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Forum » SpaceEngine » Science and Astronomy Discussions » Passage of time on a superluminal vessel
Passage of time on a superluminal vessel
MackTuesdayDate: Monday, 24.11.2014, 17:35 | Message # 1
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As I understand, within an event horizon, spacetime gets rotated so that the singularity is in the future, rather than in the distance. This is because every radial path is spacelike.

So could it be that on a superluminal vessel, the destination seems to be in the future to those on board? Maybe the only way for the trip to seem instantaneous is to go lightlike?

Maybe it depends on whether or not you carry your arrow of time with you when you go superluminal? Maybe that's the core of the question.


Edited by MackTuesday - Monday, 24.11.2014, 17:37
 
DoctorOfSpaceDate: Monday, 24.11.2014, 17:54 | Message # 2
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Quote MackTuesday ()
So could it be that on a superluminal vessel, the destination seems to be in the future to those on board?


Thats not how it works. If we are going off of real world physics then the only method of FTL that works within general relativity is the Alcubierre drive which has nothing to do with time travel. Your destination coming towards you would simply be crunched up into a single glowing point of blue light. If you could somehow translate this into useable data it would be information that has been traveling for x amount of time, depending on your location in space and proximity to destination, that is not from the future but the destination's past. What you have to realize is even at superluminal speeds, nothing is actually going faster than the speed of light and so no time travel or paradoxes.

Quote DoctorOfSpace ()
Maybe the only way for the trip to seem instantaneous is to go lightlike?


If you could make your ships mass effectively 0 then the trip from your perspective would be instantaneous, however it would take many years for an outside observer. This is why FTL travel will always be the better alternative.

Quote MackTuesday ()
Maybe it depends on whether or not you carry your arrow of time with you when you go superluminal?


If you leave Earth and turned on an FTL drive then your local time on board will roughly be the same as local time on Earth. You are not even accelerating to a large % of c, so any time dilation is negligible. You are not moving at FTL velocities from your local frame of reference inside the bubble. The easiest way to think about this is that your ship is not actually moving besides your initial velocity, the distance between point A and B is being reduced.





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MackTuesdayDate: Monday, 24.11.2014, 18:28 | Message # 3
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Cool, thanks for the response.

It leads me to another question. If the speed of light is the same in all inertial frames, how does that work with regard to the Alcubierre drive? Within the bubble, light has a certain speed, but from your point of view you're also outpacing light outside the bubble. So how is special relativity not broken?
 
WatsisnameDate: Monday, 24.11.2014, 22:39 | Message # 4
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Quote MackTuesday ()
As I understand, within an event horizon, spacetime gets rotated so that the singularity is in the future, rather than in the distance. This is because every radial path is spacelike.


Yep, this is correct. Future-directed light cones turn to face only inward from the horizon once you pass inside, so your future lies in the direction of the singularity. One way to see this is from the metric: the the components in r and t switch signs at the horizon, so r becomes a time-like component and t becomes space-like.

Quote MackTuesday ()
So could it be that on a superluminal vessel, the destination seems to be in the future to those on board? Maybe the only way for the trip to seem instantaneous is to go lightlike?


Proper time (the time that clocks everywhere measure) is zero for a lightlike path (so it works only for photons and other massless particles as Doc says). But as far as I am aware, there is no physical meaning for proper time of a space-like curve (faster than light). Proper time is related to the space-time interval (ds2). For a space-like curve, the sign of ds2 is flipped, so you end up with an imaginary answer if you take the square root. So whereas events separated by space-like paths make physical sense, space-like world lines of a particle or ship are almost certainly unphysical. If they were real then they would also allow paradoxes.

Quote MackTuesday ()
Within the bubble, light has a certain speed, but from your point of view you're also outpacing light outside the bubble.


It certainly seems that way, but this is an artifact of the curvature. Our intuitions from special relativity can easily lead us astray because the space-time is not flat. So to be correct the motions must be examined locally, not globally. Locally, all photons are moving at c, and the ship is moving at less than c. In fact, the ship is locally on a free-fall trajectory -- a time-like geodesic!

