What is Quantum?

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0:30at Whole Foods Market. If your best finance people are doing expense reports, chasing receipts, or spending time on month-end clothes, it's time to get Brex AF, a gentic finance that eliminates that work before it starts. Learn more at brex.com slash AF. My stomach is lurching around. The rain is driving into my face. There's people holding onto benches and dancing around the deck. I don't know what the collective term is for a ferry full of physicists. Hello, I'm Marnie Chesterton, and I'm a science journalist.

1:10Marnie Chesterton:Are you going to be sick? Because I think I might be. And that's my producer, Anand. You're joining us at the start of a surreal journey we made this summer. Pitching and lurching across the North Sea, surrounded by screaming quantum physicists. People from every chapter of my life as a quantum scientist are here on a boat in a stormy ocean sailing to a random island in the middle of nowhere. That island, off the coast of Germany, is called Helgeland. It's a spot with an almost mythic status. It's the birthplace a century ago of quantum theory. There's something beautiful that we've made it to 100 years of quantum science that the field has matured and evolved.

2:02So for me, I mean, this is just a party. It's more than just an anniversary party, though. This is the chance for some of the greatest minds in the quantum world yes, there are multiple Nobel Prize winners on this ferry to get together at a conference and argue about how our world works at the smallest level. And I want to talk to them. I am not a physicist, but I'm just going to say it out loud, I'm not stupid, because I do need to keep reminding myself of that. Quantum is, as far as I can tell, the beating heart of how the world works. And it's also something that's got a reputation as totally impenetrable and very weird.

2:47What do we know? Here are the headlines. Quantum physics is the study of how matter and energy, particles and light, behave. It's an incredibly successful theory. It's helped us understand everything from the structure of atoms to why the sun shines. Oh, and it underpins all modern electronics. And yet, as anyone on this ferry will tell you, it's at odds with how we experience the world.

3:14Marnie Chesterton:Quantum mechanics is the most fundamental description of the microscopic world that we have at the moment. We still have to manage to connect it to our everyday experience and physicists are still not sure how to do that. It's quite exciting to work with physics that you don't really understand. You can see why people have trouble grasping a theory that apparently says particles can be in two states or even two places at once. You may have heard of quantum entanglement, this idea that two particles can have some sort of relationship so that what happens to one instantaneously affects the other, even at vast distances.

3:58If that wasn't odd enough, it even suggests that particles seem to know if you're looking at them. or at least that's what science journalists like me tend to say about quantum when we're talking about it on the radio before moving things along by saying something like it's strange but simply calling quantum theory weird feels like a cop-out what is it really able to tell us about the nature of reality that's what I'm hoping to learn in this documentary and to get there I think I need a different strategy. What we're going to do is give ourselves one question, and I'm only allowed to ask that question.

4:42Hopefully, that's going to force me to sit with some of the very uncomfortable aspects of this weird, strange branch of physics.

4:53Marnie Chesterton:And what's the one question? What is quantum? Isn't it going to be quite difficult if you can only ask them one question? Yes, but I'm hoping that we will get something unusual that we don't normally get when we talk about difficult science ideas. There are rules. I'm allowed to recap. I can say I don't understand. That's probably going to happen a lot. But what I can't do is pretend I got it and move the conversation on. If you try and ask another question, I've got to stop that from happening. Yeah. I'm not 100 % sure this is going to work, but there is only one way to find out, which I'm about to, because the seas have calmed and our ferry full of physicists has arrived at its destination.

5:43Oh, the cliffs of Helgoland have just risen out of the cloud and rain and mist. Not a moment too soon, I'm feeling really quite seasick. Ah, we've just come into the harbour. A hundred years ago, in 1925, a young German scientist called Werner Heisenberg also arrived on the shores of Helgoland. He came to this windswept, treeless island to clear his head, kind of literally because he wanted relief from the hay fever that crippled his ability to think straight. It was a strategy that worked, and here Heisenberg made a mathematical leap of understanding that laid the foundations for quantum mechanics and changed the world.

6:32So I've come up to the highest point of Helgoland, and I can feel the wind in my face. My word is windswept here. and this point is where there's a monument to Werner Heisenberg. Meeting me at this monument to Heisenberg is Carlo Rovelli, a quantum physicist who, fittingly, wrote a book called Helgeland. I was fascinated by this young Heisenberg discovery on this island. So this is actually the beginning of my book, which starts with a page of the journal. It was around three o 'clock in the morning when the final result of my calculation were before me. I felt profoundly shaken. I was so agitated that I could not sleep.

7:25I left the house and began walking slowly in the dark. I climbed on a rock overlooking the sea at the tip of the island and waited for the sun to come up. I love this idea that we're pretty much where he was. And I think if we go somewhere with a little less wind, I've got a question for you. OK, let's go. Just the one. OK. So, Carlo, we've walked to a clifftop overlooking the main town of Helgoland. The weather's turned nice, which is lovely. It's time for my question. What is quantum? Oh, boy. What is quantum? I would say that quantum is three discoveries. Two easy and one hard. The two easy are that everything comes in little bits, because quanta, lights come in little bits of photons, energy comes in little packets.

