FTL Travel: Is it Possible to Violate C?

[Super Luminal]

As one may have deduced from my name, FTL (faster than light) travel interests me very much even if it will forever remain in the realm of Sci-Fi. I am a layman, and a deep understanding of the math and equations eludes me, I must admit.

However, my question remains. First of all, there are a few assumptions that I am making that are, in essence, the question itself.

Why can C not be violated? Is it possible that the speed of light is not C, and while we are right in assuming that there is an inviolable "speed limit" so-to-speak, we are wrong about that limit being equal to the speed of light in a vacuum? That is to say, that there is an inviolable speed limit, but it is greater than the speed of light in a vacuum? Or is that notion as worded above just absurd and indicative of my ignorance of the equations?

 

[Vorde]

No, not really. While you can argue that anything is possible, all current science stands firm that c is the unbreakable speed limit, anything that claims otherwise would have a tough time calling itself science.

That being said, there are concepts that have been introduced over the years that try to 'cheat' the universe. The two that come to mind are wormholes and an Alcuberre drive, both highly speculative concepts with absolutely no solid base, but If you are interested in learning about some of the problems with ideas like these you could look those us.

 

[Drakkith]

We have overwhelming evidence that the speed of light in a vacuum is c, and that it is unable to be broken. The LHC accelerates protons to 99.99+% c, and the fact that protons need more and more energy to accelerate as they get faster agrees perfectly with GR. That is merely one example.

 

[Nabeshin]

That being said, there are concepts that have been introduced over the years that try to 'cheat' the universe. The two that come to mind are wormholes and an Alcuberre drive, both highly speculative concepts with absolutely no solid base, but If you are interested in learning about some of the problems with ideas like these you could look those us.

I think it's important to point out that these methods aren't cheating at all, and they still respect a local speed limit of c. That is to say, that if you were using either an alcubierre drive or a wormhole to go from point A to point B, you would never see yourself moving faster than c, and yet you'd get to the destination having moved, globally, potentially faster than c. To sum up, to say that the speed of light is the ultimate speed limit is true only in a LOCAL sense.

To address the OP's question of WHY it is that c is the speed limit, see the FAQ on this issue:
https://www.physicsforums.com/showthread.php?t=534862

 

[Cosmo Novice]

Hello Super Luminal and welcome to PF!

FTL in the local sense is not possible AT ALL. That is any object with a rest mass cannot travel at C. As you accelerate an object with rest mass towards C the object becomes time dilated due to its reletavistic speed, therefore it takes correspondingly longer to accelerate any more - and also takes correspondingly more energy. Eventually as you approach the speed of light you get to a limit where further acceleration (to reach c) requires an infinite amount of energy - more energy than in the entire universe.

Just as a caveat - this is not infinite energy to exceed C but infinite energy to equal C. So if it takes infinite amounts of energy just for rest mass to travel at C then to exceed it is even further from possible.

There has been mention above of FTL using workarounds that still allow for FTL globally without breaking the local limit - Alcubierre drives and Einstein-Rosen bridges. However they still violate Gobal FTL in terms of causality violation. Anything which can travel FTL (even non locally) could violate causality because traveling FTL even globally would be seen as time travel depending on an observers frame of reference (ie: from some frames they would arrive at their destination prior to departing from their starting point.) For more understanding on this lookup a tachyon telephone.

Hope this helps.

 

[2sin54]

What about distant galaxies moving away from us FTL?

 

[Cosmo Novice]

What about distant galaxies moving away from us FTL?

Expansion speeds are not limited by GR as they are not mass traveling locally but these galaxies are carried with the scale factor expansion. They do not have kinemtaic motion exceeding C - so there is no violation of physical laws.

 

[SHISHKABOB]

shouldn't it be said that our current theories do not allow for velocities exceeding the speed of light?

 

[Cosmo Novice]

shouldn't it be said that our current theories do not allow for velocities exceeding the speed of light?

Yes if you read my previous posts I think I outline that quite clearly.

