# Question about travelling faster than light.

• Synchronised
In summary: Nor is imaginary time negative time, not under any simplification. The only reason I can see that people think going faster than light equates to going back in time is that they think its a simplification or 'close enough' to move the negative sign out of the square root and turn your imaginary number into a negative number. Other than this fallacy, where would anybody get this notion that traveling faster than light is equivalent to traveling back in time?In summary, the conversation discusses the concept of time dilation and the possibility of traveling faster than the speed of light, which could potentially lead to negative time or traveling backwards in time. However, this is currently considered impossible due to the limitations of energy. There is also mention of t
Synchronised
So when a moving object reaches the speed of light time dilates so I assume if an object that has mass somehow achieves the speed of light time stops moving (since it is traveling slower and slower) and once the object exceeds the speed of light time becomes negative so does that mean the object travels backwards in time? (I know this is impossible because we cannot create an infinite amount of energy but assume it is achievable)

Synchronised said:
So when a moving object reaches the speed of light time dilates so I assume if an object that has mass somehow achieves the speed of light time stops moving (since it is traveling slower and slower) and once the object exceeds the speed of light time becomes negative so does that mean the object travels backwards in time? (I know this is impossible because we cannot create an infinite amount of energy but assume it is achievable)

It's impossible for more than the energy issue.

Would need to read about SR and specifically about metrics. Going "back in time" is just going back in coordinate time. Not same as travel back in time to 1905, like time travel sci-fi. In other words the clock on board your spaceship wouldn't start ticking backwards...just think of the paradox there you would just keep being forced back in time to the point you started to travel ...back in time. lol oh imagination

Synchronised said:
So when a moving object reaches the speed of light time dilates so I assume if an object that has mass somehow achieves the speed of light time stops moving (since it is traveling slower and slower) and once the object exceeds the speed of light time becomes negative so does that mean the object travels backwards in time? (I know this is impossible because we cannot create an infinite amount of energy but assume it is achievable)

The question you have asked cannot be answered. It's like asking "If a pig could fly, would it be able to catch a bumblebee? I know this is impossible because pigs don't have wings, but assume that they do."

You might, however, try searching the web, wikipedia, and this forum for "tachyons", which are hypothetical faster-than-light particles with many interesting properties. Just be skeptical about the sources; there's a lot of over-simplified junk out there.

Nugatory said:
The question you have asked cannot be answered. It's like asking "If a pig could fly, would it be able to catch a bumblebee? I know this is impossible because pigs don't have wings, but assume that they do."

You might, however, try searching the web, wikipedia, and this forum for "tachyons", which are hypothetical faster-than-light particles with many interesting properties. Just be skeptical about the sources; there's a lot of over-simplified junk out there.

So it is simply impossible to 'travel back in time'.

Synchronised said:
So it is simply impossible to 'travel back in time'.

As far as we know so far, yes, it is impossible. If it ever turns out to be possible (and I would bet very very long odds against this) it won't be by somehow accelerating an object to the speed of light.

It depends on if you want to talk about observed reality as we know it now or possible pathological examples and also what you mean by "time travel". In a non time - orientable space - time you have point(s) where one cannot differentiate between future and past for example.

Synchronised said:
So when a moving object reaches the speed of light time dilates so I assume if an object that has mass somehow achieves the speed of light time stops moving (since it is traveling slower and slower) and once the object exceeds the speed of light time becomes negative so does that mean the object travels backwards in time? (I know this is impossible because we cannot create an infinite amount of energy but assume it is achievable)
You're getting answers that are being technical and not very "fun". The simplistic answer is that your speculation is correct, traveling faster than light could be considered to be the same thing as traveling backwards in time.

rjbeery said:
You're getting answers that are being technical and not very "fun". The simplistic answer is that your speculation is correct, traveling faster than light could be considered to be the same thing as traveling backwards in time.

The "fun" traveling back in time is the most remote, as in not even remotely possible.

nitsuj said:
The "fun" traveling back in time is the most remote, as in not even remotely possible.
Agreed, but the OP admitted that he understood the answer would lie outside of reality. Ya Grinch. :P

rjbeery said:
Agreed, but the OP admitted that he understood the answer would lie outside of reality. Ya Grinch. :P

Agreed

rjbeery said:
The simplistic answer is that your speculation is correct, traveling faster than light could be considered to be the same thing as traveling backwards in time.

