Who Ages Faster in the Twin Paradox?

  • #51
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".
 
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  • #52
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.
 
  • #53
indirachap said:
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.
 
  • #54
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.
 
  • #55
ghwellsjr said:
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.
 
  • #56
Darwin123 said:
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:
ghwellsjr said:
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?
Darwin123 said:
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.
Darwin123 said:
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.
Darwin123 said:
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.
 
  • #57
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.
 
  • #58
indirachap said:
Would he be actually be in slow motion at the near speed of light or not.
Yes. Although words like "actually" can cause problems.
 
  • #59
DaleSpam said:
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?
 
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  • #60
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?
 
  • #61
indirachap said:
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.
 
  • #62
indirachap said:
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.
 
  • #63
GrammawSally said:
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.
 
  • #64
indirachap said:
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.
 

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