How does FTL = time-travel?

[SCVN 2812]

The delay of light speed signals across vast distances in space is not hard to understand and I think not in need of explanation. I would also hesitate to call Planet A being able to observe ship A arriving at Planet B via FTL decades later time travel. A supernova that is happening right now in another galaxy is happening right now regardless of how long it will take for us to find out about it.

What I think the lack of comprehension lies is how FTL could cause someone to arrive before they left. Which if I understand the postings correctly, using, for example, a negative mass drive, relativity or causality dictates that time is reversed to some degree (which makes sense since time slows down as one approaches the speed of light.)

[Covenant]

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

In this case it is, again, removing relativity. It's just saying that in the normal universe, you have relativity and causality, and that in the red universe you have causality and FTL. Popping into an alternate realm where the rules do not apply doesn't circumvent the rules, it just adds more set dressing to it.

People need to just accept that FTL is the sci-fi equivalent to Zeus moving people around on that chessboard from Jason and the Argonauts. You're perfectly allowed to write about bad science, if you so desire. It should just be done knowingly, so that you don't look like you're coming off as pretentious. Hard sci-fi that relies on a lot of FTL and fancy tech explinations to be 'realistic' is just short-sighted and unnecessary. While it may make you look so smart right now, eventually it won't. The Cold Equation is a good example. While it did try to sound so edgy and realistic, you didn't see people uprooting the table and other objects to lob out the door. ;D

[Adrian Laguna]

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

The Cold Equation is a good example. While it did try to sound so edgy and realistic, you didn't see people uprooting the table and other objects to lob out the door. ;D

What table? What objects? There were three things that could be shoved out the airlock in that ship: the pilot, the stowaway, and the medicine.

[Covenant]

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

The Cold Equation is a good example. While it did try to sound so edgy and realistic, you didn't see people uprooting the table and other objects to lob out the door. ;D

What table? What objects? There were three things that could be shoved out the airlock in that ship: the pilot, the stowaway, and the medicine.

Pardon me, it's been a while since I've read it, and I can't find much online to read of it. I believe there was a chair that could have been tossed, there were other assorted items such as clothing, you could cut off people's hair and dump that too, as well as several other things. So while the story was pretty good, not only were the people who designed that rocket pod criminally insane, but there were also other ways to solve it. If you happen to have the story, I can point them out.

[andrewgpaul]

Newton's Wake by Ken McLeod has a 'cosmic something' that enforces causality during FTL. So, if you attempt to plot a course such that you arrive before you left, it doesn't work. There's some descriptions of FTL combat which use this tactically - you can box your opponent in in such a way that the only escape paths are non-causal, so he's trapped.

[Rye]

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.

Hmmmm. I would've thought something like this: At any point in time from any observer, everything else must correspond to a specific time/space position relative to it, work out the average for everything in the universe and you have the "true" position of everything relative to everything else and have everything truly plottable. Once you have that, you have an effective simultaneous map of everything, and that will enforce a limit on how truly fast you can travel as to not bugger up time. Now, I would suspect if you actually found a way to do this in real life, the speed of light would still be the speed limit, since things have a habit of turning out that way, but for fictional universes I think you could assume that it's as fast as you want with the max being instant travel anywhere within that simultaneous map.

Alternatively, maybe causality doesn't have to work on enormous (or really tiny) scales, after all, there's all the confusing stuff with virtual particles, entanglement and the origin of the modern universe itself. Or there is the idea that relativity is not the whole story since on the tiniest scales, the physical universe is connected to the warp and chaos or whatever.

[Surlethe]

This thread is particularly relevant, I think. This is mind-boggling stuff.

[skotos]

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.

How does picking a common frame of reference in order to determine the times and locations of FTL events (say, the entry and exit into "hyperspace") violate relativity? We pick common frames of reference in order to do physics all the time. If I'm solving a mechanics problem on Earth, I pick the frame of reference in which the Earth is at rest. If I'm sending a probe to Mars, I pick the frame in which the Sun is at rest, and so on.

