A Step Closer To The Alcubierre Drive!

[-Velocity-]

Lets break this down: the paradox is that an stationary observer and an observer who is moving see two different things because of relativity effects. The two realities is both true so any follow up action will also be true. In your example the paradox was caused by faster than light travel who is an easy way to generate this.

However I claim you could cause the effect in other ways: In this case get two different results as light moves in the direction of travel or opposite.

If you take two clocks who are synchronized and placed at the front and back of the flatbed before you start moving they will still be moving at the same speed, attach sensors and transmitters who return an time stamp then the beams hit, then both signals return they are stored then compared, if the timestamps are diferent the beams are turned off.

Now for the on board observer both beams hit the sensors at the same time, and the same time is returned so the beam stays on, the time the timestamp signal uses on the return is not important here.

The stationary observer see one beam hit before the other, just as before, he should wherefore get two different timestamps and see the beams turn off because of this.

You might do this simpler. The beams will get an doppler frequency shift because of the flatbeds movement from the stationary observer viewpoint but not from the onboard one.

Yes the frequencies swap around at the mirrors but its still different frequencies who hit the central receiver from the point of the stationary observer and different frequencies trigger the bomb.

This is incorrect because you CANNOT synchronize the two clocks from BOTH the perspective of the moving observer and the perspective of the stationary observer. When the clock was synchronized from the perspective of the observer sharing the inertial reference frame of the train, the observer on the ground witnesses the clock on the rear of the flatbed car slightly ahead of the clock on the front of the flatbed car, with the time difference being exactly the time of flight difference between the forward travelling light beam and the backward travelling light beam.

Don't just take my word for it though, read this-

http://galileoandeinstein.physics.virginia.edu/lectures/synchronizing.html

Anyway, the idea that I could be wrong about this is a little ridiculous, since it is a VERY BASIC example of perceived simultaneity in two inertial reference frames, and all this stuff was figured out a hundred years ago or more, and has been very rigorously tested, and found to be correct to the best measurements taken so far.

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And it makes perfect sense to say that the light from Andromeda left then humans started to evolve, this does not require synchronized clocks, only an accuracy of some hundred thousand years.

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Mostly true, BUT it misses the whole point of there being no such thing as simultaneity. A clock in the Andromeda galaxy and a clock in the Milky Way that appear synchronized from a point midway between each galaxy (and at relative rest with each) could be a few million years out of sync with each other from other inertial frames of reference.

No I'm no expert on relativity, however I have a feeling you draw the conclusions too far.

How so? That this it's nonsensical to talk about "what is happening now?" in the Andromeda galaxy, because what is happening "now" depends on what your frame of reference is, that is a well known fact. There is simply no way to synchronize two spatially separated clocks from all frames of reference.

Maybe this is a matter of interpretation, but since the speed of light is the speed of causality, and it is not possible to know about distance events more quickly than the speed of light, to me, the question of "what is happening now?" is best answered by saying that what is happening now is EXACTLY WHAT WE SEE happening now. After all, if you could ask a photon hitting telescope how long the trip from the Andromeda Galaxy to the Milky Way took, it would reply that the trip was instant.

Of course, that is not correct either, because other reference frames would disagree. This just underscores why I say that the whole question of "what is happening now?" in some distant location is just nonsensical and nonphysical.

Edited October 14, 2013 by |Velocity|

[magnemoe]

This is incorrect because you CANNOT synchronize the two clocks from BOTH the perspective of the moving observer and the perspective of the stationary observer. When the clock was synchronized from the perspective of the observer sharing the inertial reference frame of the train, the observer on the ground witnesses the clock on the rear of the flatbed car slightly ahead of the clock on the front of the flatbed car, with the time difference being exactly the time of flight difference between the forward travelling light beam and the backward travelling light beam.

Don't just take my word for it though, read this-

http://galileoandeinstein.physics.virginia.edu/lectures/synchronizing.html

Anyway, the idea that I could be wrong about this is a little ridiculous, since it is a VERY BASIC example of perceived simultaneity in two inertial reference frames, and all this stuff was figured out a hundred years ago or more, and has been very rigorously tested, and found to be correct to the best measurements taken so far.

Clocks was started before the train started to move and both start at the same time. But you are correct that the ground observer would read them different. This might cancel out the arrival time difference.

Think we has to agree to disagree here. My point of view is that I feel the paradox is to weak to be an significant barrier.

Does it exist any better/ more real life examples, almost everything of relativity has been tested true with pretty simple experiments except gravity waves who makes a lot of sense in themselves.

Mass increases time slows down, gravity bends light and so on.

