For some time I’ve pondered on the following question about time travel:
We’ll assume that energy in our universe is a constant. So if we consider the universe we live in as a closed system, what does this mean for time travelling?
Let me elaborate. If I, constituting all the atoms and constituent particles of which I am formed, were to travel back in time say a thousand years, wouldn’t those same particles already be present there, in other forms, for example in the water that is coursing through its own cycle at that time, but which is, at the moment, very much busy with being part of my body. Or any other atoms or molecules which were somewhere else at the time.
I would think that this is another argument against timetraveling into the past, but with the distinction that without branching into varied future alternatives, or into stable time loops, this would actually add energy into a closed system from the future against the laws of conservation. It would take the same energy that was in the future and tack it to the past where it already existed.
On the other hand, would this block traveling forward in time? In this case, you would take energy out of the system and put it back again later. I suppose this could be adverted with a technique that would not actually take energy out of the system, like those stasis boxes and similar fancies which are often used in science fiction. But if one where to travel back in time, one would not do it the pedentrian way, that is, through stasis or something, one would just hop into the past or future or whatever through a wormhole or what supports the fiction.
I don’t know if this has been answered exhaustively in another thread, at least I couldn’t find one and I haven’t stumbled upon one on the net either.
If two different things can interact, pretty much by definition they are in the same system as far as thermodynamics are concerned. If it turns out that time travel is possible, that just means you have to extend what you consider to be a closed system to account for it.
That’s your first mistake. The law of conservation of energy doesn’t actually imply that, and in fact, to the best of our ability to determine, the total energy of the Universe is increasing.
The law of conservation of energy is a local law, not a global law. What this means is that, if you take a box with defined boundaries, and count up all of the energy that’s crossing the boundary over some time period, that amount is the amount by which the energy content of the box has changed over that same time period. This is in fact true for every box with defined boundaries you can draw anywhere in the Universe… But the entire Universe itself is not such a box.
The same principle applies, though. Every method of time travel physicists have come up with (by which I mean write down the equations, not implementing the situations described by the equations) has that same property of conserving energy locally but not globally.
Hmm, trying to get my head around what this implies from Neother’s Theorem. So if energy is not conserved across the universe in time, that implies that time invariance symmetry is not true across the universe? (Since time symmetry -> energy conservation: TS -> EC, thus ~EC->~TS). Sort of. But only when we move in both time ands space. Ugh.
If you assume that there exists a single universe with one unique history, then the only logically consistent form of time travel is called a closed timelike curve (CTC). Such curves are possible solutions to the equations of General Relativity, which, as Chronos pointed out, does not conserve energy globally. While I personally don’t think this is directly relevant to your question, it does hint that there might be something more deeply pathological regarding energy conservation wherever GR is concerned. For instance, energy is only conserved for inertial frames of reference, and in GR you can have things like frame-dragging causing all sorts of complications. Another possibility would be a wormhole, where travel through a CTC would only be possible to some time after the wormhole had been created. Therefore, supposing you are traveling from (position,time) = (x_A,t_A) to (x_B,t_B), then 1) an enormous amount of energy would have to be present at (x_B,t_B) before you even arrive there, and 2) a mouth would exist that would allow exchange of energy between the two locations in spacetime. I personally like to think that this hints that the CTC would insure that energy is always locally conserved.
I remember when I first learned this, I thought the professor had to be mistaken. What do you mean there’s no time symmetry?!
Even weirder, you can find that time symmetry does exist, but only in really peculiar circumstances: the universe appears to be symmetric under a time inversion only if you also invert spatial directions (down is up, left is right, etc) and electric charge of particles (protons are negative, electrons are positive, etc). If you do two inversions but not the third, physical laws get fundamentally changed. But if you invert all three - charge, direction, and time, then physical laws are exactly the same.
This is known as Charge-Parity-Time symmetry, or CPT symmetry. It’s pretty wild.
