How does FTL = time-travel?

[Crom]

I think it was in a Xeelee thread that FTL travel in the Xeelee-verse equates to time travel. I was wondering, how does that work? Or is it just an in-universe method of FTL?

I was thinking about it the other day, if you could move move FTL between point A and point B, you could potentially turn around and see yourself walking up A. But that doesn't mean you could immediately move FTL back from B to A and punch yourself in the face.

[Duckie]

I think it was in a Xeelee thread that FTL travel in the Xeelee-verse equates to time travel. I was wondering, how does that work? Or is it just an in-universe method of FTL?

I was thinking about it the other day, if you could move move FTL between point A and point B, you could potentially turn around and see yourself walking up A. But that doesn't mean you could immediately move FTL back from B to A and punch yourself in the face.

Information moves at C. Nothing, beside an FTL ship and perhaps Tachyons, move at FTL speeds. From our perspective, thus, a star that was extinguished a million years ago in another galaxy still exists.

If you move FTL, you can turn around and see yourself. Then, you can move FTL back before that event actually occurs (relativistic time dilation, if taken to mathematical extrapolation, says the faster you move at the speed of light the quicker time flows backwards since it'd be like going near-c in reverse).

Or something. I understand how, but my understanding of why is a bit hazy.

[Adrian Laguna]

In other words, the author of the Xeelee verse actually understands general relativity. Because according to Einstein's famous theory, FTL travel is for all intents and purposes time travel.

[andrewgpaul]

I've seen a description of this that almost made sense, but there was some fairly convoluted world-line diagrams and evil maths involved.

[Patrick Degan]

Kuroneko could probably explain this a lot better than most of us, but imagine an FTL day-trip to Rigel: from the perspective of Earth, you've traveled to a Rigel which won't be observable to anyone on Earth for another 965 years, which means you've traveled into Earth's past while simultaneously moving toward Rigel at FTL velocities. While from the perspective of your average Rigellian, you've just popped in from the future, since you come from an Earth which won't be observable to any average Rigellian astronomer for another 965 years. After you do your shopping and lunch, take in a movie or two and perhaps a ballgame or a walk in the park, you head on back —into the past as far as anyone on Rigel is concerned. But to your friends and such, you've just returned from Earth's future.

[The Nomad]

I wonder if some FTL trajectories could bring you back to your own past... ergo could you meet your future FTL-travelling self?

In other words, the author of the Xeelee verse actually understands general relativity. Because according to Einstein's famous theory, FTL travel is for all intents and purposes time travel.

The implication of FTL travel has only been expanded to all form of Xeeleeverse FTL in recent novels : originally, only relativistic wormholes could be used that way. Though it could be attributed to a misunderstanding of proper FTL navigation resulting in a preemptive measure, a computer lock of all potential paradox-generating FTL trajectories (that's how I rationalize it anyway).

[Admiral Valdemar]

Generally, if you're doing sci-fi and want FTL, you have to decide whether to ditch relativity or causality for your drive. You cannot keep both and go and have FTL, because one negates the other. You either time-travel and causality flies out the window, or your piss on relativity.

Either way, FTL is never happening. I like how Reynolds made a thing of FTL being theoretically possible, but so difficult that it was constantly ending with Novikov Self-Consistency Principles erasing whoever attempted it from the timeline, split off from our universe.

[LaserRifleofDoom]

Kuroneko could probably explain this a lot better than most of us, but imagine an FTL day-trip to Rigel: from the perspective of Earth, you've traveled to a Rigel which won't be observable to anyone on Earth for another 965 years, which means you've traveled into Earth's past while simultaneously moving toward Rigel at FTL velocities. While from the perspective of your average Rigellian, you've just popped in from the future, since you come from an Earth which won't be observable to any average Rigellian astronomer for another 965 years. After you do your shopping and lunch, take in a movie or two and perhaps a ballgame or a walk in the park, you head on back —into the past as far as anyone on Rigel is concerned. But to your friends and such, you've just returned from Earth's future.

Just because it's not observable doesn't mean it's not happening at the same time. Just because it takes hours for a laser to reach somebody waiting at the edge of the solar system doesn't mean I'm not firing it right now. Anybody in the spaceship with a calculater can figure out how long ago the laser was fired.

I'm not arguing that time travel is part of FTL, I just have to say that there's got to be a better way of describing it.

