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Time Travel: Probability and Impossibility

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Re: Time Travel: Probability and Impossibility

#61

Post by throwawaytemp27 »

framecowbird wrote: post #60
zoratu wrote: post #59

I did not see mention in the link of the problem of space. You might very well move through time, either forward or backward, but the solar system is moving through the galaxy is moving through the universe is still expanding. Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.

This assumes some fixed, absolute "background" space through which the universe is moving; the idea being that you'll stay in the same position in "background" space but all of the things you care about (e.g the earth) have moved on. Is space not relative?

That makes no sense to me. One particle goes in one direction at a particular velocity and another particle goes in another direction at another velocity. They diverge over time. When you travel through time, which particle’s path do you follow through space?

I often think along the gp’s lines and would love an explanation of why it’s wrong?

Re: Time Travel: Probability and Impossibility

#62

Post by wyldfire »

ajuc wrote: post #34

Recently an interesting paper came out that starts by accepting all solutions to special relativity (usually the faster-than-light solutions are ignored) and goes on to derive the axioms of quantum mechanics from just that. It turns out what you need to prevent timetravel paradoxes is exactly quantum mechanics. https://www.eurekalert.org/news-releases/540753 https://arxiv.org/abs/1910.02780 I don't understand the who…

> It could be interpreted as universe having a separate family of particles going backward in time synchronizing entangled particles over distance.

Are you kidding me? I'm a big Nolan fan but I really threw up my hands at "Tenet". It didn't feel like it made a whole lot of sense.

Re: Time Travel: Probability and Impossibility

#63

Post by Jansen312 »

jnurmine wrote: post #25

I don't understand why backwards time travel is even a thing. This is how I've thought of it: Assume you have a clock with two atoms A and B some distance apart which interact somehow with mechanism C using waves, fields or whatever, which move at light speed. This interaction between A and B happens at some rate. You test the clock at rest using your arbitrary reference clock, a pendulum swinging at some rate. You o…

When clock travel faster than speed of light it catches up with the previous light showing clock earlier time. That is time travel as observed (assuming observable anyway). For observable the clock will stay fixed and then disappear as no light will ever bounce back from the clock to observer.

Re: Time Travel: Probability and Impossibility

#64

Post by hn_throwaway_99 »

new_guy wrote: post #28
hn_throwaway_99 wrote: post #6

Earlier quoted context omitted.

> Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics". Huh? Would you argue that it's "incredibly naïve" to think perpetual motion machines aren't possible purely based on "laws of physics"? Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics.

> Because I think it's incredibly naïve to think perpetual motion machines would be possible, precisely because they do violate the laws of physics. What do people mean by 'perpetual'? 1 year, 100 years, 1 million, is it expected to outlast the universe? And 'laws of physics' is such an anachronistic term, it carries such baggage with it, better to say 'what we think we know so far'. And a 'perpetual energy machine'…

> And a 'perpetual energy machine' is trivial, punch a hole through to another universe and siphon their energy out. That's probably what black holes are, another universe has punched a hole through to ours and is sucking the energy out.

I also sometimes smoke too much weed.

Re: Time Travel: Probability and Impossibility

#65

Post by remram »

xwdv wrote: post #35

Is there any sci-fi story where time travel is possible, but the government has mandated that the only legal purpose for it is to offset the effects of time dilation from near-lightspeed voyages? Basically as you travel to your destination, you are also traveling backward in time so you can arrive at the same time you started relative to when you left, essentially making a sort of “warp drive”. To an observer on Eart…

Universal War One (comic) has such a device. Spoiler warning, someone invents a way to travel faster than light and hides the fact that it is perfectly capable of traveling through time too. Most of his time developing the device is actually spent putting safeguards around this feature and concealing it.

Re: Time Travel: Probability and Impossibility

#66

Post by sogen »

Jansen312 wrote: post #63
jnurmine wrote: post #25

I don't understand why backwards time travel is even a thing. This is how I've thought of it: Assume you have a clock with two atoms A and B some distance apart which interact somehow with mechanism C using waves, fields or whatever, which move at light speed. This interaction between A and B happens at some rate. You test the clock at rest using your arbitrary reference clock, a pendulum swinging at some rate. You o…

When clock travel faster than speed of light it catches up with the previous light showing clock earlier time. That is time travel as observed (assuming observable anyway). For observable the clock will stay fixed and then disappear as no light will ever bounce back from the clock to observer.

Doesn’t this mean that the clock is just moving faster?

Re: Time Travel: Probability and Impossibility

#67

Post by RyanCavanaugh »

jerf wrote: post #56
quantum2021 wrote: post #41

Earlier quoted context omitted.

Why not? Read the particle as there/remove the particle. On/off.

You know, I've seen this discussion carried out any number of times, and the math just doesn't convince people this is impossible. So, try this on for size: If this did work, it would be easy . It is every bit as easy as the idea sounds. Any lab set up for "quantum" experimentation could do this; entangling two things is step one in any experiment that you could call "quantum". Yet this is not established, easy techn…

Obviously you're right on the physics, but this is like saying that supersonic flight must be impossible, otherwise we'd all be commuting that way.

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.

Re: Time Travel: Probability and Impossibility

#68

Post by devwastaken »

quantum2021 wrote: post #41
Strilanc wrote: post #39

Earlier quoted context omitted.

You can't do on/off manipulation of entanglement. Side note: timelike entanglement is kind of funny to read about. It's a bit like describing "just leaving an atom in the same spot for a bit" as a "timelike quantum teleportation". It's applying fancy words to make the normal case sound like the strange case.

Why not? Read the particle as there/remove the particle. On/off.

To my understanding, quantum is nature's method to some things we have had to derive artificially. Entanglement does not involve "communication". Changing the spin on one particle does not change the other in reverse. When entangled, ones spin will oppose the other, but you can only ever know what spin that is by measuring it (with scientific instruments), which "breaks" entanglement, therefore you can't do anything further to it. This is one of those problems where you're working with the very fabric of the universe, and there's no debugger. You can't know the state of the particle without measurement, but measurement changes it.

Re: Time Travel: Probability and Impossibility

#69

Post by ajuc »

zoratu wrote: post #59

I did not see mention in the link of the problem of space. You might very well move through time, either forward or backward, but the solar system is moving through the galaxy is moving through the universe is still expanding. Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.

> Therefore, maybe you moved in time, and when you’ve phased to no longer has the where you needed.

It just means you have to move at an angle. Like when you jump onto a slowly moving train. If you want to land in a specific place on that train you have to aim ahead of it.

Re: Time Travel: Probability and Impossibility

#70

Post by drran »

Jansen312 wrote: post #63
jnurmine wrote: post #25

I don't understand why backwards time travel is even a thing. This is how I've thought of it: Assume you have a clock with two atoms A and B some distance apart which interact somehow with mechanism C using waves, fields or whatever, which move at light speed. This interaction between A and B happens at some rate. You test the clock at rest using your arbitrary reference clock, a pendulum swinging at some rate. You o…

When clock travel faster than speed of light it catches up with the previous light showing clock earlier time. That is time travel as observed (assuming observable anyway). For observable the clock will stay fixed and then disappear as no light will ever bounce back from the clock to observer.

> When clock travel faster than speed of light it catches up with the previous light showing clock earlier time.

I can just record the clock and replay the record. It makes back-in-time travel much simpler then.

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