True, BUT it misses the whole point of there being no such thing as simultaneity. A clock in the Andromeda galaxy and a clock in the Milky Way that appear synchronized from a point midway between each galaxy could be a few million years out of sync with each other from other inertial frames of reference.

How so? That this it's nonsensical to talk about "what is happening now?" in the Andromeda galaxy, because what is happening "now" depends on what your frame of reference is, that is a well known fact. There is simply no way to synchronize two spatially separated clocks from all frames of reference.

Maybe this is a matter of interpretation, but since the speed of light is the speed of causality, and it is not possible to know about distance events more quickly than the speed of light, to me, the question of "what is happening now?" is best answered by saying that what is happening now is EXACTLY WHAT WE SEE happening now. After all, if you could ask a photon hitting telescope how long the trip from the Andromeda Galaxy to the Milky Way took, it would reply that the trip was instant.

You can calculate orbital mechanisms just fine if you set earth as the center of the solar system. It just makes the math an magnitude more messy.

In any practical sense we can observe time moves the same everywhere (except again for relativity effect who can be ignored outside of very fast moving or heavy objects. )

I then draws Occam's razor with an +5 damage against over complication

[K^2]

Can you cite a source? And how you reconcile that warp drives would be capable of producing the kind of impossible scenarios I describe in the thought experiment I give? It's a local causality violation, maybe even something worse than that (two observers who are in the same universe observe drastically different experimental outcomes), this problem being caused by sending signals faster than light between two causally connected regions.

Any high level GR book will cover global causality violations. To keep it simple, lets look at Hartle's Gravity: An Introduction to Einstein's General Relativity.

In short, the local light cone structure of general relativity is the same as the flat spacetime. However, the global arrangement of light cones (called the space-time's causal structure) can have interesting properties.

And two observers in different frames already observe drastically different universes. So long as it's covered by a coordinate transformation, it's not a problem. And anything to do with warp drive is. I mean, we do have the actual Alcubierre Metric here. It's not like we need to make anything up. Absolutely everything about Alcubierre's Warp Drive can be computed.

Edit: As I thought, Wald gets a lot deeper into the subject, but the language is much more technical. I'll still give you a couple of examples, just to keep the records straight.

In addition, in spacetimes with nontrivial closed casual curves[i.e., closed casual curves other than the trivial curve λ(t) = p for all t], severe consistency conditions may exist on solutions of the equations describing the propagation of physical fields.

In other words, structure of space-time places restrictions on whatever field theory you are using to solve for what happens to matter, which you'd expect in situations when time travel is possible.

Furthermore, one may make use of the causality violation occuring near the ring singularity to go "backwards in time" by an arbitrary large amount as measured by the t coordinate of (12.3.1) and thereby produce closed timelike curves passing through any point in the spacetime.

In other words, if a suitable Kerr black hole exists in your universe, you can already time-travel.

Edited October 14, 2013 by K^2

[DaveofDefeat]

I wonder if you can break the universe.

[Mr. Scruffy]

BTW, i think i can think of something that can be faster than light: Shadow. An object close to a light source and far from a projection plane, travelling way slower than c could have it shadow moving across the projection plane at a speed >c - or could it?

[K^2]

BTW, i think i can think of something that can be faster than light: Shadow. An object close to a light source and far from a projection plane, travelling way slower than c could have it shadow moving across the projection plane at a speed >c - or could it?

When people talk about faster than light, implied is ability to carry information. Shadow moving faster than light is fine, because it inherently cannot carry information.

Another illustrative example is phase velocity vs group velocity of light in materials. You probably know that index of refraction in material is related to speed of light in materials. High index of refraction means that light travels through material slower. But there are materials with index of refraction less than one. That means light travels through them faster than light. What? Well, it's the phase velocity that's faster. If you look at the beam, the waves of electromagnetic intensity really do seem to move faster than the speed of light, but if you look at where the pulse begins, that front is still moving slower than light. That is group velocity and it is what determines how fast you can send information.

(Because my explanation of phase vs group velocity is probably not very clear, take a look at top animation on the Wikipedia's article on the subject.)

[SargeRho]

I have a question on quantum entanglement when it comes to faster than light communications. You can not directly transmit information with it, right? But can you detect how many times the quantum state changes? And thus making a sort of quantum-telegraph?

[Mr. Scruffy]

Hmmm... it seems to also have something to do with info-density...

I was just thinking: I could form a sentence with shadows and make that sentence scroll over a surface with >c. But by just having the (original cardbox-orwhatever) letters magnified, i dont get any additional info across (per time-unit).

Hmmm. Probably the hardware running our universe can only cope with certain amount of info per (hyper-)volume of spacetime - or else the program hangs.

