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Isn't this backward? Usually polarization is the analogy used to explain stern-gerlach. I dont get the desire to cast light as something non quantum...

fsh 4y ago
The quantum nature of light is extremely difficult to observe. Almost all laboratory experiments can be explained using Maxwell's equations and the quantization of the electric charge (this explains why photodetectors "click"). Photons usually only show up when higher-order correlation functions are analyzed.
bowsamic 4y ago
> I dont get the desire to cast light as something non quantum. Because you can describe it entirely using classical physics in this situation
- kergonath 4y ago
  Except for the whole quantisation thing, and what happens in single-photon experiments. This thread is very weird, there are a lot of people saying nonsense about photons not existing and absorption and re-emission (things that are detailed in any quantum physics textbook in existence). As if the fact that we can also explain the movement of planets around the sun somehow makes general relativity unnecessary or somewhat invalid. Quite disturbing.
  - bowsamic 4y ago
    I have a PhD in physics and I am on my first postdoc in the field of quantum optics so I'm well aware of the quantisation of the EM field. The point is that a lot of people think this is a demonstration of a purely quantum effect, but it is not, it is a classical effect. Sure, quantum mechanics is the full theory for describing it, but that's like saying that the increase in pressure when you heat up a gas is a "quantum effect". It's basically misinformation, especially in the twitter thread that others mentioned. People are coming out thinking their mind has been blown with some crazy quantum effect, but actually this is described by boring classical EM.
    - aflag 4y ago
      I thought that experiment was something easy to do that required quantum theory to explain and the article proved me wrong. I'm the exact sort of curious, but ignorant and impressionable demographic that videos about that sort of effect target. I think that text really improved my understanding of reality.
      - bowsamic 4y ago
        Yeah it is a common misconception. There is also a minutephysics video on it (https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs) that portrays the effect of polarising filters in a misleading/wrong way, leading to further confusion. Polarisation is used in many books and courses as a familiar classical system, which we then use to springboard into Stern-Gerlach, which actually is an inherently quantum effect. Polarisation already appears when writing down the full solution to the classical EM wave equation. On the other hand, the similar effect seen in the Stern-Gerlach experiment with the angular momentum measurements is not: the non-commutivity of the angular momentum operator for different measurement axes is a purely quantum phenomenon.
    - kergonath 4y ago
      This is not why “people want to cast it as something non quantum”. If it were the case, we would see “nice, classical EM and QM are consistent here”. Instead we have “classical EM works, therefore photons are bullshit and QM is not real”. The fact that we can understand a macroscopic phenomenon using classical EM theory is thoroughly uninteresting, as the vast majority of what we see every day is a combination of quantum objects that we can understand macroscopically with classical physics. Of course people had EM theory before Planck. It’s still just a bunch of photons. It is as not a personal attack of your knowledge, just that I don’t think your conclusion about people’s behaviour was right. My Physics PhD is as relevant as yours here :)
      - bowsamic 4y ago
        > Instead we have “classical EM works, therefore photons are bullshit and QM is not real”. Anyone who is saying that is obviously wrong and of course I am not claiming that (although I do think that photons are probably bullshit) > The fact that we can understand a macroscopic phenomenon using classical EM theory is thoroughly uninteresting Why is it uninteresting? To me as a physicist this is thoroughly interesting! > It’s still just a bunch of photons. This is somewhat misleading, the wave description of light is complete and still totally preserved in quantum optics. The only deficiency is that the light's energy density should be quantised. You absolutely should not think of light as infinitely small particles that move at the speed of light, that is an inaccurate and outdated picture.

fsh 4y ago

The quantum nature of light is extremely difficult to observe. Almost all laboratory experiments can be explained using Maxwell's equations and the quantization of the electric charge (this explains why photodetectors "click"). Photons usually only show up when higher-order correlation functions are analyzed.

bowsamic 4y ago

> I dont get the desire to cast light as something non quantum. Because you can describe it entirely using classical physics in this situation

kergonath 4y ago
Except for the whole quantisation thing, and what happens in single-photon experiments. This thread is very weird, there are a lot of people saying nonsense about photons not existing and absorption and re-emission (things that are detailed in any quantum physics textbook in existence). As if the fact that we can also explain the movement of planets around the sun somehow makes general relativity unnecessary or somewhat invalid. Quite disturbing.
- bowsamic 4y ago
  I have a PhD in physics and I am on my first postdoc in the field of quantum optics so I'm well aware of the quantisation of the EM field. The point is that a lot of people think this is a demonstration of a purely quantum effect, but it is not, it is a classical effect. Sure, quantum mechanics is the full theory for describing it, but that's like saying that the increase in pressure when you heat up a gas is a "quantum effect". It's basically misinformation, especially in the twitter thread that others mentioned. People are coming out thinking their mind has been blown with some crazy quantum effect, but actually this is described by boring classical EM.
  - aflag 4y ago
    I thought that experiment was something easy to do that required quantum theory to explain and the article proved me wrong. I'm the exact sort of curious, but ignorant and impressionable demographic that videos about that sort of effect target. I think that text really improved my understanding of reality.
    - bowsamic 4y ago
      Yeah it is a common misconception. There is also a minutephysics video on it (https://youtu.be/zcqZHYo7ONs https://youtu.be/zcqZHYo7ONs) that portrays the effect of polarising filters in a misleading/wrong way, leading to further confusion. Polarisation is used in many books and courses as a familiar classical system, which we then use to springboard into Stern-Gerlach, which actually is an inherently quantum effect. Polarisation already appears when writing down the full solution to the classical EM wave equation. On the other hand, the similar effect seen in the Stern-Gerlach experiment with the angular momentum measurements is not: the non-commutivity of the angular momentum operator for different measurement axes is a purely quantum phenomenon.
  - kergonath 4y ago
    This is not why “people want to cast it as something non quantum”. If it were the case, we would see “nice, classical EM and QM are consistent here”. Instead we have “classical EM works, therefore photons are bullshit and QM is not real”. The fact that we can understand a macroscopic phenomenon using classical EM theory is thoroughly uninteresting, as the vast majority of what we see every day is a combination of quantum objects that we can understand macroscopically with classical physics. Of course people had EM theory before Planck. It’s still just a bunch of photons. It is as not a personal attack of your knowledge, just that I don’t think your conclusion about people’s behaviour was right. My Physics PhD is as relevant as yours here :)
    - bowsamic 4y ago
      > Instead we have “classical EM works, therefore photons are bullshit and QM is not real”. Anyone who is saying that is obviously wrong and of course I am not claiming that (although I do think that photons are probably bullshit) > The fact that we can understand a macroscopic phenomenon using classical EM theory is thoroughly uninteresting Why is it uninteresting? To me as a physicist this is thoroughly interesting! > It’s still just a bunch of photons. This is somewhat misleading, the wave description of light is complete and still totally preserved in quantum optics. The only deficiency is that the light's energy density should be quantised. You absolutely should not think of light as infinitely small particles that move at the speed of light, that is an inaccurate and outdated picture.