# Tag Info

46

It's the second one: the reason the speed $299792458\ \mathrm{m/s} = c$ is special is because it's the universal speed limit. Light always travels at the speed $c$, whatever that limit may be. The reason there is a "universal speed limit" at all has to do with the structure of spacetime. Even in a universe without light, that speed limit would still be ...

21

Light doesn't travel at $c+V$ (where $V$ is the speed of the source), it travels at $c$. What's the difference? It means that if you're flying towards someone at a speed $V$ and you shine a light at them, you measure the light to travel away from you at a speed $c$, but the other person measures it to fly past them also at a speed $c$ (i.e. not $V+c$). In ...

18

The short answer is that there is no known theoretical strictly positive lower bound to the speed of light. Any positive number, no matter how small, is possible, although limits are set for each candidate material, as I explain at the end of my answer. One has to be pedantic to understand the lack of limitation to a more generalized "speed of light". From ...

17

This is the fundamental postulate of special relativity: Light (in vacuum) moves at the same speed no matter what you measure it relative to. Pretty much everything in SR is just a mater of figuring out the deductive consequences of this basic fact. It is an experimental fact that it is so, and it was so even before Einstein -- in particular, light had ...

16

How the light slows down in a matter, it depends on the recipe of its refractive index. For common, ordinary materials it is in the range of 1-3. Bose-Einstein condensates have an extreme refractional index, even millions or billions. In a BEC with a refractive index of $10^9$, the speed of light is only $30~\mathrm{cm/s}$. Here is a relative old article ...

11

The simple answer is 'with respect to anything'. For instance if I am standing somewhere and you are in a spaceship then we will always measure our relative speeds to be less than $c$. Equally, if I am standing somewhere and two spacecraft are passing me in opposite directions, then I will always measure the speeds of the spacecraft to be less than $c$, ...

9

Light does not slow down during a reflection. Light is a signal disturbance in electric and magnetic fields. These disturbances propagate through space at a fixed speed $c$ in vacuum. The situation is completely analogous, in a mathematical sense, to a wave pulse that is sent along a string. When the pulse encounters a boundary, it flips direction, and may ...

8

Even if nothing propagated at the speed $c$, it would still be a universal speed limit, and we could still measure it. In fact, it's not impossible that light has a (very tiny) mass in reality. If it does, that wouldn't change anything about special relativity. It would make teaching it even more of a nightmare than it already is, because we'd have to deal ...

6

In this case, it would be useful to not consider light in its particle form as photons, but instead to consider it as a wave - see this wikipedia page. Then, the wave is simply reflected from the surface, without us having to consider the kinetics of any particle. The wave, in a vacuum, would continue to propagate at the speed of light, regardless of the ...

6

Above all, speed of light is the speed of propagation of fields through space. While light may be slowed down when crossing matter, fields (electromagnetic fields, gravity) are always propagated at c. One of the consequences is the "speed limit for causality" mentioned by DavidZ and the speed limit for transmission of information.

4

Let me explain in purely classical terms (not the description of reality, but easy to imagine). You realized that when a ball bounces off the wall, at a certain point, it has no momentum. However, it must still have all the energy of the movement (neglecting losses to environment) - where did that energy go? The ball is formed of many discrete atoms, bound ...

4

No. There is nothing to prevent such faster than light appearances. The rule is simple: No actual thing (information) can travel at a speed greater than the speed of light. When the considered particle appears to travel at a speed greater than the speed of light in your video, there is a non-local distribution of information - set up a priori. This ...

4

The answers given here make me wonder, because I sense in here perhaps a misunderstanding. Or maybe I'm wrong, which might be more likely. :-) The answers here refer to distances light travels. But as far as I understood, light is never slower than 299 792 458 m/s. I guess it may "look" like from a point of reference that light has slowed down, when a event ...

3

Nothing can prevent you from speeding up the video, but there are ways to tell if you are looking at a sped-up video or at a real video of an object traveling at a speed close to or equal to $c$ (or greater than $c$, if we can assign any meaning to this concept). Think about length contraction: if you are looking at a spaceship traveling at a speed $v$, ...

