the Earth twin age 500 + 3500 + 3500 + 500 = 8000 heartbeats. The Earth twin sees the traveling twin age 1000 + 1000 = 2000 heartbeats. Am I getting this right?

(btw, thanks for all the help so far!)

 

[ghwellsjr]

Well, yes, there are some obvious problems.

First off, distances are measured in light-years, not years and I think you want your twins to have 100 heartbeats per year to make the rest of your example work.

Maybe you can go back and edit these mistakes before it is too late.

Now as to your explanation, you are combining things that I have offered with things that Mike has offered. I'm not going to help Mike explain his stuff because I don't agree with it. He is jumping between frames of reference during the trip which he thinks is the only way to explain the "reality" of what the traveling twin experiences. This is why there are jumps in the aging. (He also believes in negative aging in different examples.)He also applies different frames for the Earth twin and the traveling twin which is a violation of SR.

I have already explained in great detail how to explain this type of example in this thread:
https://www.physicsforums.com/showthread.php?t=447605

Look at posts #2 and #58.

Basically, the answers to your questions lie in the "fact" that the images of the Earth twin's heartbeats are still in transit when the traveling twin turns around and he then starts seeing them at a faster rate (Relativistic Doppler). The Earth twin does not "see" the traveling twin's turnaround event until long after it happens at which point he starts seeing the higher rate heartbeats. It's the "fact" that for the traveler, the times during which he sees the low rate heartbeats and the high rate heartbeats are equal but for the Earth twin, he will see the low rate heartbeats for most of the trip and it only switches to high rate near the end, resulting in a lower count of the traveling twin's total heartbeats.

Please note that this explanation describes what the twins each see and measure and has nothing to do with SR. You can also use SR with different Frames of Reference (one at a time) to learn about how SR explains the Twin Paradox.

 

[idea2000]

In your example of the traveling twin counting 100 heartbeats while his Earth twin has 75 is a nebulous statement. First, I'm going to assume that both twins have hearts that beat at the same rate when they are at rest together.

If you really mean that he "sees" his twin's heart beating at 75% of his own during the outbound part of the trip, then he will "see" his twin's heart beating at the reciprocal or 133% of his rate for the inbound trip (assuming he's traveling at the same speed). So the traveling twin will experience 200 heart beats during the trip and the Earth twin will experience 208. So you can see that the traveling twin is indeed younger. You can look up relativistic doppler if you want to see why the reciprocal has to be taken for the inbound trip.

But if you really mean that the time dilation is such that the traveling twin calculates that his twin's heart beat due to time dilation is 75% of his own during both legs of the trip, then we have more calculations to do.

First we note that a heart beat running slow at 75% yields a gamma of the reciprocal of 75% or 1.333 and from that we can determine that the relative speed between the twins is .661c during both legs of the trip and from that we calculatet that the relativistic doppler for the outgoing trip is 45% and for the incoming trip is 221%. If we assume that the traveling twin counted 100 of his own heart beats for each leg of the trip, he will count 45 for his twin on the outgoing part and 221 for the incoming part for a total of 266 so once again the traveling twin is younger.

Are the values calculated for the inbound trip for the entire inbound trip or only the u-turn part of the inbound trip?

 

[ghwellsjr]

As long as the u-turn happens fairly quickly, then nothing significant happens during the u-turn part of the trip. The values calculated for the inbound trip are for the entire inbound trip.

 

[Mike_Fontenot]

[...]
1. Distant planet is 4 years away (one-way) as measured by the Earth twin
2. Distant planet is only 1 years away (one-way) as measured by the traveling twin
3. Total round trip is 8 years as measured by the Earth twin and 2 as measured by
the traveling twin
[...]

OK, you're using a gamma factor of 4. So their relative speed is 0.968c.

[...]
And let's just say, for examples sake, that each person experiences 1000 heartbeats in a year...

