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[Madman198237]

We have lots of !!SCIENCE!! threads, but only this one science thread.

[Maximum Spin]

Also. Wouldn't it be more accurate to say light sphere than cone? I get what you meant by cone, but unless you were shining a flashlight, it'd be a sphere of everywhere from that point.

Light cone is a different concept, the axis of the cone is time. (The cone shape is what you get when you simplify to two spatial dimensions, of course, you get a four-dimensional hypercone otherwise.)

Anyway, you're misconceiving the rest, but I'm too tired to explain right now, so please accept this promise to clarify further later if you want.

[smjjames]

We have lots of !!SCIENCE!! threads, but only this one science thread.

Yeah, there's the space thread and tech/engineering/automaton/environment thread further down.

[Trekkin]

Though technically a warp drive which stretches/compresses the fabric of space isn't a FTL drive in the absolute definition because you aren't physically trying to move faster than light.

People love to bring that up, but that the ship itself is not physically moving faster than light relative to the bubble doesn't mean that the bubble and ship aren't moving at FTL speeds as seen by an outside observer (granted, you don't see the bubble, but still). The bubble is just there to facilitate movement at FTL speeds without dealing with acceleration to/past lightspeed. It's a separate problem.

And yes, seeing an unexploded star that you watched explode breaks causality. In the simplest case, let's say you warp from the exploding star to another faraway star around which your friend orbits. From your perspective, the star exploded, you left (after it exploded, having observed the light from its explosion), and you arrived at their star. From their perspective, given a sufficiently powerful telescope, the sequence of events is reversed: you arrived, you left, and then the star exploded. Causes cannot follow effects. Now, you can see unrelated events happening differently from different perspectives just fine; to return to our exploding star example, let's say we have four stars in a line A-B-C-D and stars B and C explode simultaneously. A would see B explode followed by C; D would see the reverse. That's allowed by physics.

However, if A launches a (subluminal) ship to destroy stars B and C, all the stars will see that ship launch (and arrive) before stars B and C explode. The same cannot be said of a superluminal ship, and that's what breaks causality.

This isn't time travel as portrayed in science fiction, no, because science fiction writers would rather write good stories than physics textbooks.

Regarding light cones, by the way: it's a ball (that is, a solid sphere) at any one time, but it looks like a cone in Minkowski spacetime and that's what matters here. If you were to look at 2d space and display time along the third axis, as you watched the ball expand and move along time its edges would describe the light cone.

EDIT: If you just wink the light on and off, then yes, it's a sphere at any one time, but for our purposes it might help to think of the ball of space that knows you turned the light on rather than the sphere currently experiencing the flash.

[smjjames]

Also. Wouldn't it be more accurate to say light sphere than cone? I get what you meant by cone, but unless you were shining a flashlight, it'd be a sphere of everywhere from that point.

Light cone is a different concept, the axis of the cone is time. (The cone shape is what you get when you simplify to two spatial dimensions, of course, you get a four-dimensional hypercone otherwise.)

Anyway, you're misconceiving the rest, but I'm too tired to explain right now, so please accept this promise to clarify further later if you want.

I was thinking of cone as in like a gamma ray burst, quasar/supermassive black hole jets, and pulsars where the light we see is originating from a tight cone that we only see because we happen to be in line of sight of it. So, I was thinking of the wrong thing.

[Maximum Spin]

I was thinking of cone as in like a gamma ray burst, quasar/supermassive black hole jets, and pulsars where the light we see is originating from a tight cone that we only see because we happen to be in line of sight of it. So, I was thinking of the wrong thing.

Yeah, I figured that was what you were imagining. Sorry if I didn't convey that well enough, like I said, I'm very tired and I have a headache.

[Max™]

Also. Wouldn't it be more accurate to say light sphere than cone? I get what you meant by cone, but unless you were shining a flashlight, it'd be a sphere of everywhere from that point.

Imagine a sphere of light expanding from a point, then make a flipbook out of it.

Each page has a slightly larger or smaller sphere relative to the adjacent pages.

Spread the pages out and line them up from left to right, then call the left <-> right direction time.

You're sliding along that axis as you read this, and at any point you could check how far a sphere of light could have expanded since you were at an earlier or later position along it.

The series of ever larger spheres extending ahead of you towards the future can all be overlapped into a hypercone, as can the ones going upstream towards the past.

You will always be found within those cones relative to any time you specify, as you are unable to move faster than light.

Introduce some method of FTL into the game and you can now choose which way your path intersects with a given light cone because you can start inside one, cross outside of it, and return at your leisure.

Figure out the right trajectories and you end up bumping into yourself in the past, or worse, end up becoming your own grandfather... _{I wouldn't wanna do that again.}

[Reelya]

Yeah, to boil that down, the cone is 4-dimensional, the cross-sections are spheres not circles, and time is the "height" axis. if you look up the "light cone" on Wikipedia you'll see the visual example is for 2D space with time as the third-dimension, i.e. it's a simplification for the purposes of depicting it.

[Starver]

From your perspective, the star exploded, you left (after it exploded, having observed the light from its explosion), and you arrived at their star. From their perspective, given a sufficiently powerful telescope, the sequence of events is reversed: you arrived, you left, and then the star exploded.

Arrive, explode then leave, you meant.

