I’ve heard that as an objection to time travel in general: it would violate conservation of matter/energy.
Of course, that isn’t a disproof; it might be that conservation laws have to be surrendered in such cases.
Same with ftl: just maybe, instead of banning ftl, we simply have to give up our faith in causality.
But don’t we already have theoretical exceptions to time and causality, in Tipler Machines (rotating cylinders?) Hasn’t it already been established, at least in pure mathematical theory, that causality can be violated? We’re just safe, because the nearest rotating mass of sufficient magnitude is a damn long way away.
So what exactly is the problem with abandoning the parts of general relativity which conflict with the concept of absolute simultaneity? What’s wrong with the model that describes a spaceship moving faster than light relative to the planet it took off from not actually travelling backward in time?
The problem I’m having wrapping my head around this is that from every frame of reference, light itself takes time to travel from one point to another (could this be where I’m missing the point?). Light never takes zero time to travel between points. Something hypothetically traveling faster than light would just take some fraction of that time to get between the two points, not “negative time,” wouldn’t it?
I’ve seen renditions of that situation that require the wormhole (or whatever allows the time travel to the past) to lose mass equivalent to the rock in order to maintain conservation of mass. No idea if that would work.
You can’t. General relativity is a consistent mathematical framework. You can’t just throw out part of it and have the entire thing still be consistent and make sense. Once you notice that light travels at the same speed in all reference frames, and follow the mathematical consequences of that observation, you end up with relativity.
I don’t think it would be a problem, because there’s no such thing as a universal TimePoint. Every place in the universe experiences time differently, so it’s impossible to say what the mass/energy of the universe is “now” since “now” is only a local concept.
If you fling a rock at near the speed of light, it will “warp” ahead in time due to time dilation. So what happened to that extra mass in the meantime? Nothing happened to it, it was always there, just going really fast. I think it’d work the same going backwards. If you send a rock back in time, you still have to pick a path through space-time to send it, and so there’s never a moment where the rock vanishes; it’s always somewhere in space-time.
Conservation of mass or of energy would not be a problem. The law of conservation of energy is a local law, not a global one. Basically, if you have a box, then the amount of energy within the box changes only when energy flows through the walls of the box. Surprisingly, it’s possible for this to be true for boxes everywhere within a universe, while not being true for the universe as a whole, and in fact it looks like this does in fact happen in our Universe (though in a different form than with time travel).
And as an aside, my old advisor once wrote a paper concerning the Alcubierre drive, wherein one of the references cited was to “M. Scott, private communication”.
Conservation of momentum is a worse problem, I believe. Alcubierre drives are essentially inertialess drives; build enough of them and you could move the stars across the sky into pleasant or offensive configurations.
All ‘realistic’ proposals for time travel (that I am aware of, at any rate) depend on some implausibility or another such as for instance the existence of ‘negative’ or ‘exotic’ matter. In the case of Tipler cylinders, they need to be infinitely long. So at the moment, I think the question whether the laws of the universe allow time travel is open either way (personally, I think it’s a attractive to speculate that those laws might be the way they are precisely because they do not allow time travel – that any universe in which genuine future-past influence is possible simply is unstable to being retroactively erased by some future event prohibiting its formation in the first place).
I disagree that having a preferred reference frame breaks special relativity. Consider a 2-D Minkowski space, where the time-like dimension is curled into a circle. So space is the surface of a cylinder, with time running roughly along the axis. Locally, special relativity holds the same as in an unbounded 2-D space. There is only one frame, though, where a single time coordinate can be consistently define: the one where the space coordinate forms a closed circle around the cylinder. In any frame in motion relative to that, the space coordinate spirals around the cylinder.
This has observable effects, too. If you have a pair of twins, one in this preferred frame and one jetting off, if you wait until the second twin goes all the way around the loop, the second twin will have aged less than the one in the preferred frame. You can only tell what that frame is if someone circumnavigates the universe. Otherwise, it’s just special relativity like you expect.
I don’t think we know that the universe isn’t bounded, so it’s possible we live in a closed universe with such a preferred frame.
I disagree that your example is truly a preferred frame. The only thing that happens when the space coordinate spirals is vanilla time dilation. Your pair of twins example is note for note the same as the classic Minkowski space twin effect.
My argument wasn’t that a preferred frame of reference breaks SR, but rather, that what Mosier proposed does; this leads to a preferred frame incidentally. Periodic boundary conditions/compact spaces do indeed lead to universes that have a preferred frame (in a universe of finite spatial extent, it’s the frame in which it is smallest) and which are locally special relativistic. One could debate, though, whether SR truly holds, as global Lorentz invariance is broken.
The problem with that is that we have tons of good evidence that there’s no such thing as absolute simultaneity. BTW, that’s even the case for special relativity. And as mentioned above, you’d have to throw the baby out with that particular bath water.