[Technical Ben]

No. Quantum entanglement means the two particles will give a distribution of results between them that matches (opposite "spins", though they do not spin). You cannot use this to transmit information, as all you ever get is a distribution of spins. Only after bringing both sets of data together can you then see that, yes, you got a distribution of opposites. As you can never choose to get an "up" or "down", you can never send a message. You can influence the results in other ways, which strongly suggests the results are chosen at the point of detection. But as we cannot choose which result, we still cannot send a message. It's in effect like the two particles arrange a result together at the point of detection, no matter how far away they are, but only they can communicate, and the only communication is "don't do what I do, do the opposite". As we can never know what the other particle does (it's far away) or effect our particle, we can never send a message.

The shadow is not faster than light for another reasons. The "shadow" never moves. You just cast a new shadow close to it or further away at a similar time. If we measure the two ends of the shadow (the far and the near) as if they are one thing, then we would think it moved faster than light. Where as really we measured one end of a shadow, and another end of a different shadow. A bit like measuring the birth of two different people on opposite sides of the earth and mistaking them for twins would make us think the mother must be traveling very fast!

I am also not sure how the light cone is causality. Or why we cannot consider multiple orders of a replay of an event that was causally ordered. It might take some time for me to understand why an observer "seeing/detecting" the order of events in the wrong order is the same as a break in causality. :/

Edited October 14, 2013 by Technical Ben

[-Velocity-]

Any high level GR book will cover global causality violations. To keep it simple, lets look at Hartle's Gravity: An Introduction to Einstein's General Relativity.

And two observers in different frames already observe drastically different universes. So long as it's covered by a coordinate transformation, it's not a problem. And anything to do with warp drive is. I mean, we do have the actual Alcubierre Metric here. It's not like we need to make anything up. Absolutely everything about Alcubierre's Warp Drive can be computed.

Edit: As I thought, Wald gets a lot deeper into the subject, but the language is much more technical. I'll still give you a couple of examples, just to keep the records straight.

In other words, structure of space-time places restrictions on whatever field theory you are using to solve for what happens to matter, which you'd expect in situations when time travel is possible.

In other words, if a suitable Kerr black hole exists in your universe, you can already time-travel.

I don't know what "covered by a coordinate transformation" means. All I know is that I can set up a very simple thought experiment, that you still haven't tried to address, where an two observers witness two drastically different realities. Let's say the two observers are humans. Let's say that an experiment with a warp drive FTL system causes a massive consistency violation to occur. Ignoring the fact that the severe consistency violation actually occurred the moment the FTL system was turned on (causing the two observers to be subject to different and inconsistent gravity and electromagnetic fields), we will amplify the consistency violation even further by having the consistency violation to cause observer A witnesses a cat get blown to tiny bits, while observer B witnesses that same cat survive the experiment.

Now, the two observers match their inertial reference frames and walk up to each other. Observer B hands the cat to observer A. Observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B. But observer B hands the cat to observer A. But observer A watched the cat get blown to smithereens; observer A can't possibly receive the cat from observer B.............................................................................................................................

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EXPLAIN HOW TO RESOLVE THIS ISSUE. The only reasonable way to resolve it is that warp drives simply cannot exist, for some reason we haven't discovered yet. Well, other than the fact that several physicists have said that warp drives would be nearly instantly destroyed by Hawking radiation once they hit speeds exceeding c.

Edited October 14, 2013 by |Velocity|

[ThirdHorseman]

First of all, causality is very different from differing observations. According to causality, the cause must precede the effect. And this means their actual order, not their observed order. For example, if the effect of a specific cause is closer to an observer, you might observe the effect before the cause. But the cause still preceded the effect in actual order. Causality is only violated when, in some frame of reference, the effect precedes the cause.

Again, this is not just different observations of events. If I get in a rocket and travel close to c, it will feel to me like a certain amount of time has passed. That amount of time will be different for an observer in a different frame of reference. There is no violation of causality here.

Also, as I said before, you can have FTL travel without violation of causality if you have a single frame of reference. If I get in my warp ship and travel at 100c to Alpha Centauri, there is no violation of causality. The problems occur when you add additional observers in other frames of reference, i.e. observers who are moving in relation to the original observer.

If you want a real in-depth read about this stuff, I point everyone to http://www.physicsguy.com/ftl/index.html where Jason Hinson has a TON of information on FTL, relativity, etc. I'll try to summarize some of it here.

