# FTL Travel

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So I don't have much of a background in physics, since I haven't had the opportunity to take it yet, but I'm really interested in theoretical FTL methods.

So my question is- What would it take for some sort of spacecraft to accomplish the speed of light or faster. I understand and methods are probably theoretical I just don't understand the math involved just yet, so I'm hoping for some clarification.

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So my question is- What would it take for some sort of spacecraft to accomplish the speed of light or faster. I understand and methods are probably theoretical I just don't understand the math involved just yet, so I'm hoping for some clarification.

I think the most likely scenario for achieving FTL travel is for the laws of physics to change. Short of that I am unaware of any circumstance in which you can accelerate a spacecraft to the speed of light or greater.

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You can have a speed greater than the global speed of light, but not the local speed. For example, warp drives and wormholes make use of this fact.

The general problem then seems to be the actual implementation of these methods - they require exotic matter (negative energy etc) to support them. Along side that, as soon as you can beat a light signal you can create time machines. Causality may also be a problem!

Edited by ajb
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I think the most likely scenario for achieving FTL travel is for the laws of physics to change. Short of that I am unaware of any circumstance in which you can accelerate a spacecraft to the speed of light or greater.

Well that isn't very promising. What about wormholes? From what I've read the main problem would be sustaining it. Is whatever force or particle that does that exist or run with the current laws of physics?

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Well that isn't very promising. What about wormholes? From what I've read the main problem would be sustaining it. Is whatever force or particle that does that exist or run with the current laws of physics?

See what I just posted!

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You can have a speed greater than the global speed of light, but not the local speed. For example, warp drives and wormholes make use of this fact.

The general problem then seems to be the actual implementation of these methods - they require exotic matter (negative energy etc) to support them. Along side that, as soon as you can beat a light signal you can create time machines. Causality may also be a problem!

So how could negative energy be found or converted from regular energy? Since energy obviously can't be created, there has to be a way. The causualties could be avoided by using robotics at first instead of humans.

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So how could negative energy be found or converted from regular energy? Since energy obviously can't be created, there has to be a way. The causualties could be avoided by using robotics at first instead of humans.

The problem is causality, rather than casualties. If you can outrun a photon, you could return to your starting point and arrive before you left. Robots wouldn't fix that.

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The problem is causality, rather than casualties. If you can outrun a photon, you could return to your starting point and arrive before you left. Robots wouldn't fix that.

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I'd like some clarification on this as well.

You can have a speed greater than the global speed of light, but not the local speed. For example, warp drives and wormholes make use of this fact.

The general problem then seems to be the actual implementation of these methods - they require exotic matter (negative energy etc) to support them. Along side that, as soon as you can beat a light signal you can create time machines. Causality may also be a problem!

If indeed a method of FTL became possible wouldn't that in of it's self change the laws of physics as we know them? In fact wouldn't that at least show that causality is at least not quite as written in stone as well...

Along side that, as soon as you can beat a light signal you can create time machines. Causality may also be a problem!

Much like encountering an infinity in an equation wouldn't this indicate a problem with the theory...

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If you can travel at faster than light speed from point A to point B, there exists some frame for which your arrival at B precedes your leaving at point A.

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If you can travel at faster than light speed from point A to point B, there exists some frame for which your arrival at B precedes your leaving at point A.

Ok, that makes it a bit simpler, thanks

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Ok, that makes it a bit simpler, thanks

The article on the tachyonic antitelephone explains the idea a bit more:

https://en.wikipedia.org/wiki/Tachyonic_antitelephone

Will there ever be a physics term greater than "tachyonic antitelephone"? According to this message from the future, the answer is "no"

https://xkcd.com/1699/

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The talk here at the International Astronautical Congress is mostly about system projects, but I'm hoping to hear a technical paper about FTL. So far, all the talk is about shrinking spacetime in front of a vehicle while expanding it behind. From what I've heard, stopping or slowing down this way doesn't work.

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Along with generating enough gamma rays to wipe out the planet you leave and arrive at. I'll see if I still have the arxiv Alcubierre drive paper correlating the gamma ray aspects.

located it.

Edited by Mordred
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The talk here at the International Astronautical Congress is mostly about system projects, but Im hoping to hear a technical paper about FTL. So far, all the talk is about shrinking spacetime in front of a vehicle while expanding it behind. From what I've heard, stopping or slowing down this way doesn't work.

Along with generating enough gamma rays to wipe out the planet you leave and arrive at. I'll see if I still have the arxiv Alcubierre drive paper correlating the gamma ray aspects.

located it.

Engineering problems guys... engineering problems only...

Edited by Moontanman
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So how could negative energy be found or converted from regular energy?

Well, particles with negative mass might exist, but we don't have any evidence of such thing - they would break the various energy conditions imposed on general relativity.

Interestingly, negative energy densities are common in quantum field theories on curved backgrounds, one may be able to exploit quantum effects and manufacture the necessary conditions. However, it is not at all clear that this is possible as subtle effect in quantum theory may render wormholes and so on unphysical.

The causualties could be avoided by using robotics at first instead of humans.

That could avoid free will issues, but causality is a problem whatever you send through time machines.

If indeed a method of FTL became possible wouldn't that in of it's self change the laws of physics as we know them?

The speed limit is that of the local speed of light. By using non-trivial geometry and maybe topology we can beat a light ray by taking another path. A soon as you do this you have questions about causality and time travel. However, none of this violates what we know of general relativity.

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Pass the light through such a material that has an unusually high refractive index. Then, it will be possible to overtake light in that material. LOL!!!

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• 1 month later...

Pass the light through such a material that has an unusually high refractive index. Then, it will be possible to overtake light in that material. LOL!!!

You'd be surprised at how many people think that this is an actual loophole in Relativity.

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• 4 weeks later...

!

Moderator Note

Hijack about alternative propulsion methods has been split, along with my modnote, so let me reiterate:

Stay on-topic here (FTL travel, answers involving mainstream physics only), and do not hijack the discussion

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