At Least There Is Symmetry



11 June 2015

Richard Feynmann giving a physics lecture.
Richard Feynmann Lectures, CalTech
Richard Feynmann giving a physics lecture.

If an atom absorbs a photon, it’s apparent that the photon should haven been emitted by an atom or molecule at some earlier time. Alternatively, if an atom emits a photon, does that imply it should be absorbed by one other atom sooner or later sooner or later? Intuitively that looks as if a really odd thought, as if a flashlight couldn’t shine except there was a wall for it to shine upon. And but physics would appear to indicate that if absorbed photons have to be emitted, emitted photons have to be absorbed. All of it comes right down to the time symmetry of physics, and it was a subject of Richard Feynman’s doctoral dissertation.

Emission and absorption are time symmetric.
Physics Stack Change
Emission and absorption are time symmetric.

One of many primary concepts in physics is that straightforward interactions between objects are symmetric below time reversal. For instance, suppose you made a video of two billiard balls colliding and bouncing off one another. For those who performed the video backwards, it might nonetheless seem like two billiard balls colliding. For a easy interplay you don’t have any option to decide which model of the video was the “forwards” one, and which was the “backwards” one. Whenever you apply this to interactions between electrons and photons, the absorption of a photon appears to be like precisely just like the emission of a photon “performed backwards.” This implies if an absorbed photon have to be emitted, by time symmetry an emitted photon have to be absorbed.

This concept turns into much more weird should you think about astronomical distances. For those who had been to lookup on the Andromeda Galaxy within the night time sky, you see mild that has traveled for about 2.5 million years. Meaning a selected photon emitted by a star in Andromeda 2.5 million years in the past should have by some means “identified” that it might attain your eye. In actual fact, due to relativity, from the photon’s perspective your eye absorbed it the moment it was emitted by a star. You would possibly balk at that concept, but when we reversed stuff you’d haven’t any bother with the concept that the sunshine you see was emitted 2.5 million years in the past. In relativity, the 2 are the identical, since trigger and impact rely upon your vantage level.

What Feynman confirmed was that regardless of it’s oddness, the requirement that emitted mild be absorbed doesn’t violate causality. It got here to be referred to as Wheeler–Feynman absorber principle (John Wheeler was Feynman’s advisor). There have been some issues with the mannequin, nevertheless. Particularly Feynman assumed that prices couldn’t self-interact. In different phrases, an electron couldn’t emit a photon solely to reabsorb it later. In fact there’s no actual motive why that ought to be forbidden, however should you permit it within the principle you get a divergence of interactions and the mannequin breaks down. This led Feynman to desert the mannequin finally, nevertheless it was deeply influential in his improvement of quantum electrodynamics, for which he was awarded the Nobel prize.

So is it the case that any emitted photon should be absorbed? We aren’t certain, however we will’t rule out the thought both. It’d simply be that after we observe mild from probably the most distant galaxies, the photons we detect are merely arriving on the vacation spot they’d all alongside.

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