Tuesday, June 23, 2015

Feynman 6: Quantum Mechanical View of Nature

This is the second to last chapter in Richard Feynman's, The Character of Physical Law, a series of lectures he gave at Cornell in 1964. The five previous were:
1. In this lecture Feynman gives the Bohr interpretation of quantum mechanics, the interpretation of quantum phenomena held by the majority of quantum physicists. Mind you, the majority of quantum physicists are simply followers of a Kuhnian paradigm they learned in school. Much smarter than me, but not the likes of Feynman or Hawking or Kip Thorne. And Hawking is wrong as often as not these days (e.g., on the existence of the Higg's boson).

I suspect most quantum physicists get annoyed at the question of whether the dogma of uncertainty is right. But it smells like classic Kuhn to me. Logical positivism died in philosophy some sixty years ago, but it is still the name of the game among classical physicists. Feynman ends this chapter with a nice touch. It's okay to have biases as long as you're willing to change them given experimental evidence. I'm willing.

But the current situation in physics has Kuhn written all over it. Bohr, it seems to me, was an ideological bully with charisma. When de Broglie proposed that nuclear particles had pilot waves, he was shut down by the Bohr mafia, the clique that ruled the physics roost at that time. John von Neumann claimed in 1932 to have shown that there couldn't be any other hidden variables like de Broglie's pilot waves that go with particles. It's what the Kuhnian dominant group wanted to hear. Case closed.

Except it wasn't. A physicist by the name of Greta Hermann found an error in von Neumann's argument in 1935, a fact ignored till the 1980s. No one wanted to hear.

Similarly, David Bohm in the 1950s was able to solve the problems with de Broglie's original version of pilot wave theory. John Stewart Bell revived Bohm's approach in the 1980s and also clarified why von Neumann's objection didn't work. And now, John W. M. Bush at MIT has shown that analogous phenomenon in fluid mechanics demonstrate the same results as the standard quantum approach. They require more complex explanations, but the results are the same.

2. I checked some of the online physicist response to Bush and it sounds very much like what Kuhn described as the expected reaction of "normal science." "Who cares." "It's just a different interpretation." It yields the same results but the Copenhagen interpretation is simpler. "It's just about what philosophy you feel most comfortable with."

Here I suppose my theology should bias me toward the indeterminant Copenhagen, but my distaste for logical positivism is even greater. Logical positivism basically says that a falling tree doesn't make a noise in the forest unless someone is there to hear it. If you can't observe it, it doesn't exist.

On the other hand, physics has been stuck for a long time. Hawking can talk about a theory of everything but he's got nothin. There hasn't been any real progress made on a unified theory in a half a century. Quantum mechanics and relativity are just as irreconcilable as they were in the 1930s. String theory has produced NOTHING, and Sheldon was smart to give it up (Big Bang Theory).

This situation suggests to me that something needs backed up to first principles, and the Copenhagen bullies seem as good a place to start as any.

3. Of course none of this is what Feynman presents in this chapter. For all Feynman knew in 1964, von Neumann's critique of de Broglie stood. In his words, "That theory cannot be true" (146).

The double slit experiment basically shows a number of seemingly contradictory things (watch the video):
  • that electrons go through one of the two slits one at a time (and thus behave like particles)
  • that electrons going through two slits produce an interference pattern (an thus behave like waves)
Even if you send the electrons one by one, particle by particle, they will end up producing an interference pattern like a wave. This is a remarkable thing. It's like the electrons know where they need to go to make the interference pattern even though you shoot them one by one.

But if you try to observe which slit each electron goes through, it stops yielding an interference pattern. You can't tell which hole the electron goes through without in effect changing the situation (to detect is to force a different outcome).

4. Feynman again emphasizes that there is nothing that can be understood about this situation. We simply have to accept it. The equations work even though they have no meaning.

"We invent an 'a', which we call a probability amplitude, because we do not know what it means... To get the total probability amplitude to arrive you add the two together and square it" (137).
  • Nobody can give you a deeper explanation for this situation. They can only describe it in more detail. So "you can mention that they are complex numbers instead of real numbers" (145). "But the deep mystery is that no one can go any deeper today." 
  • Nature herself does not know which slit the electron will go through.
In effect, "the future is unpredictable" (147). "It is impossible to predict in any way, from any information ahead of time, through which hole the thing will go, or which hole it will be seen behind" (146).
4. I am open to the Copenhagen interpretation, mind you. At the beginning of the lecture Feynman warned that intuition and common sense are completely useless in quantum physics because there simply aren't ordinary human world analogies. "I think I can safely say," Feynman said, "that nobody understands quantum mechanics" (129).

No comments: