Who Ages Faster in the Twin Paradox?

[indirachap]

Lets say we witness the twin paradox in real life without any knowledge of relativity. On Earth we see the astronaught twin climb out of his spacecraft considerably younger than his brother and both the clock on the ground and the one in the spaceship show different times.

With no knowledge of relativity at all - how does one explain what has happened to a group of laymen onlookers?

The only explanation I can think of is that (1) the astronaught has spent his time in space in slow motion

(2) the difference in clock times is due to time and reality slowing down on the spaceship.

This is fact and reality surely - something that overrides everything else for the layman! Any other answer would seem like magic to them.

Once this fact is established then one can go into the realms of relativity theory

 

[ghwellsjr]

Lets say we witness the twin paradox in real life without any knowledge of relativity. On Earth we see the astronaught twin climb out of his spacecraft considerably younger than his brother and both the clock on the ground and the one in the spaceship show different times.

With no knowledge of relativity at all - how does one explain what has happened to a group of laymen onlookers?

The only explanation I can think of is that (1) the astronaught has spent his time in space in slow motion

(2) the difference in clock times is due to time and reality slowing down on the spaceship.

This is fact and reality surely - something that overrides everything else for the layman! Any other answer would seem like magic to them.

Once this fact is established then one can go into the realms of relativity theory

This is an exact repeat of your statement in post #27 and my answer in post #28.

OK, let's imagine that this experiment has been performed and no one yet knows about relativity so they perform the experiment that Darwin123 proposed in post #35:

Now suppose that the Earth twin gets into a different rocket, fires the engines and catches up to the first rocket twin. Then the twin from Earth is younger.

Or suppose this group of laymen onlookers hop in a spaceship and catch up to the first traveler in a subsequent experiment to see if he really is younger and they discover that he is older.

Do you understand why this would happen?

 

[indirachap]

This is an exact repeat of your statement in post #27 and my answer in post #28.

OK, let's imagine that this experiment has been performed and no one yet knows about relativity so they perform the experiment that Darwin123 proposed in post #35:

Originally Posted by Darwin123
Now suppose that the Earth twin gets into a different rocket, fires the engines and catches up to the first rocket twin. Then the twin from Earth is younger.

I don't have a problem with this. I am pleased at Darwin's affirmation of the suggestion that time travel was probably impossible presumably due to differential ageing which would preclude the violation of cause and effect.

 

[Dale]

Lets say we witness the twin paradox in real life without any knowledge of relativity. On Earth we see the astronaught twin climb out of his spacecraft considerably younger than his brother and both the clock on the ground and the one in the spaceship show different times.

With no knowledge of relativity at all - how does one explain what has happened to a group of laymen onlookers?

One realizes that there is a deficiency in their current physics since it fails to account for the observation, develops relativity as the explanation for the observation, and teaches it to the laymen.

This is a silly question, a little like how does one explain a flashlight to a bunch of cavemen with no understanding of chemistry and electronics. One teaches them.

 

[ghwellsjr]

Now suppose that the Earth twin gets into a different rocket, fires the engines and catches up to the first rocket twin. Then the twin from Earth is younger.

I don't have a problem with this.

Good. Then you also don't have a problem with the clocks that the twins carry showing corresponding differences in accumulated time, correct?

 

[indirachap]

Good. Then you also don't have a problem with the clocks that the twins carry showing corresponding differences in accumulated time, correct?

Correct. For a while I was puzzled as to why it is important to Relativity that, when traveling at the near-speed of light, the twin should appear to be experiencing normal conditions

I am told that if he isn't this would falsifiy the theory of relativity. Is this correct.?

We all experience time slowing down when we are traveling on Earth - only our speeds are so insignificant we do not notice anything but nonetheless this is happening. Is this correct?

I have been asked what reference frame I have been using. I have replied the reference frame of Earth t or any near object (even the ether)to the spacecraft with an imaginary observer. Prersumably this hypothetical could be moving but its movement nsignificant to the near-speed of light. Is this correct?

 

[Mark M]

Correct. For a while I was puzzled as to why it is important to Relativity that, when traveling at the near-speed of light, the twin should appear to be experiencing normal conditions

That's why it's 'relativity'. Different observers disagree over things, such as whose time is normal.

I am told that if he isn't this would falsifiy the theory of relativity. Is this correct.?

We have experimentally verified all of the predictions of special relativity many times.