8:28So there's some granularity in the world. That's easy. Second is that the way the quanta moves, it's probabilistic, there's probability. For a while we hoped that we had exact equations that in principle tell us for sure what happened, but that's not true. The world is really probabilistic, and that's true. The third ingredient is a hard one, and is the most interesting one, is that any system, an atom, a galaxy, whatever, we make a mistake if we think that we can describe it just by itself independently, that it has its own state, its own properties. Any system can only be described as it is perceived by something else, as it affects something else.

9:17The weirdness of quantum comes from this third discovery, that if we try to make a picture of an atom just by itself, we fail, because that's not the way the world works. That's the core of quantum mechanics, in my opinion. Now would be a good point to say I don't understand. So my idea in my head, if you look at an atom, it's all balls. It's a ball in the middle of protons and neutrons. And then it's more balls called electrons that ping around the outside. That's my picture of reality. That's exactly your picture of reality. That's a common picture of reality. And I'm sorry I have to tell you it's wrong.

10:03I know I'm not allowed to ask you another question, but talk to me more about your third point. I'm not getting it. Again, again. So quantum mechanics is a discovery that if you look one way or another way, if we make a measurement of an atom in some way, the atom responds, but we should avoid it to ask the question, yeah, yeah, but what about the atom when it's not interacting, when it's not touching anything, nobody sees it, no other object interacts with it, what about it? Well, that's an ill-posed question. that doesn't mean anything. I've had that before from physicists. They went, that's not a valid question.

10:39I'm used to getting stuff like that from physicists. Often we learn that the correct answer to a question is that the question is meaningless. Early physics was a discovery that velocity is relative. You can say that something moves or does not move, but you're always referring to something. I move with respect to the Earth. I'm still with respect to the Earth but I'm still moving with respect to the Sun and so on so somebody could insist and say well that is ok but how do I know if something really moves or doesn't move and the answer is this is a meaningless question at some point you give up from your world view the requirement of saying things move or don't move so quantum mechanics does the same just take away something more heavy it takes away the idea that every single piece of the world has its own properties, position, velocity, whatever you want, well-defined.

11:36And it replaces with the idea that these properties come up in interaction. When one of your little balls hits something else, that's when it has a position. To be somewhere is to answer the question where you are.

11:58to be somewhere is to answer the question where you are i'm now struggling to answer the question of why i'm on an island gate crashing a quantum physics conference this type of thinking hurts my brain time for a tea break back at our b &b thank you so how are you feeling weirdly tired Brain's had a battering. So, three things about quantum theory.

12:30One, it's probabilistic. So you don't know where, say, your electron is going to be in the atom. But it's a cloud of possibility of places that it could be. What's the second thing?

12:48Marnie Chesterton:Granularity. Oh, yes, granularity. energy exists as little packets tick that's fine and then we're into where is this stuff so he said that um you have to let go of this idea that the electron can be in a place when you're not observing it or making a measurement of it the concept of location is meaningless when you're not looking at an electron or when it when it's not interacting with anything else which is i find that troubling because i want to have a picture of reality as it exists on its own i don't know for example where carlo revelli is right now he's on the island somewhere yeah kind of whizzing around the island but that's not the case with the lectrons is that not that they don't exist but they aren't anywhere when we don't look it doesn't have a place it's just it's not even a valid question to ask.

13:47Oh God, why am I struggling with this? See that duvet? This duvet over here. What I'm going to do for my insanity is just hide. I'm hiding from physics under a duvet.

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15:12Marnie Chesterton:So it's a new day, Marnie. How are you feeling? I mean, yesterday was tough, but, you know, I think this one question rule is actually going to help us get somewhere. OK, well, let's see if that's true. I've got another person for you to talk to, to ask your one question to. Hi, I'm Elise Kroll. I'm Associate Professor of Philosophy at the City University of New York. But my area of expertise is philosophy of physics and, in fact, history and philosophy of quantum physics. Okay, Elise, what is quantum? Not an easy thing to answer. I take quantum theory to be our best and most fundamental physical description of the world.

15:54And at its core is this idea that no system can be described as an object all on its own. When two quantum systems interact and then fly apart or cease interacting, You can no longer describe them as separate systems, but they are entangled. And then they're not independent anymore. And this is a radically different worldview than any classical theory gave us. And so what is quantum mechanics? It's about entanglement. And I think when people talk about quantum being weird, entanglement is one of the weirdest properties. Well, there is some sense, unless the physics is incorrect or incomplete, that you are entangled with the photons coming from the sunshine right now, from the breeze.

16:41It is very strange to think that somehow the light from Sirius B or something has entangled itself with properties here on Earth, but it has. So does that mean...

16:55Marnie Chesterton:Question. Oh, so I'm taking that to mean that all of my atoms could be over on that island in the distance. But that feels like nonsense. So it's not that your atoms are elsewhere. It's that part of your information is not local to you. Think about it this way, maybe. when we're doing classical physics we sort of imagine we can draw a lasso around space time and look at a system in fact entanglement seems to say that to list the properties of one system I have to refer to some other system and in fact those other systems might be very very far away the old schools of metaphysics used to say what it is to be an object is to be a thing located in one place with this list of properties, color, mass shape Things like this.

