(Podcast Version) Einstein's Quantum Riddle | NOVA | PBS | NOVA Remix

When you hear the word quantum, what do you think of? A sci-fi movie? Maybe a physics class you avoided? Something about it feels incomprehensible. But even Albert Einstein, who helped discover it, found it hard to believe. That's because it says that things that seem totally impossible happen all the time in the real world. Like two objects seemingly connected to one another, even when they're light years apart. But quantum mechanics is real. In fact, it's all around us and at work inside modern conveniences like cell phones. So, if we're already successfully using the quantum realm, what was Einstein missing? From GBH in Boston, this is Nova Remix. I'm Caitlyn Saxs. Quantum mechanics can feel futuristic, but it was debated all the way back in October of 1927 in Brussels at the fifth SV conference. Dr. David Kaiser, physicist and science historian, takes us back. — This amazing week-long series of discussions on really what the world is made of, on the nature of matter and the new quantum theory is a collection of some of the most brilliant people in the world. We see Albert Einstein and the great Marie Cury and Mox Plunk the dapper Erwin Schroinger and these sort of brash 20-year-olds are mid20s Veror Heisenberg and Bilf Gang Powi. These scientists were the pioneers of quantum mechanics and this was one of the greatest meetings of minds in history. More than half would eventually win Nobel prizes. Their work was showing that the fundamental building blocks of matter like the nucleus of an atom and its orbiting electrons were not solid little spheres. They were fuzzy and undefined. — So this group here, these were the folks who had just been plumbing deeper and deeper to find what they hoped would be a bedrock of what the world is made of. And to their surprise, they found things less and less solid as they dug in. This world was not tiny little bricks that got smaller and smaller. At some point the bricks gave way to this mush and what looked like solidity solidness in fact became very confusing and kind of a whole new way of thinking about nature. — Remember matter is made up of atoms and atoms themselves are made up of even smaller particles like electrons, protons and neutrons. — For the first time scientists were able to probe a world that was until then quite invisible to us. Looking at the world at the scale of atoms, a million times smaller than the width of a human hair. — That's Dr. Shini Goch, quantum physicist and computer scientist. To be clear, scientists weren't looking at atoms through some special microscope. They were using new equations and mathematics that modeled the subatomic world. These objects like electrons, atoms, when we describe mathematically their behavior, the only thing we can describe is the probability of being at one place or another. At the scale of subatomic particles, the best we can do is make educated guesses about even basic things like where something is or how fast it's moving. To understand how weird that is, it would be like being pulled over by a police officer who tells you that there was a 60% chance that your car was speeding and a 40% chance that it was on the road and that paper you're being handed has a 75% chance of being a ticket. Basically, it feels like nothing is certain. In our macro world, we expect physical objects to behave better than that. But these scientists were describing a world where at the smallest scale, physical reality was just surprisingly uncertain. That uncertainty is at the heart of quantum mechanics. The theory says that a particle like an electron isn't even physically real until it's measured by an instrument that can detect it. before it's detected. Instead of being a solid particle, an electron is just a fuzzy wave. A wave of probability that doesn't resolve into a particle until it's observed or interacts with something else. Here's how theoretical physicists Dr. Sha Carroll and Dr. Robert Dyraph put it. — It's like a wave of all those different possibilities. It's not that the electron is in one place or the other. We just don't know. is that the electron really is a combination of every possible place it could be until we look at it. — Laws of nature were no longer definite statements about what's going to happen next. They were just statements about probabilities. And Einstein felt, well, that's defeat. You're giving up on the heart of what physics has been, namely to give a complete description of reality. — Einstein was troubled, and it's easy to understand why. It goes completely against common sense and classical

