# Tag Info

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I am wondering whether is it taken as a postulate or a proven phenomenon that c is constant irrespective of observer's speed? Either one. Both. Einstein took it as a postulate in his 1905 paper on special relativity. From it, he proved various things about space and time. The frame-independence of $c$ is also experimentally supported. This is what the ...

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Generally speaking they refer to the distances from us when the light as emitted. No correction is usually made to say how far away the object is from us now, because this correction would be very small and inconsequential compared to the uncertainty in the original distance measurement. For instance, taking the Andromeda M31 galaxy as an example. Riess et ...

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Long ago, English speaking sailors measured horizontal distances in units of nautical miles but depths in units of fathoms. The distance in fathoms to some point $z$ fathoms deep and $x$ nautical miles along the surface is $d^2 = 1012.68591^2 x^2 + z^2$. There's nothing physical to that factor of 1012.68591. It's solely a result of using inconsistent units ...

5

The quantity $c$ is a very fundamental constant related to space and time. It is largely independent of the existence of matter or light or any other substance. It is probably more ontologically appropriate (if not pedagogically appropriate) to define it as the number that makes Lorentz transformations between reference frames work. The quantity $c$ would ...

5

Have a look at the question Speed of light in a gravitational field ? as this shows you in detail how to calculate the speed of light in a gravitational field. I haven't flagged this as a duplicate because I'd guess you're not so interested in the details but rather how the speed of light can change at all. You've probably heard that the speed of light is a ...

4

We have to be careful about what we mean by "speed of light". It can mean two things: the speed at which light travels, which I'll write as $s_{light}$, and the maximum speed at which anything can possibly travel, which is written $c$. In our universe, in a vacuum, $s_{light}=c$, as far as we know. Now, no information can ever be transmitted faster than ...

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By using orthogonal optical resonators, laboratory tests concerning verifying the isotropy of c have come a long way. As quoted from http://journals.aps.org/prd/abstract/10.1103/PhysRevD.80.105011 "An analysis of data recorded over the course of one year sets a limit on an anisotropy of the speed of light of $\Delta c/c \sim 10^{-17}.$ This constitutes the ...

3

$\lambda = c/nf$ While in the medium, $n$ is greater than one, in vacuum $n=1$. The medium responds to whatever is driving it. The molecules of the medium oscillate at whatever frequency shakes them. Each molecule is then a tiny radiator, generating light at that same frequency. The light that the polarized, oscillating, molecule produces interferes ...

3

This is a very complicated question and a complete answer would be very deep and very, very long. Much of it stems from the fact that the speed of light is independent of the frame of reference from which it is being observed. In this sense, it is one of few "universal constants" - that is, quantities which do not depend on the observer. Since it has units ...

3

For a massive particle to move at, or faster than, the speed of light, it would require infinite energy as shown by Einstein's relativistic equation: $$E = \gamma \cdot mc^2\quad\left(\gamma = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}\right)\\ E = \frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}\\ v\rightarrow c, E \rightarrow \infty$$ If we plug in $c$ for the velocity, ...

3

It's tempting to think of spacetime as a thing, and it doesn't help that it's often represented as a rubber sheet in popular science programmes. In relativity (special and general) spacetime is a mathematical concept - it is a manifold equipped with a metric. At the risk of over-simplifying, a manifold is a thing that has dimensionality (four dimensions for ...

2

Indeed photons are massless particles, so they follow "quickest paths" during their propagation in space-time; these are called "geodesics". However, general relativity doesn't simply says how the way masses attract each other is modified, it most of all says that mass (and energy density) curve space-time; this also curves the geodesics, which photons (in ...

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It is said that photons have zero rest mass so how can gravitational force of a black hole affect light? Photons have zero rest mass so when they are at rest they have no mass. They are never at rest so this is a little misleading. And if photons do have some effective mass while traveling at speed of light then only can a black hole's ...

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Virtual particles aren't real. It's in the name. The reason people say there are "vacuum fluctuations" in form of virtual particle-pairs forming and annihilating again is because they misunderstood Feynman diagrams. In Feynman diagrams, internal lines are called virtual particles, since the external lines (i.e. the open-ended lines) correspond to real ...

