Time travel talk

[Lithrael]

Look, saying it's impossible is not playing the game right. You can't have a 'what if you could travel backwards in time' discussion if your opening gambit is 'you can't travel backwards in time.'

 

[Psi Baba]

Another problem is one called, "Information from nowhere": If you travel back in time to before Einstein developed his famous equation and tell him about it and what it means, then that means Einstein got the idea from you, but you got the idea from Einstein. So, where did the idea actually come from?

 

[gnome]

Here's an interesting one.

I find a watch behind a plant in a restaurant.

Ten years later I invent time travel, go back in time ten years and a day, and replace the original watch (before it was found) with the watch that is now ten years older, leaving it there for me to find.

If that would change the watch, it would age instantly until it was so run down I would have left it for worthless. But then it gets worse--then I haven't taken the watch, so what am I doing there ten years of my own personal time later?

Even odder--what if you went back to place the watch and found nothing there to replace--suggesting that the watch came from you in the first place.

Stable time loops give me a headache. I prefer alternate timeline explanations

 

[gnome]

Another problem is one called, "Information from nowhere": If you travel back in time to before Einstein developed his famous equation and tell him about it and what it means, then that means Einstein got the idea from you, but you got the idea from Einstein. So, where did the idea actually come from?

Alternate timeline Einstein got it from you, who got it from original timeline Einstein. It explains so much easier with alternate timelines.

One could expand the idea of conservation of matter and energy to suppose that any particular timeline is no longer a closed system, but matter and energy in the "Multiverse" is still conserved.

 

[William Parcher]

Look, saying it's impossible is not playing the game right. You can't have a 'what if you could travel backwards in time' discussion if your opening gambit is 'you can't travel backwards in time.'

Oh, you mean the thought game set up in the OP. The idea that time travel is a kind of technology not available to us as of yet.

But that game begs a question that it doesn't outline. Is mushy talking about other extraterrestrial civilizations visiting us with their time machine, or us visiting ourselves?

 

[Lithrael]

Mushy's question is pretty straightforward. First you have to define whether if viewed form a hypothetical 'outside time' any time travel events would be permanent parts of the universe, or if they'd be scribbled in ad-hoc as time travel occurred. The question assumes the former.

Imagine time-n-space is a roll of clay, and time travelers are kebab skewers in that roll of clay. Is time travel a set part of time-n-space (skewers were always there in the roll - the roll was never in an un-skewered state)? Or is time travel happening to space-n-time (skewers get pushed into the roll - so there was once a state where there was no skewer and time travel happens and now there's a state where there is a skewer)?

And then he's asking that if it's the case that time travel is set in space-n-time, does that mean there's no free will, because it would seem to follow from that that space-n-time is set in general.

 

[12AX7]

The TV series "7 days" had it that there was never enough power available to take journeys further into the past than 7 days.

I'd still take that.

 

[Checkmite]

I'm sure it's some kind of violation, but it's not a violation of creation of matter and energy - you're bringing it with you. You already existed. Yes, it's a case of some matter and energy showing up where it's not supposed to be. But that matter and energy is not being created. It's being moved. If you put a soda can down on a table, there's a can there where there wasn't before, but you didn't create it there.

You're not understanding exactly what's being argued.

Taking your time machine back in time to a place where all the materials used to build it (and you) already still exist in some other form means you would have necessarily just increased the mass of the universe by the total mass of yourself and your machine. It is impossible to increase the mass of the universe (law of conservation), therefore, time travel to the past is impossible.

 

[Soapy Sam]

Not necessarily.
Our conversation laws are assumed to hold solely for any moment in time, but maybe they hold over the lifetime / extent of the universe.
If we move a kilo of metal from 2090 to 1990, there is 2kg in 1990, but none in 2090.
It may be that this is "good enough" to satisfy the conservation police.
This raises the question of duration.

If the object moves from 2090 to 1990 and stays there ten days, we might expect our extended conservation rule to require that it be absent from 2090 for the same duration.

So what happens if it is moved back (forward) to the moment from which it went back? Or for that matter, what happens if it gets caught in a Tipler loop and spends half it's time going back to when it just started from? And if it gets back when it started from, does it do so before it starts, or after?

The process of travelling in spacetime itself takes time (unless you are a photon). If you go back a day, but doing so takes an hour, did you go back 23, 24 or 25 hours?
If I repeat your trip, starting an hour later, but arrive at the same time, did it take me less time , or more?

I think I need to lie down .

 

[William Parcher]

Mushy's question is pretty straightforward.

It seems to contain two assumptions.

1. Anything that could ever occur will occur.
2. Right now, we must automatically know that #1 is true.

I don't know that I'd agree with either of those.

 

[Lithrael]

Not necessarily. ... I think I need to lie down .

