Time Travel

[whereagles]

Re-hi,

Wormholes are just one way of time-traveling. In the last post I assumed they were the *only* way because the point I wanted to make didn't need more than that.

As for what is plausible and not.. well, again I was just transmitting what scientists currently believe. I don't think scientists dismiss other explanations, but they have to start somewhere and right now only nr.1 and nr.2 are only sensible starting points. That's certainly not nonsense.

[kandrathe]

Actually, I think the answer would be in experimentation. I think as Pete had alluded, a device is neccesary. Once you have figured out a workable hypothesis on how to manufacture and move something like a probe through the "wormhole" if that is your theory, then you need to prove that it worked by either retrieving the probe, or by some other observable method. For experimentation with time, seconds or minutes are sufficient. If you send an object back in time, you would already be past it in the time continuum and so unable to observe it. You might theorize that it exists in the past in an alternate universe, but as you imply it is unobservable. If you were able to send an object forward in time you might observe the item winking out, and then winking into existence moments later, that is of course if you are able to send it into the future of your own universe.

IMO, the secrets of the nature of time travel are embedded in a detailed understanding and ability to control gravity.

Here is an opinion from a September 2002 Article from Scientific American.-

[Kartoffelsalat]

I didn't read every post here, so if somebody already said any of this, well... never mind.

I read that a time machine would not actually travel across time but would instead warp the space-time continuum. It could only start warping time as soon as it was invented. So you could only travel back in time to the instant your time machine was complete. You couldn't kill your grandparents if you invented the machine, but you could do it (I suppose) if you used somebody else's who invented it before you were born.

Also, I read somwhere that it has been proven (I have no idea how) that one could build such a machine. The guy who proved this came up with a parts list, which included an infinitely long spinning cylinder. Nobody that I know of has found one of these yet, but if you've got one laying around, go ahead and post and let me know.

[Thecla]

No, it does not. It does not address that question at all. What it does say is that mass is a form of energy.

Well, some form of conservation of energy is a consequence of Einstein' general theory of relativity: the Einstein field equations (G = Einstein curvature tensor, T = energy-momentum tensor):

G = 8 pi T

impy that Div T = 0 (via the Bianchi identities), which is a form of energy-momentum conservation; but energy is a particularly subtle concept in general relativity, because the energy of the gravitational field cannot be localized. In special relativity (flat space-times), or classical mechanics, conservation of energy is much more straightforward, and in all cases it's connected with the fundamental concept (via Noether's theorem) of the invariance of the laws of nature under time translations.

Kurt Godel's solution of the Einstein field equations showed that general relativity allows universes in which time travel can occur, though whether it can really happen, who knows?

[Vandiablo]

The guy who proved this came up with a parts list, which included an infinitely long spinning cylinder.  Nobody that I know of has found one of these yet, but if you've got one laying around, go ahead and post and let me know.

I don't like to mention it because people get, you know, jealous, but yes I assure you the one I have is infinitely long.

But let me tell you that while you may dream that your own was infinitely long, it is not the absolute joy that you imagine it would be - there are drawbacks! The main benefit is the confidence of knowing that no one, NO ONE, has one longer than yours.

[--Pete]

Hi,

Well, some form of conservation of energy is a consequence of Einstein' general theory of relativity: the Einstein field equations (G = Einstein curvature tensor, T = energy-momentum tensor):

G = 8 pi T

impy that Div T = 0 (via the Bianchi identities), which is a form of energy-momentum conservation;

Unless I'm mistaken, it is an energy conservation in the sense that there can be no net flow of energy into or out of the universe. Making it more in the nature of a boundary condition. However, I believe that it really does not address the appearance or disappearance of energy within the universe. Which, of course, can happen for short times according to the time-energy commutation relationship. If one loophole, why not more?

In special relativity (flat space-times), or classical mechanics, conservation of energy is much more straightforward, and in all cases it's connected with the fundamental concept (via Noether's theorem) of the invariance of the laws of nature under time translations.

