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

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Why don't we substitute for $p$ in $E = pc$?

See, the energy of a photon is given out by $E = pc = hv$ why don't we substitute for $p$ in $E ^2= p^2 c^2 + m^2 c^4$ by putting $p = \gamma mv$ and then get a value for $m$ (which will be $0$ for a ...
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What's the purpose of the speed of light in $E = mc^2$?

Is $E=mc^2$ not just $E=m$. What does the speed of light have to do with this other than to give it a really big number so it looks cool? What spectrum of light is used? How can we test the speed of ...
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About accelerating particles

Can a particle moving below the speed of light be accelerated more and more until it is travelling at c? IF so does it behave like electro-magnetic radiation?
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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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The speed of light, objects moving against each other and the effects of a clash

Physics is a hobby interest of mine, so I may be asking a senseless question, but there is something I cannot get my head around when it comes to relative frames, the constant speed of light (as a ...
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How Are Galaxies Receding Faster Than Light Visible To Observers?

We know that some galaxies are moving away from us faster than the speed of light and we know it by measuring the redshift, but how's that possible? If they're moving away say at 2c, how would the ...
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86 views

Is there a connection between exclusion principle and the speed of light?

As far as I know exotic stars are composed of degenerate matter created by the balance between gravity and pressure of exclusion principle and as the mass of the star grows it goes into more ...
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35 views

Resultant wave in the Michelson-Morley experiment

I was wondering when the two waves in the experiment, don't they lead to constructive interference? Consider the first case in the image. The two combining waves should lead to constructive ...
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Detecting absolute motion inside a box

This is not a contradiction and I know it is impossible but still consider a thought experiment by me and point out if something is wrong. See the following picture and then the explanation follows. ...
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251 views

Travel at the speed of light

Is it me who have a poor understanding, or does all matter have to become 'pure energy' in order to achieve speed-of-light speed? If so, does that mean that no material can achieve the speed of ...
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21 views

Spinning theoretical object moving faster than the speed of light [duplicate]

Let's say you have the Earth, or any celestial body, spinning as it is. What if you build a tower from the surface, and extended it out into space. If it was built far enough, could the furthest end ...
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SPEED of LIGHT as photons reach black holes [duplicate]

As photons reach a black hole and begin fall in - is their speed increased as they go down (like a vacuum cleaner does to dust).
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How do we know that photons are exactly massless and travel exactly with speed $c$ in vacuum?

I know that this question may sound stupid, but what I mean is that photons have some energy and no mass, yet the mass and energy are said to be equivalent (or maybe I got that part wrong). In an ...
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62 views

Speed of light originating from a star with gravitational pull close to black-hole strength?

Imagine you have a star which is on the brink of turning into a black hole. Lets say it is infinitely close to become a black hole, but not there yet. Since there is no event horizon, but a great ...
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Would approaching a distant star at near the speed of light unfold its entire history in “fast-forward”?

The light we detect today in our telescopes from distant stars is really old. If we could travel fast (lets say, 90% the speed of light) towards a star a million lightyears away, all the time looking ...
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Is speed of light ( Sun-to-Earth ) related to the movement speed of solar system?

The speed of light has been measured to be 299 792 458 m/s. Now, the Solar System is traveling at an average speed of 828,000 km/h (230 000 m/s). Summing up the numbers we get close to 300 000 000 m/s ...
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Why does the light travel slower in denser medium? [duplicate]

Wikipedia says that "in general, the refractive index of a glass increases with its density." And the refraction index of water vapor is less than ice, and even less than liquid water. Is there any ...
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How can photons interact with anything?

I read photons do not age because they move at the speed of light. So when a photon interacts with my eyes, aren't they apart in space-time by the difference of the time in the frame of reference of ...
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Speed of light that is traveling away from the observer

The second postulate of Special Relativity states: The speed of light in a vacuum is the same for all observers, regardless of their motion relative to the source. Now imagine the observer ...
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106 views

Time Dilation at zero velocity (and zero gravity)

From what I've learned, the more an object travels closer and closer to the speed of light, the more time will slow down for that object.. at least from an outside perspective.. It was shown that ...
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46 views

Which properties of electricity and magnetism are responsible for the speed of light being what it is? [duplicate]

I have always been fascinated by the speed of light. I have wondered how it was originally calculated before we had equipment sensitive enough to clock light speed. All I know about it is that the ...
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35 views

Gravity propagation speed [duplicate]

Related to: The speed of gravity? In the related question and in many other questions here, it seems as if the propagation speed of the gravitational interaction is $c$. To my understanding, the only ...
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Weird case for relative velocities and special relativity

This has bothered me for a while. I've been taught that the formula for determining the perceived velocity on an object from another moving object's reference frame under special relativity is: $$v = ...
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72 views

What is the role of the speed of light in mass-energy equivalency? [duplicate]

Where does $c$ squared come into play in the equation $E=mc^2$. Multiplication obviously but how does energy equal mass times the speed of light?
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1answer
81 views

Does the light speed change in media? [duplicate]

Does the light speed change? The Theory of Relativity says that the speed of light in vacuum is the same and unchangeable, while I read that the speed of light in glass is lower than in air, and that ...
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29 views

Aim of the Michelson Morley Experimetn [duplicate]

What was the aim of the MM experiment? Many sources say that it was to measure the relativity velocity of the Earth through the aether. But I'm trying to understand what this exactly means, and why ...
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Speed of gravity in cosmological codes and ephemeris generation

There are few questions in Phys.SE concerning the speed of gravity, and the answers are traditionally that the speed of gravity equals to the speed of light. But in that case I have three more ...
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Theoretically if you passed the speed of light, would there be a sonic boom equivalent?