So, during the first leg of the trip, both frames are in free float and they both see each other aging slower. This is no problem from the Earth twins perspective as she measures her own heart rate and experiences 4000 of them, while the counting only 1000 for the traveling twin. Everything is fine and dandy. The traveling twin, however, by the time he counts up to 1000 of his own heartbeats, has only gotten to, say, 500 for the Earth twin.
[...]

No, the gamma factor (4) has to be the same in each twin's calculations ... in the above, you've used a gamma of 4 in the home twin's calculations, but a gamma of only 2 in the traveler's calculations. So you should have said that the traveler would conclude that while his heart beats 1000 times, the home twin's heart beats only 250 times.

Other than that one mistake, though, you've got the right idea. That one mistake does, however, propagate through the rest of your numbers, so you need to change a lot of your subsequent numbers to make things correct.

So where did all the other 3500 beats go?
[...]

The correct number is now 3750 beats. If you change all of your subsequent 3500 numbers to 3750, and all of your subsequent 500 numbers to 250 (and a few other changes in some other numbers), everything will be correct. Your basic ideas and conclusions are correct.

My only other advice is to try to be much more precise in your statements.

For example, you originally said "Distant planet is 4 years away (one-way) as measured by the Earth twin". You were mixing up terminology for distance (lightyears) versus time (years). I knew what you meant, and your misused terminology didn't end up causing any problem with your conclusions. But when doing calculations in special relativity, ANY sloppiness at all in your statements CAN come back to bite you. Sloppiness that would cause no problem at all in doing Newtonian physics can cause you LOTS of problems in special relativity. Just a word to the wise.

[...]
I'm guessing that all 4000 have already happened on Earth, but they haven't yet happened in the traveling twins frame (because of relativity of simultaneity) until he decelerates and turns around. Is this correct?
[...]

Again, be careful in your statements. When you say "all 4000 [heartbeats] have already happened on Earth", that statement BY ITSELF doesn't have any meaning. The home twin has HER conclusion about how many times her heart has beaten, between when the traveler left home, and when he reached the distant planet, but BEFORE he changed his velocity. And the traveler has HIS conclusion about how many times her heart has beaten, between when he left home, and when he reached the distant planet, but BEFORE he changed his velocity. Their two conclusions are different. They are both correct.

Mike Fontenot

 

[ghwellsjr]

I think you want your twins to have 100 heartbeats per year to make the rest of your example work.

Disregard this comment. I was working from a mobile device which wouldn't let me see your original post and I got it mixed up with another example on a different thread.

 

[flashprogram]

there is no "absolute" way for the two twins to compare their ages- the question of whether he would still be "younger" is moot.

Realism tells us that even if we cannot perform the comparison an underlying reality or truth must exist(an observer independent truth.). If one of the twins is moving at near the speed of light, and the other is merely on earth, he will have time passing slower relative to the other.

Now quantum physicists are fond of saying any object can appear spontaneously out of thin air, though very improbable. Suppose two twins spontaneously appear one inside his own space ship, a ship near lightspeed heading in some random direction, the other stationary on top of some mountain on earth.

Irregardless of whether they can communicate or not, the underlying reality or truth must exist even if neither of them can determine it.


For example suppose the spontaneously appearing twins appear in the following scenario:

Say 100 years pass from their 'appearance' and the twin on Earth dies, but the other twin say happened to be in a ship with a path that would pass near earth*(no acceleration or deceleration has taken place for this twin, it reaches near Earth location at the 100 year mark.)... as the ship approaches earth, they look at it and see a tombstone for the other twin... or better yet the tombstone was floating in space they see it and crash right through it. Obviously they must agree the other twin died first and aged faster.

Relativity of simultaneity can apply to non causal chains... but direct causal chains a causes b which causes c, must be in agreement for all observers, iirc. Death of earth-twin causes floating-tombstone which causes space-ship crash... if spaceship crash occurs a violation of causality would be required for disagreement.

 

[ghwellsjr]

there is no "absolute" way for the two twins to compare their ages- the question of whether he would still be "younger" is moot.

