I wish to hell that I could just not bring up quantum physics. But there’s no real way to explain how reality can be a perfect mathematical object and still look random due to indexical uncertainty, without bringing up quantum physics.
The Everett universe is simpler by the Bayesian version of Occam’s Razor, algorithmic information. I’ll write a post on this eventually, I think.
But imagine a physics that is just like our physics, except that the amazing new theory says that whenever conventional physics predicts you won’t be able to see an object any more (for example, you threw it away to infinity), that object ceases to exist, because it’s “no longer necessary”. This amazing new theory violates conservation laws, and worse, introduces a subjective note into physics (objects stop existing when people can’t see them), and even worse, produces no new testable predictions and in fact has to peek at the simpler theory to find out when objects ought to “vanish”.
Well, that’s just what the Copenhagen interpretation looks like relative to many-worlds—the Copenhagen interpretation says that large clouds of amplitude vanish from configuration space, at exactly the point when the simpler theory says that decoherence prevents the local you from seeing it any more. This violates unitarity, CPT symmetry, relativity, and half a dozen other basic principles of physics; plus it introduces a note of subjectivity that confused half the planet for half a century; furthermore it adds a strictly extra law of physics that produces no new predictions; and finally, it has to peek at decoherence calculations in order to find out whether or not it’s safe to declare that an amplitude cloud has “vanished”.
So the probability of the many-world interpretation relative to the Copenhagen interpretation, is essentially equivalent to the probability of the theory that your missing socks are somewhere behind your dryer, relative to the theory that your missing socks have been banished from existence by supernatural fairies that only come out when you’re not allowed to look behind the dryer.
Not all physicists agree with this, perhaps because not all physicists realize that probability theory is a technical subject, so some of them talk about “Occam’s Razor” without knowing how to do calculations that involve Occam’s Razor, or talk about “falsifiability” without knowing how to calculate exactly how much a piece of evidence falsifies something. However, I believe that polls have shown that a majority of physicists do know better than to believe in Copenhagen fairies, at this point. I don’t really feel like waiting for the other 37% or whatever of physicists to catch up. It’s not the polls that nail down many-worlds, it’s the evidence as interpreted sanely.
I know this isn’t a complete explanation, but I hope it gives you some idea of what’s going through my mind when I just speak as if many-worlds is true, without Copenhagen caveats. It’s for the same reason I don’t end all my sentences with, “Unless a magic chocolate cake is altering my mind.” Wave functions don’t collapse. It was a silly idea.
As for free will: “Free will” is a name for a state of confusion, not a name for something that either does or does not exist. Tell me an experiment I can do to find out whether someone has “free will”, and I’ll tell you whether or not it can exist in a mathematically regular universe.
It occurs to me that one consequence of learning about QM from the sequence (as many people are doing), is that you then need to un-learn wavefunction realism, if you want to think about the subject for yourself. A better way to learn QM is to approach it as an incomplete classical-looking theory. E.g. a particle isn’t really a wavefunction; it’s a particle, with a position and momentum that we only know imprecisely, and the wavefunction is a calculating device that gives you the probabilities. Once you’re clear on that picture, then you can say “this theory is manifestly incomplete; what’s the actual physical reality, and why does this wavefunction thing work?” And then you’re in a position to consider whether the wavefunction itself could somehow be the actual physical object. But because the sequence presupposes wavefunction realism from the beginning—even the Copenhagen interpretation is mostly portrayed as being about an objectively existing wavefunction with two modes of evolution—it would take an unusually careful reader to come to the sequence with no prior knowledge of QM, and still notice the possibility that wavefunctions aren’t real.
What you describe is the hidden-value theory of QM, which has been invalidated experimentally. Any interpretation of QM must be inherently “weirder” than observers merely bring in a state of ignorance about the velocity and position of billiard ball particles.
Probably true. That said, I’m not sure how many readers could approach QM as a “classical-looking theory” and notice the possibility that particles aren’t real. I’m also not sure there’s a way to approach QM—or, indeed, anything else—that doesn’t bias the reader in favor of some ontology.
Incidentally, New Scientist (“Ghosts in the atom: Unmasking the quantum phantom”, Aug 2, 2012) are now reporting that theoretical breakthroughs have disproved non-realist interpretations of QM. Its been shown that different interpretations of QM have different empirical consequences, and the naive version of the Copenhagen interpretation contradicts empirical data.
“Now Matthew Pusey and Terry Rudolph of Imperial College London, with
Jonathan Barrett of Royal Holloway University of London, seem to
have struck gold. They imagined a hypothetical theory that
completely describes a single quantum system such as an atom but,
crucially, without an underlying wave telling the particle what to
do.
Next they concocted a thought experiment to test their theory, which
involved bringing two independent atoms together and making a
particular measurement on them. What they found is that the
hypothetical wave-less theory predicts an outcome that is different
from standard quantum theory. “Since quantum theory is known to be
correct, it follows that nothing like our hypothetical theory can be
correct,” says Rudolph (Nature Physics, vol 8, p 476).
Some colleagues are impressed. “It’s a fabulous piece of work,” says
Antony Valentini of Clemson University in South Carolina. “It shows
that the wave function cannot be a mere abstract mathematical
device. It must be real—as real as the magnetic field in the space
around a bar magnet.”
The Pusey-Barrett-Rudolph result was published (and much discussed) last year. Matt Leifer has a nice, non-sensationalist discussion of the theorem, and he argues convincingly that the theorem does not rule out any interpretation of QM held by contemporary researchers.
I mean many-worlds.
