Is time travel possible?

[cps012]

Serious question: does Einstein's Theory of Relativity support the idea that time travel is possible? I recently watched a segment where physicist, Ron Mallett, PhD discusses his ideas for a real time machine, and supports his theories with mathematical and scientific research. As for the reality of the idea, I am still unsure... what do you think? I really would like to hear other people's opinion on the ideas and the video.

[Stephen]

Time travel is perfectly possible, approach the speed of light and time slows down for you ergo you travel into the future

You don't even have to travel all that fast, GPS satellites experience this phenomenon and have to adjust themselves accordingly so your GPS works on the ground.

[ronin]

Time travel is perfectly possible, approach the speed of light and time slows down for you ergo you travel into the future

Wouldn't that just be that time passes at a different rate? Not exactly time travel.

Would not quite be the same as lets hop into the tardis and go to the Battle of Hastings, or the Fall of Rome.

[Stephen]

Wouldn't that just be that time passes at a different rate? Not exactly time travel.

Would not quite be the same as lets hop into the tardis and go to the Battle of Hastings, or the Fall of Rome.

Granted, depends on the definition of time travel I guess

[NickMilner]

Forward travel happens all the time. We just did it.

Backward travel doesn't work because the universe protects causality.

[Barry]

Of course time travel is possible; we are all travelling into the future! Einstein's theories and subsequent observations have also shown that the rate of travel can be altered.

I am not in any way qualified to comment on time travel into the past but I am of the opinion that it is not possible, because no-one has come back in time (at least not that is known about - see also Prof Stephen Hawking's invitation) and because of the obvious risk of paradoxes.

Prof Mallet seems to be driven by a personal need to travel back.

Barry

[RobH]

From what I've read, it's possible, but the engineering is way beyond what we can even start to think about.

I seem to remember it involves either creating a wormhole, or using a black hole or some such thing, and you could only travel back to when you'd first activated your machine.

Checking out any out-of-place police box is probably worth a try though

Rob

[pook]

Time travel in the classic Sci-Fi sense is not possible no, and we've proved it. Einstein is absolutely correct, and travelling at extreme velocity does indeed make time travel faster for the traveller, and this has been proved. However, if this was actually "Time Travel" in the sense you refer to, when all astronauts return to earth we'd not be able to see them any more, because they'd be in the future and no longer in our "time" if that makes sense.

Time is not something linear that can be fast forwarded or rewound, it is merely something relative to us that we measure out lives by, but devices to record the passage of time differently at extremes of velocity and altitude.

[smudgeball]

Checking out any out-of-place police box is probably worth a try though

I'll keep a look out !

[brianb]

There was a young girl called Miss Bright

Who could travel much faster than light

She departed one day

(in a relative way)

And came back on the previous night

(I don't know who the author of this awful limerick is, it's been around for a terribly long time but was clearly written after 1905.)

[smudgeball]

shall I get your coat for you ????

[JamesF]

AFAIR in once of his recent interviews Brian Cox said that according to current theories it would be possible to travel into the future, but not into the past. By travel into the future I assume he meant faster than the normal rate.

James

[brianb]

AFAIR in once of his recent interviews Brian Cox said that according to current theories it would be possible to travel into the future, but not into the past. By travel into the future I assume he meant faster than the normal rate.

Well the fact that Brian Cox said it doesn't make it any more (or less) believable. However this is the truth according to special & general relativity.

Accelerate to a higher speed and you will travel into the future faster. Go into a stronger gravitational field and you will travel into the future faster.

If a time machine can be built, why are there no fossilized skeletons of time travelers from our future who have traveled into our past and died there? Even if our putative time travelling descendants were clever enough to avoid accidents, surely they'd have been careless enough to drop some litter?

Sorry guys. "The Terminator" is a great film, but the idea of sending someone or something back into the past to change history - intentionally or otherwise - is fiction and will always remain so.

[JamesF]

Sorry guys. "The Terminator" is a great film, but the idea of sending someone or something back into the past to change history - intentionally or otherwise - is fiction and will always remain so.

Yebbut the fact that *you* said it doesn't make it any more (or less) believable

And on that note, who remembers "The Flipside of Dominick Hyde"?

James

[brianb]

Yebbut the fact that *you* said it doesn't make it any more (or less) believable

Quite. Einstein said it too, but Einstein was wrong about many things. It so happens that denying the possibility of a practical HG Wells type time machine is agreed by all conventional physicists ... so why pick out Brian Cox, as though his sayings have the authority of The One True Prophet?

