Is time travel possible?

[ollypenrice]

Since we do not have an entirely satisfactory theory of time the discussion seems premature to me. It is possible that our our past-present-future theory is correct but there are some troubling quantum world observations, are there not, which give us pause? The entangled photons or even Young's double slit experiment. I'm not at all sure that the tensed theory of time (a past, a moving present and a future) is anything like the whole story. Why should it be?

But within the model of time that we have, this is an interesting thought experiment; knock a Ming vase off the table and watch it shatter. Now run the clock backwards to reassemble it. There is a huge assymetry because reassembling it needs organizational energy. Is this assymetry the cause of time's apparent flow? Is it flowing down an organizational slope?

Olly

[NickMilner]

Entropy rules, baby!

[brianb]

we do not have an entirely satisfactory theory of time

Oh? What's wrong with Einsteinian relativity?

[RobH]

Oh? What's wrong with Einsteinian relativity?

Quantum entanglement for one thing. Time and space are inextricably connectede in relativity, but entanglement seems not to have been told that.

Relativity also doesn't explain the arrow of time. We know time runs forwards, not backwards, but all the laws of physics work equally well in either time direction.

This has still not been adequately explained as far as I know.

To say that something is simply not possible is to assume that we know everything, and that, after a scant few hundred years of scientific investigation, we have a true and deep understanding of the nature of reality.

This is simply not so.

All we can say to the OP is that with our current understanding, travelling backwards in time is not possible.

Rob.

[ollypenrice]

Oh? What's wrong with Einsteinian relativity?

Surely an awful lot in the quantum world? Instantaneous communication between particles once observed, as cited by Rob?

And what of Feynman's discovery that an electron cannot be distinguished from a time-reversed positron and vice-versa? These strike me as being strongly suggestive of something not comprehended. Likewise the double slit experiment. We fire photons at a pair of slits (as you know) one at once and they seem to interfere with each other. Maybe they don't know what one at once means? Maybe they live outside time's arrow and are embedded in some other matrix than past-preset-future?

Why, of all the 'obvious' dimensions, does time alone seem to have a preferred direction?

Oh, there could easily be so much wrong with our notion of time. Past, present , future might be the flat earth of the ... ah...

Future! Whoops.

Olly

[NickMilner]

All laws of nature only apply until demonstrated to be wrong. Having said that, relativity has proven remarkably resilient to experiment, and the most successful model of the universe we have, the Standard Model of Elementary Particles which describes nature at the smallest scale, is itself a relativistic quantum field theory.

[brianb]

Surely an awful lot in the quantum world? Instantaneous communication between particles once observed, as cited by Rob?

The many universes theory (Deutsch) fixes that one. There is no instantaneous communication, they always were made in pairs & making the observation of one particle from one of a pair of universes whose history is forking at that instant automatically gives an observation of the other.

We fire photons at a pair of slits (as you know) one at once and they seem to interfere with each other. Maybe they don't know what one at once means? Maybe they live outside time's arrow

Easily understood, the wave packet is distributed in space according to the quantum wave equation of the particle, when the slits are fairly close together part of the wave packet goes through one slit, part through the other (probabilities given by the wave equation) and so self-interference is possible.

Photons do live "outside time" as there is no elapsed time for them. Same applies to anything else moving at the speed of light.

Why, of all the 'obvious' dimensions, does time alone seem to have a preferred direction?

Why, uniquely of all the forces, does gravitation appear to have only an attractive phase?

Actually there is a good heuristic (hand waving) argument as to why time appears to have a preferred direction. It's all to do with entropy (2nd law of thermodynamics) ... if there were exceptions to this law, perpetual motion machines could exist and I think it would be very likely that the existence of such machines would lead to serious instabilities in the fabric of the universe (concentrations of energy being equivalent to concentrations of mass).

[Starblazer]

Doesn't answer the question of time travel.......

But begs the question, does time really exist?

Was our past a 1 time experience that can't be recreated, did it exist and died the moment it was observed. And our future to be the same.

[brianb]

does time really exist?

Was our past a 1 time experience that can't be recreated, did it exist and died the moment it was observed. And our future to be the same.

