100 Years Later, Quantum Science Is Still Weird

IRA FLATOW: This is Science Friday. I’m Ira Flatow. 100 years ago this summer, physicist Werner Heisenberg wrote a letter to Wolfgang Pauli, and in it, he revealed his new ideas about what would eventually be known as quantum theory. And 100 years later, that theory has been expanded into a field of science that has revolutionized the way we look at the world around us, from the ultra small to the ultra big, from subatomic particles to the makeup of the universe. But it’s still really weird and often counterintuitive.

Here to help us celebrate 100 years of quantum science and separate quantum fact from science fiction is Dr. Chad Orzel. He’s the R. Gordon Gould Associate Professor of Physics and Astronomy, also chair of the Department at Union College in good old Schenectady, New York. Welcome to the program.

CHAD ORZEL: Thank you for having me on.

IRA FLATOW: OK. 100 years. But what actually are we observing and celebrating in detail?

CHAD ORZEL: So Heisenberg is really kind of a late stage in the early development of quantum. Really it’s, in some sense, it’s the 125th anniversary of the kickoff, which was Max Planck in 1900, who was desperately trying to explain the color of light from a hot object. And he had to resort to this weird mathematical trick to make the equations work out, where he essentially assigned an energy to the frequency of light.

And a few years later, Einstein picked that up and ran with it. Planck always thought it was an ugly mathematical trick. Einstein took it seriously, ran with it. And then around 1913, Niels Bohr proposed a quantum atom, sort of the cartoon solar system atom that you see in children’s books and that sort of thing with electrons going around the nucleus, which works brilliantly for hydrogen, but it’s kind of hard to make work in detail for anything else.

IRA FLATOW: Well, if this all happened 125 years ago, why are we celebrating 100 years ago?

CHAD ORZEL: So by the mid 1920s, it had gotten sort of untenable when people realized that what they were trying to do with the Bohr picture of the atom wasn’t really working. And Heisenberg is one of the people who really sat down and said, OK, let’s go back to basics and think about what really can we say about this? And realize that it didn’t make sense to talk about the electron following a definite orbit, but he had to talk about just what you could measure. And that is the first complete mathematical formulation of quantum mechanics.

IRA FLATOW: So what are the basic principles that make up quantum theory? I mean, I want you to sum it up in 25 words or less.

CHAD ORZEL: So the basic idea is you can think of everything in the universe as having both a particle like character. You can count things. You can detect things at a particular instant and a particular position. And also some wave-like characters. They’re sort of distributed over space. They’re delocalized a little bit. They interfere with each other if you allow them to take two different paths.

And that gives you this weird combination of properties. It’s not that they’re really particles or really waves. They’re really a third kind of thing that has some characteristics of each. And that’s not like anything we experience in the world around us, which is why the whole theory seems strange.

IRA FLATOW: So many of the ideas that come out of quantum theory just seem so weird. Stuff can pop out of empty space. You have two particles that can be linked across space, entanglement. How real these effects are?

CHAD ORZEL: The fascinating thing is all of these things are absolutely and verifiably real. The idea of entanglement, that’s Einstein’s last really great contribution to this debate, this notion of it being problematic that these particles can be correlated across distance in ways that seem to violate relativity.

A guy named John Bell in the 1960s showed that there’s an experiment you can do to test whether that non-locality is real. And people subsequently went and did those experiments. And every time this has been tested, the result is very clear that the sort of theory Einstein preferred is ruled out and the quantum theory where these things are correlated across space and time in ways that seem to defy our intuition is absolutely true.

IRA FLATOW: When Einstein called it spooky action at a distance, was that pejorative? Was he making fun of the idea?

CHAD ORZEL: He was talking about the notion that the things could change instantaneously over a wide area of space. And he found that idea really ridiculous and outlandish as something that simply couldn’t be allowed in large part because he had spent so much time with the development of relativity, working out exactly the restrictions on sending information through space and the relationship between space and time. And something that goes against that was really offensive to me.

IRA FLATOW: Well, that was the idea that light is a constant. Nothing can go faster than it. How could spooky action at a distance, how could something so far away react instantaneously?