Realism tells us that even if we cannot perform the comparison an underlying reality or truth must exist(an observer independent truth.). If one of the twins is moving at near the speed of light, and the other is merely on earth, he will have time passing slower relative to the other.

Now quantum physicists are fond of saying any object can appear spontaneously out of thin air, though very improbable. Suppose two twins spontaneously appear one inside his own space ship, a ship near lightspeed heading in some random direction, the other stationary on top of some mountain on earth.

Irregardless of whether they can communicate or not, the underlying reality or truth must exist even if neither of them can determine it.


For example suppose the spontaneously appearing twins appear in the following scenario:

Say 100 years pass from their 'appearance' and the twin on Earth dies, but the other twin say happened to be in a ship with a path that would pass near earth*(no acceleration or deceleration has taken place for this twin, it reaches near Earth location at the 100 year mark.)... as the ship approaches earth, they look at it and see a tombstone for the other twin... or better yet the tombstone was floating in space they see it and crash right through it. Obviously they must agree the other twin died first and aged faster.

Relativity of simultaneity can apply to non causal chains... but direct causal chains a causes b which causes c, must be in agreement for all observers, iirc. Death of earth-twin causes floating-tombstone which causes space-ship crash... if spaceship crash occurs a violation of causality would be required for disagreement.

Even if we grant you that there is an underlying realism where time is absolute (which is the assumption of LET), you still must specify the frame in which this is true in order to unambiguously define a scenario such as you have attempted to do. It appears that you think Earth fits the category of such a frame but it is constantly moving around so you need to be much more precise if you are going to be arguing on the basis of realism.

So I ask you, how do you define this frame in which time is absolute? Where is it?

 

[flashprogram]

Even if we grant you that there is an underlying realism where time is absolute (which is the assumption of LET), you still must specify the frame in which this is true in order to unambiguously define a scenario such as you have attempted to do. It appears that you think Earth fits the category of such a frame but it is constantly moving around so you need to be much more precise if you are going to be arguing on the basis of realism.

So I ask you, how do you define this frame in which time is absolute? Where is it?

Earth is merely used for familiarity's sake, but it can be disregarded.

Suppose two ships, at sufficient distance, head towards each other, in a straight line course such that they pass near each other at some point. The ships have a difference in speed, even if the travelers know not, and two twins pop into existence inside the two ships. The difference in speed causes one twin to reach 100 and die of old age, whilst the other reaches 20s. It is customary to throw tombstones into space, and they do so in such a way that it collides with the other ship.

As for the frame, it probably is some very strange frame. After all so long as causal chains are not involved everyone's supposed to be able to argue about order of events validly. But once causal chains are involved there must exist agreement on the order of events. One may imagine something like a http://en.wikipedia.org/wiki/Eternalism_%28philosophy_of_time%29" is at play.

 

[flashprogram]

Another intriguing question is whether the time relativistic effects would affect radioactive decay. We know chemical reactions and all known clocks would be affected. So I imagine radioactive decay would be affected in which case from an outside perspective half-life should be affected.

Some physicists theorize that protons could decay in the long term. While proton-decay is an open question, iirc. If decay is subject to time relativistic effect, then the entire ship, if we assume hypothetical proton decay, would be subjected to it. The ship whose protons decay last would be the fastest. If a man experiences a century of aging while a traveler on the ship experiences ten years, that's a 10:1 ratio, if decay is affected by a similar ratio both on board nuclear fuel as well as possibly protons(if they decay) would via half life effect last much longer than expected.

 

[yuiop]

Another intriguing question is whether the time relativistic effects would affect radioactive decay. We know chemical reactions and all known clocks would be affected. So I imagine radioactive decay would be affected in which case from an outside perspective half-life should be affected.