We all experience time slowing down when we are traveling on Earth - only our speeds are so insignificant we do not notice anything but nonetheless this is happening. Is this correct?

No. YOU always believe YOUR time is normal. So, we do NOT feel any difference in time.

 

[ghwellsjr]

Good. Then you also don't have a problem with the clocks that the twins carry showing corresponding differences in accumulated time, correct?

Correct. For a while I was puzzled as to why it is important to Relativity that, when traveling at the near-speed of light, the twin should appear to be experiencing normal conditions

I am told that if he isn't this would falsifiy the theory of relativity. Is this correct.?

Correct. Another way of saying this is that no matter what your past experience of acceleration was, as long as you stop accelerating and you are just "coasting", you cannot tell that you have accelerated at all. Of course, you will see that other objects are now traveling differently with respect to you, but you cannot tell whether it is because they all accelerated or because you accelerated.

We all experience time slowing down when we are traveling on Earth - only our speeds are so insignificant we do not notice anything but nonetheless this is happening. Is this correct?

Correct, if by that you mean when we conduct our own little "Twin Paradox" experiments with our ordinary watches, we can't notice any difference in the times on the watches. However, if we use very precise atomic clocks, we can notice the difference and this has been done with the very famous Hafele–Keating experiment.

I have been asked what reference frame I have been using. I have replied the reference frame of Earth t or any near object (even the ether)to the spacecraft with an imaginary observer. Prersumably this hypothetical could be moving but its movement nsignificant to the near-speed of light. Is this correct?

Correct. As Darwin123 pointed out in post #35, the Earth is not really an inertial reference frame. When we are discussing a twin traveling in space at near light speed with respect to the earth, we can ignore the slight accelerations that the twin on the surface of the Earth experiences due to the rotation of the Earth and its motion around the sun but when we are talking about earth-bound travelers at very slow speeds, we can no longer treat the Earth as an inertial reference frame.

In fact, it was these very slight accelerations of the surface of the Earth that lead to the discovery that the presumed stationary ether could not be detected and ultimately to Einstein's Theory of Special Relativity in which he predicted that a clock located at the equator would run slower than one located at one of the poles because it was traveling at a very slow speed with respect to the clock at the pole.

The whole point of SR is that you can pick any inertial frame of reference, it doesn't have to be associated with the rest state of any observer or any heavenly body or any presumed ether, as long as it is inertial, it is just as good as any other. Do you now understand what I said in post #3?

You need to pick one inertial frame and stick with it from start to finish. And you need to understand that the faster you go in that frame, the slower your clock ticks.

Now can you see that from the Earth's frame, only the rocket twin's clock will run slow?

And can you see that if you use the rocket's frame during the first half of the trip, only the Earth's clock will run slow but during the last half of the trip, the rocket has to travel much faster than the Earth in order to catch up with it and so its clock has to run even slower such that it ends up with less time on it when it gets back to earth?

 

[indirachap]

Perhaps there is no preferred frame of reference for velocity rather like the speed of light ?

 

[Darwin123]

Correct. For a while I was puzzled as to why it is important to Relativity that, when traveling at the near-speed of light, the twin should appear to be experiencing normal conditions

I am told that if he isn't this would falsifiy the theory of relativity. Is this correct.?

We all experience time slowing down when we are traveling on Earth - only our speeds are so insignificant we do not notice anything but nonetheless this is happening. Is this correct?

I have been asked what reference frame I have been using. I have replied the reference frame of Earth t or any near object (even the ether)to the spacecraft with an imaginary observer. Prersumably this hypothetical could be moving but its movement insignificant to the near-speed of light. Is this correct?

I think this is incorrect. Or rather this is ambiguous because you did not uniquely specify what the meaning of the word "movement" means.
I would say that the Earth would be hypothetically accelerating but its acceleration is at all times insignificant compared to the acceleration experienced by the rocket at large distances from earth.
One quantity that breaks the symmetry is dynamics acceleration times relative distance divided by the speed of light. In other words, look at the quantity ρ where:
ρ=(F/m)x/c^2
where F is the force on the "clock" in question, "m" is the mass of the clock in question, x is the distances between the clocks, and c is the speed of light.
If ρ is small (ρ<<1), then there is perfect symmetry between the two clocks. If ρ>>1, then the symmetry is broken.

 

[ghwellsjr]

Perhaps there is no preferred frame of reference for velocity rather like the speed of light ?

