Thought Experiments

When Einstein was 16, he appears to have conducted his first thought experiment, imagining what things would look like if he travelled on a beam of light. This was an approach he'd continue to use throughout his career of using thought experiments to investigate the mysteries of the universe, an approach other great Theoretical Physicists of the 20th century like Bohr and Schrodinger would emulate to explore the mysteries of the quantum. Indeed some of the greatest experiments of the 20th century didn't happen in the lab but in the imaginations of some of the most gifted scientists in history.


Exploring Quantum Frontiers - From Light Beams to Bell's Theorem

Segment 1: Thought Experiment (Physics)

• Introduction to the concept of thought experiments in physics.
• How these mental exercises help scientists explore and understand complex phenomena.

Segment 2: Riding a Light Beam

• Delving into the hypothetical scenario of riding a light beam.
• Theoretical implications and challenges presented by the speed of light.

Segment 3: Speeding Trams

• Exploring the concept of time dilation in speeding trams.
• Einstein's theory of relativity and its impact on our perception of time.

Segment 4: The Twin Paradox

• Discussing the intriguing consequences of the twin paradox.
• How time dilation affects the aging process for individuals in different reference frames.

Segment 5: Bronowski & Science Communication

• Reflecting on Jacob Bronowski's contributions to science communication.
• The importance of effectively conveying complex scientific concepts to the public.

Segment 6: Double-Slit Experiment

• Unraveling the mysteries of the double-slit experiment.
• The surprising results that challenged classical notions of particles and waves.

Segment 7: Wave-Particle Duality

• Understanding the phenomenon of wave-particle duality.
• The implications for our understanding of the fundamental nature of particles.

Segment 8: Bohr vs Einstein

• Examining the philosophical and scientific clash between Niels Bohr and Albert Einstein.
• Divergent views on the nature of quantum mechanics and the famous debates.

Segment 9: EPR

• Introduction to the Einstein-Podolsky-Rosen (EPR) paradox.
• Theoretical challenges and implications for our understanding of quantum entanglement.

Segment 10: Schrödinger’s Cat

• Discussing the famous thought experiment involving Schrödinger's cat.
• The concept of superposition and its role in quantum mechanics.

Segment 11: Quantum Weirdness

• Exploring the bizarre and counterintuitive aspects of quantum mechanics.
• How these phenomena challenge our everyday understanding of reality.

Segment 12: Bell’s Theorem

• Tracing the history of Bell's Theorem and its origins in the EPR paradox.
• The pivotal role played by physicist John Bell in resolving the quantum debate.

Segment 13: Non-Locality Proven (for now!)

• Examining the groundbreaking experiments by John Clauser and Alain Aspect.
• Confirmation of non-locality in quantum mechanics and the statistical significance of results.

Segment 14: Bell’s Final Assessment

• Reflecting on John Bell's final assessment of the Bohr-Einstein debate.
• The Nobel Prize in Physics awarded to Clauser,

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Music by LiteSaturation from Pixabay

Chapters

• 1:10 Introduction
• 2:18 Riding A Light Beam
• 6:32 Relativity & Speeding Trams
• 10:13 The Twin Paradox
• 14:32 Science Communication
• 17:43 The Double Slit Experiment
• 21:21 Wave-Particle Duality
• 25:34 Bohr v Einstein
• 30:14 EPR Paradox
• 32:40 Schrodinger's Cat
• 36:24 Quantum Weirdness
• 42:24 Bell's Inequality
• 45:32 Non-Locality Proven (for now!)

Transcript

Thought Experiment (Physics)

Introduction

Hello, my name is Darren. And this is The Makers Rage podcast. Thought Experiment. When Einstein was 16, so around 1895 or 1896, he appears to have conducted his first ever thought experiment. And a simple one, on the face of it. He imagined what things would look like if he was riding on a beam of light. At the time, he conjectured, as most 19th century theorists would have done, that since light doesn't travel in a vacuum, but requires a medium, or ether, to transmit it, that the beam would appear frozen. Lord Kelvin another 19th century stalwart of classical Newtonian physics, would have nodded his head and said, “of course, what do you expect? It’s what Newtonian mechanics predicts – it’s best not to develop any grand designs on making notable contributions to theoretical physics after 1899.

Space and time are absolute. Light transmits as waves. Gravity is what Isaac Newton said it was. All that's left for physicists to do is to dot the I's and cross the T's. All the great discoveries have basically been made. Kelvin was an old fogy though. He probably wasn’t even aware that in 1887, the American Albert Michelson, along with his colleague, Edward Morley, proved that there was no ether, that light travels at constant speed in a vacuum, paving the way for the special theory of relativity 20 years later. 

