# 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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### 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 undergo ...
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### Why is the speed of light in vacuum frequency-independent?

I recently came across this paper: Mainland & Mulligan, Foundations of Physics 50(5), 457–480, "Polarization of Vacuum Fluctuations: Source of the Vacuum Permittivity and Speed of Light" ...
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### Are these two definitions for propagation speed be the same?

We know for a fact that for any medium with permittivity $\epsilon$ and permeability $\mu$, the propagation speed of electromagnetic waves is given by $$v = \frac{1}{\sqrt{\mu\epsilon}}$$ We also know ...
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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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When observer sees source in actual nearest position, there is a transverse Doppler effect due to time dilation. From the view point of observer, the source experience a shorter time by $1/\gamma$, ...
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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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### 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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### Relativistic ship and base on Earth talking

Let say humans make it possible to travel so close to the speed of light that time dilation for the ship can achieve 100 times the time of a standing still object. So one of that ships starts its ...
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### Is the duration of atomic/electron phenomena proportional to the wavelength of photons produced by those phenomena?

Given that any periodic electromagnetic emission has a wavelength due to its time related oscillation and a finite speed of propagation, is it the same for atomic/electron phenomena, such as the ...
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### Does Loop Quantum Gravity predict that high-energy photons or light waves will travel faster or slower than low-energy (longer-wavelength) ones?

Matthew O'Dowd, the Australian-born host of PBS Space-Time, insists that high-energy (short-wavelength) photons (or waves) of light will travel slightly more slowly through space than lower-energy, ...
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### Restoring force of electron-nucleus spring model (Lorentz oscillator model)

The restoring force should fulfill at least two criterion Experience repulsive force when it is compressed and attractive force when it is extended The restoring force always increases with distance ...
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### Gravitation time delay

Einstein showed us that our reality is very different from what we perceive, he taught that time under the influence of matter passes slowly. When I got to know this fact and tried to find a ...
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### Lightspeed - what do we measure? Wave group, phase, front and signal velocities

I'm studying for an experiment of measuring the lightspeed c. It will be measured following the rotating mirrors method by Fizeau-Foucault, using a HeNe laser. I could understand - and visualize - ...
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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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### 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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### 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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### 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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### 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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### Deriving the Minkowski Metric from homogeneity of space-time and the isotropy of space

In this wikipedia page, it says that one can derive the spacetime interval between 2 arbitrary events from the second postulate of special relativity, together with the homogeneity of spacetime and ...
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### 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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### 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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### 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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### How is light trajectory affected by the trajectory of environment it passes through?

There's a sci-fi concept of slow light that I find very amazing: Imagine a glass material that has index of refraction $n$ say, $3,000,000,000$ which means: v_{glass} = \frac{c_{vacuum}}{n} = 0....
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### 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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### 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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### 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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### 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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### Postulate of constancy of speed of light in vacuum

I'm not of course questioning the constancy of the speed of light, just the way the postulate about it is worded. It is often stated that the speed of light is independent of the motion of the source. ...
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### Can photons ever be at rest or not?

I've read two, what appear to be, contradicting statements in my physics textbook. First it says that all the experimental observations have shown that energy of a photon and its' mass is $0$ when it'...
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### Speed of information related anomaly

Suppose I have a perfectly rigid rod of length $1$ light-year and I am running with that rod with its one end in my hand. I suddenly stop at some moment. Will the other end of the rod continue to move ...
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### How was frequency measured in Evenson, et al.'s paper on the measurement of the speed of light?

To avoid confusion, please note that I am referring to a paper published before the speed of light was defined axiomatically. I am referring to this paper, but I don't understand it. In the abstract, ...
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### Practical details on Fizeau cogwheel experiment

Fizeau measured the speed of light using the so-called Fizeau-Foucault apparatus. It consists, if I understood correctly, in sending light to a long distance (in the historical experiment, more than 8 ...
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### Is it possible to calculate speed of light in a mixture of A and B, by knowing the speed of light in both medium A and B seperately?

So we all know speed of light varies in different medium, which then makes me thinking is it possible for us to calculate the speed of light in a thoroughly mixed medium consists of X% of medium A and ...
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### Trying to derive the Lorentz factor. Provide no more than a hint

I'm trying to derive the Lorentz Factor by tackling a simple case of a thought experiment in Special Relativity: Observer $A$ is situated at a distance $d_A=1cs$ meters to the left of a mirror. ...
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When light travels in air, all the component frequencies of light travels with the same velocity $v_{air} = 1/\sqrt {\epsilon_0\mu_0}$ (where $\epsilon_0$ is independent of frequency. Then we say that ...