Muons have very short half lives and the effective extended life of muons at relativistic speeds has been extensively studied and confirmed in the atmosphere and in accelerators. So yes. In the atmospheric experiments, it is known that the half life muons is too short for the muons to traverse from the top of the atmosphere to sea level, but measurements show that a large proportion do make it to the ground. An observer co-moving with the muons would measure the half lives of the muons to be the same as when they are rest in the lab, but to an observer in a frame in which the muons are traveling near the speed of light the half lives are significantly longer. I think this was one of the earliest direct experimental confirmations of time dilation which has been refined in more recent accelerator experiments to much higher degrees of accuracy.

 

[ghwellsjr]

Earth is merely used for familiarity's sake, but it can be disregarded.

Suppose two ships, at sufficient distance, head towards each other, in a straight line course such that they pass near each other at some point. The ships have a difference in speed, even if the travelers know not, and two twins pop into existence inside the two ships. The difference in speed causes one twin to reach 100 and die of old age, whilst the other reaches 20s. It is customary to throw tombstones into space, and they do so in such a way that it collides with the other ship.

As for the frame, it probably is some very strange frame. After all so long as causal chains are not involved everyone's supposed to be able to argue about order of events validly. But once causal chains are involved there must exist agreement on the order of events. One may imagine something like a http://en.wikipedia.org/wiki/Eternalism_%28philosophy_of_time%29" is at play.

OK, if Earth is not going to define the frame in which time is absolute, and now you think there is something very strange about this frame, you need to tell us exactly how you are going to identify this frame. Saying that the future as well as the past and present are cast in concrete, doesn't help, you need tell us where is this frame. How fast is the Earth or the solar system moving through it and in which direction?

In your example, you didn't tell us the speeds of the ships with respect to this absolute frame, you just took a relativistic approach and said there was a difference in speed. And you didn't define the time at which the twins popped into existence in terms of an absolute time frame. You've got to decide if you want to promote an absolute time realism or a relativistic one. Please be specific. Tell us how you are going to identify the absolute time reference frame. No more fuzzy ideas--that won't get us anywhere.

 

[flashprogram]

OK, if Earth is not going to define the frame in which time is absolute, and now you think there is something very strange about this frame, you need to tell us exactly how you are going to identify this frame. Saying that the future as well as the past and present are cast in concrete, doesn't help, you need tell us where is this frame. How fast is the Earth or the solar system moving through it and in which direction?

In your example, you didn't tell us the speeds of the ships with respect to this absolute frame, you just took a relativistic approach and said there was a difference in speed. And you didn't define the time at which the twins popped into existence in terms of an absolute time frame. You've got to decide if you want to promote an absolute time realism or a relativistic one. Please be specific. Tell us how you are going to identify the absolute time reference frame. No more fuzzy ideas--that won't get us anywhere.

Imagine instead of ships we have identical chunks of radioactive material moving at constant but different speeds in straight lines, no one knows exactly how fast they're moving relative to each other. Clearly if the difference in speed is great time dilation will seemingly notably affect half lifes. After a time their composition should thus measurably differ in such a way that it would be possible to order them in terms of speed differences, and to tell which rock was the slowest amongst them all.

If they were on a collision course the nature of the collision would be affected not just by differences in speed but via the differing composition. Reaching the same destination at different speeds should thus result in different stuff*(compositionally) arriving at the destination. IF the stuff was originally identical, the one receiving it will be able to note compositional change and thus be able to tell and compare the speed at which it came.

 

[ghwellsjr]

More fuzzy idea that aren't getting us anywhere. I'm sure you think your ideas are clear as a bell and they make perfect sense to you but you aren't describing precisely what you are proposing and it is so open to interpretation.

Also, you keep changing your apparatus, I suppose, because you think it buttresses your position. You can keep it simple. Just describe how you think two identical clocks would work in your proposed concept of "realism" with absolute time. You don't need to embellish your story with spontaneously generated twins, radioactive rocks, colliding tombstones or anything else. Please keep it simple and describe exacly and precisely what your idea is and what would happen from start to finish.

All you have to do is say a clock has this time on it at this location and is traveling at this speed in this direction and another clock has this other time on it at this other location traveling at this other speed in this other direction and then describe what happens and how that proves your position.

