Time travel

[sparhawk]

I was thinking about time traveling and I think I should share my thoughts on them to see what others think of it. I know that physical laws currently have no prefered time direction (except in some obscure cases of particle physics, but this is IMO not completely confimed). Our current understanding of the laws does not neccessarily mean that it is technically possible. I know there are several ideas out there, which might or might not work (some of them involving negative energy, wormholes, etc.). But even if these things might be found, that does not neccessarily mean that time travel, as it is usually portrayed in popular movies or stories, will be what we can expect from it.

 

I think there are two fundemental approaches to time traveling. One would be the reversal of time (TR) and the other would be "normal" traveling only taking the fourth dimension into account as well.

 

The first approach would be pretty tough. Considering the second law of thermodynamics, this would mean that all these processes would be reversed. This alone would be an unsurmountable task, but lets consider what would happen if we could achieve this with technology. You have to keep in mind, that, for entropy to be reversered, not only would we have to find a process which DECREASES entropy (which has never been observed), we would have to do this maybe for every macroscopic particle. I don't consider quantum states as relevant for the moment, because on this scale particles can be replaced without any consequences, as they are all to be considered to be the same. A photon is exactly the same as any other photon, and the same is true for quarks.

 

So what would happen if we would manage to reverse entropy and in turn reverse time along with it. A timetraveller (TT) would enter the machine that is supposed to make this happen. Up to this point, the flow of time would be in the normal direction. So now the starts the engine, and the flow of time would still be in the same direction as before. As the engine starts to gain momentm, in whatever it needs to be doing in order to achieve TR, time is probably slowed down, just like proposed with relativistic physics, so we are still working. Now we have two effects taking place similarily. The first one is that, because of the slowdown of time for the TT, the time outside the machine woudl start to pick up. As we approach the zero point, where time starts to tilt over, all the time in the universe outside would go by. This is the same effect that would happen to a photon as well. So instead of traviling backwards in time, the TT would be travilling far into the future.

The second effect that would take place is near the zero point (where time flips over). So what would happen there is, that time reaches a zero point, and then start to reverse. Up to this point the machine always works correct according to physical and mechanical laws. And here is the catch. In the first nanonsecond (or even less), that time starts to spin in the reverse direction, the process, that the machine generats to let time flow backwards, the machine itself would also be reversed. This means that, the first action that the machine would achive in reversing the time, would also reverse the process in the machine itself, because this is part of the past of that machine, which also has to be reversed. This means that the machine gets now into the state where it was a nanonsecond before it started to reverse time, and this means that time can now not go backward any longer, because the machine needs to operate, whatever it does to achieve that, but it can not operate past the point where time starts to reverse. From the point of view of an outside observer it would probably look as if the machine didn't work, because it just stops working at this point. Of course any observer would be frozen in time by then anyway.

 

So my logical conclusion from this thought experiment is that timetravel by simply reverting the physical laws and trying to achieve timereversal by that, is not only technically impossible, it is also logically impossible. Any action that would cause time to reverse, would also cause it's own undoing and thus stops the process of time reversal.

 

 

Now we look at the second method of time traveling. We consider spacetime to be a fourdimensional vector. Mathematically it follows that this would need a static universe, because if we would be able to reach ANY arbitrary point in this fourdimensional spacetime, it follows that not only has the past to be fully determined, also the future has to be fully determined. If the future were not fully determined, it would follow that any future observer might find that one specific point in that spacetime would have two different events at the same time associated with it, which can not happen, because we assumed that we not only go this particular point in space but also in time, thus everything must be the same in this spacetime at each point and stay there.

From this it also follows, that, suppose we COULD go to such a point, then if we try to modify the events, we can not do this. If we could do this, then we would have the effect that we would see two different events at the same point, which can not happen, or we would be at a different that just looks like the same.

 

I don't really think that the universe is static like this, but from my conclusions, I have to think that timetravel is logically impossible, even though our current understanding of physical laws have no such preference.

[Order of the Hammer Bureaucrat]

Timetravel is impossible.

It has always boggled and irritated me when they have timetravel in Star Trek or some movies. They talk about weird time things and multiple event lines so surely as if it's actually true.

However, I don't mind watching well-made movies about time-travel, and one movie I've been wanting to see for years, just don't remember its name, if anybody does tell me: it's about some guy in a long coat and wide-brimmed dark hat.

[demagogue]

@Spar,

 

not only would we have to find a process which DECREASES entropy (which has never been observed), we would have to do this maybe for every macroscopic particle.

