Light inside a faster then light craft

[Sarevok]

As a layman one thing about relativity always confused me. It is said that the speed of light does not change like other quantities in relativity. Now suppose a spaceship is traveling at faster then light. If someone turns on a lamp at that kind of velocity what would happen?

[Surlethe]

As a layman one thing about relativity always confused me. It is said that the speed of light does not change like other quantities in relativity. Now suppose a spaceship is traveling at faster then light. If someone turns on a lamp at that kind of velocity what would happen?

Depends on from where you are observing; paradoxes like this are why we aren't allowed to go faster than c.

[Molyneux]

If you're travelling an infinitesimal bit slower than c, you see the beam travelling away from you at c (relative to you).

An observer 'at rest' would see the light ray travelling...at c relative to them (not at c+your speed, as you might expect).

The apparent paradox of the beam of light travelling at two different speeds is resolved by the time dilation inherent in movement, IIRC. It's weird, but eh, that's the way the universe works.

[Magnetic]

If the craft was traveling at exactly the speed of light, and the bulb was in the center of the craft, I'd expect that the aft section would be lit, but the foward section (from the bulb) would remain dark.

At faster than light travel, I'd expect that, if you were in the craft, you wouldn't see the bulb come on at all. Depending on how much faster you were going, you may see a faint glowing "line" of light aft.

Or am I "all wet"?

[Magnetic]

Ghetto Edit:

Now, a bulb glows from heating the fillament. I wonder if, at the speed of light, you'd see the filliment becoming hot. Would you see the heat radiation but not the light?

Heck, for that matter, would you see anything at all. I mean, is the speed of sight quicker than the speed of light?

[Magnetic]

Another Ghetto edit:

That was supposed to be, "at GREATER than the speed of light" in the second sentence.

[Sriad]

From the POV of someone (somehow) traveling AT lightspeed, they would arrive at their destination instantaniously thanks to time dilation, so no lamp turning on could happen en route.

If you're FTL it depends how you're doing it. All the cheatin' SF hyperspace/warp/wormhole drives don't involve actually exceeding C, neither do those which change local values of C, so light would appear to behave normally in those settings.

As for ships that "go tachyonic" like in Star Wars... no idea.

[Surlethe]

The SWTC, on its hyperspace page, suggests:

Fortunately it turns out that causality paradoxes actually cannot arise. Although nobody will see starships travelling backwards in time, other exotic effects will be observed. A particle moving backwards in time is equivalent to the motion of some antiparticle forwards in time. In relativistic quantum mechanics that's exactly what antiparticles are: time-reversed versions of ordinary particles. In those situations where naive relativistic considerations would suggest the observation of a starship travelling backwards in time what the bystanders will really witness is an antimatter starship travelling forward in time. This mirror ship will be seen to head in the opposite direction.

The implication of this passage, to my rather naive interpretation, is that the superluminal ship's velocity c+e, where e is the positive difference between the ship's velocity and c, will be negative -- the antiship is traveling backwards from its destination -- and, presumably, when a light bulb is turned on, the negative value of the ship's velocity combined with the positive value of the light and the time dilation and spatial dilation will conspire to produce a beam of light traveling at exactly -- surprise! -- c.

[drachefly]

That's not it. It will move in the micro-reverse fashion (if we apply a magnetic field, the particles within will deflect the opposite way from the way we'd expect); but as we observe it in forwards time, it will move forwards in time. No flying backwards or anything.

Anyway, a lot of particle physicists think that calling antiparticles 'normal particles going backward in time' was one of the worst memes to let loose ever, since it doesn't help explain anything that can't be explained by other means without opening up the massive cans of worms that the backwards-time explanation does.

[CmdrWilkens]

Just a thought here and anyone feel free to throw it away but if the light is in fact within the starship itself then wihtin that frame of reference it will still move at c while external observers will see some thing moving forward at c along with an anti-particle at whatever speed differential there is beyond c.

[Kuroneko]

The implication of this passage, to my rather naive interpretation, is that the superluminal ship's velocity c+e, where e is the positive difference between the ship's velocity and c, will be negative -- the antiship is traveling backwards from its destination -- and, presumably, when a light bulb is turned on, the negative value of the ship's velocity combined with the positive value of the light and the time dilation and spatial dilation will conspire to produce a beam of light traveling at exactly -- surprise! -- c.

No. Assume that the observer is stationary at the spatial origin. If the spatial trajectory of the ship is linear, then there exists a unique plane through the origin and that trajectory; introduce standard Cartesian coordinates (x,y) and time coordinate t, so that the ship trajectory is parametrized by t:(X+vt,Y+wt), where (X,Y) is the initial position and (v,w) is the velocity vector. The cone t²=x²+y² represents the set of all event that can reach or be reached by the observer at t = 0 by a signal of unit speed. The goal is to find the intersection of the cone with the ship trajectory, which is just a quadratic: t² = X²+2Xvt + v²t² + Y²+2Ywt+w²t², or [1-v²-w²]t² - 2[Xv+Yw]t - [X²+Y²] = 0. If v²+w²<1, then according to Descartes' rule of signs, there is exactly one positive root, and hence one negative. On the other hand, if v²+w²>1, there are either two positive roots or two negative roots.

What does this mean? Well, if the ship is superluminal (v²+w²>1), and if it is possible to observe it, then two distinct images of the ship will be observed, the light from which being emitted at times t that are solutions to the above equation. Moreover, it is obvious that if instead of receiving or sending signals at t = 0, we did this at arbitrary t = T, the result would still be a quadratic equation, albeit a slightly more complicated one. Since quadratics have linear derivatives, varying T varies both t-solutions in the same direction; neither is a time-reversal of the other.

[Durandal]

Listen to Kuroneko. He's like, always right.

[Dark Hellion]

Everyone know the ship will go to plaid!

[drachefly]

That's from the outside. Read the OP. INSIDE, it goes paisley.