Time travel physics explaination needed...

[kittynh]

OK, don't make fun of my nice UFO people....but I have question this morning I could use some help with. The part of the question I need help with is ...I would like a good time travel scenario. Like what is physics saying, say with string theory, about the possibility of time travel. Also make is simple enough that just about anyone could get the gist of it. I don't know enough, other than to suggest we build a Tardis.

I'll take care of the "earth sister" part, but I'd like some real hard science about time travel.

Thank you.

Here is my question...................it sounds to me like someone that is really interested in time travel, and that could find science really fascinating. More so that "earth sister" I hope.

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Ok, so according to earth sister, aliens can move back into the past in our time and affect things that happen and travel in speed that we cannot travel in. My question is, is it possible for humans to use whatever methods they use to do this, say if they taught us their ways of traveling light years in matter of weeks, would it be possible to do it from planet earth and with our resources and current technology? and what would doing this actually require

 

[Dancing David]

Um, except for the fact that you can't really time travel, unless you exceded the speed of light....

The biggest problem is that your time machine has to be a space ship, the arth rotates around the sun, the sun rotates around the center of teh galaxy, the galaxy moves in the local group... etc... ... the universe is expanding (contacting as you go back in time).

So our time tarveler has to whiz around the galaxy and inter galactic spce.

 

[Complexity]

Kurt Godel developed some solutions to Einstein's general relativity equations that resulted in a 'world without time', certainly not time as we understand it, and that some movements through this world involved 'travel through time'.

I'm sure I've garbled much of this, but it is in an interesting book entitled A World Without Time about Kurt Godel's work on general relativity, his professional and personal relationship with Einstein, and the last years of his life.

Einstein apparently accepted Godel's work as a valid solution to his equations and was intrigued by the possibilities.

I don't have a clue if this helps with your question, but it was too interesting (to me) not to mention.

 

[sol invictus]

Generally speaking, "time travel" the way it's usually conceived of in fiction is regarded as impossible by physicists. The reason is that it leads to intractable logical paradoxes - kill your own grandparents before your parents were born - that cannot be resolved in any known way. Physics is in essence the science of how, after initial conditions have been specified, the system will behave and evolve. If part of your system can go back in time and change the initial conditions, the whole question physics is set up to answer becomes ill-posed and essentially meaningless.

With that said, there are at least a few cases where time travel into the "past" might not be so catastrophic. For example, it's quite easy to construct solutions in physics that are perfectly periodic in time (i.e. they repeat themselves over and over). If you only consider solutions like that, you are free to regard them as existing in a spacetime with "closed time-like loops" - i.e., periodic boundary conditions on time. But in that case, you can't really distinguish past from future...

In the Godel spacetime Complexity mentioned there are such closed timelike loops, but they have a more complex structure than simply periodicity in time. Perhaps one could make sense of such a spacetime, but I don't think anyone ever has. And there are pretty good reasons to believe that closed time-like curves can never form from physical matter or energy.

So in short: physics probably doesn't allow time travel - and if it does, it's not at all like the kind where you can go back and change your own past.

 

[quarky]

Perception is tied to time. Altering it isn't exactly time-travel, but its pretty close.

 

[Soapy Sam]

If humans had time travel, the CIA would have deleted this post before I wrote it.

 

[quarky]

Any one seen Soapy Sam lately?

 

[Soapy Sam]

No such poster ever existed.

 

[Complexity]

Generally speaking, "time travel" the way it's usually conceived of in fiction is regarded as impossible by physicists. The reason is that it leads to intractable logical paradoxes - kill your own grandparents before your parents were born - that cannot be resolved in any known way. Physics is in essence the science of how, after initial conditions have been specified, the system will behave and evolve. If part of your system can go back in time and change the initial conditions, the whole question physics is set up to answer becomes ill-posed and essentially meaningless.

With that said, there are at least a few cases where time travel into the "past" might not be so catastrophic. For example, it's quite easy to construct solutions in physics that are perfectly periodic in time (i.e. they repeat themselves over and over). If you only consider solutions like that, you are free to regard them as existing in a spacetime with "closed time-like loops" - i.e., periodic boundary conditions on time. But in that case, you can't really distinguish past from future...

In the Godel spacetime Complexity mentioned there are such closed timelike loops, but they have a more complex structure than simply periodicity in time. Perhaps one could make sense of such a spacetime, but I don't think anyone ever has. And there are pretty good reasons to believe that closed time-like curves can never form from physical matter or energy.

So in short: physics probably doesn't allow time travel - and if it does, it's not at all like the kind where you can go back and change your own past.

Many thanks. That was the impression that I got from the book. Einstein thought that Godel had a valid solution to the equations of general relativity, but that could remain a curiosity without being realized.

