Ask A Quantum Mechanic
IRA FLATOW, HOST:
Speaking of upside-down, there is no more a topsy-turvy world, a mysterious place that's hard to believe than the quantum world. Now I'm going to warn you, it's a weird place where all our intuitions about how things should behave, they fall apart. For example, electrons can be here and there at the same time. Imagine if that happened with your eyeglasses, like in the last segment.
And when you're not looking at something, you can't be sure if it's there or not. If you do happen to turn your head to examine something, you might disturb it, and it may not appear as it was before. Sounds a little like "Alice in Wonderland," doesn't it?
Well, luckily we have a tour guide who's going to walk us through the scary stuff, answer your questions, too. Continuing our Ask an Expert series, it's Ask a Quantum Mechanic. And we have the perfect guy. His lab motto is: If your quanta are broke, we fix them. I wonder if they make crescent wrenches for quantum mechanics. We'll ask him. Seth Lloyd is professor of mechanical engineering at MIT in Cambridge. He joins us from WBUR.
And if you want to talk to Seth, our number is 1-800-989-8255, 1-800-989-TALK. You can also tweet us @scifri. You might be getting through and listening to us on the Internet today, and you can tweet us @scifri, S-C-I-F-R-I. Ask questions of a quantum mechanic. Welcome, Seth.
DR. SETH LLOYD: Hey, Ira.
FLATOW: How do you begin to, you know, explain to people what quantum mechanics is?
LLOYD: Yeah, well, quantum mechanics is the branch of science that deals with stuff when they're really small and also when they're really big because in fact quantum mechanics pervades the universe to the largest scales, as well. And I guess the way I would start is by saying that quantum mechanics is weird.
This is a technical term.
LLOYD: It means it's strange. It's like as James Brown said when asked what he was going to play next: I don't know, but it's got to be funky.
FLATOW: It's funky.
FLATOW: Funky is a nice way to describe the quantum world. Funky. And the famous line, I think that you know and we all know, that is the most famous line about quantum mechanics is the Richard Feynman line, right? About...
LLOYD: Which line would be...
FLATOW: Well, he has a lot of them but the one about if anybody who tells you why or how it works that we know it's true, but if they understand why it works, they say that, they're lying.
LLOYD: Yeah, that's right. Yeah, Niels Bohr also said anybody who thinks they can contemplate quantum mechanics without getting dizzy, like the upside-down eyeball man of the previous segment, well, they haven't understood it. Of course, he said it in Danish, which is more impressive.
FLATOW: Let me ask you then to begin with something that people have heard of all the time, but they may not quite understand it, and that is the Schrodinger's cat paradox. What is going on, and why is that even there?
LLOYD: Yeah, so in quantum mechanics, it's pretty common, in fact it's pretty much mandatory, for things to be in two places at the same time. So, something like an electron, which is, you know, a very quantum mechanical thing, can be both here and there simultaneously.
Well, you know, if you have an apparatus that takes electron over here and, you know, strokes your cat, and then the apparatus that the electron's over there, it strangles your cat, well then your cat is alive and dead at the same time.
I think when Schrodinger proposed this, I don't think the SPCA was as strong as it is now.
FLATOW: Yeah, he would never have gotten away with it. So what happens when you open the box?
LLOYD: Well, so if you open the box and look at it, you're either going to find the cat to be either alive or dead. But the funny thing comes about because it's possible to actually make a measurement in principle that would reveal the cat to be alive and dead at the same time. I was just - actually last week, I was at the University of Vienna, and I was in a kitchen, and Schrodinger's desk was in the kitchen.
And I have to say, I was very disappointed that it was only in the kitchen on the third floor and not in the kitchen on the second floor at the same time.
FLATOW: That's because you actually were looking at it at that point.
LLOYD: Yeah, yeah, that's right. (Unintelligible) cups of coffee.
FLATOW: Explain that whole idea about - I remember John Wheeler(ph), who, you know, was one of the famous - he used to talk famously about quantum mechanics. He had the idea that - and he tried to explain this to me 30 years ago, and I'm still trying to grasp it - that the act of looking at something then makes it happen.
LLOYD: Yeah, you can think of it that way. Of course, John was a wild and crazy man himself, and he used to draw this, for many of his talks, this picture of the universe like a big U with an eye on one part of it investigating the other half of the U, kind of crazed.
And so, what makes things be in one place or another is looking at it, but it doesn't have to be just looking at it. You can just have - you know, if another electron whizzes by and whacks off the electron over here, well then it's done what is called de-cohered the electron. So the first electron is now not here and there at the same time but in either here or there.