 

By the way, on entropy. As I understand it, entropy is a statistical artifact of the Brownian motion, at some level, in systems which statistically tends to wash out their organization in proportion to that randomizing motion ... but it isn't a hard rule in the sense that a system is logically barred from gaining organization over time. It's just astronomically unlikely, since it's such a small space that the random meandering would have to stumble into. And I've read somewhere that in certain very restricted domains is possible to observe a system gaining entropy. Well, I guess your idea assumes this, doesn't it?

 

But anyway, this really wouldn't be time travel per se, would it. The causal chain a->b->c, etc, would keep marching forward as ever, time's still "running forward", it just might, at best, have the illusion of reproducing what a reverse chain might look like if one were to run the video backwards. It's sort of cheating, though, since it's not "real time travel". I'm not sure why you even need to bother (as a logical matter) with an inside and outside to the time machine; it's just a shorthand for which particles/fields are getting affected and which aren't. Time is still technically moving "forward" for both.

 

And what is this "tipping point" of which you speak? You mean the singularity at the beginning of the universe? Why exactly does this flip the time scale and put us far into the future? I didn't think they knew yet, exactly, that there was going to be a big crunch, or how the universe was going to end.

 

Although, for that matter, I'd think the time machine would just stop in any event, since it's job wasn't to run time back per se, but decrease entropy, and once it's to zero it doesn't get any smaller, so it stops. Anyway, this is all terribly new to me, so forgive me if it's an obvious question...

 

----------------------------------------

 

As for my own understanding of time travel, so far my thoughts on time are influenced by the book I just read on Quantum Loop Gravity. I'm going to completely botch this explanation, I'm sure, but it's worth the mental exercise to try my best to explain what I understood.

 

At the most fundamental level, time is just a measure of things interacting, and every very basic interaction (of loops, or whatever is most fundamental) can be considered a fundamental click of time (like a Planck click). A fundamental interaction is just one which changes the state of a system at its most fundamental level, with nothing "below" the level of that change there to even change, at least as far as the physical state of the system is concerned (i.e., lower level changes -- and smaller clicks of time with them -- might technically exist, but then they would just be irrelevant to the state of the system since they don't affect that state in any way; so they'd have no real physical meaning).

 

In that sense, every fundamental interaction is its own clock, and you could see how, in systems with a "fast" rate of fundamental interactions, clicks occur more often relative to systems with "slower" rates of interactions, where less interactions occur in the same period. So there is a sense in which time runs faster in some places relative to others.

 

Note, the proper interpretation is NOT that the two domains undergo the same amount of time in the same period, with stuff merely happening slower or faster (the part of Newton that Einstein threw out with Special & General Relativity), because all that time means at that fundamental level is the measure of interactions happening. Ten clicks of time = ten clicks of time, from a system's perspective it's ten clicks older, regardless of whether or not one system gets through those 10 clicks before another system has even gotten through 2 ... in which case one system will be 10 clicks aged, the other just 2 clicks.

 

There is nothing "in between" interactions, so to speak, that could even keep track of the time between them. Time is discrete, a "Planck click" of time is as fundamental as it gets ... which it would have to be (at least in this way of thinking) if gravity (and time and space with it) is to be quantized into discrete bits to be handled under Quantum Theory ... and this way of thinking gives you a foundation for special relativity, general relativity, and quantum theory all in one idea. It also has a certain intuitive logical appeal to it, as well. In this picture, every bit in the whole damn universe is running at a greater or lesser different rate, like a gazillion independent clocks in every drop of water, although physical systems still function, for the most part, as if time were smooth because tiny differences get washed out on the classical level so that they don't matter as far as the system's classical state is concerned (at least, they're supposed to for the theory to work), except that it gives you the weird time-things you get with General Relativity.

 

Anyway, if you buy this way of thinking about time, then the sort of forward time travel you get in general relativity is easy. A system just has to put itself into a domain where less fundamental clicks occur relative to a system where more clicks occur in the same relative period, such as the earth/spaceship Twins example, where the non-accelerating twin appears to have undergone much more time than the accelerating twin when he returns to earth, who underwent much less time in the same "period" (as measured from the earth). A system accelerating relative to itself "delays" the clicks, so to speak (compared to itself without the acceleration), the more the acceleration the more the delay.

 

But backwards time travel is barred for the simple reason that backwards causation is closed. Clicks can occur at different rates relative to one another, but once an interaction "clicks", it's done; it can't unclick, so to speak. There is no way another system could come into an interaction with the clicked system once the click is done. It's basically an application of the principle of locality. Elements have to directly "interact" to send signals, and a signal sent is a signal sent. This idea could also even cope with some bending of locality that you get with quantum theory, where you get a single photon in a quantum tangle interfering with itself to create an interference pattern, as long as you don't get backwards causation where it goes back in time to interfere with its previous self.