That is correct, there is no preferred frame of reference. The measured speed of light is the same for all inertial observers.

 

[Darwin123]

Perhaps there is no preferred frame of reference for velocity rather like the speed of light ?

Here is something that you said earlier that superficially appears to contradict the above statement.

Correct. For a while I was puzzled as to why it is important to
"Relativity that, when traveling at the near-speed of light, the twin should appear to be experiencing normal conditions

I am told that if he isn't this would falsifiy the theory of relativity. Is this correct.?

Then again you said:

We all experience time slowing down when we are traveling on Earth - only our speeds are so insignificant we do not notice anything but nonetheless this is happening. Is this correct?

I already posted a comment on that statement. Here is another similar statement.
Here is what you said earlier.

" when traveling at the near-speed of light, the twin should appear to be experiencing normal conditions
I am told that if he isn't this would falsifiy the theory of relativity. Is this correct.?"

The last is an incomplete statement.
The correct statement would be:
If the twin isn't accelerating, he should see "normal" conditions. If the twin isn't accelerating and he sees something weird, then relativity is being violated.
Let
ρ=(F/m)x/c^2
where x describes the distance between two clocks on the space ship, positive is in the direction of acceleration, F is the force on the clock in question, and c is the speed of light.
If the twin is accelerating, as in |ρ|>0, he will experience weird conditions. If |ρ|<<1, he will see mildly weird conditions. If |ρ|>1, then he will see very weird conditions. The two clocks will be off if ρ≠0. The sign determines which clock is moving faster.
The problem with the traditional presentation of the twin paradox is that the effects of velocity are not completely separated from the effects of acceleration. What most "skeptics" really want to know is the effect of acceleration. The way the twin paradox is usually presented, the effect of acceleration is "hidden" by the words in the problem. This isn't a scientific problem.
However, it would be useful to think about a problem where there is ONLY a difference in acceleration, and NO difference in velocity. Let us consider two twins on the same rocket ship, with different clocks. If the acceleration is huge, then the effects of motion will become very weird even if the relative velocity is small.
Let me give an example where the effect of acceleration is completely separated from the effect of velocity. Suppose we are comparing the rates of two clocks on the rocket ships. Here, "x" would be the distance between the two clocks. Again, let,
ρ=|(F/m)x|/c^2
Note that in this case, the relative velocity between the two clocks is zero. The two clocks are forced to travel at the same speed by the contact forces within the space ship.
If ρ=0, there will be no difference in the way the two clocks tick. If ρ<<1, there still may not be a significant difference in the rate of the two clocks. However, if ρ>1, then the clock that is in the back of the rocket ship will tick far slower.
Let there be two twins in the same rocket ship. They start out in the back, near the rocket engines. One twin walks to the front and walks back at a very slow speed. The relative velocity is negligible (v<<c), but the maximum effect of dynamic acceleration is huge (maximum ρ>1).
This will be a difference that is noticed by the two twins once they get back together. The walking twin will be younger than the other, even if he is walking slowly. The reason the difference is permanent is that neither twin is in an inertial frame. The dynamic acceleration makes a big difference.
It should be stressed that the important quantity here is the product of dynamic acceleration and the distance of walking.
Some will claim that this example is technically general relativity, not special relativity. However, this is a matter of semantics. One could call it special relativity anyway, because there is no "gravitational mass" in the problem. It doesn't matter. The point is that the force breaks the symmetry, not the velocity.

 

[phyti]

There is no known method of determining the absolute speed of A or B, only the difference or relative speed. Drawing 1 shows the symmetrical time dilation observations, with the green hyperbola indicating one clock tick, which depends on the clock speed. Below the origin, A and B are converging, and each sees the others clock tick faster. Above the origin they are diverging, and each sees the others clock tick slower. Since the clock rate is a function of v/c, and v is unknown, the accumulated time on each clock can only be determined by a simultaneous comparison, at separation and later at rejoining. Since the speed of A and B will in general be different, there is no expectation of symmetry for accumulated time, i.e. equal aging.
The dependence of the clock rate on v/c also means the idea that relative speed causes time dilation is false. Only the relative difference in time dilation is observed.
https://www.physicsforums.com/attachments/49469

 

[Darwin123]

Good. Then you also don't have a problem with the clocks that the twins carry showing corresponding differences in accumulated time, correct?

