Oh dear, here we go again.
I know that there are people out there who have a hard time accepting
quantum mechanics, but yes, each "photon" really does interact with
EVERY ELECTRON IN THE CRYSTAL at once. Young's double-slit experiment
is the simplest form of diffraction, which he performed in 1801 to
settle an argument about the wave vs particle nature of light. The
tricky bit, however, was performing the experiment at such low flux that
only one "particle" is "in flight" at any given moment (Dontai et al.
1973, done with electron diffraction). However, if you do this, you
still get an interference pattern at the end of the day:
http://en.wikipedia.org/wiki/File:Double-slit_experiment_results_Tanamura_2.jpg
This is the observation, and Mother Nature doesn't care if we like it or
not. The only way we can interpret it is to conclude that the photon
must be "interfering with itself".
In fact, anyone with a Pilatus detector (and a lot of extra beam
time) can verify the self-interference of photons in macromolecular
crystal diffraction. Since the source-to-detector distance of a typical
MX beamline is about 30 m, it takes 100 nanoseconds for a "photon"
generated in the storage ring to fly down the beam pipe, do whatever it
is going to do in the crystal, and then (perhaps) increment a pixel on
the detector. So, as long as you keep the time between photons much
greater than 100 nanoseconds you can be fairly confident that there is
never more than one photon anywhere in the beamline at a given instant.
Now, one photon per 100 nanoseconds is 1e7 photons/s, which is a pretty
weak flux for an MX beamline, but not too bad. Ideally, of course, one
would want to read out images from the Pilatus that contain at most one
photon hit each. You would then sum these images all together after the
experiment to confirm that you do, in fact, get the same diffraction
pattern that you would have gotten using higher flux. I am willing to
take bets on this! Then again, this is exactly how the old multi-wire
detectors worked (at much lower flux). So, I suppose this experiment
has already been done?
The other observation that can only be explained by quantum
mechanics is the fact that electrons orbiting atomic nuclei do not emit
synchrotron radiation. Amazingly, this latter conclusion was first
reached by Debye in the introduction of his now famous paper on
temperature factors (Debye, P. J. W. 1915, Annalen der Physik 351,
809-823). http://dx.doi.org/10.1002/andp.19153510606
Yes, this is the paper where Debye first defines what we now call the
"B-factor" (although he doesn't use "B"), as well as the first
occurrence of the Lorentz factor (which appears in a second
note-added-in-proof). If you read it (perhaps with the help of Google
translate), you can tell that the conclusion about the de-localized
nature of electrons in atoms was deeply disturbing to him as well.
After all, we are all scientists, and letting go of determinism is not easy.
-James Holton
MAD Scientist