To use a concrete example, lets say that we are using Known Space style hyperspace (ie. FTL fixed speed travel through hyperspace), and lets say that the common frame of reference is the frame in which the universe has zero momentum (or, if we're feeling geocentric, just use the frame where Earth is at rest). Things move through hyperspace at 100.84c, so if the distance between two points is 100.84 light years in the common frame of reference, there will be a year between when the ship leaves point A and arrives in point B, again as measured in the common frame of reference. In other frames it might be more than a year, or less than a year, or the ship might even arrive before it left.

In your example, lets say that ship B is at rest in the common frame of reference, and ship A is moving at .84c relative to it. Ship B sends the signal at T=30, and it arrives at ship A at T=30.24. Ship A receives the signal at T=19.341312, and the signal was sent at T=60.

At this point we have time travel, but have not yet violated causality.

Ship A sends its reply at T=30.24, and it arrives at T=30.48, as measured by ship B. Ship A sees the signal depart at T=19.341312, and arrive at 60.96, so the signal arrives after it was sent, so again there is no violation of causality, nor any time travel either. In fact, there is no FTL either, the return signal traveled at a speed of about .58c.

The consequence of this is that while FTL is possible and relativity and causality are both preserved in this scenario, ship B finds it impossible to send FTL signals in the direction of ship A. The moral of the story is that in this universe, it pays to operate near the "FTL" frame of reference.

[Ariphaos]

How does picking a common frame of reference in order to determine the times and locations of FTL events (say, the entry and exit into "hyperspace") violate relativity?

*snip*

The ordering of events of spacelike separation is observer dependent.

Call two stars A and B, which go supernova in events A' and B'. From their own frame of reference, are 100 light-years apart, and effectively static with relation to eachother.

Construct two ships, C and D. C is located, static, at the exact midpoint between stars A and B. D is flying in a line, perpendicular to AB and crosses near B at a negligible distance, at a speed .98c (Lorentz contraction of 5) from B's frame of reference. It begins 49 light-years away from B (~111 light-years from A), and continues past B to the other side. Note that D sees itself as only being ~9.8 light-years from B at the start due to contraction.

So, at tA0, both stars explode. At tA100, A sees B'. At tB100, B sees A'. At tC50, C sees A' and B'. Naturally, in all these frames of reference, they can conclude that the events were simultaneous.

I need to run so I'll have to do the math later but, here's a question - would D conclude that A' and B' are exactly simultaneous?

[phongn]

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

In this case it is, again, removing relativity. It's just saying that in the normal universe, you have relativity and causality, and that in the red universe you have causality and FTL. Popping into an alternate realm where the rules do not apply doesn't circumvent the rules, it just adds more set dressing to it.

Yeah, but then you jump back into realspace so the "pick two" rule rears its ugly head since you've effectively broken the lightspeed barrier since your displacement/time value > c. If you went into hyperspace and stayed there it might not be an issue.

People need to just accept that FTL is the sci-fi equivalent to Zeus moving people around on that chessboard from Jason and the Argonauts. You're perfectly allowed to write about bad science, if you so desire.

Nobody here is criticizing FTL in fiction - it is just a plot device nicely handwaved away. We're arguing about its realistic feasability.

[skotos]

The ordering of events of spacelike separation is observer dependent.

I understand that. FTL automatically means time travel if relativity holds, it just doesn't have to mean causality is violated.

Yeah, but then you jump back into realspace so the "pick two" rule rears its ugly head since you've effectively broken the lightspeed barrier since your displacement/time value > c. If you went into hyperspace and stayed there it might not be an issue.

The pick two rule posted earlier was relativity, FTL, and causality. I still don't see why you can't have all three, as in the example I posted (I see now that drachefly proposed the same system in the thread Surlethe linked, for whatever that's worth).

It seems to me the rule ought to be relatvity, FTL, and forward only travel in time, pick two.

[phongn]

It seems to me the rule ought to be relatvity, FTL, and forward only travel in time, pick two.

