If a photon does not experience time, what is the interaction like when it meets an electron? What is it like for an electron to absorb or emit a timeless entity?

A2A#1. Photons are massless and cannot have co-moving observer. There is no way that a photon (i.e. a co-moving observer), can experience anything.

A2A#2. A photon is absorbed when it meets an electron. Poof. Gone.

A2A#3. A2A#3. This energy difference+ (?) must be resolved somehow and within the time limit (determined via Heisenberg’s indeterminacy relation


  1. The material in which the electron was originally placed absorbs the excess energy or, alternatively, the recoil momentum. This is the photoelectric effects

The 1905 explanation earned it the title of. Albert Einstein

  • The 1921 Nobel prize.
  • The electron emits some thing and then gets back “on shell”. Because of conservation of angular momentum and charge, choices are limited. Instead, the electron emits (different) photons. This is the Compton scattering

The 1923 explanation earned the title of “The Secrets of the Universe”, Arthur Holly Compton The 1927 Nobel Prize in Physics

  1. .
  2. There is no other option.

The Quantum electrodynamics is more in-depth.

Calculation requires computing the matrix elements that correspond –to lowest order (a.k.a. “tree-level”)–to two Feynman diagrams.

[In the diagrams, time moves upward; cribbed form “ Advanced Concepts Particle & Field Theory

” (Cambridge U. Press, 2015, ISBN: 9781107097483)]

This illustration shows the location of the above (heuristic description) of the process. The precise interpretation of the diagram is dependent on the observer’s choice. Some observers believe that the 1-3 interaction is after the 2-4 one (photon absorption), in which case it is identical to the diagram. Other observers think that the 1-3 interaction is before the 2-4 one (photon absorption), in which case historical interpretation is that an electron emitted the “other”, before it absorbed and emitted the “incoming”. However, the two matrix elements are algebraically related and only one must be computed.

† Since the OP asks “what is it like for an electron,” we’ll work from it’s point of view—i.e., the point of view of an observer for which the electron was at rest before it absorbed the hapless photon. The rest energy of the electron is , while its momentum is . The incoming photon has energy , and the linear momentum (we’ll ignore the other directions) of . Conservation of energy-momentum then dictates that

The electron’s total energy and linear momentum is determined right after it has been ababsorbed by the photon. These values of energy, linear momentum and total energy clearly don’t satisfy the “on-shell” condition.

The difference is


Expanding in powers of, assuming that. The Reader will be able to determine the electromagnetic radiation wavelength which serves as the boundary between the two regimes. 🙂

++ The “Heisenberg-time duration” () that is allotted for settling the energy gap is not due to Ms. Vacuum is offering this lay-away plan at a generous pay-back, but Heisenberg’s indeterminacy relationship

It is stated that energy and time can not be measured simultaneously with greater precision than the.

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