 

[Drakkith]

shouldn't it be said that our current theories do not allow for velocities exceeding the speed of light?

That is correct. However if we had to resort to saying "according to theory" every time we explained anything, it would get old real quick. I take it for granted that this applies whenever I talk about anything in science. That is one of the downfalls of discussing science with some of my friends. They hear me say "x cannot do y" and they immediately start yelling "But that's just according to our own theories! We could be wrong!". It's very frustrating because I already know this and I just don't say it every other sentence.

 

[SHISHKABOB]

That is correct. However if we had to resort to saying "according to theory" every time we explained anything, it would get old real quick. I take it for granted that this applies whenever I talk about anything in science. That is one of the downfalls of discussing science with some of my friends. They hear me say "x cannot do y" and they immediately start yelling "But that's just according to our own theories! We could be wrong!". It's very frustrating because I already know this and I just don't say it every other sentence.

Huh, that's a very reasonable thing to assume and I will now assume it whenever I hear a scientist talking.

 

[Super Luminal]

Well, I am familiar with tachyons but as far as I know tachyons are a Sci Fi invention, or perhaps highly speculative science at best. And also am I familiar with wormholes, but I agree with the sentiments that wormholes are more about circumventing (or cheating the universe as someone worded it) c rather than violating it--if that even makes sense. One thing that confuses me, and it was briefly touched on in this thread, is that the universe is seemingly expanding at a rate greater than c, am I misunderstanding something?

 

[H2Bro]

Once galaxies are moving away from us faster than light their light will no longer reach us, correct? would this be equivalent to saying their light has been redshifted off the spectrum?

 

[Drakkith]

One thing that confuses me, and it was briefly touched on in this thread, is that the universe is seemingly expanding at a rate greater than c, am I misunderstanding something?

Expansion is a rate. That means that closer objects are moving away slower than objects further away. The rate of expansion causes objects to receded from us at an increasing rate of about 74 km/s per megaparsec in distance. Past a certain distance objects are receding faster than c from us. The key is that the objects are not moving THROUGH space, but that space is expanding and carrying them away as it expands. That's not the exact answer, but it's a close one that involves no math. To be exact would require an understanding of GR and all the math involved with it.

Once galaxies are moving away from us faster than light their light will no longer reach us, correct? would this be equivalent to saying their light has been redshifted off the spectrum?

I'm unsure. I think that light would eventually reach us from some of the galaxies, but I don't know how redshifted it would be.

 

[SHISHKABOB]

can it even get shifted off the spectrum? Does the EM spectrum even end?

 

[Drakkith]

can it even get shifted off the spectrum? Does the EM spectrum even end?

No. Wavelength can always get longer or shorter.

 

[Fredrik]

Once galaxies are moving away from us faster than light their light will no longer reach us, correct? would this be equivalent to saying their light has been redshifted off the spectrum?

No, most of the galaxies we can see are moving away from us faster than c. (I think Marcus said so in a post in the cosmology forum, and I have no reason to doubt it).

Edit: I tried to find that comment, but Marcus has made hundreds of posts in the astronomy & cosmology forums that include the word "expansion". I did however find a post where he says that the oldest light we can see, the background radiation, is redshifted by a factor of about 1100. Everything else is redshifted by a much smaller factor. (I know he has mentioned what that factor is in some other post, but he didn't mention it inthis one). I was also reminded about this Scientific American article that answers the question about expansion faster than c.

http://www.mso.anu.edu.au/~charley/papers/LineweaverDavisSciAm.pdf

No. Wavelength can always get longer or shorter.

That's just according to our current theories.

 

[H2Bro]

No, most of the galaxies we can see are moving away from us faster than c. (I think Marcus said so in a post in the cosmology forum, and I have no reason to doubt it).

