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I would disagree, I think we do not understand quantum mechanics at all. We have the equations and get the correct results, but we still don't know what's reall

I would disagree, I think we do not understand quantum mechanics at all. We have the equations and get the correct results, but we still don't know what's really behind measurements, how entanglement can be non-local, we are not even agreeing on whether time evolution is unitary or not.

BeetleB 8y ago
>We have the equations and get the correct results, but we still don't know what's really behind measurements That's pretty much true for classical physics as well.
- danbruc 8y ago
  In classical physics there is no limit to how gentle a measurement can be, you can essentially measure everything without perturbation and to arbitrary precision, so measurements are not really a concern. Or am I misunderstanding what you wanted to hint at?
  - fjsolwmv 8y ago
    Measurements aren't a concern, but what's "behind" them is unknown.
    - danbruc 8y ago
      What do you think is missing in classical physics? There are physical objects with real properties an we are determining their values by carefully interacting with the physical object using a measurement device. If we, for example, want to know the position of an electron - and in classical physics the electron is just a classical particle with a definite position - we carefully throw something, for example photons, into the rough direction of the electron and look at the scattering result to find out where the electron is. I don't see what this picture is missing.
  - BeetleB 8y ago
    I'm referring to the notion that we understand classical mechanics beyond merely "having the equations and getting correct results" - we really do not. Because classical mechanics prevails on the scale that our brains operate on, we do not find it unintuitive. However, at the end of the day, it is merely that we have a bunch of equations that give us correct results - just as quantum mechanics does. This is true of all of physics - reducing observations to the level of predictability. Beyond that we are wading into philosophy. (BTW, classical physics has warts as well - what is the force between a charge of 1 Coulomb and -1 Coulomb when they are touching each other?)
    - danbruc 8y ago
      I see where you are going and you probably have a point but I also will have to think about that for some time. On the other hand my ad hoc reaction is that the problem is much more severe for quantum mechanics than it is for classical mechanics. In classical mechanics we use a slightly wrong model of the world but then everything is consistent and works as long as you are not stepping into regions where the approximations are no longer good. Quantum mechanics yields correct predictions for a wider range of phenomena because we no longer pretend that Planck's constant is zero, it explains some strange features of classical physics, but it is - at our current level of understanding - also no longer self-consistent. And while quantum mechanics is certainly less intuitive I think that is not really the issue. Once you leave Newtonian mechanics behind and step into the Lagrangian or Hamiltonian formulation, even classical mechanics becomes a quite abstract theory. I am certainly still better at picturing phase space flows than superpositions in momentum space but I have no fundamental problem with abstract representations. But that time evolution is supposed to be unitary while I am not looking and then suddenly goes out of the window, that is something you can not simply ignore. It's not not intuitive, it's not consistent. And it's not limited to a region of the theory where we might expect the theory to break down, it's right in the center of it.

BeetleB 8y ago

>We have the equations and get the correct results, but we still don't know what's really behind measurements That's pretty much true for classical physics as well.

danbruc 8y ago
In classical physics there is no limit to how gentle a measurement can be, you can essentially measure everything without perturbation and to arbitrary precision, so measurements are not really a concern. Or am I misunderstanding what you wanted to hint at?
- fjsolwmv 8y ago
  Measurements aren't a concern, but what's "behind" them is unknown.
  - danbruc 8y ago
    What do you think is missing in classical physics? There are physical objects with real properties an we are determining their values by carefully interacting with the physical object using a measurement device. If we, for example, want to know the position of an electron - and in classical physics the electron is just a classical particle with a definite position - we carefully throw something, for example photons, into the rough direction of the electron and look at the scattering result to find out where the electron is. I don't see what this picture is missing.
- BeetleB 8y ago
  I'm referring to the notion that we understand classical mechanics beyond merely "having the equations and getting correct results" - we really do not. Because classical mechanics prevails on the scale that our brains operate on, we do not find it unintuitive. However, at the end of the day, it is merely that we have a bunch of equations that give us correct results - just as quantum mechanics does. This is true of all of physics - reducing observations to the level of predictability. Beyond that we are wading into philosophy. (BTW, classical physics has warts as well - what is the force between a charge of 1 Coulomb and -1 Coulomb when they are touching each other?)
  - danbruc 8y ago
    I see where you are going and you probably have a point but I also will have to think about that for some time. On the other hand my ad hoc reaction is that the problem is much more severe for quantum mechanics than it is for classical mechanics. In classical mechanics we use a slightly wrong model of the world but then everything is consistent and works as long as you are not stepping into regions where the approximations are no longer good. Quantum mechanics yields correct predictions for a wider range of phenomena because we no longer pretend that Planck's constant is zero, it explains some strange features of classical physics, but it is - at our current level of understanding - also no longer self-consistent. And while quantum mechanics is certainly less intuitive I think that is not really the issue. Once you leave Newtonian mechanics behind and step into the Lagrangian or Hamiltonian formulation, even classical mechanics becomes a quite abstract theory. I am certainly still better at picturing phase space flows than superpositions in momentum space but I have no fundamental problem with abstract representations. But that time evolution is supposed to be unitary while I am not looking and then suddenly goes out of the window, that is something you can not simply ignore. It's not not intuitive, it's not consistent. And it's not limited to a region of the theory where we might expect the theory to break down, it's right in the center of it.