Does Einstein's Theory of Relativity Support Time Travel?

[Carrie]

Ronald Mallett, PhD is a renowned physicists known for his proponents on the idea of time travel. Mallett uses his background in physics and mathematics to support his idea to build a time machine, using Einstein's Theory of Relativity as a basis.

Do you think this is even possible? Right now, Mallett is testing and measuring the speeds of neutrons in light beams in attempts to create a valid and working time machine within the next ten years...

Ron Mallett

 

[timo]

Ten years? Time machines were invented much later. Around 2042, if I remember correctly.

[DrRocket]

Ronald Mallett, PhD is a renowned physicists known for his proponents on the idea of time travel. Mallett uses his background in physics and mathematics to support his idea to build a time machine, using Einstein's Theory of Relativity as a basis.

 

Mallet is renowned for being a nut.

 

 

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

 

You too can join Spike Lee and contribute to this "research". Or, ....

[ajb]

It is fair to say that within classical general relativity it is not obvious that time travel is not allowed.

[DrRocket]

It is fair to say that within classical general relativity it is not obvious that time travel is not allowed.

 

Nicely weasel worded.

 

 

http://prd.aps.org/abstract/PRD/v46/i2/p603_1

[ajb]

Nicely weasel worded.

 

 

http://prd.aps.org/abstract/PRD/v46/i2/p603_1

 

So, Hawking's conjecture is based on semi-classical gravity, that is quantum field theory on a curved background space-time. Here is seems that the expectation value of the energy-momentum diverges near CTCs. Thought, to my knowledge, there is no general proof of this.

 

Also, quantum effects of gravity could presumably make this much more complicated. One possibility is that the expectation value of the energy-momentum gets regulated near CTC's and then time machines could be realised! Or at least on the microscopic scale.

 

So, right now I would say that

1. classical general relativity probabily does not disallow CTCs
   
2. Semi-classical gravity suggests that CTCs are not allowed
   
3. Quantum gravity (whatever that is!) will have the final say on CTCs
   

[DrRocket]

So, Hawking's conjecture is based on semi-classical gravity, that is quantum field theory on a curved background space-time. Here is seems that the expectation value of the energy-momentum diverges near CTCs. Thought, to my knowledge, there is no general proof of this.

 

Also, quantum effects of gravity could presumably make this much more complicated. One possibility is that the expectation value of the energy-momentum gets regulated near CTC's and then time machines could be realised! Or at least on the microscopic scale.

 

So, right now I would say that

1. classical general relativity probabily does not disallow CTCs
2. Semi-classical gravity suggests that CTCs are not allowed
3. Quantum gravity (whatever that is!) will have the final say on CTCs

 

I don't have access to the whole paper, but I read the abstract a bit differently.

 

1. GR plus the weak energy condition disallows CTCs

 

2. Quantum theory might allow some violation of the weak energy condition, but (the expected value of) the stress energy tensor would be very large (indicating large local curvature).

 

3. Quantum gravity (whatever that is!) will have the final say on CTCs.

 

 

Note: 1. GR by itself is known to admit spacetimes with CTCs.

 

2. Mallet is still a nut.

[ajb]

I don't think we are disagreeing on anything fundamental.

 

In the context of classical GR you have to decide if space -time with CTCs are "physical". To decide if a space-time is physical you have to add other constraints like various energy conditions or insist that the space-time be globally hyperbolic. So, in this sense classical GR by itself does allow CTCs. But adding other physical conditions may mean they are not realised in nature, like the weak energy condition you mention. So, in classical GR it is not at all obvious that time travel is not allowed.

 

In fact the situation for semi-classical gravity is very interesting. It is very difficult to build quantum theories that do not violate the weak energy condition. So, the weak energy condition is not likely to hold at the microscopic level.

 

As for Mallet, I have not really made my mind up about him. However, his is certainly out on a limb actually trying to build a time machine. I have not looked into his analysis of the proposed experiment. I suspect that some effect he has not taken care of will spoil the time machine. It could be very interesting to find out exactly why it won't work.