17:51And so for thousands of years, we've thought of metaphysics as describing well-defined objects. But nature is telling us in quantum theories that it refuses to be carved up in a principled way into the objects that we experience every day or stuff we don't experience like little quanta. But we don't see that, right? I mean, further interactions sort of damp down that entanglement so that we don't see it. and it maybe in fact doesn't physically change the way we deal with coffee cups and cats. Yeah, I'm not sure I understand at this point. I'll try to supply a metaphor for you. Classical metaphysics has tried to interpret the world, and to do that they've used a Lego brick.

18:37Little pieces that you can sort of tesseract together and build up a world. Quantum mechanics seems to say that the world is more like Jell-O. So I'm working on a paper right now called We All Live in a Jello Submarine. I know, right? So that's the big question. It's like, if I see all this classical stuff around me, why the heck should I adopt a metaphysical view that's Jello instead of one that's Legos? I'm very easily sold on the idea that we live in a Jello world. Like, it's good.

19:11Okay, so Elise has gone back to her next conference session. We all live in a jello universe. Simples.

19:18Marnie Chesterton:I can't believe how easily you swallowed that. Like, what does that even mean? Well, what she said is part of my information is not local to me. So I think that means some of the properties of my atoms aren't in the same place as the actual atoms. They're kind of smeared across the universe. I think I get that. I don't really know how I feel about it. It sort of feels so abstract and theoretical to the point of being ridiculous. I think this stuff is getting really intangible. So I think what we need to do next is go and call us someone who actually has a more practical perspective on this question.

19:57Marnie Chesterton:So my name's Michelle Simmons, and I'm the CEO and founder of Silicon Quantum Computing in Australia. So what is quantum? For me, I'm looking at quantum computing. so I'm really looking at information. I'm a very visual person, so imagine you're sitting in the middle of the Earth and you're pointing to the North Pole, that's your digital one of information, or you're pointing to the South Pole and that's your digital zero of information. But you can also point to anywhere on the surface of the Earth, in which case you're in a combination of some fraction of the upstate and some fraction of the downstate.

20:35Marnie Chesterton:And in the quantum world, we use those states in between. so one quantum bit called a qubit has two classical bits of information to describe it so if you've got one qubit question oh no statement you had an inflection okay yes i don't think i understand what you meant when you said it can be a bit of up and also a bit of down it is essentially a mathematical formulation that describes that quantum state and we can prove that the mass that describes them is what we're seeing and so once you get comfortable with the fact that the world is not just black and white there is states in between then in the quantum world you can start to use it because you understand it but then the most amazing thing is you can take two quantum bits two qubits and entangle them so that anything you do on one part of that automatically impacts the other part so any kind of coding that you do any operation, you do it in all the individual components at the same time.

21:38Marnie Chesterton:So it's a new paradigm of computing that we're looking at. The stuff that you've been telling me all seems very rooted in reality, and yet I was talking to our philosopher, Elise, about entanglement and the world potentially being made of jelly.

22:03and yes.

22:04Marnie Chesterton:So I'm a very practical person and we control entanglement in our labs and for quantum computing you need to be able to do that and so for me I see entanglement as a very practical way of accessing those states in between 1 and 0 and that's where this massively parallel computing comes from.

22:31cool all packed all good um thoughts okay my takeaway from talking to michelle is the quantum entanglement is real uh because it has to be because people like michelle are using it to build computers but i've still got this idea that quantum allows my atoms to connect up to faraway atoms in the jelly universe.

23:01Marnie Chesterton:Which is a kind of amazing image. But do you think that you like it because it's an image that you can kind of picture in your head? Yes. I mean, it is a wonderful image, but maybe we should just let go of having images. I mean, Carlo Rovelli said that he kind of said it's meaningless to try and have a picture of an atom as it exists on its own. But I like my picture of zoom in on things and you get to tiny balls whizzing round balls. And I think I should mention at this point that we are leaving behind lots of people arguing about different interpretations of quantum. So there's a lot to unpack.

23:40Marnie Chesterton:There is a lot to unpack. We, on the other hand, are fully packed. Nice. Because we've got to catch a ferry. But there is one more person I would like to talk to. I think we can get him on the way to the docks. He's written multiple books on the topic and he's probably the smartest person I know. I'm Philip Ball and I'm a science writer. So, Phil, I want to talk to you down at the dockside because I've been trying to get my head around what quantum is by asking people one question. What is quantum? So I have a way of thinking about it that helps me. And the way I think about it is what it seems to be telling us is that we can ask more apparently reasonable questions of the world than the world can give us answers to.

24:31So if we're in a car, we can ask at any point as we're driving along, where are we and how fast are we going? And those are reasonable questions and there are answers to both. if we ask that about an electron those aren't reasonable questions anymore and it's worse than that because the questions only become meaningful when we actually look to see where the electron is before we look saying where is the electron how fast is it going doesn't have an answer all quantum mechanics can give you is well there's a certain probability that it's this and a certain probability that it's that. Not because you don't yet know, because that is simply all you can say about reality.