Time Travel: Probability and Impossibility

> One way to make it crash is making the simulation crawling to a stop by requiring always more computation to be done by preventing the simulator to take shortcuts in the computing process.

Isn't this the reason for mass-induced time-dilation? When you add more mass (computational complexity) into a region it runs slower, to the point of being removed completely from the sim, hidden behind an event horizon and described by just a couple of parameters.

"so almost nobody has tried, and nobody expects it to work because it that's not how entanglement is understood to work."

Thank you.

I'm thinking you have schrodinger's box, you have particles set inside it you know are all entangled with other particles in the future. You read the initial 'code'.

Can you change the particle from on/off? What happens if you eliminate a particle in the future that is entangled with a past particle/vice versa; will that particle change states or somehow escape the box?

How would you know if the code had changed? If you observed it in the past the past would have changed so it would seem like nothing had changed. Or would both the past observation and future (if possible) be changed at the same time?

So I guess what I'm asking is the theory saying it's actually impossible, or just saying we can't currently figure out a way to see it?

If the past and future changed at the same time we probably wouldn't currently be looking at that as communicated information, even if information was being communicated to the past from the future. Now what about the initial communication that changed the past? Does it even need to occur any longer once the past and future have changed, or do we just sort of slide into the new future by altering the past?

Could be, but it only works in a non-adversarial context. Biological numerical instabilities are a tougher beast to tame.

When you have to simulate a big star you can do it quickly by approximating it by a sphere of a certain temperature. The bigger the star, the more the law of large numbers apply and the better your approximation get.

When you have to simulate a computer that would be the size of a star, you can't take any shortcut. And the amount of computation that a computer the size of a star would allow you to do according to the limits of physics is huge, and would allow to simulate the full universe many times (see https://en.wikipedia.org/wiki/Limits_of_computation).

One way to avoid a simulation crash would be to introduce time-dilation effect to throttle the quantity of computation locally, but fundamentally it's a very hard problem equivalent to the halting problem.

This is why I hammered so hard on if it worked, this would be easy.

Colonizing other planets may or may not be possible, but if it is, everyone expects it will be very difficult. If supersonic flight is easy, it's obvious that it isn't necessarily easy. (It is, of course, possible, and it isn't easy. It's solved, but it's not easy.)

If FTL communication was just a matter of entangling particles and poking them to collapse this way or that, thus sending a message on the other side, it would be easy. It would be a lab demonstration in every college-level quantum mechanics class, right next to the double-slit experiment.

"If you can send a regular signal around the planet faster than you can read a quantum entanglement state, which seems entirely likely (to say nothing of bandwidth), then the technology would never have a useful terrestrial application."

If you haven't read those links ajuc linked in this conversation, you should. It's a very similar idea, getting to QM by taking superluminal relativity seriously. (I suggest this as an "interesting followup" to you point, not disagreement.)

Still, given that the "entangled state" is likely to be as easy as detecting polarization on a photon, since that's the whole point, it doesn't seem likely this would be the case. More likely the case is simply that FTL communication is impossible. You can send an entangled state around the world and read it in two places, no problem, today, plenty quickly. That's how the quantum key distribution works. You just can't communicate with it because you have no influence over how the entangled state collapses.

(In fact, there's a sense in which quantum key distribution works precisely because the properties you'd need for FTL communication don't exist. If they did, quantum key distribution wouldn't work safely!)

IE could you do an experiment with two boxes of quantum entangled particles, one box is read, the other box in a 'sealed room' for a set amount of time. If the entanglement through time is possible, and altering a entangled particle so it's sister particle responds is possible, would there be a chance that when you changed the entangled particle in the sealed room after it was unsealed it would change the particle in the observed room?

Or would it be like where entanglement through time may be possible, the original measurements in both boxes would change instantaneously if the 'later' box was changed so it's, as far as we know, unmeasurable?

Could you create a black box of entangled particles and a code to read them and post it and just hope someone would write to you from the future and you could read it? Sort of like Hawking's time travel party nobody showed up to but for information and just hope something shows up someday. So the code never changes, but the entangled particles in the box can. This could potentially get around the problem of not knowing if the particles were switched in the box. You know the code which doesn't change so you could simply read the box every day and hope that someone from the future had set the particles to a definite state that was readable. I understand that if the particles are observed it doesn't automatically change the particle, but if the particles are observed one way and are continuously observed then they must stay that way, and then the entangled particles must be the opposite of that. From what I understand you're saying it'd be a one and done transmission, but then it wouldn't break the no communication theorem and would be ftl (using a slower than light method, the code which would travel through time at regular speed, to jumpstart the process).