zlacker

Lot's of quantum related phenomenon here, but what keeps bothering me is that while I get it that light is both a wave and a particle, but I have no clue what that means. I mean, a wave of sound, is made from air particles, a wave of ripples in a pond is made of movement of water molecules. In the double slit experiment, it's explained that the single photon has to be "interfering with itself", so I don't get it if by being a "wave" it means that the single photon is basically a bunch of "magic" photon ghosts that behave like a wave, but once it is measured or any other reason to "collapse" these ghosts "disappear". I just don't get what the "wave" of light/radio wave is. Is it just an abstract concept of something that behaves like a wave but not the same as sound waves / ripples since we simply don't know? Or is it just a wave of these "not yet collapsed" probabilities of the photons locations that are interfering with each other right until we ask them to choose a location, then they just collapse magically into a single "real" photon. Another thing I don't get is in the double slit experiment, a LOT of the measure before, measure after, etc, are told to be thought experiments, but it's also claimed that someone managed to actually replicate them. Why isn't there a video showing it? I obviously believe they happened, and understand why more or less (e.g. in the one photon at a time experiment, it's spooky that over time you get the same pattern that indicates interference as if you shoot many) but the more spooky result is that thought experiment, that if you measure which slit the photon actually traveled through, you'll see 2 slits on the screen vs the famous pattern. e.g. you'll cause the wave to collapse back to particles. So any video of reproducing of that thought experiment or explanation why it's so hard to reproduce, will be super helpful.

>>eranat+(OP)
I think this is more about the terminology than the actual concepts. This is like in biology, where you classify things as living and non-living, but then you encounter things like viruses, which would depend on the criteria for a "living thing." Or, is a gel a solid or a liquid? Again, it has properties of both states of matter. Similarly, in order to classify phenomena as waves or particles, you look for common attributes between the things you observe. But light exhibits properties of both.

>>eranat+(OP)
Sorry, don't have time to unpack all this, as some are things that scientists understand, but some are so deep that ... not.

When talking about light or radio waves, you're probably talking about Classical Electrodynamics, which does not include the notion of a photon. In Classical EM, light is an EM wave in the sense that if you look at the electric field E or magnetic field B (of a plane wave) at a fixed point in space it is varying sinusoidally. So the waviness is in the amplitude of the E and B fields.

Once you talk about photons, you're in the realm of Quantum Mechanics (QM), and yes things are harder to understand.

It's actually all just fields according to the Standard Model (particle physics), a quantum field theory (QFT).

In QFT there's a field for each fundamental particle that permeates the whole universe. E.g. an electron field, a photon field, etc. Disturbances in these fields are what one would call particles in non-relativistic QM.

So Classical -> QM (quantum system, classical observer/apparatus) -> QFT (quantum everything)

In classical EM, light is a wave. In QM, light is particles. In QFT particles are just disturbances in the all-pervading fields.

Binney has said that QM is just measurement for grownups, or some such. What is a measurement? It's when the system you're observing becomes entangled with the measuring device. We don't know the exact state of every atom in our measuring device, but these could all perturb the system we're measuring. So QM is a hack where you treat the system as quantum but the observer/measuring device as classical which is why you need this confusing wave-function collapse. It was a conscious choice in the development of the theory. This last bit might give some insight into why trying to sense the photon at one of the double-slits ruins the interference pattern.

>>eranat+(OP)
> while I get it that light is both a wave and a particle, but I have no clue what that means.

Because it's wrong. It's a quantum of the electromagnetic field. It's neither a wave nor a particle. It just happens to have some properties of both.

>>eranat+(OP)
I found these two videos very helpful in understanding the quantum nature of light after being stuck in the same spot: https://www.youtube.com/watch?v=zcqZHYo7ONs https://www.youtube.com/watch?v=MzRCDLre1b4 (Watch those in order, because they're a collaboration between two YouTubers)

>>eranat+(OP)
A sibling comment has mentioned this but the problem is with the terminology and how it manifests itself in your mind. You’ve mentioned it in your comment: trying to view light as a ripple or as air particles... light is neither of them, it’s a different thing entirely. Light exhibits certain properties that are wavish and particle-ish which is why it’s said to be both, technically. But trying to visualize light through analogies with phenomena in the macro world that you have intuition for is wrong. Mathematics is the only “eye” with which you can really understand light for what it is.

>>SAI_Pe+r8
I've done plenty of layman reading into QFT, and I'm truly good with this. All particles are really waves. There are no particles. There's self-interaction, and all other kinds of weird stuff. (it's important to qualify that stable orbits and molecules and stuff exist)

But for the duality, there's something bigger that the responses always seem to blow past. Is wave-like nature for explaining behavior (wavy double-slit intensity pattern), or is it something to have a mathematical mapping to measured probabilities?

Quantum stories always seem so backwards. The root phenomenon is some sort of irreducible probability. But then the mechanical part (inference in double-slit) goes a totally different direction. Instead of just turning the situation into a probability of one-or-the-other slit, it STAYS as a wave.

Okay, now you have a new hole in the story. If the photon refuses to choose just 1 slit to go through, why does it choose 1 spot on the photo paper to land on?

Why do we not still have to consider interference in outcomes after the photon makes its mark on the paper? Why does there appear to be like a limit on entanglement, such that it goes away beyond a certain scale? Why are quantum computers hard?

>>Jabavu+d5
Wow, any chance with a few MooCs in physics I’ll be able to understand this better? Or it needs years of study. I’m worried about the math mostly (Ms in CS, but I got Bs in all the math classes :))

>>eranat+uo
The math should be no problem if you go to the right sources. Some people like to play up the math, but it sounds like you're well prepared.

>>AlanSE+kj
> If the photon refuses to choose just 1 slit to go through, why does it choose 1 spot on the photo paper to land on?

The photon (as a field excitation) goes through both slits, but is quantized so only has enough energy to trigger a mark at 1 spot on the photo paper.

> Why do we not still have to consider interference in outcomes after the photon makes its mark on the paper?

If we want to be completely accurate, we should. However so many interactions happen so quickly that the law of large numbers quickly takes over and obfuscates the quantum reality. Technical term for this is decoherence.

> Why are quantum computers hard?

Exactly because of this decoherence. It is very difficult to keep the qubit state isolated from the environment throughout the computation.