Riding a Light Beam

Of course, Einstein himself didn't know this when he was riding a light beam at 16. And if it was ever mentioned at school, he likely wasn't paying attention. With 50 years of hindsight, he recollected the fledgling thought experiment thus, “a paradox upon which I had already hit at the age of 16. If I pursue a beam of light with velocity c (velocity of light in a vacuum), I should observe such a beam of light as an electromagnetic field at rest, though spatially oscillating. There seems to be no such thing, however, neither on the basis of experience nor according to Maxwell's equations. From the very beginning, it appeared to me intuitively clear that judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the Earth, was at rest. For how should the first observer know or be able to determine that he is in a state of fast uniform motion, one sees in this paradox the germ of the Special Relativity Theory is already contained.” Well, as they say, hindsight is 20-20, giving the impression that he intuited the special theory of relativity at 16, before he knew anything about the constancy of the speed of light as a cosmic limit, before he learned Maxwell's equations, which he wouldn't learn until 1898, two or three years after this thought experiment reputedly happened. He also claimed that the idea for general relativity came to him when he saw a painter falling off a ladder. Ronald W. Clarke, in his definitive biography of Einstein, said that accounts like these are typical of what Einstein’s eldest son called, his “delight in making up good stories.” And he was certainly an example of a great scientist who was also a great science communicator. But does it really matter if he was as precocious as the light beam anecdote seems to indicate? At 25, he wrote the great paper that sealed his immortal fame: ‘On The Electrodynamics Of Moving Bodies’, which introduced The Special Theory of Relativity. He wouldn't even win the Nobel Prize for this paper, since even by 1921, when he won the Nobel Prize, The Special and General Theories Of Relativity were quite contentious. Instead, the citation reads that he won it for, quote, “his services to theoretical physics, and especially for his discovery of the law of the Photoelectric Effect.” The Photoelectric Effect being just one of the papers he published in that year of wonders. 

But he didn’t need a degree to conduct his first thought experiment, he needed no knowledge of advanced mathematics, no familiarity with the latest breakthroughs in theoretical or experimental physics. Just his imagination, which you would later say, is more important than knowledge. Because unlike imagination, knowledge is limited. Once he became the grand panjandrum of physics and science in the 20th century, he would continue to use thought experiments to illustrate and reinforce his ideas, especially in dialogue with his colleagues and rivals, asking them to imagine scenarios that could be feasibly created according to physical laws, experiments that could be feasibly setup, if the technology was available, or the funding. The only limit that must be agreed upon, the only constraints or rules that are required to play this game – and the thought experiment is a kind of game – is that the known laws of physics must be obeyed. This is what distinguishes a thought experiment from a flight of fancy. And the best thought experiments can be apprehended by any lay member of the public, or a 16-year-old who has yet to learn the physical laws and the mathematics that must bridle or discipline his unbounded imagination.

Speeding Trams

In his popular work, Relativity: The Special and General Theory, Einstein translates the formal presentation of those theories into a thought experiment involving a train, a railway embankment, and lightning flashes. I still remember the first time I saw this explained by Jacob Bronowski, when as a teenager in the 90s, I used to stay up late, until the early hours of the morning actually, waiting for the BBC Learning Zone to start, and Jacob Bronowski’s Ascent Of Man. In the episode entitled ‘The Majestic Clockwork’, Bronowski climbs aboard a tram in the city of Bern in Switzerland, where Einstein had his Annus Mirabilis or ‘Year of Wonders’ in 1905, writing the five papers that, had he written nothing else thereafter, would have secured his reputation as among the greatest physicists the world has seen. So, to illustrate the puzzling features of one of those papers, ‘On the Electrodynamics of Moving Bodies’, Bronowski wants to speed this rickety tram up to approach the speed of light. And using somewhat crude and yet charming 1970s special effects, the tram speeds up and starts getting squashed horizontally, illustrating, as the special theory predicts, that as an object approaches the speed of light, it's length contracts in the direction of motion. And likewise, for the passenger in the tram, looking out the window, the buildings seem to get squashed together. Although, significantly enough, their heights remain the same. But if the passenger looks ahead, the tops of building seem to bend inwards and forwards towards him, forming a kind of tunnel. And of course, people and cars are distorted in the same way. You can imagine how the old Newtonians would have reacted to this news. One of the implications of the speed of light being a cosmic limit, that not only can nothing go faster than the speed of light, but no matter the velocity of a person in the tram say, or in a spaceship, they will measure the same speed of light as someone on the embankment or back home on Earth, or in any stationary reference frame, or any reference frame for that matter. It is the same speed of light in all reference frames – 300,000 kilometers per second, 196,000 miles per second, whether you're standing on a platform, or trundling along on a rickety tram, or aboard the Millennium Falcon. 