You cannot understand causality violations, paradox, and FTL travel without looking at Minkowski diagrams. This is a simplified 2D representation of movement in space and time for a given observer in a specific frame of reference:

The observer (O) is "at rest" at the origin, with the x axis measuring travel in space and the t axis measuring travel in time. By adding a grid and identifying a point on that grid, you can determine when and where any event occurs in relation to the at-rest observer at the origin:

So if you measure the grid using seconds as the unit of time, and light seconds as the unit of distance, the event "@" occurred 3 light seconds away and 1 second in the past for the observer (O) at the origin. The key here is that any event that occurs below the x axis (-t) occurred in the past for any observer in that frame of reference.

Now what happens if you add a second observer (O')? Well if that observer is "at rest" with respect to the original observer (O), then they share the same frame of reference and thus the same coordinate system. Any event that happens in the past for one observer (-t) will be in the past for the second observer.

NOW what happens if that second observer (O') is moving in relation to the original observer (O)? You need to apply a Lorentz Transformation to the coordinate system of the moving observer, which shifts the axis depending on the how fast O' is moving in relation to O:

Now what we can do is say that O and O' both share the same point in space and time at x=0 and t=0. Basically, the two observers pass each other at a specific time and at a specific point in space common to both of them. By making the "passing event" be at the origin we can superimpose the two coordinate systems, and you get this where the black lines are the coordinate system for O and the blue lines are the coordinate system for O':

The important thing to note here is that the event marked as * happens in the past for O (it is below the black x axis) but happens in the future for O' (it is above the blue x axis). So when they meet at the origin the two can argue all day about whether * happened yet or not, but there is still no causality violation because * really HAS NOT HAPPENED yet for O'...there is no cause before effect. And as long as FTL does not exist you cannot have a causality violation, because O cannot send any signals or information to O' at the time of the event that will reach him before he actually encounters the event.

Now once you introduce FTL you throw all that out and can easily create a causality violation and a paradox. Using the same diagram above, we say:

1) Observers O and O' are in space stations, with O at rest and O' moving at a high speed in relation to O

2) As the two space stations pass each other (the "passing event") O' sends out a robot in a FTL shuttle to kill the brother of O. That death is the event *. The death happens after the passing event according to O' (which makes sense, since he sent out the shuttle) but it happens BEFORE the passing event according to O (it is below the black x axis)

3) The shuttle arrives at a third space station where the brother of O lives. This space station is at rest in relation to O station, so is in the same frame of reference and shares the same spacetime coordinate system (the black axis) as his brother. The robot kills the brother, and third observer O'' records the event. The observer O'' now knows that at some point in the future, O' will send a robot to kill the brother, because the passing event HAS NOT HAPPENED YET in their frame of reference.

4) O'' then fires up his FTL radio and sends a signal to O. Remember these two observers are in the same frame of reference so for both of them the passing event has not happened yet. O'' tells O that at the passing event, O' will send the robot.

5) When the passing event happens, O is ready and when the robot leaves on his shuttle, O destroys it with a laser blast, thus saving his brother.

Now you have a paradox. If O stops the robot, then his brother isn't killed. But if his brother isn't killed then he doesn't know the robot will leave, and he doesn't blow it up. If he doesn't blow it up, the robot kills his brother......etc. Note that all this happens because the two observers are moving in relation to each other, and it happens regardless of the type of FTL used. The robot could have used a Alcubierre Drive shuttle, or gone through a wormhole, or even just send a FTL signal to an assassin on the third station. As long as you can send information FTL then you can get causality violations and thus unsolvable paradoxes.

The idea of self-consistency principles is that any event that might cause this kind of paradox has a probability of occurring of zero. Something will happen, the FTL signal from O'' to O wont go through. The robot will break down. Something will prevent the paradox from happening. So you could get FTL travel, and you could even get causality violations (or closed timelike curves), but any attempt to create a paradox will not happen.

TL;DR Go to Jason Hinson's page and at least read Part I Chapter 2 where he talks about Minkowski diagrams and Part IV wher he talks about FTL travel and the issues it causes.

[-Velocity-]

...

But wouldn't any FTL signal at all cause consistency violations, where two observers in two different inertial frames observe two different outcomes? It might start off as a very minor consistency/causality violation (slightly different gravity waves and electromagnetic signals would be received) but over time, it could become huge difference, as the universe is a chaotic system. And yet the observers would still be in the same universe, they could still match each other's inertial reference frames, and directly interact with each other. How could they be subjected to different physical realities and yet still be in the same physical reality? K^2 keeps dancing around this question, even though I present it in a simple thought experiment. If warp drives are possible then where is my thought experiment wrong?

