Philosophy and the Twin Paradox

Mike Fontenot

The famous twin paradox of special relativity involves a scenario where one twin (he) rockets away from the home twin (her), coasts to a far-away turnpoint, reverses course, coasts back, and comes to a halt when they are reunited. At the reunion, both twins agree (by inspection) that she is older than he is.

There is no dispute about the outcome at the reunion. But physicists DO differ about what HE concludes about HER current age DURING his trip. One school of thought is that he says that she is ageing more slowly than he is, on both the outbound leg and on the inbound leg, but that he concludes that she instantaneously ages by a large amount during the instantaneous turnaround. But that conclusion can be shown to imply that he will have to conclude that it is possible for her to instantaneously get YOUNGER when he changes speed in certain other ways. THAT result is abhorrent to many (maybe most) physicists. The most extreme reaction is to conclude that simultaneity at a distance is simply a meaningless concept. Other physicists react by embracing alternative simultaneity methods, that don't result in instantaneous ageing (either positive or negative).

So what does the above have to do with philosophy? For many physicists, there is no place for philosophy in special relativity. Philosophical arguments are usually banned on physics forums. But philosophy has always played a role in my thinking on the subject, even though I'm a physicist, not a philosopher (and I have only a VERY limited knowledge of philosophy). Philosophy has entered into my thinking about the twin paradox in this way: when some physicists contend that simultaneity at a distance is meaningless, I have a philosophical problem with that. IF I were that traveler, I don't think I would be able to believe that she no longer EXISTS whenever I am not co-located with her. (And I doubt that many other physicists believe that either). BUT many physicists DO believe that she doesn't have a well-defined current AGE when he is separated from her (at least if he has accelerated recently). THAT'S the conclusion that I can't accept philosophically: it seems to me that if she currently EXISTS right now, she must be DOING something right now, and if she is DOING something right now, she must be some specific AGE right now. So I conclude that her current age, according to him, can't be a meaningless concept. That puts me at odds with many other physicists.

What say the philosophers on this forum?

ovdtogt

One school of thought is that he says that she is ageing more slowly than he is, on both the outbound leg and on the inbound leg, — Mike Fontenot

I don't know that much about physics but don't they both age at the same rate as they are traveling at the same relative speed? Were he however be traveling in a circle around her he would be aging slower than her.

Isn't an atomic clock that has returned from the space station absolutely behind in time?

Mike Fontenot

Special relativity says that, for two perpetually-inertial observers (meaning that they have never accelerated, and never will accelerate), they EACH will conclude that the other is ageing more slowly. (I believe that all physicists believe that). So those two inertial observers DON'T agree with each other. But special relativity says that they are BOTH CORRECT, even though they disagree with each other! And it's impossible prove that an actually INCONSISTENCY results in that situation. To what extent the above can be extended to also apply to an observer who has accelerated at some point in the past, or may accelerate at some point in the future, is the source of disagreement among physicists.

ovdtogt

Special relativity says that, for two perpetually-inertial observers (meaning that they have never accelerated, and never will accelerate), they EACH will conclude that the other is ageing more slowly. (I believe that all physicists believe that). — Mike Fontenot

Where do you get that from? You and I are 'perpetually-inertial observers' and do not age differently.

Mike Fontenot

↪ovdtogt

I forgot to respond to your other two points.

For circular motion by the traveler, the two twins Do agree with one another. Both say the twin at the center of the circle is ageing faster than the twin who is moving in a circle. The standard twin paradox scenario assumes that their relative motion is one-dimensional (although it CAN be extended to two of three spatial dimensions).

As to your last point, the undisputed results of special relativity HAVE been experimentally confirmed in lots of ways. We have never been able to check the most dramatic predictions by accelerating large objects (like humans) to relative speeds that are large fractions of the speed of light, and probably never will ... it simply requires WAY too much energy.

ovdtogt

For circular motion by the traveler, the two twins Do agree with one another. — Mike Fontenot

For the traveler traveling away or the observer staying behind there is no difference in their relative speed so they age at the same rate.

Mike Fontenot

↪ovdtogt

"Where do you get that from. You and are 'perpetually-inertial observers' and do not age differently."