Edit: I tried to find that comment, but Marcus has made hundreds of posts in the astronomy & cosmology forums that include the word "expansion". I did however find a post where he says that the oldest light we can see, the background radiation, is redshifted by a factor of about 1100. Everything else is redshifted by a much smaller factor. (I know he has mentioned what that factor is in some other post, but he didn't mention it inthis one). I was also reminded about this Scientific American article that answers the question about expansion faster than c.

This seems silly to say, but light can't go faster than the speed of light, so if something was receding away faster than light could light from it reach us? The light we receive now was emitted at a time when they were not moving away faster than c, but I would think light emitted after their relative velocity >c would be "cut off", so to speak.

Or is there something about relative velocity due to expansion and the velocity of light through space that I am missing?

 

[Matt Benesi]

This seems silly to say, but light can't go faster than the speed of light, so if something was receding away faster than light could light from it reach us? The light we receive now was emitted at a time when they were not moving away faster than c, but I would think light emitted after their relative velocity >c would be "cut off", so to speak.

Or is there something about relative velocity due to expansion and the velocity of light through space that I am missing?

You're correct. Once distant objects are receding faster than c, their photons will never reach us (except, like you said, the photons emitted prior to their recession velocity exceeding c). Of course, if there is an eventual collapse of the spacetime (bubble) those photons won't be too far away.

In a distant galaxy with a recession velocity greater than c, a spaceship traveling away from Earth is traveling faster than c away from earth. So, if you launch a spacecraft and travel far enough away that the recession velocity of Earth is greater than c, you've done the FTL trick- not that this is the answer people want.

When we learn to manipulate spacetime, FTL travel will be possible. We'll simply have to recede FTL from our departure point.

 

[Fredrik]

This seems silly to say, but light can't go faster than the speed of light, so if something was receding away faster than light could light from it reach us?

As I said, yes. You should read that article.

The light we receive now was emitted at a time when they were not moving away faster than c,

I don't think that's correct.

 

[H2Bro]

I don't think that's correct.

If light emitted from objects receding away faster than the speed of light does not reach us, then light that reaches us must be emitted by objects not receding away faster than light. So, the light from objects we see would have been emitted at a time when their relative receding velocity was not greater than c.

Is this an improper inference?

 

[Fredrik]

If light emitted from objects receding away faster than the speed of light does not reach us, then light that reaches us must be emitted by objects not receding away faster than light. So, the light from objects we see would have been emitted at a time when their relative receding velocity was not greater than c.

Is this an improper inference?

Yes. Let F="it was emitted by an FTL object", and R="it reaches us". (Here "it" refers to some light that reaches us). Your argument is of the form

F implies not R.
Therefore, R implies not F.
Therefore, not F.​

Compare this to the following argument about an arbitrary real number x.

x=1 implies x^2≠0.
Therefore, x^2=0 implies x≠1.
Therefore, x≠1.​

 

[Super Luminal]

You're correct. Once distant objects are receding faster than c, their photons will never reach us (except, like you said, the photons emitted prior to their recession velocity exceeding c). Of course, if there is an eventual collapse of the spacetime (bubble) those photons won't be too far away.

In a distant galaxy with a recession velocity greater than c, a spaceship traveling away from Earth is traveling faster than c away from earth. So, if you launch a spacecraft and travel far enough away that the recession velocity of Earth is greater than c, you've done the FTL trick- not that this is the answer people want.

When we learn to manipulate spacetime, FTL travel will be possible. We'll simply have to recede FTL from our departure point.

With all due respect, some of what you say seems silly to me, perhaps I do not understand. In my mind, if I envision an object such as a star inhabiting a distant galaxy retreating away from "us," well then, even if the object is receding away at a rate greater than c why could we not eventually observe the light originally projected from it? ...The photons were emitted and should proceed on their course regardless of how fast their source is moving in any given direction, that is unless the photons are gravitationally bound to their source...That reasoning eludes me.

 

[Antiphon]

can it even get shifted off the spectrum? Does the EM spectrum even end?

The EM spectrum bottoms out just before DC. But at DC you no longer have radiation so I'd call that off the spectrum.

