The Twin Paradox

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  • #26
JesseM
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Ok, so we go from I am wrong to it's neglible. :smile:
Your original post was wrong in saying "acceleration is not handled by SR for the simple reason that acceleration is mitigated by curved space-time" because an object's path is not only determined by the curvature of spacetime (and your later posts seemed to argue that it was). Anyway, in practice it's standard to treat a negligibly-curved spacetime as equivalent to a flat one--otherwise the only flat spacetime would be one devoid of all particles (even without force fields, their masses would curve it slightly), and the equivalence principle couldn't be used in any finite-sized region.
MeJennifer said:
So let me get this right; are you claiming that the total amount of energy applied to accelerate an object is not equal to the amount of space-time curvature induced? :confused:
I'm not sure about the technical details of how spacetime curvature is quantified or whether it is proportional to "the amount of energy applied to accelerate an object" even in pure GR without other forces. (Since everything moves on geodesics in pure GR, does it even make sense to talk about an object 'accelerating' if its path is always locally inertial?) But put it this way: if an object is accelerated by non-gravitational forces it takes far less energy than it would to alter its path in a similar way using only gravity. You can use the muscles in your legs to jump upwards, but you'd need a vast density of energy over your head to pull you away from the Earth at the same speed based only on the gravitational force. Also, consider the fact that a force fields will curve spacetime the same way for everyone, so why is it that two particles with identical masses and initial positions and velocities but different charges will move in different directions in an electromagnetic field? Why is it that a neutrino can travel straight through the entire planet as if it were empty space, while a proton or electron cannot? If objects' paths were determined only by spacetime curvature, then even if particles still generated fields with the same energy densities as real fields in our universe (but with the fields having no other effects besides curving spacetime), then every particle would be even more "ghostly" than the neutrino, since the neutrino does at least interact with matter via the weak nuclear force (but the electromagnetic and strong forces have no effect on it, apart from how they curve spacetime of course).
 
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  • #27
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But put it this way: if an object is accelerated by non-gravitational forces it takes far less energy than it would to alter its path in a similar way using only gravity.
Sorry but that defies all common sense! :smile:
Instead it takes exactly the same amount of energy!
 
  • #28
JesseM
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Sorry but that defies all common sense! :smile:
Instead it takes exactly the same amount of energy!
I guess it depends how you define the "energy used" to move something. I was thinking in terms of the total energy of the system doing the attracting, including the energy due to its mass. If a paperclip on a table would move upward at the same speed in response to the electromagnetic pull of a small magnet held above it as it would to the gravitational pull of a miniature black hole at the same distance, doesn't this mean more energy is being used in the second case? If not, how do you define the "amount of energy it takes"?

In any case, do you agree that the path of objects is not due only to the curvature of spacetime, but also to the effects of non-gravitational forces? If not, could you address the example of two particles with opposite charges but identical masses and identical initial positions and velocities, and why they take different paths in identically-curved spacetimes?
 
  • #29
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IIn any case, do you agree that the path of objects is not due only to the curvature of spacetime, but also to the effects of non-gravitational forces? If not, could you address the example of two particles with opposite charges but identical masses and identical initial positions and velocities, and why they take different paths in identically-curved spacetimes?
In GR, when an object accelerates it means that the space-time curvature is modified in that region. Mass, but also energy, changes the curvature of space-time. Technically you would want to inspect the stress-energy tensor to see how.
For instance, if you launch a rocket you have a lot of EM energy density, and what happens when you have a lot of energy density with flux and momentum, you curve space-time, alot! :smile:

In this context it might be interesting to lookup Noether's theorem.
 
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  • #30
JesseM
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In GR, when an object accelerates it means that the space-time curvature is modified in that region.
Not if you are including non-gravitational forces such as EM.

edit: sorry, depends what you mean. I assumed you meant that the object could only be caused to accelerate by a change in spacetime curvature, but perhaps you meant that the motion of the object itself would change the curvature of spacetime, which is of course true unless we consider a "test particle" with infinitesimal mass (and anyway, for an object on the human scale the changes in curvature caused by the object's motion is, again, negligible).
MeJennifer said:
For instance, if you launch a rocket you have a lot of EM energy density, and what happens when you have a lot of energy density with flux and momentum, you curve space-time, alot! :smile:
Are you claiming that a rocket moving away from the earth is following a geodesic path in curved spacetime caused by the "EM energy density"? If so you are badly mistaken. And you still haven't answered my question about why you think two particles identical in every way except their charge would follow different paths in an electromagnetic field, if their paths were determined only by the curvature of spacetime.
MeJennifer said:
In this context it might be interesting to lookup Noether's theorem.
I'm aware of Noether's theorem, what relevance do you think it has to this debate?
 