Newton thought light moves from one point to another instantaneously, which is why he believed space was absolute, and time. Einstein showed neither was the case: that on the contrary, space contracts. And time… dilates. So, while someone may see the tram flying by, getting squashed as it approaches the speed of light, if they can somehow see a clock in that tram, the seconds will actually tick more slowly. Time moves more slowly. The person in the tram ages more slowly than you do. Of course, the person in the tram doesn't notice this effect. They may see your thin contracted figure fly past, spectral, like an El Greco portrait, but for the passenger, the clock on board is ticking just as they'd expect it to, the second having the same interval as the famous clock tower in Bern, or any of the wristwatches of the people on the platform waiting for the train to arrive. Now, you may remember when I said earlier, that the person on the tram – let's say it's Bronowski himself – notices that the buildings get squashed and the cars and the people on the platform, the way the people on the platform see the tram getting squashed. Does the time dilation effect therefore not cancel out? The answer is no. 

The Twin Paradox

Paul Langevin proposed another famous thought experiment to illustrate this, ‘paradox’. Paul Langevin an interesting character. One of his doctoral students was Pierre Curie. Marie's husband, who died prematurely after getting run over by a horse and carriage. Langevin would go on to have an affair with a widowed Marie. Her daughter by Pierre Irene Joliot Curie would become another of Langevin’s doctoral students and go on, like her mother, to win the Nobel Prize in Chemistry. Langevin is seated next to Einstein in that famous 1927 Solvay Congress photo, which features pretty much all the Avengers of mathematical physics at the time – one of the most extraordinary gathering of minds in history. To Einstein's right, is Hendrik, Lorentz, who's transformations imply that length contraction and time dilation are a consequence of the speed of light being constant. And to his right is Marie Curie herself, the only woman in the photo and the only person with two Nobel Prizes.There were 29 attendees in total, 17 would go on to win the Nobel Prize. Among them were other members of the old guard like Max Planck, seated next to Curie, on whose work younger arrivistes like Max Born, Erwin Schrodinger and Niels Bohr were developing the theory of the quantum. And representing the future, Heisenberg, Dirac, Pauli – What a headache these would cause Einstein in the coming years, especially Bohr. Well, we'll get these.

First, back to time dilation and Langevin’s famous thought experiment involving identical twins. We’re asked to imagine one of them, Franz, say, staying on Earth, while is twin brother, Hans, climbs aboard the Millennium Falcon and travels to Alpha Centauri or some other not too distant star, stays long enough to drop a probe or something then makes a return voyage. According to Einstein, when only a few years will have elapsed for Hans (for even at near light speed, it would take that long to reach even the nearest star that isn’t our sun), when he returns, he discovers centuries have gone by, and to his dismay, that his twin brother, the nieces and nephews he never met, etc. are long dead and buried.

But the question I asked earlier was, why doesn't the effect cancel out? If space and time are relative, can't we imagine the Millennium Falcon standing still in space and earth zipping away from it at 99% speed of light? The point to remember here is that this only applies if we cannot determine who is moving and who is stationary (like when you’re on a train moving slowly at constant speed – you don’t know if you’re moving or the train opposite). But since we were able to set things up so that we knew that Franz back on Earth is stationary, and Hans was the one moving in the Millennium Falcon, Special Relativity predicts the time dilation only occurs for the moving object. Because being stationary, Franz doesn’t cross reference frames. As the moving observer, Hans exists in both the stationary and the moving reference frames. And as a result, ages more slowly relative to Franz. This is a consequence of the Lorentz transformations. Hendrik Lawrence, if you remember, sitting beside Einstein that Solvay photo, appropriately enough. And what is it the equations named after him transform? They show once we know who was moving, and who's at rest, that space and time are in a sense, distorted for a stationary observer when they perceive something approaching the speed of light. 

