#### matheinste

Here is a thought experiment similar to the one given by Passionflower with an added twist, that gives a perhaps non-intuitive result. A and B are initially adjacent and at rest wrt each other. B accelerates to 0.99c in a couple of minutes (ignore the practical difficulties) and then cruises for aproximately one year according to A, who up to now has not undergone any acceleration. Now instead of B coming to rest in A's frame, A does a rapid burst of acceleration (lasting a few minutes) in B's direction and comes to rest in B's frame, but they are still far apart.

The slightly surpising result, if I have worked it out correctly (and I might not of, because I am a bit tired) is that A (who has a spent a year lazing around while B hurtles off at 0.99c relative to him) has aged less than B. It seems that the twin that ages the most, is the one that does the short final burst of acceleration to come to rest in the other twin's rest frame.

As I'm sure you know, a spacetime diagram shows exactly what is going on. Without giving it too much thought I would imagine that you can, by using appropriate figures, set up a similar scenario where they age by the same amount.

As usual its just about path lengths. Spacetime diagrams let you visualize what is going on and calculation confirms it.

Matheinste.

#### Demystifier

2018 Award
Hey whoa what's going on here? It seems to me like Demystifier could save my thread https://www.physicsforums.com/showthread.php?t=404650".

Demystifier, I have a headache over this problem which to me is unsolved yet, could you help me out over there if you feel like it?
But that is really easy. It is only Newtonian mechanics, no Einstein relativity is involved. From their point of view, one is rotating around the other (e.g. Moon around Earth or vice versa). They miss each other because the direction of force does not need to coincide with the direction of velocity. I do not see what is your problem with that.

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#### Demystifier

2018 Award
You are the "go-to" man when it comes to accelerated frames. I have also used extensively your other excellent paper, the one dealing with rotating frames.
Thanks! Have you also published something on that stuff? (arXiv preprints also count as published.)

#### starthaus

Thanks! Have you also published something on that stuff? (arXiv preprints also count as published.)
No, but I have used extensivelyin order to derive new results. I consider your two papers on the subject the "nec plus ultra" :-)

2018 Award

#### Jonnyb42

But that is really easy. It is only Newtonian mechanics, no Einstein relativity is involved. From their point of view, one is rotating around the other (e.g. Moon around Earth or vice versa). They miss each other because the direction of force does not need to coincide with the direction of velocity. I do not see what is your problem with that.
Yeah but, the coordinate system you are talking about does not rotate with the particles. The frame is centered around the center and both particles lie on the same axis, the x axis for example. In that system, what explains the particles not colliding? I know you might say it is because the frame of reference is non-inertial, but what truly makes it non-inertial? Only a third observer would have to say that the reference frame is non-inertial.

#### Demystifier

2018 Award
I know you might say it is because the frame of reference is non-inertial, but what truly makes it non-inertial? Only a third observer would have to say that the reference frame is non-inertial.
That is not correct. Being in a non-inertial frame is an absolute statement. If you are in a non-inertial frame for one observer, then you are in a non-inertial frame for ANY observer. In particular, you can determine experimentally whether you are accelerating or not. Velocity is relative, but acceleration is absolute.

Now you will probably ask: But if I accelerate, then I accelerate with respect to what? In Newtonian mechanics it was not completely clear. For example, Mach thought that acceleration is defined with respect to (all) distant stars in the Universe. But in the relativity theory, this question can be answered: You accelerate with respect to the metric field (metric tensor).

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#### JesseM

That is not correct. Being in a non-inertial frame is an absolute statement. If you are in a non-inertial frame for one observer, then you are in a non-inertial frame for ANY observer. In particular, you can determine experimentally whether you are accelerating or not. Velocity is relative, but acceleration is absolute.
To put it simply, inertial observers feel weightless, non-inertial observers feel G-forces (which can be measured with an accelerometer)

#### Mike_Fontenot

[...] It seems that the twin that ages the most, is the one that does the short final burst of acceleration to come to rest in the other twin's rest frame.
No, it's the opposite. The twin who does the accelerating, WHEN THEIR SEPARATION IS NON-ZERO, will be the younger, after they are again stationary with respect to one another.

The initial acceleration by B (when their separation is zero) has no effect: you can reformate the problem as two unrelated newborns, who are moving at a constant speed with respect to one another, and who happened to be momentarily adjacent at the instant they are both born, with neither one accelerating then.

