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 light continue traveling after the source has stopped producing light? [closed]

Will light continue traveling after the source has stopped producing light?
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104 views

How can travelling at light speed affect light itself? [duplicate]

~Hypothetical Scenario: Your driving in a car (with the headlights on high beam) and you gradually get faster until you reach the speed of light. Raised Assumptions: You can see in and out of the ...
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82 views

Observing light speed

Consider an observer B is moving with speed $0.8c$ relative to another observer A in standard-configuration (I think it is called that in english: that the system B is only moving along the $x$-axis ...
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4answers
183 views

Why is the speed of light in vacuum what it is?

Three-pronged question on the speed of light. One: simply, do we know why the speed of light in a vacuum is what it is and why nothing is allowed to go faster? Two: if we knew why the speed of ...
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38 views

Dispersion for a highly energetic electron

The dispersion relation can be expressed as: $$w=\frac{\hbar k^2}{2m}$$ The energy of a free particle is: $$E=\sqrt{p^2c^2+m^2c^4}$$ For a highly energetic electron we have $E\approx pc.$ The ...
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673 views

Why is light in vacuum so slow? [duplicate]

Accepting the speed of light in vacuum and infinite mass and all that, I wonder why light does not have infinite speed? I know light does not have rest mass, but people use the term infinite mass ...
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102 views

Equal and opposite reaction at light speed

My knowledge of physics is very limited. And this question is only really to help me put to rest a question i have had for some time. I would like the answer to be a simple one as i am not really in ...
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3answers
182 views

What experiments have been done that confirm $E=mc^2$?

What experiments have been done that confirm $E=mc^2$? Are there experimental results that contradict $E=mc^2$? Or are experimental results consistently showing this famous formula to be true?
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62 views

Speed of light violation?

While reading this article Discovery of the companions of millisecond pulsars, it mentions a pulsar rotating multiple times a second, and I was wondering if this doesn't exceed the speed of light?
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143 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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Speed of light in vacuum - definition and Constancy [duplicate]

This question has been nagging me for sometimes now, and even after studying all those relativity still cannot find an appreciable answer. The speed of light is $c$ defined and measured as constant $...
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4answers
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How does light travel?

How does light travel, does this not contradict the idea that going the speed of light stops time? Because if going the speed of light stops time and light goes the speed of light shouldn't it be dark?...
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11answers
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What happens if a super fast rotating ball accelerates near speed of light?

Assume we have a ball with diameter 1 meter and mass 1 kg rotating at 99,5% speed of light (...
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2answers
153 views

Why don't constant motion charges produce waves?

I'm a little confused about the origin of electromagnetic waves. Although I can understand their origin mathematically, I get a little confused about the physical intuition of... Information ...
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136 views

can a spaceship decelerate relativistically? [closed]

Say I have a spaceship, and on board is a laboratory set to accomplish some sort of experiment, simulation, or computation which might take upwards of a few hundred years or more to complete. Instead ...
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1answer
58 views

Speed of entangled particle changes [duplicate]

If two particles are entangled at a distance of light years, does changing one affect the other instantaneously, or does it take years to occur? What gives us the answer to this question?
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1answer
93 views

Could observed accelerating expansion be an illusion based on distance?

The thought occurred to me that if we just happened to be born into a time when the the universe started slowing down, it would take some time for us to observe this - especially in more distant ...
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1answer
283 views

Does laser beam expand along distance or not? [duplicate]

Why does laser dot become bigger at bigger distance? e.g: from millimeter to inches so that it distracts pilots in aircrafts. I knew that laser beam should't expand over distance, but remain stable. ...
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2answers
94 views

Electromagnetism and a simple doubt [duplicate]

Why don't electromagnetic waves need a medium to propagate? How does light get such a magnificent velocity?
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107 views

The speed of information

Does this expression have any rigorous meaning? Intuitively ,I feel that information about an event or a system in my environment propagates to me at a certain "speed". Does this correspond to ...
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73 views

Can light travel with a speed less than its speed at same time?

I know that the velocity of light with respect to anything is constant ($c$) . What, then, is velocity of light with respect to light?
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132 views

Is there any way to measure the one-way speed of light?

Is there any way (practical or theoretical) to measure the one-way speed of light? The two methods that come to mind are: Stellar aberration, and Using adiabatic clocks: synchronize clocks, then ...
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Alongside a light ray (in another medium)

It is a said fact that the speed of light is insurmountable. But can the validity of this fact be questioned in another medium? If I pass a beam of light in, say water (here the light speed is ...
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8answers
622 views

Why is there a universal speed limit?

I am looking for an answer that does not rely on Special or General Relativity -and without recourse to the fact that the speed of light is frame invariant. Is there another way of showing this ...
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1answer
66 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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Experiment measuring vacuum permittivity and permeability in high speed

Suppose that we do NOT know anything of constancy of speed of light, to say nothing of special relativity. It is that we acquired a constant speed of light from Maxwell's Equations that we finally ...
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47 views

Can photons be converted to dark matter in black holes? [duplicate]

I'm a pharmacy student so if it's not a good theory let's just not laugh at me :) I was wondering if it is possible that when the light enters a black hole and gets trapped in its gravity, its energy ...
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5answers
264 views

Why isn't time just a function of the speed of light being finite

Time is generally referred to as the fourth dimension and seems frequently to be treated as an equivalent to the 3 spatial dimensions. But then people seem to get tied in knots as to why time has an ...
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1answer
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Can a beam of light be modulated close to it's frequency?

