Space is dead: A challenge to the standard model of quantum mechanics
Since the idea of locality is dead, space itself may not be an aloof vacuum: Something welds things together, even at great distances.
Lee attended Harvard University for graduate school receiving a Ph.D. in theoretical physics in 1979. He held postdoctoral positions at the Institute for Advanced Study in Princeton, The Institute for Theoretical Physics (now KITP) in Santa Barbara and the Enrico Fermi Institute at the University of Chicago. This was followed by faculty positions at Yale, Syracuse and Penn State Universities, where he helped to found the Center for Gravitational Physics and Geometry. In September of 2001 he moved to Canada to be a founding member of the Perimeter Institute for Theoretical Physics, where he has been ever since.
Lee's main contributions to research are so far to the field of quantum gravity. He was, with Abhay Ashtekar and Carlo Rovelli, a founder of the approach known as loop quantum gravity, but he has contributed to other approaches including string theory and causal dynamical triangulations. He is also known for proposing the notion of the landscape of theories, based on his application of Darwinian methods to Cosmology. He has contributed also to the foundations of quantum mechanics, elementary particle physics and theoretical biology. He also has a strong interest in philosophy and his three books, Life of the Cosmos, Three Roads to Quantum Gravity and The Trouble with Physics are in part philosophical explorations of issues raised by contemporary physics.
LEE SMOLIN: So let's first define what we mean by being a realist. What I mean when I say I'm a realist is that I believe that there is an exactly understandable way the world is, which is independent of our intervention, independent of our existence, our knowledge. It's possible for everything that happens in nature, to give a complete individual description to that process or those events.
And thence, to understand what -- the causal processes behind those events, to understand exactly what's behind, what goes on in a physical process. Now, for reasons which have a lot to do with World War I, and a lot to do with philosophy and things that I'm not a scholar of, the predominant view among educated people in Europe in the 1920s and 1930s was not-- was not realism. They didn't believe that realism was possible.
They thought that science was a way of speaking about our interactions within our interventions in nature. That the concepts that we use, like wave or particle or law or causality or energy, represent our own intuitions, and are useful to describe what happens when an atomic system interacts with a measurement device. But are not -- do not extend to concepts that stand alone, have meaning when applied just to the atomic system without the context of the experiment.
And I'm trying to talk here the way that Niels Bohr talked because he was the most radical of these anti-realist thinkers. Let's talk about what it means to be not a realist. What it means to be not a realist is that realism is too ambitious and too hard. We don't have it -- they would say something like, "Our concepts come from our experience." Our experiences of the world are the big things we throw them. They bounce back and forth. We play with balls. We sail, we have some intuition about water and wind and so forth.
And then we go into the laboratory and we try to take apart an atom and understand what its components are well, there are things called electrons and nuclei and protons and neutrons and quarks. But what are these things? What-- and we get very confused when we try to understand what they are. And it is indeed confusing. They're not balls like a baseball or a soccer ball. They're not waves like a breeze kicking up some ripples on a lake. But there's something that, to describe them, sometimes we can use the first kind of concept and sometimes it seems like we need to use the second kind of concept. So they're stretching the limits of what our concepts allow us to describe.
So it's natural that some people just get impatient with trying to understand what's really there, and trying to invent a concept that fits all the cases of what's really there. And instead says, "Well, let's lower our ambition. Let's just say we're pragmatically describing what happens when we measure and interact with these things. So we don't know what an electron is exactly, but we know that we put it in a certain kind of experiment and we can diffract and measure its wavelength just as if it was a water wave or a sound wave. We put it in another experiment and we can bounce and measure its energy and momentum.
So it seems to be a material particle. And then we wonder, can we contradict ourselves?" Again, I'm trying to fake being Neils Bohr, which is hard to do. Not just the accent, but the -- his whole way of thinking was, so he was certainly mystifying and mystical, in the proper sense of the word of being a mystic. But he was reaching for some accommodation and which concepts were useful, but he didn't claim that the concepts described nature. They described our interaction with nature. And they could contradict each other. Because something can't be both a wave and a particle because a wave is spread out and a particle is localized on a trajectory.
So it seems like you're in danger of contradicting yourself. And he said, well, but maybe the situation is so beautiful that there are subtle barriers that prevent you from ever saying it's a wave-like thing and it's a particle-like thing at the same time. Then he said-- because he generalized right away because he saw bigger applications. And he said, well, when people talk about God's love, they -- then they talk about God's justice the next Sunday. [INAUDIBLE] between God's love and God's justice, there is a contradiction. If God's justice is true and strict, then-- or as a parent, we run into this all the time. How do we be just and loving at the same time?