Maybe warp drives are possible, but in order to prevent consistency and causality violations, warp drives simply never work. IMO, I find that hard to believe, I don't like a universe that protects its integrity with nothing more than "fate"- things are just fated to never work if they cause a problem. That's just messy and inelegant. There should be a physical LAW that prevents FTL from working, ever. Honestly, I think that there is, and we just haven't found it yet- the perceived "loopholes" in Relativity simply being holes in our understanding. But that's just IMO.

But you know what- who knows. Maybe the "fate" thing actually works. If the MW interpretation of QM is right, then for every parallel universe where a warp drive works, then there is a nearly identical universe where that warp drive failed. Maybe all the universes where the warp drive works are simply destroyed, meaning someone trying to use a warp drive to go faster than c always observes the warp drive failing for some not-so-random reason. Maybe the universe really does BSOD if you exceed c with a warp drive. But that's still messier than a universe that has a physical law preventing such a warp drive from working...

Edited October 14, 2013 by |Velocity|

[K^2]

No. You'll never have that. You'll have disagreement on which is cause and which is effect, but both observers will always see the same events. Just in different order. Order of events is the only thing that can come into question with FTL. If you came up with a situation where it's otherwise, you are wrong and you missed something. If you'll link me to original post (sorry, too lazy to dig through them) I'll see if I can tell you what exactly goes wrong.

[-Velocity-]

No. You'll never have that. You'll have disagreement on which is cause and which is effect, but both observers will always see the same events. Just in different order. Order of events is the only thing that can come into question with FTL. If you came up with a situation where it's otherwise, you are wrong and you missed something. If you'll link me to original post (sorry, too lazy to dig through them) I'll see if I can tell you what exactly goes wrong.

No, that's not a full statement of the rules. Observers can disagree on the order of events, but only if those events do not occur at the same point in space. For events that DO occur at the same point in space, then ALL observers must agree to the order of events. For example, you can't observe the new One World Trade Center building being constructed before you observe the original Twin Towers being constructed.

So what I did is set up a simple thought experiment, using instantaneous communicators (though warp drives would work just as well), where two different observers disagree on the order of events that occur at a single point in space. This causes each observer to witness entirely different events to take place. It's not that just that they disagree on the order of events, they disagree on the very physical realities that each observer is experiencing. This disagreement in itself fundamentally breaks the universe, but I further amplify the severity of the problem by making a drastic set of consequences for this disagreement (one observer witnesses the other observer die).

http://forum.kerbalspaceprogram.com/threads/47034-A-Step-Closer-To-The-Alcubierre-Drive%21?p=688102&viewfull=1#post688102

Edited October 14, 2013 by |Velocity|

[K^2]

Erm. Two events at the same location but separated in time in one coordinate system are separated in space and time from a moving coordinate system. Saying "two events happened at the same place" has all the same problems as "two events happened at the same time". Same as there is no such thing as absolute simultaneity, there is no such thing as absolute collocation in relativity.

What you are thinking of is an extreme case of time-like separated events. In flat space-time, they cannot switch order because they can be casually related. In curved space time, however, any two events can switch time. See the quote I've posted earlier from Wald which mentions closed time-like curves. In some parts of these curves you are literally going backwards in time. Relative to you, everything in the whole universe is happening backwards. And this isn't some crazy science fiction scenario. Rotating black holes allowing this are predicted to actually exist. So there is a place in the universe viewed from which the Twin Towers are built before the new One WTC. Well, to be precise, they are seen to be disassembled some time after growing from a cloud of dust.

(Man, did you have to pick such a morbid example?)

[Mr. Scruffy]

Yeah, Vel, i dont see the issue with observed order of events either.

Wouldnt breaking casuality and the paradox be caused by something that would bring back information about an event to the place where it takes place, before it happens? If that was possible, then, in the end, two events would have taken place at exactly the same spacetime. That would imply a multi-layered nature of it: At least one of the dimension would have to be two (or more) ´in disguise´. It´d be like savegames in video-games, beyond which you have already played.

But, i´d know a type on universe, in which this would work. A totally deterministic one, in which ´free will´ is just an illusion: The Universe wouldnt have to go BSOD, if everything was happenening the way it must, anyways ; if spacetime is a static picture painted over god-knows-how-many dimensions. Time travel, then, even when going all SF-warp-drive, is a mere curve that the universe as a whole (seen over time) had always featured, has nothing decisive about it, and is just a looping upwards on an otherwise flat track.

Philosophically speaking, this might be a struggle over wether what ever created the universe took all the freedom and thus allowed us unconcious time-travel, but not the ability to decide or change anything in its grand static picture, or wether there is freedom for us in it, and the ability choose your pens´ paths, but only one each and for all.