CHAD ORZEL: Exactly.

IRA FLATOW: Was there an answer to that?

CHAD ORZEL: The modern understanding is that this very definitely does happen. And then there’s questions about how you interpret that. What do you say goes on inside the box? We can do these experiments and we see that you have a particle in Vienna and another particle in London, and you measure them very close together in time and their properties turn out to be extremely well correlated. But why that is, is open to debate.

Some people say it’s really all just about information. All we’re doing is we’re learning things about what we know about the outcomes of measurements. Other people say that there is some weird sort of collapse that happens that doesn’t respect the boundaries of space in the way that we normally think of and that we just have to deal with.

IRA FLATOW: If these early papers that we’re celebrating 100 years of now, if they were the introduction to the field, and even going back, as you say, 1900, 1910, 1920s, if they were the first chapters of a quantum physics book, let’s say, what chapter would you say we are in now?

CHAD ORZEL: I would say your Planck Einstein is chapter one. The Bohr old quantum theory is two. What we’re celebrating is really the start of chapter three. And then I would say there’s probably three more after that. There’s the solidification of quantum theory and the development of QED in the 1950s. Kind of ’60s and ’70s, you get the particle physics revolution and the development of the standard model, which is really closed off with the discovery of the Higgs boson in 2012. But the basic framework is really pretty well nailed down by the mid ’80s.

And then ’80s, ’90s, up into today, you get these experiments where we have the ability to control the states of atoms and light with sufficient precision that we can start to really probe these bizarre quantum things. And that leads to things like quantum information science and quantum computing that are some of the most exciting things going now. And I think that’s its own chapter, the ending of which hasn’t been written yet.

IRA FLATOW: So that’s the frontier now is what you’re saying.

CHAD ORZEL: Where we’re at now is sort of a combination of getting these applications of the weirdness, the things that allow you to do kinds of calculations quickly that you wouldn’t be able to do with a standard classical computer. That’s really one of the great frontiers of this.

Another that kind of brings in general relativity is the notion of trying to reconcile quantum mechanics with general relativity, which is an incredibly hard problem.

IRA FLATOW: Are there limits to how big a thing quantum rules apply to? Can it govern big things above x, let’s say?

CHAD ORZEL: So that’s an open question. And there are people working very hard on those kinds of problems. There’s some groups that have made very nice careers for themselves out of demonstrating the wave nature of progressively larger molecules. They’re up to these big almost borderline viruses that they can do interference experiments with, send them through multiple slits and see that they interfere like waves afterwards.

And one of the open questions is how far can you push that? Can you get to things the size of red blood cells? Can you get to bigger than that? And there are some of these models of how to combine quantum with gravity or how to address these problems of what’s going on with quantum measurements. Some of them say that there should be an upper limit, that if you get to a certain size, that other effects should kick in and destroy the quantumness of things. And so that’s an open question that people are working very hard to probe.

IRA FLATOW: You brought up the magic word slits, because that’s where a lot of this all started. And I want to bring that up because recently, MIT researchers did an experiment that was a version of the famous double slit experiment. And some of the interpretations of their experiment says that Einstein was slightly wrong. Can you tell us about that? What was going on there?

CHAD ORZEL: Yeah, so this is a famous experiment where you send light through or an electron, because it behaves like a wave, you send it through a slit. And that light will diffract and it will behave in a wave-like manner. And there are certain places where you’ll see a high probability of observing the electron later, and certain places where you’ll see 0 probability.

IRA FLATOW: Right.

CHAD ORZEL: And Einstein’s claim was, well, could put that slit on some kind of spring that let it move back and forth, and that would– measuring how much the slit moved would tell you which direction the particle was kicked and how hard, and then you would know both the momentum of this thing and the position. And Niels Bohr pointed out that, no, if you do that, then you’re smearing out where the slit is in a way that destroys that interference pattern. You no longer have these places where there’s 0 probability of finding the electron.

What this MIT experiment does is they scattered light off in an array of atoms that were held in position, and they could hold them in position very loosely or very tightly. If they hold them very tightly, they show that there’s this clear wave behavior. There are these places where there’s 0 probability of seeing any light.