My statement applied to the case where we are moving at non-zero speed. I forgot to state that. Sorry.

Mike Fontenot

↪ovdtogt

"For the traveler traveling away or the observer staying behind their is no difference in their relative speed so they age at the same rate."

As I've already said, special relativity says that if they are perpetually inertial (and moving at a non-zero speed), they will each conclude that the other is ageing more slowly. It's time now to address my philosophical comments. I'm not here to teach or defend special relativity. I want to know what philosophers on this forum think about my philosophical comments.

ovdtogt

non-zero speed — Mike Fontenot

I have googled non-zero speed but only find non-zero velocity and that means something quite different.
Non speed means you are standing still. You are not in motion.

perpetually inertial — Mike Fontenot

Nor can I find anything on perpetual inertia.
Inertia a property of matter by which it continues in its existing state of rest or uniform motion in a straight line, unless that state is changed by an external force.

So in your hypothetical nothing is moving.

Pfhorrest

↪Mike Fontenot

On the one interpretation, does she actually get younger, or does she just age much more slowly while he continues aging much more quickly, resulting in him catching up to her age similarly to if she had gotten younger? Like how passing a car on the road looks similar to that other car backing up past you even though you’re both still moving forward.

Mike Fontenot

↪Pfhorrest

She actually gets younger, according to him.

Bartricks

↪Mike Fontenot

I am not sure I understand the problem. Is it that situations can arise in which one person, Jill, has apparent evidence that James has aged more slowly than she, and James will have apparent evidence that Jill has aged more slowly than he?

If so, that as it stands is not really a problem as such, for we can simply say that one of them is mistaken. They may both be equally justified in their beliefs, nevertheless, one of them is incorrect.

Devans99

↪Mike Fontenot

I wonder if anything could be deduced by a constant, mutual, radio broadcast of each other's heartbeats to each other. That would allow verification of the existence of the other twin and also act as sort of body clock by which they might be able to judge each others relative speeds of ageing.

Maybe even quantum entanglement could be used to transmit, instantaneously this time, the beat of each other's hearts.

Mike Fontenot

↪Bartricks

"

I am not sure I understand the problem. Is it that situations can arise in which one person, Jill, has apparent evidence that James has aged more slowly than she, and James will have apparent evidence that Jill has aged more slowly than he?

If so, that as it stands is not really a problem as such, for we can simply say that one of them is mistaken. They may both be equally justified in their beliefs, nevertheless, one of them is incorrect. — Bartricks

No, they are actually BOTH correct. And the evidence that they each have is valid evidence fro THEMSELVES, but it contradicts the evidence of the other person. That's strange, but it can't be shown to lead to an actual logical inconsistency.

Mike Fontenot

↪Devans99

I wonder if anything could be deduced by a constant, mutual, radio broadcast of each other's heartbeats to each other. That would allow verification of the existence of the other twin and also act as sort of body clock by which they might be able to judge each others relative speeds of ageing.

Maybe even quantum entanglement could be used to transmit, instantaneously this time, the beat of each other's hearts. — Devans99

Your first paragraph doesn't work, because the distance between them is constantly changing, and so the actual period of the heartbeats is distorted by the varying travel times of the messages. The same thing happens if she continuously transmits a TV image of her, holding a sign that gives her exact age. When he properly allows for her ageing during the transit of the message, he gets the correct current age for her. And that is the same answer that can be obtained analytically from the Lorentz equations.

Your second paragraph doesn't work, because special relativity and quantum mechanics are mutually inconsistent ... neither theory recognizes the legitimacy of the other. Einstein never accepted quantum mechanics.

But NOW, I want to know what you and other philosophers think of my intuitive philosophical reasoning about the question of whether simultaneity at a distance is meaningful or meaningless. THAT'S why I posted here. This isn't the appropriate place for me to either teach special relativity or defend special relativity.

Devans99

Your first paragraph doesn't work, because the distance between them is constantly changing, and so the actual period of the heartbeats is distorted by the varying travel times of the messages — Mike Fontenot

But distorted in a predicable / pre-calculable manner?

