[jiggawatts]

I’ve never quite understood why this is so certain.

If I instantly teleported to Alpha Centauri, that wouldn’t put me in the future.

Sure, if I turned a telescope back towards our system and watched Earth, I would see myself wandering around as I was four years ago and then… after four years… I could watch myself step into a teleporter.

This is entirely consistent and in no shape, way, or form would this let me get super rich on the stock market.

You could only ever know information from your present or your past.

Imagine a hypothetical universe with a maximum speed ‘s’. The creatures in this universe could develop relativity and everything, the same as us. But what if ‘s’ is the maximum speed of sound in the gas that fills this toy universe? What if the creatures are all blind and use only sonar to get to know their world? Would travelling faster than ‘s’ be violating causality somehow? Or would it simply be the same as a supersonic plane, leaving a sonic boom behind it?

Having said all that, I very strongly suspect that FTL will never be possible. However, I don’t agree that it would result in time travel if it were possible.

[mr_mitm]

If you "instantly" teleported to alpha centauri, then there would be a frame of reference in which you arrive before you left. To another observer, you would travel back in time. It's not something you can disagree on. Instead, convince yourself by studying the spacetime diagram: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...

Note that "instantly" is ill-defined in SR. Simultaneity of events is observer-dependent.

Edit: this is also known as the tachyonic antitelephone. It's described here: https://en.m.wikipedia.org/wiki/Tachyonic_antitelephone

Instead of warning about shrimp you could easily tell your past self to buy a certain stock.

[jiggawatts]

> there would be a frame of reference in which you arrive before you left

There would be a frame of reference in which you appear to arrive before you left.

The analogy is with a sonic boom, where an observer using only sound for sensing the world would hear the plane arriving before it left.

This does not allow closed timelike loops, the type that would allow you to predict the stockmarket.

The observers can't create paradoxes through via a third observer with a high celerity in the same way that QM "action at a distance" doesn't allow causality violations either, or how moving a laser pointer across the surface of the moon doesn't allow FTL motion either.

It just looks like it does, that doesn't mean that it actually does.

PS: I'm fully aware that in the SR/GR model, appearance is reality, and hence the paradox within the framework of relativity. Clearly, if FTL were allowed, then relativity would be falsified by that, and a new framework would be required. I'm saying that that framework could be consistent, allowing FTL and relativity without allowing travel backwards in time.

[gtech1]

Does this assume a single universe ?

In other words, can causality be broken if we assume we live in a multi-verse and achieving FTL from Time-PointA in Universe 1 to Time-PointB in Universe 2 ?

[mr_mitm]

Yes it assumes a single universe. I'm not aware of any multiverse solution to this. There is, however, the Novikov self-consistency problem which could solve this. It's entirely speculative, of course.

Edit: IIRC Novikov only covers wormhole-type time travel, i.e. configurations of curved spacetime, while we were only talking about flat spacetime until now.

[superposeur]

And allowing non-flat space time refines but doesn’t alter the picture.

For instance, it’s a common misconception that wormholes circumvent the relativistic prohibition against ftl travel. Actually, if a wormhole could facilitate travel between two events faster than a light ray moving between the same two events on the outside of the wormhole, then all the same causality troubles would ensue. This just highlights that that the particular mode of transport between events is not important to the argument. (Whether it be warp drive or secret tunnel.)

[gnad_sucks]

Studying a spacetime diagram is hard work. It's a lot easier to just say "I never understood why it's so certain" and leave it at that.

[avmich]

> If I instantly teleported to Alpha Centauri, that wouldn’t put me in the future.

If the teleport is using a wormhole - a device which connect two points in space - we can consider this.

We have experimental confirmation of time dilation, if we have an accelerated motion. In other words, let's make the wormhole entry on Earth move for some time so that its time is behind, say, by 10 years from the Alpha Centauri.

Then, if you instantly teleport to Alpha Centauri by stepping from Earth into the wormhole, the time at Alpha Centauri is 10 years before. You may use, say, 7 years to fly back to Earth through "ordinary" space, with sub-light speed, and you'll arrive to Earth 3 years before you left.

[Jaygles]

True teleportation would not hit the same limits. The idea is, in conventional travel, your velocity has a time and space component that are hard linked, the faster you are traveling through space, the slower you are traveling through time. The speed of light is the maximum speed you can travel through space, because you've run out of the time component you must take away from to get a larger spatial component. An object that has a spatial speed greater than the speed of light must have a negative time component, aka traveling into the past, which under current understanding isn't possible.

[andrewflnr]

It doesn't really make sense until you grok how relativity thinks of events and "observing" them, and then stare at the relevant spacetime diagram for a while.

Note that the time when the light arrives from (in your scenario) Earth is not the same as "observing" it; observing is a much stronger sort of hypothetical measurement, more like assembling all the evidence in retrospect and deducing a consistent physical story. That's the story where, if stuff is moving FTL, you start seeing cause and effect reversed.

Ed: the key difference between c and your hypothetical speed of sound is that light is the same speed no matter how fast the observer is moving. Two observers both have to see a laser moving at c, even if A also sees B moving in the same direction at c/2. With your example, B can actually see the relative speed of an object moving at s as s/2.

[shsbdncudx]

You could do something on Alpha Centauri, instantly teleport back, and then make a bet on what’s going to be observed in 4 years time.

[jiggawatts]

You'd be seeing Earth's past at Alpha Centauri. You have only the information you take with you through the wormhole (the present), and information that has been travelling through space for 4 years (the past). You have no information from the future, and I've never seen a convincing argument as to how it would be possible to cheat this via instant travel.

[shsbdncudx]

That’s not what I meant, read the comment again