Correct, I don't see any problem with corresponding differences in accumulated time.
Maybe I should add some qualifiers.
1) I don't see any problem with differences in "accumulated time" so long as there is no force applied to either twin once they are apart.
2) I don't see any problem with differences in "accumulated time" so long as a large force is applied to one twin, and one twin only, while they are far apart.
I think we may have a disagreement as to the relevance of the force that turns the rocket around. Thus, you may feel most uncomfortable with statement #2. The twin in the rocket feels this force, while the one on Earth doesn't feel this force. What happens to the rocket twin in this interval determines the difference in accumulated time when the two twins are back together again.
Of course, maybe I don't understand what you mean by "accumulated time". One has to know when to begin counting in order to accumulate anything. In order to measure "accumulated time", one has to synchronize two clocks which are very close together.
What is not understood by many is that the word "twin" implies another type of synchronization. There is an assumption that the two twins started aging inside their mother's uterus. Their time of conception occurs when the eggs were fertilized by sperm. Thus, their biological clocks started ticking at the same minute while they were no more than 10 centimeters apart. This is a synchronization.
Synchronization and accumulation are connected. If we can't synchronize, we can't compare accumulations.
If the problem didn't specify twins, and they were born a large distance apart, then we couldn't feel any discomfort in the fact that they are different ages when they get back together. The reason we feel discomfort with differences in accumulated time is that we "know" the two boys were born at the same time in the same place. If the time starts accumulating at different initial times, then the problem isn't so clear.
It has been pointed out to me that there are experiments where it is not obvious that force breaks the symmetry. For instance, the cosmic ray experiment where the lifetime of the muon was measured as a function of velocity.
I would argue that there were hughe forces at work here. A fast moving proton (i.e., cosmic ray) hits an atom high in the atmosphere. The atom is "stationary" relative to both a photon detector and a muon detector on the surface of the earth. The photon detector first measures the flash of light that reaches the ground, and the muon detector detects the muon when if finally
I would argue that the force between proton and the atom is large enough to account for the asymmetry. The muon can be pictured as being initially embedded in the atom. The proton accelerated the muon downward.
Basically, the "synchronization" is being done by the flash of light. The clocks were set when the proton makes the atom emit light. So the force of the proton is setting the :initial" time of the clocks.
There are many variations on the muon experiment. However, there never has been a direct test of relativity. By "direct", I mean an experiment where an entire laboratory travels along with the muons. However, the real muon experiment "indirectly" tests special relativity.
The results of the experiment are analyzed from the point of view of an observer at rest relative to the surface of the earth. This is basically an inertial frame. The array of detectors that never see a force are hypothetical. The results are consistent with special relativity.

 

[Darwin123]

There is no known method of determining the absolute speed of A or B, only the difference or relative speed.

Correction. replace the word speed with velocity. "absolute velocity" "...only the difference of relative velocity".
I understand what you mean. However, it will help if we distinguish vectors from scalars. Velocity is a vector, and speed is a scalar. Velocity includes direction as well as speed. Direction is very critical in these discussions.
There is no known way to determine the absolute velocity of A and B. There are very well known methods of determining the absolute acceleration of both A and B. You may not be able to tell how fast the elevator is moving from inside, but one can easily determine when it is accelerating. I would rather use the phrase "dynamic acceleration" instead of "absolute acceleration", just to highlight the role of force in the determination. From the force, one can determine the absolute acceleration.
One can determine the external force acting on the instruments of A and the external force acting on the instruments of B. For example, the dynamic acceleration of body i is:
a_i=F_i/m_i
where a_i is the dynamic acceleration of body i, F_i is the external force on body i, m_i is the mass of body i, and index i can be A or B.
One can also call a_i the absolute acceleration of body i. It is absolute in the sense of special relativity because if a_i≠0, then the observer associated with body i can't be part of an inertial frame.
The concept of absolute acceleration has a bit more provenance than the concept of space-time. The geometric way of looking at relativity problems is really a short cut through the concept of force. Einstein was discussing forces and impulses before he talked about space-time. He was very careful to define inertial frame as if it was a real frame.
Maybe you should show a series of force diagrams for every part of the journey in addition to the space time diagram. I find that force diagrams, with the forces labeled, can be very helpful in special relativity problems.

