Time Travel... the fly in the ointment

[bilby]

The force that is pushing us towards the future at a constant rate is just like the force that pushes the solar system along at a constant rate - nonexistent.

Isaac Newton would be turning in his grave (except that a body at rest remains at rest until acted upon by a force).

Newton knew diddly-squat about time and even less about time travel. Although his Principia Mathematica is amazingly insightful, his lesser known Principia Tempus was a childish effort. It rightfully has vanished into the archives along with works like the work of Pope Leo I, Jesus, the Party years, that describes the hedonistic antics of Jesus in his late teens and early twenties (Copies of either of the last two are now impossible to find). Cronon drag (like wind resistance or friction) means that a constant temporal force be applied to maintain a constant motion through time.

I’m thinking of starting a career as a hand waving apologist for really bad sci-fi.

Wait, what?

You can get paid for that??

I am in the wrong career.

We should start a conslutancy (sic) firm.

 

[Loren Pechtel]

We are familiar with the science fiction model of time travel. Dr Who sets the coordinates of the Tardis and in a few seconds, he is someplace else in the universe, in some other point in time. It's effortless.

Imagine you had a time traveling car, or airplane. You can go backwards in time, but to go back an hour, you have to drive an hour. Would it be really worth it.

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A Greater Infinity

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While it's not an important part of the plot they do have time travel that works in a fashion similar to this--you can't actually move around time, all you can do is go to a universe where time is running backwards.

 

[DBT]

We are familiar with the science fiction model of time travel. Dr Who sets the coordinates of the Tardis and in a few seconds, he is someplace else in the universe, in some other point in time. It's effortless.

Imagine you had a time traveling car, or airplane. You can go backwards in time, but to go back an hour, you have to drive an hour. Would it be really worth it.

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A Greater Infinity

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While it's not an important part of the plot they do have time travel that works in a fashion similar to this--you can't actually move around time, all you can do is go to a universe where time is running backwards.

Surprisingly, I once heard someone say that time in the Universe goes into reverse when gravity overcomes expansion and becomes the Big Crunch.

 

[T.G.G. Moogly]

Won't that cause issues with displacement of the stuff already occupying the space and time that you move into?
If you travel backwards in time, that could be rather painful for your past self, as your future self tries to (re)occupy the same space...

No doubt it would. Some authors of time travel novels include a pop or a bang as the air is displaced by the arrival of their intrepid time traveler, including positioning their time machine in spot that is known not to have a hill or mountain at the targeted time of arrival, or during any time between departure and arrival. H G Wells comes to mind, but his time traveler stayed on location while buildings and hills arose and decayed around him and his time machine/time bubble.

But I am also space aren't I? Or is the space me? Or are we the same? Time travel seems to necessitate the transportation of space itself into another spacetime. Seems weird to me, and therefore impossible except a la Hollywood.

 

[untermensche]

No doubt it would. Some authors of time travel novels include a pop or a bang as the air is displaced by the arrival of their intrepid time traveler, including positioning their time machine in spot that is known not to have a hill or mountain at the targeted time of arrival, or during any time between departure and arrival. H G Wells comes to mind, but his time traveler stayed on location while buildings and hills arose and decayed around him and his time machine/time bubble.

But I am also space aren't I? Or is the space me? Or are we the same? Time travel seems to necessitate the transportation of space itself into another spacetime. Seems weird to me, and therefore impossible except a la Hollywood.

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Where is it except the human memory?

 

[DBT]

But I am also space aren't I? Or is the space me? Or are we the same? Time travel seems to necessitate the transportation of space itself into another spacetime. Seems weird to me, and therefore impossible except a la Hollywood.

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Where is it except the human memory?

Logically, the past no longer exists, nothing to go back to, time in the form of particle position, entropy (the arrow of time) has moved on. But then there are different models of time, block time/eternalism, which may allow travel to the past. It seems highly unlikely.

 

[Bomb#20]

But I am also space aren't I? Or is the space me? Or are we the same? Time travel seems to necessitate the transportation of space itself into another spacetime. Seems weird to me, and therefore impossible except a la Hollywood.

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Somewhere just sitting there waiting, yes, probably. It doesn't make a whole lot of sense for it to be gone. How could the state of Mars a minute ago have come and gone forever from reality even though it hasn't happened yet from the point of view of an observer moving past Earth at relativistic speed? The state an hour ago could be gone, since it's in our past light cone so it's past from the point of view of all nearby observers; but that would suggest the past stays around for a while and then gets cleaned up -- a sort of cosmic janitor following behind the present at a safe distance. No, if the past is going to stick around at all then it's simplest to suppose it sticks around permanently. But to suppose the converse -- that the past doesn't exist at all and there is only the present -- would seem to imply that there has to be a preferred frame of reference, a possible observer who's at absolute rest. That's a very Newtonian view of the universe.

But possible to return to it? No, probably not in any useful sense. Something existing somewhere doesn't mean you can get to it -- try to get from inside to outside a black hole. True, there are quantum mechanical phenomena that look like "call by need". I.e., a measurement now appears in effect to change the past from having been in a superposition of states to having been in a definite state. But since there's no way to control which way the past collapses, you can't copy yourself into it, or even send your past self a message telling you to bet on a last minute interception by the Patriots. The best you could do is cause the past to have had definite random noise instead of yet-to-be-randomized random noise.

Where is it except the human memory?

Well, since time is the 4th dimension, where it is is, quite literally, in a parallel universe.

 

[DBT]

Particle position shifts with time/space/events.
Consequently the Mars of a moment ago is altered irrevocably by the progression of accumulated change.
The Mars of a moment ago is not identical to the Mars of the present.
The Mars of a moment ago no longer exists.

