This just occurred to me, while watching the first Terminator on IFC this weekend.
IF matter cannot be created or destroyed, then the total matter in the universe at any given moment is constant, right? What would the consequences of altering that constancy?
If you send a Scharzenegger-sized chunk of matter out of the present and into the past, isn’t that matter now MISSING from the present and SURPLUS to the past?
Is this proof that time travel is not possible, or can this “obstacle” be overcome?
if you assume time is absolute and that the universe really cares about the flow of energy through time, perhaps.
but time is not absolute. time does not “flow” the same throughout the universe. parts of the universe might be billions of years older (say, very near a black hole) than other parts, despite their genesis coming from a common event (the big bang).
there are other ways, though, to show that time travel isn’t possible. at least, not while maintaining our current view of many things (such as causality).
Disregarding for the moment that it’s complete fiction, couldn’t there be an energy drain where the Terminator appears in the past, and an energy surplus where the Terminator leaves in the future? In fact, didn’t they show bolts of lightning surrounding him when he appeared? Perhaps energy in the atmosphere is converted to matter (and vice versa), thus maintaining the conservation.
Quantum Mechanics allows matter to be created and destroyed, although it’s usually just elementary particles. Also, Einstein’s famous equation e=mc^2 equates mass and energy, so it’s more correct to say that the total amount of mass plus the total amount of energy (on a macro scale) must be preserverd.
Well, first off it’s not just mass, it’s the conservation of mass and energy. We can get around this, thematically by assuming that the time travel device simply swaps Schwarzenegger for a Schwarzenegger size chunk of mass/energy. That is, it takes a piece of the past and swaps it for a piece of the future. As long as each chunk is the same mass/energy we have no problem.
Secondly, the creation and destruction of mass/energy isn’t really a problem from a modern quantum theory standpoint. Virtual particles are created and destroyed spontaneously all the time and there is no balance between the two.
Finally, and most importantly, Quantum Physics has demonstrated conclusively that the future won’t miss Schwarzenegger at all.
In one of those tricky dick paradox thingies it comes clear.
When the future sends Schwarzenegger back in time it creates a deficit in mass/energy, right?
There’s too much mass/energy in the past and not enough in the future, right? There’s a defecit in the future and a surplus in the past.
Eventually though time goes on in the past. The years go by and finally we are at the point where Arnie was sent back. At that point in time when they send Arnie back they correct the surplus and balance everything out.
So actually it would violate the conservation of mass/energy if they didn’t send Schwarzenegger back. Why? Because they did.
The only way that you could screw things up would be by sending Schwarzenegger back so that he changes the past so that they end up not Schwarzenegger back in the future.
Of course, by definition this is impossible. If they sent Arnie back then they did. If they didn’t, they didn’t.
So, to answer your original question, not only does it not violate the laws of physics to send Schwarzeneger back in time, it is specifically required that it happen by the laws of physics.
This is one of those things that’s obvious if you’re a Conservative. That’s why liberals shouldn’t be allowed to mess with the space/time continuum.
I’ve heard that theory before, but I guess I don’t get it. It depends, perhaps, on your concept of time. Does it depend on a static and rigid concept of time? I am no scientist, but if time is relative, it might not be a matter of “creating” and “destroying” matter, but simply moving it around in a dimension.
Imagine the flatland world, where the world is flat like a table top and the beings are all two-dimensional shapes. suppose a string were passed through the plane, and then turned back to go through it again. To the flatlanders, this would seem like matter had been “created,” because their perceptions can’t conceive of what really happened. At least for the point of speculation, this could what it is like for us with time.
I second eli’s notion, although I’d phrase it differently. If the 4th dimension is a palpable realm that is merely beyond our perception, then there’s never a deficiency or a surplus. All we’re doing is relocating the matter.
I mean, assume you could perceive yourself in 4th-dimensional form. You’d probably look like a multi-armed and legged snake, with a fetus at one end and an elderly corpse at the other. (We can get really absurd and have your component particles at either end as well, so we’d see, for example, the sperm and egg of your parents, the food your parents consumed that generated into those cells, yadda yadda.) Time travel would really just bend the direction of the “snake” so that you’d loop back into an earlier axis that you’d already been through before. From a 4th dimensional standpoint, nothing’s been gained or lost.
My head hurts. I’m wondering how I’d draw a 4-D picture…
Yes; I tend to think this problem of mass does rule out time travel; the citizens of the far future will come back to visit us, and to escape the heat death of the universe; this will increase the mass of the universe now, and perhaps increase it so much that the universe starts to collapse.
Additionally some of the atoms that go back in time will persist long enough to go back in time again * on the same time machine*; you will find that some atoms only exist in closed timelike loops- the proportion of these will increase and add mass to the universe…
believe me you don’t want to go there… http://www.orionsarm.com/intro/WhyNoTimeTravel.html
The physics explanation is rather final on this point: time travel to the past is possible for a few bizarre solutions to Einstein’s Equations. While it has been proposed that there are logical inconsistencies to time-travel to the past, there are working solutions that avoid all the problems presented.
