David Reffkin is a radio host at KUSF in San Francisco. His usual gig is classical music, but once a month he hosts a special called Static Limit where he delves into physics and cosmology. Here’s an interview he did with me a short while back. Right at the beginning we’re talking about this very blog, which I am now using to plug the interview, which is mostly about my book. This is what’s known as “synergy.”
(Those viewing in an RSS reader, you have to visit the page to click the audio link.)
David assumes the listeners have been following along previous shows, so we don’t spend too much time defining general relativity and the Big Bang; we go right for the cutting edge. But we also covered a lot of meta ground, about the process of doing physics. He also gave me the most comprehensive list of errata (mostly minor typos) for my book, so I know he read the whole thing!
Ray–
You write: “Ironically, I would take that as an argument in favor of MWI. My principle of local realism rules out all sorts of things that are logically possible but not observed, while your principle of no signaling does not. This means my principle has more predictive power.”
That’s certainly untrue; you know full well that just as quantum mechanics is only one of many conceivable no-signaling theories, so it is likewise only one of many conceivable many-worlds theories. (Although I doubt that anybody’s been looking for them, since they’re much less interesting and falsifiable than new non-signaling theories.) Neither imposing no-signaling nor many-worlds uniquely defines quantum mechanics. That is, after all, part of the point of that information causality paper, to find some principle that actually does uniquely define quantum mechanics! (Although I would argue it’s not the only way; indeed, I don’t take no-signaling as the fundamental definition of quantum mechanics, even though it certainly satisfies that property.)
One of the deep problems with attempting to define probability in many-worlds (and there are many; Blake Stacy cited one paper that provides more examples) is that it relies on a version of frequentism, i.e., branch counting. But frequentism is circular: You need to perform a literally infinite sequence of measurements to get it to work, via the central limit theorem. Because if you perform only a finite (even if large) number of measurements, then you always have nonzero probabilities for all possibilities, and you have to assume they’re “unlikely” to make them go away. This is the one the key troubles with trying to “derive” probability from quantum mechanics, rather than employing an interpretation, like the one I’ve been advocating, that simple assumes Bayesian/ignorance probability from the beginning.
And it bedevils all the papers that you mention, like the Farhi et al. argument, where they define frequency ratios for large numbers of identical experiments and show that Born-rule-violating outcomes have amplitudes that “go to zero” when the number of experiments “goes to infinity.” Well, great, but there are never an infinite number of identical experiments, so in truth the amplitudes of those bad branches are never actually zero, and thus they have to argue that their small amplitudes make them “unlikely,” which is again circular.
And actually many-worlds is still worse than the usual troubles with frequentism, because the simple fact is that every branch is there; it exists! No branch “goes to zero.” If two branches separate with respective amplitudes sqrt(1/3) and sqrt(2/3), what is the meaning of the statement that one is “more likely” than the other?
These are only a few of the many reasons why many-worlds and probability don’t work well together, even if at a purely mathematical level, where you can take limits, measure-theoretic probability theory seems to work. But see that paper mentioned earlier for more reasons for logical confusion. I’m not going to list all the logical issues here. But they are many, and they are unresolved. (And perhaps unresolvable.) It’s not merely a “raw revulsion at the idea that you have millions upon millions of near identical twins whom you will never get to meet.”
Then you say “And of course theres no fundamental principle which prevents you from choosing the values of nonlocal hidden variables, such that the one world of your interpretation grossly violates the Born rules prediction either.” Well, sure. But that’s true of classical probability theory as well. There’s no fundamental principle that prevents a fair classical coin from (by chance) landing heads a million times in a row, and thereby “violating” the predictions of probability theory. That’s the reason why you can’t “derive” probability from deterministic assumptions; with only a finite number of measurements, there’s no hard and fast prediction that you can make that doesn’t cite circularly probability theory itself. (“The more experiments you perform, the unlikely outcomes become less likely.”) But the point is that you don’t have to accept anything more than we already accept when doing classical probability theory.
In the approach I’ve been advocating, there’s a superficial and aesthetic downside because non-signaling phenomena can break the cosmic speed limit, but that presents no logical contradictions. That’s important: There are no fundamental logical contradictions. But many-worlds presents the same logical confusions as classical modal realism when it comes to probability theory itself, and that’s why I don’t prefer it. Probability theory needs a logical bed to sleep in, and many-worlds logically doesn’t provide one.
And unless you’re advocating classical modal realism, then you’re furthermore admitting that there are two kinds of probability in Nature: classical ignorance probabilities, and quantum many-worlds probabilities. In my approach, they’re unified; there’s just one kind of probabilities, namely, ignorance probabilities. That’s one more sense in which the approach I advocate is more minimal and conservative than yours.
But we’re clearly never going to agree; how could we ever know which one is ultimately correct? My point, again, was just that! Yours is not the only road. The universe really is big enough for both us. (Right, Mark?)
So let’s call a truce, shall we? You take that side over there, and I’ll take this side over here, and we’ll have somebody else (Mark? Blake?) patrol the border. 🙂
I gotta go. It’s been a great chat, but rather time-consuming, wouldn’t you say?
Steve B,
I think you got confused what Ray’s principle was; he meant local realism as his principle, not many worlds. Although I don’t think I can accept what he means by “local realism” if there are in fact many worlds. I simply don’t know what “realism” itself is when all possibilities are simultaneously real!
And local realism itself isn’t a strong enough principle, either. Steve was wrong about confusing many worlds and local realism, but it’s similarly true that local realism alone isn’t strong enough either to get quantum mechanics as the unique theory.
But I do agree with Steve’s point that just because you can slap down a probability measure on something at the level of abstract math doesn’t mean that thing is really probability. And I have a lot of trouble with the idea of probability and many worlds going together, too.
And I suppose it’s just personal preference for me, but I find many worlds to be a bit too religious for me. It’s like an alternative gospel for science/atheist people, and people cling to it for a lot of the same reasons. (I know I wish there was another world out there in which I was more successful, or worlds in which I was immortal! 🙂 ) That doesn’t mean it’s necessarily wrong, but it does explain why people seem so attached to it when there are other interpretations that solve the measurement paradox too.
This really has been quite a conversation, hasn’t it? I hope everyone here had some fun, and nobody took this stuff too seriously! And everybody seems to have corrected some mistakes in their thinking along the way, and their misunderstandings of each other’s ideas.
The real question is, if a few smart people have a deep and intense conversation/debate about the fundamental meaning and interpretation of quantum mechanics, and nobody else reads it, then did it have any significance at all? 🙂
But I think I’d like to call it quits, too. Everybody’s made some good points, and I have a lot I’d like to chew on now. 🙂 So long, and thanks for all the fish!