Bronowski & Science Communication 

None of this is easy to explain without the maths. And even with the maths, it's difficult to wrap your head around. And as Bronowski illustrated all this in his rickety tram and crude 1970s special effects to a curious 14-year-old, who stayed up until 2am to watch him, before I knew anything about the Lorentz transformations, or the mathematics that undergirds the special theory of relativity, I was as wonder wounded as Einstein must have been at 16, imagining himself riding upon that beam of light. Seven years after the Ascent of Man first premiered in 1973, in another acclaimed 13 part series directed by Adrian Malone, Cosmos: A Personal Voyage, Carl Sagan tries to explain the same phenomenon, this time using a bicycle in Tuscany. But I always preferred Bronowski’s version, probably because I encountered it first, and I always had a soft spot for that peculiar plummy delivery, and that rolling rhoticism that was perhaps a remnant of the Polish accent to shake off early in life. He stood barely five feet tall, and spoke with a calm assurance of his calling, and a confidence in the breadth of his learning in both science and the arts, which never gave the impression of pedantry, but always of his humanity and enthusiasm to impart without condescension all the wisdom he’d accrued. And having lived through both world wars and travelled to Hiroshima to see the aftereffects and destruction wrought upon that city by the bomb: a tragic consequence of the advances made by scientists in a 20th century, by many of the people in that famous Solvay Congress photo, including Einstein, he knew was important for scientists like him, for public intellectuals, to remind us that science may produce much that is good, but it is human activity. And scientists, human beings, who occasionally forget the power they wield; the power they confer on to the world for others to wield. For good or bad. But crucially, science itself is neither good nor bad. Gravity isn't there to make us wicked. Length contraction doesn't care about our politics, or the speed of light, our sexual habits. It is human beings who decide to make a transistor or a bomb.

He died at 66 of a heart attack in 1974. In New York. In a taxicab, just after the Ascent of Man was released. He didn't have long to enjoy the fruits of his labor. Still, it's there in all its glory on the internet for anyone who's interested. Of course, both Bronowski and Sagan are examples of truly great science communicators. They may not be as famous for the contributions they made to their respective fields of study, but they inspired 1000s, perhaps millions to enter those fields of study. They moreover shared with Einstein a facility for elucidating difficult scientific concepts in a visually arresting and often entertaining way. And although most of the thought experiments that were bandied about by Einstein and others a century ago, do require some knowledge of physics, the best ones evoke enough in there presentation to illuminate any non-physicists who may have been listening in during one of those train journeys when Bohr and Einstein were so engrossed in conversation they missed their stops. And on the return journey, missed their stops again. 

Double-Slit Experiment

The famous double slit experiment, although not a thought experiment, resembles one in the simplicity of its setup. Of course, what Thomas Young didn't know in 1802 when he first conducted the experiment to investigate the behavior of light waves, is that the simple apparatus: a light source, a screen with two parallel slits, and a viewing screen, has the potential to show that light also behaves as particles, as quanta to be exact. Although, of course, the term wouldn’t be used in this sense until a hundred years after Young. And I bet it would have blown his and all his contemporaries minds. That said, he would have known Newton believed that light was composed of particles or ‘corpuscles’, as he called them. While Newton's contemporary, Christian Huygens of the Netherlands, believed like him, and most of Europe that it propagated in waves. It wasn't until 1897 That JJ Thompson discovered the electron and tentatively proved Newton correct. His son, George Paget Thompson, proved that these very electrons, these very particles, diffract like waves, proving Huygens correct. The fact is, the double-slit experiment demonstrates both were right, and that both Nobel Prizes were deserved. 

Imagine you're firing a machine gun at a bulletproof screen, and only two slits for bullets to pass through and strike the opposite wall. After firing enough bullets to ensure enough got through, what pattern would you expect to see on that wall? Two vertical strips of bullet holes, of course. And on looking behind the screen, you're satisfied that your expectations have been met. Now, instead of bullets, try firing photons through the two slits. If they are indeed like particles, what do you expect to see? Well prepare to be disappointed (or amazed).

Not two strips of light, but a series of them. Light and dark, light and dark, growing fainter the farther away from the center of the screen you go, an interference pattern that we get with waves. Hold on a minute, Planck & Einstein said light consists of discrete packets of energy, or quanta. We can even fire them one at a time, like bullets. So, let's fire photons by by one and see what happens. And just to be sure, we'll wait an hour between each shot. And gradually, as the hours and days go by, but a single photon firing each hour, we are disappointed to discover are amazed that this very same interference pattern begins to emerge. It’s as if the experiment is playing a trick on us. The thing is, if we cover up one slit, we get the expected pattern: a single strip of light. But once we uncover the other slit, the interference pattern returns. And it doesn't matter if you wait an hour, or a year or a millennium between each photon firing, it will always produce that same pattern. It's as if the photon is a particle when we’re looking at it and a wave when we’re not; or a particle when we fire it and when it lands, and a wave on the journey between the light source and the screen. 

