# Question about travelling faster than light.

by Synchronised
Tags: faster, light, travelling
 P: 47 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 travelling 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)
P: 1,103
 Quote by 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 travelling 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
Mentor
P: 3,973
 Quote by 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 travelling 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.

P: 47
Question about travelling faster than light.

 Quote by Nugatory 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'.
Mentor
P: 3,973
 Quote by Synchronised 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.
 C. Spirit Sci Advisor Thanks P: 5,661 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.
P: 270
 Quote by 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 travelling 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.
P: 1,103
 Quote by rjbeery 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.
P: 270
 Quote by nitsuj 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
P: 1,103
 Quote by rjbeery Agreed, but the OP admitted that he understood the answer would lie outside of reality. Ya Grinch. :P
Agreed
P: 1,105
 Quote by rjbeery 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?
P: 47
 Quote by ModusPwnd 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.
P: 1,105
 Quote by Synchronised 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?
PF Gold
P: 5,091
 Quote by ModusPwnd 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.
 P: 1,105 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.
PF Gold
P: 5,091
 Quote by 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", 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.
P: 270
 Quote by 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.
Think in terms of tipping over light cones. We all agree that it's impossible, but the notion is not 100% without merit.
 P: 1,105 I'm not talking about reputable books. I'm talking about non-reputable people (no offense to the original poster :P)

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