After viewing the links provided by eburacum45 and JS Princeton, I wonder if there is a necessary balance between pro-time-travel and anti-time-travel websites.
If time travel were possible, then theoretically it would be possible to move ALL matter, backward or forward, to the same point in time. This would mean a universe that was wholly without matter except in one brief moment when it contained infinite matter: surely this is impossible, and proof against the viability of time travel? And it time travel ever does become possible, doesn’t it stand to reason that, like any other means of travel–roadways, riverbeds, ocean currents–there would develop, over a time, patterns of heavier and lighter travel? Well worn paths and unexplored wildernesses? Wouldn’t we have seen evidence by now, unless all such well worn paths exist only in the future and none cross the past or present? I mean, if time travel were possible, wouldn’t someone have come back and told us so by now? In the way that mechanical travel technology (trains planes and automobles) have made the world seem to be a much smaller place, wouldn’t time travel eventually make TIME seem like a much smaller universe? Isn’t the fact that this has never happened proof that it never will happen?
Not entirely. Suppose your time travel machine can only send stuff back through time as far as when the machine was first turned on. Since it hasn’t been invented yet, nobody can come back this far, but the moment it is invented and turned on, 17 billion people might come pouring out of it.
The phrase “the same point in time”, the way you are using it, is meaningless in terms of relativity. Measurements of time are dependent on position and velocity; there is no “point in time” that exists for the entire universe. Let’s say your friend leaves in a rocket ship for another galaxy at near the speed of light, but you and your friend wish to simultaneously drink a toast on his birthday, while he is midway through his journey and you are still on Earth. That would be impossible, because there is no absolute frame of reference for both of you to measure when his birthday occurs. His perception of time will be completely different than yours. It’s not just that you can’t communicate with your friend, relativity holds that there is literally no common time frame.
In fact, all the matter of the universe does exist simultaneously; it’s called “the universe”. What I suspect you are really wondering is what would happen if all matter occupied the same point in spacetime. Is that possible? Well, considering that it actually happened, and is called the Big Bang, I would say yes. But a qualified yes, because, if I understand it correctly, the math breaks down at that point, and so the jury’s still out as to exactly what goes on at the Big Bang.
Remember that the universe is expanding, so the further back in time you go, the smaller it gets. If you somehow succeeded in moving all the matter of the universe backwards in time into a single point, time would not exist; there would be no “other” times during which the universe was “wholly without matter”. Everything the universe is, would be, and ever was would exist at that one point.
And then we have the possibilty of multiple instances of mass;
an immortal time traveller could come back to the present, live in forward time untill the last proton decays, and come back to the present again; a universe with repeat tourists would quickly - instantly - fill up.
The moral of the story is - please don’t pull that switch on the first time machine…
But each time you travel back, while you would add to the mass of the present-day universe, you would also subtract from the mass of the future universe, so the net amount of matter in the universe would still be equivalent. IANA physicist, and I have no idea how the math works, but I’m guessing that a mass exodus from the future to the present would cause the universe to implode. I believe one theory holds that once the universe begins to contract, that time reverses direction. So if enough people made the journey back, time might reverse and they would no longer be able to “live in forward time”.
Where’s SentientMeat? Get in here and explain this, man.
I agree somewhat with lissener. I think the conservation of mass/energy precludes any notion of time travel and, in fact, time itself. Bodily movement through time becomes a patent absurdity when you realize the slightest imagined “travel” would require an infinite creation of substance from nothingness.
I think it is safe to say that nothing comes from nothing. Because of this intuitive understanding we eventually conclude that the stuff of the universe is neither created nor destroyed. It simply changes form.
There is a “before” and an “after” in motion/time but these are merely proximal shifts following the physical laws of the universe. The fact is that there is no place (in time) to go. There is only the “now”.
The tendency to conceive of “time travel” at all is merely a symptom of the time illusion.
I know this because it was (and will be) explained by the Talking Rings of the Eloi.
Matter is present in space-time and not in just space so it is inappropriate to say conservation of matter/energy is violated by time travel. The fact of the matter remains that any energy density that is taken from the future to the past will necessarily effect the universe in the very way to counteract the future “problems” of having removed the energy density by compensating for it in the past. If the universe is consistent then time travel will have happened already at the point at reaching that particular event in spacetime and there is no energy density issue. The conservation of energy “violation” is actually not an issue at all because it doesn’t occur.
Don’t think of energy occupying a point in space and time marching on. Time and space are intimately connected. Energy density is involved in both space AND time. That’s why you can have cosmological redshifts, for example, which seem to indicate that the energy is changing (but really it’s the volume which is changing and the energy as a non-transformable value in conformal time stays the same).
Complicated stuff, I know, but it works out. Time travel to the past is possible. The solutions are available and follow Einstein’s Laws. Those who argue against it might have the practical edge, but the theory allows it.
