Is Faster than Light Time Travel Possible?

[abledpilot]

Hello, I'm a high school senior interested in studying physics in college. I was listening to a podcast by Neil deGrasse Tyson in which he stated that if one could travel faster than the speed of light, it might be possible to travel back in time. He didn't really expand on that so I was wondering if someone here could explain this to me.

 

[bcrowell]

Either you misunderstood him or he made a mistake. Material objects can only travel at less than the speed of light.

 

[abledpilot]

I think he was speaking theoretically, and that if you plug in a time faster than the speed of light into Einstein's equations time starts to flow backwards.

 

[nitsuj]

Hello, I'm a high school senior interested in studying physics in college. I was listening to a podcast by Neil deGrasse Tyson in which he stated that if one could travel faster than the speed of light, it might be possible to travel back in time. He didn't really expand on that so I was wondering if someone here could explain this to me.

Things that have happened cannot be undone. no matter the frame.

Simularly, I can add numbers to 299,792,458m/s, however that doesn't impact the reality. What point would it be to see SR/GR equations compute time reversal at FTL speeds, it's beyond the predictability of those equations I'm sure (let alone reality).

I don't know in what context Neil deGrasse Tyson was speaking from. It seems a very odd statement for SR/GR.

 

[bcrowell]

I think he was speaking theoretically, and that if you plug in a time faster than the speed of light into Einstein's equations time starts to flow backwards.

No, that's incorrect, so either he made a mistake or you misunderstood him.

-Ben

 

[AndyUrquijo]

Well, as you tend to the limit of traveling at the speed of light time dilations increases. That is, time appears to 'slow down' (on the outside of your spaceship). If you extrapolate this tendency just for the fun of it you would say that time stays 'still' at the speed of light, and taking this to a further step would lead you to say that time goes 'backwards' at higher speeds.
So its not crazy talk, just a baseless or "hypotetical" speculation. You can't even travel at the speed of light, much less at a higer speed*.

*at least as our observations appear to confirm us so far

 

[bcrowell]

Well, as you tend to the limit of traveling at the speed of light time dilations increases. That is, time appears to 'slow down' (on the outside of your spaceship). If you extrapolate this tendency just for the fun of it you would say that time stays 'still' at the speed of light, and taking this to a further step would lead you to say that time goes 'backwards' at higher speeds.

No, that's incorrect. If you plug v>c into the equation \gamma=1/\sqrt{1-v^2/c^2}, you don't get a negative result, you get an imaginary one.

-Ben

 

[BruceW]

In special relativity, causality is limited to one direction because of the speed limit of c.
(It is possible to reverse the order of two events, by looking at them through a different reference frame, but this requires that the two events must be separated such that one event could not have caused the other).
But, if there was a wormhole between the two events, then you could in principle reverse the order of two causally connected events.
But of course, a wormhole doesn't mean traveling faster than c. Nothing can travel faster than c. So you'd be more correct to say that in certain situations, the curvature of spacetime could lead to the reversal of the cause-and-effect of two events.

 

[AndyUrquijo]

No, that's incorrect. If you plug v>c into the equation \gamma=1/\sqrt{1-v^2/c^2}, you don't get a negative result, you get an imaginary one.

-Ben

Yeah, my bad. I didn't mean to say that was correct. I just think this was probably the (naive) reasoning that went on to make such claim. You are right of course. And you get an undefined result when you use v=c. Which is irrelevant since you can't get to that speed to begin with (if you have non-zero mass).

 

[DrGreg]

Hello, I'm a high school senior interested in studying physics in college. I was listening to a podcast by Neil deGrasse Tyson in which he stated that if one could travel faster than the speed of light, it might be possible to travel back in time. He didn't really expand on that so I was wondering if someone here could explain this to me.

It's possible he was referring to the tachyon, a hypothetical particle which, if it existed, would always travel faster than light and would never go slower. There are many good reasons for believing that tachyons don't exist, but nevertheless we can consider how they might behave if they did.

If a tachyon is traveling faster than light forwards in time relative to one inertial observer, it is traveling backwards in time relative to some other inertial observers. If you could relay a message via at least two tachyons, you could send a message into your own past. See Tachyonic_antitelephone for details. That would give rise to all sorts of grandfather paradox-like problems, one reason that we suspect tachyons don't exist -- there are other reasons, too.

 

[byron178]

It's possible he was referring to the tachyon, a hypothetical particle which, if it existed, would always travel faster than light and would never go slower. There are many good reasons for believing that tachyons don't exist, but nevertheless we can consider how they might behave if they did.

If a tachyon is traveling faster than light forwards in time relative to one inertial observer, it is traveling backwards in time relative to some other inertial observers. If you could relay a message via at least two tachyons, you could send a message into your own past. See Tachyonic_antitelephone for details. That would give rise to all sorts of grandfather paradox-like problems, one reason that we suspect tachyons don't exist -- there are other reasons, too.

Why could virtual particles travel faster than light but yet i don't hear them violating causality.

 

[kamenjar]

... If you could relay a message via at least two tachyons, you could send a message into your own past. See
Oh ok, so FTL message transfer is paradoxic because formula says so )) I don't mean to offend - But do you realize how hilarious that sounds to someone who doesn't rely on formula doctrine? In just another thread on this forum ("A photon's time") it is agreed that photon's frame can not be expressed with Lorentz transformations which are used in the Tachyonic Antitelephone example. It's the same as saying that GR is wrong because Newtonian formulas give wrong results at relativistic speeds.

Does anyone have a better thought experiment that doesn't involve formulas that are likely not accounting for FTL? I'm sure there has to be something.

And FYI, I am not saying that FTL is possible... All I am saying is that formulas are not useful answers to reply to thoughts outside of things that we can experiment with.

 

[Aimless]

No, that's incorrect, so either he made a mistake or you misunderstood him.

If, by FTL, we mean the ability to travel between two spacelike-separated points, then in that sense there is no distinction between faster-than-light travel and time travel, because one can easily draw a spacelike curve which ends within the past lightcone of its origin. (Assuming Lorentz invariance; if there's a preferred foliation which restricts such trajectories, then that's another matter.)

 

[abaio]

Actually a few things travel faster than light but are not helpful or useful. For example, the EPR parardox that information between 2 electrons is shared instantly and faster than lightspeed, however it proves no use if the information cannot be controlled and only random.

 

[GrayGhost]

Actually a few things travel faster than light but are not helpful or useful. For example, the EPR parardox that information between 2 electrons is shared instantly and faster than lightspeed, however it proves no use if the information cannot be controlled and only random.

Yes, but I'm supposing we don't really know that the other entangled particle changes state instantly upon the 1st being observed, even though we might casually assume such. There's no proof it's instant, yes?

GrayGhost

 

[abaio]

I believe we do know its instant. Thats why Einstein himself referred to the event as "spooky". Either way, there are still other ways to conquer the speed of light.

 

[nkadambi]

Coming from a very pure math background, I see why traveling at or faster than light screws up all the math formulas, but physically why can't an object travel at (or faster) than light?

What fundamental law of the universe could you be breaking? Thanks.

 

[abaio]

Well photons (the particles that make up light) have 0 mass so it is very easy for them to stay at a constant speed through the vacuum. It takes extremely high energies to harness the speed of light, energies that we cannot quite reach yet (suck as the Planck energy). However certain theories have been originated where negative energy and exotic matter may propel faster than light travel. But as far as breaking any laws it would just really screw up mathematic equations as you mentioned before. (You should look up a particle called tachyons which may be able to break the light barrier)

 

[mathfeel]

Coming from a very pure math background, I see why traveling at or faster than light screws up all the math formulas, but physically why can't an object travel at (or faster) than light?

What fundamental law of the universe could you be breaking? Thanks.

It take infinite work to accelerate any massive particle from subliminal speed to speed of light.

 

[DaveC426913]

It take infinite work to accelerate any massive particle from subliminal speed to speed of light.

That's pretty much it in a nutshell.

There's several ways to look at it.

Mass increases without limit as you approach c. Mass would exceed total mass of universe before you got there.
Energy required to accelerate that mass would exceed total energy of universe.

 

[CJames]

Coming from a very pure math background, I see why traveling at or faster than light screws up all the math formulas, but physically why can't an object travel at (or faster) than light?

What fundamental law of the universe could you be breaking? Thanks.

The invariance of the speed of light. Every experiment that has been performed to measure the speed of light has determined that it is always c. This measurement is independent of the "velocity" of the observer, and the velocity of the observer can only be stated relatively speaking.

In short, even if an observer were traveling at 99.9999% the speed of light in comparison with the earth, that observer would still measure the speed of light to be c in comparison to them. In other words, the speed of light is c relative to any observer. It is the only speed that has this unique property, and it is the reason for the "time warps" that occur for observers who are traveling near c.

Obviously you can never travel faster than something which is always 300 million m/s faster than you.

 

[CJames]

Actually a few things travel faster than light but are not helpful or useful. For example, the EPR parardox that information between 2 electrons is shared instantly and faster than lightspeed, however it proves no use if the information cannot be controlled and only random.

I think the standard interpretation of this is that the information is non-local, which is more like saying that the system is in two places at once than saying that information is traveling from one particle to another at faster than the speed of light.

 

[CJames]

Hello, I'm a high school senior interested in studying physics in college. I was listening to a podcast by Neil deGrasse Tyson in which he stated that if one could travel faster than the speed of light, it might be possible to travel back in time. He didn't really expand on that so I was wondering if someone here could explain this to me.

Most likely he was referring to "spacelike" trajectories. Suppose you observe two events taking place "simultaneously." There are reference frames in which event A occurred before event B. There are also reference frames in which event B occurred before event A.

The catch here is that event A couldn't possibly have caused event B, or visa versa, because light hasn't had enough time to connect the two events.

For this reason, if somebody could travel from point A to point B, they would be traveling backward in time from the perspective of some reference frames. This means that if they had a method of doing this, they could theoretically travel backward in time to any event that they wished.

Obviously this is all theoretical and there is no reason to expect that it is possible given the current laws of physics. For sure, it is impossible to travel from event A to event B, or visa versa, simply by accelerating faster than the speed of light, unless the laws of physics as we know them break down at extremely high energies.

 

[GrayGhost]

Coming from a very pure math background, I see why traveling at or faster than light screws up all the math formulas, but physically why can't an object travel at (or faster) than light?

My understanding, is that the mathematical problem arises only at v=c, whereby an anomaly occurs. Physically, no material particle can accelerate to speed c, because it would take infinite energy and the universe is assumed to possesses finite energy. If I recall correctly, "faster than c" involves an imaginary spacetime from our perspective. There is a theory (or hypothesis maybe) that any mass traveling at >c might be possible assuming it were never c or less to begin with. It could not attain a relative light speed either. However, there has never been any physical evidence of such in nature to date.

GrayGhost

 

[BruceW]

CJames has written a good explanation of what people are often talking about when people say "exceeding speed of light would mean time travel."
Of course, locally exceeding the speed of light is impossible, which is very explicit in Einstein's relativity. Which is why I think this phrase would be more accurate: "the speed limit of c means that in special relativity, the reversal of causality is not allowed".
In general relativity, you still can't locally exceed c, but due to curvature of spacetime, it is possible to imagine wierdly shaped universes where you could effectively time travel.

 

[DaveC426913]

CJames has written a good explanation of what people are often talking about when people say "exceeding speed of light would mean time travel."

Agreed. I'm going to make note of his post and will likely refer to it many times when trying to explain this issue.

 

[abaio]

Dont some physicists believe tachyons travel faster than the speed of light because they actually travel faster the fewer the energy they have?

 

[CJames]

Thank you Bruce and Dave for your kind words.

abaio,

Dont some physicists believe tachyons travel faster than the speed of light because they actually travel faster the fewer the energy they have?

As I believe somebody else already mentioned, tachyons have all kinds of imaginary numbers associated with them. (An imaginary number is the square root of a negative number, which has no known physical meaning.) This, plus the fact that they would violate causality, plus the fact that they have never been observed, makes them unlikely. Then again, reality has a way of surprising us.

 

[DaveC426913]

Dont some physicists believe tachyons travel faster than the speed of light because they actually travel faster the fewer the energy they have?

Well, hypothetical particles could do that, which are given the name tachyons. But not only is there no evidence for them, there is not even any reason to think they might exist. Simply, relativity does not explicitly forbid them.

 

[GrayGhost]

CJames has written a good explanation of what people are often talking about when people say "exceeding speed of light would mean time travel."
Of course, locally exceeding the speed of light is impossible, which is very explicit in Einstein's relativity. Which is why I think this phrase would be more accurate: "the speed limit of c means that in special relativity, the reversal of causality is not allowed".
In general relativity, you still can't locally exceed c, but due to curvature of spacetime, it is possible to imagine wierdly shaped universes where you could effectively time travel.

Wrt the highlight ...

Causality would be violated given superluminal motion from the vantage of our sub-c POV. However, it seems to me that one should expect casuality to be reversed given tachyon speeds, since time should run in reverse for the tacheon relative to our own sub-c POV. From the POV "of the other (tachyon) side" it deems itself sub-c, and causality should seem normal from its own sub-c POV. Yes?

GrayGhost

 

[BruceW]

I would assume so. The problem about tachyons, is we really don't know how they would act (whether they are too unstable to be called particles, or whether they will allow causality reversal). Wikipedia's page is pretty good.

 

[CJames]

Wrt the highlight ...