physics. He asked, "Do you really believe the moon is not there when you are not looking at it? " Outside of the formal setting of the conference, he challenged the most vocal supporter of these ideas, the great physicist Neils Boore. — Einstein would show up to breakfast at the hotel and Neils Boore would be there and Einstein would present his latest challenge. Neils Boore would sort of mumble and wonder and confer with his younger colleagues. They'd head off to the formal meeting at the institute and somehow every night by supper time bore would have an answer. One of the observers said that Einstein was like a jack in the box. Every day he'd pop up with a new challenge and bore would flip this way and that and the end by supper have crushed that one and it would start all over again. — Even though Einstein had issues with what quantum mechanics implied for our universe, he hadn't found flaws in the equations themselves. Bor and his colleagues left the SVY meeting feeling more confident than ever in their ideas. But Einstein didn't give up trying to show that the new quantum view of nature was incomplete. At the Institute for Advanced Study in Princeton, New Jersey, he recruited physicists Boris Podilski and Nathan Rosen to write a paper with him. With some additional research, their paper known today by their initials EPR argued that the equations of quantum mechanics predicted a strange type of connection between particles. If the particles were linked in this way, then observing one would cause the other to snap out of its fuzzy state. It would be like having two particles, each hidden under a cup. Looking at one would mysteriously cause the other to instantly reveal itself with matching properties. Seems impossible, right? Einstein thought so, too. He figured any theory that predicted this behavior, which defied common sense, must be wrong, or at least halfbaked. And yet that 1935 EPR paper that described this weird relationship between particles known as entanglement has become Einstein's most cited work of all. It has captivated generations of physicists including Dr. Anton Zylinger. — Suppose I had a pair of quantum dice. I put these two quantum dice in my little cup. Throw them. I look at them. They show the same number six. I put them again in the cup. Throw them again. Now they both show three. I put them in again. Show again. Now they both show one. Point now being what I see here is I see two random processes. Namely each die showing some number but these two random processes do the same. That's really mindboggling — for Einstein. Entanglement conflicted with the most basic concept we use to describe reality. Space. How could one particle influence another without sending some kind of physical signal? According to his theory of special relativity with its famous equation E= MC^², nothing can move through space faster than the speed of light. And that includes information. So if two particles can instantly communicate and influence each other, quantum mechanics renders time and space meaningless. Then it blows a huge hole in our understanding of the universe. Essential to the EPR argument is that these particles can be separated at an arbitrary distance. One could be here at Princeton, one could be in the Andromeda galaxy. And yet, according to quantum mechanics, a choice to measure something here is somehow instantaneously affecting what could be said about this other particle. You can't go from Princeton to Andromeda instantly. And yet, that they argued is what the equations of quantum mechanics seem to imply. Einstein, of course, was the master of spaceime. He thought that if something happened here, that shouldn't immediately and instantaneously change something that is going on over there. The principle of locality as we currently call it. — Rather than a world where linked particles are undefined until they're observed, Einstein thought that particles must have a hidden layer of deeper physics that determine their properties from the start. With Pedulski and Rosen, he argued that the theory of quantum mechanics must be incomplete if it was missing these hidden variables that gave particles definite properties. Einstein dismissed entanglement as spooky action at a distance, a ridiculous feature revealing the shortcomings of quantum theory at the time. Of course, no one could think of an actual experiment to test whether Einstein or Boore was correct. But quantum mechanics proved useful in the real world. Physicists and engineers steamed ahead, using it to develop transistors, the first lasers, the atomic bomb. With so much innovation, it seemed Einstein's questions could wait. Nova and Nova Remix are supported by Carile Companies, a manufacturer of innovative building envelope systems. With buildings responsible for over a third of total energy use and energy demand on the rise, Carile's mission is to meet the challenge head on. Carlilele's advanced energy efficient and laborsaving solutions can help reduce strain on the grid. Operating

