279 - Paul Davies: The Second Quantum Mechanics Revolution

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**Robinson Erhardt** (1:10)
Your new book, Quantum 2.0, is about what you think of as the next quantum revolution. But quantum physics itself has been around for about 100 years. So I'm just curious, why is this what's been on your mind lately?

**Paul Davies** (1:25)
Well, I've actually worked in quantum mechanics, as we call it, for my entire career, ever since I did my PhD as a student many, many decades ago. And I've mostly worked in technical fields like quantum gravity and quantum field theory.
And this has been a way of life. But the thing, if you know anything at all about quantum mechanics, you'll know that it's pretty weird and it's deeply mysterious in what it tells us about the nature of reality. And in daily life, if I'm going about doing a calculation, for example, about the heat emitted by a black hole, which is a quantum process, I don't trouble myself to think what does this all mean.
But then when I'm in conversation with people, they always want to know what's actually going on. And it seemed to me that having written textbooks on quantum mechanics and having given quantum mechanics a mention in pretty much all the books I've written, it's an obligatory mention. I really ought to just tackle the whole thing seriously and do a proper job, do an entire book on the history and the future of quantum mechanics, and all of the philosophic and conceptual problems that swirl around it. And the centenary seemed like a very good time to do that, a celebration of the most successful scientific theory in history. But coincidentally, we're on the cusp of the next great quantum leap, if I may use that expression, with which we call quantum information science, or quantum information technology. And that could prove as revolutionary as the original quantum mechanics. So this seemed like the perfect time to take this theory, which has already transformed our society, and given us most of the technology we take for granted, and is poised to do perhaps even more in the coming decades. And yet, fundamentally, this theory seems to make no sense. It seems to paint a picture of reality, which is weird. Some people even think bizarre and flat out contradictory. And so there are a lot of people, including some professional physicists, who simply think quantum mechanics, as it stands, has unfinished business. That we can sort of use it, because we know technically what to do, but something has been left out, because it really doesn't seem to make any sense.

**Robinson Erhardt** (4:04)
Well, I'd like to spend the majority of our conversation talking about the 2 in the title.
But most of our listeners are scientifically literate, and I am not a physicist, but I've done plenty of reading and had plenty of conversations about quantum mechanics. Nonetheless, I love hearing individual physicists take on what the problem is and why the revolution was itself so revolutionary. I think that would be a great way to start, since your book is also largely about history. So what was the world of physics like before the quantum revolution, and why was it so revolutionary?

**Paul Davies** (4:44)
Well, what we now call quantum mechanics, which was developed in the mid-1920s, was a response to a handful of things in basic physics that didn't seem to make any sense. We don't really need to list all of those things because they eventually fell into place.
But one obvious one, I've just mentioned this, is that physicists had some idea about the turn of the 30th century that atoms consisted of charged particles, a nucleus, positively charged nucleus, a heavy thing, electrons whirling around it. The problem about whirling electrons, they go round and round. If you have whirling electric charges, they emit electromagnetic waves. They emit light, for example. And so how is it that these electrons could go whirling round and round in the atom and not be drained of energy and spiral into the nucleus? So it seemed like all atoms would be unstable, that the electrons would enter a death spiral and just emit a flash of light and the atom would exist. So that was one of the problems that people worried about. But the big conceptual change, and I should point out that in the mid-1920s, this was really regarded as a technical issue. How could one describe the nature of matter or the atomic and subatomic level? But very soon, within a year or two, it became clear that there was something deeply, deeply strange. And the best way that I can explain that, my particular take, is that in daily life, we assume there's a real world out there and that this world exists whether we observe it or not, and that objects in the world have properties which belong to them. So for example, if I say, well, a golf ball is round, it doesn't matter whether I'm looking at it or not, it's a property of the golf ball. It's not a property of me observing the golf ball. That seems sort of like common sense. But at the atomic level, if that golf ball is an atom or an electron, that statement is no longer correct. That subatomic and atomic particles simply do not possess well-defined properties in advance of us inspecting them. And the peculiar thing is that we human beings can decide what we're going to inspect. We might decide that we believe an atom should be somewhere even though we may not know where it is, and then we can do a position measurement, we find the atom at a place.