View Full Version : When Does Atomic Recoil Occur?

Peter Wilson
2006-Apr-05, 10:25 PM
Quantum entanglement of a certain sort has been demonstrated in the lab. A pair of photons, say, is created with no net polarization. So if you measure the polarization of one, the other is "instantly" endowed with the opposite polarization. Although mysterious, "conservation" is evidently behind this, in this case, conservation of polarization.

But has anyone measured quantum entanglement of recoil? Here is the thought experiment: a single atom of hydrogen in space emits a photon. Theory says this creates a spherical wave, like a stone in a pond creates circular waves. In my understanding of QM, the direction the photon takes is not determined (or poorly defined) until something absorbs it. Like the entanglement of polarization, the spherical wave emitted by the atom suggests that the atom does not recoil until the photon is absorbed, and the "direction it went" is determined.

This seems really wierd, but typical of QM. Is this correct? If yes, have experiments been done to verify it?