Yes! That's exactly the sort of fun I wish to have in a conversation such as this. I wish to play around with exactly what the Law of Conservation may be taken to mean. If any of our lovely local physicists would like to come in and explain which bits I have got arse over teakettle they are welcome to, but I am not going to have my parade rained on by my fellow pub-quiz winners.

For instance that the Law of Conservation means you cannot increase the mass of the universe - I understand that to mean that you cannot have mass poof into a closed system from nowhere. But in the case of time travel the mass would be coming from somewhere - a different time - and as such we wouldn't be describing those moments in time in the universe as a closed system.

If you want to say 'but you cannot do that' that is fine. I'm saying 'if you could do that would it technically be a violation of what this principle actually means?'

Like if I asked 'if you could run 60 mph would you get a speeding ticket on the highway', I'd be looking for responses like 'I don't think you can get a speeding ticket without operating a vehicle, but you'd definitely get a fine for being on the highway without a vehicle' and not responses like 'you can't run 60 mph'.

 

[sadhatter]

The TV series "7 days" had it that there was never enough power available to take journeys further into the past than 7 days.

I forget if that show sucked or not, all i know is i had really strong feelings for it one way or the other...

 

[sadhatter]

I have always held the notion that if time travel is possible it will be soley on a per object ( or, lets say kilogram, or pound, etc.) basis. Meaning that one could, for example, regress their cup of coffee back 5 minutes to when it was hot, with relative ease, but the more stuff being regressed, the harder it would be to do so.

Essentially one would not be "going" anywhere, but almost retrieving or restoring an object ( or possibly person, animal, etc.) from a previous state. Wrapping up the issue of "changing the past" fairly nicely.

Somewhat the same way that when you drive a car really fast, your not moving the entire earth, but only your car.

All said and done though this is me talking out of my ***, but imho, this is the most likely form of " Time travel.".

 

[William Parcher]

"If time travel were ever possible, it would have happened in the future, so we would have come back to visit ourselves already" imply that the future has already happened, therefore our lives are pre planned?

Yeah, but we may never know that the future time-travellers did it. If they go back to visit 1932 we can't know now that they did that. They'd have to visit us right now for us to know right now that they had accomplished it. The folks in 1932 have no way of informing us that they were paid a recent visit by some futurefolks.

 

[Drewbot]

Yeah, but we may never know that the future time-travellers did it. If they go back to visit 1932 we can't know now that they did that. They'd have to visit us right now for us to know right now that they had accomplished it. The folks in 1932 have no way of informing us that they were paid a recent visit by some futurefolks.

I wouldn't believe them anyway.

 

[Checkmite]

Not necessarily.
Our conversation laws are assumed to hold solely for any moment in time, but maybe they hold over the lifetime / extent of the universe.
If we move a kilo of metal from 2090 to 1990, there is 2kg in 1990, but none in 2090.
It may be that this is "good enough" to satisfy the conservation police.

I'm not sure why this would work the way you propose. You're saying it's allowable to add mass to the universe at one point so long as you subtract an equal amount at some point in the future?

At any rate, Novikov's self-consistency principle eliminates the possibility of successfully taking any action which would change the past during an episode of time travel; thus the principle itself forbids time travel altogether since increasing the mass of the universe at any point of time would also increase the rate of entropy at that point in history.

 

[gnome]

I wouldn't believe them anyway.

This is where time-travel passwords come in handy

 

[Squeegee Beckenheim]

You're talking about it like it was an actual thing. Whatever kind of violation backward time travel is, it's not a run of the mill 'this particle cannot exist in two places at the same time' violation. For starters it's not the exact same particle, it's an older version of the same particle. If you had an atom of plutonium, waited for it to decay, then sent it back in time, and set it next to its younger self, they wouldn't be the exact same atom existing in two places at once.

I'm not sure that the objections rely in any way on it being the exact same particle because once you get down to the atomic level, all particles are identical in any case. All electrons, for example, are exactly the same as all other electrons.

As for the conservation of mass (and conservation of entropy) argument, I'm not sure that they prohibit time travel either. They're based on observations about the universe as it is. But, if we could travel in time, those observations might not still hold - the same kind of way that Newtonian physics turned out not to apply for the very fast or the very small.

I do believe that relativity prohibits time travel (or, IIRC, it allows time travel to the future, but not to the past) but, again, I'm not sure that just because it should be impossible according to relativity means that it must be impossible, because there are things about quantum mechanics which aren't compatible with relativity.

 

[Kopji]

Does the argument "If time travel were ever possible, it would have happened in the future, so we would have come back to visit ourselves already" imply that the future has already happened, therefore our lives are pre planned?

No. The time travel paradox unfolds something like in HG Wells Time Machine. If we went back in time to prevent an event that caused us to go back in time, a paradox would occur if we changed history so that we would have not needed to go back in time. The best we could hope for is creation of different reasons for needing to go back in time.

I am actually a fan of Douglas Adams, who pretty much showed that time travel would make everything we did increasingly meaningless.