Your second paragraph doesn't work, because special relativity and quantum mechanics are mutually inconsistent ... neither theory recognizes the legitimacy of the other. — Mike Fontenot

Well it might be a chance to shed light on the inconsistency?

I want to know what you and other philosophers think of my intuitive philosophical reasoning about the question of whether simultaneity at a distance is meaningful or meaningless. — Mike Fontenot

I would agree that her heartbeats continually in his absence, so she must always have a well defined biological age.

Mike Fontenot

↪Devans99

I would agree that her heartbeats continually in his absence, so she must always have a well defined biological age. — Devans99

Thanks.

Bartricks

↪Mike Fontenot

I do not understand.

Bartricks

↪Mike Fontenot

If two people have equally good evidence for contradictory positions, it does not follow that they are both correct. For instance, there may be just as good evidence that Jack-the-Ripper was Tim Baloney as there is that he was John Junk. It does not follow that both judgments are true, just that they are equally justified.

So, are you saying that they will both have equally good evidence for contradictory judgments?

Or are you saying that both of their judgments will be true?

If the latter, then they don't contradict (unless you are claiming that contradictions can be true). Yet you seem to be suggesting that their judgments 'do' contradict.

That's strange, but it can't be shown to lead to an actual logical inconsistency.
— Mike Fontenot

But that 'is' a logical inconsistency - that is, if their judgments contradict but you claim they are both true, then we have a logical inconsistency.

So I am not yet understanding what the difficulty is.

Bartricks

↪Mike Fontenot

To continue, if I may, you say this:

The famous twin paradox of special relativity involves a scenario where one twin (he) rockets away from the home twin (her), coasts to a far-away turnpoint, reverses course, coasts back, and comes to a halt when they are reunited. At the reunion, both twins agree (by inspection) that she is older than he is. — Mike Fontenot

How is this different from me putting one piece of cheese in the fridge and another on the sideboard and then reuniting them on the sideboard a week later and noting that the piece of cheese on the sideboard seems to have 'aged' considerably more than the piece I put in the fridge?

It is not, of course, that time has slowed down for the cheese in the fridge. No, it is just that processes involving one have sped up relative to the other.

So, the twin who rockets away has, in effect, put himself in a fridge. Or is that a mistaken way to think about it?

jgill

Human bodies age at different rates according to genetics and other factors. In high school at the age of 17 I could barely grow enough facial hair to shave, while a friend of the same age had a thick black beard. Nothing to do with advanced physics I assume. In a recent note I explained chronological age vs "imaginary" (i^2=-1) age (goofy mathematical speculation). I wonder if imaginary age would also be affected by gravitation and acceleration?

SophistiCat

BUT many physicists DO believe that she doesn't have a well-defined current AGE when he is separated from her (at least if he has accelerated recently). THAT'S the conclusion that I can't accept philosophically: it seems to me that if she currently EXISTS right now, she must be DOING something right now, and if she is DOING something right now, she must be some specific AGE right now. So I conclude that her current age, according to him, can't be a meaningless concept. — Mike Fontenot

Her current age is not a meaningless concept, just under-defined. Pick a reference frame - any reference frame - and the ambiguity will disappear. The problem is that there is no absolute reference frame, so that you could say that her age is this many years, without any further qualifications, as you would in Newtonian world. In the relativistic world you must specify the reference frame to go with the age figure, and there is no right or wrong answer.

Pfhorrest

↪Bartricks

You could also just talk about two light clocks or any other physical systems to judge the time passage on the ship and on Earth. All processes on the ship, even elementary physical processes like light moving through a vacuum, happen as though less time has passed. A physicist on the ship would still calculate the speed of light from Maxwell's equations to be exactly the same as one on Earth, and both would measure the speed of light bouncing around their labs to agree with their calculations, but when the ship got back to Earth one would find that a light beam bouncing around their lab had bounced around a shorter distance even though both would agree that it was moving at the same speed, so the only way they can reconcile that is to say one bream of light was traveling for less time -- and sure enough, clocks and all other measurements of time on the ship would agree that less time had passed.

prothero

It is not just a matter of clocks running at different speeds. It is also a matter of physical (and hence chemical and biologic processes) running at different rates due to differences in acceleration and gravitational systems. Thus the traveling twin (given sufficient acceleration and gravitational) effects indeed does age slower in biological terms than the stay at home twin. One can show such changes in the lifetimes of accelerated atomic particles, in atomic clocks taken on airplane trips or placed in space at altitudes where the gravitational field is weaker. It brings up the whole question of what is the nature of time and the fact that there is no universal fixed eternal time or present moment. The rate of all processes are affected by acceleration and gravity not just the running of a clock.

