Who Ages Faster in the Twin Paradox?

[Visceral]

Hi,

I am a little confused with this paradox. I asked my professor about it and he didnt really give a convincing answer. So the scenario basically seems to be some twins on earth(or anywhere) at rest, and then one leaves at relativistic speed for some time then comes back to see that his/her twin is much older than them.

My question is, how come you can tell which one would age more? Why couldn't it just as well be the one on the ship? From the twin on Earth's reference frame, they are at rest and then the rocket flies away from them, while in the rocket frame it is at rest and the Earth flies away from them. From each of the twins perspective the other one moves and they are stationary in their own frames. How come the same thing wouldn't happen to the twin on Earth and find the rocket twin older when the Earth arrived back at the rocket?

I just took this paradox for granted for a long time but now I seem to be confused.

Thanks for reading.

 

[TheEtherWind]

In order for twin on the rocket to return they must undergo acceleration, and therefore do not have an inertial frame of reference during the entire round trip.

 

[ghwellsjr]

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?

 

[yuiop]

If you are at rest in a given inertial reference frame then you do not feel any proper acceleration. When the traveling twin turns around to head back he experiences proper acceleration and he can now be certain that he is not at rest in inertial reference frame. The stay at home twin does not feel proper acceleration during the turn around event, so the situation is not symmetrical.

 

[mathman]

There is a simple way to understand it based on the Lorentz transformation. Say the traveler goes to a place ten light years away from earth. He quickly speeds up to almost the speed of light. The distance in his frame to his destination is now foreshortened and the time to get there is is greatly reduced. Once he gets there, stops and turns around to go back to earth, the same foreshortening of distance and time takes place. So in his frames the total time would be a lot less than twenty years, while the Earth bound twin would age more than twenty years.

 

[PeterDonis]

Visceral, you might want to read the Usenet Physics FAQ entry on the Twin Paradox:

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html

There are a number of different ways of understanding what's going on in this scenario (most of which have been mentioned in this thread), but the FAQ entry ties them all together.

My personal preference is to look at everything using a spacetime diagram (the FAQ entry discusses this in some detail). Looking at it this way makes the solution obvious, in my view: you have two twins, each of which takes a different path through spacetime between the same pair of events (the event where they part, and the event where they come back together). The two paths they take have different lengths, so they experience different amounts of proper time passing between those two events (since the "length" of a worldline is just the proper time elapsed for someone following that worldline). Working out the actual math tells you that the stay-at-home twin's path is longer, so he ages more and is older when the two meet up again. It's no different in principle than the fact that two paths through Euclidean space that start and end at the same point can have different lengths; it's just geometry.

 

[gmax137]

...
My question is, how come you can tell which one would age more? Why couldn't it just as well be the one on the ship? ...

I think that's why it is called the twin paradox

 

[HallsofIvy]

I think that's why it is called the twin paradox

Actually, no, it isn't. The term "paradox" is being used ironically- it is only a seeming paradox which can be resolved as others have said here.

 

[DrGreg]

Well, actually, the word "paradox" has multiple meanings including:

1. a seemingly absurd or self-contradictory statement that is or may be true
2. a self-contradictory proposition

(my emphasis). Most of the paradoxes in physics and maths (including the twins paradox) turn out to be of the first type.

Source: Collins Concise Dictionary, 4th Ed 1999

 

[gmax137]

Well, the OP said he suddenly thought to himself, why isn't this story symmetrical -- why is one twin different from the other?? But that's exactly the point of the story, isn't it? That's why it is the 'twin paradox' not the 'twin effect' or some such. As DrG points out, it seems contradictory, but it isn't.

 

[Underwood]

If the traveler twin, on both his outgoing trip and on his returning trip, says that his home twin is aging slower than he is, then how can he find the home twin to be older when he finally gets back?

I think the answer is that the traveler twin says his home twin ages a lot during his turnaround.

 

[PeterDonis]

If the traveler twin, on both his outgoing trip and on his returning trip, says that his home twin is aging slower than he is, then how can he find the home twin to be older when he finally gets back?

I think the answer is that the traveler twin says his home twin ages a lot during his turnaround.