Causality would be violated given superluminal motion from the vantage of our sub-c POV. However, it seems to me that one should expect casuality to be reversed given tachyon speeds, since time should run in reverse for the tacheon relative to our own sub-c POV. From the POV "of the other (tachyon) side" it deems itself sub-c, and causality should seem normal from its own sub-c POV. Yes?

GrayGhost

Tachyon's have an imaginary rest mass. Like a beam of light, they can't be at rest. I don't know if it's actually possible to try to construct an inertial reference frame from the POV of a tachyon.

 

[GrayGhost]

Tachyon's have an imaginary rest mass. Like a beam of light, they can't be at rest. I don't know if it's actually possible to try to construct an inertial reference frame from the POV of a tachyon.

Hmmm, interesting. I did just take a look at Wiki. I had never assumed "imaginary mass" of superluminal particles to mean "effective mass" as in the photon. I figured an imaginary mass could be at rest. Speed c exists "at the cosmic speed limit", but superluminal speeds do not. I had assumed that the imaginary spacetime would be "orthogonal" to our spacetime, since neither side can have a particle accelerate to c.

GrayGhost

 

[jiohdi]

FTL travel is still forwards in time not backwards... by definition you are traveling some distance in some forward unit of time. Just because you are outside a light cone does not mean you are traveling backwards in time... one part of the equation that seemingly gets ignored is the mass... if you could in theory travel your mass to a prior spacetime coordinate what do you suspect you would find? what is missing is the rest of the universes mass... your traveling to a prior spacetime coordinate would not drag that mass back with you... you would undoubtedly find yourself in hyperspace alone or with some other hyperspacial objects but the past you were looking for would not be there... that only exists in records.

 

[CJames]

Hmmm, interesting. I did just take a look at Wiki. I had never assumed "imaginary mass" of superluminal particles to mean "effective mass" as in the photon. I figured an imaginary mass could be at rest. Speed c exists "at the cosmic speed limit", but superluminal speeds do not. I had assumed that the imaginary spacetime would be "orthogonal" to our spacetime, since neither side can have a particle accelerate to c.

GrayGhost

I don't know enough about tachyons to really say for sure. The only way that I could imagine picturing a tachyon's reference frame would be if you were to say that the x-axis for matter is the t-axis for a tachyon. If that were the case then all normal matter would look like tachyons, some of which would appear to be traveling backward in time.

 

[CJames]

FTL travel is still forwards in time not backwards... by definition you are traveling some distance in some forward unit of time. Just because you are outside a light cone does not mean you are traveling backwards in time...

The point is that different observers would disagree about the order of events. Some observers would say that the tachyon was present at all points along the trajectory simultaneously. Others would say that it traveled from event A to event B, and still others would say that it traveled from event B to event A.

one part of the equation that seemingly gets ignored is the mass... if you could in theory travel your mass to a prior spacetime coordinate what do you suspect you would find? what is missing is the rest of the universes mass... your traveling to a prior spacetime coordinate would not drag that mass back with you... you would undoubtedly find yourself in hyperspace alone or with some other hyperspacial objects but the past you were looking for would not be there... that only exists in records.

I'm not sure what you're getting at here.

 

[jiohdi]

The point is that different observers would disagree about the order of events. Some observers would say that the tachyon was present at all points along the trajectory simultaneously. Others would say that it traveled from event A to event B, and still others would say that it traveled from event B to event A.



I'm not sure what you're getting at here.

anyone who could measure a tachyon as moving faster than light would be measuring a forwards time by definition... otherwise it would not make any sense to say it was faster than light... a major point of relativity theory is the ability to normalize any frame of reference ot any other by means of the lorenz transformations and come to an understanding of a true order of how reality unfolded.

 

[DaveC426913]

anyone who could measure a tachyon as moving faster than light would be measuring a forwards time by definition...

The point is that, because it could be arranged such that event B preceded event A, it is possible that effect can precede cause i.e. violating causality, i.e. a form of time travel.

 

[CJames]

anyone who could measure a tachyon as moving faster than light would be measuring a forwards time by definition... otherwise it would not make any sense to say it was faster than light... a major point of relativity theory is the ability to normalize any frame of reference ot any other by means of the lorenz transformations and come to an understanding of a true order of how reality unfolded.

Except that this isn't the case if trajectories outside of the light cone are allowed. The ability to travel between two events with spacelike separation is the ability to travel between two events that should not be causally connected to one another. This is one of the main reasons why they are suspected not to exist.

 

[abaio]

Thanks for the information.

 

[CJames]

Thanks for the information.

You're welcome.