A Seagull

You and I are 'perpetually-inertial observers' and do not age differently. — ovdtogt

You are not inertial, you are accelerating towards the centre of the Earth. (There is an equivalence between gravity and acceleration.)

Bartricks

↪Pfhorrest

I do not follow you, so need more detail.

My original point was that it would be clearly absurd to suggest that time had slowed down for the cheese in the fridge. Fewer events have occurred in the refrigerated cheese than in the cheese on the sideboard, that's all.

Perhaps that analysis cannot be given of the case you describe, but I do not grasp that case yet so can't assess it.

noAxioms

But physicists DO differ about what HE concludes about HER current age DURING his trip. — Mike Fontenot

This seems to conflict with this subsequent statement:

Special relativity says that, for two perpetually-inertial observers (meaning that they have never accelerated, and never will accelerate), they EACH will conclude that the other is ageing more slowly. — Mike Fontenot

OK, maybe not, since both are wrong.

Physicists do not differ about this, although they might word it differently. At any point during the exercise, there is no ambiguity of the age of one person relative to the other in the frame of either person (or more exactly, in the frame in which the person is currently stationary). Without the frame specification, statements about simultaneity are ambiguous
.
It has nothing to do with being perpetually inertial. An inertial frame is defined by a reference to an object at a specific moment, so if that object is accelerating, its current inertial frame is changing.

"For the traveler traveling away or the observer staying behind their is no difference in their relative speed so they age at the same rate." — Mike Fontenot

That was an example of a statement without a frame specification, and thus wrong. In either frame in which one person is stationary, the other ages more slowly.

If two people have equally good evidence for contradictory positions, it does not follow that they are both correct. — Bartricks

There are no contradictory positions in this scenario. Both parties agree on all facts at all times. Confusion only arises when the frame references are omitted.

Sophisticat seems to be the first poster with a sensible reply.

You are not inertial, you are accelerating towards the centre of the Earth. — A Seagull

Gravity is not part of special relativity. That said, under GR, on Earth you are accelerating upward, not downward, else the water in your cup would stay in only if inverted. The force on me from my chair pushes me up, not down.

Mike Fontenot

Please remember that my reason for posting here is to get opinions from the philosophers on this forum on my "intuitive" philosophical comments. I'm not here to teach, or debate, or defend, my views on special relativity and the twin paradox.

ovdtogt

ou are not inertial, you are accelerating towards the centre of the Earth. (There is an equivalence between gravity and acceleration.) — A Seagull

Why aren't we at the centre of the earth by now then?

SophistiCat

↪Mike Fontenot

I think that your main worry here is the relativity of simultaneity, the conventionality of clock synchronization protocols in SR. The only time when one can unambiguously match the chronology of the twins is when they are collocated, i.e. before or after the journey. The rest of the time the question "How long has my twin been in traveling?" does not have an unambiguous answer: it depends on the reference frame from which the length of the timeline is measured.

There is only one frame that has a special significance here: the comoving frame, the frame associated with one of the twins. Since all of the aging processes will be synchronous with this frame, this is the frame that you want to use if you want to know how much a twin has aged over time. However, that answer will only be useful to the other twin at the end of the journey, when the two twins meet, because at any other time there is no non-arbitrary way for the one twin to tell how much time has elapsed in the other twin's comoving clock as of this momement.

Pfhorrest

↪ovdtogt

Technically we are accelerating away from the center of the Earth, not toward it. If we coasted in an inertial frame (=freefall) we would end up orbiting the center of the Earth, but there’s a bunch of rock in the way pushing us off course from that path thanks to the electrostatic repulsion between the molecules that make up both it and us. That repulsion is the acceleration you feel pushing your chair into your butt right now.

Philosophy and the Twin Paradox

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