Yes, yes and infinitely many of them (only one of which will be observed).

[rocketandroll]

I think the old 'if it were possible why is there no evidence of time travelers in the present/past' question is generally answered by the suggestion that you can only travel backwards as far as the date of creation/activation of the time machine.

Not that I want to suggest fiction as a good source of scientific ideas but... Go and watch the film 'Primer'. Apart from being an awesome film... It has one of the best thought out depictions of time-travel ever used in a film... and works on tha principal that you can effectively 'travel backwards' by turning on the machine, letting it run for a few days, getting in it, and getting out of it again when you turned it on.

I wouldn't want to comment on the feasability of that as a concept... But it's more believable than flying Deloreans :-)

Ben

[brianb]

I think the old 'if it were possible why is there no evidence of time travelers in the present/past' question is generally answered by the suggestion that you can only travel backwards as far as the date of creation/activation of the time machine.

This restriction is illogical and gives rise to a whole series of secondary paradoxes. Suppose that, having gone back to the moment you activated your time machine, you decided not to activate it? The only solution seems to be that, once the prototype time machine exists, the universe is caught in a loop with no freedom for variations (like Groundhog Day without the reasuuring ending). I'm not sure how e.g. radioactive decay or light emission/absorbtion from electron energy level change could work in such a universe ... such a universe may be possible under the laws of physics as we know them, but I thing living in one would be impossible, or at least very very boring.

[rocketandroll]

...I think the point is you can only travel back to a point where the machine is running, not a point before you started it where you could decide not to.

The bigger problem would be that there would then be two of you in the past...meaning you had effectively created matter. At that point in time the universe would have 150lbs or so of extra matter in it... which I think is also impossible... Well, not without converting it froman anormous amount of energy.

Anway, don't shoot the messenger, it's not my theory... Just an interesting slant on things and a possible answer to the old 'why are there no time travellers' question.

Ben

[rocketandroll]

....and it doesn't stop Primer still being the best time travel movie and well worth a watch :-)

[ollypenrice]

I'm not sure that the many worlds hypothesis fixes anything for me!

What of Feynman's time reversed positrons/electrons, Brian? Don't they trouble you with respect to our comprehension of time?

Olly

[SteveKinder]

I read a voraciously; high, low and middle brow, including a lot of science fiction. For me one of the most reliable measures of the quality of SF is how the author approaches the Einsteinian speed of light limitation / time travel challenge. The best of these books, while obviously still fiction are extremely well researched and have the benefit of artistic license, imagination and extrapolation.

If you haven’t read it already I strongly recommend Galileo’s Dream by Kim Stanley Robinson it addresses time travel brilliantly. In fact it is a fabulous book all round as it is also a well researched biography of Galileo with a SF thread interleaved into it. I can’t recommend the book highly enough. A “must read” for all Astronomers.

I apologise for the rather lateral response to your question but it is nice to be able to post something other than a please for help for a change.

Oh and while I am on book recommendations The City and the City by China Meilville! One of the best books I have ever read full stop!

Cheers,

Steve.

[brianb]

What of Feynman's time reversed positrons/electrons, Brian?

Theoretical ... just a model ... entropy increaseth (but not at a usefully steady rate)

[Karlos]

I always thought it is a feasible idea, only if we can go faster than the speed of light. Many times I've heard this speed is impossible, but it will only stay impossible if we don't keep trying. Theoretically possible, but not practical! (As of present) I think we have along way to go yet

[Earl]

On SKy Player there is a program which has a section regarding time travel using a laser to manipulate gravity. Interesting but as for good science? I have no clue.

[msinclairinork]

Very intresting thread here.

Here's my thoughts;

At present, theoretical physics sudgest we could at least send a message back in time using particles like Neutrinos or maybe Higgs singlets.

This then leads me to conclude that if we could one day send information then how long would it be until we could send ourselfes at least as information?

So it would go like this...Upload your consciousness to the computer - transfer to particle - send back in time (but send to a computer that is equiped to handle you) Like an e-mail with a LARGE attachment??

Any way joking aside, I have traveled back in time on our flight from holland to the UK.

Await your thoughts...

Michael