Yes, if the laws of nature are invariant under time translations then a quantity is conserved that can be identified as energy. However, there is some question of the time invariance of physical laws. Hence the question of the constancy of G over the age of the universe.

It's been almost 30 years since I've thought of Emmy and her theorem. Every invariance in four space corresponds to a conservation law -- a beautiful theoretical concept. So, displacement gives us momentum, rotation gives us angular momentum, and so on. In practice, it just gives us an additional way of finding or testing conservation laws. It's been too long. How does Noether's Theorem apply to the quantum conservations? It seems that I remember it as part of a classical mechanics course and the direct application to quantum, if any, eludes me. Of course, quantum has a very similar basis as reflected in the various commutative relations.

So, yeah, perhaps my statement was too general. However, neither special not general relativity really address the question of a sudden change in energy caused by an object being translated in time. Perhaps, we could extend the theory of an anti-particle being a particle going back in time (CPT conservation). The world line of the particle forms an "N" with extended start and finish. Before the "loop back" and after, there is only the one particle. During the "loop back", there are two particles and an antiparticle. When all the conserved quantities are summed, the two conditions are "identical".

Considerations of that type could, possibly, lead to some postulated limitations on time travel.

--Pete

[Thecla]

Unless I'm mistaken, it is an energy conservation in the sense that there can be no net flow of energy into or out of the universe. Making it more in the nature of a boundary condition. However, I believe that it really does not address the appearance or disappearance of energy within the universe.

Well, standard conservation of energy says that: rate of change of energy in an arbitrary region = flux of energy through the boundary of the region; but in general relativity it doesn't quite work that way because you can't localize the energy in the gravitational field (you can always choose a reference frame in which the gravitational field vanishes locally). But you can assign a definite mass/energy to an isolated object, like an isolated black hole, even though it's just one item in the universe.

Yes, if the laws of nature are invariant under time translations then a quantity is conserved that can be identified as energy. However, there is some question of the time invariance of physical laws. Hence the question of the constancy of G over the age of the universe.

Even if G is constant, then you don't have invariance under time translations if the universe is expanding, and this is closely related to the difficulties in defining energy conservation in general relativity (space-time is not invariant under time translations).

It's been almost 30 years since I've thought of Emmy and her theorem. Every invariance in four space corresponds to a conservation law -- a beautiful theoretical concept. So, displacement gives us momentum, rotation gives us angular momentum, and so on. In practice, it just gives us an additional way of finding or testing conservation laws. It's been too long. How does Noether's Theorem apply to the quantum conservations? It seems that I remember it as part of a classical mechanics course and the direct application to quantum, if any, eludes me. Of course, quantum has a very similar basis as reflected in the various commutative relations.

Yup, there's a beautiful quantum mechanical version of Noether's theorem (described, for example, in Vol III of Feynman's lectures on physics), which in many ways is simpler than the classical version: any operator that commutes with the Hamiltonian (which generates the time evolution of a quantum system) generates a conservation law. But no one really knows what's the physically correct combination
of quantum theory and general relativity (gravitation), though string -- or membrane -- theorists may disagree.

So, yeah, perhaps my statement was too general. However, neither special not general relativity really address the question of a sudden change in energy caused by an object being translated in time. Perhaps, we could extend the theory of an anti-particle being a particle going back in time (CPT conservation). The world line of the particle forms an "N" with extended start and finish. Before the "loop back" and after, there is only the one particle. During the "loop back", there are two particles and an antiparticle. When all the conserved quantities are summed, the two conditions are "identical".
Considerations of that type could, possibly, lead to some postulated limitations on time travel.

Well, my only real point is that there's no conclusive objection to time travel based on energetic considerations: the cosmologies that allow time travel allow it by global, topological means -- you travel around the universe, or some portion of it, and return earlier than when you started. There's no mention of ancestors, or descendents that kill them, in the theory of general relativity, any more than Schrodinger's cat is an integral part of the in the theory of quantum mechanics.