And so Neils Bohr said, "Maybe there's a general concept to carry these way -- to express the way that these contradictory concepts seem part of the whole. And he called that complementarity. And he proposed that this was a new principle for science, for human life, for religion, for art, for everything." He got quite ambitious, [INAUDIBLE]. The story that I tell in the book, and that first 2/3 of the book is built around, is the battle between the realists and the non-realist, or the anti-realists.
And in the way that quantum mechanics was textbook formulated, and the way that we teach it now, was finalized in 1927, roughly. And that was done along an anti-realist line. But simultaneously, there were a handful of physicists -- and these were not quacks. These were good physicists. Some of them were Nobel Prize winners, or shortly to be, who invented realist alternatives. And one of those that I talk about is what's called the pilot wave theory of Louis de Broglie, who was a Frenchman, which apparently put him socially a little bit in the outgroup anyway at this period. And he was wealthy.
In fact, he was an aristocrat, which also put out of the social group. So he wasn't really hanging out. And there's a whole lot to say about how that separation let him have an idea that nobody else had, which was an obvious idea. First, an idea that eventually everybody embraced, that the wave particle paradox -- the wave particle duality, as Einstein called it, applies not only just to light, but to electrons, and he was the first person to say that the duality between waves and particles applies to everything -- all material particles, as well as radiation and light. And that was accepted, and for that, he was given the Nobel Prize.
But he went on to say that he could explain the paradox that Bohr wrestled with by saying there are waves and there are particles. Both exist, both are real. The waves propagate and the particles follow the waves. And this is what he called the Pilot Wave theory because the waves are guiding, or piloting, the particles around. And this works. He developed it in the late '20s at the same time as the other theory was developed. It predicts -- it gives all the same predictions, and explains at least as much, if not more, than the standard version of quantum mechanics. It was done by somebody who was quite famous, and to his other work, and yet it was ignored -- completely ignored. So there's no textbook.
Except for a small school of accolades in France, in Paris, around him, there was virtually nobody who developed his idea. And in fact, he even came to disbelieve in it. Everything that ordinary quantum mechanics explains, the Pilot Wave theory explains, but the reverse is not true because there's a long list of things that the regular theory does not explain. Regular theory does not explain, if you have a radioactive nuclei, exactly when it will decay. It just predicts that there is a certain probability per unit time that the atom or the nuclei will decay. But it doesn't tell you why and when it decays.
It chooses some moment, but it doesn't tell you why that moment rather than another moment. If you want to describe an electron and its motion within an atom, the standard theory just says there is a wave function, which is a probability distribution, to find the electron somewhere in the vicinity of the atom. But it doesn't tell you where it is or how it's moving. And then a photon comes and knocks it out of the atom. It doesn't tell you exactly how it absorbed the energy of that photon, how it reacted to it, and how it jumped out of the atom. The "it" being the electron. The pilot wave theory tells you, in detail, everything. Exactly how, why, and when. So the regular theory is probabilistic.
Chance is fundamental. Uncertainty is fundamental. Statistics is necessary. The pilot wave theory is deterministic, like old-fashioned Newtonian physics. And if you employ it, you can explain everything that happens, and exactly how and why and when and how. In physics, we have something where you think of a particle or a photon, and we ascribe properties to it, like energy or position or momentum or wavelength or polarization. We have various properties we ascribe to particles. Then, supposing we have two particles, and they're in some kind of interaction together. They maybe are charged in each and are repelling each other or attracting each other.
They each have a long list of properties. But in quantum mechanics, something new happens. In quantum mechanics, the first thing that happens is you take away from the description. So an important principle of quantum mechanics is that if you have the list of all the properties some particle has, you can only know half of them at any one time. That's called the uncertainty principle. That's part of the uncertainty principle. But you can choose which half it will. Now, if you have two particles making up a system, that's true of a description of each of them and the two put together. And it turns out that you can choose properties to describe the two put together that are not properties of either of them separately.
For example, there is a property called opposite. If you measure the same thing on each particle, it's totally random. It's totally unpredictable what you'll get for each of those measurements. But you can guarantee, in the state opposite, that they'll be opposite. So if you measure the momentum of particle one and it's going that way, particle two will be going this way. Or if particle one is going this way, particle two will be going this way. But if you just look at one of them, the probability is completely random which way it will be going. So in this state called opposite, there's no thing which is true for certain about particle one or particle two separately, but there are things which are true about their combination. And we call this entanglement. It's a property of quantum systems that Einstein first chanced upon in the 1930s as part of his mission to basically find a flaw a logical inconsistency in the theory.