3

To see what's going on, it's enough to do this in two dimensions, with the Lorentz form $\pmatrix{-1&0\cr 0&1\cr}$. (I've set $c=1$.) The Lorentz group is the group that preserves this form. A typical element is $$\pmatrix{\pm\sec\theta&\tan\theta\cr \tan\theta&\pm\sec\theta\cr}$$ where $\theta$ runs through the open interval from $-\pi/... 3 That light moves with a fixed speed in vacuum, in all reference systems is an experimental fact. Maxwell's equations fit so well all macroscopic electromagnetic data that the speed of light is fixed is not under question. It is inherent in the construction of the classical theory. Light is made up by a zillion of photons. Photons are elementary particles in ... 3 The speed of light in vacuum is 299,792,458 m/s - that is an unalterable quantity. However, light doesn't always travel in vacuum. The concept of a refractive index describes the relationship between speed of light in vacuum vs a particular medium, with the value for glass around 1.3 - meaning that the speed of light in glass is about 1.3x slower than in ... 2 The point is that the considered postulate states that the speed of light in the vacuum is$c$with respect to each and every 'inertial' observer. It is independent of not only the source that is emitting the considered light quanta but also of the observer who is observing it as long as it is an inertial observer. It is true that for a given observer, in ... 2 Regarding the experiment mentioned with Francois Arago in 1810 measuring the speed of light when it hit the telescope, we are only measuring the speed of light once it hits earth's atmosphere. This does not tell us the speed of light out in space. 1 First of All, I am confused by your question. Light travels at a constant speed in all reference frames. What is this "stationary observer in the Aether Field"? That doesnt make any sense to you or me. Forget about that. The experiment supports the non existence of a medium through which light moves in because of the null result. If you look back to ... 1 Well, I'm not going to tell you my opinion, because that would be irrelevant to the actual science. But, what I can do is assess your premises and conclusions. What you should note first, and I'm unsure if you know this or not already, is that the four dimensions of spacetime are the three spatial dimensions and time. We can define a velocity through the ... 1 The other answers seems to answer most of your questions, but I think one confusion remains: The speed of light as a maximum speed in the Universe (which is not the case). First off, redshift doesn't go to infinity for objects receding at$v = c$. We easily see galaxies recede at superluminal velocities. In fact, this is the case for all galaxies with a ... 1 Lets imagine for a moment that for some reason or other only one object was left existing within the universe, and it was a spaceship. It can accelerate and decelerate, thus movement is in effect here, movement across space. However, whether it is alone in the universe or not, it has a maximum speed of which it can move across the vacuum of space. That being ... 1 The Lorentz transformation may shed some light on this... $$\gamma = \frac{1}{{\sqrt {1 - \frac{{{v^2}}}{{{c^2}}}} }}$$ Assume one body is, dare I say, "stationary" and the other is traveling away at velocity v. If the relative velocity between these two bodies moving apart is equal to the speed of light, then the denominator in the Lorentz transformation ... 1 There is no way to explain this without explaining relativity first. In Galilean universe (the "classical" physics, which is what most people intuitively assume and think about), light speed cannot be explained. Indeed, the Maxwell equations which is describe how light works, were the first clue that our understanding of space-time was flawed. So the real ... 1 In the special relativity, as explained by Einstein, there are two possibilities. Either the speed of any particle is limited at all or not. Now if the speed is not limited, the Galilean theory pops out. But as we already know, that does not properly explain the transformation of velocities from one to another reference frame accurately when dealing with ... 1 Aside from simply looking at the Lorentz transformation, seeing a divergence and concluding "meh, it doesn't work", another way to gain insight into the divergence is through the statement: no finite sequence of finite boosts will get you to a speed$c\$ relative to your beginning inertial frame. Imagine yourself in a spaceship with orientation controls ...

1

To test Lorentz invariance rigorously, one has to consider theoretical models where Lorentz invariance is violated that are not already ruled out. One can do that by considering the Standard Model and then adding terms that violate Lorentz invariance and studying the most general such model that is physically plausible. This has been done in this article ...

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