Edited May 11, 2007 by demagogue

[oDDity]

Well this is obvious. Forward time travel is easy, we do it constantly, if slowly, and speeding it up can be a matter of cryogenics or travelling fast, but going backward is impossible, because the past doesn't exist.

You can no more 'go to' the year 1381 than you can 'go to' the number 3 or the colour red.

What you would have to do to go to a point 300 years ago, is rearrange all the subatomic particles in the universe in the exact place and direction they were travelling at that time, and that's impossible, and will always be impossible.

Don't despair though, because things like the holodeck in Star Trek TNG will obviously be a reality in the future, and that will be as good as going back in time. Better in fact, since it won't be dangerous.

[sparhawk]

I do not despair. Maybe you are missunderstanding my motivation for these thoughts. It's not because I want to find a loophole that gives me hope of having backward time travel. I'm only interested in the backward time travel aspect, because, as you already pointed out and I neglected to do so, forward time travel is pretty trivial (logically). When I was about 16 I read my first article in a popular science magazine about relativity, and forward time travel was the first implication that I deduced from that description. It's glaring obvious IMO.

Quite on the contrary, I was always intrigued by that "paradox" - I go back in time, prevent my mother from meeting my father, and suddenly I'm undone.

I never really bought into this kind of logic, because I don't believe that the physical laws would work like this, but I never really tried to follow it through and find a good explanation of why this wouldn't work. I don't think that backward time travel will be possible. The more I think about it, the more I run into logical consistency problems. That doesn't mean much. Of course it could mean that I'm just not smart enough, but I don't have the feeling that this is the case here.

 

Demagoge, I like that "click" approach, but I think it might still have a problem. After all, such a statechange would happen in time, and I have a bit of a problem at the moment to conceptualize how such a fundemental click would come to pass. Also I don't see why such a click system would prevent backward time travel. This would assume that such a click can only happen with a preferred mode of movement, but what would that be? Why would be one statechange allowed but not the other?

 

That "tipping point" I meant was the point in time, which the assumed machine would reach before it actually starts to reverse time. Obviously we have to start the machine somehwere, so the time doesn't flow backward yet. When I press the trigger and start the engine, the machine would do whatever it has to do, so at this moment it would also still run forward. As it gains momentum, there would be a point in time, where the reversal would start to happen and this would be that "tipping point" I was refereing to. It was not neccessarily the bing bang event, if that ever happened.

[Komag]

Maybe there is some sort of "record" of all history as is really happened, like stored in the cells of all things, in the other dimension or whatever, so we could, at best, travel to the past and see it as a hologram, not a man-made creation like the holodeck, but a discovery of the actual history as tapped into from the natural record. Sort of a geology on super-steriods.

[sparhawk]

Well, this record indeed exists to some extent, in the form of photons being emitted into space. Of course this is limited, because it would only record what is on the surface of earth, because only such photons have achance of getting into space. As soon as a photon hits something it would stop and thus get out of existence.

[oDDity]

Photons only show you want the universe looked like anyway, nothing more, plus you'd have to travel faster than them to overtake the ones that were emitted 500 years ago, and that's impossible also.

An interesting point, is that time doesn't exist for a photon when it's travelling at maximum speed though a vacuum. It experiences no time, it doesn't age, and if you could travel that quickly, neither would you, so essentially, you could travel anywhere in the universe instantly.

[Order of the Hammer Bureaucrat]

I could, if I bought a Macleans magazine to lose mass, and got some Iraqi oil for some infinite energy.

[sparhawk]

What I also find intersting is that there are remarkable similarities in various objects. For example, a black hole has the same three parameters to be described as a particle. Mass, spin and charge. Another intersting thing is when I thought about what happens if you reach the border of our universe. Assuming that the big bang happened, and that the universe is exapnding with the speed of light, it means that at the border the time would also stand still. Same as with a black hole event horizon.

[oDDity]

It's not surprising that everything behaves in the same way with the same principals, since it's all part of the same system, which used to be all in a very small space at the start.

Everywhere in the universe is the same as everywhere else.

I like this image of the hubble telescope ultra deep field.

http://www.spacetelescope.org/images/screen/heic0406a.jpg

It shows 10.000 galaxies, and this is just one tiny area of the sky equivalent to holding a small coin 10 metres away.

Some of these blobby red galaxies are forming nearly back at the beginning of the universe.