 

[fuelair]

There is also the problem that if you had a time machine and stepped in it and turned it on and moved a day forward or back the point where the machine stopped would be in a hard vacuum and you would be shortly very dead and uncomfortably so - an event that none of us would be at all happy about.

Reason is that if you are removed from current time you are also removed from current geography which moves along merrily on it's way around the sun. You stay in the place you left from in the universe but the Sun, Earth and the rest of the galaxy have moved on a small bit.

The further forward or backward in time you go, the further you are from where you left but you are still just as dead - you merely die much earlier than you otherwise would have or (technically) much later.

Most GOOD SF writers know this, but it would make really lousy time travel stories so they tend not to talk about it much. This relates to teleportation also - which, if you could do it, woiuld result in unfortunate things if, for example, you teleported from a car starting to crash to your front yard. As the laws of motion state, you would as you ported to your front yard, still be travelling at the speed of the car and would crash into the house, a tree, the ground very swiftly and damagingly.

 

[Complexity]

There is also the problem that if you had a time machin and stpped in it and turned it on and moved a day forward or back the point where the machine stopped would be in a hard vacuum and you would be shortly very dead and uncomfortably so - and event that none of us would be at all happy about.

Reason is that if you are removed from current time you are also removed from current geography which moves along merrily on it's way around the sun. You stay in the place you left from in the universe but the Sun, Earth and the rest of the galaxy have moved on a small bit.

The further forward or backward in time you go, the further you are from where you left but you are still just as dead - you merely die much earlier than you otherwise would have or (technically) much later.

Most GOOD SF writers know this, but it would make really lousy time travel stories so they tend not to talk about it much. This relates to teleportation - which, if you could do it, woiuld result in unfortunate things if, for example, you teleported from a car starting to crash to you front yard. As the laws of motion state, you would as you ported to your front yard still be travelling at the speed of the car and would crash into the house, a tree, the ground very swiftly and damagingly.

Well, obviously, time machines need the time-travel equivalent of 'Heisenberg compensators'.

 

[Roboramma]

There is also the problem that if you had a time machine and stepped in it and turned it on and moved a day forward or back the point where the machine stopped would be in a hard vacuum and you would be shortly very dead and uncomfortably so - an event that none of us would be at all happy about.

Reason is that if you are removed from current time you are also removed from current geography which moves along merrily on it's way around the sun. You stay in the place you left from in the universe but the Sun, Earth and the rest of the galaxy have moved on a small bit.

That seems to imply some sort of preferred reference frame: here's a question for you: where do you think you would end up, relative to the earth, if you could somehow time-travel one day into the future without passing through the time between today and tomorrow?

I don't think physics has an answer to that: there is no preferred reference frame. If we assume that you coordinates in space would remain unchanged, unchanged in which reference frame?

Maybe there's a way to find the solution with some conservation principle? Conservation of momentum might imply an answer, but I can't see it, and to be honest I think that's a problem with any sort of teleportation. If I teleport ten meters away in zero time, where do I end up? It seems to me that because of the relativity of simultaneity we don't have an answer to that question, though perhaps I'm not quite thinking this through clearly enough.

Anyway, my point is that it's not obvious to me that you should end up somewhere "out in space" rather than simply on the earth, given that a reference frame that's stationary relative to the surface of the earth is just as "valid" as any other.

 

[Cuddles]

Ok, so according to earth sister, aliens can move back into the past in our time and affect things that happen and travel in speed that we cannot travel in. My question is, is it possible for humans to use whatever methods they use to do this, say if they taught us their ways of traveling light years in matter of weeks, would it be possible to do it from planet earth and with our resources and current technology? and what would doing this actually require

This can really be generalised to any claimed alien technology - if aliens can do it, there's no reason we wouldn't be able to do it here on (or near) Earth as well. There's nothing special about our particular location that would prevent things from working here that work elsewhere. And we have the technology to do a lot more than most people assume - it's funding, politics, and in many cases simply the lack of any reason to do it, that are the biggest limiting factors. For example, let's say the alien technology needs some weird mineral only found in remote asteroids. Well, we've already returned samples from the Moon and at least one comet. We're not mining asteroids because it would cost a huge amount of money and resources and is completely pointless. But if we could be sure of a big pay-off, we absolutely could start right now.

The problem is simple the "if". Sure, if aliens can time travel and teach us how to do it, we could almost certainly do it. But in the absence of any evidence for time travelling aliens, it's really a nonsensical question. How could we possibly answer what would be required, when the question starts off by making explicitly clear that we don't actually know what would be required?

 

[Correa Neto]

Well, it is undeniable that we already are travelling in time... We know we can also travel at different (relative) paces through time. All it takes is speed. The faster I go, the slower my clock will tick from your perspective and the faster your clock will tick from my perspective. It seems we can't, however, travel back in time, we can only travel forward.

