Similarly, if wormholes are real, and you construct one in a gravity well, what happens to gravity? Especially if both ends are in a gravity well?
I'm not sure what to think about gravity "reaching through". Gravity and the wormhole are both just curved spacetime. Maybe the larger body distorts the wormhole throat in a way that looks like "reaching through"? Certainly, you need to have some effect that causes energy conservation when you make a perpetual water wheel by suspending one mouth over another and running water through it...
[0] https://en.wikipedia.org/wiki/Alcubierre_drive [1] https://en.wikipedia.org/wiki/Chronology_protection_conjectu...
It turns out that if you use any remotely sane numbers on the speed of an FTL craft, it is extremely difficult to travel in time. To get time travel, you have to accelerate on the other end to very significant fractions of the speed of light, which itself takes so much time that it's hard to recover it via the FTL drive again. You have to put in absurd numbers for the speed of the drive and your ability to accelerate (and your ability to carry energy for conventional acceleration!) before you can get time travel out of most FTL drives, because bending your light cone enough to put the past into your FTL drive's path takes a lot of energy.
The only fictional drives that have serious time travel possibilities are the ones in universes where accelerating to significant fractions of the speed of light is easy-peasy, and the FTL drive is very, very fast (some are instantaneous) and can be arbitrarily steered. It's actually not that long of a list; a lot of FTL drives can hypothetically be used for time travel but the engineering in the universe still doesn't practically permit it. Just adding a cosmic censor post facto would preserve the story just fine. Many of the universes that meet those criteria are already so fantastical that they do in fact have time travel, so... those work out too.
(There's also several video universes where it's nominally possible, but the video wildly contracts the documentation. Both Star Wars and Star Trek have documentation that their ships can get to pretty significant fractions of the speed of light in small amounts of time, with ludicrous amounts of acceleration available to them, but on screen we only see them moving with absurd levels of acceleration like 50 or 100G, but which still takes a long time to get to light speed as a result, and with refueling being necessary often enough that it's hard to believe they could actually make it.)
I don't know about Wars, but Trek strongly implies that impulse drive reaches significant fractions of light speed with near-zero acceleration time; there is certainly nothing to indicate that the on-screen non-warp acceleration is limited to 50-100G, and there is little on-screen indication that refueling (as opposed to drive system maintenance, and replacing non-fuel consumables used onboard) is regularly needed.
The indication I refer to is that when you see a ship moving on screen from the perspective of another ship, it never suddenly blinks to "near lightspeed", because what that would look like is that in one frame the ship is there, and the next it isn't. That is never what happens. What happens is that the ship lumbers off, and slowly turns over the course of a couple of seconds, and can be seen to visibly accelerate in a way that the ship doesn't even cover its own length in less than five or six seconds. If you work the math it comes out on the order of 50-100G. Accelerating to near-lightspeed in a couple of seconds is several orders of magnitude larger than that.
Of course the ships don't blink around at near lightspeed; it wouldn't be photogenic. So all ships on all shows in all screens move within roughly a single order of magnitude of screen speed, no matter what the tech manuals say. Similarly, no matter what the claimed ranges of the weapons are, all engagements take place within a range where the two ships can clearly see each other, meaning no more than a handful of kilometers at most. No matter how far the ships are nominally apart from each other, a bolt of energy fired from one ship to another will take on the order of half a second to arrive. Almost no matter how zoomed in we are or are not, the energy bolts will subtend roughly the same amount of our eyes. If the ship is, say, firing on a planet, the bolts will perspective shrink maybe by a factor of two or three, if they shrink at all, rather than doing what they should do which is disappear entirely (except for perhaps one lit pixel if the bolt is sufficiently emissive). There's this one ST:TNG episode where the Enterprise uses phasers to quickly mine holes in the ground and when you start to see this sort of thing it's hard not to see the Enterprise as firing on a roughly 5-mile-diameter ball sitting right next to it, because if you follow the perspective of the "beam" that's what it implies about the size of the target.
Space isn't photogenic. Even today on earth, real combat takes place at ranges where you don't have a direct visual on the target a lot of the time. Any even vaguely realistic space combat will take place across ranges where you can't see a mile-sized ship with the naked eye. But that's not what they want to show. Consequently there's a massive, massive mismatch between the numeric claims of performance and what we actually see on the screen. So you might say something like "oh, the screen is maybe being symbolic", but then they routinely do things that imply what's on the screen is the real truth, such as in the one TNG movie where Data leaps between two ships. That's just one example, there's plenty of other situations like that.
> GIVEN: That the universe of discourse permits both time travel and the changing of the past.
> THEN: A time machine will not be invented in that universe.
> For, if a time machine is invented in that universe, somebody will change the past of that universe. There is just too much future subsequent to the invention of a time machine: too many people with too many good motives for meddling with too many events occurring in too much of the past.
thus
> Niven's Law: If the universe of discourse permits the possibility of time travel and of changing the past, then no time machine will be invented in that universe.
https://www.hindawi.com/journals/isrn/2013/482734/
I suspect using GR to design a warp drive is like using a spoon to build a freeway. Without a theory of quantum gravity - which appears to mean not just a theory of quantised space time, but also of the non-locality and complex distributed relationships that haunt QM - you're probably not going to get very far.
The same thing happens with sound regularly. A thing happens, and we receive light before we receive sound. Some third faster thing that gets to us before light would just be some other thing.
The distance of travel requires time relative to both reference frames.
If a star explodes at A and I travel to B at 2x the speed of light to tell folks at B, so what? Some time has elapsed and those two frames are still chugging along with causality.
