On the Physics of Time Travel

[ydoaPs]

Is there any theory in physics in which the Grandfather Paradox could actually occur? As far as I can tell, there isn't.

 

In General Relativity, for example, time is co-ordinatized. Time and space are inseparable, so an event is just a place in a manifold. The time machines usually talked about are closed timelike curves. That's just a trajectory through spacetime that does a loop to back before the initial endpoint of the trajectory. It's not like going through a closed timelike curve lets one get a do-over. It's the same event. It is what it is. To get the do-over, you'd need some sort of meta-time which is absent in GR. It's simply a fact that, if you go back in time, you cannot kill your grandfather before your parents are conceived. Our present is already the result of any future time travel to our past.

 

There is another kind of time travel that is possible in a multiverse. This is analogous to the "timeline" talk in sci-fi, but not exactly. Time travel to the past, in this time travel, isn't actually time travel at all. It's a wormhole to a different spacetime altogether. You leave our universe and enter a similar one. There's no strict notion for your destination of before or after, since our spacetimes are incommensurable. One could travel to an extremely similar universe to a point in that universe where goings on that have already happened in our universe have yet to occur at one's point of entry into the other universe. This is functionally similar to the "timeline" version of time travel, in that you could "time travel" and kill "your" grandfather before "your" parents were ever conceived.

 

As far as I can tell, the only physics that allows the grandfather paradox scenario is folk physics.

[ajb]

Our present is already the result of any future time travel to our past.

This is one resolution to the problems - consistent histories. It is known to mathematically be okay within classical general relativity due to the work of Russian physicist Igor Dmitriyevich Novikov. But it does not seem 'watertight' (due to technical restrictions) and not everyone agrees with Novikov. The real output of the self-consistency principle/conjecture is that no new physics occurs when CTCs are present.

 

There is the question of free will here, for example.

 

There is another kind of time travel that is possible in a multiverse.

This seems another common resolution. You are right in that you don't time travel back into your history - and so in what sense is it really time travel?

 

 

As far as I can tell, the only physics that allows the grandfather paradox scenario is folk physics.

Or we have to allow some modification of the notion of causality. But few people want to do that

 

Right now - mod the obvious paradoxes - it is not clear that time travel is not allowed in classical general relativity. That is, we don't know if space-times with CTCs are physically realised or not.

 

Then you go to semi-classical gravity (quantum field theory on a curved background) and see that the theory is now well behaved as you approach a CTC. This suggests that time machines are just not allowed - Hawking calls it the chronology protection conjecture. I do not think that this is fully proved, but no counter examples have been found - at least with reasonable fields.

 

The consensus of experts seems to be that quantum gravity is need for the final say.

[Daecon]

Wouldn't time travel violate the 2nd law of thermodynamics? With the Universe as a closed system, wouldn't sending matter and energy back through time cause jumps and dips in the total amount of entropy in the Universe?

Edited June 29, 2016 by Daecon

[ajb]

Wouldn't time travel violate the 2nd law of thermodynamics? With the Universe as a closed system, wouldn't sending matter and energy back through time cause jumps and dips in the total amount of entropy in the Universe from a pre-timetravel moment to a post-timetravel moment?

You might well see energy in and out flow locally. And I don't think anyone has really seen anything like this. Another explanation for such flows could be large extra dimensions and brane worlds - gravitons could remove energy from our brane.

 

Globally as the Universe is expanding we do not have conservation of energy on a global scale.

[Daecon]

I see. I also just realised that there's a difference between a closed system and an isolated one (thanks Wikipedia), I should have referred to the Universe as an isolated system instead, shouldn't I?

[Markus Hanke]

Join two Krasnikov tubes with opposite orientation together - effectively you would get a CTC, without having to get singularities or event horizons involved.

This is an interesting topological structure - I am sure that there is some principle of nature which would prevent this from actually occurring in the real world, but so far as pure classical GR is concerned, this seems like a doable ( and survivable ! ) concept.

[ajb]

I should have referred to the Universe as an isolated system instead, shouldn't I?

I think so.

[ydoaPs]

Wouldn't time travel violate the 2nd law of thermodynamics? With the Universe as a closed system, wouldn't sending matter and energy back through time cause jumps and dips in the total amount of entropy in the Universe?