nearly 100 manufacturing facilities across North America, Carile is dedicated to helping the transition to a smarter, more sustainable future. Learn more at carile. com. Nova and Nova Remix are sponsored by Viking Cruises. Ocean voyages, expeditions, river journeys. Viking is dedicated to bringing travelers closer to the destination, offering a small ship experience and a shore excursion in every port. Learn more at viking. com. The world had moved forward using quantum mechanics and Einstein's questions remained unanswered until the 1960s when a physicist named Dr. John Bell made a remarkable breakthrough. — Quantum mechanics has been fantastically successful. So it is a very intriguing situation that at the foundation of all that impressive success there are these great doubts. Bel is a very talented young physics student but he quickly grew dissatisfied with what he considered almost a kind of dishonesty among his teachers. He got into shouting matches with his professors. Don't tell us that Boris solved all the problems. This really deserves further thought. It's a very strange thing that ever since the 1930s, the idea of sitting and thinking hard about the foundations of quantum mechanics has been disreputable among professional physicists. When people tried to do that, they were kicked out of physics departments. And so for someone like Belle, he needed to have a day job doing ordinary particle physics. But at night, you know, hidden away, he could do work on the foundations of quantum mechanics. — Belle continued to explore the debate between Einstein and Boore. And in 1964, he published an astonishing paper arguing that Bors and Einstein's ideas made different predictions. Predictions that could be experimentally tested and reveal whether the particles were entangled or not. In other words, it's possible to experimentally answer the question, do we live in Bor's world or Einstein's? — We now know with hindsight this was one of the most significant articles in the history of physics. It's not just the history of 20th century physics in the history of the field as a whole. But Bell's article appears in this, you know, sort of out of the way journal. In fact, the journal itself folds a few years later, literally collects dust on the shelf. — A few years later, completely by chance, an experimental physicist stumbled upon Bell's article. His name was Dr. John Clauser. — I thought this is one of the most amazing papers I had ever read in my whole life. And I kept wondering, well, gee, this is wonderful, but where's the experimental evidence? — With the help of a few fellow physicists, Klauser created a very clever experiment to test Bell's theorem. He had a talent for tinkering in the lab and building the parts he needed. Piece by piece, they constructed the world's first Bell test experiment. It was genius, but it does require some explanation. They focused a laser onto calcium atoms which caused them to emit pairs of photons. If quantum theory equations were right, these photons should be entangled. The photons traveled in opposite directions, passing through detectors fitted with filters which measured their angle of polarization. If Einstein was right and entanglement wasn't real, then the results on each side would not match any more often than predicted by the laws of classical physics. But after hundreds of thousands of measurements, a statistically significant correlation appeared in the data. Strong evidence that the photons were in fact entangled. — We saw the stronger correlation characteristic of quantum mechanics. We measured it and that's what we got. — As Bore predicted, the experiment implied that the spooky connection of quantum entanglement did exist. Could it be that Einstein was really wrong about how the world actually worked? Skeptical physicists, including Clauser himself, pointed out possible loopholes in the experiment. But it's important to remember that this is normal for science. A pioneering physicist like Clauser comes along with an amazing discovery, and the rest of the scientific community investigates, tries to find flaws in the work, kick the tires, see if it holds. For instance, the filters that measured the photons in Clauser's experiment. Could something have influenced them and created a false correlation in the data? This is how scientists think. Is there any common cause deep in the past before you even turn on your device that could have nudged the questions to be asked and the types of particles to be emitted? Maybe some strange particle, maybe some force that had not been taken into account so that what looks like entanglement might indeed be an accident, an illusion. Maybe the world still acts like Einstein thought. — One way to address this scrutiny would be to randomize the filters that measured the photons in Clauser's experiment. Think about it. If the data found a significant correlation between the photons even when the filters were randomized, that would further prove that the photons were entangled. To do that, you could randomize the filters

many different ways. You would just need to be sure that whatever guided that randomization was truly independent. But what could that be? Whatever controlled the filters and their randomization would need to be completely separate from the experiment itself. When some scientists offer a challenge, others rise to it. In 2018, a team led by Xylinger Kaiser and Dr. Dominique Groush performed the ultimate bell test at two high alitude observatories in the Canary Islands. Rather than being contained in one room or even a single building where mysterious forces could influence the results, the detectors were separated from the photon source by a third of a mile. But more importantly, the random filters that are so critical to the experiment were set by information coming from opposite sides of the universe. That's right. That's