Thanks for the answers. The theory behind closed time like curve sounds interesting, if it means, as I gathered, that there would be a transfer of energy between the two points in time.
But what I was thinking was a bit different as an obstacle to timetraveling. Since I do not really understand all there is to understand about energy and how it is formed, I intuitively think of it as small blocks of something and for example an atom is matter formed of energy. So, for example, all the carbon and other atoms constituting a single nervous cell in my brain came from somewhere prior to my birth and growth. So if I travel back in time, the carbon and other atoms would be somewhere else, in the soil or in some other animal or as gas or something. What I start to think then is that this would somehow duplicate something already existing in the past…
I’m getting confused. Is my problem in thinking that energy or atoms or such have an, I don’t know, individualistic nature? That time duplicates would somehow overlap or something, or is all the energy just stuff which can be exchanged between systems without anything weird happening? And this is assuming that there is just this one timeline where the past, present and future is the same universe and basically the energy would be the same.
And I figured that the universe was a closed system, because I thought that all the energy that is contained in our universe has been present and been the same amount since the big bang, so that’s why I thought it mattered for my question.
I suppose, although I still can’t wrap my mind around the idea that by going to the past would somehow cause overlapping, but I may be mistaken with assuming that energy has a particular, still I don’t know what word to use, identity, which moves through time in whatever direction.
Invert charges? How does inverting charges do anything at all? I was under the impression that the labeling of positive and negative is entirely arbitrary and all that matters is that they are opposite.
That’s true for most of physics but not for the weak force. Beta decay happens at a slightly different rate. Electromagnetism is unchanged, and so is gravity and even the strong force, but that pesky weak force looks a little different.
I should add before someone corrects me that to flip the signs of the charges you are really flipping the signs of all the internal quantum numbers. Really, you’re replacing all matter with antimatter in a charge inversion.
Also note, that even if we needed to be worried about mass-energy (electromagnetic charge, or lepton number, or color charge, etc) conservation, we could get around it by bringing forward in time the exact quantities to balance what we’re sending back in time. That might be an engineering challenge, but it’s not a fundamental theoretical problem.
That the molecules you replace travel forward in time to fill in the vacuum left. If the material is less dense than you then the temperature increases to match the energy in your body thus flash-frying the Earth like a gamma-ray burst. If more dense then you cause a new Ice Age. Either way, you single-handedly have killed us all. Good going Hitler.
If you “disappeared” from “here” and magically “appeared” in the past, then you would have a good point. But this is not realistic. Something more realistic might be something like:
3000 AD: A Star collapses, forms a black hole
3050 AD: Scientists realize they can turn the black hole into a worm hole. The worm hole is unstable, and needs an incredible amount of energy in order to “stay open”.
3050 AD: Fred pops out of the worm hole.
3050 AD: Worm hole collapses.
3050 AD: Scientists note that the amount of energy added to the universe by Fred appearing is the exact amount of energy they lost into the wormhole by trying to keep it open. (Energy is conserved)
4000 AD: Fred is instrumental in discovering the “other end of a worm hole.”
4000 AD: Fred jumps into the worm hole and disappears. Simultaneously, a burst of energy emits from the worm hole as it collapses. Energy is conserved.
The energy of the universe technically is not constant because space is expanding due to the cosmological constant. When space expands, galaxies move away from each other. Since their relative speed is increasing, they are being “given” energy. In this particular sense the universe is not a “closed system”.
It should be emphasized that C, P, and T by themselves are all really good approximate symmetries: The weak force (and only the weak force) violates P; CP violation only shows up at the level of about one part per thousand and only in a few specific reactions, and T symmetry violation has never actually been observed directly, just derived from the observed CP violations and the mathematically-proven CPT symmetry.
In general relativity, the conseravtion of energy is extremely limited. It only works in some reference frames and is not guaranteed to work for a spatially extended system.
So for example a small system will usually have some kind of conservation of energy, but there’s no reason a large system should have.