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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 l

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.
  - kergonath 4y ago
    > Anyone who is saying that is obviously wrong and of course I am not claiming that This thread has fine examples of people doing just that, I did not invent it. > Why is it uninteresting? To me as a physicist this is thoroughly interesting! It is interesting in that it’s an example of QM agreeing with classical EM theory. The opposite would be quite an issue actually, because it would mean that QM does not behave at the classical limit. There are lots of other examples of QM agreeing with classical mechanics, there is nothing magical about polarisers. > 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. In a narrow range of quantum optics maybe. I work on radiation/matter interaction, and you definitely cannot account for a lot of things if you posit that light is a wave. Unless you add some particle-like characteristics to your wave, in which case it’s not much better than assigning arbitrary wave-like properties to a particle. > You absolutely should not think of light as infinitely small particles that move at the speed of light I don’t. > that is an inaccurate and outdated picture It is not a wave either. The problem is when we try to pigeonhole it in either character. I mean, really, are we arguing the whole wave/particle duality concept again? That was novel a century ago.
    - bowsamic 4y ago
      It’s not a symmetric picture. The wave picture is far more useful than the particle one generally. Also I have to ask, how do you think of a particle?

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.
- kergonath 4y ago
  > Anyone who is saying that is obviously wrong and of course I am not claiming that This thread has fine examples of people doing just that, I did not invent it. > Why is it uninteresting? To me as a physicist this is thoroughly interesting! It is interesting in that it’s an example of QM agreeing with classical EM theory. The opposite would be quite an issue actually, because it would mean that QM does not behave at the classical limit. There are lots of other examples of QM agreeing with classical mechanics, there is nothing magical about polarisers. > 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. In a narrow range of quantum optics maybe. I work on radiation/matter interaction, and you definitely cannot account for a lot of things if you posit that light is a wave. Unless you add some particle-like characteristics to your wave, in which case it’s not much better than assigning arbitrary wave-like properties to a particle. > You absolutely should not think of light as infinitely small particles that move at the speed of light I don’t. > that is an inaccurate and outdated picture It is not a wave either. The problem is when we try to pigeonhole it in either character. I mean, really, are we arguing the whole wave/particle duality concept again? That was novel a century ago.
  - bowsamic 4y ago
    It’s not a symmetric picture. The wave picture is far more useful than the particle one generally. Also I have to ask, how do you think of a particle?