As the two twins initially move apart (say, at a relative speed of 0.866c, to give a simple value of gamma = 2), they EACH will (correctly) conclude that the other is ageing more slowly (by a factor of 2). They are BOTH correct. Neither can adopt any other conclusion, without contradicting their own elementary measurements.

So, given the complete symmetry of this initial phase, you can actually consider B to be "the home twin", and A is "the traveler". If you do that, you can apply the CADO equation that I gave earlier, to get their two conclusions about their corresponding ages at various instants.

Suppose that the traveler (A) is 10 years old at the instant that he accelerates (call it point P), and they were both zero years old when they were co-located at birth.

The home twin (B) concludes that she is 20 years old when the traveler accelerates at point P, so she concludes that their separation is 20 (0.866) = 17.32 lightyears.

The CADO equation then says that CADO_T immediately before the acceleration is

or

CADO_T = 20 - (17.32)(0.866) = 20 - 15 = 5 years old.

This is B's age right before the acceleration by A, according to A. So, right before A accelerates, B says she is 20, but A says she is 5. (And they both agree that A is 10 then).

Immediately after the acceleration, the CADO equation says

CADO_T = 20 - (17.32)(0) = 20 years old.

So now, the twins AGREE on their relative ages (as they always will whenever their relative speed is zero). They both agree that when A is 10 (immediately AFTER his acceleration), B is 20.

And thereafter, they both agree that their rates of ageing are equal, and that B is always 10 years older than A.

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#### Al68

Now you will probably ask: But if I accelerate, then I accelerate with respect to what? In Newtonian mechanics it was not completely clear. For example, Mach thought that acceleration is defined with respect to (all) distant stars in the Universe. But in the relativity theory, this question can be answered: You accelerate with respect to the metric field (metric tensor).
Isn't this equivalent to Mach's principle, if the metric field is determined by distant masses?

#### Jonnyb42

But in the relativity theory, this question can be answered: You accelerate with respect to the metric field (metric tensor).
WHAT!?

Why haven't I been told this before!???

Thank you very much Demistyfier, I really really need to learn general relativity.
I am still a bit confused, (because I have not learned about Mach's principle or general theory of relativity.) For instance, another question I have is, why isn't the metric field considered an 'absolute reference frame'?

#### JesseM

WHAT!?

Why haven't I been told this before!???

Thank you very much Demistyfier, I really really need to learn general relativity.
I am still a bit confused, (because I have not learned about Mach's principle or general theory of relativity.) For instance, another question I have is, why isn't the metric field considered an 'absolute reference frame'?
It's like an electromagnetic field, defined at every point in spacetime, but it doesn't have a rest frame (maybe Demystifier could clarify what it means to 'accelerate with respect to' something that has no rest frame?)

#### Demystifier

2018 Award
Isn't this equivalent to Mach's principle, if the metric field is determined by distant masses?
GR does not obey the Mach principle because the metric field is not determined ONLY by distant masses. Instead, when the matter is fixed, you still have a freedom to choose initial conditions for the metric field. In particular, even if there is no matter at all, there is still a metric field with respect to which acceleration is defined.

#### Demystifier

2018 Award
For instance, another question I have is, why isn't the metric field considered an 'absolute reference frame'?
Because the velocity is still relative. Only acceleration is absolute. In other words, you cannot talk about velocity of the metric field, but in a sense you can talk about acceleration of the metric field. This is related to the fact that metric field is not a first-rank tensor (i.e., a vector), but a second-rank tensor. This is just a rough, hopefully intuitive explanation, while a proper explanation requires more math.

#### Demystifier

2018 Award
It's like an electromagnetic field, defined at every point in spacetime, but it doesn't have a rest frame (maybe Demystifier could clarify what it means to 'accelerate with respect to' something that has no rest frame?)
Note that a photon moving with the velocity of light does have a rest frame, but that this frame is not a Lorentz frame. Instead, it is a light-cone frame.
Note also that the velocity of photon is related to the Poynting vector of the EM field. Finally, note that not all EM fields have a non-zero Poynting vectors.