I might not quite understand your question, but let me try to fight ignorance with some real light weaponry. Hand-to-hand combat perhaps. You guys that know the math, please feel free to correct any mistakes on my part. I learned the math once long ago …
The best analogy that helped me conceptualize this is from Brian Greene’s book, “The Fabric of the Cosmos”: “slicing the spacetime loaf”.
Imagine the universe as a long loaf of bread. Slice it and look at the sliced surface, that’s the (2D) universe at some point in time. The loaf on one side is the past, the bread on the other side is the future (or, later than the slice point).
Without special relativity, everyone would always slice the loaf at 90 degrees to the length of the loaf, like normal bread slices. There would be no argument about “now” being different for different observers.
OK, now factor in special relativity. In that case, you always slice the loaf at 90 degrees. But if someone is travelling towards or away from you, they’d slice the loaf at what would seem to you to be an angle.
There’s a limit to the angle based on the speed of light. Depending on units conversions, that could be say 45 degree slice.
The nice thing about this model is it allows us to look at reality without being too blinded by our “stationary” perspective, and still see the differences that observers at different speeds (and/or at different locations) would perceive – albeit in a 2D universe.
However, I haven’t tried to figure out how to illustrate the above paradox using this spacetime loaf. Maybe some better-armed fighter can attack this.
I’m not sure I’m understanding this correctly. I get that the idea is that a tiny difference in relative motion, magnified by millions of light-years, can lead to a significant difference in simultaneity. But this would apparently mean that two observers on opposite sides of the Earth, one rotating toward a distant galaxy and one rotating away, could disagree by days on the timing of an observable event such as a supernova or the light curve of a variable quasar. I’m sure I would have heard if such is the case.
Note that there is only ambiguity if there hasn’t been enough time for light from the event to reach the observers.
Whenever there’s enough time for light to go from event A to event B, all observers report A as preceding B.
Going back to the spacetime loaf, you can’t slice the loaf more than 45 degrees.
Stick a sphagetti through the loaf, longwise, parallel to the sides. That’s one point in space, through time. If there are two events at the same spot, all observers report them occurring in the same order.
Take one point and cut the loaf in a 45 degree cone from that point – sort of like shaving the loaf like a pencil point. The resulting surface is the set of times and places where light could have reached the point. Anything before that surface, all observers will agree occurred before the point.
Cut the cone the opposite way. That’s the cone where light from the event at the point could reach. All points forward of that are reported by all observers to be later than the event.
Now let’s think about the stuff between those two cones we cut away above. Its basically a section of the loaf with two longitudinal concave cones cut into it, meeting at the center. That’s the set of events where the order of events is reported differently by different observers. That is, for any point in that stuff, there will be some observers who say that point happened before the “cone” event, and others who will say it happend after (and some who saw them at the same time).
Yup, it seems strange, but it’s not so strange.
Now regarding the other galaxy. It’s in the questionable section. Only now, we have a BIG loaf of bread! Two people near the center of that loaf. If one is walking away from the other, his “slice” of “now” is at an angle to the one who’s sitting still, and vice versa. If some event in that galaxy far far away is between the two slices, well …
… it really doesn’t mean anything to say what they observed, since they can’t observe it …
… but in our model, for one guy that event could be in his future, and the other guy, in his past. But it does NOT matter, because neither can observe it anyway. Not that that matters much either, because we can come up with examples where the two CAN observe two events, and report them as happening in a different order.
But to come up with that example, the distances have to be great, or the relative speed has to be great, or the time between the events has to be miniscule. So, we can’t come up with an everyday example. (If we could, Newton would probably have noticed it!)
Are you saying that observers on Earth will observe the event at the same time, they’ll simply disagree on exactly how long ago the light left the distant galaxy? (i.e., exactly to the kilometer how far away the event was?)
I know that the original idea required an infinitely long cylinder, but I had been told that this is only an ideal, and that, in the region of a finite rotating cylinder (of absolutely stupendous and gargantuan mass) trajectories could be defined that went back in time.
You’re still right, in that we aren’t going to be able to shoot spaceships at relativistic speeds very close to rotating black holes. It’s never actually going to happen.
But haven’t the abstract physics been shown merely to be absurdly (even laughably) difficult, yet not flat-out impossible?
(I agree with you, when it comes to wormholes depending on “negative energy.” And yet…as I understand this…and that ain’t saying much!..“negative energy” could be accumulated. It isn’t absolutely impossible…)
I think it was a Larry Niven essay, where he talked about time travel (and in particular the invention of a time machine). He went through various scenarios of how people would go back in time and change things, modifying the timeline, until eventually the only universe that is stable is one where a time machine is never invented.
“The Theory and Practice of Time Travel” in which he states Niven’s Law Niven's laws - Wikipedia “If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.”