Yeah, but how do you prevent information from being transmitted backwards in time if you have FTL? Take a look at the thread Surlethe linked to - Kuroneko has a diagram and explanation on why casualty violations are inevitable.

[Admiral Valdemar]

I believe Baxter uses that in some of his novels as well, so that you can actually plan for battles that haven't taken place yet in your time frame, but where you already know the outcome. It's fluid, so the wars are often fought with people affecting how time plays out with different information sent back and forth by ships.

[skotos]

Yeah, but how do you prevent information from being transmitted backwards in time if you have FTL? Take a look at the thread Surlethe linked to - Kuroneko has a diagram and explanation on why casualty violations are inevitable.

You can't prevent information from being transmitted backwards in time if you have FTL. But merely being able to send information back in time is not sufficient to violate causality.

Take the example that Xeriar outlined, but instead of ship B sending a signal to ship A ship A's FTL sensors detect that a bomb has gone off on ship B. The bomb would go off at T=60, but ship A detects it at (roughly) T=19.

So ship A fires up its FTL radio and sends a message to ship B warning it that a bomb is going to go off. But ship B does not receive the message until T=60.96, after it has already exploded, thus there is no contradiction and no violation of causality. As I mentioned, you now have some FTL travel, relativity, and causality. What you lose is "no more time travel", and the ability to send FTL signals in all directions.

[skotos]

Crap. I fail at physics. I forgot that ship B is moving away from ship A when I measured the velocity of ship A's signal. So instead of moving at .58c, ship A's signal (as measured by ship A) travels at .58c + .84c = 1.42c, and FTL is possible in all directions, just not at the same speed.

[Ariphaos]

I understand that. FTL automatically means time travel if relativity holds, it just doesn't have to mean causality is violated.

You argued that a common Newtonian frame of reference was possible. In Minkowski spacetime, it isn't - time itself is also relative, and two clocks will almost always tick at different rates.

For example, in your Known Space example, the very perception of a light-year is velocity dependent, as is the idea of time's progression. You acknowledge this, but making it Earth's frame clearly screws over people in neighboring galaxies. The further they are from Earth, or the common rest frame as seen from Earth (pretty close to Earth), the less effective their version of hyperspace will be. Earth, then, is experiencing something close to true time, and no one else in the Universe is, establishing a common frame of reference (Earth's), and relativity does not apply in Known Space, because Earth - and Earth alone - is the proper frame of reference.

[Ariphaos]

Ghetto Edit: Known Space also has the problem with people beyond the horizon moving at negative speeds in hyperspace. Whatever that means.

[skotos]

You argued that a common Newtonian frame of reference was possible. In Minkowski spacetime, it isn't - time itself is also relative, and two clocks will almost always tick at different rates.

I think you've misunderstood what I was trying to say. I'm not sugesting that any Newtonian frame exists, merely that there could exist some frame that is used to determine when and where FTL departure and arrival events take place. The speed of light would be the same in this frame as in all others. Clocks will still tick at differenent rates, etc.

You argued that a common Newtonian frame of reference was possible. In Minkowski spacetime, it isn't - time itself is also relative, and two clocks will almost always tick at different rates.

For example, in your Known Space example, the very perception of a light-year is velocity dependent, as is the idea of time's progression. You acknowledge this, but making it Earth's frame clearly screws over people in neighboring galaxies. The further they are from Earth, or the common rest frame as seen from Earth (pretty close to Earth), the less effective their version of hyperspace will be. Earth, then, is experiencing something close to true time, and no one else in the Universe is, establishing a common frame of reference (Earth's), and relativity does not apply in Known Space, because Earth - and Earth alone - is the proper frame of reference.

If I was using Earth as the "FTL frame" then I would indeed be saying that Earth, and Earth alone is the proper frame of reference, as far as determining how long it takes things to travel FTL. For other purposes, the frame wouldn't have any special significance.

Just to clarify, I'm not saying that Known Space uses the idea of using a special frame to determine speeds in hyperspace, I just thought it be easisest to demonstrate how causality violations can be avoided by having a fixed rate of travel in hyperspace, which is one of the distinguishing characteristics of Known Space FTL.

