Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Questions re Compton scattering

  1. Mar 17, 2004 #1
    I am not sure this is the right topic for these questions. They are for the people who are familiar with Compton scattering. Let


    be the angle that the trajectory of the scattered photon makes with the trajectory of the incident photon.

    is zero. Does that mean that the incident photon misses the electron (or some target charged particle) altogether or it hits the electron but does not interact with the electron at all? I can imagine the incident photon usually misses the electron, but surely there should be some hits now and then. Therefore, Compton forward transmission (CFT - not really scattering) as I shall term it should happen. The formula predicts in this case that there shall be no redshift. I find this hard to believe. It's though the cue ball goes right through the 8 ball into the pool pocket without changing the position of the 8 ball at all.

    Has anyone done any detailed QFT analysis of CFT? If so, what were the results? As for experiments, I suppose it would be very hard to measure any redshift in CFT.
  2. jcsd
  3. Mar 17, 2004 #2


    User Avatar
    Science Advisor

    The angle of scatter for Compton scattering can assume any value between 0 and 180o. The probability of it being exactly 0 (or any other angle specified in advance)is 0. In other words, the probability density has no Dirac delta function component.
  4. Mar 17, 2004 #3
    Yes, a theoretical reason CFT is all but impossible to check cleanly. Let me see if I understand: As the angular spread around zero theta is made smaller, the redshift results become less reliable for verification purposes. I suppose one of you can put that in more technical language.

    What I think I can say: Redshift does happen, though we cannot ever know for sure whether that is from CFT or from theta very close to zero.
  5. Mar 18, 2004 #4


    User Avatar
    Science Advisor

    I am not sure what your question means, but essentially, if there is no energy change we can't tell if is was scattered straight forward or missed - there really is no difference. If there is any energy loss, there was scatter.
  6. Mar 18, 2004 #5
    My understanding is that we cannot measure radiation coming from theta = zero only. Rather we have to measure radiation integrated over a range of angles, zero <= |theta| < constant. Actually, a cone centered on the initial direction of the incident photon. The constant may be small but cannot be zero. Hence, we cannot know which photon in that range is a "straightforward" photon, and cannot thus verify the prediction of zero energy loss for theta = zero.

    Not only would an experimentalist say that, but a quantum mechanics theorist would.

    Am I correct?
    Last edited: Mar 18, 2004
  7. Mar 19, 2004 #6


    User Avatar
    Science Advisor

    The wavelength (or energy) can be measured quite accurately. Threfore if there was a change, there had to be a scatter.
  8. Mar 20, 2004 #7
    So, if you measured close to theta = zero and found a change in wavelength = scatter, you would conclude that the Compton formula value for theta = zero is wrong?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook