Question about travelling faster than light.

[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)

 

[nitsuj]

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

 

[Nugatory]

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.

 

[Synchronised]

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'.

 

[Nugatory]

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.

 

[WannabeNewton]

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.

 

[rjbeery]

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.

 

[nitsuj]

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.

 

[rjbeery]

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

 

[nitsuj]

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

Agreed

 

[ModusPwnd]

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?

 

[Synchronised]

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.

 

[ModusPwnd]

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?

 

[PAllen]

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.

 

[ModusPwnd]

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.

 

[PAllen]

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.

 

[rjbeery]

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.

 

[ModusPwnd]

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

 

[nitsuj]

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

Amazing! +

 

[rjbeery]

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.

 

[PAllen]

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.

and that beyond the EH time runs backwards from +inf from A's perspective.

.
This is nonsense. Do you have reference?

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).

 

[rjbeery]

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.

 

[Mordred]

This article is related to this post, it describes extending Einsteins theory to faster than speed of light using Lorenz transforms.

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

 

[Nugatory]

... 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.

 

[PAllen]

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.]

 

[Mordred]

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

 

[Nugatory]

looks like I got the link working
http://rspa.royalsocietypublishing.org/content/early/2012/09/25/rspa.2012.0340.full.pdf+html

Wasn't this discussed here a few months back? I have a vivid memory of reading a thread about it somewhere, but can't find it right now.

 

[PAllen]

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]

Wasn't this discussed here a few months back? I have a vivid memory of reading a thread about it somewhere, but can't find it right now.

Here's the thread. It appears to be bunk.

https://www.physicsforums.com/showthread.php?t=642823

 

[Mordred]

Ah didn't know there was a thread already on that article