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Time Travel: Probability and Impossibility

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Re: Time Travel: Probability and Impossibility

#21

Post by cjg »

hn_throwaway_99 wrote: post #6
vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

> 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.

We can already see how the laws of physics might accommodate energy not being conserved - and from there a small hop to perpetual motion machines.

Energy conservation relies on time translation symmetry. In other words, that the laws of physics do not change over time. Well, perhaps they do change slowly.

A prime example of this is the expansion of spacetime, which can reduce the energy of a photon travelling through empty space - it's redshifted. This has been experimentally observed.

See https://www.preposterousuniverse.com/blog/2010/02/22/energy-... for more details.

Re: Time Travel: Probability and Impossibility

#22

Post by Koshkin »

vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

It is incredibly naïve to think that a thought based on knowledge has the same value as a thought based on ignorance.

Re: Time Travel: Probability and Impossibility

#24

Post by Koshkin »

cjg wrote: post #21
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.

We can already see how the laws of physics might accommodate energy not being conserved - and from there a small hop to perpetual motion machines. Energy conservation relies on time translation symmetry. In other words, that the laws of physics do not change over time. Well, perhaps they do change slowly. A prime example of this is the expansion of spacetime, which can reduce the energy of a photon travelling through…

Isn't redshift a results of the change in the observable wavelength, i.e. the photon's momentum?

Re: Time Travel: Probability and Impossibility

#25

Post by jnurmine »

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 observe one interaction in the clock once every swing update rate. (Let's call this 1 Hz)

Now, send the clock to travel at light speed and have a magic camera to see 4k live stream of what happens. The clock behaviour would appear to slow down. The interaction between the atoms becomes slower, because C has to travel a longer distance to transport effect from A to B and vice versa. Therefore, C travelling from A to B will take a longer time (i.e. more swings of your reference clock).

Here there was no time dimension, just rate of change in something you're observing, using some arbitrary duration (pendulum swing) as a reference "clock".

So, I don't understand how one could travel backwards in an observed rate of change? How would that even work?

Re: Time Travel: Probability and Impossibility

#26

Post by leephillips »

godelski wrote: post #20
nikhilgk wrote: post #5

Earlier quoted context omitted.

That's "relatively" easy to do. Get in a space ship that could quickly get to a fraction of c, travel for a short amount of time and then return back to the starting point. You would have traveled to the future from the point of view of some one at the starting point.

This is actually harder to do in practice and is a common misunderstanding of the twin paradox, which is resolved by incorporating acceleration into the model. You'll have better luck with gravity wells.

It’s not only not a misunderstanding of the twin paradox, but is correct and verified by experiment.

Re: Time Travel: Probability and Impossibility

#27

Post by nonameiguess »

vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

The laws of physics as we know them today don't forbid time travel. In fact, they propose a fairly specific means of doing it via accelerating one side of a traversable wormhole to near the speed of light. The only reason this isn't actionable is it requires exotic matter that may not exist, and if it does, we certainly don't know where to find it or how to make it (and we don't currently know how to make a wormhole, but we know they're physically possible).

Usually, when people rule out time travel as impossible, it's because it has the potential to introduce paradoxes that totally break any possible understandable notion of causality, i.e. the inventor of time travel going back and killing himself before he invents it.

Thankfully, the known physically possible forms of time travel don't introduce that specific issue as they would not permit traveling back to any point before the wormhole was created, which also goes far to explaining why, if time travel is ever going to be invented in the future, we have never seen anyone come here from the future.

Re: Time Travel: Probability and Impossibility

#28

Post by new_guy »

hn_throwaway_99 wrote: post #6
vanillax wrote: post #4

This is like saying we will never fly a plane. Do you really think people in the 1200's could fathom a phone or the internet or access all information that ever has been on a device that fits on your phone? Its incredibly naïve to think that time travel isn't possible purely based on "laws of physics".

> 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' 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.

Is it perpetual? Well, it'll last a billion years or so. Does it violate our naive understanding of physics? Not at all.

Re: Time Travel: Probability and Impossibility

#29

Post by LegitShady »

Koshkin wrote: post #24
cjg wrote: post #21

Earlier quoted context omitted.

We can already see how the laws of physics might accommodate energy not being conserved - and from there a small hop to perpetual motion machines. Energy conservation relies on time translation symmetry. In other words, that the laws of physics do not change over time. Well, perhaps they do change slowly. A prime example of this is the expansion of spacetime, which can reduce the energy of a photon travelling through…

Isn't redshift a results of the change in the observable wavelength, i.e. the photon's momentum?

The energy of a photon is defined as

E = (hc)/λ

where h is the planck constant, c is the speed of light, and λ is the wavelength.

As a redshift is the increase in wavelength it is an actual decrease in the energy of the photon. This makes more sense when you consider that a photon is mass-less, and so its total, energy is bound up in its momentum rather than momentum and mass.

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