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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Is our physics and system of units based on time (second) or the speed of light?

The speed of light in vacuum is stated in our physics as a universal physical constant, c, when measured locally, in vacuum. The speed of light in vacuum, commonly denoted c, is a universal ...
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Undoing problems caused by setting $c = 1$ { or “Undoing $c = 1$” }

In the mathematical derivation of equations for physics, and involving wave propagation in particular, the propagation speed at the start of the derivation is often set to one (c = 1). I am working ...
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Why is the equation $E=mc^2$?

The equation $E=mc^2$ never made any sense to me. c is a constant (speed of light), therefore c squared is also a constant. We're not specifying any units so surely the equation should be reduced to ...
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How can I show that the speed of light in vacuum is the same in all reference frames?

I have regularly heard that the Michelson-Morley experiment demonstrates that the speed of light is constant in all reference frames. By doing some research I have found that it actually demonstrated ...
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Is it possible that the universe is not expanding faster than the speed of light? [closed]

What if the speed of light increases proportionately to the expansion of space? Is it possible that light traveling in the medium of space gets to travel faster if the medium is expanding? An analogy ...
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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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To Be Forever Young [duplicate]

It is clear from the Lorentz Parameters, applied to Einstein's Equations, that as velocity, v approaches speed-of-light, c, the denominator (1 - [v/c]) tends to zero; when v=c, time, t=0: time stops?! ...
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How does the Higgs field prevent the electron from going the speed of light?

Both the Photon and the Electron are point-like particles. Most massive particles can be thought of "bound collections of massless particles", but the electron assumed to be a "point particle with a ...
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Is it possible that “$c = 1$?” [duplicate]

Can c = 1 and if so, what does this mean for Physics? If you were to take any Physics equation, and let c = 1, will it work?
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Is Lorentz transformation the only way to preserve speed of light?

Doesn't this transformation also preserve speed of light?: $B$ is moving with speed $v$ relative to $A$ in the $+x$ direction. $B$ passes $A$ at $t=0$, and $A$ shines a torch at that moment. Let $(x'...
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How to understand $E=mc^2$?

I am trying to understand Einstein's mass-energy equation $E=mc^2$. Knowing about the atomic bomb I am inclined to believe that $E$ is proportional to the mass: $E=Cm$. For this equation to be ...
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Messages at near-light speed

I’ve been reading up a bit on time dilation and near-light-speed travel for a personal project, and I’m having trouble wrapping my head around some of the numbers involved. Any help would be ...
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Correlation between the refractive index and the density

The $v$ propagation speed of light in a transparent medium is related to the $c$ speed of light in vacuum through the relationship, $$\boxed{n=c/v}$$ The constant $n$ is a pure number called ...
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Is the value for the speed of light in vacuum irrelevant? [duplicate]

Does it matter that the speed of light, $c$, is 299,792,458 meters per second in a vacuum? Would the universe be different if $c$ were, for example, 1 meter per second? Or should things be the same, ...
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Is 'photons being massless' the reason everyone observes their speed to be the same?

When a man on a train throws a ball, the resultant velocity of the ball (as seen from ground) is the sum of the velocity of the train and the velocity imparted to the ball from the throw. However, if ...
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Has the Michelson-Morley experiment been performed while moving in relationship to Earth?

I'm currently trying to debunk some geocentrist claims the Michelson-Morley experiment proves Earth isn't moving and I'm wondering - has this experiment ever been performed while in motion relative to ...
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Is the speed of light in a vacuum constant?

The news site is titled, “Speed of Light May Not Be Constant, Physicists Say.” It talks about how the speed of light in a vacuum is not constant and that estimates of the size of the universe might be ...
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Astronauts in a spaceship moving close to the speed of light

There is a spaceship, moving at a speed close to the speed of light. The control room of the spaceship is in the front and the sleeping quarters are at the back. One of the astronauts, Joe, decides ...
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Can we get the values of $G$, $h$ and $c$ to be numerically equal if we use a convenient system of measurement?

The speed of light in vacuum is approximated to be $3×10^8\ ms^{-1}.$ But, if we change the units, we can get a different number. For example, it won't be $3×10^8$ if we used $ft\,s^{-1}$ instead of $...
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How to travel between two stationary worlds in the least amount of proper time?

I was intrigued by this question about minimizing the travel time between two worlds that are at rest with respect to each other, and disappointed that the answer turned out to be trivial. However, I ...
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How did Einstein know the speed of light was constant?

I often hear the story of how Einstein came up to the conclusion that time would slow down the faster you move, because the speed of light has to remain the same. My question is, how did Einstein ...
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How to travel between two stationary worlds in the least amount of time? (time dilation)

Let's imagine there are two, isolated, stationary worlds in space (called A and B), very far apart from each other. I live on World A, and some aliens live on World B. I want to learn about the ...
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What will happen if a stick and a beam of light is to race [duplicate]

I don't know whether the question is asked before on not on this website as I can't found it anywhere so I have to ask. What will happen if a place a long stick with a length of a light year. Rather ...
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Does a static electric field travel at the speed of light?