As for screwing people over, any choice of the frame will screw somebody over. I wouldn't use Earth's frame personally in a story unless I wanted it to have some kind of mystical element where Earth is of great importance. I would probably use the frame in which the universe has zero momentum (something I asked about in a SLAM thread a little while ago). As it happens, the Milky Way is moving at about 600 kps relative to this frame, so FTL here would be about the same speed no matter what direction one went in.

[Star-Blighter]

Admittedly my knowledge of casuality and its relevence to FTL/timetravel is rather shakey but I'll give this a go.

The arguement that FTL=timetravel to me stems from the fact that the only "evidence" one can use to find an object in space are the photons and radiation that reflect off it which is percieved by someone. FTL might allow a vessel to reach a distant star in a fraction of the time the light will take to get there, but the light will reach it eventually, and if you stay too long you will see your ship approaching and I guess some horrible paradox is likely to occur.

Since c. is the same in all reference points, but can't reach all reference points in the universe instantaniously, something has to give and I reckon what gives is time in the form of dialation. Thus preventing you from ever reaching c. in the first place, much less breaking the lightspeed barrier.

Feel free to correct as I'm a little dazed and confused here...

[NeoGoomba]

*head explodes*

Okay, so if I go to Rigel by FTL and land on the planet, and go jump in the swimming pool, eventually, I'll be able to see my ship showing up, me getting out of the ship and jumping into the pool, I got that much. But if I stand around to watch this happen, I wouldn't be able to interact with myself because it already happened, would I? Would it just look like some kind of illusion, like a time-delay on everything I did from that point on?

*head explodes*

[Admiral Valdemar]

*head explodes*

Okay, so if I go to Rigel by FTL and land on the planet, and go jump in the swimming pool, eventually, I'll be able to see my ship showing up, me getting out of the ship and jumping into the pool, I got that much. But if I stand around to watch this happen, I wouldn't be able to interact with myself because it already happened, would I? Would it just look like some kind of illusion, like a time-delay on everything I did from that point on?

*head explodes*

That assumes you somehow move faster than light within the visible realm still, which is impossible anyway as I have no idea how wacky it'd be to just break light speed with pure brute force like that. If it was a jump drive or hyperspace, then you'd not see yourself since you effectively jumped from one time frame to another with no journey to be observed.

[skotos]

Okay, so if I go to Rigel by FTL and land on the planet, and go jump in the swimming pool, eventually, I'll be able to see my ship showing up, me getting out of the ship and jumping into the pool, I got that much. But if I stand around to watch this happen, I wouldn't be able to interact with myself because it already happened, would I? Would it just look like some kind of illusion, like a time-delay on everything I did from that point on?

Keep in mind that the first thing you would see would be the last thing you did, because that light would reach you first. So, I think if you ended your FTL trip by jumping in the pool, and then turned around to watch what happens, you'd see yourself running backwards in time: Jumping out of the pool, running backwards to your ship, your ship "unlanding" and then flying backwards back to Earth. It'd be like watching a recording of your actions while rewinding the tape.

[Lord Zentei]

I'll have a go at this:


The fact that superluminal speeds violate causality stems from the fact that lightspeed is equal in all frames of reference. For instance: if you are in a car that moves at 50 m/s and fire a bullet in the direction of motion that moves at 400 m/s, the bullet will move at 450 m/s in the reference frame of the ground. Not so with light: if you are in a spacecraft that moves at 0.5 c with respect to some planet or other, and fire a beam of light (obviously moving at c with respect to the ship), that beam of light will move at c with respect to the planet also, not 1.5c. The fact that c is equal in all frames of reference is a postulate of the theory of relativity, and its implications are corroborated by observation.