 

[ghwellsjr]

Darwin123, my comment in the above quote from post #50 was directed at indirachap, not at you. Since he had acknowledged that there was a difference in the accumulated age of the two twins, one of which traveled at high speed, I just wanted to make sure he also agreed that the clocks they carried with them would indicate the same difference in accumulated time because I then wanted to direct him to the Hafele–Keating experiment done with slow speed clocks which is a direct confirmation of Einstein's prediction of time dilation so I'm surprised that you would then say that "there never has been a direct test of relativity".

 

[indirachap]

Say one was going around a distant star and back to Earth in a spaceship. The clock in the spaceship once back on Earth has run say 12 hours slow to the one on the ground, thus the hands on the clock in the ship surely would have moved in slow motion compared to those on the ground just as human hands (and other parts) would have moved in slow motion irrespective of not being able to tell if one was moving or not. I know that onboard the spaceship in flight a second would still be a second but because of dilation that second would take longer to pass. So at the near speed of light the astronaught would be vitrually frozen in time.

Differential aging would ensure that cause and effect was not violated and that time travel could not occur. In fact time travel would be impossible.

 

[HallsofIvy]

Say one was going around a distant star and back to Earth in a spaceship. The clock in the spaceship once back on Earth has run say 12 hours slow to the one on the ground, thus the hands on the clock in the ship surely would have moved in slow motion compared to those on the ground just as human hands (and other parts) would have moved in slow motion irrespective of not being able to tell if one was moving or not. I know that onboard the spaceship in flight a second would still be a second but because of dilation that second would take longer to pass. So at the near speed of light the astronaught would be vitrually frozen in time.

You are still not specifying a reference frame for these statements and that is why you are making strange statements like "that second would take longer to pass". On board the spaceship (i.e. reference frame is the space ship), "a second would be a second" and they would not see themselves moving slowly, aging slowly, etc. But as viewed from Earth (i.e. the frame of reference is the earth), their clock would appear to move slowly, so that it would take much longer than a second (on an Earth clock) for the second hand (on the spaceship clock) to move, everyone would appear to be moving slowly, etc.

Differential aging would ensure that cause and effect was not violated and that time travel could not occur. In fact time travel would be impossible.

No, "Differential aging" has nothing to do with "cause and effect". And, while I will not assert that it is possible, relativity does not assert that time travel is impossible.

 

[indirachap]

You are still not specifying a reference frame for these statements and that is why you are making strange statements like "that second would take longer to pass". On board the spaceship (i.e. reference frame is the space ship), "a second would be a second" and they would not see themselves moving slowly, aging slowly, etc. But as viewed from Earth (i.e. the frame of reference is the earth), their clock would appear to move slowly, so that it would take much longer than a second (on an Earth clock) for the second hand (on the spaceship clock) to move, everyone would appear to be moving slowly, etc.

In his famous work on special relativity in 1905, Albert Einstein predicted that when two clocks were brought together and synchronized, and then one was moved away and brought back, the clock which had undergone the traveling would be found to be lagging behind the clock which had stayed put.(wikipedia)

Do you not deny that when the clocks were brought back to Earth the hands of the space-clock had run in slow motion in comparison to those of the ground-clock? Why wouldn't human hands (and body parts)have run slow as well? Everything would have been in slow motion during the journey.

 

[Darwin123]

Darwin123, my comment in the above quote from post #50 was directed at indirachap, not at you. Since he had acknowledged that there was a difference in the accumulated age of the two twins, one of which traveled at high speed, I just wanted to make sure he also agreed that the clocks they carried with them would indicate the same difference in accumulated time because I then wanted to direct him to the Hafele–Keating experiment done with slow speed clocks which is a direct confirmation of Einstein's prediction of time dilation so I'm surprised that you would then say that "there never has been a direct test of relativity".