Suppose we have a large area where the net electric field is zero. We place a charge q somewhere in this area. How long will it it take for another positive charge q' standing at a distance d from q ...
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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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Light speed in vacuum over space and time [duplicate]

Is the light speed the same, constant, in all the universe locations? I mean, is it he same in the surface of the Sun compared with speed in space inter galaxies? Also: is light speed now the same as,...
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Do photons interact or not directly?

All I am asking about is photon (EM wavepacket) photon (EM wavepacket) interaction. I have read this question: If photon-photon interactions are impossible, how are higher harmonics generated? ...
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4answers
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Relativistic Doppler Effect from source’s frame

Could someone please help me understand the derivation of the relativistic Doppler effect of light for objects moving away as derived from the source's reference frame? There’s no problem for me to ...
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Why frequency does not change when light passes through the denser medium? [duplicate]

as far as I noticed always people in physics have a predefined assumption that frequency is constant. whereas we know that the c is the outcom of product of wavelength and frequency. we have different ...
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Relativistic doppler efect

I got a little confused with the derivation of doppler effect for light as showed on wikipedia. The derivation states that when the source of light and the receiver are moving away from one another, ...
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1answer
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The nearly unmoving heavy star?

The interior Schwarzschild metric $$c^2 {d \tau}^{2} = \frac{1}{4} \left( 3 \sqrt{1-\frac {r_s}{r_g}}-\sqrt{1-\frac{r^2 r_s}{r_g^3}} \right)^2 c^2 dt^2 - \left( 1-\frac{r^2 r_s}{r_g^3} \right)^{-1} ...
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Black Holes without Schwartzschild Radius condition? [duplicate]

Reading this comment makes me ask what does that actually mean that In fact, if you are close enough to $c$, the mass-equivalent of your kinetic energy may be enough to make you appear as a black ...
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Reason for change in colour of light

We all know that light has constant speed, and variable wavelength, and frequency. Suppose I have a big red glass square sheet. When white sun rays pass through it, their colour changes from white to ...
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Gravitational field (metric tensor) and speed of light between two massive plates

Suppose I have two massive plates of size $l\times h\times w$ mounted parallel to each other with a distance of $d$ and with a mass density of $\rho$. I send a light beam in the middle between them ...
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1answer
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Why has Einstein gotten the credits for “inventing” Special Relativiy, while Lorentz wrote down the Lorentz tranformations first? [closed]

At the heart of Special Relativity lie the Lorentz transformations, invented by the Dutch physicist Hendrik Antoon Lorentz (the name of the transformation was given by Poincaré in 1905). He wrote them ...
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Would a relativistic collision create a black hole?

Imagine an ultra relativistic object with a very small rest mass, but an extremely high kinetic energy in our reference frame. So high that a stationary object with the same amount of energy would ...
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Help understanding the speed of light in media with refractive index < 1

I have read the standard explanations on this, but I still have trouble with convincing myself that information is not propagating at faster than the speed c in media of refractive index less then ...
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Meaning of the term speed of light in context of an object of non-zero mass

We know from GR that an object of non-zero mass cannot propagate at light. I have a problem with the term speed of light. Does it mean the relative speed of that object with another object or frame of ...
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Changing Refractive Index

Is it possible that the refractive index of a material could change?if so,what are those factors and what kind of variation could they bring?
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1answer
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The Meaning of $u=c\sin\theta$ Equation in Special Relativity [closed]

In the Introductory Special Relativity book, by W G V Rosser, page 19, the author is formulating the relativistic kinetic energy equation. In his derivation, he writes the following equation: $$u=c\...
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What forces are experienced when someone running near the speed of light jumps?

Suppose I am running near the speed of light, say 0.87c so that my gamma factor is exactly 2. What forces will I and the ground experience if I jump? Imagine it's a normal two-footed jump where I take ...
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The facts of travelling at the speed of light [duplicate]

I’m travelling at the speed of light. If I hold a mirror in front of me could I see my face?
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Effects of Relativistic/Inertial Mass

What would happen to a ball of ice in a remote region of space if spun in a circle accelerating toward the speed of light? As the ball of ice approaches the speed of light, does it melt/sublimate as ...
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A thought experiment about time dilation

Suppose you are on am infinitely long train traveling on tracks and an atmosphere that has no resistance whatsoever. No this train is traveling at fractionally below the speed of light, and you are on ...
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Would close to light speed travel be possible if all atoms in the system were instantly given “forward motion”?

I have heard people say that based on our current understanding and ideas for possible/current technology no person would ever be able to even travel at a solid (70-80%) the speed of light because ...
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Which of these two is Einstein's greater contribution? [closed]

One, speed of light is constant and nothing can be faster than it. Two, space and time should not be viewed as two things. They are one thing called spacetime. Presumably, Einstein brought these ...
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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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Is speed of light continuous when entering a medium?

I know that light has the speed $c$ in vacuum and speed $c/n$ in a medium with refractive index $n$. I wonder how this exactly happens - is there some kind of smooth transition? If so, on which scale?...
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Why don't spacecraft accelerate to the speed of light, or just fractionally below that?

In space there is almost nothing to slow you down, and your proppelent/S will always be traveling relatively faster than your craft so you should accelerate infinitely close to the speed of light ...
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$c$ slowing down rather than universe expanding?

Can the Friedmann equations be reformulated with taking the universe as being rigid and instead have the vacuum-speed of light in the void decrease over time? So, not changing any equations, just ...