Personally: I tend to believe in this determinstic version, and so tend to believe that free will might be a mere illusion, psycholigally somehow required to keep us scientients from going nuts. Which is the reason why i also tend to not implore too deep on the matter. I read Nietzsche - but only the first 100 pages or so. Then it went on my ´poison shelf´. He might be right - that´s why nobody should read what he wrote.

BIG EDIT: Maybe the part of physics that we call ´quantum physics´, with all its probabilities and uncertainties is caused by the few (wholly deterministic) time-travels within the whole of spacetime and the in-reality also determinsitic causes for its phenomena hidden in the ´layers´ those travels cause (will have caused, will have had caused, did cause... - doesnt matter).

So: Our 4D-universe didnt know probabilities and chance until something, someone, information or whatever travels back in time (as was bound to happen). From that time of destination on then, there is another layer ´close to´ the original one. Those two interfere in a seemingly random fashion (when seen from only one side). This happens a couple of times (still, when seen across all, predetermined to happen) and you end up with a universe which consists out of, say, 10 dimension, 6 of which are just tinily curled up and interfere with the rest only in a way which cannot be determined from yonder and seems random from ´the other side´. Then, Quantum Physics phenomena could not be predicted precily, because their cause is in the future. Schrödingers cat would have been put in the box by someone traveling backwards in time. The by-many postulated multiverse just diffuses to a probability field in our reality.

Rambling on: Or maybe all this random stuff in physics is just proof that someone, sometime will have almost succeded in traveling backwards through time - the closest anyone ever got. The attempt was to initiate a negative-time phase-freeze that would allow the traveler to ´drop back´ in time. The calcs proofed wrong though, and the freeze didnt propagate into greater volumes. Instead, it stayed on tiny scales and propagated backwards through time agressively. And now, a billion years earlier, we got this mess we call quantum physics, dark matter and whatnot. (Pro-SF-authors: To the Helm!)

Edited October 14, 2013 by Mr. Scruffy

[ThirdHorseman]

But wouldn't any FTL signal at all cause consistency violations,

No. FTL signals between observers in the same frame of reference will not result in causality violations. There needs to be time travel to cause that, and you only get time travel when you have FTL communication (either by signal or by actual movement) between observers in two different frames of reference. And that's even not always true. The reason the situation I outlined above caused a paradox was because the event happened in the past for one observer and the future for the other. If the event is below the x axis for BOTH observers (in both of their pasts) then you won't get a paradox. So it's not just the existence of FTL travel that causes the issue, it's the particular WAY that the system is utilized.

I don't like a universe that protects its integrity with nothing more than "fate"- things are just fated to never work if they cause a problem. That's just messy and inelegant. There should be a physical LAW that prevents FTL from working, ever.

It's not fate, it's probability. There is no doubt that time travel is mathematically possible, since closed timelike curves have been shown to exist at least on paper. So if time travel is possible, and self-contradictory situations are not, then there must be a mechanism to prevent them. You have ideas like Hawking's chronology protection conjecture, and Novikov's principle. These aren't relying on "fate" to stop paradoxes...they use mathematics. Hawking's proposes that a buildup of energy in the system increases as you approach the point where closed timelike curves are possible, eventually destroying the system before it can actually be used.

And Novikov, as well as the other consistency principle ideas, show that the probability of an event occurring is zero. This has actually been "somewhat" proven by using a situation where a ball is sent through a wormhole so that it time travels back and hits itself before it goes in the wormhole, causing a paradox. An analysis was run using every possible initial situation for the ball, and there were no results that were not inconsistent. They could even show probability curves with all possible results, with inconsistent futures being zero. I don't pretend to understand a lot of it, but you are welcome to read "Cauchy problem in spacetimes with closed timelike curves" the PDF which is available here: http://authors.library.caltech.edu/3737/1/FRIprd90.pdf You'll see that they don't rely at all on "fate" or "destiny" or any other vague notion, only cold hard math.

So what you have is not a Law that prevents FTL communication or travel, but prevents those systems from causing illogical self-contradictory situations.

[Mr. Scruffy]

´This has actually been "somewhat" proven by using a situation where a ball is sent through a wormhole so that it time travels back and hits itself before it goes in the wormhole, causing a paradox.´

What if it became a matter of those probabilities in which state you´d find the ball after it had fallen back thorugh before it entered the hole? So: The ball falls into the hole, it bounces back on the other side, reenters the hole - and then what? It doesnt come back into our reality as a ball, it comes back as a probility wave. It might rematerilize in myriad of random ´creational´ events in a trillion different locations. The rule is: ´Dunno where and when it has to go? Split it up and throw a million dice´

[ThirdHorseman]

The ball falls into the hole, it bounces back on the other side, reenters the hole - and then what?