This is one way of looking at it, yes. Check out the Usenet Physics FAQ entry I linked to in post #6.

 

[Underwood]

This is one way of looking at it, yes. Check out the Usenet Physics FAQ entry I linked to in post #6.

Thanks. I read it, but it says that the gap is an accounting error. Seems like if the gap is an error, then it would also have to be an error for the traveler twin to say that the stay-home twin ages slower when the traveler's going away, and when he's coming back. Is that an error too? I hear lots of people talk like that's true.

 

[PeterDonis]

Thanks. I read it, but it says that the gap is an accounting error.

I assume you're referring to the "Time Gap Objection" page...

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_gap.html

...which says

The apparent "gap" is just an accounting error, caused by switching from one frame to another.

The "accounting error" is basically changing the "zero point" of time; the outbound reference frame has a "zero" of time that is about 13 years and 8 months earlier than the "zero" point of time in the inbound reference frame. So when you switch frames, you have to switch zeros of time as well. That adds 13 years and 8 months to Terence's clock as seen by Stella.

Seems like if the gap is an error, then it would also have to be an error for the traveler twin to say that the stay-home twin ages slower when the traveler's going away, and when he's coming back.

Why do you think the two have to be connected? "Ages slower" refers to the *rate* at which time "flows" in one frame compared to the other; it doesn't say anything about where the "zero point" of time is set. On the Time Gap Objection page, it says:

During the Outbound Leg, Terence ages less than two months, according to Stella. (12 Stella-months / time dilation factor of 7.) During the Inbound Leg, Terence also ages less than two months, according to Stella, by the same computation.

So Terence does "age slower", according to Stella, on *both* legs, in this interpretation, even though he ends up older at the end (because of the change in "zero point" of time).

I hear lots of people talk like that's true.

I suspect that's because they're trying to use a different interpretation than the "Time Gap" one. For example, check out the Doppler Shift Analysis page in the FAQ.

The key is that there is not a single unique "right answer" for most of these questions; how you answer them depends on how you interpret various observer-dependent quantities. The only question that has to have a unique answer is, how do the two twins' ages compare *when they meet again*. That answer is unique because both twins are at the same location at the same time, so none of the ambiguities in interpretation come into play.

 

[Underwood]

Why do you think the two have to be connected?

I'm just saying that if the traveling guy says that his home-staying twin's total aging while he is gone is the sum of 3 parts, then if one of the 3 parts is an error, the other 2 parts can't be right either. Because the sum is right.

 

[PeterDonis]

I'm just saying that if the traveling guy says that his home-staying twin's total aging while he is gone is the sum of 3 parts, then if one of the 3 parts is an error, the other 2 parts can't be right either. Because the sum is right.

Ah, I see. You are interpreting "error" as "wrong" ; but the FAQ really means "error" as in "something that has to be compensated for by adding in an additional term". In this case, what has to be compensated for is the difference in the zero of time between the two frames, i.e., the "error" is a "correction" you have to apply to convert one frame's time into the other's. The term "accounting error" is an unfortunate choice of words; the FAQ is not trying to say that the 13 year 8 month difference is an incorrect value; all 3 parts, as given in the FAQ, are correct values, and are added together correctly to give the correct final answer. It's just that one of them, the 13 years 8 months, doesn't correspond (in the Time Gap version, where the turnaround is instantaneous) to anything "observed" by Stella; it's just a change in zero of time that she has to apply when she changes inertial frames.

 

[whiskybob]

so has anyone actually flown a clock in space at relativistic speeds(or really fast speeds at least - I know clocks can be very accurate tools nowadays) and then collected it to see if it's fast or slow?

 

[Nugatory]

Yes, many times. Look at the FAQ on experimental support, a stcky thread at the top of this sub forum.

 

[Underwood]

all 3 parts, as given in the FAQ, are correct values, and are added together correctly to give the correct final answer.

OK, so it's right for the traveler guy to say that the homey girl gets a lot older when he turns around. It all comes out right. Thanks.

