Cambridge University Press has two good books: Exact Spacetimes in Einstein's General Relativity (Griffith & Podolsky) - good readable (if you speak GR) account of the more common solutions Exact Solutions of Einstein's Feild Equations (Stephani, Kramer, MacCallum, Hoenstelaers, Herlt) - encyclopedic, authoritative and mathematical. In my case an aspirational purchase
Unless the theory of relativity is superseded by something quite radically different.
Does Quantum Entanglement have any promise for instantaneous communication (theoretically) ?
There was a time when the current model of physics couldn't allow for traveling to different planets.
Never say never.
It is possible that relativity will be refined by further developments (quantum gravity?), but it is reasonable to expect that the refined understanding will reduce to relativity in an appropriate limit, one core feature of which is the causal structure of events in space time.
Hoping that exotic physics will allow ftl travel is like hoping that an exotic theory of gravity will give a radically different prediction for the time it takes an apple to drop from tree to ground. True that “never” is a strong word but this is just about as never as any fact we know.
I use this strong word as a corrective to the impression you get reading discussion threads, where you may suppose ftl is next up on musk’s bucket list, or just limited by sluggish tech development.
To be able to move those sorts of distances and arrive at a time comparable to the origins absolute time would require the “time travel” aspect of ftl, right?
Yes, the point is that any way to send a signal between space like separated events A and B (in sr or gr) would also allow a return signal to be sent back from B to event C in the past of A. So any means of ftl signaling (wormhole, warp drive or whatever) is tantamount to building a time m achine. No amount of techno optimism or can-do cleverness will ever be sufficient to surmount this obstacle in real life.
Incidentally the Gödel metric runs into causality violation in a different way: closed timelike curves. No need for ftl , it’s even less realistic as Einstein is quoted pointing out in the article.
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.
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.
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.
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.
Retroactive causality implements the same functionality without requiring information to propagate faster than c.