 

[bilby]

As there are no preferred reference frames, it makes no more sense to talk of the past not existing once we move on than it makes sense for me to say that the United Kingdom no longer exists now that I have moved to Australia.

 

[DBT]

As there are no preferred reference frames, it makes no more sense to talk of the past not existing once we move on than it makes sense for me to say that the United Kingdom no longer exists now that I have moved to Australia.

I'll try this; the United Kingdom existed yesterday and exists today, but the UK of today is not identical to the UK of yesterday. The UK of tomorrow will be not be identical to the UK of this moment. The conditions of yesterdays UK cannot be repeated or revisited.

 

[bilby]

As there are no preferred reference frames, it makes no more sense to talk of the past not existing once we move on than it makes sense for me to say that the United Kingdom no longer exists now that I have moved to Australia.

I'll try this; the United Kingdom existed yesterday and exists today, but the UK of today is not identical to the UK of yesterday. The UK of tomorrow will be not be identical to the UK of this moment. The conditions of yesterdays UK cannot be repeated or revisited.

Yeah, sure; you can't cross the same river twice. But I have crossed the Brisbane river twice just today. Go figure.

 

[untermensche]

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Where is it except the human memory?

Logically, the past no longer exists, nothing to go back to, time in the form of particle position, entropy (the arrow of time) has moved on. But then there are different models of time, block time/eternalism, which may allow travel to the past. It seems highly unlikely.

I go further. I say it is absurd to imagine that some "copy" of the past exists eternally so that at any moment in the future any moment in the past can be returned to.

Where would this "copy" exist and for what purpose?

It is far closer to a religious belief than a scientific.

 

[Malintent]

Logically, the past no longer exists, nothing to go back to, time in the form of particle position, entropy (the arrow of time) has moved on. But then there are different models of time, block time/eternalism, which may allow travel to the past. It seems highly unlikely.

I go further. I say it is absurd to imagine that some "copy" of the past exists eternally so that at any moment in the future any moment in the past can be returned to.

Where would this "copy" exist and for what purpose?

It is far closer to a religious belief than a scientific.

The "copy of the past" "exists" in the same "place" as the copy of the "future yet to be".

There exists only now. This is not a problem for the past any more than it is a problem for the future.

 

[untermensche]

I go further. I say it is absurd to imagine that some "copy" of the past exists eternally so that at any moment in the future any moment in the past can be returned to.

Where would this "copy" exist and for what purpose?

It is far closer to a religious belief than a scientific.

The "copy of the past" "exists" in the same "place" as the copy of the "future yet to be".

There exists only now. This is not a problem for the past any more than it is a problem for the future.

There exists a particular now.

To go to the past means to return to a series of particular nows that don't exist anymore.

They have been replaced by other nows.

 

[Shadowy Man]

The "copy of the past" "exists" in the same "place" as the copy of the "future yet to be".

There exists only now. This is not a problem for the past any more than it is a problem for the future.

It's arguable that "now" even exists. It's basically the infinitely thin edge between the past and the future.

 

[skepticalbip]

The "copy of the past" "exists" in the same "place" as the copy of the "future yet to be".

There exists only now. This is not a problem for the past any more than it is a problem for the future.

It's arguable that "now" even exists. It's basically the infinitely thin edge between the past and the future.

...and no two people can agree on that infinitely thin "now" because of the spacetime separating them. Any one person will see others (and everything else) existing in the past - though the very recent past.

 

[Jimmy Higgins]

BTW, not all means of time travel have this problem.

Robert L. Forward, Timemaster has time travel via wormhole. The wormholes are physical objects in space, moving them around does not change the linkage between them. (In fact, when created they are not time travel devices at all--the time travel aspect comes from hauling one end around at relativistic velocity.)

While there is a certain amount of handwaving to create the basic technology he was a hard-science author and apart from the negative matter & wormholes I can't find any scientific flaws. (And, yes, that includes the stardrive he uses to haul them around.)

But wouldn't the two openings have to not be in motion? You open a worm hole on Earth and it appears to rotate away from you as your position changes due to the Earth's movement.

 

[Loren Pechtel]

But I am also space aren't I? Or is the space me? Or are we the same? Time travel seems to necessitate the transportation of space itself into another spacetime. Seems weird to me, and therefore impossible except a la Hollywood.

Does anybody actually believe that every moment of the past is somewhere just sitting there waiting so that it is possible to return to it?

Where is it except the human memory?

Modern physics treats time as a dimension.

The question is whether we are able to travel on that dimension or not.

Also, even if we can't that doesn't mean that it might not be possible for some higher dimensional being.

 

[Bronzeage]

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A Greater Infinity

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While it's not an important part of the plot they do have time travel that works in a fashion similar to this--you can't actually move around time, all you can do is go to a universe where time is running backwards.

Surprisingly, I once heard someone say that time in the Universe goes into reverse when gravity overcomes expansion and becomes the Big Crunch.

That's gotta hurt.

 

[DBT]

I'll try this; the United Kingdom existed yesterday and exists today, but the UK of today is not identical to the UK of yesterday. The UK of tomorrow will be not be identical to the UK of this moment. The conditions of yesterdays UK cannot be repeated or revisited.

Yeah, sure; you can't cross the same river twice. But I have crossed the Brisbane river twice just today. Go figure.

Taken in isolation, but the World itself is moving, once you have crossed the river the World does not have you crossing the river at that moment on that day ever again. Once upon a time there was an England at war with Germany with Churchill as Prime Minister. England still exists but that England has gone forever. The world still exists, but the world of WW2 has long gone.

How can we revisit the events of WW2 with Churchill as PM, a time in the world that (apparently) no longer exists?