Added: To give a couple further examples of why this doesn't violate special relativity, just consider motions within a black hole. Viewed globally, it looks like photons within the horizon are moving faster than c with respect to you. This is not a violation because the photons aren't really moving faster than c. Rather, they are moving at exactly c within their local space-time, and it is the space which is flowing inward faster than c (the strong space-time curvature). Another example is the apparent superluminal motions of galaxies at very large distances in the universe. We actually can see galaxies whose radial velocities are faster than c! But this is again a consequence of general relativity, not special. The galaxies are not actually moving (much) through the space.





 
MackTuesdayDate: Tuesday, 25.11.2014, 17:32 | Message # 5
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I've asked this question in several different places on the Net, and yours is by far the best answer I've gotten. Thanks and kudos.

Quote
But as far as I am aware, there is no physical meaning for proper time of a space-like curve (faster than light).


Surely this is why I can't wrap my head around it. I just wonder if someone smarter than me hasn't really thought about it and come up with something interesting.

There are clues to be found when you study the theoretical behavior of tachyons. They can't transmit information faster than light! They're unstable too. I'm not sure why, but I imagine that stable oscillatory behavior following some exp(iwt) can diverge if t is imaginary. I'm just not sure how to incorporate these facts into this study.

Quote
Rather, they are moving at exactly c within their local space-time, and it is the space which is flowing inward faster than c (the strong space-time curvature).


Ah right, we can also imagine galaxies beyond the Hubble horizon that would appear to be moving faster than light if we could observe them.
 
WatsisnameDate: Wednesday, 26.11.2014, 04:31 | Message # 6
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Quote MackTuesday ()
Ah right, we can also imagine galaxies beyond the Hubble horizon that would appear to be moving faster than light if we could observe them.


Oh no, the reality is even more profound than that. smile There are galaxies in the observable universe with recession velocities faster than c. I chose my wording in the last post very deliberately.

I think this is one of the finest demonstrations of how intuitions from special relativity can fail us when we try to apply them to what are actually general relativistic effects. Most everyone knows that "relativity" says that "nothing can go faster than light". Since special relativity assumes a globally flat and static space-time, it doesn't matter how far apart two objects are for this consideration -- they cannot have a relative velocity measured between them that is faster than c.

But when things like curvature or expanding space-time metrics are involved, that assumption is no longer valid. The correct statement in relativity is that space-like trajectories are forbidden. To determine if a trajectory is space-like, its motion must be examined locally. And that's how we find that galaxies, particles inside of black holes, and Alcubierre drives are not violating this principle. Their motions are time-like (or light-like if massless).

As for tachyons, a lot of what you say is new to me! I am certainly not an expert on them though. (I did have one professor at my old uni who studied them, he was a really cool guy). I'm just not sure I grasp how any particle with a space-like trajectory (a tachyon by definition) can avoid the potential of paradoxes. Even if unstable, as long as its path traces out some nonzero-length space-like interval, then it conceivably allows for a signal to return to its origin at an earlier time than when it originated. Nor do I grasp how tachyons, if they both exist and interact, don't allow for the transmission of information. But there is probably a lot of theoretical work in these areas that is simply beyond my understanding. It's very interesting stuff to contemplate. smile





 
HarbingerDawnDate: Wednesday, 26.11.2014, 04:38 | Message # 7
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Quote Watsisname ()
There are galaxies in the observable universe with recession velocities faster than c.

"We analyze apparent magnitudes of supernovae and observationally rule out the special relativistic Doppler interpretation of cosmological redshifts at a confidence level of 23σ"

Showoffs tongue





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Edited by HarbingerDawn - Wednesday, 26.11.2014, 04:39
 
WatsisnameDate: Wednesday, 26.11.2014, 04:50 | Message # 8
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LOL yes, 23-sigma is an impressive figure. tongue That's a one in 2x10116 chance of being a statistical fluke.




 
MackTuesdayDate: Wednesday, 26.11.2014, 18:38 | Message # 9
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Quote
There are galaxies in the observable universe with recession velocities faster than c.
Fascinating paper! Those diagrams are illuminating. At some point I will have to study them more closely. I always wondered how photons from the past could be moving toward us. That's another of the things those diagrams explain.

Regarding tachyons, it seems I misspoke. A tachyonic *field* is unstable. The Wikipedia article on tachyon condensation has a discussion that I understand only superficially.

Ignorant guesswork:

Regarding causality, I was thinking that FTL could work if you it forbade you to "go askew"; e.g. if your origin and destination are comoving, and your path is spacelike, when you return it would need to be along the same spacetime path. A wormhole with comoving termini would do this, but then all wormholes would need to be properly separated so no closed timelike loops are possible. (Or would mere separation not suffice?) It gets hard to imagine if things aren't comoving.