I'm wondering if it would be possible to modulate a signal using photon pulses - where the frequency of the signal would be close to the frequency of the light. Also would it be possible to transmit ...
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Can we measure speed of light in a non-transparent material

As I think even photons are absorbed by a non-transparent material, they aren't absorbed exactly at the same time. Light does propagate though a (sufficiently thin) opaque material. So can we measure ...
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194 views

Why does this treatment of the Michelson-Morley experiment assume the speed of light is constant?

In the Michelson-Morley experiment as per the above picture, the difference between the time for light to travel from $P$ to $R$ and back and the time for light to travel from $P$ to $Q$ and back is ...
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How do we know the speed of light is constant and spacetime dilates rather than vice versa?

Some conspiracy nut was telling me that Einstein was BS and there's a giant conspiracy that he's wrong but scientists would loose all their jobs if they admitted it. Of course this is all baloney, ...
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112 views

Velocity of approach of a beam of light and a moving object

I came across a problem on Brian Greene's course on Special Relativity. (http://www.worldscienceu.com/courses/6/elements/TnRUSU) From George's perspective, how fast does a beam of light approach ...
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Speed of inflation [duplicate]

The speed at which matter separated and formed our universe as we know it today was faster than the speed of light (I think its called Planck time)? Anyway how does this differ from saying that '...
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What are some ways to measure variations in the speed of light in vacuum?

To avoid confusion, I am not talking about measuring differences in the speed of light in vacuum as a result of a particular Lorentz boost. What I mean is, suppose the "constant speed of light" were ...
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Difference in Light's speed in various mediums [duplicate]

Light, despite not needing a medium and traveling due to oscillations of the electric and magnetic fields, slows down in mediums such as glass. I used to think it intuitively made sense, as we ...
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1answer
175 views

Einstein on a train paradox: what's the resolution?

Okay, so Einstein is on a train looking at a mirror. The train is moving at $c - 1 \frac ms$. Einstein is facing in the direction of motion. All along the side of the train are stationary clocks. ...
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1answer
33 views

The effects of special relativity when accounting for two reference frames

If reference frame A is travelling past reference frame B could one argue that it is in fact B that is motionless as their motion is relative to each other? If this is the case then how does one ...
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2answers
470 views

Why is the speed of light limited to (only) 299.792.458 m/s? [closed]

The speed of light (in a vacuum) is limited to 299.792.458 m/s. This sounds very fast, and it is on a global scale. But if you look at the Universe on a larger scale than it sounds quite slow, For ...
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1answer
63 views

Could light be limiting towards a more fundamental speed limit? [closed]

Could it be possible (as in strictly speaking, no evidence going against this) that light could be limiting towards a more fundamental speed, and that light truly does slow down when a mass goes a ...
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23 views

Would it be ok to say that not all light travels at the same speed when you consider its own perspective? [duplicate]

The reason I am asking is because higher energy would cause greater gravitational time dilation. Therefore I would expect, from the perspective of a radio wave, a gamma ray having more energy would ...
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3answers
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Does the accelerated rate of expansion of the Universe have any effect on the speed of light in vacuum?

So I was just wondering about this as I finished reading about Michelson-Morley's experiment which disproved the ether theory. My question is since the Universe is constantly expanding and that too ...
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Distance and Time

I understand that if an object is 10 light years away then the light I am seeing from that object is actually from 10 years ago. If this is the case, which a few posts on this forum seem to concur, ...
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time distortion

Observer A is standing on a planet, and observer B is on a ship. At t=0 B is 1 light year away from A. Also at t=0 B will accelerate its ship to a 0,999999999...$c$ instantaneously (yes, its a very ...
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2answers
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Time taken by a point in the light to cross the length of an object moving with a high velocity wrt to an observer at rest [closed]

I am new to the field of special theory of relativity and while understanding its concepts the following question popped into my mind... Suppose there is an object O moving with a very high velocity, ...
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57 views

Does light accelerate when it meets a surface? [duplicate]

Does light accelerate when it meets a surface like mirror, we know that light changes path when it meets a surface like mirror.If light changes path,even if travelling at c,this means light has ...
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1answer
52 views

Light photon direction in movement

I recently watched a couple science videos on YouTube and left me with a doubt. If a spaceship goes to a really fast speed in the (horizontal) $x$-direction, what would happen if it shoots a light ...
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2answers
61 views

Does light ever experiences acceleration? [duplicate]

Till now I have learned that light has fastest speed. But I have also noticed that wherever it travels it only experiences retardation. So I wanted to know if light ever felt acceleration naturally in ...
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94 views

Why is this method valid in special relativity

Here is the question Two clocks are positioned at the ends of train of proper length $L$. They are synchronised in the train frame. The train travels past you at speed $v$. It turns out that if ...
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How does a world line of an Alcubierre drive look like?

In my recent question ”Speed of light and warp drives in general relativity” I asked exactly how an Alcubierre drive worked and exactly what "FTL travel" meant. One of the comments I got stated that: ...