 

[Underwood]

For the problem we've been talking about, we can figure out how much older the homey girl got while the traveling guy was turning around by figuring out how much extra she had to age so that she would be the right age when he gets back. But is there any way to figure it out before he gets back? Can he figure it out right after he finishes turning around?

 

[indirachap]

With reference to the twin paradox: in order for one twin to end up younger than the twin on the ground the astronaught twin would have had to have spent his time in space in "slow motion" to account for difference in time between the clocks on the ground and on the spaceship.

Is this correct?

 

[Darwin123]

Hi,

I am a little confused with this paradox. I asked my professor about it and he didnt really give a convincing answer. So the scenario basically seems to be some twins on earth(or anywhere) at rest, and then one leaves at relativistic speed for some time then comes back to see that his/her twin is much older than them.

My question is, how come you can tell which one would age more? Why couldn't it just as well be the one on the ship? From the twin on Earth's reference frame, they are at rest and then the rocket flies away from them, while in the rocket frame it is at rest and the Earth flies away from them. From each of the twins perspective the other one moves and they are stationary in their own frames. How come the same thing wouldn't happen to the twin on Earth and find the rocket twin older when the Earth arrived back at the rocket?

I just took this paradox for granted for a long time but now I seem to be confused.

Thanks for reading.

What breaks the symmetry is the external force on the observer. The twin on Earth never has an external force applied to him. In order for the twin in the rocket ship to turn around, a large external force has to be applied to him.
Other people on this forum claim that the acceleration of the rocket breaks the symmetry. They are talking about a dynamic acceleration, not the kinematic acceleration. The kinematic acceleration is defined in terms of geometry, and so is symmetric to both observers. The dynamic acceleration is roughly the external force divided by the mass of the observer.
The magnitude of kinematic acceleration is the same for both twins, since the geometry is quite symmetric. This is probably what you are talking about. However, the dynamic acceleration is quite different. The twin in the spaceship, when turning around, is going to experience a large external force.
The twin who turns around is going to experience a "speed up" of his Earth twins aging no matter how long the turn around time is. The asymmetry of the time dilation increases with the linear impulse and distance between observers. Contrary to some critics, there is no way an observer that turns around won't be aware of his twins "extra age".
There is a little confusion due to the following misunderstanding. Some scientists have written that there is "no such thing as force in relativity". This is technically true only in a very narrow sense. Actually, the nature of the forces is critical to understanding relativity. In the case of special relativity, it is important to understand that the external impulse on the observer that breaks the symmetry.
So long as both observers do not experience an impulse, their observations will be symmetric. The impulse necessary to turn the spaceship around "causes" the Earth twin to age relative to the spaceship twin. Understand this is not a "cause" in the classical sense. Nothing happens to the Earth twin due to the impulse. However, the timing of the internal forces in the space rocket twin are changed by the impulse in a way that is inconsistent with Principia.

 

[Darwin123]

With reference to the twin paradox: in order for one twin to end up younger than the twin on the ground the astronaught twin would have had to have spent his time in space in "slow motion" to account for difference in time between the clocks on the ground and on the spaceship.

Is this correct?

Not for the entire trip. For example, the twin on Earth can "jump" ahead as seen by the rocket ship twin during the external impulse that turns him around.
This description of the Earth twin "jumping ahead" in age is a little oversimplified. Of course, there is Doppler effect and signal propagation issues. I am also ignoring the basic issue of how the rocket ship twin avoids getting squashed during the impulse. However, the experiences of the rocket twin during the turn around impulse are significantly different from any twin in an inertial frame.
If the rocket engines never fire, the twin in the rocket ship is going to go in a straight line and never meet his twin. Then, there is no paradox. However, the rocket engines cause the rocket to turn around. Contact forces in the floor or ceiling then apply an external impulse to the rocket twin. It is the external impulse, caused by the rockets, that cause the asymmetry.
There are several ways to say it. Some say acceleration, some say external forces, some say impulse, others talk about equivalent gravitational fields and some talk about space-time. However, the bottom line is that the external force on an observer distorts the measurements of time and distance.
An observer experiencing an impulse is not in an inertial frame. Therefore, special relativity without modifications doesn't work while the observer is undergoing dynamic acceleration. General relativity is the topic which describes precisely what the rocket twin sees.

