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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Will the speed of gravity and the speed of light always be the same?

I have been considering the following though experiment: If the entire sun were to disappear at once, would we feel it first due to the sudden drop in gravity or see it first? The answer I have come ...
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34 views

Does the definition of the metre have to take into account spacetime curvature?

The definition of the metre is: The metre is the length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second Does this have to be modified in a ...
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2k views

Is traveling at the speed of light the same thing as teleportation? [on hold]

If I were on one side of the room and moved at the speed of light to the other side of the room, to an observer it would appear that I teleported. If time stops at that speed, it would be ...
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39 views

Reference for Reichenbach synchronisation and non-standard special relativity

My professor introduced in the last lesson a new method for clock synchronisation, which he called "Reichenbach synchronisation". In this new method, two clock A and B synchronise themself with the ...
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111 views

After proving that the photon remains stationary in the fourth dimension, must we conclude that the fourth dimension is moving at c? [closed]

Firstoff, in his general relativity Einstein showed that dimensions could bend, curve, and move. This is an experimentally proven fact. Dimensions can, and do, move. In an earlier post we ...
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How does a photon experience space and time?

To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere. ...
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39 views

How did the Michelson-Morley experiment really confirm the universal speed of light? [closed]

So we know the earlier physicists largely assumed light moved in the aether as a fixed reference frame, and the Michelson-Morley experiment successfully disproved that. But it appears that the ...
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2answers
378 views

What does $v=c$ in the Lorentz transformation for time tell us?

For the simpler cases as boost in the x-direction, the time dilation formula following the Lorentz transformation for time is $$\Delta t'=\gamma(\Delta t-v\frac{\Delta x}{c^2})$$Now, we observe that ...
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25 views

Why is metre defined in terms of distance covered by light in 1 second? [duplicate]

Why is the unit of length defined with the help of unit of time? (1m=x covered by time in 1/3*10^8 s) Isn't length a fundamental unit too, why is it defined in terms of an other unit?
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1answer
122 views

What would we see if a camera orbiting close to light speed beamed down images of us on earth?

If a camera was sent up in a satellite, put in orbit at close to light speed, and instructed to beam down images to earth, what would we see if we watched its video feed? (Ignoring the fact that ...
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27answers
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Do we know why there is a speed limit in our universe?

This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is? I'm not asking why the speed limit is equal to ...
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80 views

Are photons massless? [closed]

If anything that has no rest mass will travel at the maximum possible speed. Hence, light being massless travels at this speed. Does this means that photons are massless?
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74 views

Speed of gravitational waves and light

We all know that speed of gravitational waves (GW) and that of light in space are exactly the same (= $c$). We also know that space is medium for GW. Does that mean space is also the medium for ...
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1answer
26 views

Is the time for light to go a certain distance the same amount of time sped up through relativity?

If you were to stand on the tower of my Lemmon and had an atomic clock that was exactly synchronized with one at tucson, (6734 feet difference) for 24 hours, it was shown to be roughly a 20 nano ...
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0answers
23 views

Special Relativity and its relation to refractive index [duplicate]

As is known, light slows down (compared to vacuum) whenever it enters a medium. Also, Special Relativity effectively puts a limit on the max. speed of any body. So is it possible for a body (in a ...
237
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15answers
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How does gravity escape a black hole?

My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
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1answer
71 views

Speed of a particle approaching that of light relative to itself

There's a particle moving at a certain speed $v$ with respect to a reference frame $A$ through an axis I will call $x$. We thus know that for the frame of reference located at the particle itself, the ...
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1answer
132 views

Can a black hole move at speed of light?

Black hole comes in all sizes ranging from microscopic black hole on a quantum scale to supermassive black hole that resides in the center of probably every galaxy. Photon have no rest mass hence they ...
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1answer
333 views

Application of special relativity on sound waves

What is the difference between an electromagnetic wave and other waves that amounts to the EM wave following the Special Relativity. I have been reading about the Special Relativity for some time, and ...
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what do we mean by speed of light dependent on direction?

I have a statement in textbook saying "When the speed of light is independent of direction,the secondary waves are spherical". Can someone tell when it is dependent on direction and how the secondary ...
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3answers
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How is light affected by gravity?

Light is clearly affected by gravity, just think about a black hole, but light supposedly has no mass and gravity only affects objects with mass. On the other hand, if light does have mass then ...
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1answer
59 views

How do electromagnetic waves/light travel at the speed of light?

When something reaches the speed of light, then time is said to stop, or at least is perceived to have stopped. The reason is that at that speed, the object become infinitely heavy and requires ...
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0answers
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A point on a disc travelling at the speed of light [duplicate]

This might be a duplicate of this. I had an interesting thought experiment: suppose that you have a disc of a 1 m radius, where a point P2 inside the disc travels at the speed of light. You have a ...
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2answers
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How can photon have wave properties if they travel at the speed of light? [duplicate]

I am not a physicist so please excuse me if this is a dumb question. As far as I understand Relativity, as observer (in this case a photon) travels at the speed of light, time stops. So how can the ...
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1answer
121 views

Fermi velocity $v_F$ and speed of light

Can Fermi velocity $v_F$ be equal to speed of light, $c$, for ultra-relativistic electron plasma waves, in plasma physics, by using Fermi-Dirac distribution function?
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How can we know that the speed of light is constant?

