Nialler

The example about travelling back in time – say, a week – to prevent an accident raised a question in my mind. What if I managed to travel back that week, but found myself returned to that time in the past *and with precisely the knowledge that I had at the original time to which I returned*? Thus, time would unfold in exactly the same way that it did first cycle round, as I would no longer be fore-warned. Of course, maybe chance would drive it in a different direction, but it wouldn’t be as a result of any prompting by me.

I ask this, because surely to travel back in time in an intact way would violate the preservation of mass/energy.

Taking this further, let’s say that I have invented a machine to travel to any specific point in time, and I decide to test it out by going back maybe approximately 2,000 years, and to a small middle-eastern town. (May as well settle some arguments on this board while I’m at it!). My feeling is that once I have made the jump my constituent atoms will all be as they were at that time; i.e. some in the ground, some as part of a living being, some in vegetation, some in the air, some in the sea.

My return may well be eagerly anticipated, but beyond experiencing what it was like to pass through a Snail’s digestive tract, I probably won’t have been able to learn anything at all from the time trip.

Thus will Time have preserved its integrity.

excreationist

I think the idea of windows into the past wouldn't involve any paradoxes... it would involve a screen and maybe speakers and smell and temperature generators, and the person would select the time they want to view. It could freeze at that time, or it could playback in realtime or slowmotion or reverse or high-speed. In the "Masters of the Universe" movie they had a device like this - but the graphics were quite poor (not full-colour, etc). In that movie, the device had to be pointed at the location you want to view.... for convenience maybe you could change the view's location using the device's controls to any position in the universe. The device could use echos or something to determine the positions of objects in the past. It would need to be very accurate to be able to detect sounds in the distant past.
You wouldn't be able to change the past but at least you'd be able to fly around it, like a fly, and pause/rewind/play/etc.

Soul Invictus

I'd always been intrigued by time travel and I saw Contact. Excellent movie! I'd heard of wormholes via Art Bell. So there is merit to the aging at the event horizon?

goat37

Yep, there are merits to it.

The gravitational pull would still be large enough create a large enough distortion of spacetime for the 'aging' factor to really occur.

The event horizon is really the 'point of no return', it is at that point once you pass it that the gravitational pull is so strong that not even light could escape. Gravity IS the distortion of spacetime. Therefore what you have is a gravitational pull so strong that that spacetime becomes infinitely curved and thus light cannot escape.


The book goes over more in detail how that process occurs.

Jesse

It's black holes where you age slower as you approach the event horizon--I don't think wormholes even have an event horizon, since you can travel in one mouth and out the other and report what you saw inside, while I think an "event horizon" is supposed to be a boundary that once you cross, you can never communicate with the other side of the boundary again. It's possible I'm wrong about wormholes not having an event horizon though, if there are any general relativity experts here feel free to weigh in...

Of course, to a limited extent you'll age a little slower in any gravitational field, including the earth's, than you would in deep space. But you need a pretty powerful field before the effect becomes significant.

HeatherD

Sorry for the delay in getting back to this, I'm taking a math class this summer session. I have about 6 hours of homework every night!

Well, I do lack the the language, mathematical and otherwise to describe what it is I'm thinking, I'll admit that. Hopefully, I'll be able to better describe what's been floating around in my head after I get my physics degree. That is still a few years off.

What I am considering is only a hazy idea formed from non-mathematical books I've read on physics, time, black holes and the like. It's a composite idea that I can't describe better until I do understand the math behind it.

I have seen the term "motion in time" used by other physicists but it might be a rather vague idea. I think I even remember a suggestion by Feynman about particles moving through time, and anti-particles moving in the "opposite" direction through time.

I get the feeling you think I'm some crackpot who thinks that Einstein was wrong and I have all the answers.

No, I'm just someone who has read a lot about science but until now was unable to get a science education beyond high-school. I'm also someone who has a vague idea about the structure of the universe, formed from things I've read. I don't consider thought experiments to be a bad thing, even Einstein used them before he developed his theories.

I obviously can't fully describe what I'm thinking but there are some considerations:

1. While relativity "suggests" a number of things, there are still many unanswered questions. Even in this thread we see discussions about time travel and paradoxes as if they are a foregone conclusion. Until someone builds a time-machine we have very little to go on. If you "travel through time" were do you go? That brings me to my second consideration.

2. I think traveling through time means some very disturbing things if it's possible:

a. Every instant in time past is forever frozen at the instant it occurred. That would mean there are an infinite number of copies of every particle, every person, every thought. It would also mean that you could go back in time to any point in the past. One question would be, does going back in time disturb the future of all those infinite instants in time.

b. Does every possible future instant already exist? Do they exist now or are they deterministic. How can you travel to a time that hasn't happened? If I go into the future that future doesn't have my contributions in it's past. If I now go back to my "present" will the future I saw be changed?

I tend to think that there really isn't any "real" past or future. I think that there is the instantaneous "now", "moving" though space-time. To really travel through time, to the past or the future, you'd either be "behind" or "ahead" of the rest of the universe. If you go I don't think you'll find anything waiting for you.

I think that time travel does come down to either of two possibilities, either the past is frozen in time, giving you the ability to drop-in at any moment, or you need to reverse the entire universe to the exact state that it was in at the moment you want to return to. Since I don't think either one is a possibility, I don't think time travel is practical if it is possible.

(Sorry if this is so vague, as I work toward my physics degree I hope to flesh out what I'm thinking about. As I get a better handle on the concepts and the math I expect to integrate present theories into my own thoughts.)

tronvillain

JakeJohnson:
While it is not a wish I try and support, it is not "illogical at its core." It may or may not be impossible, but it is not inherently illogical. Anyone claiming that it is is simply experiencing a failure of imagination.

One way to avoid a paradox with a single universe and complete causality is similar to NumberTenOx's idea: the only stable changes to the past that can be made are ones that create a future in which those precise changes are made and anything else simply causes the universe to snap back the way it was before. If the universe snaps back to precisely the way it was "before" the time travel, a time machine might appear to do absolutely nothing or simply vanish altogether. If the universe only snaps back to approximately the way it was "before" the time travel, the process will repeat until the time travel results in a stable change to the past or until time travel does not occur.

Another way to avoid a paradox with a single universe require incomplete causality: travel back in time results in a universe in which the time machine and what it carries seem to appear from nothing spontaneously. There would be absolutely no explanation for its existence in terms of anything in the universe, but it could be explained in terms of the universe which "previously" existed. In this view the spacetime universe would be a four dimensional object that changes over meta-time. Has anyone read James P. Hogan's Thrice Upon a Time? It describes a similar scenario, in which only information can be sent back in time.

Then we have the possibility of multiple time lines, but from within an individual timeline this would be indistinguishable from a combination of the possibilities I mention above. In the time line where time travel is first discovered, time travel will appear to simply be a way of blinking yourself out of existence. In the time line created by the occurence of time travel, a time machine will seem to appear from nothing, completely without causal explanation. Without a way to travel from one timeline to another, no time traveller could be sure that they weren't simply replacing futures rather than creating new ones.

tronvillain

HeatherD:
Yes, I once took that view, but eventually I realized that it simply does not hold up. Or rather, it leaves you with exactly the same problem you had before, only with "meta-time" in place of "time." Now, perhaps a meta-time exists and the present is a wave moving forward in time when viewed from meta-time perspective, but unless you propose another level of time on top of that, you will simply be left with a "frozen" five dimensional universe. The problem is that no matter how many layers of time you propose, the problem of the frozen universe remains to force you to propose yet another layer of time. So, it is far simpler to propose that the universe is a frozen four dimensional object, for which every moment in time is "now."

sophie

tronVillain says time travel is not "illogical at its core." It may or may not be impossible, but it is not inherently illogical. Anyone claiming that it is is simply experiencing a failure of imagination.

ME : My imagination fails me when the science is unfounded. It seems tronvillain places more value on his imagination rather than hard facts, science facts or natural facts of life.

Jesse

Even this is too weak, I think, because it still allows for the possibility of "changing" the past, which would suggest some sort of physically implausible "meta-time" and also allow for small paradoxes. The resolution of paradoxes favored by most physicists who speculate about time travel in the context of general relativity is the fixed block time idea in which the whole of spacetime is a fixed four-dimensional entity, with the constraint that history must be self-consistent (see the Novikov self-consistency principle), so any interaction a time traveler has with the past was already part of this self-consistent history all along. This does open up the possibility of weird causal loops, like a time traveler who grew up fascinated by the mysterious disappearance of Amelia Earhart, until finally he goes back to investigate and the shock of the appearance of a time machine causes Amelia Earhart to crash the plane. Still, there is nothing inconsistent about such loops.

The self-consistency principle also suggests that any attempt to change the past is bound to fail--if you try to assassinate Hitler in 1935, the gun will misfire, or you'll kill the wrong guy, or your time machine will blow up in transit, or you'll have second thoughts before going through with it. People sometimes object to this on the grounds that it would seem to require some sort of intelligence or strategy on the part of the universe, but perhaps it's better to say that the laws of physics demand that among all logically possible sets of events, only the self-consistent ones can become a reality.

Imagine you want to write a computer program to generate a possible chess game. One way is to start with the pieces in their starting configuration, then have the program generate each successive configuration on the next turn from the configuration on the previous turn, using only legal chess moves. But here's another, more elaborate way to do it: have the computer generate an entire series of configurations at once, completely randomly--it just picks randomly which pieces to put in which positions on which turn. It is very unlikely that the resulting series will look like a legal chess game--a piece might randomly be on a particular square on one turn, but then the next turn randomly be on some totally different square that it shouldn't be able to get to in one move by the rules of chess. But suppose you have access to God's own home computer with nearly infinite speed and memory, and you have it generate a gigantic number of random series this way--if your number is large enough, chances are at least some of the series would just happen to satisfy the rules of a legal chess game. So you could specify to the computer to throw out all series which violate the rules of chess, and be left only with series that represent legal chess games. But since you are dealing with an entire series at once, you could also place other constraints on them, like "throw out all series where white wins", or "show me only series where the black rook checkmates the king in 25 moves", whatever you want. For sufficiently detailed conditions, it might be very hard to generate a chess game that matched them in the traditional way of starting from the beginning and basing each new configuration of pieces on the configuration of the previous turn, but using this brute-force method of generating a near-infinite number of entire histories, and throwing out all but the ones that satisfy your constraints, it's easy to get a game that satisfies any conditions you like without even having to think about it or plan the details of the game.

Similarly, suppose you were using God's PC to generate a simulation of an entire universe--instead of picking some initial conditions and then letting it evolve forward according to some set of laws of physics, you could again specify your "laws" in terms of conditions on entire histories, with the computer generating a huge number of random histories and then throwing out all the ones that don't satisfy the conditions. If the "laws of physics" you pick happen to allow time travel, you can also impose the condition that everything about the history is self-consistent, and the computer will find some. But the computer has no intelligence, it's just randomly generating a huge number of possibilities until it finds one that satisfies the constraints.

I'm not really suggesting that reality generates a huge number of "virtual histories" randomly, and then picks out one that satisfies certain conditions and magically makes it "real" somehow...my point is just to illustrate the idea of laws of nature as timeless "constraints" on an entire history, rather than as a rule that constantly generates the "next" state of the universe from the "previous" one. The question of how reality would make sure that our universe obeys such a constraint may not be answerable, but then neither is the question of how reality makes sure our universe obeys the regular laws of physics we're familiar with. There's no reason to think one should involve any more "intelligence" on the part of the universe than the other, though.

Note that physicists are already used to thinking of laws of physics that take the form of constraints on entire histories rather than "dynamical" laws that give you the next state of the system from the previous one--the principle of least action is one such example. In most cases it can be proven that such global laws are mathematically equivalent to some set of dynamical laws--this article talks about this idea, starting with the example of deriving Snell's law in optics from the wave theory of light, and moving on to a similar derivation of the principle of least action from quantum wave mechanics (although the principle of least action can also be derived from ordinary Newtonian mechanics, and Newtonian laws can themselves be derived from the principle of least action).

The theory of general relativity, on the other hand, seems to inherently be a law of global constraints on entire spacetime histories rather than a "dynamical" law. In many cases it is possible to "foliate" a curved spacetime into a series of curved space-slices and recast the laws of GR into dynamical laws describing how curved space changes over time, but not all spacetimes can be foliated in this way (According to this article, spacetimes that can be foliated and thus recast in dynamical terms are called "globally hyperbolic".) Spacetimes including closed timelike curves, ie objects that travel into their own past, are one type of spacetime that cannot be foliated, according to this abstract.