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i have been watching a bit of Dr. Who and SG1 lately and became fascinated with the concept of time. now, i am just a high school student, and have no idea what i am talking about, but i felt i needed to ask this question because i want to know the answer if there is an answer.
this is how my logic worked:  energy and matter cannot be created or destroyed and therefore the total amount remains constant  when people travel through time  weather forwards or backwards, they take mass with them because they are of course, made of mass knowing this i am asking: what happens to the mass that disappears? if the amount of mass/energy remains constant and someone travels through time taking mass with them what would happen? the amount of matter in that point of time has increased and the amount of matter taken from the original point of time would have decreased. And according to the first law of thermodynamics this is impossible. i had a second question but i forgot it, if i remember i will post it. any questions? :D 
The mass bends! It stretches along time through dimensions so it is always there, but infintesimally small at each point in time. i... i.. don't understand that. ARGH MY BRAIN HURTS! i have been watching a bit of Dr. Who and SG1 lately and became fascinated with the concept of time. now, i am just a high school student, and have no idea what i am talking about, but i felt i needed to ask this question because i want to know the answer if there is an answer. Who said it's correct in all instances? Einstein isn't all knowing. Maybe timetravel is a loophole to that theory, or maybe energy is released in current time and mass/energy is replaced with that person in the future/past.
Edited by EvanorEthan on 5/17/2011 12:49 AM PDT

Time travel works, we are right now travelling through time into the future. Traveling into the PAST is the real problem. We have no idea how that would work if it were possible, therefore, we don't know how it would correlate with mass/energy conservation. Time traveling deeper into the future, however, is possible and explained by Einstein's theory of special relativity. According to special relativity, the closer you are to the speed of light, time passes at a slower rate for you, which means you age more slowly (not that you would be aware of that... time would feel the same for you). This way, if you move fast enough, 1 year for you could be 100 years for a different observer. You would have travelled 100 years into the future and would not violate mass/energy conservation at all, since you would have existed at every point in time between these 100 years. 
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Your problem is that you are thinking of each slice of time (and as it pertains to the universe) as a closed system. It isn't. The universe as a whole *may* be thought of as a closed system....but I time isn't applicable as a way to define that system.

I'm on my 4th year on my Ph. D. in physics. I will try to go a little deeper in explanation than "it's how it is, accept it" but still make it understandable for someone with basic physics knowlegdge. Feel free to ask more questions.
Special relativity will not really help here as it is actually invariant under timereversal (t > t). This implies that you could move backward through time the same way as forward. General relativity is what you what to talk about. I'm not an expert on that but if I remember correctly, you cannot find a space metric (how spacetime is curved) such as you travel back in time. Basically, the metric is a 4x4 matrix and you would have to change the sign of its determinant by an unitary transformation. This cannot be done as anyone with some knowledge of linear algebra will tell you. Now, according to quantum mechanics, time doesn't really exist (according to some theory at least). Time is actually a macroscopic quantity, like temperature that doesn't exist for "small" system (2, 3 particles). Quantum physics is actually invariant in time so time travel is of course possible since it doesn't matter. However, due to our macroscopic nature, we are bound by the 2nd law of thermodynamic (entropy increases in time). To answer to OP: the laws of physics, as we currently understand them, forbids traveling through time in the past. So the question of what happens to the mass is irrelevant. 
E=MCsquared in layman's terms To send a 180 pound person back in time requires a #!@%load of energy.

To answer your question, it's called entropy. The Universe (theoretically) has contracted and expanded several times in the past, and it loses mass with each contraction/expansion cycle. When it loses mass to the unknown, it is called 'entropy.'
Mass can not disappear outside of those parameters, though. Edit: The theory I mentioned is the most prominent one, but it is by no means 100% correct.
Edited by Xugos on 5/17/2011 6:43 AM PDT

So you can't define entropy and energy for 1 particle, given that it exists in a sea of other particles? I was assuming 23 isolated particles. If they are in a sea of other particles, then you can defined a temperature for the system. @ Xugos : wtf? What are you talking about? I have no clue. @ Astrai: That's not what entropy is exactly. Entropy is a measure of the disorder of a system. Technically, S= k_B \log \Omega. S= entropy k_B = Boltzmann constant \Omega = number of available state for the system. The 2nd law of thermodynamics state that a closed system will tend towards a maximal entropy state. 
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