(i.e. To be seen leaving prior to the explosion you observed, you must have observed the explosion by a superluminal method that itself got you moving before the light arrived at your position, if you were directly between star and observer. Even more in advance if you aren't, and from every point equidistant (perpendicular) and further (behind) you additionally need it to be a time-travel-viewer (that predicts things beyond any sort of observable simultaneity) in order to have any chance of of out running the explosion-observation wavefront.)

[Trekkin]

From your perspective, the star exploded, you left (after it exploded, having observed the light from its explosion), and you arrived at their star. From their perspective, given a sufficiently powerful telescope, the sequence of events is reversed: you arrived, you left, and then the star exploded.

Arrive, explode then leave, you meant.

(i.e. To be seen leaving prior to the explosion you observed, you must have observed the explosion by a superluminal method that itself got you moving before the light arrived at your position, if you were directly between star and observer. Even more in advance if you aren't, and from every point equidistant (perpendicular) and further (behind) you additionally need it to be a time-travel-viewer (that predicts things beyond any sort of observable simultaneity) in order to have any chance of of out running the explosion-observation wavefront.)

Yes; apologies for the error.

In any event, it should probably be pointed out that FTL is only problematic when it's FTL relative to an outside observer. You can move between two points at relativistic speeds and experience a shorter trip time than the speed of light would indicate is the minimum, but that's just time dilation and only affects the ship; importantly, everyone will still see you arrive after you left. This is about the point in the discussion, bikeshedding about light cones aside, where someone usually confuses the two.

[redwallzyl]

https://www.npr.org/sections/thetwo-way/2017/11/28/567014007/robot-muscles-inspired-by-origami-lift-1000-times-their-weight
Soft and squish robots. quite cool.

[Bumber]

However, if A launches a (subluminal) ship to destroy stars B and C, all the stars will see that ship launch (and arrive) before stars B and C explode. The same cannot be said of a superluminal ship, and that's what breaks causality.

Am I right in assuming:
A: Sees superluminal ship travel to B, blow up B, travel to C, then blow up C.
B: Sees ship appear at B, blow up B, simultaneously travel to C and backwards to A, then blow up C.
C: Sees ship appear at C, blow up C, remain at C while a copy travels backwards to B, blows up B, then travel backwards to A.
D: Same as C.
And in all instances, the ship looks like it's traveling at only the speed of light?

It's interesting, but I don't see it as all that problematic, nor time travel.

Suppose the ship returns to A after successfully destroying C.
From A's perspective, the ship departs towards B, and is on route when suddenly two ships appear out of nowhere, one of which is flying backwards in the direction of C.
The remaining ship's pilot reports mission complete, stars B and C destroyed. The two ships traveling to B and C won't respond to hails, not even FTL messages.

Now we get to the good part. C's FTL capacity just isn't as great as A's. They learned of A's new weapon that can destroy stars, and sent a ship to declare their surrender.
It was too late, as we know, and their star was destroyed minutes later. Back on A, where the mission has been reported completed, but is not confirmed, a ship arrives declaring C's surrender.
A observes their ship still on route to B, and decides to use an experimental faster FTL drive to outrun their ship and call off the destruction of C.

Unfortunately, what the pilot of the new ship experiences is this:
As he travels to intercept the original ship, it suddenly accelerates and he can't keep up with it. He sees B and C destroyed before he can reach them.
Dejected, he returns to A and reports his failure. A is very upset and confused why their pilots are reporting the destruction of C while the original ship is still clearly on route.

[Trekkin]

Well, the exact optics are dependent on how you see FTL working, but the reason it's time travel is because it involves effects following causes that aren't actually in their past light cones. It would be like seeing a flowerpot on your windowsill fall today because you will push it tomorrow. What are you going to push tomorrow, exactly, now that the flowerpot is on the floor?

I'll draw the Minkowski diagram for the exploding stars as soon as I can, but the problem mostly comes up when, for example, B sees the ship appear at B before it leaves A; you can easily set the time, distance, and loiter time at B so it experiences two ships at once, for example*, and even if that's not a problem, when the ship leaves from A to travel to and explode B (from B's perspective), where is it going? Bear in mind we have to care about B's perspective even after it's exploded, because that's relativity, and it has to make sense according to causality for physics to make sense at all.

*incidentally, before someone brings up anything to do with rubidium and laser grids, that's not this thing.

Okay. Assuming the ship travels ~4x the speed of light, the stars are 10 years apart from each other in a line ABCD, and it lingers motionless for five years at each star in a trip going A->B->C->A, (and omitting the actual destruction events to simplify) each of them sees this:

Ship: Depart A, Arrive B, Depart B, Arrive C, Depart C, Arrive A. All good so far.
A: Depart A, Arrive B, Depart B, Arrive A, Arrive C, Depart C. Oh hey, two ships at once.
B: Arrive B, Depart B, Depart A, Arrive C, Depart C, Arrive A. It arrives before it leaves! It leaves before it leaves!
C/D: Arrive C, Arrive B, Depart C, Depart B, Depart A, Arrive A Good grief! The ship's in two places at once and it's arriving before it leaves besides!

See the problem? Mass-energy equivalence isn't conserved, effects precede causes... I can post the diagram if you want.

[McTraveller]

<morbo>Causality doesn't work that way!</morbo>

[Trekkin]

<morbo>Causality doesn't work that way!</morbo>

It gets even better if you have two spaceships and can talk to your own past. EDIT: or go round-trip, actually...
Or are you saying my explanation was wrong?