“Light is a wave and a particle”, I say glibly, to show that the world is too complex for people to understand, especially using language. So I was disappointed to learn that an electron is a particle, and not a wave as I had thought.
I started in The Guardian, which recommends various youtube channels for science and maths. I went to the Looking Glass Universe, where a woman who in 2018 was doing a PhD on quantum computing explains quantum mechanics. Why can an electron be in two places at once but not, say, an apple? Because the apple is continually located by light and other particles hitting it, but an electron is small enough not to be located like that. So the point where superposition ceases to be true of an object is where it becomes large enough to be located by light or by particles, and as I have heard that the nucleus’s ratio to the atom is like an orange to a football stadium, I am unsurprised, now, that classical mechanics does not apply to the electron. That just fascinated me. I can grasp it.
I learn from her that I had completely misunderstood Heisenberg’s Uncertainty principle, which might be better understood if the German unschärfe was translated as “fuzziness”. I thought it was her second suggested explanation: To find the particle’s position, you bombard it with light… which has high momentum so it gives some to the particle, and now we’re uncertain about the particle’s new momentum. But no, it is not that at all. Rather, the particle has lots of positions and momentums, but if it is “mostly” in a small range of positions then it mostly in a large range of momenta.
It has a “superposition” of momenta and locations- apparently more than one possibility for each- but “what that word actually means is the biggest open question in the field”. I had not understood. The idea I had made sense to me, and was completely wrong; it’s probably from some populariser, years ago. Once you measure the particle’s location, it will just be in one place, a random selection from the range of possibilities. So if you have a two slit experiment with an electron detector at the slits, it will only go through one. And then you can measure the particle’s speed, but again will find one particular speed, which is randomly one of the possible speeds. You don’t know from the measurement what the range had been before the measurement.
Two slit experiment? I remember around 1981 at school seeing interference patterns with waves in a water tank, and have seen explanations on the TV regularly since. So, I hope you have the general idea, like me; and since 1981 researchers and theorists have had different explanations of what particles are doing. The electron is
not a wave!
At least, according to this PhD student, whose animation is charming and whose enthusiasm is infectious. As a commenter says, her voice sounds as if she is smiling all the time. The range where the particle is likely to be is expressed as Δx, and if the range Δx of location is small, the range Δp of momentum is large.
Quantum mechanics can tell us what happens, but not why. And there is a joke about what theoretical physicists understand about experimental physics. Well, they could just do the beautiful maths (which I don’t have a clue about) but at some point some observations might be helpful. It’s not a wave, creating interference patterns, because one electron fired at a detector with a barrier with two slits in it gives one crisp dot. So again my understanding of quantum mechanics was wrong.
This video goes into the maths more deeply, then goes back to the double slit experiment. Why do physicists talk about the wave/particle duality? Possibly because it makes quantum mechanics sound mystical and difficult. She says it annoys me because it makes the world sound so paradoxical that we couldn’t possibly hope to comprehend it. Well, that was what pleased me about it- not because I wanted to sow confusion where understanding was possible, but because I wanted not to pretend to understand anything I really didn’t.
Youtube is not an ideal place to see these things. If I see them on BBC4, I trust that they are a good explanation of current understanding, and if they are not they will be criticised. Youtube has anti-vaxxers sounding as plausible as anti-vaxxers can sound, and even flat-earthers (I read somewhere).