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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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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Doppler effect and light

Approaching the speed of sound in an aircraft is relatively difficult, because the closer you get to Mach 1, the denser the pressure is around you (sound accumulates causing vibrations). Is there a ...
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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: ...
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Concerning infinite energy and the speed of light [on hold]

From Wikipedia: "Under the special theory of relativity, a particle (that has rest mass) with subluminal velocity needs infinite energy to accelerate to the speed of light, to accelerate beyond the ...
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“If the astronaut found that light slowed down relative to his motion, he would have detected the very ether wind that Michelson failed to detect”?

The following extract is from Relativity Simply Explained by Martin Gardner (Amazon link): Consider an astronaut in a spaceship that is racing alongside a light beam. The ship is traveling with ...
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Why can't we see light travelling from point A to B?

Lets say we have a cloud of dust which is a lightyear across and someone shoots a beam of light from point A to B , why it is not possible for an observer far far away to see the light while it ...
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Is the speed of light in vacuum always the same value?

The escape velocity of different planets and stars vary. If they vary, the velocities of bodies escaping from the respective stars or planets should also vary. Like, if I want a ball to reach 10 ...
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101 views

Inertia on relativistic mass when particle is near speed of light

Inertia is directly proportional to mass but what happens when something travel to speed near to light. Its relativistic mass tends to infinity but that is false mass so I want to know if inertia is ...
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1answer
354 views

Theoretically if you passed the speed of light in a medium, would there be a sonic boom equivalent?

I know that it is technically impossible to reach the speed of light in vacuum since the mass of the object traveling would reach infinity. However in a medium, would there be some sort of theoretical ...
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The velocity of the image if both the mirror and the object start moving [on hold]

Suppose a vertical plane mirror and a object which is placed at a distance of 4 m from the mirror. If we start moving the mirror with a velocity of 2m/s and also start moving the object at 3m/s. Then ...
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Measuring the speed of light and defining the metre - absolute or relative?

If the metre is now defined as the distance light travels in vacuum in 1⁄299,792,458th of a second and the speed of light is accepted to be ...
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How to explain the homogeneity of the universe through a physic model [duplicate]

If in general, two objects homogenize themself by combining it's parts, why the horizon effect, based on a big bang model, excludes such an important physical interaction process? I wonder if this ...
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Constancy of speed of light $c$ with perpendicular motion [duplicate]

Suppose light is travelling in a straight line parallel to $y$-axis takes time $t$ to reach from $y_1$ to $y_2$ in a reference frame $S$. Let there be another frame, $S'$, which is travelling parallel ...
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How can the orbit of Jupiter's moons be used to calculate the speed of light? [duplicate]

How can the orbit of Jupiter's moons be used to calculate the speed of light? It seems this was one of the first methods and goes back to 1656.
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How did Rømer measure the speed of light by observing Jupiter's moons, centuries ago?

I am interested in the practical method and I like to discover if it is cheap enough to be done as an experiment in a high school.
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Rømer's determination of the speed of light

I am trying to understand Rømer's determination of the speed of light ($c$). The geometry of the situation is shown in the image below. The determination involves measuring apparent fluctuations in ...
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1answer
54 views

Light speed measured by observing Jupiter moons [duplicate]

The famous discovery that the light has speed measures the times of the immersion and emergence of Io, one of Jupiter's moons. However, I still can't comprehend this. Even when the Earth is further ...
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687 views

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 the speed of light in vacuum constant or does the math just happen to work out?

My apologies if my question is really idiotic, but I ask sincerely because I want to learn. Based on this question and lots of other places on the web, this topic seems to be really confusing. Let's ...
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How is it possible for the wavelength of light to change in a medium?

So my physics class has just finished a long unit on optics while at the same time I've been trying to teach myself relativity. I admit my understanding is probably rudimentary, but I figured all the ...
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Why does the speed of light in vacuum have no uncertainty?

I could understand that the definition of a second wouldn't have an uncertainty when related to the transition of the Cs atom, so it doesn't have an error because it's an absolute reference and we ...
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1answer
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Are we all moving at $c$? [duplicate]

Is it true to say that all matter in the universe is travelling with velocity c through spacetime, but that for baryonic matter most of that velocity is through the time dimensions rather than the XYZ ...
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Measurement of the speed of light form different perspectives

I've been showing a special interest in Einstein's theory of relativity and how he proved the speed of light to be always the same. At first it was a bit hard for me to understand, but now I THINK I ...
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Propagation speed of photons when taking higher-order QFT corrections into account

In our group of experimental physicist who have nothing to do with and know very little about quantum field theory, we recently had a question concerning the propagation speed of photons in vacuum: ...
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The movement with the speed of light [duplicate]

Light moves at $3\times 10^8$ m/s, but is this speed always so?, imagine if I am in a dark room that is $1$ light year long and I am standing at one extreme end of the room, and there is a powerful ...
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How is it that the voltage wave along a circuit is a tenth of the speed of light?

