Light Q

[vrus]

This might sound dumb; or paradoxical depending on how u look at it.

 

We all know that light has the highest speed practically.

 

Einstein enlgihtened us by telling us that anything at the speed of light has infinite mass and will be too heavy to move. He used the speed of light as a constant. BUT ! And a very big BUT too!

 

How is it that light itself manages its speed ? Is there any explanation for this ? Or have I just lost it ?

[mezarashi]

The most simple explanation for this is that light particles have no mass! Anything with mass will not be able to achieve the speed of light. But photons along with all its zero mass cousins such as the graviton also travel at the speed of light. This means that changes or fluctuations in the gravitational or electromagnetic fields are felt at the speed of light.

[Yggdrasil]

Maxwell's equations predict that electromagnetic waves (i.e. light) in a vacuum must have a certain speed of propagation, defined by the permittivity of free space and permeaility of free space. Einstein reasoned that since Maxwell's equations must be valid in all inertial frames of reference, light must propagate at the same speed in every inertial frame of reference, and after a few thought experiments, he came to the conclusion that nothing could go faster than the speed of light.

[vrus]

So, that explains why gamma rays and all other electromagnetic waves travel at the speed of light and why they have no mass!

[Edward]

Can EM "stuff" ie light and radio waves travel "faster than we have observed is the speed always constant? For EM is there any acceleration or deceleration?

[mezarashi]

No, to any observer, light will always travel at the speed of light in a vacuum. As far as I know, light is not known to speed up, but it can most definitely slow down when propagating through a medium as with any other wave. Upon a change of medium, the wave changes velocity. That's where you get refraction and this constant known as the index of refraction. Diamond has one of the highest indices, greater than 2, meaning that light travels through diamond at half the speed it does through a vacuum.

[Edward]

What slows the light down? It is massless so mass shoulden't slow it down.

[mezarashi]

Well, that requires a bit of an explanation on the propagation of light through matter. Generally, light doesn't even go through matter in the first place (opaque items), but we have a class of optically transparent or atleast translucent materials. The photons will strike the medium's atoms and excite the electron to a higher state, when the electron comes back down again, it releases a photon of the same frequences in the same direction as the original photon. This continues in a chain until light exits the medium or finally gets absorbed somewhere. So in fact when light propagates through a medium, you are continuously creating and destroying light. The "mechanism" for which this happens will determine the speed of light through the medium.

 

There is also a matter of loss. That's why things don't seem as bright through glass. Your sunglasses for example will restrain the vibration of electrons and thus the ability of light to pass through. Polarized glass will restrict vibration of electrons in a certain direction thus polarizing the light waves.

[Edward]

Thankyou that was a vary good explanition I understand our univers a little bit better now:P

[FrankM]

It is recognized that the speed of light is dependent upon the medium in which it is propagated. The speed of light we use as a reference was measured in a vacuum on the Earth's surface. We also know that light can be bent by gravity, just like light is bent going through other mediums, like glass.

 

Do we know whether light is "slowed" while being bent by gravity or does it just bend while keeping the same speed? I think that violates something.

 

What assurance do we have that the speed of light measured in a vacuum on the Earth's surface is representative of "free space"? I would like to denote "free space" as being well outside of the Sun's heliosphere.