[johnb]

YES of course its possible just not for us, both back and forward - our issue is that given current thinking and know how etc we say no you cant etc... give it time. Science has moved a long way in the past 100 years - expand thinking free the mind and yet none of us can even contemplete what life, understanding and technology will be like in say 1000 years. So possible yes just not for us - YET

[NickMilner]

Well the fact that Brian Cox said it doesn't make it any more (or less) believable.

Than who? I'd say his word carries more weight than, say, mine.

[JamesF]

Because I happen to remember him specifically talking about the current views on the feasibility of time travel last week, or the week before. And whilst he may get as many, if not more, things as wrong as Einstein, he is perhaps more likely to be aware of the current views on it. I mentioned that it was him purely to convey that the comment was made by someone who might reasonably be expected to have some kind of clue as to what they were talking about. If I'd said it was Katie Price somehow I don't think anyone would be particularly inclined to give significant weight to her opinion.

No idolatry of any kind was intended.

James

[ixalon]

As far as I'm aware there's no proof that backwards time-travel is impossible? (though the consensus and good old occam's razor seem to suggest that it is impossible).

There's a lovely hypothetical method of backwards time travel by moving one end of a (hypothetical) traversable wormhole, so it moves into the future faster than the other end (thanks to relativity). Then sending information from the 'future' end to the 'past' end.

Makes me think that either traversable wormholes can't exist, or moving one end of them would be impossible. But... you never know!

[tonyh66]

Yes it is

[tonyh66]

I'll tell you in a minute.................

[tonyh66]

Im back.

[yeti monster]

Because I happen to remember him specifically talking about the current views on the feasibility of time travel last week, or the week before.

Maybe it was both...

Or was it next week? How can we be certain?

[George Jones]

Serious question: does Einstein's Theory of Relativity support the idea that time travel is possible?

It is certainly allowed by Einstein's theory of general relativity, as there are numerous solutions to Einsten's equations that have closed timelike curves. A closed timelike curve means that for at least one observer, time loops back on itself.

There's a lovely hypothetical method of backwards time travel by moving one end of a (hypothetical) traversable wormhole, so it moves into the future faster than the other end (thanks to relativity). Then sending information from the 'future' end to the 'past' end.

Makes me think that either traversable wormholes can't exist, or moving one end of them would be impossible. But... you never know!

Yes, some of these solutions involve traversable wormholes, there is no observational evidence for the existence of traversable wormholes. A stable wormhole requires "exotic" matter to hold it open, and exotic matter is such that some observers measure its density to be negative. There are some hints from quantum theory that exotic matter is theoretically possible, but not everyone agrees on how much is needed to hold a wormhole open, how much is possible, etc. This is an area of active research.

There are other solutions of Einstein's equations that allow time travel, but that don't require wormholes or exotic matter, e.g., Godel's universe.

How does one deal with the paradoxes associated with time travel? Matt Visser has written about this. He talks about four possibilies:

1. Radically rerwite physics from the ground up.

2. Permit time travel, but also invoke consistency constraints that situations like going back in time and killing one's grandparents.

3. Quantum physics intervenes to prevent time travel.

4. Invoke the Boring Physics Conjecture, where we assume (until forced not to) that our particular universe is globally hyperbolic (a technical nicety condition), and thus doesn't have closed timelike curves.

In the past 4. was often assumed, but since global hyperbolicity is a very strong global condition and Einstein's equations are local equations, many physicists have moved to 2. and 3.

If a time machine can be built, why are there no fossilized skeletons of time travelers from our future who have traveled into our past and died there? Even if our putative time travelling descendants were clever enough to avoid accidents, surely they'd have been careless enough to drop some litter?

Stephen Hawking likes 3., for example, and has formulated the Chronology Protection Conjecture, "It seems that there is a Chronology Protection Agency wich prevents the appearance of closed timelike curves and so makes the universe safe for historians. ... There is also strong experimental evidence of this conjecture from the fact that we have not been invaded by hordes of tourists from the future."

This roughly states that near regions of spacetime that allow time travel, either matter or radiation density become infinite, thus preventing (by wall-of-fire barriers) physical objects from crossing into these regions. There seems to be some theoretical evidence for this conjecture, but a fully quantum theory gravity is likely needed to know for sure.

I recently watched a segment where physicist, Ron Mallett, PhD discusses his ideas for a real time machine, and supports his theories with mathematical and scientific research. As for the reality of the idea, I am still unsure... what do you think? I really would like to hear other people's opinion on the ideas and the video.

I don't think that the physics community takes Mallet's work very seriously,

[brianb]

I'm open to suggestions as to how backwards time travel might be achieved but unless and until someone demonstrates a practical device that unambiguously transports at least one particle a measurable time into the past, I'm going to stick to my belief that quantum physics disallows it, except perhaps in a universe where "history" repeats itself in an endless loop. Fortunately we do not appear to occupy such a universe.