FLATOW: So then the act of observation, not just by you but even another electron coming off, I guess observation would be if you shine a light on it, you're shining a lot of electrons on it. That would be sealing its fate.
LLOYD: It's really de-coherent. Yeah, the famous physicist Eugene Wigner once claimed that it was only when consciousness perceived quantum things that they went from being here and there at once to being either here or there. But it doesn't take much consciousness. An electron is not a very conscious thing last time I checked.
FLATOW: Well, is there any evidence in the macro world that's not in the subatomic world? Can we see any evidence of it?
LLOYD: For sure, in fact this year's Nobel Prize in physics was given to Dave Wineland at the National Institutes of Standards and Technology and Serge Haroche at the École Normale Supérieure for demonstrating not Schrodinger's cats because of the things they were making dead and alive at the same time, but what they called Schrodinger's kittens.
So these are relatively macroscopic sets of particles, in their case particles of light, or photons, that are both here and there simultaneously. And I myself participated in a great experiment done by Casper Vandervole(ph) in which we took a superconducting loop and had a gajillion electrons going clockwise and counterclockwise simultaneously. A gajillion is, again, a technical term.
FLATOW: Yeah, and so what happened?
LLOYD: They went both ways at the same time. I mean, I don't know. We weren't looking at them, I'll tell you that much.
FLATOW: Both ways at the same - how do you know they're going both ways at the same time? If you can't look at it and observe it, right, which would ruin the experiment, how do you know they're both going in...
LLOYD: Yeah, so it depends on what question you asked them. So if you asked them are you going clockwise, or are you going counterclockwise, they're just going to show going clockwise or counterclockwise. But the guts of this experiment was to come up with a sneaky way of saying: Hey, are you going clockwise and counterclockwise at the same time? And they said: Yup, uh-huh, yes we are.
FLATOW: Yes, the old joke quantum-mechanically placed. 1-800-989-8255. Seth Lloyd is with us to talk about questions about quantum mechanics, if you want to ask him. You can also tweet us @scifri, @-S-C-I-F-R-I. And maybe we'll get into a little bit of Einstein and quantum science when we come back after the break. Stay with us.
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FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.
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FLATOW: I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.
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FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow. We're talking about quantum mechanics, everything you've ever wanted to know. And someone who knows everything we want to know is Seth Lloyd, he's our guide on quantum mechanics. And we asked some interesting questions. We posed a question on Reddit, asked people to ask questions on Reddit, and one interesting one, has always boggled my mind, and I've only been reading this for 40 years, so here it is:
It's often said that quantum entanglement, quantum entanglement is spooky action at a distance and instantaneous. However, it is impossible to transmit information faster than the speed of light. So, one, what is meant by instantaneous in this sense; and two, what is being transmitted between the entangled particles? I'm putting my feet up on this one, so...
LLOYD: Yeah, yeah, so quantum entanglement or spooky action at a distance, or as Einstein put it, (speaking in German), sounds cooler in German somehow...
FLATOW: Well, he had a Nobel Prize for it.
LLOYD: Yeah, actually, in fact, you know, Einstein got his Nobel Prize for his work on quantum mechanics, not in special relativity. And the funny thing about Einstein and quantum mechanics is he never believed it. He always, you know, he said basically, the problem is that quantum mechanics is very counterintuitive.
It's counterintuitive to me, it was counterintuitive to Einstein, the difference here being that Einstein deserved to trust his own intuition. So he did, and he said it's got to be wrong.
In my case, you know, my intuition is usually wrong. So if my intuition says hey, things can't be, you know, two places at the same time or have spooky action at a distance, I say what the heck, it could be true.
FLATOW: Well what is it? What is instantaneous? Why is it not violating this speed-of-light concept?
LLOYD: So entanglement comes from taking this notion of things being two places at the same time and having things in two places and in two different states at the same time. So you can have a state where, you know, photon over here is wiggling from left to right; it's horizontally polarized, correlated with photon over on this other place being horizontally polarized, as well, or they could both be vertically polarized.
But now the funny thing in entanglement is that they're both horizontally polarized and both vertically polarized at the same time. Now on the face of it, that doesn't sound so bad. But the problem is if you measure one photon, say the photon on the left, and you find it to be horizontally polarized, that means instantaneously the photon on the right has gone from being in some sense vertically polarized and horizontally polarized to being just horizontally polarized. So just wiggling back and forth rather than wiggling up and down.