 

[flashprogram]

More fuzzy idea that aren't getting us anywhere. I'm sure you think your ideas are clear as a bell and they make perfect sense to you but you aren't describing precisely what you are proposing and it is so open to interpretation.

Also, you keep changing your apparatus, I suppose, because you think it buttresses your position. You can keep it simple. Just describe how you think two identical clocks would work in your proposed concept of "realism" with absolute time. You don't need to embellish your story with spontaneously generated twins, radioactive rocks, colliding tombstones or anything else. Please keep it simple and describe exacly and precisely what your idea is and what would happen from start to finish.

All you have to do is say a clock has this time on it at this location and is traveling at this speed in this direction and another clock has this other time on it at this other location traveling at this other speed in this other direction and then describe what happens and how that proves your position.

The problem is, taking any two arbitrary objects, letting some time elapse... and measuring the rate at which their clocks differ should be practically possible. If we have a collection of objects, we can take any two arbitrary objects and do so, thus looking at how they differ relatively to each other. Trying to expand this so that we can relate all objects to each other probably presents some serious problem. The problematicity of this is probably in some way similar to what occurs with gravity, in the http://en.wikipedia.org/wiki/N-body_problem" .

As you asked: putting the two things simultaneously in two objects, even if impractical is either impossible or it is possible. The question is can we relate one instant on one object with one instant or sets of instances in the other? I don't see why not, why should it be impossible to relate moments in one to one or more moments in the other? but I honestly cannot tell how this would be represented.

 

[JesseM]

As you asked: putting the two things simultaneously in two objects, even if impractical is either impossible or it is possible. The question is can we relate one instant on one object with one instant or sets of instances in the other? I don't see why not, why should it be impossible to relate moments in one to one or more moments in the other? but I honestly cannot tell how this would be represented.

Are you familiar with the idea of the relativity of simultaneity?

 

[ghwellsjr]

Flashprogram, here is what I was trying to get you to do:

Suppose we have two clocks, A and B, that are separated by 1 light year. A is stationary and B is traveling at a speed of .5c toward A and they start out with the same time on them, which clock will have elapsed more time when they meet? The answer is A and can you see that I have implied a specific frame for this scenario?

On the other hand, you have proposed scenarios such as:

Suppose we have two clocks, A and B, that are separated by 1 light year. They are traveling toward each other with a relative speed of .5c and they start out with the same time on them, which clock will have elapsed less time when they meet? The answer is indeterminate because you have not stated or implied a frame in which to describe or analyze your scenario.

Do you see the difference between these two scenarios? The second one, typical of your scenarios, is not specific enough, it has no frame. It could be interpreted like the first scenario or it could be interpreted like the first one but with the clocks, A and B, interchanged. So the answer could be either A or B if we had more information.

Now you are supposing that nature has an answer to the question for your type of scenario; all we have to do is wait until the clocks meet and we will see which one has elapsed more time and that will determine which one was traveling slower and that will establish a preferred frame for nature.

And therein lies the problem: you are defining scenarios without frames and then talking as if nature is also defining scenarios but with a frame. You can't have it both ways. Either you get to define specifically what you want to consider happening in a scenario OR you have to let nature do whatever it wants and you draw whatever conclusions are valid from the measurements you can make.

So let's go back and see what's wrong with your combination of your partially defining a scenario and letting nature define the other part: When you say that two separated real clocks start out with the same time on them, you are saying something that we cannot know by making a measurement until those two clocks with a known relative speed and a known relative distance between them come together and we compare their times. After that happens, we can pick a frame, calculate backwards and determine, in that frame, what the times were on the two clocks when they were separated by a given distance. But it is totally arbitrary. We could pick a different frame and calculate backwards and show that the two clocks had totally different times on them when they were separated by a given distance.

So it is not legitimate for you to claim that two clocks start off with the same time on them without specifying a frame and then making the claim that you can determine the correct frame by waiting until they unite to compare their elapsed times and assuming that the choice of frame is determined by nature and not by you.