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You are pushing very hard on the limits of this site, trysis. This site focuses on established physics. Julian Barbour is far removed from established physics, so far removed that he can't get a job as a physicist. That said, you are very much misreading that pop sci article. Barbour's complaint is that general relativity is not fully Machian. He has no ...

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A photon cannot be said to have its own inertial reference frame, because inertial reference are defined to be a family of coordinate systems that satisfy the two fundamental postulates of SR, one of which is that light moves at c in all frames. You could construct a coordinate system where the photon was at rest, but since this coordinate system wouldn't be ...

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It is a well-substantiated observed phenomenon. Science deals only with provisional truths, but this hypothesis has undergone (and passed) immense amounts of scrupulous experimentation and mathematical formulation. In a Neo-Lorentzian interpretation, physics works differently in all reference frames except for one single, undetectable, privileged reference ...

2

The size or path-length that the wave has nothing to do with it. There are two explanations on offer for why the frequency of the radiation remains unchanged as it crosses the boundary between different media. (a) If a medium is considered to be lots of driven oscillators, then they will oscillate at the frequency of the driving force - the electric field ...

2

Sound is a pressure wave, and the generation of a pressure gradient requires atoms/molecules to move to create a density difference. No particle can move faster than light, so it's impossible to create a pressure gradient that propagates faster than light. Nathaniel's argument that sound waves could travel faster than light in a system like a Bose Einstein ...

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There is a "universe" where the speed of sound is greater than the speed of light (or at least the speed of electromagnetic wave propagation), and that is inside conductors. In a conductor, the EM wave velocity is $$v = c \left(\frac{2 \omega \epsilon_0}{\sigma}\right)^{1/2},$$ where $\sigma$ is the conductivity and $\omega$ is the angular wave frequency. ...

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There are a few results of variable speed of light: Ellis claimed that any varying c theory must redefine distance measurements must provide an alternative expression for the metric tensor in general relativity might contradict Lorentz invariance must modify Maxwell's equations must be done consistently with respect to all other physical theories

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The speed of light can't be measured anymore (in SI units) because it has had a defined value since 1983. See Why do universal constants have the values they do? Before 1983, the meter was defined in terms of the wavelength of a certain emission line of krypton 86. The second is also defined in terms of an atomic standard (the frequency of a transition in ...

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This is a reasonable enough question. Imagine that you are looking at the object and behind you, you have a very large mass of clear, but very high index of refraction material (with index of refraction $n$). Then you could easily say "I wonder how this looked 100 years ago", and then very quickly run behind your large block of material at some speed $c/n ... 1 People may say things such as "... we do not have a theoretical justification for the constancy of the speed of light... ", or say " These things simply are, therefore there is no deeper, more fundamental, explanation. ". Thus such people in general accept the effect, yet they have no desire to find the cause. However, it is to be noted that if you analyze ... 1 I often try to comprehend exactly why light wants to keep it's frequency, yet alter it's wavelength as it travels through a medium Have you not heard the standard (macroscopic, continuum approximation) explanation? To whit: the electric and magnetic fields must both be continuous at the boundary, and must therefore have the same time variation on ... 1 "Using the equation of the time unit I derived for time, we can say now a unit of our time for an observer will be the distance (light will cover) for a unit of time to pass for us (observer)" Is this observer meant to be the one who sees the object with the walls A and B moving at velocity v? If so, then although it's true this observer will see the length ... 1 I'd like to add to Chris White's excellent answer and summarise things thus:$c\$ is a number that parameterises the family of all possible linear transformations that follows from Galileo's relativity with the assumption of absolute time relaxed. If you do Galileo's relativity with the assumption of an absolute time (that two relatively moving ...

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Firstly, you need to calculate how much your hypothesised effect will change the optical delay in each of the interferometer's arms and check that you expect to see any result with your proposed experiment. Otherwise put: what are the specifications of the interferometer (arm lengths, light source requirements etc, vibrational tolerances) that will let you ...

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Light can only travel at one speed (in a vacuum), approximately 300,000 km/s. It doesn't matter what frame of reference it is created in, it never goes faster or slower than this speed, and it doesn't matter what frame of reference you are measuring it from, you will always measure it to be the same speed. This is given by Maxwell's equations, Einstein's ...

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