And so, when our classical intuitions try to contemplate this state, or this funny phenomenon called entanglement, it seems to - our intuition is that information is being sent instantaneously from one place to another.
FLATOW: But you're saying it's not being sent.
LLOYD: That's right. So - well, there's a caveat here having to do with black holes and closed time-light curves. But we can get to that in a moment.
LLOYD: I know you like to talk about that stuff, Ira.
FLATOW: Stay tuned. No I do, but I still - you know, I still can't get past this - there's no information going between one to the other. So it doesn't violate the law, right?
LLOYD: Right. So if you actually try to send information, what you have to do is you have to do something over on Point A, which changes the probabilities of something happening at Point B. And the measuring over Point A, you have a 50-percent chance of finding the photon wiggling back and forth rather than up and down.
And before you measure the photon at Point B, it also has a 50-percent chance of being wiggling back and forth or up and down. Now afterwards when you measure, you know for sure, if you get the answer back and forth, that the other photon (technical difficulties) back and forth. But for an observer on the other side who makes a measurement, they don't know that it should be back and forth, so when they make a measurement for them, it's still 50-percent back and forth and 50-percent up and down. So you can't send information.
The problem is that for classical intuitions, it appears as if something is happening instantaneously, but quantum mechanically, nothing is happening instantaneously.
FLATOW: Thus Einstein called it spooky action at a distance, 'cause it makes your hair hurt.
LLOYD: Yeah, that's right.
LLOYD: Exactly or stand on end. If I had hair, I would have it stand on end.
FLATOW: I'm sorry, I didn't mean to bring that up.
LLOYD: Yeah, well, in another quantum world out there, I have a full, bushy head.
FLATOW: Are there - I mean, are there supposedly many, many quantum worlds out there, many quantum universes, or is that a different theory?
LLOYD: This is another strange and counterintuitive feature of quantum mechanics, which is related to entanglement. So in this case of the photons, or when I measure one, I - you know, before they're both up, both back and forth or both up and down at the same time, and when I measure, I only get one of these possibilities.
But if you look at what happens quantum mechanically, including a quantum description with me, what's happened quantum mechanically is the world has effectively split in two, and there's two me's, one of which got them going back and forth and the other one of which got them going up and down.
So this is a phenomenon that goes with the name of the many worlds theory, which says that every time I or you or anybody makes a quantum measurement, the world splits into different possibilities, in each one of which one of the possible things has happened.
FLATOW: Even though we can't understand it intuitively, it's widely accepted because we can test it out, and it works?
LLOYD: Well, the many worlds theory...
FLATOW: Would that be fair? I mean and all of quantum mechanics in general?
LLOYD: Oh all of quantum mechanics, yeah. So, well, you know, there are some folks like Einstein who never really accepted quantum mechanics, and there are even Nobel laureates alive today who think that quantum mechanics isn't the end, the be-all and the end-all. And that's just because they trust their intuitions, which says it's wrong.
The thing is that quantum mechanics is the - if you look at all of our physical theories, with the possible exception of natural selection, it has the most number of pieces of confirming evidence. You know, every time the Large Hadron Collider, you know, in the course of one second it collects trillions of bits of evidence that quantum mechanics is the case.
And nobody's ever collected a single piece of evidence that quantum mechanics is not the case. So from the kind of preponderance of evidence perspective, you've just got to suck it up.
FLATOW: Let's - that's a segue I'm not going to touch. Let's go to the phones, to Jerry(ph) in Columbia, South Carolina. Hi, Jerry.
JERRY: Hi Ira, how are you?
FLATOW: Fine, how are you?
JERRY: I - the question that I have for the guest is that from everything that I understand about quantum mechanics, fundamental particles that are so small we have those instances when we observe them, and that seems to affect them, we know that. But it seems like scientists are making an assumption. Basically, there are those moments when this particle is not being observed, and there are those moments when it is being observed.
What leads scientists to say OK, here's an electron that's just made contact with something, and now after that we're going to assume that it's still an electron and it's still behaving the way that it was behaving when we observed it?
Isn't it possible that subatomic particles are not the way that we think they are when we're not observing them?
LLOYD: Well Jerry, that's a very good question. So of course anything is possible, and lord knows we don't know everything about subatomic particles. Like, we don't know what the dark energy in the universe is, we don't know what the cold dark matter is. But from the quantum mechanical perspective, basically it says when you observe this particle, you kind of pin it down.