[sparhawk]

IMO that's not as clearcut as you make it. The more particles get together, the more complex the system gets and the combined system can have quite different properties, then each individual particle. If you combine particles in a certain way, you get water, and in another way you get steel. Now what you claim is that these complex system would be predictable by observing the individual particles and their behaviour. Of course you might be able to use a reductionist view to explain how you go from metal or water to particles, but the opposite side prediction is IMO not very realistic.

 

The image is quite impressive though.

[oDDity]

Well yes, you have to say it almost seems as though there was some intelligent design involved, for everything to have worked out so perfectly and end up with the situation we currently have on Earth.

There are so many factors that had to be exactly the way they are for it to work.

However, if you use the hypothesis that there are an infinite number of universes, or dimensions, then the reasoning is that this is just one that worked, where everything did go right, and there are countless ones that 'failed'.

In other words, it's all just random stuff that happened to work out perfectly in this one universe or dimension, but from our single point of view it seems quite amazing.

Since it's random, it could never have been predicted, no.

[sparhawk]

Well yes, you have to say it almost seems as though there was some intelligent design involved, for everything to have worked out so perfectly and end up with the situation we currently have on Earth.

There are so many factors that had to be exactly the way they are for it to work.

 

That is only the impression if you point to the universe and say "give me the parameters that are needed to evolve human beings in that particular form" obviously this is the wrong approach, because we were not planned in that way or at all. The only assumption that we can make from our existence is, that we are in a universe that is parameterized in such a way that it allows for our existence, but there can be an infinite number of parameters that would also allow us.

 

However, if you use the hypothesis that there are an infinite number of universes, or dimensions, then the reasoning is that this is just one that worked, where everything did go right, and there are countless ones that 'failed'.

 

I can't believe in that infinite number of universes to account in a particular subset for us. That seems to be the easy copeout for me. I don't believe that our universe had such special parameters, because I don't believe in a prefered set of universes.

[oDDity]

Well, if you don't believe in any intelligent design, or in infinite universes, then I don't see what other theory you can have.

There is only this universe, it's the only one that ever has or ever will exist, and it just happened to have the correct properties to produce what we currently see, and life?

Nope.

Infinite universes/dimensions is the most likely explanation, whether you think it's the easy way out or not.

[sparhawk]

Infinite dimensions is quite different from infinite universes. My idea with an infinite universe is maybe a little bit different though. Because I think that these infinite universes are bascially all the same, only they are isolated from each other so they may not be able to communicate. Physically they would share the same properties though. I don't really believe that there exists an infinite number of universes, all with different physical properties.

[Crispy]

You don't need a theory of infinite universes or a Creator to explain the seemingly-miraculous way that our conditions in the universe are just perfect to produce us. It gets a lot simpler than that: If the conditions weren't right, then we wouldn't be here to observe them!

[demagogue]

That (what Crispy just said) is the crux of the anthropic argument. I actually don't like it very much because it doesn't really explain anything and is hovering a bit in pseudo-science itself.

 

More particularly, it doesn't really explain why the values of the universal constants are what they are, because apparently it is incredibly unlikely that the universe would pick just the right numbers to support carbon-chemistry (life) in the vast possibility-space of possible numbers it might choose ... the universe is old and big, but not really that old (only about 3x our own earth's age) and not that big (light can only travel so far in that time), and anyway the constants were set incredibly early on and, so far as "we" know, are consistent throughout the space and history of the (at least observable to us so far) universe.

 

It doesn't really "explain" how those parameters were set just for our single, particular (not so big, not so old) universe to say, well, if they weren't those numbers we wouldn't be here. *Something* made the ball tip this way rather than the significantly more likely that way (whether "it" realized it was contributing to the formation of life or not), and we aren't really doing the problem justice to throw the anthropic argument back at it, IMO.

 

If it really was a random fluke and the dice came up lucky, I'd like to at least see the random number generator mechanism that did the work. There's *something* there that did the work, a glorified random number generator, a seeding blackhole that naturally selected the number (Smolin's theory of Cosmological Natural Selection, interesting but I'm skeptical), or infinite inflation, in which the universe really is just a smaller part of a bigger universe, and the numbers periodically change in universe-sized clusters, eventually you are going to get the numbers that can support carbon chemistry (not sure about this one, either, though).

 

Anyway, something should be behind the numbers, and any "answer" (like the anthropic arg) that just throws back the fact that a number was selected, end of debate, I don't see it is any much better than the other theories that merely end up explaining the question away by unsatisfying fiat.

Edited May 12, 2007 by demagogue

[oDDity]

You don't need a theory of infinite universes or a Creator to explain the seemingly-miraculous way that our conditions in the universe are just perfect to produce us. It gets a lot simpler than that: If the conditions weren't right, then we wouldn't be here to observe them!