My definition of direct was very narrow. I am sorry that I said it this way.
What I meant to say is that the experimental tests of relativity did not use two symmetric "observers". There has been no test with two twins.
For example, consider one possible test of time dilation using mesons. A cosmic ray hits an atom in the atmosphere. A photomultiplier on the ground detects the flash of light, a scintillation counter with other gadgets detects a meson from the event and measures the mesons velocity. Many such events are recorded. Statistical analysis shows that the Lorentz time dilation formula "works".
I don't have references. I don't know how many times it was actually done this particular way. However, I think that it is clear to everybody that this method validates the Lorentz time dilation formula from the point of view of an observer at rest with respect to the surface of the earth.
This experiment did not validate the Lorentz time dilation formula from the point of view of an observer traveling with the meson. Hypothetically, suppose there was a photomultiplier and a scintillation counter with the same gadgets at rest with respect to the meson. The atom in question is hit by the cosmic ray at the very moment when the PMT and counter are passing the atom. The two detectors travel with the meson until it decays. The two detectors are then hoisted back into the atmosphere and accelerated to do the measurement again.
The principle of relativity says that a statistical analysis of the second procedure should validate the same Lorentz time dilation formula as the experiment where both detectors are fixed on the ground.
The second procedure would be prohibitively expensive. Aside from accelerating the detectors to near light speed relative to the ground, one would have to do something to prevent the detectors from hitting the ground near the speed of light. However, this experiment could hypothetically be done.
Comparing the results of this experiment, where the detectors are moving relative to the ground, with the results of the traditional experiment, where the detectors are fixed relative to the ground, is what I call a direct test of relativity. The direct test would validate the Lorentz time dilation formula twice with detectors that are identical but for their state of motion.
What I meant by a direct test of special relativity is where two experiments are done twice with nearly identical detectors in different inertial frames. One set of detectors is moving at high speed relative to the other set of detectors. The same process is being examined by both. This experiment would be testing the relativity part of relativity.
The experiment where the Lorentz time dilation formula was validated once is sufficient proof for me. I can follow the simultaneity argument, unlike some other people. However, not everybody follows the simultaneity argument. The simultaneity argument is logical, but it takes too many steps.
This is why the OP asked for a "simple explanation". There is an implication in some of the responses that special relativity is based on circular reasoning. He needs a "simple" experiment, by which he means symmetric observers, to convince him that there is no circular reasoning.
My point is that no test of special relativity has used "symmetric observers" in two different inertial frames. The theory of special relativity has been validated many times using just one observer in one inertial frame. The Lorentz time dilation formula has been validated many times.
You brought up the Hafele-Keating experiment as an experiment that validated special relativity. The HK experiment did not use "symmetric observers" in two inertial frames. In fact, none of the detectors used in the HK experiment were part of an inertial frame. What the HK experiment involved was actually three frames that were accelerating relative to the inertial frame.
Some people have claimed that the HK experiment is a test of general relativity, not special relativity. They argue that any test that involves both acceleration and gravitational mass has to be a test of general relativity. I have argued with some of these people on line. However, I have argued that it really is a test of special relativity.
My argument is that the direction of travel (East-West) in the HKE was chosen in such a way that gravitational mass canceled out. Furthermore, the results of the experiment could be analyzed from the point of view of an inertial frame located on the Earth's axis. Therefore, it is a test of special relativity. However, I admit that my argument is a bit tenuous.
In the HKE, none of the detectors was in an inertial frame. So some would argue that this was not a "direct" test of special relativity. The results were consistent with special relativity, using mathematical analysis. However, The HKE was not an experiment that used identical detectors in two different inertial frames.
I don't think any such "direct" test is practical. In actual practice, all the tests of relativity will involve at most one inertial frame. If you know of any direct test using detectors in two completely different inertial frames, then tell us. I don't need it for myself, but it would relieve a lot of other people.

 

[ghwellsjr]

My definition of direct was very narrow. I am sorry that I said it this way.
What I meant to say is that the experimental tests of relativity did not use two symmetric "observers". There has been no test with two twins.
...
This is why the OP asked for a "simple explanation". There is an implication in some of the responses that special relativity is based on circular reasoning. He needs a "simple" experiment, by which he means symmetric observers, to convince him that there is no circular reasoning.
My point is that no test of special relativity has used "symmetric observers" in two different inertial frames. The theory of special relativity has been validated many times using just one observer in one inertial frame. The Lorentz time dilation formula has been validated many times.

The OP was asking about the Twin Paradox which does not have symmetric observers so I don't know why you expect the answer to include a test involving symmetric twins. He simply wanted to know how you can determine which twin ends up older. I gave him a simple answer in post #3:

You need to pick one inertial frame and stick with it from start to finish. And you need to understand that the faster you go in that frame, the slower your clock ticks.

Now can you see that from the Earth's frame, only the rocket twin's clock will run slow?

And can you see that if you use the rocket's frame during the first half of the trip, only the Earth's clock will run slow but during the last half of the trip, the rocket has to travel much faster than the Earth in order to catch up with it and so its clock has to run even slower such that it ends up with less time on it when it gets back to earth?