So the way in quantum mechanics this works is that every particle, like an electron, has a wave that's associated with it. And the wave - the electron is like a point-like particle. It's just in one place, you would think. But the wave tells us where it is. And the wave can be in many places at once.
And so when you pin down the electron, you also pin its wave down to a particular point. But as soon as you've pinned it down, once you stop looking anymore, that wave starts spreading out and rippling and going many places at once.
FLATOW: Can you describe it as there being an infinite number of possibilities in the quantum world, but when you then observe one of them, all the rest of them go away, and you were left with the one you're observing?
LLOYD: Yeah, I think that that's a very good way to describe this. And this actually tells us a nice way to think of this many worlds theory. So in the many worlds, there are all these different possibilities. There are - actually, interestingly, it's only a finite number of possibilities, not an infinite number. It's just a very large number of possibilities.
FLATOW: I failed that test.
LLOYD: That's OK. But so one way to think of this many worlds theory is that all of these realities exist simultaneously. But another way, Ira, is the way you just described it, to say when we make an observation, we just get one piece of this, and the other ones might be there in some funny quantum mechanical sense, but so far as we're concerned, they're not there because we, you know, they're basically the things that are not the case. You know, the electron, I measure the electron. I find it here.
LLOYD: The other part of the wave function, the electron is over there. To heck with that. I don't care about that other part of the wave function.
FLATOW: Yeah. Is there something about - possible about quantum biology?
LLOYD: This is a really interesting question. So, for many decades, most people thought, well, you know, in quantum mechanics, it's tough for things to have this two-places-at-once kind of thing going on if they're constantly being bombarded by the environment because it's these interactions with the environment that so-called decohere the quantum system and prevent things from being too places at once.
But remarkably, over the last five years, a whole bunch of experimental evidence has accumulated, starting with work from the Fleming Group in Berkeley, that in photosynthesis, for example, that as energy makes its way through photosynthetic complexes, after, you know, photosynthesis, light comes in, creates a particle of energy called an exciton. It's kind of a nice name for a...
LLOYD: ...particle of energy. That this exciton, as it's going through the photo complex to get turned eventually into chemical energy, that it's taking multiple paths at once. So the exciton really is in more than one place simultaneously. And moreover, this funky quantumness is what's responsible for the tremendous efficiency of energy transport in photosynthesis, which is upwards of 99 percent.
FLATOW: So it is actually working in a macro world, but we just don't see it in action.
LLOYD: Yeah. That's right. I mean, that's right. Every time - actually, every time we put on sunblock, we're responding to this because when we - our skins also absorb energy from light, and excitons are created. And the problem is that excitons can be bad and mess things up, so...
LLOYD: ...we're actually responding to quantum weirdness when we slather ourselves with white stuff.
FLATOW: Hmm. Speaking of weirdness, Dana(ph) is on the phone from Plymouth, Mass. Hi, Dana. Welcome to SCIENCE FRIDAY.
DANA: Hi. Thank you. Well, you kind of stole my thunder. You answered my question, actually, about Einstein and why he couldn't fully embrace quantum physics. So can I ask another question?
FLATOW: Sure. Sure.
FLATOW: Free one today.
DANA: Can you please spell out Planck's constant? I read about it, and I kind of understand. I can't wrap my head around it. Maybe if I hear another, you know...
DANA: ...a different explanation. Yeah.
FLATOW: I know. You know, particle physics, you got to read it 40,000 times.
FLATOW: I get you. I get you. I'm with you on this, you know? How many books I have read on this stuff over and over? It's like reading it new the first time. Thanks a lot, Dana. We'll see if we can get - what is Planck's constant?
LLOYD: Yeah. So Planck's constant dates to the very first paper on quantum mechanics written by Max Planck in 1900. And he was concerned because when people did calculations of how much energy was being emitted in the light coming out of a hot stove, you know, when it's glowing red-hot or white-hot, that if you do the classical calculation of how much energy is coming out, the amount of energy should be infinite, which would be kind of a bummer. It meant, you know, you could really - you don't - you don't just want to get warmed by the stove. You want - don't want to get vaporized.
LLOYD: Actually, also the amount of information coming out of the stove is infinite, which is also bad from the computational perspective. So he said, hey, you know, how about we just see what happens if the energy doesn't come out in a continuous form, in the form of waves, as Maxwell's equation would suggest, but it comes out in a - bits of chunks, chunks of energy? So - or which are he called quanta. So, you know, this is kind of the chunk style version of energy.