That doesn't actually explain anything though. How it all started, if this universe is all there is, etc.

I find that highly doubtful, since if one big bang can happen from seemingly nothing, and produce the universe as we know it, then there's no reason to assume that it only happened once, just like if life can happen here on earth then it's logical to assmue it has happened on other planets as well.

I always liked the idea of evolution pertaining to universes. The good universes with the right proerties to produce sustainable matter also produce black holes, which in turn spawn more universes. It's seems plausible that a big bang is the reverse of the singularity of black holes. Supergiant black holes will eventually suck up most of the matter in the universe, as galaxies merge, and who knows what happens to it in a singularity. What we see on a small scale such as evolution of life on earth does tend to translate to bigger cycles. There's seems to be very few laws and forces that operate the universe, so there's no need to look for new ones perhaps.

I mean, the universe might actually be nothiing, it cancels itself out, the positive being all the matter and antimatter, and the negative being the gravational attraction, the net result being zero, so in a sense it doesn't actually exist.

[sparhawk]

I also find the evolution of univeres theory quite attractive, because I think that the evolutionary mechanism, is not restricted to life, itself, rather to all kind of structures that tend to organize in more complex structures over time.

My personal opinion about the start of our universe is, that I think that black holes explode every now and then. Maybe when they aggregated enough mass, and that this event would create kind of "universe bubbles". They might be seperated from each other, or not, depends on how one views it.

 

I think this also would be supported by the following thought. Lets assume that the univers is expanding all the time. This means that matter drifts apart ever more, and there will be a point, where space is so stretched out, that these parts, formerly sharing the same universe, wouldn't be able to coomunicate with each other anymore. Thus they would become a seperate "universe". With this szenario there wouldn't be need for a big bang, nor would it make any sense. I guess that blackholes (or some other mechanism) can explode every once in a while, just like supernovas happen under certain conditions, and this will create new disruptions in spacetime. This would mean that we only have this one universe, but it would be never ending though.

[Crispy]

I certainly think there may be more universes besides our own... we can't know for sure, but point-blank assuming that our observable universe is all that exists smacks of self-centredness. To discount the possibility of other "universes" out of hand would be arrogant. Though I guess it depends how you define a "universe"... strictly there can only be one "universe", by definition.

 

The idea of black holes creating new universes is an intriguing one, though it strikes me as somewhat difficult to prove... it's not like you can go inside the black hole and find out while remaining intact.

 

I do think the anthropic argument (thanks for reminding me of the name, demagogue) is a nice neat answer to the question of "why can the universe support us?" - but in many ways it's less of a scientific argument and more of a philosophical one (though the lines do tend to blur at this scale). And of course the investigation into the nature of the universe(s?) shouldn't stop just because of it!

[sparhawk]

The anthropic argument doesn't help anything at all. It doesn't explain anything, it doesn't predict anything, and it is trivial and obvious. It's like saying that lions are lions because they are lions, and if they were not lions, they would be something else.

 

As for black holes, of course you can investigate them with math, and observation.

[Crispy]

The anthropic argument doesn't help anything at all. It doesn't explain anything, it doesn't predict anything, and it is trivial and obvious.

Indeed. As such, it's a very useful argument to use against people who wave around the improbability of our existence as evidence of either intelligent design or infinite universes - evidently these people have failed to see the obvious.

 

Which is exactly what I was using it for; it was in response to oDDity saying:

 

Well, if you don't believe in any intelligent design, or in infinite universes, then I don't see what other theory you can have.

There is only this universe, it's the only one that ever has or ever will exist, and it just happened to have the correct properties to produce what we currently see, and life?

Nope.

Infinite universes/dimensions is the most likely explanation, whether you think it's the easy way out or not.

 

Of course the anthropic argument doesn't actually explain anything, I never claimed it did. It does, however, show why you can't make the above argument.

[oDDity]

It's a pointless wordplay argument, of no consequence or usefulness to anyone, like the ontological argument in religion - 'god is perfect, and perfect things must exist, so if he didn't exist he wouldn't be perfect, and therefore he must exist'

All these arguments with formal, fancy names are just philosophical wordplay, used by people with no thoughts of their own, and which don't get us any further forward.

[Springheel]

I'm reading an interesting book on cosmology right now, but there is a question it isn't answering (at least yet):

 

1. Is the actual *universe* expanding, or is it just the matter *inside* the universe that is getting further apart? If the former, what is beyond the edge of the universe?

 

I always thought a universe was infinite by definition, but infinite things can't expand, so....