This one was inspired by a post by John Baez, reminding us that Maxwell’s equations are the seat of the photon and electromagnetism, bringing us back to his classic “What is a photon” which I encountered in some form on the newsgroups in apparently the late 90s. I was about to say how all the great online and education work of this guy and his stellar reputation shows that publication records aren’t really everything.. then I went and checked and it turns out he has an off the scale h-factor as well. Wow, salute to John Baez. I guess Tom Roberts, who I encountered at around the same time, salute to Tom Roberts, is a better example of how reputation is not well reflected by such indices, at least in his influence which I saw on my path through the physics literature.
So, on to “what is a photon?”. Capturing the thing in another “light”…. is Kim Stanley Robinson writing in “Ministry for the Future” :
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I zing and I ping and I bring and I bling. Freed to self in the heart of the sun, I banged around in there for a million years before popping out the surface and zipping off. Then eight minutes to Earth. In the vacuum I move at the speed of light, indeed I define the speed of light by my dance.
Into Earth’s atmosphere, exhibiting aspects both wave and particle but not either, I am a four-space conceptualized as an hourglass shape in three-space, where time and space cross in the human mind. Hitting things again, slowing down, breaking off brothers and sisters trapped in things, all the same, all without mass, all spin one, and so bosons, not fermions; one three-hundred-sixty-degree turn and I’m back where I began in that regard, while for fermions it takes seven hundred and twenty degrees of turning to get back to its original position; fermions are strange!
I am not strange, I am simple. Banging into atoms and moving them as I too move, simple as Newton, bang bang bang, the atoms of the atmosphere move more than they would have without my kick in their pants. That’s heat. Until I kick into something that captures me and I stop moving on my own. That or I might bounce off something I hit and head back out into space, become the light in the eye of some lunatic observer, looking up at the big blue ball and seeing me bang something in their retina. A blue pixel of the utmost granular fineness, so much so that the wave I make is easier to detect, maybe even easier to imagine. The wave/particle duality is a real thing, and both at once is hard to think, hard to see. It doesn’t really work, four-space in your three-space mind, no. I am a mysterious thing, massless but powerful. There are more of us than there are of anything else. Well, perhaps that’s not true. We don’t know about dark matter, which should be called invisible matter, we don’t know them or what’s going on there. Presumably its individual constituent elements are something like me, but maybe not; no one knows. All that stuff flies around as if in a parallel universe slightly overlapping ours, maybe just waves, in any case gravity works on it for sure, because its very existence has been revealed to us by its gravitation effects. If we are similar to those dark-matterinos, it is in the way that light and dark are similar. Two parts of a whole, perhaps. I am visible, I embody light itself; dark matter is not actually dark, it is invisible, and we don’t know how or what it is. Our absent self, our shadow, our twin. Although there are a lot more of them than us, maybe. A mystery among all the other mysteries, we fly through each other like ghosts.
But me, me, me, me, banging Earth, bouncing back up into the eye of an observer on the moon, then off again, immortal, immutable, I bang bluely on and on and on, and in the process, passing by just this once in a cloud of my brothers and sisters, we hit Earth and light it up, and the gas wrapping the planet’s hydrosphere and lithosphere gets warmer from our touch. My brothers and sisters follow me and they continue that warming.
What am I? You must have guessed already. I am a photon.
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Very nice! So then what is a photon? Well that depends on what the meaning of “is” is. When we say what “is” a note, for example, correct answers could be “that dot on the music with a tail”, or “the sound waves which emanate when a key is pressed a harpsichord” or “a short bit of text written with intention of later reading for recalling or for reviewing information” or “a pulse of audible sound”. Similarly in the physics of quantum electrodynamics, the right answer to the question “What is a photon?” was given by our 42nd president: that depends on what the meaning of “is” is.
For example, lets consider that we see a signal on our oscilloscope:
Is this a photon? Well, it’s an oscilloscope, not a photon. But is it a photon showing on the screen? Well the mechanism of light emission from the screen is one of excited electrons in phosphorus emitting light, so those are photons, yes. But what are they representing that came into the antenna? Well it appears to be a continuous electromagnetic wave, probably not from a quantized emitter such as an electron in an atom, so it appears this isn’t really a photon. But wait, how have the signals registered in our retinas? Well the light from the oscilloscope has induced quantized absorption of light via electrons in the atoms in our eyeballs. So, those are photons. What about the signals from those visual cells which arrive at our brain? Well nerve cells transmit signals in a more continuous fashion, so we might say those light impulses are not photons. However some signals are mediated by calcium ions, so if a single ion moves across a synapse, perhaps we have a quantized signal and again a photon.
As for the elusive faux-ton, that might have to await explanation from another head of state. Thanks for processing the photons from your screen and turning them into meaning in your mind!