 

[H2Bro]

Yes. Let F="it was emitted by an FTL object", and R="it reaches us". (Here "it" refers to some light that reaches us). Your argument is of the form

F implies not R.
Therefore, R implies not F.
Therefore, not F.​

Compare this to the following argument about an arbitrary real number x.

x=1 implies x^2≠0.
Therefore, x^2=0 implies x≠1.
Therefore, x≠1.​

Hmm, I thought I was saying something more like this: O = we receive light from this object and ~O we do not receive light from this object. F = the object is receding FTL and ~F the object is not receding FTL.

I thought what I said was of the order:

​

if O then ~F

​

if F then ~O

​

O(t)

​

~O(t+1)

Therefore

​

~F(t) and F(t+1)

In other words, if we observe light from an object THEN the light was emitted at a time when it was not receding FTL away; if at a later time this light does not reach us THEN it may be inferred (assuming there are only two objects in the universe, us and it) its rate of recession is FTL.

I don't think this line of reasoning assumes the consequent of the contrapositive, as you set out... unless I am missing some serious screws.

 

[Fredrik]

Hmm, I thought I was saying something more like this: O = we receive light from this object and ~O we do not receive light from this object. F = the object is receding FTL and ~F the object is not receding FTL.

I thought what I said was of the order:

​

if O then ~F

​

if F then ~O

​

O(t)

​

~O(t+1)

Therefore

​

~F(t) and F(t+1)

In other words, if we observe light from an object THEN the light was emitted at a time when it was not receding FTL away; if at a later time this light does not reach us THEN it may be inferred (assuming there are only two objects in the universe, us and it) its rate of recession is FTL.

I think the first two statements should mention time as well, like this:

For all times s, if O(s) then ~F(s)
For all times s, if F(s) then ~O(s)
O(t)
~O(t+1)

Therefore~F(t) and F(t+1)​

I assume that lines 1,3,4 are to be thought of as "facts", and line 2 as an equivalent way to express the content of line 1. I agree that if 1,2,4 are true, then we can conclude 3 and ~F(t). However, we can't conclude F(t+1). That's not the biggest problem with your argument. It's that your first "fact" is false. (And isn't that statement precisely what you wanted to prove?)

Let's try it with real numbers:

For all real numbers x, if x=2, then x²≠4
For all real numbers x, if x²=4 then x≠2
y=2
z≠2

Therefore y²≠4 and z²=4​

I'm sure you agree that a variable y that represents a real number can't have both of the properties y=2 and y²=0, and that when z=100 for example, it's not true that z²=4.

 

[PAllen]

Just to add to the question of superliminal speed of galaxies:

1) Fredrik is correct that light emitted from many galaxies we see was emitted when they were 'moving away from us' faster than c. If this isn't covered in the Lineweaver and Tamara Davis article he mentioned, it is definitely covered in the references given in that article.

2) However, we should emphasize a point made by many in this forum, many times. Velocity at a distance has no unambiguous definition in GR. The separation speed that can be a multiple of c in expanding universes is based on growth of proper distance measured in the simultaneity hypersurfaces of the coordinates comoving with the expansion. This is in some ways analogous to a separation speed in SR, which can reach 2c. Recession velocity can greatly exceed 2c even.

3) Another way to specifying relative velocity, more analogous to 'you must compare velocities only locally' is to parallel transport the 4-velocity of a remote galaxy along its light path to 'us'. If you do this, its relative speed at emission is always less than c, and, in fact, is exactly that of the red shift interpreted purely as SR doppler.

Note that coauthor of Fredrik's reference (Tamara Davis) makes explicit note of the the result described in (3), available in:

http://arxiv.org/abs/0808.1081

in the following later article for SciAm:

http://www.scientificamerican.com/article.cfm?id=is-the-universe-leaking-energy

(I don't know where to get the whole thing on line; I have this last one in the relevant issue).

 

[H2Bro]

Thanks for the answer and references, looks like I have some reading to do :)