 

[Nugatory]

An observer experiencing an impulse is not in an inertial frame.

Yes

Therefore, special relativity without modifications doesn't work while the observer is undergoing dynamic acceleration. General relativity is the topic which describes precisely what the rocket twin sees.

No. SR works just fine even for accelerations as long as you're in a flat space-time, one not curved by gravitational effects.

At a hand-wavy level... Even when accelerations are involved, at any given instant the speeds have definite values so SR will accurately describe the effects of those speeds at that instant. Google "Rindler coordinates" and "Bell spaceship paradox" for examples of how the math works out.

 

[Nugatory]

With reference to the twin paradox: in order for one twin to end up younger than the twin on the ground the astronaut twin would have had to have spent his time in space in "slow motion" to account for difference in time between the clocks on the ground and on the spaceship.

Is this correct?

No.

Consider that you could just as easily have said that the at-home twin would have had to spend his time in "fast motion" to account for the difference in time. What's to make the one description better than the other?

You're going wrong when you start thinking in terms of one twin being in slow (or fast) motion. When you're thinking that way, you're implicitly assuming that there's some true correct time out there so that you can compare the two twins's travel time against that time to see which one is "really" fast or slow.

 

[indirachap]

No.

Consider that you could just as easily have said that the at-home twin would have had to spend his time in "fast motion" to account for the difference in time. What's to make the one description better than the other?

You're going wrong when you start thinking in terms of one twin being in slow (or fast) motion. When you're thinking that way, you're implicitly assuming that there's some true correct time out there so that you can compare the two twins's travel time against that time to see which one is "really" fast or slow.

For me this is absolutely huge - I don't have a problem with anything else but this. I can't get my mind to accept the fact that "the books of time do not have to be balanced" like in accountancy.

From my Earthly point of view, it is essential to explain the discrepancy in the time of the two clocks - this is cardinal for my sanity! And the only explanation I can fnd is the fact
that the astronaught twin spent most of his time in space "in slow motion". This supplies the answer for the "missing time" between the two clocks.

The answer given to me by the scientists, on the face of it, sound just too metaphysical!
I suppose I will just have to accept it! Thanks

 

[ghwellsjr]

For me this is absolutely huge - I don't have a problem with anything else but this. I can't get my mind to accept the fact that "the books of time do not have to be balanced" like in accountancy.

From my Earthly point of view, it is essential to explain the discrepancy in the time of the two clocks - this is cardinal for my sanity! And the only explanation I can fnd is the fact
that the astronaught twin spent most of his time in space "in slow motion". This supplies the answer for the "missing time" between the two clocks.

The answer given to me by the scientists, on the face of it, sound just too metaphysical!
I suppose I will just have to accept it! Thanks

You are correct in saying that the astronaut twin spends most of his time "in slow motion" from an Earthly point of view, if by that you mean from an inertial Frame of Reference in which the Earth is at rest, the Earth twin and his clocks run at normal speed while the astronaut twin and his clocks run slower. However, you should not think that the astronaut twin can be aware that he and his clocks are running slower, in fact, as far as he is concerned, everything is normal because when he measures his heart rate using his clock, he still gets around 1 beat per second.

Also, you should be aware that there is nothing sacrosanct about the Earth's inertial rest frame, you can analyze the situation from any other inertial frame, including the ones in which the astronaut is at rest during either the first half or during the last half of his trip. This is what I pointed out in post #3. There is no "missing time" or "time gap" to be concerned about if you use just one inertial Frame of Reference. Please read it and see if it makes sense to you. If not, ask.

 

[phyti]

What breaks the symmetry is the external force on the observer.

What if the Earth twin decides to accelerate into space to hasten their reunion?

 

[Mentz114]

What if the Earth twin decides to accelerate into space to hasten their reunion?