I know that it is technically impossible to reach the speed of light since the mass of the object traveling would reach infinity. Despite this if it were possible, would there be some sort of ...
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65 views

Expected result of vertical Michelson-Morley Experiment

What would the result of a vertical variation of the Michelson-Morley experiment be? I.e., if one were to compare light traveling along a horizontal arm with light traveling along a vertical arm ...
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What will be the relative speed of a photon in a light ray to another photon of opposite direction light ray?

If two light rays start simultaneously in the space from exactly opposite ends in opposite direction that is separated by a distance of 600000 km in a way they meet at the mid point (300000 km from ...
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twin paradox inferring cosmic speed limit

As I understand it, the twin paradox was integral to Einstein's conclusion that nothing can travel faster than the speed of light. Why wasn't there a similar conclusion for a hypothetical scenario ...
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61 views

Question about relative speeds on a different “scales of perception”

I was kind of puzzled yesterday when thinking about this. If we observe an object moving away from us with 10m/s we would say the object is moving away from us really slowly But if we (humans, ...
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49 views

Where / what is the source of the force that causes refraction?

So we know that as light moves from, say, a glass block to air, it speeds up and refracts, but if there is to be acceleration then surely there must be a resultant force to cause it? So where does ...
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193 views

Why light can't go faster then 300 000 km/s? What prevents it? [duplicate]

Why can't the speed of light be faster then approx 300 000 km/s? What prevents it?
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101 views

A vertical variation of modern versions of Michelson-Morley

For almost a year now, I have been in the uncomfortable position of having an idea. However, there is one nice thing about this idea. It makes a concrete, exact and relatively easy to test physical ...
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Light & Observer moving perpendicular to each other

Light is the yellow arrow. Observer is the black arrow. Observer is moving at a constant speed of v, w.r.t to a Galilean frame of reference. Now from the point of view of the observer (O), how ...
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170 views

Why is light speed the limit? [duplicate]

Why is the light speed a limit? Why can't anything go faster than light? Not even a single atom?
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Difficulty in understading a part of the book “A Brief History of Time”?

Sorry if the question is not upto the standard of the site but i really can't understand what the following para says. I am reading the book "A Brief History of Time" by Stephen Hawking and in the ...
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Is there a delay in the effect of gravitational force?

Let's suppose there is a very massive object and a small object that are 1 lightyear apart. The massive object is large enough that the gravitational force pulling the small object is easily ...
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32 views

Twin paradox on a cosmic scale

I am referring to yet another version as the classical twin paradox. In my version the moving apart of the twins is entirely induced by space expansion between them and they move apart each other at ...
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What changes occur while an atom approches the speed of light?

My knowledge of atomic dynamics is a little superficial. But to my understanding an electron travels an orbital path around a nucleus of an atom. "correct?" So let's say that if a hydrogen atom were ...
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Does the speed of light in different mediums affect the lorentz transformation?

We know that the Lorentz transformation is derived using the speed of light in vacuum. But if we were to use it in water, would it change since the speed of light in water should also remain constant ...
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Why the speed of light is represented by $c$? [closed]

In almost every textbook, I've found that the speed of light is $c \approx 3 \times 10^8\: \mathrm{m/s}$. I wonder why it's just $c$ ?
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Would a beam of light moving through a vacuum be considered an example of perpetual motion? [closed]

I have a very limited understanding of Physics but if the beam of light was moving through a vacuum like space and had saw no obstuctions during its path, would that be perpetual motion in action?
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Theoretical Upper Bound on Processor Speed?

Barring aside considerations such as heat dissipation, capacitance, etc... (aka any sort of technological issue) what is the fastest speed of a processor? I am told that at distances of 1 planck ...
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38 views

How to apply equations to figure out how Hubble's constant affects the speed of light?

For my thought experiment, I will create my very own infinite expanding universe similar to our own assuming current cosmological theories are correct, except for a couple things: $$\text{(Hubble's ...
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59 views

What is the mass required for a star to have the gravity equivalent to a black hole?

At what mass does the light from stars ( I am talking about stars and not black holes) fail to escape the star's gravity? Is it the same (minimum) mass required for an object to be called a black ...
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178 views

Has anyone checked whether the speed of light varies according to gravitation

My physics is fairly basic, but I hope someone can answer without being too rude. A transparent medium such as water or glass refracts light and also reduces its speed, so I was wondering whether ...
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Why does special relativity talk about the speed of light in a vacuum?

What's the importance of the speed of light being in a vacuum? Does not being in a vacuum cause a difference?
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Resolution of the EPR paradox using relativity of simultaneity

A simple explanation for photon entanglement experiments Example: Quantum teleportation La Palma-Teneriffa in 2012 (distance 143 km) - Photons were entangled in such a way that when measuring ...