Regardless, from some points of view there would be a gargantuan local violation of the conservation of energy, unless (I think) the vessel appeared to have negative energy density when it's moving backward in time.

Or you could have a propulsion system that just says, "F--- all y'all, the universe is Newtonian and I'm at the center."

That would make a good sci-fi story: Only one object in the universe can be moving FTL at any given time, and people are fighting to control it.
 
NikolaAnicic007Date: Saturday, 21.11.2015, 17:46 | Message # 10
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Quote MackTuesday ()
As I understand, within an event horizon, spacetime gets rotated so that the singularity is in the future, rather than in the distance. This is because every radial path is spacelike.

So could it be that on a superluminal vessel, the destination seems to be in the future to those on board? Maybe the only way for the trip to seem instantaneous is to go lightlike?

Maybe it depends on whether or not you carry your arrow of time with you when you go superluminal? Maybe that's the core of the question.


Depends on the type of FTL engine. The way the engine achieves superluminal speeds determines the time the crew experiences on-board.


Edited by NikolaAnicic007 - Saturday, 21.11.2015, 17:50
 
JackDoleDate: Saturday, 21.11.2015, 18:28 | Message # 11
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Somewhere I read that anti matter moves backwards in time. If that's true, you just need a spaceship that flies nearly the speed of light, and a method to transform the whole ship and passengers in antimatter. Then you fly halfway, converts everything back to normal matter, and flies the second half of the route. biggrin




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DoctorOfSpaceDate: Saturday, 21.11.2015, 18:45 | Message # 12
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Quote NikolaAnicic007 ()
The way the engine achieves superluminal speeds determines the time the crew experiences on-board.


There are only 3 types that one could bring up under hypothetical situations. I don't know of any others that could apply or have any physics to actually describe them.

1. Wormhole.
Time is relative to departure location and only varies depending on ship velocity relative to c.

2. Warp drive.
Time is relative to departure location and only varies depending on ship's velocity relative to c inside the bubble.

3. True FTL.
Ship somehow achieves negative mass just as the effect of space is reversed the effect on time is reversed, ship is now moving backwards in time.

The last one is pretty much a definite impossibility.

Quote JackDole ()
Somewhere I read that anti matter moves backwards in time


Best not to believe everything you read.





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parameciumkidDate: Saturday, 21.11.2015, 20:09 | Message # 13
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JackDole, I think what you read about was Tachyons, which is a name given to a hypothetical type of particle that can move faster than light and/or backward in time. It's consequently a favorite McGuffin for time-travel authors, like Unobtanium. Remember though that, also like Unobtanium, Tachyons aren't any specific species of theorized particle so much as a placeholder used whenever an imaginary physics-defying particle is brought up.




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WatsisnameDate: Saturday, 21.11.2015, 20:10 | Message # 14
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Quote JackDole ()
Somewhere I read that anti matter moves backwards in time. If that's true...


Do antimatter particles vanish the moment they are created in the lab?

parameciumkid, no, he's talking about an interpretation of antimatter where they can be considered to be regular matter particles except with a time reversal.

This idea first appeared from some speculation by Feynman (actually earlier, but Feynman made it well known). The properties of anti-particles are consistent with regular matter particles "moving backward in time", if you choose to formulate physics with acausal rules (which is silly), instead of using the forward-time propagating Hamiltonian operator (which works, and describes antimatter just fine.)





 
JackDoleDate: Saturday, 21.11.2015, 21:30 | Message # 15
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parameciumkid,
I just had a little problem with my computer, so I'm a little late to answer you, 'Watsisname' has it already done for me.
Tachyons are no Unobtainium. Tachyons are indeed purely speculative and there are in the known physics as far as I know no reason for their existance, but that does not mean that it does not exist. While by definition Unobtainium is something that does not exist. If one were to 'discover' Unobtainium, it would not be Unobtainium.

Watsisname, to me it is not clear why a particle should disappear if it lives backward in time. Perhaps the particle was indeed in the future already there, and it has been in the 'production' destroyed in reality?
Well, that's probably complete nonsense! tongue





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Edited by JackDole - Saturday, 21.11.2015, 21:33
 
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