Wave-Particle Duality

The idea that a photon or an electron or any of the elementary constituents of matter, could behave both as a particle and a wave still disturbs any one today who tries to wrap their head around it. 100 years ago, when physicists were still coming to terms with the implications of the new quantum theory, it was clear a new interpretation of reality was needed, at least reality on the scale of elementary particles. And so, in an attempt to reconcile the opinions of many of those present at the 1927 Solvay congress referenced earlier, it would be Niels Bohr and Werner Heisenberg, who’d eventually present what would later be called the Copenhagen interpretation of Quantum Mechanics, after the city in which the two physicists did their greatest work in the 1920s. To put it bluntly, we must simply accept that there's a veil between us and the quantum world behind which there is no solid reality, as we understand it. Only by looking does it adopt a semblance of the real. But until we look, there is only a fuzzy soup of potentialities, possibilities, probabilities: the photon is a wave until we measure it, until we look and see where it struck the screen. As a particle, the act of measuring, in a sense, makes it real, like slamming one's hand down on a spinning coin, makes it definitively heads or tails. And it was this aspect of the interpretation that Einstein never came to terms with. “He does not play dice”, he once said to Bohr, referring to God, and perhaps thinking of the double slit magic trick that caused so many physicists to reel, also said, “God may be subtle, but he's not malicious.” Finally, an exasperated Bohr retorted, “Stop telling God what to do.”

And Einstein’ God wasn't Allah or Yahweh. He believed more in Spinoza’s God, God as Cosmos, order, harmony: “God is everything. And everything is God”, said Spinoza. “The Cosmos is all that is, or ever was, or ever will be,” said Carl Sagan. Anyway, despite Einstein's misgivings, it was at the Solvay Congress in 1927 that Max Born and Werner Heisenberg declared, “We consider quantum mechanics to be a closed theory, whose fundamental physical and mathematical assumptions are no longer susceptible of any modification.” Einstein didn’t agree. And I think they knew the peremptory pronouncement would’ve rubbed him the wrong way. Anyway, what seemingly happened next was a kind of intellectual jousting, and this began in earnest at the fifth Solvay Congress in 1927. Einstein had already expressed his misgivings shortly before the Congress in a letter to Max Born, saying, quote, “quantum mechanics is very impressive. But an inner voice tells me that it is not yet the real thing. The theory produces a good deal, but it hardly brings us closer to the secret of the old one.” There he goes telling God what to do again. Anyway, at the Congress, it soon became clear it was the great Dane, Niels Bohr versus Einstein. Although Bohr had more support, since most present at this conference on electrons and photons agreed, at least in principle, with the Copenhagen interpretation. The debates began as dining room discussions, where Einstein challenged Bohr and his supporters on the assertion that quantum mechanics in its present form can be called ‘complete’. It wasn't just the probabilities that irked him, but Heisenberg's Uncertainty Principle, which states that you can't know simultaneously both where a particle is, and where it's going with arbitrary precision: the better you know one, the less well you know the other. 

Bohr vs Einstein

Einstein's challenges came in the form of increasingly clever thought experiments intended to prove that position and momentum could in principle be simultaneously known to arbitrary precision. For example, one thought experiment involved sending a beam of electrons through a shuttered screen, recording their positions as they struck a photographic screen. Bohr and his allies would go away and think about it, and return usually by the end of the day, with an adequate counter. On the last day of the conference, Einstein voiced his objection to another aspect of the Copenhagen interpretation that bothered him, one that appeared to allow action at a distance, or the ability for two objects separated by a great distance to communicate at speeds greater than light. This would violate the principle of relativity, the theory for which he was most famous, and on which his reputation as the Newton of the 20th century rested. And he would continue working to challenge this feature of quantum mechanics in particular over the next several years. For the time being, though, Einstein found himself perhaps for the first time in the minority among his peers, and despite sitting front and center in a famous Solvay Congress photo of 1927, when the disagreement between he and others placed less prominently, the consensus was that Einstein had lost the debate and even his closest allies during the conference, for example, Prince Louis the Broglie, whom Wolfgang Pauli seems to be looking at with disdain in the photo, conceded that quantum mechanics appeared to be complete. 

Of course, Einstein wasn't satisfied with the consensus. So, a couple of years later at the sixth Solvay International Conference on magnetism in 1930, he came armed with a new thought experiment. He described a box full of light and suggested that both the energy of a single photon, and the time it was emitted could be determined precisely. This would violate Heisenberg's Uncertainty Principle, since time and energy, like position and momentum mentioned earlier, are according to the Copenhagen interpretation of quantum mechanics, variables governed by uncertainty. In his thought experiment, Einstein says first, the box can be weighed, after which a single photon can be released at a particular instant, to a shutter operated by clockwork inside the box. After this, the box can be weighed again, at which point we know the change in its mass and, thus, the energy of the photon can be calculated from you guessed it, E = mc2. The energy change would then be known, as would the precise time when the photon was emitted. So that's the end of your uncertainty principle. You can imagine Bohr wincing at Einstein using his famous equation like a sledgehammer against quantum uncertainty. And he was indeed stumped initially. He reputedly stayed up all night, not thinking about magnetism, which is what the sixth solve a Congress was supposed to be about but trying to find a flaw in Einstein's formidable challenge and rescue the Copenhagen Interpretation. 