Maybe there's some physics I've not learned that makes this obvious, but I've never grasped exactly why violating light speed prevents entropy from continuing. Even if at two different rates.
If we perfectly timestamped everything happening on two sides of the world and sent a telegram at twice lightspeed from A to B, it would still arrive after it is sent.
Congrats, you have just traveled “back and forth” through time. To people in both the Point A and Point B reference frames, you have “seen the future” and acted on it.
Now, the Legbreakers aren’t stupid. Once they know someone is screwing around with FTL they’ll just hold bets and payoffs until light can travel. Yes, but… how do they synchronize their clocks between A and B? And how can they distinguish between a lucky person and a time traveler? Etc etc, ad regressum.
"Now" and "Here" as fundamental concepts only make sense inside a single reference frame. If you break the idea that information has a speed limit, then both of those ideas shatter into tiny little pieces.
If you start being able to travel >c those relationships immediately get impossibly and paradoxically twisted.
See e.g.
http://www.theculture.org/rich/sharpblue/archives/000089.htm...
That’s not time travel. You could get the same effect by just blocking the photons before they make it to B, and then traveling at conventional speed.
It sounds like you are referencing a special reference frame where FTL could prove such a special reference frame exists, and that such a frame would allow FTL to be possible without time travel.
Can you explain a little more about this idea?
That said, the usual disclaimers about confidence intervals apply. Also, not checking because you have better things to do is perfectly ok.
I've heard that many times now, but I'm not sure I fully understand it. So, could someone ELI5 how, given a starship equipped with a warp drive (or in range of a series of cleverly put stable wormholes), how can I give myself winning lottery numbers, and meet my great-great-grandfather?
https://physics.stackexchange.com/questions/128075/how-would...
Better job, but less accessible: https://physics.stackexchange.com/questions/52249/how-does-f...
The classic version involving FTL ships: http://www.physicsmatt.com/blog/2016/8/25/why-ftl-implies-ti...
A little math, but useful because it applies to the ability to send signals FTL, regardless of means: https://www.quora.com/Why-would-traveling-faster-than-light-...
I like to think of time and space as two different directions you can go i.e. forward/backward (time) or left/right (space). A stationary observer travels only forward, i.e. through time. So they experience lots of time but very little or no space. As you get closer and closer to the speed of light you're moving less in time and more in space. Your vector gets closer and closer to pointing right (using this analogy). An observer traveling at light speed experiences only space, i.e. no time at all.
Then, once you exceed the speed of light your arrow starts pointing a little bit backwards. You're still traveling mostly in space, very little in time but now the time you do travel in is actually negative.
I forget where I read this vector explanation but it's one that makes the concept tractable in my mind :)
Yep. If a photon could talk, it might say "What is this 'time' thing you speak of?"
And since time has no meaning, neither does 'traveling'. A photon is always 'everywhere it's ever been or ever will be' so it might also say "What is this 'space' thing you speak of?"
If you managed to actually achieve the speed of light, the universe would be completely flat. Everything would be contained in a single plane perpendicular to your direction of travel.
So now, what would it mean if you went even faster? The universe would no longer be flat, but would be achieving a negative size. What that really means is not clear to me, but I suppose it's here where the traveling back in time starts to make sense mathematically.
My impression was that the wormhole was used as a "shortcut" between two distant places to avoid having to travel very fast. Imagine a race where everybody else runs the whole course but you take shortcut. You haven't physically travelled fast but your path has been shorter.
I'd guess that Big Banking is blocking the development of this, it'd be a nightmare for inter-galactic ATM machines.
We think of time being this thing that just moves forward around us, and we can't stop it, time just goes.
But in this interpretation, it's us who are falling through time, literally.
I find that fascinating because it changes entirely how I think about the universe. I used to think of the "big bang" as a big explosion of little balls of energy (quarks, electrons, whatever) that eventually coalesced into matter, when enough time had passed and there was enough room for things to cool down. The part that I never understood before this interpretation is how "enough time could pass", when time itself was also supposedly expanding.
But in fact with this interpretation, which I guess is probably more correct (?), space and time simply exploded and began expanding.. which is what we're told but it's hard to picture, exactly. But if you think about energy always moving at the speed of light, and initially all of it flying off in the same direction (time), then matter becomes those particles which were somehow slightly diverted onto the spatial axes -- momentum is conserved, just redirected. And if you think about particles being ripples in fields of non-zero constant energy, an interactions between the fields causing ripples to bump away from each other in non-time directions which we describe as space, this whole relativity thing, and the fact that electric field particles (photons) do not have mass and move at c, starts to make a bit of sense.
It's crazy to think that what we perceive as time passing, the very possibility of things happening, is actually an emergent property of simple geometry and interactions of pure energy.
The way I visualize it is: start two marbles rolling towards each other down an inclined slope. They bump, and bounce off in opposite directions on the X axis. Now imagine there is no table. Would the marbles "feel" anything in the vertical direction? Or would they only feel something in the horizontal direction? Time, for them, emerges from the fact that they can get closer, bump, and move off in the other direction. The other axis, which they are both moving very quickly in, they have entirely in common and they might agree with each other that it is simply a stationary frame which allowed the "bump" to happen. Same for us falling, unrelentingly and unstoppably, through time. (Maybe?)
Gluons are also massless like light, but don't exist on their own. If there's a particle that carries gravity, it would also be massless. You could say light travels at the speed of gravity.
Unfortunately, I am confused about implications of that.
https://www.amazon.co.uk/Why-Does-mc2-Should-Care/dp/0306819...
And it makes it obvious why FTL travel implies time travel
It’s chock-full of a-ha moments and is a pleasure to read