# 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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### Relationship between poynting vector and radiation pressure

I know that When an electromagnetic wave is absorbed or reﬂected by a surface, the momentum of the wave is transferred to the surface. I also know that for absorption the radiation pressure is 〈S〉/c ...
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### If we know the distance of a distant light emitting object, how could we know the age without knowing how long the light has been here?

I have been contemplating the age of our observable universe and I was unsure as to how we could tell the age by only divdeing the distance by the constent with out knowing how long the light has been ...
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### Why do two moving inertial frames of reference atribute the same velocity to each other?

In both relativity and Newtonian mechanics, it is a self-evident axiom according to which two inertial observers who move relative to each other measure the same velocity of, say, $v$. I want to know ...
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### How was the value of vacuum permittivity originally found?

The vacuum permittivity appears originally in Maxwell's equations, used to describe electric fields. The permeability of vacuum was defined using Ampere's force law (itself derived from Biot-Savart ...
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### Relativity thought experiment - spaceship travelling at speed of light [closed]

I have basis knowledge in special relativity, but I haven't studied it for too long and I'm stuck in a simple thought experiment. Assume that there is a spaceship departing from the earth at speed of ...
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### Trying to understand velocity addition and time dilation [duplicate]

I'm familiar with Einstein's formulae $V=\frac{u+v}{1+ \frac{uv}{c^2}}$ and $\Delta t'=\frac{\Delta t}{\sqrt{1-\frac{v^2}{c^2}}}$, the former being velocity addition and the latter being time dilation....
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### Rule of addition of velocities in water

Is the speed of light still the same for all inertial observers in water? If not, what are the rules of addition of velocities in water according to special relativity?
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### Exceeding the speed of light [closed]

I understand that the speed of light c is derived from the self-interaction between elections/photons, and is thus the maximum speed of anything composed of electrons/photons. Suppose that there is a ...
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### Einstein's frozen wave?

Here's the quote from Stephen Hawking's latest book. At the age of sixteen, when he (Einstein) visualised riding on a beam of light, he realised that from this vantage light would appear as a ...
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### Thought experiment - faster than speed of light travel [closed]

I came up with a thought experiment and it has been on my mind for quite some time now. Would like to hear the thoughts of others as my wife has no clue what I'm talking about. Given two points of ...
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### Do Maxwell's equations predict the speed of light exactly?

I know that $\frac{1}{\sqrt{\mu_0\varepsilon_0}}$ is equal to the speed of light but is this prediction accurate? I mean is it 100 percent accurate?
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### Paradox of the magnetic field's collapse [closed]

Imagine two strong electromagnets (X and Y) enough far from each other. Facts: 1.) We know the magnetic fields spreading with the speed of light. 2.) When the magnetic fields reach the other ...
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### How can individual neutrinos have different amounts of energy?

Photons have no mass, travel at the speed of light, and their energy is related to their frequency. Neutrinos have a very small mass, travel at almost the speed of light; what is their energy related ...
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### If the speed of light is a limit, is there a way to tell which direction we are moving through space and how fast?

Say two photons start at point A, and go in opposite directions. They will be traveling apart at 2x the speed of light correct? So if this limit is how fast you are able to travel through space (and ...
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### Can someone travelling close to the speed of light witness the end of the universe? [closed]

I have read that objects with non-zero rest mass cannot attain light speed $c$. But for the sake of this question let us assume that Mr. X can get as close to $c$ as he wants. So this means that an ...
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### Why the speed of light is independent of the relative motion of observer? [duplicate]

As we use Lorentz transformation equation to relate velocity of particle measured by observer which is in frame s this frame is in relative motion having some velocity to that particle so Why the ...
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### How close should you get to speed of light, in order for time to be dilated?

Recently I was watching Carl Sagan's Cosmos: A Personal Voyage. In episode 8 ("Journeys in Space and Time") there is a scene presenting the idea of time dilation, due to traveling close to the speed ...
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### Infinite Speed of Light

I recently watched a video that stated that Newtonian Mechanics assumed an infinite speed of light. That same video, "PBS Space Time: The Speed of Light is Not About Light", stated that if the speed ...
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### Is spacetime defined mathematically without using $c$ speed?

Is there a mathematical definition of spacetime that does not use $c$ speed as a conversion factor or involve the spacetime interval? If not why?
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### Velocity of massless particles

Due to the equation $E = \frac12{mv}^2$, can a massless particle travel at an infinite speed in a vacuum (as its mass would be $0$ so its energy would also be $0$)?