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  • #31
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Are you claiming that a rocket moving away from the earth is following a geodesic path in curved spacetime caused by the "EM energy density"?

I am not saying that, and hopefully you are not thinking that a body moved by an EM force is traveling in ambient space. :smile:
It is traveling on the manifold, and the EM force provides exactly the correct change in curvature to explain the acceleration.

Now I am not saying that anybody has provided a complete and verifiable theory on how to calculate the induced space-time curvatures for EM, weak and strong forces. But if we can't or if we can demonstrate it is impossible then GR is in serious trouble.
But we simply cannot have some attitute of: "well, EM forces just ignore the background theorized by GR, but that's ok, it will still work, EM forces just fly over the manifolds".

We have attempts made, for instance in the Kaluza-Klein theory and of course Einstein tried for a long time to find it as well and then there is string theory.
 
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  • #32
JesseM
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I am not saying that, and hopefully you are not thinking that a body moved by an EM force is traveling in ambient space. :smile:
It is traveling on the manifold, and the EM force provides exactly the correct change in curvature to explain the acceleration.
I don't understand what you mean by "traveling in ambient space" vs. "traveling on the manifold". When you say "manifold", don't you mean the manifold of curved spacetime?

To rephrase the question: do you believe that the accelerating rocket is following a geodesic in curved spacetime (whose curvature of course includes the slight contribution from the EM field)? Or would you agree that EM fields can cause charged particles to follow non-geodesic paths? (in terms of our current most successful theories, not more speculative ideas like Kaluza-Klein).
 
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  • #33
JesseM
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Now I am not saying that anybody has provided a complete and verifiable theory on how to calculate the induced space-time curvatures for EM, weak and strong forces. But if we can't or if we can demonstrate it is impossible then GR is in serious trouble.
But we simply cannot have some attidute of: "well, EM forces just ignore the background theorized by GR but that's ok, it will still work, it'll just fly over the manifolds".
What are you talking about? When physicists use EM forces in GR, they don't ignore the background theorized by GR or have the EM fields "fly over" it somehow, they define the EM fields on the curved spacetime. The distance in curved spacetime between any two points would presumably affect the EM force between charges at those points, for example, and EM waves would always locally move at c as measured by nearby freefalling observers, which explains how EM waves can get "trapped" at the event horizon of a black hole, without violating the rule that EM waves can never "stop".

And why do you think that any theory which doesn't explain other forces in terms of curved spacetime would cause GR to be in "serious trouble"? There needn't be any violation of the equivalence principle, as you seemed to suggest earlier--as long as the non-geodesic paths seen by an observer in a room falling through a gravitational field look just like the non-geodesic paths seen by an observer in a room moving inertially in empty space, the equivalence principle is fine.
 
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  • #34
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Well it is clear to me that I am certainly not the person to explain this to you.
 
  • #35
JesseM
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Well it is clear to me that I am certainly not the person to explain this to you.
The things you're saying seem to be your own idiosyncratic notions, not standard ideas or problems that would be recognized by physicists, so if you're not willing to explain what you mean I don't think anyone else can either. If you think your ideas or arguments would be recognized by physicists, can you cite any sources? I'm still not sure if you disagree that the standard understanding is that EM fields cause charged objects to follow non-geodesic paths, but if you do I looked up some sources on arxiv.org which might help convince you otherwise, like this one which discusses the non-geodesic worldlines in the neighborhood of a collapsing magnetized medium.
 
  • #36
Chris Hillman
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Acceleration

Hi again, MeJennifer,

Well, running the risk of getting stuck in the middle of this discussion, strictly speaking acceleration is not handled by SR for the simple reason that acceleration is mitigated by curved space-time.
In a Lorentzian manifold (curved or flat), the acceleration vector is a property of a curve (or a congruence of curves), and is identified with the covariant derivative of the tangent to the curve, taken along the curve. This has nothing to do with the curvature of the Lorentzian manifold itself, or with general relativity. In fact, the decomposition of a vector field into acceleration, expansion, and vorticity is sometimes called the kinematical decomposition. In a locally flat spacetime, it makes sense to regard this as part of special relativity, although to some extent this is a matter of convention, I suppose. The important point is that no-one should get the idea that this kinematical decomposition "requires general relativity" or is "a technique which only makes sense in a curved manifold"!
 