The following morning, he showed up with a drawing of Einstein's box of light and offered the following refutation. When the photon is released, there will be a recoil, causing an uncertainty in the position of the clock and the Earth's gravitational field. This will produce a corresponding uncertainty in the time recorded due to believe it or not, Einstein's own general theory of relativity. Einstein had brilliantly invoked his own special theory of relativity to refute quantum uncertainty. Bohr had rebuffed his challenge by invoking Einstein's general theory of relativity. An extraordinary intellectual duel, which on this occasion ended in victory for Bohr, who in retrospect, had the advantage of being correct. Quantum mechanics today is the most successful theory in the history of physics. Indeed, in the form of quantum electrodynamics, it correctly predicts the magnetic moment of the electron to an accuracy of about one part and a trillion, making it in fact, the most accurate theory in the history of science. But in the 1920s and 30s, the ground wasn’t so firm. And despite losing the uncertainty battle, Einstein wasn't done challenging the implications of the Copenhagen interpretation, and the one implication that troubled him more than uncertainty – non-locality. 

EPR

It would take him five years though, to produce his next challenge, five years during which the quantum theory had achieved more or less general acceptance in the physics community. And despite its troubling implications, which even its most ardent defenders had to agree, were unsettling, they couldn't ignore its success in the lab, where time and again, experiments vindicated the results of theoretical predictions. Moreover, between 1930 and 35, a lot had transpired outside the world of physics. Hitler had risen to power in Germany, European physicists who congregated in places like Göttingen (where Born and Pauli were), Berlin (Einstein), and Copenhagen (Bohr, Heisenberg, Dirac), were now dispersed all over the world, including Einstein, who in the mid-30s ended up at the Institute of Advanced Study in Princeton. And it was here that he collaborated with two younger colleagues, Boris Podolsky and Nathan Rosen, on a paper whose title echoed what Max Born had said, eight years previously, at the fifth solve a congress i.e., that quantum mechanics was closed and complete. The title of the Einstein-Podolsky-Rosen paper – or EPR, as it became known – ‘Can Quantum Mechanical Description of Physical Reality Be Considered Complete?’ 

One of the properties of electrons is spin. An electron can have spin up or spin down, and we can detect this. In fact, it is possible to obtain a pair of electrons in a so-called singlet state where their spins cancel each other to give a total spin of zero. Let's consider such a state we can call the two electrons, A and B. Now let’s say the spin of A is measured and found to be in the up state, because the two spins must cancel to zero, it follows that particle B must have spin down. Before the Quantum Revolution, one might have drawn a comparison here with a pair of gloves: you put the left in one box ,the right and another and move them wide apart, even light years apart, if you open one, and discover the left glove, you immediately know the one near Proxima Centauri is the right one. So, what's the problem, you may be asking? To illustrate what the problem really is, I will cite perhaps the most famous thought experiment in physics: Schrodinger Cat. Schrodinger was another who was dissatisfied with the Copenhagen Interpretation and would later repudiate his involvement in the quantum revolution, despite having contributed as much as anyone to it. You see, he believed that phenomena like wave particle duality weren't intrinsic to the particles but a reflection of our incomplete knowledge of them. By looking, we determine where the particle landed on the screen in the double slit experiment. By opening the box, we determine whether the glove is the left or the right one. Max Born, who if you remember annoyed Einstein by saying Quantum Theory was ‘complete’, stated that Schrodinger’s own wave equation determined only the likelihood or probability that an electron say will be found in a certain position. And that, moreover, this is not probability due to ignorance. This probability is all we can never know about an atomic system because it is the reality of the system, despite what we can and cannot know about it. 

