New answers tagged

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 ...


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 ...


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 ...


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 ...


0

Light emitted from the sun is gravitational time dilated, which means it travels slower than the speed of light as it is measured here on earth. I don't know if the difference is as great as the questioner assumes, but it would be significant. Einstein's relativity theories are PRINCIPLE-BASED theories. They are based upon established scientific ...


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 ...


0

Heather is right and it is not much more complex than that. Except you might need to follow the math to understand it. Dodelson certainly has the math. Light goes at c. Period. If you want to find the geodesics of light you set the metric ds^2 = 0. But space itself expands, and it can expand at any spee, it is not a particle or wave or object, it is just ...


0

The problem with the assumptions in your second paragraph is that space is moving, not the galaxies. Space itself can travel faster than the speed of light - that is not forbidden by general relativity. The speed of light as a constant therefore still holds, removing the implications you bring up. As an analogy, imagine you have a coordinate grid, and you ...


0

Light as a Particle The photons in the beam of light are continuously being absorbed and re-emitted by the glass atoms (though this is also true in the other mediums light slows in). The level by which the light is slowed is dependent on how often this happens. As @garyp commented below this question, the delay also depends on how long the photon stays in ...


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 ...


-10

Articles published in Science and Nature say the speed of light is not constant: http://science.sciencemag.org/content/347/6224/857 "Spatially structured photons that travel in free space slower than the speed of light" Science 20 Feb 2015: Vol. 347, Issue 6224, pp. 857-860 http://www.nature.com/nature/journal/v406/n6793/full/406277a0.html Nature 406, ...


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

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 ...


-2

How would you define speed? In case it has a direction: The maximum slowdown you can get is 2C, this is achieved by using a mirror. In case it is defined by the time taken to get from A to B In case you would define speed by looking at the width of an object, where the light enters at point A, and leaves at point B, the following can be derived: The ...


0

The numerical value of $c$ does not have any fundamental significance. Rather it is the number we get based on the experimental fact (according to the number & unit system employed) . If some alien civilization ended with some different value of $c$ compared to us. Even that is not a problem. They will reach the conclusion that this is upper bound of the ...


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 ...


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 ...


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 ...


0

I would like to get some help concerning the definition of information [in the context of sending information] Suppose Alice and Bob are playing a game. The communication game. The communication game goes as follows: A referee flips a coin, and tells Alice the result. (Alice and Bob apply some strategy X.) Elsewhere, Bob tells a second referee what he ...


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 ...


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.


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 $v$, ...


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 ...


-1

Potentials do not have retarded time effects. If electron is removed from the sun its effect on the potential on esrth is immediate . So the electronics will be likely tip top. The limit of speed nowadays in space is set by the rocket gas engines that they use. Basically they use the chemical bonds energy to accelerate and push gas outside.


0

Your logic is very good, indeed. And it makes sense, what you describe. I agree. But then we try to measure it. And here comes the big problem: what we measure in our world doesn't follow this logic! It seems very logic, but the world just doesn't behave that way. Weird, yes. But apparently it is true. A famous example: Put a light measurement device ...


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 ...


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 ...


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 $c$, ...


-3

When you talk about the speed of light, regardless of what you say the speed of light is relative to, the speed of light remains the speed of light which is 1,86,000 miles/s. We just cannot travel at the speed of light because we humans are made up of particles which have mass, and according to Einstein's theory of relativity, the closer an object gets to ...


0

No, the light was not slowed down. They made it interact and travel more. The slowdown in a refractive index material is as Floris and CuriousOne described. It is similar in for instance a light fiber, in essence a waveguide, where it slows down because of reflections against the Fiber edges or walls (i.e., material, so longer distance and also whatever time ...


-4

Yes the speed of light is not constant - it can be slowed down IN VACUUM. The discovery was published in 2015 in Science (but is unpopular, for obvious reasons): http://www.upi.com/Science_News/2015/01/23/Scientists-slow-down-light-particles/1191422035480 "The speed of light is a limit, not a constant - that's what researchers in Glasgow, Scotland, say. A ...


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

actually light moves at different speeds not only dependenig on the density of the environment but also the kind of light i.e red light is faster than blue, thats why stars seem to twinkle or change colour its because the defferent coulors rech your eyes at diferent times because of moving at different speeds


-4

The assumption that the speed of light is independent of the speed of the light source is false but sounds reasonable, the reasonableness due to the fact that the assumption is valid for all waves other than light. However, when combined with the principle of relativity, this assumption entails that the speed of light is independent of the speed of the ...


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 ...


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 ...


0

Actually intensity becomes less in any medium like water, which can be verified by experiment. It is also an accepted fact that velocity of light decreases in medium like water. Does it not mean that velocity of light and intensity are dependent on each other? It is not safe to say that intensity depends on the refractive index of the medium. Certainly there ...


0

Yes and No. Yes because it can be "postulated" and No because, apart from all experimental subtleties of the notions involved, there are also inequalities outlining the region of its applicability. Point-likeness, classicality, etc. - all are approximations with the corresponding inequalities. Inequalities are an essential part of any branch of theoretical ...


0

The constant speed of light is confirmed by the Michelson-Morrey experiment . But I don't know why it is constant. It's a measured quantity, just as G is.


-1

The speed limit c for massless particles is a characteristic of space. In accordance with Maxwell's equation, speed of light which is the speed of massless particles is reflecting the characteristics of vacuum vacuum permittivity $\mu_0$ and vacuum permeability $\epsilon_0$. $$ c = \frac {1}{\sqrt {\mu_0 \epsilon_0}}$$ Such a speed limit of Maxwell'...


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 ...


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 ...


0

The speed of light is invariable but the number or ratio of the frames of reference are variable as determined by the amount of gravity, the result of which is gravitational lensing.


0

Yes, Einstein knew about the Michelson-Morley experiment. Toward the end of his life, he saw it as increasingly important for relativity theory. Here's an excerpt from Clark, Ronald. 1971. Einstein: life and times. New York: World Pub. Co. p. 78: As Einstein said years later, talking to Sir Herbert Samuel in the grounds of Government House, Jerusalem:...


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 ...


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/...


-1

Lorentz transformations apply to objects with nonzero mass. For an object with mass, it would require an infinite amount of energy to reach light speed.



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