How so? What simplifications are you making to conclude this? As far as I know its not a simplification, rather its flat out wrong. Its a conclusion that comes from misunderstanding the time dilation formula rather than a simplification. Is this not the case?

ModusPwnd said:
How so? What simplifications are you making to conclude this? As far as I know its not a simplification, rather its flat out wrong. Its a conclusion that comes from misunderstanding the time dilation formula rather than a simplification. Is this not the case?

Yes, time would become imaginary which is impossible.

Synchronised said:
Yes, time would become imaginary which is impossible.

Nor is imaginary time negative time, not under any simplification. The only reason I can see that people think going faster than light equates to going back in time is that they think its a simplification or 'close enough' to move the negative sign out of the square root and turn your imaginary number into a negative number. Other than this fallacy, where would anybody get this notion that traveling faster than light equates to going back in time?

ModusPwnd said:
Nor is imaginary time negative time, not under any simplification. The only reason I can see that people think going faster than light equates to going back in time is that they think its a simplification or 'close enough' to move the negative sign out of the square root and turn your imaginary number into a negative number. Other than this fallacy, where would anybody get this notion that traveling faster than light equates to going back in time?

How about because FTL in one inertial frame is a back in time path in another? How about that if FTL exists, and the principle of relativity applies to it, then communication with the past is trivially possible. It's got nothing to do with imaginary anything.

Perhaps... I have never heard either of those as claims for why FTL in SR allows for time travel. Neither really is time travel. Not in the sense of the original post.

The notion I get from people and non-physics students is that they take the fact that time dilates as you approach "c" and conculde that time stops at "c" and then goes backwards when you are over "c". This is an error of course and is usually how I see the time travel idea in SR justified. Look at the original post, "once the object exceeds the speed of light time becomes negative ". Confusing imaginary time with negative time is the source of his error.

ModusPwnd said:
Perhaps... I have never heard either of those as claims for why FTL in SR allows for time travel. Neither really is time travel. Not in the sense of the original post.

The notion I get from people and non-physics students is that they take the fact that time dilates as you approach "c", stops at "c" and then goes backwards when you are over "c". This is an error of course and is usually how I see the time travel idea in SR justified. Look at the original post, "once the object exceeds the speed of light time becomes negative ". Confusing imaginary time with negative time is the source of his error.

If you travel FTL in one frame, accelerate quickly to speed near c, and travel FTL back, then purely by Lorentz transform, you arrive back before you left. Just look up tachyon anti-telephone. Basically, if the Lorentz transform applies to FTL paths, and if FTL travel is possible in any frame, then travel to the past is possible. Since both of these assumptions are core to relativity, it is correct to say that relativity + FTL travel implies travel back in time.

FYI: I've never heard any other explanation than the above. Reputable books would never mention the nonsense argument you refer to. The above argument goes back a 100 years.

ModusPwnd said:
Perhaps... I have never heard either of those as claims for why FTL in SR allows for time travel. Neither really is time travel. Not in the sense of the original post.

The notion I get from people and non-physics students is that they take the fact that time dilates as you approach "c" and conculde that time stops at "c" and then goes backwards when you are over "c". This is an error of course and is usually how I see the time travel idea in SR justified. Look at the original post, "once the object exceeds the speed of light time becomes negative ". Confusing imaginary time with negative time is the source of his error.
Think in terms of tipping over light cones. We all agree that it's impossible, but the notion is not 100% without merit.

I'm not talking about reputable books. I'm talking about non-reputable people (no offense to the original poster :P)

PAllen said:
then purely by Lorentz transform, you arrive back before you left.

Amazing! +

Under the Schwarzschild coordinate system and a classical analysis, the distant observer A could make the case that the infalling object B has a coordinate velocity of c at the EH; that this occurs at t=+inf; that beyond the EH B has a coordinate velocity > c (because B is still being accelerated); and that beyond the EH time runs backwards from +inf from A's perspective.

In other words, there is an interpretation of BH's which suggests that an infalling body reaches the singularity "before" it crosses the EH. Please discount all of the above with the caveat that we're being highly speculative with no "reality restrictions" here, but I have in fact seen an analysis of BH's done in this manner.

rjbeery said:
Under the Schwarzschild coordinate system and a classical analysis, the distant observer A could make the case that the infalling object B has a coordinate velocity of c at the EH; that this occurs at t=+inf; that beyond the EH B has a coordinate velocity > c (because B is still being accelerated);
This much is true, but is mixed up. The coordinates in which you can say object falls through horizon at c and continues inside at > c are one set of coordinates (Gullestrand-Panlieve, for example). The coordinates that say t->+infinity on approach to horizon are different coordinates (Schwarzschild coordinates). So this mixes coordinates, and fails to note that coordinates only get meaning through computation of observables via the metric. In which case all these coordinate statement lose their exotic appearance. They are coordinate artifacts.
rjbeery said:
and that beyond the EH time runs backwards from +inf from A's perspective.
.
This is nonsense. Do you have reference?
rjbeery said:
In other words, there is an interpretation of BH's which suggests that an infalling body reaches the singularity "before" it crosses the EH. Please discount all of the above with the caveat that we're being highly speculative with no "reality restrictions" here, but I have in fact seen an analysis of BH's done in this manner.

Again, I've read many BH treatments, both popular and mathematical, both by scientists, science writers, and cranks, and I've not seen such claims. Would be interested in seeing a reference to a writer who is that loony.

The truth (per GR - who knows in the real world), is that clock falling through the horizon ticks forward normally through the horizon and up to the singularity. Light always appears to move at normal speed relative to this observer, locally. As for 'from the point of view of a distant observer', the only thing you can say physically is that the distant observer can never see or get any information about the history of the infaller crossing and beyond the horizon. So it is hard to know what 'point of view' [about these events they can't detect] to ascribe to them. Note, however, that the distant observer can send signals to the infaller that are received by the infaller (until the infaller reaches the singularity).

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PAllen said:
This much is true, but is mixed up. The coordinates in which you can say object falls through horizon at c and continues inside at > c are one set of coordinates (Gullestrand-Panlieve, for example). The coordinates that say t->+infinity on approach to horizon are different coordinates (Schwarzschild coordinates). So this mixes coordinates, and fails to note that coordinates only get meaning through computation of observables via the metric. In which case all these coordinate statement lose their exotic appearance. They are coordinate artifacts.
One thing at a time here. I'm still at work, technically! :)
If we consider the B's "coordinate velocity" to be A's calculation of the required escape velocity at that point then A would consider B to be traveling at c at the EH. No G-P coordinates required.

Thought some ppl may be interested in it I've been unable to get the original paper however.
http://phys.org/news/2012-10-physic...early/2012/09/25/rspa.2012.0340.full.pdf+html

rjbeery said:
... traveling at c at the EH. No G-P coordinates required.

What coordinates ARE you using at the event horizon? Not Schwarzschild - they don't work there.

rjbeery said:
One thing at a time here. I'm still at work, technically! :)
If we consider the B's "coordinate velocity" to be A's calculation of the required escape velocity at that point then A would consider B to be traveling at c at the EH. No G-P coordinates required.

First coordinate velocity, now escape velocity. Escape velocity is the speed relative to a local static observer required to reach spatial infinity; or equivalently, the speed a free faller from infinity would have locally, relative to said static observer. There are no static observers at or inside the horizon, so the concept is undefinable, not c, or > c.

[edit: For the fanciful purposes of this thread, you can note that tachyons (any particle presumed to follow spacelike paths = > c) could escape from inside to outside and event horizon. However, that doesn't define any concept of escape velocity. Specific to escape velocity is the feature that a radial, timelike geodesic with escape velocity relative to some static observer has zero speed relative to a static observer in the limit at infinite distance. A spacelike geodesic that crossed the horizon would remain spacelike everyhwhere - i.e. it would still be moving FTL at infinity. Thus there is no possible concept of an object starting < c relative to static observer and getting to be > c relative to static observer. A timelike geodesic is timelike up to the singularity; a spacelike geodesic is spacelike everywhere. This is related to the notion that tachyons are as strongly prohibited from slowing down to c as normal particles are prohibited from reaching c.]

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PAllen said:
This is related to the notion that tachyons are as strongly prohibited from slowing down to c as normal particles are prohibited from reaching c.]

Brings up an interesting thought. As tachyons are often descibed as traveling back in time. which brought up the law of causality. If by the above statement above. The tachyon would never be able to change its timeline vector either. So it could never violate its own spacetime causality Would it still violate causality in our spacetime?

I know they have never proven the existence of the tachyon nor likely to do so lol. Just curious on how to interpret the above. Not to hijack the thread lol

Mordred said:
Brings up an interesting thought. As tachyons are often descibed as traveling back in time. which brought up the law of causality. If by the above statement above. The tachyon would never be able to change its timeline vector either. So it could never violate its own spacetime causality Would it still violate causality in our spacetime?

I know they have never proven the existence of the tachyon nor likely to do so lol. Just curious on how to interpret the above. Not to hijack the thread lol

This is an interesting point. The causality violated by a process using tachyons is causality along some timelike (normal) world line that sends tachyons to someone else and gets a reply back before they sent. Or, in the case of tachyon rocket, the rocket returns before it left - from the point of view of the world line it left. Causality along the tachyon path itself is undefined.

PAllen said:
How about because FTL in one inertial frame is a back in time path in another? How about that if FTL exists, and the principle of relativity applies to it, then communication with the past is trivially possible. It's got nothing to do with imaginary anything.

Everything you said before "It's got nothing to do with imaginary anything" had everything to do with imagination.

PAllen said:
Again, I've read many BH treatments, both popular and mathematical, both by scientists, science writers, and cranks, and I've not seen such claims. Would be interested in seeing a reference to a writer who is that loony.
PAllen said:
First coordinate velocity, now escape velocity. Escape velocity is the speed relative to a local static observer required to reach spatial infinity; or equivalently, the speed a free faller from infinity would have locally, relative to said static observer. There are no static observers at or inside the horizon, so the concept is undefinable, not c, or > c.
If you're going to claim that Newtonian escape velocity has no meaning at the event horizon then you're either intellectually sandbagging or simply not being fanciful enough on an admittedly fanciful subject.

Considering time to move backwards within the event horizon is not difficult; pick a radius and keep it constant. I'm not here to convince you of this, I don't really care one way or the other, but you asked for other references so http://curious.astro.cornell.edu/question.php?number=652.

EDIT: That link seems to be flaky, sometimes it works and sometimes it does not. Below is the article...

What kind of time reversal takes place inside the event horizon of black holes?

Many people are fascinated by the famous "event horizon" of a black hole, the boundary of the region out of which nothing can escape. The mechanism that gives it this property is strange and amazing--it has to do with the idea of causality.

Merlin from the King Arthur legends was supposed to have lived his life backwards--the first thing he experienced was his death and the last was his birth, hence his ability to foretell the future. To us, however, time feels as though it flows only forward. This feeling actually comes from the more general property of causality. In a region like the one here on Earth, you can only remember events that meet 2 criteria: (1) it has to have been in the past, and (2) it has to have happened at a distance no more than what light could have traveled since it happened. The second rule is just the familiar light speed limit. The first is called causality, and it's why you won't meet anyone like Merlin here at home.

Here comes the strange part. General relativity (that same theory supported by so many experiments and needed to make the GPS system work) predicts that, simply by compressing any piece of matter down enough to make a black hole, you create a region where this just isn't true. Inside the event horizon, time and space change places. Therefore the new restrictions go like this: in order for you to remember something, (1) it has to have happened farther from the center of the black hole than where you are now, and (2) if T is the time that it would take light to travel to you from the location of the event, then it happened either no more than T hours ago or T hours into the future.

I recommend thinking about this at least until your head starts to hurt. First of all, note that restriction #1 prevents you from moving away from the center of the black hole, and therefore from going back across the event horizon. Also note that it says "farther", not "at least as far". This means that not only can't you move away from the center, you can't even stand still. Also we see that everyone inside the event horizon is a psychic. This happens because light can travel to you from events in the future, so you can quite literally see them. You can't see anything closer to the center than you are because light can't travel away from the center. If you look away from the center, though, you see two images of everything--one from T hours in the past and one from T hours in the future. For nearby objects, these two images will look just the same, since T will be very small due to the large speed of light. For faraway objects, though, they could be completely different. For instance, if both you and Tolstoy were in a black hole and were separated by 3 light years, you could be watching him start and finish War and Peace at once. At that point in time, he would only be done with half of the book. Of course, you'd want to try sending him a message with the text of the book, to save him some work writing it, but you couldn't--he can't see you at all, since you're closer to the center of the black hole than he is. Pity.

If you think about it for a while, you'll be able to come up with loads of strange situations that can happen inside a black hole--but none of them will be logically inconsistent (such as would be the case if you had been able to send Tolstoy the last chapter of his book before he had written it). There are even more when you consider that realistic astronomical black holes should actually have 2 event horizons--the causality flip discussed above happens at the outer event horizon, and then flips back at the inner event horizon.

January 2005, Sara Slater (more by Sara Slater) (Like this Answer)​

rjbeery said:
Think in terms of tipping over light cones.

"Tipping over light cones" only happens in curved spacetime (in fact it's one way of describing what spacetime curvature *is*). It's a completely separate concept from "traveling faster than light", which can be analyzed purely in flat spacetime.

rjbeery said:
Considering time to move backwards within the event horizon is not difficult; pick a radius and keep it constant.

No, this doesn't work, because inside the EH, a curve of constant radius is spacelike, not timelike. So there's no such thing as "time" along such a curve.

rjbeery said:
Inside the event horizon, time and space change places.

This is only true in a particular coordinate chart, the interior Schwarzschild chart. It is not true in other charts. Also, it is not necessary to ground the further statements made; see below.

rjbeery said:
Therefore the new restrictions go like this: in order for you to remember something, (1) it has to have happened farther from the center of the black hole than where you are now, and (2) if T is the time that it would take light to travel to you from the location of the event, then it happened either no more than T hours ago

All of this is correct, *and* it is independent of any coordinate chart; and it does *not* require that "time and space change places".

rjbeery said:
or T hours into the future.

This is *not* correct, and I don't understand where the author is getting it from. Light signals have to move inward inside the horizon, just like everything else. There's no way for light "from the future", meaning at a smaller radius, to get to you at a larger radius. It *is* possible for you to see light that was emitted "below" you, if it was emitted radially outward; but that's because you "catch up" to the light, i.e., you decrease in radius faster than the light does. That doesn't mean the light was emitted "in the future" relative to you; that's impossible.

rjbeery said:
You can't see anything closer to the center than you are because light can't travel away from the center.

No, but you *can* "catch up" to light emitted radially outward from someplace "below" you. See above.

rjbeery said:
If you look away from the center, though, you see two images of everything--one from T hours in the past and one from T hours in the future.

Again, I don't know where the author is getting this from. It is true that there are light paths which go "around" the hole, so you can in fact see multiple images from the same source, which have gone around the hole different numbers of times. But none of those images come from "the future". (Also, such light paths are not purely radial, obviously, since they go around the hole. That makes them more complicated to analyze, and I don't think the author fully appreciates those complications.)

rjbeery said:
For instance, if both you and Tolstoy were in a black hole and were separated by 3 light years, you could be watching him start and finish War and Peace at once.

Possible, yes, but only if both events were in your *past* light cone. This should be obvious, since you're watching them both.

rjbeery said:
At that point in time, he would only be done with half of the book.

Nope--both events ("start the book" and "finish the book") are in your past light cone, so "at that point in time", *every* event on Tolstoy's worldline between the two events must also be in your past light cone, including the one where he is halfway done.

Also, since all of those events are on Tolstoy's worldline, they are all timelike separated, so there is *no* "point in time" by *anyone*'s clock which contains more than one of them.

rjbeery said:
Of course, you'd want to try sending him a message with the text of the book, to save him some work writing it, but you couldn't--he can't see you at all, since you're closer to the center of the black hole than he is.

Which also means that *none* of the events on his worldline are in your future--events in your future are all closer to the center of the hole than you, and Tolstoy is not. I think this person is failing to completely analyze their own scenario.

rjbeery said:
There are even more when you consider that realistic astronomical black holes should actually have 2 event horizons--the causality flip discussed above happens at the outer event horizon, and then flips back at the inner event horizon.

It is true that rotating holes and charged holes have two horizons, not one, but "causality flip" is an erroneous way to describe what happens at those horizons. See above.

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