# Tag Info

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The speed of light can be slower than c. wikipedia: Propagation_of_light_in_non-inertial_reference_frames So there can be things going faster in one frame than the slow light of another frame.

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There is a subtle difference between "local" and "global" (or apparent) superluminal travel. Kaku is only correct in the global sense. Local superluminal travel, in the sense of increasing one's speed to exceed the speed of light, is strictly prohibited as special relativity holds in any sufficiently local frame. However, general relativity (with the most ...

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All mater we see and touch and manipulate macroscopically is composed of atoms, molecules and solids which are held together by intermolecular forces. All these forces are electromagnetic, i.e. any disturbance in the end of one end of a string cannot be transmitted faster than the velocity of light, and usually transmission is much slower ( acoustic, ...

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If (and that's a big if) tomorrow we had a $70\sigma$ detection in a repeatable experiment of a particle that travelled faster than $c$, then one of several things would be true. 1) We would be forced to conclude that $c$ is not, in fact, the limiting speed of information transfer; everything based on this assumption would have to be scrapped (pretty much ...

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But what if tomorrow we happen to observe a particle X that travels with a speed V>c? We would have made the first observation of a tachyon. In special relativity, a faster-than-light particle would have space-like four-momentum, in contrast to ordinary particles that have time-like four-momentum. It would also have imaginary mass. Being ...

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Unfortunately, this result was later found to be caused by faulty electronics, according to the CERN press release.

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Alice places herself at system C. Now she detect A and B approaching each other from opposite directions, while A and B both have velocity 0.9c. She can safely say, B is approaching A with velocity 1.8c. However, if Bob is located at A and detect the velocity B towards him, he will find that B is approaching with velocity ~0.994c. If Alice and Bob can ...

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The Special Theory of Relativity deals with two observers, well... observing a single event, and comparing their observations. So: You observe two spaceships, S-A and S-B, occupied by Obs-A and Obs-B moving towards you at $0.75c$ from opposite directions. You observe S-A travelling towards you at $0.75c$; Obs-A observes you moving towards him at $0.75c$. ...

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No, unfortunately this is one of those "It should be, but it isn't." Ship A shines a light at ship B. The light leaves ship A at $c$ and it arrives at ship B at $c$. Even if the 2 ships are traveling at 99% $c$, the light still leaves ship A at $c$ and arrives at ship B at $c$. I believe that the reason lies with the time dilation when approaching $c$.

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The addition of relative velocities is $$\frac{a+b}{1+ab}$$ so $\frac{.75+.75}{1+.{75}^2}$ = .96 c. Consider that you are travelling almost the speed of light (1-x) c and you see a space ship pass you at an equal speed. Combining (1-x) c with (1-x)c. This gives $$\frac{2-2 x}{2-2 x+x^2}$$ Since $2-2 x < 2-2 x+x^2$, we have $\frac{2-2 x}{2-2 x+x^2} ... 0 (This was intended to be a comment but ended up being too long) A related "paradox": Consider the diurnal motion of the stars as they appear to rise and set each night (the non-circumpolar stars at least) traversing circular paths on the celestial sphere. An obvious back-of-the-envelope value for their angular velocity ... 0 If you tell me that an object has velocity$\omega r$, it will have velocity$\omega r$. There's nothing stopping me from imagining a nonphysical/theoretical point moving this fast. If you tell me that a physical particle of something, or something that moves at this speed, has this velocity, I will tell you that you're wrong when$\omega>c/r$, and I'll ... 2 Can we pass the information faster than light? No, as a matter of principle. Because the quickest "passing of information" (i.e. the signal front) is what's called "light" (or "light signals being exchanged") for the purposes of kinematics according to (S)RT. Referring then to specifics of the proposed setup: Suppose we have a "scissor", which is ... 1 A different take on the question from Martin Beckett's answer is the variability of light speed in general relativity. In GTR, the lightspeed as measured by an observer in a small enough region of spacetime that is local to the observer is always exactly$c\$. This is because spacetime is assumed in GTR to be a smooth manifold, which means that you can always ...

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Light always travels at the speed of light. It appears to be slowed down in a material because of the effects described What is the mechanism behind the slowdown of light/photons in a transparent medium?

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if you consider a rigid rod and apply a torque at one end, by definition of being rigid, the whole body must start rotating at the same instant. But [...] I'm wondering if one can even define a rigid body in a relativistically correct manner In the strict sense indicated: one can not. An important clue in the argument has been stated by J. L. ...

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Remember, the Big Bang theory is just that- a theory. I predict astrophysicists will soon discover galaxies 15,20,25, billion years old. Then what?

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Perhaps your question is whether the speed of approach of the two particles is 2c, is this so? Yes, it is 2c and this does not violate the principles of relativity, because such speed is not the speed of a particle, but it is just a derived value. On the other hand, the speed of a photon is c regardless of the inertial frame, and is calculated by the ...

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Besides the comments, stars have formed well after the Big Bang. Depending on where they are in the universe, it will take a while for us to see them.

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In curved spacetime, you can no longer compare velocities at different points in the straight-forward manner we use in flat spacetime. Thus the claim that recession velocities should not be considerer 'real' (as in relative) velocities, but rather rates of expansion of space. If you want to get at the former, you need to parallel transport the source's ...

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