Who ages most depends on the proper time ( also called proper length) of their journeys through spacetime. See here for an explanation of proper time

http://en.wikipedia.org/wiki/Proper_time

 

[Mark M]

It only seems metaphysical to you because you aren't working out the specific calculation. See 'specific example' here:

http://en.wikipedia.org/wiki/Twin_paradox

Even though two observers moving relative to each other should both say the other's clock is ticking slower, this doesn't account for other relativistic effects. When you work out the calculation as in the article, it works out fine. The real problem behind the twin paradox is the fact that it seems that one inertial observer is singled out as *really* moving, although this seems to violate the principle of relativity. The solution is that the observer isn't an inertial frame, since he had to accelerate up to his speed. So, he is distinguishable from the observer on earth.

 

[indirachap]

You are correct in saying that the astronaut twin spends most of his time "in slow motion" from an Earthly point of view, if by that you mean from an inertial Frame of Reference in which the Earth is at rest, the Earth twin and his clocks run at normal speed while the astronaut twin and his clocks run slower

From an Earthly frame of reference( in other words the FOR of reality), does this mean that near- light travel has to be spent in extreme slow motion? Is this possible?

However, you should not think that the astronaut twin can be aware that he and his clocks are running slower, in fact, as far as he is concerned, everything is normal because when he measures his heart rate using his clock, he still gets around 1 beat per second.

But what if at the near speed of light each second had slowed to the point when 1 beat per second would lead to blackout and eventually death of a biological system? How would the apppearance of normality be maintained?

Even if it were possible for a biological system, wouldn't the time spent in slow motion cancel out the gain in time into the future.

Wouldn't this suggest that significant time travel is virtually impossible?

 

[Mentz114]

But what if at the near speed of light each second had slowed to the point when 1 beat per second would lead to blackout and eventually death of a biological system? How would the apppearance of normality be maintained?

You have not been paying attention. Relativity is what happens to other observers. I am at this moment traveling at great speed relative to many objects in the universe but I'm unaware of it.

Even if it were possible for a biological system, wouldn't the time spent in slow motion cancel out the gain in time into the future.

It's called differential ageing as in the twin 'paradox'.

Wouldn't this suggest that significant time travel is virtually impossible?

Yes.

 

[Nugatory]

From an Earthly frame of reference( in other words the FOR of reality)

What makes that frame the "frame of reality", or any more or less "real" than the frame of the traveling twin? For that matter, how is it any more the "frame of reality" than the frame of the slimy and multiple-limbed aliens who may or may not live on a planet somewhere in the Andromeda galaxy and may or may not be sitting in an Andromedan classroom considering a thought experiment that starts "Two twins born on this planet called Earth, somewhere in the Milky Way galaxy..."?

 

[Darwin123]

What makes that frame the "frame of reality", or any more or less "real" than the frame of the traveling twin? For that matter, how is it any more the "frame of reality" than the frame of the slimy and multiple-limbed aliens who may or may not live on a planet somewhere in the Andromeda galaxy and may or may not be sitting in an Andromedan classroom considering a thought experiment that starts "Two twins born on this planet called Earth, somewhere in the Milky Way galaxy..."?

