Does the electron “know” of an impending collision?

Recent research suggests that it acts that way:

Albert Einstein received the Nobel Prize for explaining the photoelectric effect: in its most intuitive form, a single atom is irradiated with light. According to Einstein, light consists of particles (photons) that transfer only quantised energy to the electron of the atom. If the photon’s energy is sufficient, it knocks the electrons out of the atom. But what happens to the photon’s momentum in this process?…

The question of which reaction partner (electron or atom nucleus) conserves the momentum of the photon has occupied physicists for over 30 years. “The simplest idea is this: as long as the electron is attached to the nucleus, the momentum is transferred to the heavier particle, i.e., the atom nucleus. As soon as it breaks free, the photon momentum is transferred to the electron,” explains Hartung’s supervisor, Professor Reinhard Dörner from the Institute for Nuclear Physics. This would be analogous to wind transferring its momentum to the sail of a boat. As long as the sail is firmly attached, the wind’s momentum propels the boat forward. The instant the ropes tear, however, the wind’s momentum is transferred to the sail alone.

However, the answer that Hartung discovered through his experiment is—as is typical for quantum mechanics—more surprising. The electron not only receives the expected momentum, but additionally one third of the photon momentum that actually should have gone to the atom nucleus. The sail, (electron), of the boat, (nucleus), therefore “knows” of the impending accident, (collision from the photon), before the cords tear and steals a bit of the boat’s (nucleus’s) momentum….

Goethe University, “Beyond Einstein: Physicists solve mystery surrounding photon momentum” at Phys.org October 1, 2019

Or something.

Hat tip: Philip Cunningham

See also: Electron’s near perfect roundness stymies the search for new physics

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