Let’s say I fire up my DeLorean, hit 88 MPH and travel back in time 24 hours. Effectively, for those 24 hours, I have increased the mass of the universe by the amount of a DMC-12, flux capacitor, and a fat American.
As I understand it, time travel is mathematically possible, but wouldn’t going backwards violate the Second Law?
Grr. I meant First Law. At least I got it right in the title.
It may or may not violate the first rule of fight club, depending on what you said when you got back there.
I’d imagine that it depends on how you accomplish it. Theoretically, you could just swap equivalent matter from the past to the future as you move from the future to the past. But seeing as time travel is, itself, beyond the scope of our knowledge, it’s possible that the current laws of physics aren’t applicable to begin with.
Hawking has stated that there is not yet a known law that would prevent time travel into the past (we know travel into the future, relative to someone else is possible). This isn’t to say there isn’t a law preventing it, we just haven’t found one yet other than the obvious causality problems.
If you think of the universe as a four dimensional space (with time being the fourth dimension) then it looks to me as though you will not actually have changed the mass of the universe, just shifted some of it about a bit, along the time dimension. Is that any more problematic than, say, driving your Delorean further down the street?
I’ve thought about that, but I really don’t understand it. Wouldn’t the fact that time is an arrow muck up shifting mass along the time dimension? At the very least, isn’t it reversing entropy since you’re going against the flow?
Sage Rat: I do like the idea of swapping equivalent mass. I could see sci-fi writers using that to humorous or dramatic effect.
This came up in another thread recently: Conservation of mass and/or energy is a local phenomenon, not global. Basically, it says that if you take a box of constant size, and the energy content of that box changes, that amount of energy flowed through the walls of the box. The most plausible ways of making a time machine would all be consistent with this.
Matter travels pastwards routinely. Whenever Monday’s photon at A goes to B on Tuesday, that can be viewed equivalently as Tuesday’s photon travelling pastwards from B to A arriving Monday.
The difficulty in getting meaningful “time travel” is thus not simple conservation problems, but rather the need for backwards-moving matter to retain organization and information. The Second Law of Thermodynamics could pose difficulties for that, but no one really believes in it anymore, do they? 
I don’t know; is not believing in the Second Law a higher-entropy state than believing in it?
By the standards of the first law, I don’t think it’d be any different than going really fast and “warping” forward in time. One thing that is glossed over in many time travel stories is that with relativity, there’s no such thing as time and space, only spacetime. So inputting “yesterday, 4:00 pm” on the time machine isn’t gonna do any good, you’ll have to specify a location too. Well that’s not a big deal, but you’ll have to keep in mind that yesterday at 4 in New York isn’t the same as yesterday at 4 in London, and certainly not the same as yesterday at 4 on Mars, or Alpha Centauri. When we specify a certain time, it’s purely a local idea. There is no universal 4 PM yesterday, at which we can say there was x amount of mass in the universe, and if we travel back, there will be x+mass of time machine mass in the universe. All you’re doing is moving the mass to another location in spacetime.
The first law itself is only a local phenomenon. On the scale of the whole universe, energy disappears all the time, from the redshift of photons due to expansion.
Also, consider that the “proofs” of the laws of thermodynamics simply boil down to “An occurrence of a violation of the law has never been observed.”
Not to knock the laws, they seem pretty damn ironclad. But throw in something like time travel, and who knows?
/thermo was always my weakest subject in college
Yes! Now I understand. I was visualizing this as a textbook heat transfer problem, which was obviously a gross simplification.
ETA: Gamehat, that’s a good point. The sad thing is that I aced my two semesters of Thermodynamics as well as my two semesters of Heat and Mass Transfer. Doc Brown would scold me with, “You’re not thinking fourth dimensionally!”
Damn, are you a ChemE too?
Mechanical, although I’ve never actually worked in the field.