  • #37
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In a Lorentzian manifold (curved or flat), the acceleration vector is a property of a curve (or a congruence of curves), and is identified with the covariant derivative of the tangent to the curve, taken along the curve. This has nothing to do with the curvature of the Lorentzian manifold itself, or with general relativity. In fact, the decomposition of a vector field into acceleration, expansion, and vorticity is sometimes called the kinematical decomposition. In a locally flat spacetime, it makes sense to regard this as part of special relativity, although to some extent this is a matter of convention, I suppose. The important point is that no-one should get the idea that this kinematical decomposition "requires general relativity" or is "a technique which only makes sense in a curved manifold"!
So you don't think that EM and other forces modify the stress-energy tensor and through this modify the curvature of space-time?
 
  • #38
Chris Hillman
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The Lorentz group acts on each fiber of the frame bundle

It's accurate in the sense that the ordinary algebraic equations of SR like [tex]\tau = t \sqrt{1 - v^2/c^2}[/tex] can only be used in inertial frames
This is potentially seriously misleading, although I see that you immediately added a caveat:

as I understand it you can put SR into tensor form so it'll apply in any frame, or you can figure out a different set of algebraic equations for an accelerating frame.
Better yet, consider frame fields on any Lorentzian manifold. Each frame field is a section in the frame bundle, a mild elaboration of the notion of the tangent bundle. In the tangent bundle, the fibers are the tangent spaces to each event, which are "bundled" together smoothly to make a smooth manifold. Similarly, in the frame bundle, the fiber over an event is a vector space which allows us to define, at that event, an orthonormal basis of vectors in the tangent space at that event (in a Lorentzian manifold, this will consist of one timelike unit vector and three spacelike unit vectors), and these fibers are "bundled" together to make a smooth manifold. If the "base manifold" is a four dimensional Lorentzian manifold, the tangent bundle is an eight dimensional manifold, and the frame bundle is a ten dimensional manifold (because it only requires six components to specify the orthonormal frame over each event, so the fibers are six dimensional--- three for the timelike unit vector, two for the first spacelike unit vector, one for the second, leaving no remaining degrees of freedom for the third).

The Lorentz group acts on each fiber of the frame bundle, because we can smoothly rotate/boost the frame at each event. In less fancy language, in the context of physics, frame fields provide the generalization of the kinematics of str to any Lorentzian manifold. The point is, we can certainly apply the Lorentz transformations at the level of tangent spaces, or better, in the fiber of the frame bundle.

Frame fields (elaboration of vector field) can be regarded as a generalization to arbitrary manifolds of the "frames" of str, but even in flat spacetime they are significantly more complicated than the frames used in elementary str (which correspond to "constant frame fields", hence the perennial terminological confusion).

In a given Lorentzian manifold, curved or not, a special propery which a frame field may or many not enjoy is the property of being an inertial frame, in the sense that the timelike vector field is a timelike geodesic vector field. Likewise, an independent property which a frame field may or may not enjoy is the propery of being an irrotational frame, in the sense that the vorticity tensor of the timelike vector field vanishes. Still a third property: some frames are nonspinning frames in the sense that the Fermi derivatives of the spacelike vector fields, taken along the timelike vector field, all vanish.

The "nicest" frames are the nonspinning inertial frames; these are close as we can get, in a curved manifold, to the "Lorentz frames" of elementary str. I stress that even in flat spacetime, there are nonspinning inertial frames which are not Lorentz frames! Irrotational frames enjoy another nice property: they are associated with a family of spatial hyperslices. So the very very nicest frames are inertial nonspinning irrotational.

For example: in the Schwarzschild vacuum, the world lines of the Lemaitre observers (freely and radially falling in "from rest at infinity") can be extended to define (in the right exterior and future interior regions only, or left exterior and future interior regions only!) a nonspinning inertial irrotational frame; the hyperslices are then locally isometric to three-dimensional euclidean space. The world lines of the static observers can be extended to define (in the left or right exterior region only!) an nongeodesic nonspinning irrotational frame.

Just as the tangent bundle has a "dual" notion, the cotangent bundle, the frame bundle has a dual notion, the coframe bundle. In a four dimensional Lorentzian manifold, an orthonormal coframe consists of four covector fields (or one-forms) which are orthonormal at each event, and the Lorentz group also acts on each fiber of the coframe bundle. That is, we can apply Lorentz transformations at each event in the cotangent bundle to rotate/boost a one-form or covector field event-wise, so likewise we can apply Lorentz transformations to rotate/boost a coframe at each event.

I am oversimplifying all of this stuff a bit, in order to try to convey some flavor of this essential construction. For some of the details, see for example Nakayama, Geometry, Topology, and Physics.
 
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