LLOYD: And he said, let's suppose, just for the heck of it, that of a - that light with - that's - with some frequency F comes out within a chunk of energy, a quantum with energy some constant times F. And he came up with this constant, which since been called Planck's constant, which sets the chunkiness scale of nature. And by doing this, he was able to show - get exactly the right answer for the spectrum of energy coming out of hot bodies.
FLATOW: Interesting. Interesting. Jason(ph) write - tweeting me. He's begging me. He's begging me to talk about quantum computing and the quantum mechanics relation. What is the future of quantum computing? We'll get your answer after I remind everybody that I'm Ira Flatow. This is SCIENCE FRIDAY from NPR. Seth Lloyd, take it away.
LLOYD: Yes. OK. So, yeah. So quanta computing, which is - it's kind of what I do on a daily basis, you know, getting our hands and arms and legs and entire faces dirty, messing around with those quanta. You know, that's why our lab motto is that we're - if your quanta are broke, we'll fix them.
FLATOW: You need a quanta washing machine then.
LLOYD: So - that's right. Well, actually, you know, we haven't gotten to quantum teleportation, which allows you to vaporize the particle over here and then have it rematerialize over there, which we - if we could apply to human beings, we wouldn't have to talk about quanta computing. We could talk about quantum commuting.
FLATOW: Wow. They figured that out on "Star Trek" already, I think.
LLOYD: That's right. So quanta computers are computers. You know, have - they flip bits, like any other computer, but in quanta computers, the bits are - all exist at this quantum mechanical level, so, you know, a quantum bit could be an electron that's over here, an electron that's over there. We'll call electron over here zero and electron over there one. But quantum bit or qubit, which, by the way, I used to believe was the distance from my elbow to my extended finger. But a qubit and...
FLATOW: I got it. I got it. I got it.
LLOYD: Yeah. OK. The electron can...
FLATOW: Behind the eight ball. I get it.
LLOYD: Yeah. The electron can be both here and there simultaneously. This is a normal quantum mechanical thing. So the resulting qubit then, in some funky quantum sense that we can't intuit, registers zero and one simultaneously. And when you turn this into a whole computer doing this, then you're going to have a computer, a quantum computer that's doing multiple things at once.
So, you know, if the electron is over here, that means, OK, calculate two plus two. Electron over here, that means calculate three plus one. Electron over here and there at the same time, that means calculate two plus two and three plus one simultaneously.
FLATOW: And so you can get all these electrons working at the same time and have mega computations.
LLOYD: That's right. And so the quantum computers can actually, in principle, at any rate, solve all kinds of problems that classical computers can't.
FLATOW: Is there any code then that they could not break?
LLOYD: Yeah, so the most famous problem that quantum computers could solve is code breaking. Essentially, if you look at the kinds of codes we use to, say, buy stuff over the Internet - and this is an appropriate time of the year to talk about this - those are based on certain problems being hard to solve, in this case, factoring large numbers. So you're given a large number and it's a product of two smaller numbers and you need to find those smaller numbers. Well, in 1994, Peter Shor at AT&T, now my colleague at MIT, pointed out that there's a kind of hidden wave-like nature to this problem. And if you could work on this wave-like nature using quantum mechanics, which, let's face it, is all wavy, then you could solve this problem in a flash, which would be kind of disruptive for the next time, like, I go online and try to buy green coffee beans or something like that.
FLATOW: Yeah. Well, Seth, we're very happy that you took a time out of your day to, you know, quantum - do they have quantum tools? You have crescent wrenches and things for your mechanics?
LLOYD: Yeah, you know, your average quantum crescent looks a lot like, you know, a laser or, you know, a helium dilution fridge. But, yeah, sure, you can get in there and, like, whip those quanta around with click and clack.
FLATOW: Well, when Seth sees a broken quanta, he fixes it. Thank you very much, Seth. Seth Lloyd is professor of mechanical engineering at Massachusetts Institute of Technology up there in Cambridge, Mass. Have a good holiday, Seth. Thanks for taking time to be with us today.
LLOYD: Thank you very much, Ira.
FLATOW: You're welcome. We're going to take a break. When we come back, we're going to talk about the Polaroid camera. Wouldn't you love to have instant photos? Aren't you just tired of doing all those Internet when you can just click it, comes right out? We'll talk about the history of Polaroid and Dr. Land behind it. So stay with us. We'll be right back after this break.
I'm Ira Flatow. This is SCIENCE FRIDAY from NPR. Transcript provided by NPR, Copyright NPR.