It doesn't bounce back. It has to do with accelerating one mouth of the wormhole at relativistic speeds so that it experiences time dilation in relation to the other mouth. So this way something entering mouth A will exit mouth B earlier in time than it entered mouth A. So the ball goes in mouth A and exits mouth B before it even entered mouth A. If it exits mouth B on a trajectory that causes it to impact itself before it enters mouth A, you have a paradox.

They did a Feynman sum-over-history calculation, where you create a probability amplitude from all possible paths, and found that what you could have is a probability of zero for any path where the paradox would occur. It's all pretty complex, and anything that refers to "Lorentzian foam" is way past my pay grade...

[-Velocity-]

You guys still are ignoring my thought experiment. It shows how different observers experience entirely different physical realities when a FTL signal is sent. There is no time travel. Simply that one observer witnesses a bomb explode and the other observer does not. This cannot possibly be, since the two observers are still causally connected to each other- hell, they are only a few hundred meters apart with velocities that differ by only 10 m/s. You guys still have not answered what is WRONG with this simple thought experiment. Instead, you start talking of curved space time, closed time-like curves etc... when that is not the issue at all.

If you claim it is possible to exceed c, and that it does not cause the kind of problems I clearly illustrate in my example, then you need to actually refute my example.

Would it help if I turned my example into a series of illustrations rather than a big wall of text? I am willing to do this, but it will take me a few days as I am rather busy right now.

[Awaras]

You guys still are ignoring my thought experiment. It shows how different observers experience entirely different physical realities when a FTL signal is sent. There is no time travel. Simply that one observer witnesses a bomb explode and the other observer does not. This cannot possibly be, since the two observers are still causally connected to each other- hell, they are only a few hundred meters apart with velocities that differ by only 10 m/s. You guys still have not answered what is WRONG with this simple thought experiment. Instead, you start talking of curved space time, closed time-like curves etc... when that is not the issue at all.

If you claim it is possible to exceed c, and that it does not cause the kind of problems I clearly illustrate in my example, then you need to actually refute my example.

Would it help if I turned my example into a series of illustrations rather than a big wall of text? I am willing to do this, but it will take me a few days as I am rather busy right now.

Here's the thing that bothers me with this thought experiment: Yes, to the outside observers it might SEEM like the light beams/ftl signals/whatever arrive at the bomb at different times depending on the observers frame of reference, but surely the bomb exploding or not depends exclusively on what the bomb itself 'sees' in it's own frame of reference, right?

I mean, it seems to me that all that would happen would be that one of the observers would say 'hmm, the bomb SHOULD have exploded just now but didn't for some reason.' (or vice versa).

[-Velocity-]

Here's the thing that bothers me with this thought experiment: Yes, to the outside observers it might SEEM like the light beams/ftl signals/whatever arrive at the bomb at different times depending on the observers frame of reference, but surely the bomb exploding or not depends exclusively on what the bomb itself 'sees' in it's own frame of reference, right?

I mean, it seems to me that all that would happen would be that one of the observers would say 'hmm, the bomb SHOULD have exploded just now but didn't for some reason.' (or vice versa).

You do not understand the point that the thought experiment makes. You're getting all caught up on the bomb, when the bomb is just there to underscore how severe of a problem this is.

Let's remove the bomb entirely.

The actual problem is that both observers see the same initial physical system at the center of the flatbed car (we'll call it the center detector system).. They all agree on center detector system's configuration of atoms, the state of the electronics, etc.

If they were watching the center detector/bomb system's state carefully, they will have to see the center detector system's atoms respond in a causal way to outside stimuli. The system can't magically change states without something to force it to change states.

The problem is that, according to the frame of reference, observers will see this same system respond to different stimuli, and thus, have a different ending configuration. Most observers (those who are not sharing the reference frame of the flatbed car) will be able to watch as the center detector system receives one FTL signal, do some computations, timestamps the arrival time, and THEN receives the other FTL signal, does some computations, and stores a DIFFERENT timestamp for the other FTL signal's arrival time. Observers on the flatbed car will observe the two FTL signals be received at the same time, and receive the same timestamp!

Furthermore, the differing computations would result in a different amounts of electromagnetic radiation leaking out of the center detector system, as the different computations would result in different electric current flows. That also means that different reference frames would witness a different heat output, a different energy consumption, etc.

Thus, as long as the universe follows physical laws, where information does not magically appear, violating conservation of energy (and mass), causality, and making all the laws of physics meaningless, observers on the train observe a different reality than those with a different inertial frame.