And it turned out that what he was on the trail of was not a logical inconsistency, but this new feature of nature which he chanced upon and wrote about in the 1930s. Locality means that to affect a system, you have to go over and tickle it or touch it. I can't affect something in Alpha Centauri here just by something that I do here. I have to send a light signal or a piece of information, or a bomb or something to Alpha Centauri. And then, when it gets there, it has the effect. So that principle is called locality. And John Bell made a precise formulation of locality as follows. If you have two particles and you want to measure particle A and particle B, then the probabilities for the outcome of measuring probability B -- whatever you measure probability B -- can't depend on what you choose to do with particle A.
That's Bell's version of locality. Bell derived a consequence from that, a certain mathematical inequality that we don't need to get into. And then he proved one thing about that consequence. It was contradicted by quantum mechanics. So if quantum mechanics is true, that assumption of Bell Locality, as Bell formulated it, is false. Then some experimentalist got in the game. So Bell is the 1960s. His theorem is 1964. And by the early 1970s, there were some experimentalists who realized they could test this consequence of Bell's theorem in a real experiment. You could make a pair of particles or a pair of photons, let them separate and study their polarizations or their energy, or the direction of their flight. And you could actually test the principle of locality very directly.
And so these were experiments carried out first at Berkeley, I think, and they were muddy, and the results weren't reliable. And then at Harvard. Then finally, a group of people in Paris led by [INAUDIBLE] -- I think we're talking about roughly 1980 now -- got definitive results. And the definitive results are that locality is false. This has nothing to do with whether you believe in quantum mechanics or not. Nothing to do with anything because Bell's theorem has nothing in it but a few obvious assumptions, like the probabilities are numbers between 0 and 1. And this one assumption of locality. It's a beautiful theorem in its sparseness.
And the experiments, together with that theorem, tell us that locality is false. That is, there are states of pairs of particles where what the properties of particle B over here turn out to do depend on what you choose to measure or manipulate with particle A. And that's just true about nature. That's the most -- if that's not the most crazy shocking thing you've ever heard in science, I got to repeat everything I just said. In quantum mechanics, there is no explanation of it. It just comes out that way. If you want to have a theory like Pilot Wave theory that gives a complete description-- in other words, if you're not a realist, it's set up so that one of the questions you can't answer is how the information is transferred back and forth.
Quantum mechanics is structurally built so you can't get at that question. But Pilot Wave theory, and other alternatives to quantum mechanics that are realistic, have to address that question explicitly. They have to show you how the two systems talk to each other. And when they do, yes, they do violate the possibility that nothing can travel faster than light. Is that testable? The answer comes in two parts. At first, no, because these measurements are all random. Just taking what you measure on this side or what you measure on this side, just taken by themselves just look like random distribution, so you can't get any information about them.
About anything, let alone what happening over here. So it looks bad for sending information faster than light. But a few physicists -- for example, a guy called Anthony [? Valentini-- ?] have speculated you could throw the pilot wave theory into a kind of phase, like those complex systems people talk about different phases, order and chaotic, and equilibrium and non-equilibrium. He speculates that you can throw the particle wave system into a kind of disordered, far from equilibrium phase, where all of a sudden, you can send real signals between the two particles.
And it proves that you always have to be able to do that. Although practically, we may not be able to throw it into a non-equilibrium state, that it is a possible state of the system. And that tells you that if you're a realist, then locality is dead, and the idea, basically, that space is dead. Space is now a discredited concept. And something deeper is going on. Something welds the world together, which has to do with the histories of who talked with who. It doesn't have to do with how far away things are from each other. That logic of that histories telling you who talks to you, rather than locality telling you who talks to who, is the real lesson here, many of us think.
- Realists believe that there is an exactly understandable way the world is — one that describes processes independent of our intervention. Anti-realists, however, believe realism is too ambitious — too hard. They believe we pragmatically describe our interactions with nature — not truths that are independent of us.
- In nature, properties of Particle B may depend on what we choose to measure or manipulate with Particle A, even at great distances.
- In quantum mechanics, there is no explanation for this. "It just comes out that way," says Smolin. Realists struggle with this because it would imply certain things can travel faster than light, which still seems improbable.
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