Why nobody understands quantum physics
Richard Feynman once remarked that nobody really understands quantum physics, and this is quoted often enough for it to become a bit of a cliché. Unfortunately, it’s quite accurate.
The main reason it has such a reputation for being incomprehensible is because it is so counter-intuitive. Take wave-particle duality, for instance. This basically goes that all particles display wave-like characteristics, and all waves display particle-like characteristics. So what?
Take the classic Young’s two-slit interference experiment:
When done with light waves, sound waves or water waves, an interference pattern can be observed on the screen (at F in the diagram). With light waves, this is made of alternating dark and light bands; with sound waves, the volume of the sound will vary correspondingly.
Now, try this with photons or electrons or ANY subatomic particle. If you fire a stream of them through the slits, an interference pattern like that of light waves can be seen. That’s expected, as light IS a wave and is made of photons.
However, the strange part comes when you try this using single photons, like I did for a lab earlier this year. Instead of going through as a particle, the photons seem to go all over the place on the screen. Eventually, you get an interference pattern like with the visible light. But wait a moment: I just said “the photons seem to go all over the place on the screen”.
WHAT?!?! I hear you cry. They’re individual particles, shouldn’t we be able to predict where they’re going? According to Newtonian mechanics, yes. But with subatomic particles, that just is not possible. All you can do is try to predict where it has the highest probability of being – which is where the bright bands in the interference pattern are.
That is exactly WHY it’s so strange: probability has come into what was previously a very deterministic branch of science. No less a figure than Einstein had trouble accepting this, and since the non-zero cosmological constant he created turned out not to be a mistake, this is arguably his greatest blunder ever: his remark in a letter to Max Born, dated the 4th of December 1926, that “Quantum mechanics is certainly imposing. But an inner voice tells me that it is not yet the real thing. The theory says a lot, but does not really bring us any closer to the secret of the “old one.” I, at any rate, am convinced that He does not throw dice” is often paraphrased as “God* does not play dice with the universe.”
Niels Bohr’s response was allegedly “Stop telling God what to do with dice”, although this is also attributed to Enrico Fermi.
Ethan Siegel of Starts With A Bang commented on this here, if you want another (possibly clearer) explanation.
*NB: Einstein was actually secular and used “God” as a metaphor for Nature.