Can someone explain how QED explains refraction?

[ashesfall_x]

I am really confused about how to interpret refraction from a QED standpoint.

[Klaynos]

If you consider the spacial, momentum uncertainty principle.

 

You have a slit, a photon moving towards it in the x direction. As it passes through the slit it's y position component is restrained to the slit size. This means the uncertainty in y is known, and finite (non-zero), so the uncertainty in the y momentum must be finite and non-zero. So the photon now has a range of positions it can be within this uncertainty...

[BenTheMan]

Well the asymptotic states in QED are plane waves, so it explains refraction in the same way that electromagnetism does.

 

The scale of the problem is ALWAYS important. So here goes---a lesson in the power of dimensional analysis.

 

If you KNOW the QED lagrangian, let me ask you---how many dimensionful parameters does it have? Just one---the mass of the electron. So where do you expect the theory to be important? Well, mass is the same as inverse length (using God's units), so the length that you expect to see QED effects become important is 1/m, or about 10^-12 meters. So you would only expect QED to be important on those length scales. AND, seeing as how 10^-12 meters is about a tenth the Bohr radius, you're not likely to ever build a diffraction grating this small.

 

For all other lengths, QED effects aren't important.

 

ashes---if you're interested in physics, it is very important to know how to use these naive dimensional analysis arguments. They seem stupid, I know, but they are right almost all of the time---at the very least, they can give you bounds on things that you wouldn't have otherwise been able to calculate!

 

Also, you should learn your orders of magnitude too

http://www.matpack.de/Info/Tables/meter.html

You would be surprised at how much physics is actually done this way.

[BenTheMan]

ashes---was this explanation good enough or do you want me to clarify anything?