Schrodinger’s Cat

Schrodinger responded thus: imagine an experiment in which a live cat is placed in a box with a radioactive source, a Geiger counter, a hammer, and a sealed glass flask containing deadly poisonous fumes. When a radioactive decay takes place, the counter triggers a device releasing the hammer, which falls and breaks the flask. The fumes will then kill the cat. And now suppose the radioactive source is such that quantum theory predicts a 50% probability of one decay particle each hour. After an hour has passed, there is an equal probability of either state, the live cat state, or the dead cat state. Quantum Theory, according to Max Born, would predict that exactly one hour after the experiment began, the box would contain a cat that is neither wholly alive nor wholly dead, but a mixture of the two states, or a superposition of two wave functions. And this is intrinsic to the system because quantum probability is not based on the ignorance of the observer. Schrodinger must have expected to get a good laugh out of his colleagues. The idea of a cat in a box that’s both dead and alive is clearly ridiculous. Max Born wasn't laughing. His response was something along the lines of “Exactly, we have a superposition of states that is intrinsic to the system. As soon as we, as the observers, lift up the lid of the box to find out if the quantum prediction is correct, the impasse is resolved. The act of observation collapses the superposition of the two wave functions to a single one, making the cat definitely dead or alive.” And if Schrodinger had cause to be turning in his grave, it would be to know that this famous thought experiment, or so called paradox, is used to teach the concepts of quantum probability and the superposition of states at universities to this day. Now if this is your first exposure to quantum superposition, and you find it hard to believe that Schrodinger was actually wrong, and Max Born correct, that the system must be interpreted as one in which a box contains a cat that is both dead and alive, or more precisely, a system that contains a superposition of these two states, you haven't been alone these 100 years. But I asked you to suspend your disbelief for a little while as we return to the EPR paradox. 

Quantum Weirdness

So, we have two electrons, one with spin up and the other spin down in a singlet state (i.e., created in the same event) so that our total spins cancel to zero. And we imagine they're moving apart at close to the speed of light. And once they're sufficiently far apart, that it would take some time for light to travel from one to the other. Doesn't have to be as far as Proxima Centauri. It can be light minutes, like the distance from the Earth to the Sun, or even light seconds. In the EPR paper, Einstein contended that the spin up was like the right glove, say, hidden in a box (although he didn’t use this analogy). And as soon as we open one box and determine what glove, is there, we instantly know what the other is. And that no information need travel faster than the speed of light. There is no superposition of states, the glove is either left or right from the start. Similarly, the electron spin is either up or down. Opening the box or checking the spin doesn’t change this. If there is a superposition of states, however, then we have a big problem. Because then there is no left glove in one box and right glove in the other from the start, but a superposition of the two. And only by opening one of the boxes does the wave function or superposition collapse to left or right glove, up or down spin. And by opening one box and collapsing that wave function, the wave function in the other box instantly collapses. So, our interfering with one box directly affects the other, and notably this effect is instantaneous. It is spooky action at a distance. And what's worse, being instantaneous, action that transmits faster than speed of light. Another analogy will be a spinning coin that is neither definitively heads or tails. By slamming our hand down on one and seeing it as heads, another spooky hand way off in a distance telekinetically slams down and determines the other coin is tails. This is very different to a coin that’s heads from the start, the other tails. 

I think it's fair to say Bohr had no adequate response to this challenge. The physics world had by then accepted that quantum mechanics is correct, despite its weirdness, and I think Bohr had grown weary of defending it. His response in the end was more a metaphysical apologia exhorting Einstein and other sceptics to simply accept that our hominin minds which evolved in the savannah is simply not equipped to understand the world at the level of the quantum; we should consider ourselves lucky we have the insight we do; it’s best to draw a veil on what we cannot grasp; it's best to simply ‘shut up and calculate’. And this became the general attitude towards quantum mechanics for the next few decades. Don't get bogged down in the philosophy. We measure a theory’s success by the accuracy of its predictions. And quantum mechanics is the most successful theory in physics. Even Richard Feynman, another quantum pioneer, who developed the path integral formulation of quantum mechanics and was a major contributor to the theory of quantum electrodynamics – which he called “the jewel of physics” for its extremely accurate predictions – even he discouraged students from getting too bogged down by the metaphysics of the quantum. And he was right in a way, the Copenhagen interpretation, or Einstein's EPR paper didn't impinge very much on the development of lasers, the transistor, the microchip, the electron microscope, Magnetic Resonance Imaging, or Global Positioning Systems. Even the developers of modern computing and the Internet weren't thinking very much about the Copenhagen interpretation, while making their breakthroughs decades after both Einstein and Bohr had died. 

But I suppose we should be glad to have those personalities who are never content to accept on faith the counterintuitive. What they can’t make sense of, fundamentally, or adequately explain, despite whatever accurate predictions; those who won't concede to the notion that simply because we evolved from Homo Erectus, that we’ll never understand what happens at the singularity of a black hole; or why electrons behave the way they do. The telescope and the microscope were both only invented about 400 years ago – look at what we've discovered since. Should we really simply accept that the universe of which we are a part, and which has already disclosed so much to us, will keep some secrets to itself; that human beings thinking about nature isn't, as Spinoza said, “Natura Naturans”, “nature naturing”, in other words, we small beings thinking about the natural world is “nature doing what nature does”. We may very well be the only beings in the universe who can think. But we are still Stardust, a concourse of atoms become conscious, become self-aware, and which, moreover, can reason. When Einstein referred to God or ‘the old one’, he meant the universe and its laws. And on our little planet in the suburbs of a single galaxy, and I’d like to think on other planets in other galaxies as well, the universe woke up and, through our ape species, began to think about itself. God is subtle, but not malicious. The universe may perform a few card tricks, but it doesn’t play dice. 