Science say that within a constant time frame the light travels a constant distance. But if time is relative how can it be possible to use it as a reference to say that something is constant?
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Why does red light travel faster than blue light?

I know that light of all frequencies travel at the same speed in vacuum. But I wonder why their speed differ in any other medium, why does red light travel faster if it has less energy than blue ...
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3answers
652 views

Speed of gravitational waves vs speed of light

I own an educational YouTube channel on physics and astronomy. I am currently working on a gravitational waves video extension to my "How Fast Is It" video book on relativity theory. I have a question ...
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1answer
84 views

If photons end up having a tiny mass, say $10^{-54}~\rm kg$, what would be the universal speed of massless particles?

First, I'm no expert so sorry if I get anything confused but I tried to research as much as could before asking this. So while it is pretty much accepted that photons are massless (Despite this ...
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4answers
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If a 1kg mass was accelerated close to the speed of light would it turn into a black hole?

I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever ...
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62 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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2answers
87 views

What particles travel near the speed of a photon? [closed]

I know photons travel at the speed of light. How does the size of a particle determine its maximum velocity? Phonons, Preons, Electron ect. Is there a scale in from the smallest particle's maximum ...
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2answers
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Why does Special Relativity apply to more than just light?

It is my understanding that time dilation is derived from the constancy of the speed of light in vacuum. I would assume this implies that the quirky consequences would therefore apply only to light. ...
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4answers
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Am I just some energy traveled at light speed? [closed]

I don't understand $E=mc^2$ very well, and here is my question: Does this equation mean masses are just condensed energy? And does this mean that the extra energy an object has when traveling at ...
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329 views

What happens when light moves perpendicular to a moving object?

Imagine the folllowing situation: A coherent light source is attached to a car such that the emitted light beam path is "being crossed over" by the car i.e. the long parallel light beams are struck by ...
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1answer
75 views

Where is the mistake in the following rationament [duplicate]

Well... kind of hard to translate in English so bare with me :). Let's consider a wheel that spins in the void. Each point of the wheel has the speed $v = ω r$. That means that for any $ω$, there is ...
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Why are black holes black? [duplicate]

If heard two explanations for this. One explanation is that the gravity is so strong that space is being stretched inward faster than the speed of light and thus no photons could possibly escape. The ...
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3answers
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Deriving the speed of the propagation of a change in the Electromagnetic Field from Maxwell's Equations

I've been told that, from Maxwell's equations, one can find that the propagation of change in the Electromagnetic Field travels at a speed $\frac{1}{\sqrt{\mu_0 \epsilon_0}}$ (the values of which can ...
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3answers
672 views

What if a faster-than-light particle is found?

What will be the consequence (severe ones) on laws of physics if a particle that travels faster than light is discovered? I am looking for a more general answer so that a high school student would be ...
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1answer
34 views

How are the frequency and wavelength of electromagnetic waves affected within an event horizon?

I apologize if this has been asked previously or if my thinking is way off base, but I am inexperienced with relativity (and this is my first question on the site). I am wondering; as one (not ...
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2answers
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Does the constancy of the speed of light in vacuum hold for any observer in GR as well?

From SR, we know that the speed of light in a vacuum is the same for all observers, regardless of the motion of the light source. But in GR, does it still hold for all observers? I mean the constancy ...
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3answers
168 views

Relative velocity of light beams

Assume two light beams moving in the same direction i want to use the relativistic velocity transformation equation to find the relative velocity with each other, Note : i let $C$=1 ...
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1answer
123 views

How far does a photon move in the 4th dimension when it travels one light second? [closed]

In one second a photon moves 3x10^8 meters through the three spatial dimensions. Light's velocity is 3x10^8 m/s. If the photon moved at all in the fourth dimension, it's velocity would no longer be ...
2
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2answers
390 views

Stanford: “Objects in spacetime all move at constant speed $c$.” Are they right? [duplicate]

In this Stanford University lecture on Relativity, it is stated: Likewise, objects in spacetime all move at constant speed c in spacetime but if you change its direction, say by moving at speed v in ...
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1answer
78 views

At what speeds does time dilations start to occur?

My question is, is it possible for time dilation to occur at a speed less than that of speed of light?
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29 views

How much time will for the observer last sound of thunder? [closed]

We have height of the cloud 5 km. The observer stands 20 km away. The speed of sound depends only on the temperature. $C=a\times\sqrt T$ Temperature on 5 km is $-33^o C$ and on the ground $7^oC$. ...
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1answer
603 views

Does a photon travel through space exactly at the speed of light?

In practice, given that in the vacuum of the universe is not perfectly empty, does the photon actually reach exactly c? Is it even possible? If it travels at exactly c, then from the photon ...
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57 views

Cherenkov Luminescence Imaging, Cherenkov radiation,

Is it possible to see glowing from Thin Layer chromatography (TLC silica gel on Aluminium) when we image it by IVIS spectrum (no radioactive source)?