Yes, some (controversial?) sets of equations seem to indicate that certain very special conditions could allow travelling back in time. Now, this is a far cry from being a practical possibility. Remember, you must engineer it, you must have the technology to build it. It must be "buildable". As far as I know (and I admit I know very few things), for all the backwards time travel theories, no one has the foggiest idea on how to actually build machines based on them. How could you, for example, actually not only just create a wormhole, but also make it big enough, spin it and stabilize it so something can cross through? Don't forget you must also "aim" it to avoid its' other end to be too close to a supernova, for example. Oh, yes, usually people just invoke some future tech-in-the-gaps... But let's face it, its nothing but fantasy.

 

[sol invictus]

There is also the problem that if you had a time machine and stepped in it and turned it on and moved a day forward or back the point where the machine stopped would be in a hard vacuum and you would be shortly very dead and uncomfortably so - an event that none of us would be at all happy about.
...
Most GOOD SF writers know this, but it would make really lousy time travel stories so they tend not to talk about it much.

I agree with Roboramma - I don't understood this particular objection.

"Time travel" into the future doesn't move us off the earth, so why should time travel into the past?

 

[sol invictus]

Well, it is undeniable that we already are travelling in time... We know we can also travel at different (relative) paces through time. All it takes is speed. The faster I go, the slower my clock will tick from your perspective and the faster your clock will tick from my perspective. It seems we can't, however, travel back in time, we can only travel forward.

That's kind of right, but not exactly. What physics really tells us is that time is a dimension, much like the three of space. Our lives can be thought of as lines, with a thickness of a meter or so in the space direction and a length of 70 years or so in the time direction. And while there's no fundamental law of physics that distinguishes future from past, we can still attach an arrow to the line (pointing from birth to death, say).

In that view there is really no passage of or traveling through time - different positions along the line correspond to the different times, but that's all. Your comment about time dilation and speed simply means that two such lines can be at an angle (that's relative motion), and then it's obvious then from the geometry that one ages slowly relative to the other.

"Time travel" would be a loop in the line, where it doubles back on itself. As I said above, such things are believed to be impossible, although they do exist in certain solutions to Einstein's equations.

 

[Correa Neto]

Well, yes. But its easier to say that way, I think, even if not 100% correct. Now, I pretend to extend my life for much more than 70 years...

Serious now. Since I first read about this particular view, I like to think of a simple 4D view of me, increasing in size, changing shape and covering lots of space, sometimes forming a complex ball. Its there, it will always be. Its the closest thing to immortality I can think of.

 

[kittynh]

ohhh thanks these are all going to help. Kochanski suggested I say "Build a Tardis". (actually my first response does say "it's too bad we can't build a Tardis". Hope the person is a Dr.Who fan!)

Book suggestions are VERY GOOD. People like it when I direct them to a book or web site. I sometimes find that the REAL theories and science is general ends up being way more interesting than "earth sister".

This person is from Finland. I thought you people in Finland had "perfect" schools!

 

[tensordyne]

Closed time-like loops using Goedel's solution, or use a wormhole. Problem with Goedel's solution is no material can withstand the rotational stresses needed to spin a cylinder fast enough (maybe other problems) to allow for time-travel, and wormholes require exotic states of matter that probably do not exist (negative pressure to the rescue).

On the flip-side, if you could cause a copy of some object to exist as anti-matter, an anti-copy as it were, then according to some ideas in Quantum Field Theory, the anti-copy would go backwards in time. Maybe it would make for a mildly entertaining science-fiction story. You create a machine that makes a anti-copy of yourself, you carefully float around in a magnetic bubble (real chance of that and the anti-copy most likely zero) for a while, and then make a copy from the anti-copy.

Eh, Forward Time-travel is very easy (you are doing it already!), going back in time is most likely impossible. Fiction writers love the idea though! Fate and the human condition, various supposed solutions to paradoxes, that all sure can sell some books and movies.

 

[dlorde]

... Anyway, my point is that it's not obvious to me that you should end up somewhere "out in space" rather than simply on the earth, given that a reference frame that's stationary relative to the surface of the earth is just as "valid" as any other.

I'd agree with that. The problem I see is that during initial testing with short durations, you'd switch on, and send the hamster back, e.g. one millisecond, where it would end up messily co-existing with its earlier self. The combined mess would move forward one millisecond in the normal way and arrive at switch-on time, when the two interlocked hamsters would be sent back one millisecond, arriving at the same time as the original hamster was tangling with its earlier self. Things go downhill from there... Before you can switch off, your lab looks like an explosion in a pet shop. But where do all the messed up hamsters come from, if you only start with one?