The laws of thermodynamics aren't stricly true. They're great thubrules, but there are 'violations'. Conservation of energy, for example, is only guaranteed when there is time translation symmetry. GR tells us that this is not the case. The second law is a theorem of neither classical mechanics nor quantum mechanics. One can construct a theoretical "Maxwell's Demon" in both classical and quantum systems.

[Daecon]

Thanks for that, a couple of new things for me to read up about.

 

Other than the Grandfather paradox, another paradox that has always caught my imagination is the Bootstrap paradox, where some information is received from the future, and then later on that same information is what's sent back in time. The paradox being the origin of that information.

[Strange]

Other than the Grandfather paradox, another paradox that has always caught my imagination is the Bootstrap paradox, where some information is received from the future, and then later on that same information is what's sent back in time. The paradox being the origin of that information.

 

 

How do Bill & Ted know Rufus' name ... ?

[lukemcleod]

Ok I have a question about time travel and I'm no scientist so you guys are all a great deal smarter then I am but I do have some understanding of things so no need to dumb it right down for me. So hypothetically speaking let's assume the multiverse and universe never existed untill the Big Bang happened creating the universe but not a multiverse, as time passed intelligent live evolved to a point similar to where we are now and begun contemplating time travel they realise the paradox's that can occur but decide to time travel anyway believing they can avoid any paradox. However they fail and accidentally kill say a grandfather of there's before he has any children. Currently we believe this can't happen because if you killed your grandfather before he had children you could never be born to travel back to kill that grandfather, but what if it could happen and the result of it wasn't just that you had travelled back in time but instead you had created a new universe at the same time as traveling back in time starting the multiverse. You are now trapped in this new univers' time line unable to get back to the future you came from. Is this a possibility? Because if it is don't we have a strong possibility as to why we're not receiving visitations from the future for example the very act of traveling back in Time and doing absolutely anything even as innocent as moving a cup from one side to another or picking up some litter and throwing it in the bin would create a new addition to the multiverse. This would remove you from the universe you come from effectively leaving your universe exactly the same way it was when you traveled back in time. This would mean that when you travel forward in this new universe you created you'll end up in a different future where there are now two of you your faced with a choice now kill the other you take his place and live his life or live on the outskirts of society as homeless person or move far away from where you lived and start a new life for yourself in this new universe. Meanwhile in the universe you came from it would be established that you had simply dissapeared like so many people do or that you discovered time travel and you were lost in time or died trying to make it a reality leaving no trace of your body? Or in the case of killing your grandfather you end up in a future where you were never born meaning u don't technically exists there even though your are physically there this means you would have one option become a homeless person or a criminal as you would not be able to get a bank account or job or anything like passports! Does this idea make any sense at all is it even possible

Edited July 13, 2016 by lukemcleod

[TyroJack]

A couple of corrections(?) to your post lukemcleod (I have had very similar thoughts).

In the primary universe you travel back in time; you are no longer part of that Universe - you time travelled out of it but arriving in your past you do not change the future you create a new one - a new multiverse system.

That new multiverse edition continues from the time you travelled to with you as an integral part of it. You kill your Grandfather yet you do not disappear for you are from another multiverse where you were born. It is in this new multiverse that you cannot be born as your 'Grandfather' had no offspring. No conflicts anywhere.

No, the difficulty comes with the information loop where, for example the secret of time travel is passed from a future self and no one ever discovered it.

[michel123456]

The very simple and logical solution for the grandfather paradox is the following: if you travel 10 years back in time, you will become 10 years younger. That is most natural. Why would you expect to be extracted in a time bubble and suddenly duplicated, looking at yourself as a baby playing with a ball in the back yard?

You will be 10 years younger and having erased from memory 10 years of your life. Of course that would prevent you to travel before your birth (it would be the end of your existence, something like death, surprisingly, before your birth)

After thinking about it, it would be exactly as it is now.

Edited August 18, 2016 by michel123456

[swansont]

!

Moderator Note

This is the physics section, and a physics question was asked. Please post speculation in the speculations section, rather than here.