 

People I know who investigate time machines (my old MPhys supervisor was one) are not really trying to contact the past, rather they are trying to push GR to its limit in order to point at new physics. Maybe even hints at what quantum gravity should be.

Edited February 26, 2011 by ajb

[DrRocket]

I don't think we are disagreeing on anything fundamental.

 

In the context of classical GR you have to decide if space -time with CTCs are "physical". To decide if a space-time is physical you have to add other constraints like various energy conditions or insist that the space-time be globally hyperbolic. So, in this sense classical GR by itself does allow CTCs. But adding other physical conditions may mean they are not realised in nature, like the weak energy condition you mention. So, in classical GR it is not at all obvious that time travel is not allowed.

 

In fact the situation for semi-classical gravity is very interesting. It is very difficult to build quantum theories that do not violate the weak energy condition. So, the weak energy condition is not likely to hold at the microscopic level.

 

As for Mallet, I have not really made my mind up about him. However, his is certainly out on a limb actually trying to build a time machine. I have not looked into his analysis of the proposed experiment. I suspect that some effect he has not taken care of will spoil the time machine. It could be very interesting to find out exactly why it won't work.

 

People I know who investigate time machines (my old MPhys supervisor was one) are not really trying to contact the past, rather they are trying to push GR to its limit in order to point at new physics. Maybe even hints at what quantum gravity should be.

 

There is no question that GR, without some sort of additional constraint like an energy condition, admits CTCs. Several solutions are known. A prohibition of physical CTCs, i.e. CTCs in the real universe, in the context of general relativity is the "Chronology Protection Conjecture". That conjecture remains a conjecture, but I think it fair to say that it would be a major surprise if it were shown to be false. We have been surprised before.

 

There are certainly legitimate physicists who have worked on or are currently working on the problem. Hawking and Thorne leap to mind. Mallet doesn't.

 

Mallet's approach has apparently been looked at in detail and errors identified -- see comments in the wiki article noted in an earlier post.

 

You might find this piece by Thorne interesting.

 

http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf

 

One question that I have, and have no clue to the answer, is whether CTCs are allowed in Einstein-Cartan theory. I know no one who knows much about EC theory.

[ajb]

A prohibition of physical CTCs, i.e. CTCs in the real universe, in the context of general relativity is the "Chronology Protection Conjecture". That conjecture remains a conjecture, but I think it fair to say that it would be a major surprise if it were shown to be false.

 

Using the chronological protection conjecture to outlaw CTCs as unphysical is just the statement that the conjecture is true.

 

There is some evidence based on semi-classical gravity.

 

But I don't think one should employ the conjecture absolutely. This of course does not mean that I think CTCs are necessarily physical in the context of GR.

 

We have been surprised before.

 

I would not be surprised if one could cook up things, but they would probably be declared unphysical on some other grounds. To my knowledge all time machines simply fail or require exotic matter or some other unphysical conditions. In fact understanding these strange configurations is itself a worthwhile pursuit.

 

 

http://www.its.caltech.edu/~kip/scripts/ClosedTimelikeCurves-II121.pdf

 

I will have a look Thorne's words. (I met Thorne a couple of years ago at Cardiff, he is a good speaker)

 

One question that I have, and have no clue to the answer, is whether CTCs are allowed in Einstein-Cartan theory. I know no one who knows much about EC theory.

 

I expect they would still be allowed, again without the imposition of other constraints. But i don't know this, have a good hunt through the arXiv. Promise to tell us what you find.

[DrRocket]

Using the chronological protection conjecture to outlaw CTCs as unphysical is just the statement that the conjecture is true.

 

There is some evidence based on semi-classical gravity.

 

But I don't think one should employ the conjecture absolutely. This of course does not mean that I think CTCs are necessarily physical in the context of GR.

 

Absolutely.

 

A conjecture should never be taken as a fact.

 

I tend to think the conjecture is true, but that simply means that I think an attempt to prove the conjecture has a better chance of success than a project to build a time machine. If I were looking for a counter-example I would start by learning about Kerr black holes.

[ajb]

If I were looking for a counter-example I would start by learning about Kerr black holes.

 

The Taub-NUT space-time is a counter example to everything.