#### JesseM

Note that a photon moving with the velocity of light does have a rest frame, but that this frame is not a Lorentz frame. Instead, it is a light-cone frame.
Note also that the velocity of photon is related to the Poynting vector of the EM field. Finally, note that not all EM fields have a non-zero Poynting vectors.
OK, but what about my question about how you define acceleration "relative to" a tensor field that has no rest frame of its own? I suppose by the equivalence principle, in the local neighborhood of any point in curved spacetime we can define locally inertial frames, so can we say that any worldline that passes through that point must have either zero or nonzero instantaneous acceleration relative to these local inertial frames?

#### Al68

Isn't this equivalent to Mach's principle, if the metric field is determined by distant masses?
GR does not obey the Mach principle because the metric field is not determined ONLY by distant masses. Instead, when the matter is fixed, you still have a freedom to choose initial conditions for the metric field. In particular, even if there is no matter at all, there is still a metric field with respect to which acceleration is defined.
Fair enough, but to my knowledge, Mach's Principle was never specific enough to say that GR doesn't "obey" it.

GR certainly seems very "Machian" to me, at least compared to Newtonian physics, even if it doesn't actually explain the source of inertia.

#### yuiop

The slightly surpising result, if I have worked it out correctly (and I might not of, because I am a bit tired) is that A (who has a spent a year lazing around while B hurtles off at 0.99c relative to him) has aged less than B. It seems that the twin that ages the most, is the one that does the short final burst of acceleration to come to rest in the other twin's rest frame.
No, it's the opposite. The twin who does the accelerating, WHEN THEIR SEPARATION IS NON-ZERO, will be the younger, after they are again stationary with respect to one another.
Yep, your right. I made a typo in the final sentence. Thanks for picking it up and have now edited the original post. The second sentence obviously contradicts the sentence immediately preceding it. Knew I shouldn't have been posting stuff when I was tired. Good to see someone is paying attention

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#### Demystifier

2018 Award
Fair enough, but to my knowledge, Mach's Principle was never specific enough to say that GR doesn't "obey" it.

GR certainly seems very "Machian" to me, at least compared to Newtonian physics, even if it doesn't actually explain the source of inertia.
Fair enough. But there are even theories of gravity which are more Machian than GR. The Brans-Dicke theory for example.

#### Demystifier

2018 Award
OK, but what about my question about how you define acceleration "relative to" a tensor field that has no rest frame of its own? I suppose by the equivalence principle, in the local neighborhood of any point in curved spacetime we can define locally inertial frames, so can we say that any worldline that passes through that point must have either zero or nonzero instantaneous acceleration relative to these local inertial frames?
Yes.

#### Jonnyb42

This is related to the fact that metric field is not a first-rank tensor (i.e., a vector), but a second-rank tensor. This is just a rough, hopefully intuitive explanation, while a proper explanation requires more math.
Daaang. Just guessing, but would an acceleration be relative in a third-rank tensor??
I am ashamed to not know what tensors are and what these things mean. I want to know enough math. I have to finish up some projects for the end of the year (I am a senior right now.) and when summer starts I am going to study vector calculus and general relativity like mad.

Thanks for all the help.
This stuff is crazy.

#### Demystifier

2018 Award
I am ashamed to not know what tensors are and what these things mean. I want to know enough math. I have to finish up some projects for the end of the year (I am a senior right now.) and when summer starts I am going to study vector calculus and general relativity like mad.

Thanks for all the help.
This stuff is crazy.
There are many excellent textbooks on this stuff, but if you need a recommendation, I suggest
http://xxx.lanl.gov/abs/gr-qc/9712019

#### Jonnyb42

Thank you very much, I will definitely read that, whenever I do!

I have another question that comes to my mind,

Is the theory of General Relativity being expanded on/updated, or has it been the same since Einstein published it?
I wonder the same with quantum mechanics.

#### Fredrik

Staff Emeritus
Gold Member
GR hasn't really been modified since 1915, but people are still working on ways to find new interesting solutions to the main equation of the theory. That equation (Einstein's equation) describes the relationship between how things are distributed in the universe and how things move. It's so hard to solve that it took 48 years to find the solution that describes a universe that's completely empty except for a single rotating star.

QM hasn't really changed since the early 1930's, but a lot of work has been done on theories of matter and interactions in the framework of QM (mostly quantum field theories), and people are still working on them.

The big thing right now is of course to figure out how these two fit together.

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