Bell’s Theorem

And so, the EPR paper was put in a drawer and not spoken about until, ten years after Einstein’s death, John Bell, the Irish physicist who, while on academic leave in 1964, took up the challenge of once and for all settling dispute. He wrote a remarkably short paper titled ‘On the Einstein Podolsky Rosen Paradox’, and in it, amazingly, provided a way for experimentalists to test if Einstein or Bohr was correct. It took a further eight years, in 1972, for someone to take up the gauntlet. That person was John Clauser, at the University of California, Berkeley. Initially, he wanted to prove Einstein was correct – that there is no spooky action at a distance; that entangled states exhibit such weird behavior because we have incomplete knowledge of them, that there are in other words ‘hidden variables’ we’ve yet to uncover, and when we do, we’ll have a more thorough understanding of what’s going on in the box with the dead and/or alive cat. Clauser’s thesis advisor discouraged this, of course, as did the great Richard Feynman who said, the work was, quote, “totally silly. You're wasting everybody's time and money.” 

But after receiving John Bell’s blessing to pursue the idea, he collaborated with Stuart Friedman on an experiment that focused a laser on calcium atoms, making it emit particles of entangled photon pairs, that shoot off in opposite directions, and used filters set to decide to measure whether they were correlated. After hundreds of thousands of runs, they found the pairs in fact correlated more than Einstein would have predicted, proving the reality of spooky action with hard data. Perhaps because the young Clauser began the project hoping to prove Einstein correct, made some of the old guard a little nervous. And even though this first major experiment indicated the Copenhagen interpretation was indeed correct, he wasn't exactly commended for his efforts. I think Einstein's EPR skeleton in the closet, made physicists feel uneasy, including the magnanimous Richard Fineman, who threw Clauser out of his office at one point. This suggests to me he knew quantum mechanics rested on shaky foundations. And perhaps he was nervous that this young turk would in proving Einstein correct, bring needless controversy to the field he did so much to develop. Well, in the end, Clauser didn't get what he wanted. But the data spoke for itself. Between 1980 and 1982, Alain Aspect in France, resolved to conduct an even more decisive experiment, one that would provide as clear a violation of Bell’s inequalities as possible and prove once and for all, that despite his ingenious objections to quantum weirdness, Einstein was definitively wrong. 

Non-Locality Proven (for now!)

The results were unambiguous. Not only was quantum physics nonlocal, as Einstein feared, but Aspect’s experiment confirmed this in the exact way predicted by quantum mechanics with a statistical agreement of up to 242 standard deviation. Given the technical quality of the experiment and the near perfect statistical agreement, this is what convinced the scientific community, and the numerous experiments performed since the 80s only further confirmed this, which is why Clauser and Aspect won the Nobel Prize in Physics in 2022. The third recipient was Anton Zeilinger of Austria, whose experiments not only reinforced and expanded on the work of Clauser, Aspect and bell, but would pave the way for quantum computing, quantum networks, and quantum encrypted communication. 

Had he lived, I'm sure Bell would have won the prize outright. He'd already been nominated shortly before his death in 1990. at the age of 62. Bohr was 37 when he won it; Einstein 42. And many of those who were seated in that famous Solvay Congress photo I've referenced so frequently we're also in their 30s 40s and 50s when given the accolade with Curie living long enough to get it twice, despite being taken away prematurely at 66, due to Leukemia likely caused by her exposure to the radioactive elements she discovered. But, since then, it seems the Nobel Committee has gotten in the habit of waiting until potential laureates having gotten too old to enjoy the accolade, like waiting until they’ve exited the stage to start applauding, or in Bell’s case, exited the building. But if he has been out there somewhere in the continuum refereeing an ongoing duel between Einstein and Bohr, I'm sure despite the judges having finally called the contest in Bohr’s favor, neither would be happy with Bell’s final assessment quote, “Bohr was consistent, unclear, willfully obscure and right. Einstein was consistent, clear, down to earth, and wrong.” And after graciously accepting the results of Clauser’s and Aspect’s experiments, and noting that their Nobel was deserved, I'm sure Einstein wouldn’t take too long to throw a spanner in the works in the form another ingenious thought experiment.