# Light speeds

What gives light its speed? And why does it travel at c, not faster or slower?

## Answers and Replies

Tide
Homework Helper
The magnetic permeability $$\mu _0$$ and electric permittivity $$\epsilon _0$$ of space determine the speed of light. That's about as fundamental as you can get since those are properties of empty space and I don't think anyone knows why they have their particular values.

Personally, I think the velocity of light through space has to do with the rate at which the energy is emitted and absorbed. Like in space, it is emitted and absorbed at a rate that allows it to go $3x10^8~m$ in one second. This value differs from water. It is emitted and absorbed to a lesser extent which gives it a slower speed. Those are just PERSONAL thoughts. I strongly agree with the magnetic permeability and electric permitivity; I just think it is easily explained through emission and absorption.

Gonzolo
h8ter said:
Personally, I think the velocity of light through space has to do with the rate at which the energy is emitted and absorbed. Like in space, it is emitted and absorbed at a rate that allows it to go $3x10^8~m$ in one second. This value differs from water. It is emitted and absorbed to a lesser extent which gives it a slower speed. Those are just PERSONAL thoughts. I strongly agree with the magnetic permeability and electric permitivity; I just think it is easily explained through emission and absorption.
Be careful with personal theories, there is a special section for that. I'll just point out that it doesn't hold if only one photon is emitted, in which case you cannot define a rate.

The value of c is only a matter of what units you use, and it is quite simply the asymptotical value that a mass cannot attain. It is entirely possible to define c = 1 with the appropriate units. If there was another "special speed", one could ask why a certain factor between both, but since it is quite unique, I don't see a meaning to the question.

I dont think we will ever know why the constants of our universe are what they are, not only the speed of light, but also, planks constant, the gravitational constant, ect.

According to Special Relativity, particles with zero mass must travel with the speed of light. So the reason why photons travel with that speed is because their massless. Gravitons (if they exist) also travel with speed of light. So you could also call it "speed of gravity", and say photons travel with that speed. Same idea.

The question why the speed of light is about 3*10^8 m/s is still open ofcourse...

phenylalanine said:
What gives light its speed? And why does it travel at c, not faster or slower?
Well, it travels slower through atmosphere...

Is it true that light travels slower though a medium because the photons are absorbed and emited?

phenylalanine
What gives light its speed? And why does it travel at c, not faster or slower?
These are questions so basic to physics that they may never be literally known. That c has the units of velocity or its current numerical value most probably does not translate to some other regions of infinite space (likewise for other fundamental constants). Isn't it said that physics answers only "how" questions?

ArmoSkater87
Is it true that light travels slower though a medium because the photons are absorbed and emited?
I believe you are close to correct.