What's the problem with bremsstrahlung?

[Bashyboy]

Hello,

I am reading about the bremsstrahlung phenomenon, and am having difficulty seeing how this phenomenon clashes with classical physics. Could someone possible explain the process, and why there is no classical physics description for it?

 

[stevendaryl]

I think bremsstrahlung is a classical phenomenon. It's just the tendency of accelerated charges to radiate, which follows from Maxwell's equations. The details might be different in quantum mechanics, but I don't think that the effect is exclusively quantum.

♫ Bremsstrahlung my friend, don't you start away uneasy...

 

[vanhees71]

That's true. Bremsstrahlung is one of the few German words making it into English. Literally translated it means "braking radiation" ;-)). The name comes from the early days of electrons (discovered by J. J. Thomson 1897) and electron theory. It just describes the phenomenon that accelerated charges radiate. It is not specifically quantum but a prediction of classical electromagnetism (Maxwell's greatest discovery). It's described by the solutions of Maxwell's equations with accelerated particles as the four-current source of the electromagnetic field, the retarded potentials (or the electromagnetic field itself, i.e., the Jefimenko equations). This special solution is also known as the Lienard-Wiechert potentials.

Quantum theoretically the only correct description is relativistic quantum field theory as is always true as soon as the electromagnetic field is involved. Often it's said that bremsstrahlung is described by a Feynman diagram, where a photon line is emerging from a charged-particle line (e.g., an electron or positron line in fermionic QED). Ironically this is a prime example for the fact that it can be very misleading, when the "photon explanation" is invoked in a somewhat careless way. It's of course the leading-order diagram to describe, e.g., the emission of em. radiation for an electron moving in a static field of a very heavy particle like an atomic nucleus (which is a good approximation in this case, as can be proven from QED; see Weinberg, QT of Fields, Vol. I). What comes out from this single Feynman diagram is usually the first encounter with an infrared divergence in QFT. The solution is the resummation of soft photon diagrams (including the leading-order radiative corrections for the elastic scattering of the electron on the external field), and this leads more to a field-like picture of the bremsstrahlung than a particle picture in terms of naively understood photons. So the classical-field picture is a good intution for bremsstrahlung even from the point of view of QFT!

 

[Bashyboy]

The reason I ask is because I am reading French and Taylor's Quantum Physics, and evidently there is a problem with this phenomenon with respect to classical ideas. I did not quite understand what they wrote on this phenomenon, and how it clashes with classical ideas, thus the reason I asked that question on physicsforums.

 

[Vanadium 50]

French and Taylor discuss this on page 21, and explain it on page 22. What part of 22 is confusing to you?

 

[Khashishi]

The tricky thing about bremsstrahlung is that there is this integral over the impact parameter which diverges if you don't set proper limits to the integral. In some situations, you need quantum mechanics to set these limits. That's the only way it "clashes".