Questions tagged [speed-of-light]

The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.

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73 views

Are these two definitions for propagation speed be the same?

We know for a fact that for any medium with permittivity and permeability $\epsilon$ and $\mu$, the propagation speed of electromagnetic waves is given by $$v = \frac{1}{\sqrt{\mu\epsilon}}$$ We ...
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Why would speed of light be directional if spacetime is discrete?

In Feynman's Simulating Physics with Computers, Feynman states that "we might change the idea that space is continuous to the idea that space perhaps is a simple lattice and everything is ...
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Relative speed when getting close to the speed of light

I was thinking about the relative speed of an observation reference frame and an object which has been accelerated to a speed close to the speed of light. I'm by no mean an expert and the last physics ...
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Has this group something to do with the cone of light?

Consider the group $V=(-1,1)$ with addition $+_{rel}:V\times V\to V$ defined as: $$v+_{rel}w=\frac{v+w}{1+vw}$$ This group is analogous to the relativistic velocities where the speed of light equals ...
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What is the upper bound for the index of refraction of space?

It seems that gravitational waves and gamma waves travail at about the same speed, arriving within seconds of each other over distances in the ranges of $10^6$LY. Naively, I would assume this caps the ...
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Why is refractive index equals to speed of light upon phase velocity and not group velocity?

We have been using group velocity for most purposes as its the velocity with which the wave packet travels, they why do we use phase velocity for calculating refractive index.
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Can we expect to relate the fundamental constants G and c?

Newton's gravitational constant G tells us about the strength of the static gravitational interaction within our spacetime. The speed of light c can be derived from the properties of spacetime itself, ...
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202 views

Photons when passing through matter

I am wondering whether there is a simple—or complicated—way to explain from a purely quantum mechanics (or QFT) viewpoint what happens to the photons when they go through some material whose index of ...
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98 views

Observable Universe from two different perspectives

My layman understanding is that the universe is much bigger than that we can observe however due to the cosmological constant the observable universe is the matter etc. that is not moving away from us ...
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171 views

The speed of light during the inflationary period

Introduction: As a thought experiment, suppose I modified the value of $c$ (speed of light) in some local region and attempted to measure it with a clock placed in that same region. I will denote ...
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57 views

Could harm due to spaghettification be mitigated by entering a black hole at almost the speed of light?

Say that we wanted to send a probe into a black hole, perhaps hoping to see if it's actually a wormhole that might transport the probe elsewhere. Upon approaching a black hole, the probe would ...
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40 views

Increasing the view of observable universe

I have read that the maximum observable universe is about 92 billion light years in diameter by measuring the microwave background waves emitted after some few million years ago from the big bang and ...
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Can we get the values of $G$, $h$ and $c$ to be numerically equal if we use a convenient system of measurement?

The speed of light in vacuum is approximated to be $3×10^8\ ms^{-1}.$ But, if we change the units, we can get a different number. For example, it won't be $3×10^8$ if we used $ft\,s^{-1}$ instead of $...
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Hypothetical video feed of time dilation

As a thought experiment, I'm imagining an observer on Earth video conferencing with an observer traveling at a high velocity away from Earth, or in a very large gravitational field relative to Earth. ...
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The nearly unmoving heavy star?

The interior Schwarzschild metric $$c^2 {d \tau}^{2} = \frac{1}{4} \left( 3 \sqrt{1-\frac {r_s}{r_g}}-\sqrt{1-\frac{r^2 r_s}{r_g^3}} \right)^2 c^2 dt^2 - \left( 1-\frac{r^2 r_s}{r_g^3} \right)^{-1} ...
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CalTech has actually filmed light crossing a space of 1 mm of space. How?

In the video https://www.youtube.com/watch?v=7Ys_yKGNFRQ, CalTech has a camera capable of filming the picoseconds it takes for light to travel 1mm in distance. It can do 10 Trillion frames per second....
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$c$ slowing down rather than universe expanding?

Can the Friedmann equations be reformulated with taking the universe as being rigid and instead have the vacuum-speed of light in the void decrease over time? So, not changing any equations, just ...
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Do condensed-matter field theories with multiple fields generically have multiple speeds of sound?

It is well known that the low-energy physics of many non-relativistic condensed matter systems can be described by field theories that display emergent Lorentz symmetry. The heuristic way to figure ...
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Can the Dirac Hamiltonian accommodate a variable speed of light?

The Dirac Hamiltonian has the form1 $$\left[\beta m c^2+c\sum_{n=1}^3\alpha_np_n\right]$$ where $\alpha_n$ and $\beta$ are Hermitian matrices, and $c$ is the speed of light. My question: Is there a ...
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How do we know that light cannot travel faster than it does?

We assume the speed of light in vacuum is its maximum speed but can we not assume that it could be faster, or slower?
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118 views

Elevator moving near the speed of light

Imagine that you are in an elevator that is moving downward at a constant speed that is near the speed of light (let's say close to the surface of the earth: toy problem, needless to say). Now say ...
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Can we assume that in vacuum in an expanding universe all photons travel faster than light away from their source?

Asking this question I assume the example photons travel in vacuum. Once a source emits a photon they get separated by some space. Doesn't that space expand with the Hubble constant like the rest of ...
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How fast does the electrostatic force travel?

Suppose an electron is created somehow in the universe -- through particle-pair production or what have you. Suppose it is stationary so that electrodynamics (and specifically propagating ...
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Is Poplawski's “Cosmology with torsion” a “variable speed of light” cosmology?