I think calling "earth" the frame of reality is not only incorrect, but a bit insulting.
In the usual presentation of the "twin paradox", the frame where the Earth is stationary is considered an inertial frame. In reality, Earth isn't an inertial frame. However, this is an approximation. In the fantasy world of precision, observers on the surface of the Earth are undergoing a dynamic acceleration that probably would influence the measurements slightly. However, this isn't the metaphysical problem. As an approximation valid for this hypothetical example, the Earth is an inertial frame.
For purposes of discussion, let us approximate the surface of the Earth as an inertial frame. Corrections, if needed, can be done after we agree on what we are talking about.
Please understand that a frame can't be a single point. A frame is a set of hypothetical measuring instruments that are spread out over the entire universe. The frame where the Earth is stationary is not only those potential measuring instruments on the Earth's surface, but every potential measuring instrument that is stationary relative to the Earth's surface. Similarly, the reference frame of the rocket isn't just the measuring instrument on the rocket, but every potential measuring instrument that is stationary with respect to the rocket.
Note that when Einstein defined a frame, he really defined a hypothetical frame. There was a rigid frame spread out through the universe that held a clock and a ruler at every corner. So a frame is really a three dimensional array of measuring instruments stationary with respect to an object that I will call the observer. The observer can be a human being, but can really just the origin of the coordinate system of the frame.
The word frame has been defined. What is important in the twin paradox is the definition of an inertial frame. The question is not what is the real frame, but what is an inertial frame. There can be any number of frames equally inertial, but a criterion is needed to distinguish any inertial frame from an "accelerating frame".
So it isn't a matter of why the Lorentz transformation is valid in the Earth frame. What is really needed is a quantitative test for whether the Earth frame is approximately inertial.
Ready?
An inertial frame is a frame where each measuring instrument does not receive an impulse (FΔt) of linear momentum. If we have a set of measuring instruments distributed throughout the universe that is stationary with respect to each other, then this is a frame. What makes it inertial is that there is no impulse provided to any of the measuring instruments.
For that interval of time where no measuring receives an impulse (FΔt), then that frame is inertial. If each measuring instrument receives a nonzero impulse, so that the measuring instruments are still stationary with respect to each other, the measuring instruments become a new inertial frame.
The twin on Earth is part of an inertial frame that extends through the entire universe. No external force acts on him or any of the instruments that are stationary with respect to him.
The rocket twin between impulses is in an inertial frame. When a large impulse is given to him and the rest of his frame, he becomes part of a new inertial frame. The laws of special relativity apply to him for the duration between impulses. However, the laws of special relativity don't apply to him while he is under the influence of the impulse. He is in a different inertial frame before and after the impulse.
If one wants to guess at what rules the rocket twin during the impulse, one needs a condition in addition to the usual assumptions of special relativity. One possible "fix" to special relativity is that the clock can't be reset during an impulse. However, it is important to understand that the rocket twin can't be treated as being in one inertial frame no matter what is assumed.
Although this isn't totally Newtonian mechanics, as described in Principia, it isn't completely different from the situation in Newtonian physics. In Newtonian physics, the third law of motion does not apply to the rocket man during the impulse. Absolute space can be considered a frame where each measuring instrument is not given an impulse.
What makes Earth different in the example is merely that the Earth twin was not exposed to an external impulse that was sufficient to affect his measuring instruments. The rocket twin was given a very big impulse by his rocket engines. Without that big impulse, rocket twin couldn't turn around. The same would apply to any alien on any planet in the galaxy.
During the impulse, the rocket twin experiences a force which does not seem to come from any partner. The third law of motion is violated for the twin in the rocket. Any alien that experiences a big impulse will be in the same situation as the rocket twin.
Suppose the rocket engines of the rocket twin are never turned on once he blasts off. Therefore, rocket twin never experiences a large impulse. 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.
The twin that receives the most impulse (i.e., FΔt) is the younger twin. It doesn't matter if he is in a rocket, pulled by a tractor beam or any other force. The one being jerked around the most is younger, the one isolated from impulse is older. That is the resolution of the twin "paradox".

 

[Mark M]

From an Earthly frame of reference( in other words the FOR of reality),

There is no such thing as the 'frame of reality'. In special relativity, there is no preferred frame of reference. EVERY inertial observer is justified in saying that they're at rest, so ALL reference frame are ALL equally valid.

does this mean that near- light travel has to be spent in extreme slow motion? Is this possible?

No. YOU always read YOUR time as being normal. YOU can't even tell that you're moving, let alone detect a difference in time. However, an observer who you are moving with respect to says your time is slow.

For example, nothing is strange for you right now. However, you are moving 99.999999 percent of the speed if light in SOME reference frame. This reference frame will measure YOUR time as being slow. Do you feel any different?

But what if at the near speed of light each second had slowed to the point when 1 beat per second would lead to blackout and eventually death of a biological system? How would the apppearance of normality be maintained?

Time dilation is only observed by OTHER observers. Again, in your frame of reference, everything is normal. That's what matters. If your heart rate was slow in YOUR frame of reference, that would be a problem. However, time dilation never affects time in YOUR reference frame.

Wouldn't this suggest that significant time travel is virtually impossible?

No. However, the extreme acceleration required to reach the necessary speed (with respect to earth) would kill any human.

 

[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.