You brought up the Hafele-Keating experiment as an experiment that validated special relativity. The HK experiment did not use "symmetric observers" in two inertial frames. In fact, none of the detectors used in the HK experiment were part of an inertial frame. What the HK experiment involved was actually three frames that were accelerating relative to the inertial frame.

No, it was not three frames but rather three clocks that were accelerating relative to one inertial frame, any arbitrary frame you want to pick. That's the point I was making. You can use any inertial frame and you will get the same answer for the final outcome of the differential aging of the clocks.

Some people have claimed that the HK experiment is a test of general relativity, not special relativity. They argue that any test that involves both acceleration and gravitational mass has to be a test of general relativity. I have argued with some of these people on line. However, I have argued that it really is a test of special relativity.
My argument is that the direction of travel (East-West) in the HKE was chosen in such a way that gravitational mass canceled out. Furthermore, the results of the experiment could be analyzed from the point of view of an inertial frame located on the Earth's axis. Therefore, it is a test of special relativity. However, I admit that my argument is a bit tenuous.

The idea that HKE was a test of only SR or only GR is wrong when in fact it included both.

In the HKE, none of the detectors was in an inertial frame. So some would argue that this was not a "direct" test of special relativity. The results were consistent with special relativity, using mathematical analysis. However, The HKE was not an experiment that used identical detectors in two different inertial frames.
I don't think any such "direct" test is practical. In actual practice, all the tests of relativity will involve at most one inertial frame. If you know of any direct test using detectors in two completely different inertial frames, then tell us. I don't need it for myself, but it would relieve a lot of other people.

All detectors, all observers, all clocks, all objects, all things are in all inertial frames. I don't know why you are struggling with this or why you think other people should be.

 

[indirachap]

Can anyone tell me if time dilation would be actually experienced by the astronaught twin.
Would he be actually be in slow motion at the near speed of light or not. I am getting conflincting info from different people. I understand that any trip involves time dilation and an element of slow motion.

 

[Dale]

Would he be actually be in slow motion at the near speed of light or not.

Yes. Although words like "actually" can cause problems.

 

[indirachap]

Yes. Although words like "actually" can cause problems.

Thanks. How is it that the law of physics is the same in every FOR when in the spacecraft FOR the astronaught clearly is not experiencing the same law of physics behaviour as that being experienced in the Earth FOR?

 

[indirachap]

Would it be fair to say that during a journey at the the near speed of light a second would still be a second but it would take longer to pass due to time dialation?

 

[Dale]

Thanks. How is it that the law of physics is the same in every FOR when in the spacecraft FOR the astronaught clearly is not experiencing the same law of physics behaviour as that being experienced in the Earth FOR?

In the spaceships FOR clocks that move fast time dilate also. The law is the same in all inertial frames.

 

[GrammawSally]

How is it that the law of physics is the same in every FOR when in the spacecraft FOR the astronaught clearly is not experiencing the same law of physics behaviour as that being experienced in the Earth FOR?

They both use the same laws of physics. But the results they get from those laws depend on whether or not they are accelerating. The person back home never accelerates, but the astronaut does accelerate, at the turnaround.

Anytime the spacecraft is traveling at a constant velocity with respect to the inertial person back home, the astronaut will say that the person back home is aging more slowly than she is. But while the astronaut is reversing course at the turnaround (which requires her to accelerate), she will say that the person back home is aging much faster than she is.

 

[MikeLizzi]

They both use the same laws of physics. But the results they get from those laws depend on whether or not they are accelerating. The person back home never accelerates, but the astronaut does accelerate, at the turnaround.

Anytime the spacecraft is traveling at a constant velocity with respect to the inertial person back home, the astronaut will say that the person back home is aging more slowly than she is. But while the astronaut is reversing course at the turnaround (which requires her to accelerate), she will say that the person back home is aging much faster than she is.

Well said, GrammawSally.

 

[Dale]

Would it be fair to say that during a journey at the the near speed of light a second would still be a second but it would take longer to pass due to time dialation?

There is a way that statement could be interpreted to make it correct, so I would say "yes", though I am not sure that you understand yet.

Let's say you have a spaceship with a lab onboard that has an array of different science experiments, each designed to measure one second using a different physical principle, and an atomic clock. In addition, there is an array of synchronized atomic clocks. The ship is traveling at .6 c relative to the array of clocks, and begins all of the experiments as it passes a clock. All of the physics experiments end at the same time, just as the spaceship passes the synchronized clock .75 light seconds away, which reads 1.25 s.