From this, one can infer that simultanety is relative: events seperated in space that are simultaneous in one frame of reference are not simultaneous in another. This can be seen with the classic train thought experiment:

Consider a moving train of length L and with speed V. There is an observer A in the center of the train, and another, B, standing on the ground next to the train tracks. Two lightning strike the tracks at the position of the two ends of the train, one at the front end, and the other at the rear end at the moment that the observer in the train passes the position of the observer on the ground. The lightning strikes leave a mark both on the ends on the train and on the tracks, thus providing a verifiable record of these events. The flashes of light move towards each observer at c.

The observer on the ground sees the flashes of light simultaneously. This is because the light flashes move towards travelling the same distance of L/2, at the same speed c, this speed being constant. The fact that the flashes moved a distance L/2 can simply be measured by a measuring tape.

The oberver on the train sees one flash before the other. This is because the oberver moves towards the front flash and away from the second one. Basically, the front flash needs to move a shorter distance than the other. However, the observer was equidistant from the two points where the lightning strikes hit, as we have required, and again, which can be observed with a measuring tape (since the lightning left marks).

But (and this is CRUCIAL): THE FLASHES OF LIGHT MOVE AT THE SAME SPEED FROM THE POINT OF VIEW OF THE OBSERVER IN THE TRAIN, BECAUSE LIGHT ALWAYS MOVES AT C.

Observer A thus has:

• The flash from the front was seen by A before flash at rear.
• The distance between observer A and the points where the lightning strikes hit the two points on the train are equal (distance L/2).
• The flashes of light move at the same speed from the point of view of A.

This means that observer A must conclude that the flash at the front occoured BEFORE the flash at the rear, unlike observer B - simultanety is therefore relative and a function of your velocity relative to the events observed.

This may seem like fucked up reasoning, placing the cart in front of the horse as it were. But it is not. That c is always the same is a postulate of relativity; it is NOT the conclusion. That time (and distance) is relative is.


However, the order of events is always the same for all observers - assuming that you move slower than light. If you were to actually move faster than light - the only non-relative velocity, you would actually see certain events happening in reverse compared with the observations of another observer, i.e. causality gets fucked up.

[Ariphaos]

I think you've misunderstood what I was trying to say. I'm not sugesting that any Newtonian frame exists, merely that there could exist some frame that is used to determine when and where FTL departure and arrival events take place. The speed of light would be the same in this frame as in all others. Clocks will still tick at differenent rates, etc.

You can have such a frame, yes - but it renders portions of relativity false. Relativity becomes a subset of some higher law, like Newtonian mechanics is of relativity. You aren't wrong in the sense of 'not possible', you are wrong in the sense that you are calling the result relativity.

If I was using Earth as the "FTL frame" then I would indeed be saying that Earth, and Earth alone is the proper frame of reference, as far as determining how long it takes things to travel FTL. For other purposes, the frame wouldn't have any special significance.

Yes it would, because it determines a frame that experiences, for certain, the most proper time, and from it we can measure the 'hyperstructure' of the Universe.

As for screwing people over, any choice of the frame will screw somebody over. I wouldn't use Earth's frame personally in a story unless I wanted it to have some kind of mystical element where Earth is of great importance. I would probably use the frame in which the universe has zero momentum (something I asked about in a SLAM thread a little while ago). As it happens, the Milky Way is moving at about 600 kps relative to this frame, so FTL here would be about the same speed no matter what direction one went in.

No, a better frame would be to have it be a hyperframe - that is, everything in existence has some fifth-dimensional movement (not time), that is fairly close to the speed of light. Thus everything in the Universe actually experiences very little true, proper time, and a lot of relativity still holds, but-

The result is, you've subsumed relativity. Equations, order of events, etc. become secondary to this greater law.

[Xon]

I believe Baxter uses that in some of his novels as well, so that you can actually plan for battles that haven't taken place yet in your time frame, but where you already know the outcome. It's fluid, so the wars are often fought with people affecting how time plays out with different information sent back and forth by ships.

An amusing result of this is being punished for crimes a potential future self commits when they travel back in time.

So future version of Person A gets punished for commiting the crime, and past version of Person A gets punished since they almost garrientied to have commited the crime if the future version didnt travel back.