[OkGoDoIt]

I don’t think that actually works. In this case you’re talking about a round trip, with the switch’s outbound signal traveling fast (airborne/vacuum light speed) and the return signal of the object visually changing traveling slower (water light speed). The total round-trip where you see the effect of flipping the switch would take longer if either leg involved water, but it wouldn’t cause the perception of it to happen ahead of the act of flipping the switch.

[withinboredom]

Observer A and observer B are mermaids. A throws a switch that turns on a light on a light house. Relative to each observer, the can see the cause and effect. B invents a periscope that allows them to see faster than light. Now B will be able to see the light turn on before the switch is flipped.

Replace periscope with “wormhole” and you get a more traditional experiment. The question of can we use this to violate casualty is non-sensical, because we can’t violate casualty (even with faster than light travel). In the traditional experiment, if I see the light turn on, the cause has already happened; sending a message “back in time” won’t change that.

However, this is only because all frames of reference stay the same. If you could actually travel back in time, who knows what would happen. That’s largely why this whole conversation makes no sense. You can’t violate casualty with FTL, only with time machines and FTL isn’t a Time Machine.

[MattPalmer1086]

FTL is shorthand for "Faster than Light" but here it really means "Faster than Light in a Vacuum".

Light actually has nothing to do with it; it just happens to travel at the max speed allowed by the universe when there's nothing that impedes it's motion (i.e. in a vacuum).

So the acronym should really be "FTLIAV"!

[withinboredom]

There are many things that can go faster than light, most of which we don't know about yet. But one thing is for sure, quantum entanglement can be undone faster-than-light. It's just that nobody has yet figured out how to send information through that medium, and it may even be impossible. But clearly, causality isn't being violated here and it goes faster than light in a vacuum.

[andsoitis]

> But one thing is for sure, quantum entanglement can be undone faster-than-light. It's just that nobody has yet figured out how to send information through that medium, and it may even be impossible. But clearly, causality isn't being violated here and it goes faster than light in a vacuum.

In quantum entanglement, two particles can be entangled in such a way that measuring one particle instantly determines the state of the other, even if they are light-years apart. This "instantaneous" connection seems faster than light, but it cannot be used to transmit usable information in a meaningful way.

The phenomenon does not violate relativity because no classical information can travel between the particles faster than light. Entanglement is a correlation, not a means of communication and hence NOT a means of causation.

[MattPalmer1086]

In general relativity, FTL can be used as a time machine.

[withinboredom]

Then you should be able violate casualty with simple water, otherwise FTL cannot be a Time Machine because it is quite simple to go FTL.

[MattPalmer1086]

Faster than light in a vacuum. Not faster than light in other mediums. See the many other explanations on this thread.

[withinboredom]

It's literally the same thing. The medium doesn't matter, see Einstein's equations. It all boils down to "relative to what". Light moves slower around a black hole, and that is in a vacuum. None of the arguments make any actual sense.

[MattPalmer1086]

There is a maximum speed, "c" that anything can move relative to something else in this universe.

If light happens to move slower than c under some conditions, that is irrelevant. It isn't the speed of light we care about, it is c.

Essentially, when we say FTL, it means "faster than c", not "faster than light".