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You're not alone : "I think I can safely say that nobody understands quantum mechanics." Richard Feynman, The Character of Physical Law (1965)

spacehome 11y ago
And when he said it, it was true.
- jeffwass 11y ago
  Are you disputing it's not true now? Btw, I can safely tell you that having studied QM in some depth, I usually avoid these types of HN threads. Ie, quite often they're dominated by people who think they know what they're talking about, and throw around buzzwords and make seemingly convincing arguments based on logically or physically unsound bases.
  - spacehome 11y ago
    Yes, I'm claiming that it's not true now and that MWI, while it may not be captial-T Truth, is a concise and deterministic explanation of experimental results.
s-macke 11y ago
I was never happy with this statement. Because obviously we can make very accurate predictions by applying quantum mechanics. The problem with this statement is, that the meaning of "understanding" is unclear here. When I say "I understand that, when I drop an apple it falls on the floor", What exactly do I "understand" here? Does "understanding" mean here, that our brain is trained to predict what would happen? Actually there is a huge understanding on the mathematical level of quantum mechanics. To apply these equations to what we call "reality" is a problem. But then again, what do we mean exactly with "reality" here? If some of the effects of quantum mechanics would be visible for us somehow from birth on, would we "understand" quantum mechanics then?
- jeffwass 11y ago
  FYI, I have a PhD in physics and spent many a late night hour solving theoretical quantum mechanics problems, as well as those applied to statistical mechanics and solid state physics. And yet I'm still inclined to agree with Feynman.
  - s-macke 11y ago
    For me quantum mechanics is something like a complex cooking receipe. We have equations, we know what we have to put into these equations to solve certain systems. The rest is just an endless variety of coordinate transformations, model assumptions and approximations. And this is the part most physics students struggle.
    - jeffwass 11y ago
      Feynman is not talking about just "turning the crank" as it were, but actual deep intuitive understanding of the 'why'. I agree with you that after you know how to setup the system you want to analyse, turning the mathematical crank can be straightforward and sometimes not so interesting.
lisper 11y ago
Feynman was wrong. It is actually not hard to understand QM, it's just taxing on the intuition. The key insight is that measurement and entanglement are the same physical phenomenon. Once you understand this, everything else is conceptually straightforward. See: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf
- saganus 11y ago
  I'm far from understanding QM for any definition of "understanding", but from your key insight, I would interpret it as, "whenever you measure something, you are actually entangling with it". Is this what you are trying to say, when you state they are both the same phenomenon? or what exactly does it mean for entanglement and measurement to be the same phenomenon? It's actually a very interesting take on QM, from my limited understanding of it.
  - lisper 11y ago
    "whenever you measure something, you are actually entangling with it" Exactly right, but there's a little more to it than that. Because you are entangling with it, all of the "mysterious" properties of measurement (Shroedinger's cat and whatnot) can all be understood in terms of entanglement, and in particular, in terms of what happens when you get large systems of mutually entangled particles.
- jeffwass 11y ago
  Here's another quote. Take from it what you will. "I can safely say that the fraction of people on Internet forums who understand as much quantum mechanics as they think they do is a non-negative number much smaller than one." jeffwass, Hacker News (2016)
- spacehome 11y ago
  Once you understand that, you've accepted MWI.
  - 11y ago

spacehome 11y ago

And when he said it, it was true.

jeffwass 11y ago
Are you disputing it's not true now? Btw, I can safely tell you that having studied QM in some depth, I usually avoid these types of HN threads. Ie, quite often they're dominated by people who think they know what they're talking about, and throw around buzzwords and make seemingly convincing arguments based on logically or physically unsound bases.
- spacehome 11y ago
  Yes, I'm claiming that it's not true now and that MWI, while it may not be captial-T Truth, is a concise and deterministic explanation of experimental results.

s-macke 11y ago

I was never happy with this statement. Because obviously we can make very accurate predictions by applying quantum mechanics. The problem with this statement is, that the meaning of "understanding" is unclear here. When I say "I understand that, when I drop an apple it falls on the floor", What exactly do I "understand" here? Does "understanding" mean here, that our brain is trained to predict what would happen? Actually there is a huge understanding on the mathematical level of quantum mechanics. To apply these equations to what we call "reality" is a problem. But then again, what do we mean exactly with "reality" here? If some of the effects of quantum mechanics would be visible for us somehow from birth on, would we "understand" quantum mechanics then?

jeffwass 11y ago
FYI, I have a PhD in physics and spent many a late night hour solving theoretical quantum mechanics problems, as well as those applied to statistical mechanics and solid state physics. And yet I'm still inclined to agree with Feynman.
- s-macke 11y ago
  For me quantum mechanics is something like a complex cooking receipe. We have equations, we know what we have to put into these equations to solve certain systems. The rest is just an endless variety of coordinate transformations, model assumptions and approximations. And this is the part most physics students struggle.
  - jeffwass 11y ago
    Feynman is not talking about just "turning the crank" as it were, but actual deep intuitive understanding of the 'why'. I agree with you that after you know how to setup the system you want to analyse, turning the mathematical crank can be straightforward and sometimes not so interesting.

lisper 11y ago

Feynman was wrong. It is actually not hard to understand QM, it's just taxing on the intuition. The key insight is that measurement and entanglement are the same physical phenomenon. Once you understand this, everything else is conceptually straightforward. See: https://www.youtube.com/watch?v=dEaecUuEqfc https://www.youtube.com/watch?v=dEaecUuEqfc http://www.flownet.com/ron/QM.pdf http://www.flownet.com/ron/QM.pdf

saganus 11y ago
I'm far from understanding QM for any definition of "understanding", but from your key insight, I would interpret it as, "whenever you measure something, you are actually entangling with it". Is this what you are trying to say, when you state they are both the same phenomenon? or what exactly does it mean for entanglement and measurement to be the same phenomenon? It's actually a very interesting take on QM, from my limited understanding of it.
- lisper 11y ago
  "whenever you measure something, you are actually entangling with it" Exactly right, but there's a little more to it than that. Because you are entangling with it, all of the "mysterious" properties of measurement (Shroedinger's cat and whatnot) can all be understood in terms of entanglement, and in particular, in terms of what happens when you get large systems of mutually entangled particles.
jeffwass 11y ago
Here's another quote. Take from it what you will. "I can safely say that the fraction of people on Internet forums who understand as much quantum mechanics as they think they do is a non-negative number much smaller than one." jeffwass, Hacker News (2016)
spacehome 11y ago
Once you understand that, you've accepted MWI.
- 11y ago