[Kartoffelsalat]

I don't like to mention it because people get, you know, jealous, but yes I assure you the one I have is infinitely long.

But let me tell you that while you may dream that your own was infinitely long, it is not the absolute joy that you imagine it would be - there are drawbacks! The main benefit is the confidence of knowing that no one, NO ONE, has one longer than yours.

OK, so you have an infinitely long cylinder. But is it spinning? :blink:

[Vandiablo]

OK, so you have an infinitely long cylinder. But is it spinning?

Of course. Not much point in keeping an infinitely long one if you never use it.

[--Pete]

Hi,

you can't localize the energy in the gravitational field (you can always choose a reference frame in which the gravitational field vanishes locally).

IIRC, as measured from an accelerated frame that cancels the local field, the gravitational energy of the rest of the universe increases just enough that the total stays constant. Thus, it is meaningless to ask for the energy distribution but not for the total energy in the universe.

But you can assign a definite mass/energy to an isolated object, like an isolated black hole, even though it's just one item in the universe.

If you are talking rest mass, I agree. However, just from the special theory that statement is not true for total energy. Consider two observers, each in an inertial frame moving relative to each other. Each will assign a different value to the total energy of that black hole. So either I'm missing something or I disagree with that statement.

Even if G is constant, then you don't have invariance under time translations if the universe is expanding, and this is closely related to the difficulties in defining energy conservation in general relativity (space-time is not invariant under time translations).

I'm a little bit confused here. The system (the universe) is not invariant under time translations. That has been pretty well established, the expansion of the universe not really being in doubt. But simply because the system is evolving does not mean that the laws governing the system are changing. From what I remember of Noether's theorem, the basic premise is that if the *origin* of the reference frame can be moved, then something (the conjugate variable in the Lagrangian sense, IIRC) related to that axis is conserved. Since the absolute time of an event doesn't matter, just the relative time between two events as measured in some frame, then unless the physical laws change between those two events, energy is conserved.

Yup, there's a beautiful quantum mechanical version of Noether's theorem (described, for example, in Vol III of Feynman's lectures on physics),

Thanks. I'll have to pull those red bastards out, blow the dust off, and give them another read :) Last time I went through them was Summer '76 for my prelims. I wish I knew as much physics as Feynman forgot during his life :)

But no one really knows what's the physically correct combination of quantum theory and general relativity (gravitation), though string -- or membrane -- theorists may disagree.

From the literature I've perused, I'd say that a lot of people think they know how to combine them. Of course, they all have different ideas, many contradictory, and none developed to the point that will meet the empirical test. But it gives a lot of smart folks a great game to play for now ;)

Well, my only real point is that there's no conclusive objection to time travel based on energetic considerations

In that we are in complete agreement.

There's no mention of ancestors, or descendents that kill them, in the theory of general relativity, any more than Schrodinger's cat is an integral part of the in the theory of quantum mechanics.

Right. However, sometimes, it is necessary to relate things which we can neither observe directly nor test conclusively to our everyday experience. It isn't so much a test of a theory as of its interpretation. Gedanken experiments are a useful way of exploring possibilities or clarifying concepts. Neither too much nor too little emphasis should be placed on them. After all, nature's logic may not be (probably isn't) ours.

--Pete

[--Pete]

Hi,

Or perhaps, more to the point, "Where's the point?" For without a point, it seems pretty pointless.

--Pete

[Mark]

The history of physics is a long timeline of enlightened thinkers, the noted highlights being Newton and Einstien (this of course is up for debate)

I'm curious who will be the next brilliant man (or woman) to 'redefine' our outlook and change the facts we now understand by broadening our horizons or changing our persepectives. Perhaps what we know now is wrong or only half the story, due to our limited point of view.

It won't be me, I'm afraid of the consequences of being that brilliant. Then again, what choice does anyone have in that regard.

Mark