why they were at an observatory. Telescopes were pointed at two different quazars billions of light years apart. Quazars are bright light sources in the night sky powered by super massive black holes. But that isn't really important. What matters is that the random variations in the light coming from one quazar would determine the setting of one filter on one side of the experiment and random fluctuations in light from the other quazar would set the other filter on the other side of the island. This means that for the measurements to be rigged, it would have required collusion between two quazars on opposite sides of the sky billions of light years in the past when light from each quazar began its long journey toward Earth. And so the entire universe became this experiment's laboratory. Two months later, the team analyzed the experimental data. — The experiment we did is just fantastic. The big cosmos comes down to control a small quantum experiment. That in itself is a is beautiful. — The result showed that the photons emitted by atoms were entangled. And since the quazars were separated from each other by extreme distances, the odds that they could have been working together in some way are effectively zero. Any lingering doubts about quantum mechanics and entanglement had all but evaporated. — You know, honestly, I still get chills when I realize what our team was able to do in this intellectual journey that stretches back to the early years of the 20th century. There's hardly any room left for a kind of alternative Einstein-like explanation. We haven't ruled it out, but we've shoved it into such a tiny corner of the cosmos as to make it even more implausible for anything other than entanglement to explain our results. — Accepting that entanglement is part of the natural world around us has profound implications. It means we must accept that an action performed in one place can have an instant effect anywhere in the universe. As if there's no space between them at all. — Science is stepping outside of all of our boundaries of common sense. It's almost like being in Alice in Wonderland, right? Where everything is possible. — With quantum entanglement on firm experimental footing, what does that mean for our technology? Engineers and innovators had been using quantum mechanics before the first bell test. But as Goch explains, much bigger things may lie ahead. — In our everyday computers, the fundamental unit of computing is a bit, a binary digit, zero or one. And inside the computer, there's all these transistors which are turning on and off currents. On is one, off is zero. And these combinations lead to universal computing. With a quantum computer, you start with a fundamental unit that's not a bit, but a quantum bit, which is not really a zero or a one, but it can be fluid. A cubit, as it's known, can be a zero or one, or a combination of both. A particle or tiny quantum system can be made into a cubit. And today, it's not just pairs of particles that can be entangled. Groups of cubits can be linked with entanglement to create a quantum computer. The more cubits, the greater the processing power. Let's take an example. Imagine a salesman has to travel to 30 cities and wants to find the shortest route to hit them all. Sounds easy, but even with just 30 cities, there are so many possible routes that it would take an ordinary computer, even a powerful one, hundreds of years to try each one and find the shortest route. But with a handful of entangled cubits, a quantum computer could resolve the optimal path in a fraction of the time. Quantum entanglement was first seen as an unwelcome but unavoidable consequence of quantum mechanics. Now after nearly a century of disputes and discoveries, it is finally at the heart of modern physics and a world of exciting new technologies. — Truly understanding quantum mechanics will only happen when we put ourselves on the entanglement side and we stop privileging the world that we see and start thinking about the world as it

actually is. The basic motivation is just to learn how nature works. What's really going on? Einstein said it very nicely. He's not interested in this detailed question or that detailed question. He just wanted to know what were God's thoughts when he created the world. — Oh, and by the way, in 2022, not long after Nova premiered Einstein's Quantum Riddle, Zylinger Clauser and Dr. Alan Aspe were awarded the Nobel Prize in Physics for experiments with entangled photons and pioneering quantum information science. This has been an episode of Novo Remix. Subscribe for more episodes and drop us a comment with your thoughts or questions wherever you get your podcasts. And if you want to go further into the quantum world, check out the documentary Einstein's Quantum Riddle on Nova's website, YouTube channel, and the PBS app. Nova Remix is a series from GBH and Nova and it's distributed by PRX. Executive producers are Julia Court and Chris Schmidt and senior director of digital media is Nadia Peks. Devon Maverick Robbins is managing producer of podcasts for GBH. This episode was produced by Chris Neighbors with sound design by David Porter. I'm Caitlyn Saxs.