# Collision between a photon and an electron

Looking through this AP Physics question, I was struck by how the 'collision' between a photon and electron looks so much like a macroscopic collision. Is this even physically possible?

Look at the last page of this pdf: http://lodischool.tripod.com/dovesol/DOVE02SOL.pdf

EDIT: Some more questions:

How could a photon collide with an electron, when their positions cannot be determined exactly? Also considering how very small the electron is, I doubt that it is even possible to make the two collide; and if it is, how could you possible detect that? It also seems as though the photon and electron are acting as particles, which seems to me not to be the whole story.

What if I put the electrons behind a double slit apparatus, and treat individual photons as particles? Based on this "compton scattering," it's possible for the photon to be deflected any which way. I could claim that the diffraction pattern observed in the double-slit experiment is due to compton scattering, among other factors. Prove me wrong!

• Why the -1? Is this a bad question? I would like an explanation of how a collision at such small scales would work.
– Greg
Commented May 22, 2013 at 13:59
• I can't speak to the downvote, but adding a link to a n-page pdf and expecting you reader to page through it is not making things easy for someone you are asking to help you. The question appears to concern itself with the backwards limit of Compton scattering. Commented May 22, 2013 at 14:17
• I specifically pointed to the last page, not asking the reader to try and find what I'm referring to.
– Greg
Commented May 22, 2013 at 14:24
• "I could claim that the diffraction pattern observed in the double-slit experiment is due to compton scattering, among other factors." The cross-section for Compton scattering can be measured and is a simple function of angle that does not correspond to diffraction patterns. I know that pop-sci books make it seem reasonable to think about physics in terms of word pictures, but you must learn the math. Commented May 22, 2013 at 16:32
• Does this answer your question? What happens if an electron and a proton fall into each other?
– Babu
Commented May 16, 2023 at 6:59

Here are real events relating to the last page of the pdf link you gave:

Fig.1 This bubble chamber picture shows some electromagnetic events such as pair creation or materialization of high energy photon into an electron-positron pair (green tracks), the Compton effect (red tracks), the emission of electromagnetic radiation by accelerating charges (violet tracks) (bremsstrahlung) and the knock-on electrons or delta ray (blue tracks)

Photons are invisible in bubble chambers as they interact only with direct collisions with electrons, called Compton scattering, or pair production in the field of a nucleus. Charge particles turn in the perpendicular to the plane magnetic field; this allow us to measure their momentum and charge and the ionisation of the tracks allows the identification of masses.

At the lower left of the picture, there is an electron( identified by its ionisation) which loses energy into a photon, and the photon pair produces some centimeters away, an electron positron pair.

in the middle right side, we see a positron that loses energy into a photon and the photon kicks an electron from the atoms of the chamber, this is a Compton scatter. This corresponds to the diagram in the last page of your link, except it has been reduced to one dimension. In reality there are two dimensions, because the photon gives part of its momentum/energy kicking at an angle . The following is the correct diagram kinematically:

It should not be surprising that classical scatters and particle scatters kinematically are the same, because momentum and energy conservation hold both classically and quantum mechanically. It is the probability of interaction that is different in the microcosm of elementary particles to the billiard ball particle scattering. In simple scattering experiments the kinematics are not different ( except that special relativity holds in the microcosm).

Edit after edits in question. I think the image , from a real experiment, answers whether a photon can hit an electron or not.