And that if instead they hold them very loosely, the atoms are able to recoil in a way that in principle could give you information about which way the light was deflected. And when that happens, your interference goes away and you start to see the disappearance of these regions where there’s interference and no probability of seeing the light. And it agrees beautifully with the quantum prediction and so goes against what Einstein was hoping to be able to do with that apparatus.

IRA FLATOW: On this hundredth anniversary, so to speak, of quantum mechanics, if you ask the average person if they anything about quantum mechanics, they’ll probably say if they do, Schrodinger’s cat.

CHAD ORZEL: Yep.

IRA FLATOW: Right? What year was that and why was that so important?

CHAD ORZEL: Schrodinger’s cat. He first puts that out in I think 1935. He and Einstein at about that same time were both really dissatisfied with quantum mechanics on a philosophical level and leaving the field. Einstein’s parting shot was this paper with Podolsky and Rosen that introduced the idea of entanglement. Schrodinger wrote this article about how he also thought that quantum mechanics was fundamentally incomplete. And one of his illustrations is this cat thought experiment.

One of the big points he was making was that you could take something that everybody agreed was quantum, like an atom, and put it in this superposition state where it’s either decayed or not decayed. And then you can tie that inextricably to the state of something very big that everybody agreed was not quantum, like a cat. And the cat is either alive or dead, but the state of the cat depends wholly on the state of the atom.

And he was saying that this division between quantum and not quantum just doesn’t make any sense, because you can do things like that. And that’s really philosophically troubling and also enormously productive, because people have been arguing about that for decades now.

IRA FLATOW: Yeah, it’s still kind of mind numbing when you think about it.

CHAD ORZEL: Yeah, it’s amazing, but inspired a huge number of really cool experiments over the years. So it’s great stuff.

IRA FLATOW: Yeah. Yeah, let me move from there to one of my favorite topics, which is the dark universe. We are in a universe that we don’t know what most of it is made out of, right? Like 90%, 95% is dark energy, dark matter. Do we need new physics to make all of this work, or just better understand the rules we already have?

CHAD ORZEL: Pretty definitely we need new physics for that. The dark matter, you can show there are some good arguments that whatever the dark matter is, it can’t be made up of particles that we already know about. And the idea of dark energy is you can make some plausible guesses as to what would be the cause of this sort of pressure that’s causing the universe expansion to accelerate. But getting the numbers to work out requires some really implausible hand-waving, unless you have some other new physics acting.

So the best current guess is that the dark matter is probably made up of some kinds of particles that we haven’t yet been able to detect. And so there’s a huge variety of experiments out there looking in different ranges of different types of new particles, different masses, different interaction strengths, and so on, and really trying to search as many places as we can to find what the thing is that’s accounting for the dark matter.

IRA FLATOW: I’m going to ask you to pull out your crystal ball. I have my own right here. And look 100 years into the future for quantum science. Tell me what you see.

CHAD ORZEL: 100 years into the future for quantum science, I think I’m hopeful that somebody will have thought of an experiment that distinguishes between these different interpretations, that allows us to say once and for all, OK, are we really in a world where we have this complicated branching wave function in this massive superposition state that effectively looks like different universes. Or is there something going on that causes a real collapse of the wave function and changes reality in a fundamental way?

These are ultimately questions of philosophy that somebody needs to think very hard about. What are the premises that underlie these different approaches? And is there a way to push those to a limit where we see different answers to the same question and could test that experimentally?

That took 30 years with the EPR experiment, this Einstein Podolsky Rosen entanglement paper. Took 30 years before John Bell came up with an experiment you could do that would distinguish between these. So another 100 is probably good for this much harder question of what’s going on and make these measurements.

IRA FLATOW: Well, let’s all meet back here 100 years from now and see if you were right.

CHAD ORZEL: I would love to.

IRA FLATOW: Thank you, Dr. Orzel, for taking time to be with us today. It’s fascinating.

CHAD ORZEL: Thank you very much. This was fun.

IRA FLATOW: Dr. Chad Orzel is the R. Gordon Gould Associate Professor of Physics and Astronomy, and also Chair of the Department at Union College in Schenectady, New York.

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