But what happens when the different physical realities, which somehow still lie in the same physical reality, start to interact with each other??? The laws of physics lose all meaning as particles start responding to stimuli that do not exist in their version of reality? If they don't exist in their version of reality, then how are they responding? Causality dies, and the laws of physics become meaningless, as anything at all can now happen, with no stimulus to cause it.

----------------------------------------------------------------------

But what you're saying is essentially the same thing- maybe worse, in fact. You're saying that observers on the ground should witness the center detector system receive one FTL signal, wait some time while its internal clock increments, receive the FTL signal at a different interal clock time, and then, it malfunctions, giving the timestamp on the arrival of the second FTL signal as being the same on the timestamp for the arrival time of the first FTL signal. If the observers on the ground watched closely, according to you, they would see a logic gate fail in the system somewhere, consuming slightly more or less energy during the failure. Furthermore, this would be a different flow of electricity though the system, and flowing electrical energy emits electromagnetic waves. The observers on the train do not witness this happen, they see everything work as normal. Thus, the end states of the two systems would be slightly different. Observers on the train would be in a different physical reality than all other observers in different frames of reference, as depending on the frame of reference, the center detector system would consume differing amounts of electrical power, output differing amounts of electromagnetic noise, and have differing final temperatures (as electric current flow generates heat). In fact, ALL frames of reference would experience their own, different version of reality, as, depending on the reference frame, differing amounts of time would have passed between the arrival time of the two FTL signals. And yet, they would still be in the same physical reality.

In other words- you're saying that the observers on the ground should end up in a different physical reality than the observers on the train, and yet, still be in the same physical reality. Which is not any better than the original example with the bomb.

So, the laws of physics lose all predictive power, and the universe no longer even needs causes for there to be effects. Out of absolutely nothing, energy can appear or disappear. Nothing means anything anymore, and anything means nothing. With FTL between causally connected regions, the universe no longer needs to follow any rules at all!

Edited October 15, 2013 by |Velocity|

[K^2]

Velocity, we've already established that FTL and time travel are not only possible, but they are a feature of our Universe due to rotating black holes. So we are not looking for a problem with FTL movement, but instead, we are looking for a problem in your thought experiment.

Could you either link to the original post, or better yet, re-iterate the entire expriment cleanly here? I want to figure out what's going on, but I don't want to dig through ten pages of this thread.

[-Velocity-]

As an aside, the only way I can imagine these causality and inconsistency problems being reconciled is if the universe is truly some kind of simulation, or something like it. In this case, the laws of physics would just be a set of rules that are computed within some kind of... system that is subject to a higher law of physics. Causality can be broken in our system as long as it is preserved in the higher system, as our system does not actually follow "real" physical laws.

As an example, imagine a video game where objects exist that follow the video game's laws of physics. A person playing such a game could enter a cheat code to magically spawn a new object. That breaks causality in the world of the video game, but causality is perfectly preserved in the "real" laws of physics that govern the working of the computer on which the game is being played.

(This is also the thought experiment that proved to me that a magical, physics-defying, non-causal God is certainly possible. The Biblical stories could even be literally true, if he re-wound the "simulation" and started it in a different initial state that eventually led to the universe we now inhabit. I consider this possibility highly, highly, highly, unlikely of course.)

[-Velocity-]

Velocity, we've already established that FTL and time travel are not only possible, but they are a feature of our Universe due to rotating black holes. So we are not looking for a problem with FTL movement, but instead, we are looking for a problem in your thought experiment.

Could you either link to the original post, or better yet, re-iterate the entire expriment cleanly here? I want to figure out what's going on, but I don't want to dig through ten pages of this thread.

Thanks for being willing to look it over. As I said, I am willing to turn it into illustrations, as I think illustrations would better... illustrate the example than a big wall of text. I think in pictures, not words, anyway.

I'm not sure if time travel is possible or not, but I tend to say no, as from what I've heard, every time they come up with some kind of time travel mechanism, someone figures out a way in which it would not be usable in the real universe. In my opinion, the universe is screaming at us that we can't time travel and we can't go faster than light, and I think we should take the hint. Of course, this is an opinion and I well understand the difference between an opinion and what the facts are. Loopholes might be possible, and should be explored, but, IMO, in the end I think all will fail.

As long as all observers can agree on the final configuration of the universe after time travel has been completed....

Anyway, I relinked the post for you yesterday, after you requested it. You must have missed the link. Here is is re-relinked: http://forum.kerbalspaceprogram.com/threads/47034-A-Step-Closer-To-The-Alcubierre-Drive%21?p=688102&viewfull=1#post688102

BTW, if this is like most forums, you can increase the number of posts shown per page under the user settings section somewhere

Edited October 15, 2013 by |Velocity|