I have heard that the voltage wave (pushing the electrons) along a circuit is a tenth of the speed of light. Can someone please explain with an illustration how it is possible?
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Why does larger permittivity of a medium cause light to propagate slower?

I was wondering about what physically happens when light is transmitted through a non-magnetic medium. Specifically, I’m trying to visualize how materials slow down light as the electromagnetic wave ...
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Is it really possible to “discover” the speed of light with a microwave oven?

I've seen a number of sites/videos online that describe a method for measuring the speed of light, using a microwave oven and a chocolate bar. For example, this video on youtube. The basic idea is to ...
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Can light travel slower than the maximum?

First of all, I know that light does indeed travel slower in a medium like air or water, but that's because the photons are bouncing off of the medium's particles and in different directions so the ...
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If all motion is relative, how does light have a finite speed?

I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds ...
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Would a giant see our year as a second? [closed]

If there were an enormous being whose arm span is one light year across, how would that being perceive time? Wouldn't what we perceive as a year be virtually nothing to that being? Explanation: ...
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1answer
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How do we know that fundamental constants don't slowly change in time? [duplicate]

Let me get one thing straight first. I am not saying that fundamental constants like the speed of light don't have the value that we know they have today. What I am asking is whether the value of a ...
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Do you encounter more photons (per unit time) when moving forwards at a constant velocity?

Let's say you have rain hitting you evenly on all sides (not very realistic, I know). If you were to move forwards at a constant speed, there would be more droplets of rain hitting you per second on ...
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The origin of the value of speed of light

Meaning, why is it the exact number that it is? Why not 2x10^8 m/s instead of 3? Does it have something to do with the mass, size or behavior of a photon? To be clear, I'm not asking "how we ...
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Forces and the light

Do external forces can affect the light? Can any external force make the light accelerate? And if it can, will it accumulate mass? (according to the second Newton's law of motion $m = F/a$ ) We know ...
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Newton's laws and the maximum speed

According to Newton's second law of motion : $F = ma$ In an certain occasion, we exert 2 forces (the magnitudes of the forces are the same) on 2 different objects, Object A and Object B, in the same ...
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Light travels in a medium

According to Snell's law : $${n_1 \over n_2} = {v_2 \over v_1}$$ $v_2 = v_1 n_1 / n_2$ Assuming that $n_1$ is vacuum , we will find the following equation: $$v = c / n$$ (We may find the same ...
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What is the speed of light relative to?

Consider the scenario where you measure the time it takes for light to travel to the left 10 meters and to the right 10 meters. Both measurements will take the same time, even though we are moving ...
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In superluminal phase velocities, what is it that is traveling faster than light?

I understand that information cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but ...
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Why can't we increase the speed of sound in a constant Temperature-pressure condition?

We know the speed depends on the kinetic energy. Imagine that we have 3 iron balls equally spaced and kept in space (zero gravity condition). If i push the first ball slowly it would travel slowly ...
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How do I sum up speed though space and time to obtain c (in terms of units)?

I'm still learning SR and only recently I could scratch the surface of such beautiful, complex and deep topic. This post here was really enlightening, and even though it is somewhat basic, it helped ...
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Relative Velocity Problem: Light? [duplicate]

Assume 2 rockets coming towards each other at each other at 99% of speed of light, and so as they pass each other the relative velocity between 2 rockets would be 198% of Speed of Light and so what ...
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Relativistic deceleration and energy

It's common knowledge that due to relativistic effects, accelerating from 0.8c to 0.9c takes a lot more energy than accelerating from 0.1c to 0.2c. However, what's the case with deceleration? Does it ...
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The relation between mass and speed

Massless particles (or luxons) have no (rest) mass ($m = 0$) and a speed equal to the speed of light ($v = c$). Massive particles (or bradyons) have mass ($m > 0$) and a speed lower than the speed ...
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How is light slowing down in a medium thought of in the photon picture? [duplicate]

The speed of light in any medium besides vacuum is smaller than $c$. In a classical way, I just look at that as a wave that propagates less fast, the change in EM-field is passed on slower. How should ...
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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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Use something that slower than light in Michelson–Morley experiment

Follow by the Michelson–Morley experiment, What happen if we use something that has a speed lower than light instead of light in Michelson–Morley experiment? How about the result?
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Has anyone tried Michelson-Morley in an accelerated frame?

After doing much more digging than I thought I had to do, I found out that the speed of light is NOT invariant in an accelerated reference frame. Has anyone done any experiments to confirm this? In ...
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I know light's speed in vacuum is constant, but what about its velocity?

Is the velocity of light in vacuum constant? It seems it would be different depending on whether it is coming toward you or away from you, but I just want to make sure. Does the direction of light ...
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If time stops for an object, does that object feel gravity?

As far as I understand The GTR, it is said that Mass bends space-time which causes gravity. So every Mass in this universe is flowing through space-time example earth is moving along with space-time ...