Speed of light - why is it a constant?

[ravisastry]

the way I am looking at this is...speed of light, is not its inherent characteristic, rather it is soley dependent on the permittivity and permeability of the medium.
If we start thinking in this way, then we probably can have a reasoning for the 2nd postulate of SR...
No matter what the speed of the source emitting light, photons will travel with a speed depending on permittivity and permeability of environment surrounding the source. Hence, in vaccum, its always C

 

[juanrga]

What makes the speed of light a constant. I read the FAQ on special relativity but still don't understand why c (speed of light) exists as a constant.

It's like a rule like many others, why do they exist? Is there a part of space-time that limits this speed. Why are all the photons that ever existed limited by this speed?

Why a cat is not dog, because it is not a dog. In our universe c is a constant, because this is how our universe is.

 

[ghwellsjr]

the way I am looking at this is...speed of light, is not its inherent characteristic, rather it is soley dependent on the permittivity and permeability of the medium.
If we start thinking in this way, then we probably can have a reasoning for the 2nd postulate of SR...
No matter what the speed of the source emitting light, photons will travel with a speed depending on permittivity and permeability of environment surrounding the source. Hence, in vaccum, its always C

No, photons travel at c. Why? Because we define them to travel at c. Einstein's second postulate defines all light to propagate through empty space at c. We cannot measure the speed of a photon or the propagation of light since these are one-way traverses and we have nothing with which to communicate back to the timing device at the source when the photon or light has reached its destination. We can only measure the round-trip speed of light because it now has arrived back at the source and we only need one timing device to make the measurement. Einstein postulates that the one-way speed equals the two-way speed for any inertial observer but it can never be measured, proven or demonstrated. So, there is no reasoning or explanation for the 2nd postulate, Einstein just uses this definition to devise his concept of a Frame of Reference.

Photons can only travel through empty space, even if it is only the short distance between a proton and an electron in an atom. As I said before, the apparent slowing down of light through a medium is due to the photons not taking a straight, unhindered path through the medium but rather interacting with the protons and electrons in the medium, being absorbed and re-emitted. But nobody can keep track of all those particles and so we don't model light in this way as it travels through a medium, rather we look at the net effect and part of that model includes permittivity and permeability which explains why an untold trillions of photons average out to travel at a speed less than c. But you're never going to use that explanation to tweak light to go faster than c by fiddling with the permittivity and/or the permeability, whatever that could possibly mean.

 

[PatrickPowers]

No, photons travel at c. Why? Because we define them to travel at c. Einstein's second postulate defines all light to propagate through empty space at c. We cannot measure the speed of a photon or the propagation of light since these are one-way traverses and we have nothing with which to communicate back to the timing device at the source when the photon or light has reached its destination. We can only measure the round-trip speed of light because it now has arrived back at the source and we only need one timing device to make the measurement. Einstein postulates that the one-way speed equals the two-way speed for any inertial observer but it can never be measured, proven or demonstrated. So, there is no reasoning or explanation for the 2nd postulate, Einstein just uses this definition to devise his concept of a Frame of Reference.

Photons can only travel through empty space, even if it is only the short distance between a proton and an electron in an atom. As I said before, the apparent slowing down of light through a medium is due to the photons not taking a straight, unhindered path through the medium but rather interacting with the protons and electrons in the medium, being absorbed and re-emitted. But nobody can keep track of all those particles and so we don't model light in this way as it travels through a medium, rather we look at the net effect and part of that model includes permittivity and permeability which explains why an untold trillions of photons average out to travel at a speed less than c. But you're never going to use that explanation to tweak light to go faster than c by fiddling with the permittivity and/or the permeability, whatever that could possibly mean.

You might be right. I thought that it could be done like this: send out a nice generous light pulse and throw some dust into the space. An observer can follow the light pulse by observing the reflections off of the dust. Presumably the photons that were not reflected were moving at the same speed.

By the way, there is a neat effect of this. Imagine light is very slow. You have a mirror one light/second away. You would see the reflections from the pulse take two seconds to travel to the mirror, then a big flash since all the reflections on the way back would arrive at the same time.