Paradox? (split from Is FTL actually possible?)

[Moontanman]

1 hour ago, Genady said:

But if you can travel in space so that you reach Moon in 0.5 s, causality is violated.

I can't argue with the point of causality being violated but how is it any different than someone detecting the asteroid in advance with a telescope and getting the area evacuated before impact? In both cases the deaths are prevented, in both cases the terrorists survive that wouldn't have. In one case the antiphone is used and the other a telescope is used. Methods are different, results are the same.   

[studiot]

2 hours ago, Genady said:

But if you can travel in space so that you reach Moon in 0.5 s, causality is violated.

So ?

[Genady]

1 hour ago, Moontanman said:

I can't argue with the point of causality being violated but how is it any different than someone detecting the asteroid in advance with a telescope and getting the area evacuated before impact? In both cases the deaths are prevented, in both cases the terrorists survive that wouldn't have. In one case the antiphone is used and the other a telescope is used. Methods are different, results are the same.   

The difference is that in the case of a telescope there is only one history and no time loop is possible.

[Genady]

2 hours ago, studiot said:

So ?

You did not say it, but the statement,

Quote

Nobody thinks that causality is violated because you can travel in space and kill your grandfather.

might make an impression that causality sets a restriction on time travel, but not on space travel. I want to point out that space travel is also restricted by causality. If you are on Earth and your grandfather is on Moon, and you travel there in half a second and kill him, the causality is violated.

[studiot]

13 hours ago, Genady said:

You did not say it, but the statement,

might make an impression that causality sets a restriction on time travel, but not on space travel. I want to point out that space travel is also restricted by causality. If you are on Earth and your grandfather is on Moon, and you travel there in half a second and kill him, the causality is violated.

Actually this is not correct.

It does correctly suggest that causality (which is a temporal relationship) does restrict time travel, but not space travel, regardless of other considerations.

Space travel is not restricted by causality, there is always another factor in play which does this.

In the case of your example, your restriction applies to the travel speed, not to the fact that the Earth-Moon distance is traversed. The traverse and murder could easily the accomplished by suitable travel speeds without violating causality.

 

[Genady]

3 minutes ago, studiot said:

restriction applies to the travel speed

Right. However, this restriction applies to the combination of distance and time rather than only to time.

[studiot]

4 minutes ago, Genady said:

Right. However, this restriction applies to the combination of distance and time rather than only to time.

So what ?

speed is not time or distance, it is something else.

 

[Genady]

13 minutes ago, studiot said:

So what ?

speed is not time or distance, it is something else.

 

My emphasis is that it is not only time. It does involve distance.

[Moontanman]

Does anyone have a personal Oxford Academic account? If you do you can access this paper on the antiphone disaster paradox.

 https://academic.oup.com/book/36741/chapter-abstract/321838499?redirectedFrom=fulltext

[MigL]

Causality restricts ( possibly ) yravel in time.

Travel in space is restricted by the speed of light.

( although since relativity joins space and time, violating the SoL effectively violates causality )

[geordief]

6 minutes ago, MigL said:

Causality restricts ( possibly ) yravel in time.

Travel in space is restricted by the speed of light.

( although since relativity joins space and time, violating the SoL effectively violates causality )

Are there any possible universes(I know  this " other laws universe"  is sometimes discussed) where space and time could   be not  joined?

 

Could there be a model of a different universe  where that was part of the set up  and would it hold together?

 

I was also wondering if time loops might be permitted somewhere on the  micro level.(in this universe )?

[Genady]

52 minutes ago, geordief said:

Are there any possible universes(I know  this " other laws universe"  is sometimes discussed) where space and time could   be not  joined?

 

Could there be a model of a different universe  where that was part of the set up  and would it hold together?

Such a universe would have almost nothing in common with this one. No particles as we know them, no gravity as we know it. No electricity and magnetism as we know them. No light, friction, chemistry, etc. Maybe "it" wouldn't be a "universe" as we understand this word. For example, such a mathematical construction is perhaps possible.

 

56 minutes ago, geordief said:

I was also wondering if time loops might be permitted somewhere on the  micro level.(in this universe )?

I don't think that would necessarily contradict something that we already know.

[geordief]

12 minutes ago, Genady said:

a universe would have almost nothing in common with this one. No particles as we know them, no gravity as we know it. No electricity and magnetism as we know them. No light, friction, chemistry, etc. Maybe "it" wouldn't be a "universe" as we understand this word. 

Would such a universe have to be one where everything  happens at the same time and in the same place? (perhaps to paraphrase in reverse a couple of well known definitions)

16 minutes ago, Genady said:

I don't think that would necessarily contradict something that we already know

Have there been any speculations along those lines?

With time and causation being such perennial fascinations it wouldn't surprise me.I may have seen something similar a few years back but can't recall where I saw or whether it was purely  theoretical or if there was any more to it.

[Genady]

40 minutes ago, geordief said:

Would such a universe have to be one where everything  happens at the same time and in the same place?

Not necessarily. It's basically a Newtonian universe with everything non-Newtonian in this universe being replaced by something else.

 

42 minutes ago, geordief said:

Have there been any speculations along those lines?

Feynman's original idea of antiparticles was that they are particles moving back in time.

[studiot]

6 hours ago, Genady said:

My emphasis is that it is not only time. It does involve distance.

But my counterpoint was that you are mistaken about this.

I noted that there is no distance, no matter how small or how large that cannot (in principle) be traversed in either direction.

There is no precedence theorem such as causality which only applies to time.

That is therefore quite a different situation from time where, for any A < B there is only one way to get from time A to time B and there is no way to get from time B to time A.

That is what causality says.