[Xerxes]

Sorry to butt in on a very interesting discussion between guys who obviously know their stuff, but I have a slight worry.

 

First I am not a physicist and I am most emphatically not a philosopher, but I am familiar with Einstein's field equations.

 

So it seems there are solutions to these equations (I believe Goedel found one) that allow CTCs, which you guys are more-or-less dismissing as "non-physical".

 

My question: What is the basis for this dismissal as being non-physical? Is it experimental? Or is it that you just don't think the universe works that way? Is this is a good argument? Is there a better one? Or am I just making a fool of myself in an area where I am a complete baby?

 

Don't get me wrong: I am as sceptical about time travel as the next moron, but I am struggling to follow the argument here

[DrRocket]

Sorry to butt in on a very interesting discussion between guys who obviously know their stuff, but I have a slight worry.

 

First I am not a physicist and I am most emphatically not a philosopher, but I am familiar with Einstein's field equations.

 

So it seems there are solutions to these equations (I believe Goedel found one) that allow CTCs, which you guys are more-or-less dismissing as "non-physical".

 

My question: What is the basis for this dismissal as being non-physical? Is it experimental? Or is it that you just don't think the universe works that way? Is this is a good argument? Is there a better one? Or am I just making a fool of myself in an area where I am a complete baby?

 

Don't get me wrong: I am as sceptical about time travel as the next moron, but I am struggling to follow the argument here

 

The solutions of the Einstein field equations depend on the distribution of matter/energy in the universe. Exact solutions, and Godel's spacetime is an exact solution, are known for only a handful of assumed mass/energy distributions. Godel assumed a homogeneous distribution of swirling dust (particles that interact only through gravity. This solution does not exhibit Hubble expansion, unlike the "real" universe. http://en.wikipedia.org/wiki/G%C3%B6del_metric

[Xerxes]

DrRocket flatters me with his link. I only partly understood it, and even then only after much beard-tugging.

 

Ah well. First forum rule: do not ask questions whose answers you are not equipped to understand!

[Schrödinger's hat]

DrRocket flatters me with his link. I only partly understood it, and even then only after much beard-tugging.

 

Ah well. First forum rule: do not ask questions whose answers you are not equipped to understand!

 

Nonsense! What better way to get encouragement to go find the equipment? Just don't make presumptions about what the answers should be and you'll do fine.

[ajb]

My question: What is the basis for this dismissal as being non-physical? Is it experimental? Or is it that you just don't think the universe works that way? Is this is a good argument? Is there a better one? Or am I just making a fool of myself in an area where I am a complete baby?

 

 

You normally have to worry about if the matter content is "physical", so obeys what you would expect it to (have positive mass for instance) or impose some topological constraint like global hyperbolicity (or both).

[DrRocket]

You normally have to worry about if the matter content is "physical", so obeys what you would expect it to (have positive mass for instance) or impose some topological constraint like global hyperbolicity (or both).

 

If you assume global hyperbolicity the question of existence of CTCs is settled immediately -- they don't.

[ajb]

If you assume global hyperbolicity the question of existence of CTCs is settled immediately -- they don't.

 

Exactly.

[The Conqueror]

If time travel was possible, then certainly if anyone invents time travel machine say in 2042 then he should be talking with us!!

Edited February 28, 2011 by The Conqueror

[ajb]

If time travel was possible, then certainly if anyone invents time travel machine say in 2042 then he should be talking with us!!

 

Not if you can only go back as far as to when the time machine was turned on.

 

Anyway, one should also be interested in the possibility of microscopic time machines. It may be the case that macroscopic time machines exists and fundamental particles are wizzing back and fore in time!

 

Really, time machines should be seen as pushing our understanding of classical, semi-classical relativity and our notion of causality to the limit, rather than real attempts at contacting the past, Mallett is rather the exception than the rule.

 

Graham Shore (my old MPhys supervisor) wrote a nice review of time machines a few years ago, "Constructing Time Machines". Int. J. Mod. Phys. A, Theoretical. The arXiv version can be found here.

 

To paraphrase Brian Cox " even if the laws of physics do not prohibit time travel, that doesn’t mean to say it’s going to happen".