In his 2010 paper "Cosmology with torsion", Poplawski mentions that, in the hypothesized formation of a new temporally-local universe within a black hole, its "expansion is not visible for observers ...
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Naturalness in Physics and fundamental constants

From the "flatness problem" in Cosmology to the "strong CP problem" and "hierarchy problem" in the Model standard, a lot of problems in physics deal with Naturalness: a certain parameter in our theory ...
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539 views

Bandwidth-length product of an optical fiber

A multimode step index fiber has a numerical aperture of 0.2 and a core refractive index of 1.47 . Estimate the bandwidth-length product for the fiber assuming only intermodal dispersion when: (a)...
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668 views

What happens when light is reflected from a surface moving in a medium with a huge refractive index?

Imagine a mirror is moving away from a light source in a substance through which the speed of light is very slow -- so slow that the speed of the mirror is close to being the same as the speed of the ...
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1answer
69 views

Subluminal speed when going in a circle around someone

Some time ago I thought about such situation: There are two people in the room. Both have synchronized watches on their wirsts. Then the person A starts running around person B with the speed of 0....
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Are there limits to human/devices perception?

As far as i know, measurement devices present measurements based on something that affects the device's particles, for instance, forces, heat, tension, voltage... My question is, given that every ...
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About the speed of light

If Mr. E is aboard a spaceship traveling near the speed of light the usual reason for the spaceship not going faster than $c$ is the (relativistic) mass of the ship increases without bound, I think. ...
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How does this paper relate to standard QED?

This paper proposes a microscopic mechanism for generating the values of $c, \epsilon_0, \mu_0$. They state that their vacuum is assumed to contain ephemeral (meaning existing within the limits of ...
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Question on “new CNGS/OPERA measurement of neutrino velocity” (hep-ex/1212.1276)

A "new measurement of the limit on muon neutrino velocity with respect to the speed of light " by the OPERA neutrino experiment in collaboration with CERN has become public recently, as preprint http:/...
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What happens with photon when it is slowed down substantially?

In a dispersive media light's velocity can change substantially. Imagine we can slow it down to near 0 what the wave would look like? Frequency of light does not seem to change even at v=0 (at least ...
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237 views

Do photons have absolute motion?

Absolute rest is not possible as the concept of motion is relative. But can we assert that photons have absolute motion as the observers in all the frames of reference would agree to the same value of ...
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1answer
229 views

Refractive index for gamma rays, is it $1$?

In (1) it says (in the context of PET imaging): The speed of gamma-radiation in every material is the same as in the vacuum: $c=3\cdot 10^8 m/s$. I realize that if this is true then it is only an ...
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2answers
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Why can't we use this method to achieve extreme speeds?

Turn an axle with diameter 1 inch at 1 mile per hour, and an attached wheel with diameter 5 inches will spin at 5 miles per hour. Why can't we apply this principle on a much bigger scale? Say you put ...
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57 views

Will the log of wood sink in the water?

Imagine the water in a river is flowing with speed very close to the speed of light. There is a log of wood tied to any object outside the river so that it doesn't go ahead with water and is at rest ...
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Where can I find this particular problem (which is popular and historical) regarding Maxwell's laws of electromagnetism?

Where can I find the problem talked about in the following paragraph regrading Maxwell's laws of electromagnetism - Every time Maxwell rearranged his equations to make the speed of light the ...
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316 views

Is it proven experimentally that the speed of causality (light) is the maximum speed?

Is it possible to have an alternative theory which is consistent with experiments supporting GR, but which doesn't mandate a speed limit on causality? In other words, I understand there is ...
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If light speed determines the rate of time, could the speed of light be varying over time and we be unaware of it?

Is this a valid implication/alternative explanation of universal red-shift? I thought I'd ask this when I read this question about light clocks. I had speculated this way: If light speed determines ...
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Can massless particles change their quantum state? What about with entanglement?

It is well established that any massless particle, by special relativity, must travel at the speed of light, e.g. Special relativity and massless particles. Accordingly, this means that massless ...
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Does light emitted in opposite direction hit equally distant objects at different times?

I am a physics student just learning about special relativity for the first time, and was wondering what would happen if you emitted two photons of light from the center of a boxcar in opposite ...
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How to travel between two stationary worlds in the least amount of proper time?

I was intrigued by this question about minimizing the travel time between two worlds that are at rest with respect to each other, and disappointed that the answer turned out to be trivial. However, I ...
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Gravitational field (metric tensor) and speed of light between two massive plates

Suppose I have two massive plates of size $l\times h\times w$ mounted parallel to each other with a distance of $d$ and with a mass density of $\rho$. I send a light beam in the middle between them ...
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39 views

Cherenkov radiation relativistic correction

I have to write a paper on Cherenkov detection. And got a bit of an issue on the relativistic particle/recoil correction of the Cherenkov angle formula. Normaly $$cos( \theta_c) = \frac{1}{n\beta} $$...
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1answer
80 views

How does temperature affect the refractive index of a vinegar?

I’m conducting an experiment on the relationship between temperature of vinegar and its refractive index.
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Black Hole Speed of Information Paradox?

Suppose you have a non-charged, non-spinning, spherically symmetric super massive black hole. Suppose also you could manipulate a sizable mass distribution within 10% of the Schwarzchild radius ...
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1answer
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Relativistic equations are medium dependent?

Einstein proposed that light travels with a universal velocity from Maxwell's equations on electromagnetism. But light has different velocity in different mediums, so does this imply that the ...
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110 views

Cookie Cutter Relativity

A situation similar to this arose in my high-school modern physics course, and I had a question to ask that I hope someone is willing to answer. Suppose a circular pad of cookie dough is being rolled ...
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1answer
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Is there a material which slows down light to the point where the “head” of a laser beam can be seen as it passes through?

Is there a material which slows light down to the point where the front tip, or "head" of a laser beam, can be seen passing through, either with high-speed photography, or even the naked eye? Imagine ...