I wish to hell that I could just not bring up quantum physics. But there’s no real way to explain how reality can be a perfect mathematical object and still look random due to indexical uncertainty, without bringing up quantum physics.
The Everett universe is simpler by the Bayesian version of Occam’s Razor, algorithmic information. I’ll write a post on this eventually, I think.
But imagine a physics that is just like our physics, except that the amazing new theory says that whenever conventional physics predicts you won’t be able to see an object any more (for example, you threw it away to infinity), that object ceases to exist, because it’s “no longer necessary”. This amazing new theory violates conservation laws, and worse, introduces a subjective note into physics (objects stop existing when people can’t see them), and even worse, produces no new testable predictions and in fact has to peek at the simpler theory to find out when objects ought to “vanish”.
Well, that’s just what the Copenhagen interpretation looks like relative to many-worlds—the Copenhagen interpretation says that large clouds of amplitude vanish from configuration space, at exactly the point when the simpler theory says that decoherence prevents the local you from seeing it any more. This violates unitarity, CPT symmetry, relativity, and half a dozen other basic principles of physics; plus it introduces a note of subjectivity that confused half the planet for half a century; furthermore it adds a strictly extra law of physics that produces no new predictions; and finally, it has to peek at decoherence calculations in order to find out whether or not it’s safe to declare that an amplitude cloud has “vanished”.
So the probability of the many-world interpretation relative to the Copenhagen interpretation, is essentially equivalent to the probability of the theory that your missing socks are somewhere behind your dryer, relative to the theory that your missing socks have been banished from existence by supernatural fairies that only come out when you’re not allowed to look behind the dryer.
Not all physicists agree with this, perhaps because not all physicists realize that probability theory is a technical subject, so some of them talk about “Occam’s Razor” without knowing how to do calculations that involve Occam’s Razor, or talk about “falsifiability” without knowing how to calculate exactly how much a piece of evidence falsifies something. However, I believe that polls have shown that a majority of physicists do know better than to believe in Copenhagen fairies, at this point. I don’t really feel like waiting for the other 37% or whatever of physicists to catch up. It’s not the polls that nail down many-worlds, it’s the evidence as interpreted sanely.
I know this isn’t a complete explanation, but I hope it gives you some idea of what’s going through my mind when I just speak as if many-worlds is true, without Copenhagen caveats. It’s for the same reason I don’t end all my sentences with, “Unless a magic chocolate cake is altering my mind.” Wave functions don’t collapse. It was a silly idea.
As for free will: “Free will” is a name for a state of confusion, not a name for something that either does or does not exist. Tell me an experiment I can do to find out whether someone has “free will”, and I’ll tell you whether or not it can exist in a mathematically regular universe.
I find this way too amusing in retrospect.
It occurs to me that one consequence of learning about QM from the sequence (as many people are doing), is that you then need to un-learn wavefunction realism, if you want to think about the subject for yourself. A better way to learn QM is to approach it as an incomplete classical-looking theory. E.g. a particle isn’t really a wavefunction; it’s a particle, with a position and momentum that we only know imprecisely, and the wavefunction is a calculating device that gives you the probabilities. Once you’re clear on that picture, then you can say “this theory is manifestly incomplete; what’s the actual physical reality, and why does this wavefunction thing work?” And then you’re in a position to consider whether the wavefunction itself could somehow be the actual physical object. But because the sequence presupposes wavefunction realism from the beginning—even the Copenhagen interpretation is mostly portrayed as being about an objectively existing wavefunction with two modes of evolution—it would take an unusually careful reader to come to the sequence with no prior knowledge of QM, and still notice the possibility that wavefunctions aren’t real.
What you describe is the hidden-value theory of QM, which has been invalidated experimentally. Any interpretation of QM must be inherently “weirder” than observers merely bring in a state of ignorance about the velocity and position of billiard ball particles.
Probably true.
That said, I’m not sure how many readers could approach QM as a “classical-looking theory” and notice the possibility that particles aren’t real.
I’m also not sure there’s a way to approach QM—or, indeed, anything else—that doesn’t bias the reader in favor of some ontology.
Incidentally, New Scientist (“Ghosts in the atom: Unmasking the quantum phantom”, Aug 2, 2012) are now reporting that theoretical breakthroughs have disproved non-realist interpretations of QM. Its been shown that different interpretations of QM have different empirical consequences, and the naive version of the Copenhagen interpretation contradicts empirical data.
“Now Matthew Pusey and Terry Rudolph of Imperial College London, with Jonathan Barrett of Royal Holloway University of London, seem to have struck gold. They imagined a hypothetical theory that completely describes a single quantum system such as an atom but, crucially, without an underlying wave telling the particle what to do.
Next they concocted a thought experiment to test their theory, which involved bringing two independent atoms together and making a particular measurement on them. What they found is that the hypothetical wave-less theory predicts an outcome that is different from standard quantum theory. “Since quantum theory is known to be correct, it follows that nothing like our hypothetical theory can be correct,” says Rudolph (Nature Physics, vol 8, p 476).
Some colleagues are impressed. “It’s a fabulous piece of work,” says Antony Valentini of Clemson University in South Carolina. “It shows that the wave function cannot be a mere abstract mathematical device. It must be real—as real as the magnetic field in the space around a bar magnet.”
The Pusey-Barrett-Rudolph result was published (and much discussed) last year. Matt Leifer has a nice, non-sensationalist discussion of the theorem, and he argues convincingly that the theorem does not rule out any interpretation of QM held by contemporary researchers.
Probably better to cite a description rather than blurbs. Scott Aaronson’s post was linked on LW, and is a really good description.
Quick summary: what the paper shows is that the “wave-function as knowledge” description is incompatible with QM.