Which 'known' laws of Physics would fail to work if we could actuallu Time Travel?

Just wondering !!!

If you search here on the boards for time travel, I’m sure your question will be answered in the numerous threads that result.

But, off the top of my head, I would think the law that matter cannot be created or destroyed would fail to work. If you were to travel through time, you would leave your current point in time, thus destroying matter, and upon entering your new point in time, you have just created new matter.

Define “time travel”.

I will assume for the sake of the following example that you are over the age of 10, and therefore were alive in 1991.

When you speak of “time travel”, does your notion of what that term means imply that you could travel back 10 years and be “you” as you are right now (memories of the 90s and the Y2K bug and WTC bombing and SDMB all intact) and occupy a separate physical space from the “you” that is/was busy doing whatever you were doing 10 years ago with memories and experiences that only go to 1991?

That’s the way most science fiction and fantasy treat time travel, although most of them avoid having the time traveler come across his/her own self at a different time (and, necessarily, space).

You will please notice that this model of time travel involves a lot more than travel through time.

Think, momentarily, of time travel like the “progress bar” of a quicktime or AVI file, or an MP3 tune. As it plays, you see the bar move (generally from left to right, at a constant and appropriate rate). Time is a dimension represented on-screen as a different dimension (length from left to right). You can “time travel” the song or movie simply by moving the progress bar back or forward from its current playing position.

When you do, the resulting playback is the same as it was (or will be) when the movie or song was there (or will be there) before (or yet to come). If time travel were to work like that and you went back in time 10 years, you would be the same “you” that you were 10 years ago, with no memories of anything happening later than 1991; you would occupy the same physical space you were in 10 years ago; there would not be two of you walking around in 1991, the “time traveler you” and the “original you”, there would just be “you, as you were in 1991”. Your behaviors and thoughts would presumably be the same as they were the first time you experienced 1991 as a live event, so you would be unaware that you were time traveling.

Note that this model doesn’t give you any opportunity to time travel to the year 4 AD because there is/was no “you, as you were in 4 AD”.

The second model does not violate any laws of physics and may be as good a description of how things actually are as one in which time really does proceed in a strictly linear fashion.

The first one necessarily involves at least one more dimension, so that at one point on that dimension’s coordinates there is a 1991 that contains one and only one “you” (and the events and experiences as they unfolded in a world without a second “you” doing time travel) and a second 1991 that contains two “yous” each doing their own thing, one with memories of years yet to come.

Seems unlikely, given that there is nothing in need of explanation that is explained by such a parameter except for the science fiction scenario of time travel.

If you’re talking about a Back to the Future kind of time travel, then conservation of energy might be a problem, but there are ways to get around this. If you are talking about a Quantum Leap kind of time travel, there wouldn’t be any such problem. What you have, though, in both cases, is a violation of causality. Causality states that if an event happens at a certain point in space-time, it can affect only the events at certain other points in space-time. There are certain points, for example, the past, on which it can not have an effect. Note also that traveling faster than light, like in Star Trek or Star Wars, is, according to relativity, another form of time-travel which also violates causality. Traveling into the future, like in the beginning of Back to the Future Part II, is not a violation of causality, though the conservation of energy thing also comes up.

If you violate causality, you wind up with things like the Grandfather Paradox, Sam Beckett teaching Michael Jackson to moonwalk, and Marty McFly inventing the skateboard. Things which could never take place in a universe that makes sense.

So though that may be, I’m not totally certain that causality is a “known” law of physics, and I’m not sure that anybody knows.

I think that if you could travel back in time in the fashion often presumed in the movies, the most remarkable broken law would be the second law of thermodynamics. So, for example, you could observe a random deck of cards being shuffled into an ordered deck.
Another “law”. which some would say was a law of physics, would be causality. The point of causality is that it is possible for things to cause one another. For example turning on the light switch makes the lights go on. It would be violated if effects happened before the their causes.

I think there are some particles, or particle decay patterns, which aren’t time-symmetrical.

I do a lot of time travel. I go into the future all the time and a steady speed. Fortunately, I like it that way.

The Second Law follows from the notion of definite causality, so that’s only one “law” that you’d be “breaking”. One can, of course, consider time travel scenarios that violate other laws, but they’re not (we think) a necessary part of time travel. Take conservation of mass, for example: The way that’s expressed scientifically is that the change in the mass contained in some arbirarily small volume is equal to the amount of mass that passed through the boundary of that volume. In other words, if I have a room with five people in it, and I see one person go out the door, then I’d expect there to be four people left in the room. Now, it’s conceivable that a person might get into a spaceship in 2401, fly somewhere a great distance away, and return to Earth in 2001 (the famous mathematician Gödel proposed a scenario where this might be possible, for instance). The folks on Earth 2401 wouldn’t see him disappearing, they’d just see him leave for places far away. Similarly, the folks here now who saw him arriving wouldn’t think he appeared from nowhere, he’d just be coming from someplace far away. Conservation of mass would be satisfied.

It’s important to note, by the way, that there’s no theoretical reason to believe that causality only works in one direction; it’s just that we’ve never seen an exception. That might mean that causality loops (i.e., time travel) are impossible, but then again, it might also just mean that they’re really difficult. We don’t know yet.

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Time slows with increases in velocity, correct?

If there are two identically set watches and 1 stays on Earth and the other goes up to Alpha, How much slower or “out” would the persons watch be thats been on the Space Station after 6 months compared to its Earthbound twin? Would it even be measurable?

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bernse, according to this page, the ISS is trucking along at a speed of 17,000 MPH. At that rate, the time dilation due to special relativitiy is about 1 part in 3 billion. Over 6 months, this would amount to about 5 milliseconds, which is definitely measureable. Experiments have been done along the lines of what you’re saying, and SR has been vindicated. You should also note, however, that with going higher in the Earth’s gravity well, there are general relativity effects that you have to take into account. These effects have also been supported by experiment.

The GR effects are larger than the SR effects, and in the opposite direction. Actually, the GR effects include SR as a special case.

I don’t have the numbers handy, but if I recall correctly, the time difference was a few seconds for some of the cosmonauts (the ones who stayed up longest) on Mir.

From my sci-fi experience, I believe time travel to be impossible. I feel that the trauma would literally rip a person apart. If it actually did happen, would there be people who told Kennedy to not go in that motorcade in Dallas? How would that change our world, would some sicko that found Hitler and make him the god of the world? Scary stuff, I don’t want to go back in time and revisit Caligula.

If in the future they do travel in time wouldnt they come back and tell us. Or even stop terrible events from happening.

That much? Wow. I’d never have thought it.

Thanks