[Admiral Valdemar]

Just because it's not observable doesn't mean it's not happening at the same time. Just because it takes hours for a laser to reach somebody waiting at the edge of the solar system doesn't mean I'm not firing it right now. Anybody in the spaceship with a calculater can figure out how long ago the laser was fired.

I'm not arguing that time travel is part of FTL, I just have to say that there's got to be a better way of describing it.

Since existence is limited to lightspeed, it's not like that. As far as you're concerned, moving to the point source for that laser instantly is essentially moving through time at an altered rate to the outside observer.

It's the same with time-dilation on STL. You could circumnavigate the observable universe in a ship going just under c in a decade, but it'll be billions of years to everyone else. That is time manipulation with regards to specific reference frames.

[Big Orange]

So FTL is doable on paper, but in practice there is too much time and space distortion? You arrive at your destination before you leave, or you go into the future?

[SirNitram]

Just to reassure myself: It's similar to Dialation, but not linked? If you can overcome dialation(And the math, at least, works for such), you wouldn't overcome the funky casuality effects?

I ask mostly because when I write about how Enclave FTL works, it's playing with inertial mass, and they still have multiple safeties to prevent arriving before they depart or otherwise completing the ugly little triangles which cause paradoxs.

[Admiral Valdemar]

So FTL is doable on paper, but in practice there is too much time and space distortion? You arrive at your destination before you leave, or you go into the future?

For FTL to be possible you'd have to get past the lightspeed barrier. That ain't happening, which means wormholes or nothing.

[Batman]

I must admit that one always bothered me. FTL=time travel, in my probably flawed understanding, seems to rest on the notion that information can't travel faster than c. Doesn't the very existance of FTL disprove that notion?

[Admiral Valdemar]

If you're operating with the premise that physics is a part of it, then you've got to either assume one thing or the other, as I stated above. You can't have FTL and not have time-travel in one case, for instance. So every time the Millennium Falcon jumps, it's fucking up time lines or something else fundamental to physics.

[Vendetta]

I must admit that one always bothered me. FTL=time travel, in my probably flawed understanding, seems to rest on the notion that information can't travel faster than c. Doesn't the very existance of FTL disprove that notion?

The simplest way to see how travelling faster than light gives you time travel is to plug a velocity larger than c into the standard equation for relativistic dilation. It produces a negative figure for delta-t.

Of course, at c the equation resolves to a divide by zero. Which is why you can't do it.

[Admiral Valdemar]

That's why the only way to go at lightspeed is to have no mass, essentially you become photonic. How that's supposed to work, I don't know. The experiments I mentioned before with regards to Reynold's universe have you being able to manipulate inertia which in his timeline is proven to be linked to quantum vacuum energy (which is looking somewhat likely today).

Different states can be acquired, some produce a small field that, via the inverse square law, lowers the mass of whatever is in the field, sort of like a gravity well in ways, but reversed. A state one and two are basically different scales of lowering actual mass, so you can get higher gees on limited drive technology, provided your biology kept ticking. A state three transition is essentially no mass, which means you turn your matter to photons and bang goes your ship. No one could attain a state four transition.

The only real alternative is wormhole travel, and even that would still play hell with causality, but get around the problem of reaching FTL velocities given it goes around the light barrier.

[Rye]

Okay, I've started to write this like 3 times now and I keep saying such convoluted, self-referential language that I get annoyed with myself and just want to make a plainly speaking simple description of what I have envisaged in my mind.

The way I deal with faster than light is I assume that everything, irrespective of the speed or accelleration it's going through has a definitive point in space and time that's the universal moment of the present. If you froze the universe at one point in time and rearranged your location to anywhere else in it upon resumption, I don't really see what problems with causality that would engender. So, if you slowed that process down, so instead of being instantaneous, it took a few weeks to travel from on side of the galaxy to the other in "universal time," you shouldn't violate any causality, just be moving really really fast. I would say for the sake of it that you would be unobservable while going at these speeds.

I always liked the idea of hyperspace, too, say you had a gate that stretched out a local area of the universe so that it was posible to extend your ship into a bit of the universe that doesn't interact normally with the normal universe as we know it, and every meter your ship traverses in there is equivalent to moving a kilometer in some corresponding points where we live, still bound by the same passage of time, then you just pop out somewhere elsehaving traversed an equivalent 1000 times the distance, just because that's where it corresponds to.

I don't think this has any causality issues. If you could get the technology to go straight through the Earth, so you could wave at some guy at the other end and the light would arrive there 1000 times earlier than an actual hole through the planet, I wouldn't see that as violating causality, since the light is going at the same speed, just has less distance to cover through one of the holes. You won't have any events being visible before they actually happened, just a removal of lag and a limitation to universal "real time" and whatever energy constraints you wish such technology to have.

[Xon]

I ask mostly because when I write about how Enclave FTL works, it's playing with inertial mass, and they still have multiple safeties to prevent arriving before they depart or otherwise completing the ugly little triangles which cause paradoxs.

3 attributes a universe can have; Relativity, FTL and causality - pick two.

Xeelee-verse "has" causaility, but it only propagates at the speed of light.

[Covenant]

I ask mostly because when I write about how Enclave FTL works, it's playing with inertial mass, and they still have multiple safeties to prevent arriving before they depart or otherwise completing the ugly little triangles which cause paradoxs.

3 attributes a universe can have; Relativity, FTL and causality - pick two.

Xeelee-verse "has" causaility, but it only propagates at the speed of light.

Yeah, it's mostly mathmatical wierdness. It's quite possible that we just do not have any damn clue how things work, but then what you're doing is tossing relativity, and that's taking two of the three.

The snag is people think this way: "Well, I want to go to another star, but I don't want to time travel, I just want to get there in 10 seconds, rather than 10 years. Since I'm still moving forwards in time, and could never fly back to where I started and arrive before I got there, why is that time travel?" And the thing is, that's removing relativity. If you travel under the current rules, at a maximum speed of light through X medium, but get there faster than you should, then you did some timetravel.

If you want to set up a system where you are able to fly faster than light through one way or another, then you've invented a situation that relavity hasn't addressed. FTL is only Timetravel under the laws we believe to actually exist in the universe. It's not to say that it's always timetravel.

[SirNitram]

The way I understand it, is it's fine except there's always some setup, usually of three or more points, where you could arrive before you begin the journey, from the origin point's POV. I figure if you load down a ship with enough safeguards to prevent the engine from engaging unless enough time has passed to go there safely.

Should you actually get there, every version of you from every possible future arrives there. And this understandably is bad, and explosive. However, with FTL signals, it'll just burn out the receivers. 'Zot', I beleive, is the term.

[Ariphaos]

One way to think of it is: Time itself is relative.

Two ships moving apart from eachother at .84 c, will each perceive themselves experiencing two seconds of time when the other ship experiences a single second.

To elaborate, we have ship A and B passing eachother at t=0, moving apart at .84 of c. The Lorentz factor is 2.

At t=30 seconds, the ships are 24 light-seconds apart. Ship B sends a signal to Ship A, which sends its response immediately. If the signals travel 'instantaneously', we have a problem:

Ship B sends its signal at T=30. To Ship B, Ship A is at T=15, and thus, that's when it should get the transmission. Ship A gets it at T=15, sends the response to B's message which receives it at T=7.5.

...and so on. In order for this specific type of situation to occur and prevent causality from breaking, a common frame of reference of some sort needs to exist, which goes against a fundamental tenet of relativity.

Kuroneko mentioned that the Alcubierre metric does not violate causality either, though the energies involved for that are insane (mass-energy conversion of the known Universe), and the proposed cost-cutting measures usually involve creating pocket dimensions which are similarly costly. This on top of the whole negative energy bit.

[18-Till-I-Die]

So what about something like Hyperspace on B5, where you pop into another universe where time flows the same but distance is shorter (IIRC that's the idea of it anyway). The way it was explained was, two points in Hyperspace match any two points here, so going from Point A in normal space to Point B in normal space takes X time, lets say for conevience ten mins.

But in Hyperspace, the distance between Point A and B are vastly shorter, so it takes, basically, five mins instead of ten, but from the perspective of both the people on the ship and in normal space because time is the same in both.

Or thats the gist i got. From a website, i dont recall if it was ever adressed on the show so my bad if it was descrived differently there and i forgot.

[phongn]

All FTL systems - including that of B5's hyperspace - are subject to the "pick two" rule.

[Beowulf]

The notion of simultaneity only applies on non astronomical scales. Therefore you can't pick a point in time, say the universe freezes, move the object, and resume time.

[Winston Blake]

This is a neat layman's explanation of why FTL = Time travel.