There was an air of melancholy about the theoretical physicist David Bohm when he came to speak at the University of Toronto in the mid-1980s – or so it seemed to me at the time, a student with only a passing understanding of Bohm’s place in history. Born and educated in the United States, Bohm had, by then, spent half his life as a political and scientific exile. The politics was bad timing. In 1949, during the peak of McCarthy-era paranoia, Bohm’s prior membership in the Communist Party while a graduate student had landed him in front of the U.S. House Un-American Activities Committee. He was later arrested after refusing to testify against colleagues. The episode crippled his career. Stigmatized and unable to find work at home, Bohm left for a teaching position in Brazil. By the time I saw him speak he was in his 60s and based in London.
What fascinated me then, and what filled the Toronto lecture hall, was not Bohm’s political history, but his status as a scientific heretic. He was an unapologetic opponent of quantum mechanics – the strange but powerful theory that physicists use to explain the behaviour of matter and light. That did not mean he questioned the theory’s results. On that score, few would argue – then or now – that quantum mechanics is one of the most successful theories in history, with practical applications that encompass lasers, computer chips and other indispensable components of the modern world. But Bohm was certain that what the theory’s founders thought quantum mechanics had to say about the nature of reality was dead wrong.
Fast-forward 35 years and we find Lee Smolin, another physicist in his 60s, reviving Bohm’s objections. Like Bohm, Smolin comes from the U.S., but, since 2001, has lived in Canada as a founding faculty member of the Perimeter Institute for Theoretical Physics in Waterloo, Ont.
As a scientist, Smolin is best known for working on ideas that try to reconcile quantum mechanics with general relativity – Einstein’s pre-eminent theory of gravity. Relativity has proved to be a remarkably reliable description of nature with some useful side benefits. For example, GPS satellites can’t pinpoint your location on Earth without taking Einstein’s equations into account. Yet, relativity and quantum mechanics do not play well together because they do not treat reality in the same way.
The weirdness of quantum mechanics comes down to our apparent inability to know everything about a physical system. A particle has both a position and a momentum and in the classical mechanics of Isaac Newton, knowledge of those properties makes it possible to predict, with complete certainty, the particle’s location at some point in the future. But, in the mathematical language of quantum mechanics, a fundamental level of uncertainty creeps in with the act of measurement. The more one tries to pin down a particle’s position, for example, the less certain one is about its momentum, and vice versa. The particle’s future status can only be expressed as a probability. It is literally everywhere at once with varying degrees of presence, until we choose to check.
The logical consequences of this have been well explored in popular science writing. They include the idea that particles sometimes behave as waves and that cats can be both alive and dead at the same time. More broadly, it seems that the universe exists as a cloud of possible outcomes that we call into being by observing them. For the better part of a century, physicists have been willing to put up with this to get on with the business of doing physics. Their approach, as Cornell University professor David Mermin wryly summed up, was “shut up and calculate.”
This is what Bohm objected to and what he tried to counter with his own “pilot wave” theory, in which particles are guided in their motion in ways that we can’t see. I still recall the metaphor Bohm used to describe this: For him, reality is a ballet in which some of the dancers are hidden behind the curtain.
Smolin is not sold on the details of Bohm’s approach, but in his previous books, including The Trouble with Physics and Time Reborn, he has explored why physicists have been stalled in their efforts to come up with one overarching theory that includes both relativity and quantum mechanics and reveals the hidden architecture behind the particles and forces that govern our existence. With his new book, Einstein’s Unfinished Revolution: The Search for What Lies Beyond the Quantum, he makes his strongest case yet that the blame lies squarely with those who developed quantum mechanics in the 1920s, and their insistence that reality is effectively what we choose to measure. For Smolin, a staunch realist, their position is not just unsatisfying – it amounts to a betrayal of the scientific enterprise.
“I don’t want a theory of myself intervening with nature,” Smolin says. “I do science because I want to understand how nature is in our absence. What is really going on?”
Philosophically, Smolin aligns himself with Einstein, who played a central role in launching the quantum revolution starting in 1905, but who became increasingly disturbed by its outcome. Rather than Einstein, it was the Danish physicist Niels Bohr and his younger colleague Werner Heisenberg who led the development of the new physics by building the mathematical framework of quantum theory and, by 1927, introducing the “Copenhagen interpretation” – the dominant view of what that theory means.
That view, as Smolin writes, essentially holds that “physics does not give a description of what exists … but is only a way to keep track of what is observable.” In retrospect, it’s easy to imagine why such an idea would have appealed to a generation of young theorists steeped in the modernism of Picasso and Joyce and distrustful of the rationalism that had led Europe into the horrors of the First World War. But in the current context, when public acceptance of basic scientific facts about climate change, vaccination and other policy-relevant matters is undermined by the toxic churn of social media, a theory that appears to challenge objective reality poses real risks. Smolin argues that it’s time for realism to reassert itself at the core of physics, the pedestal upon which the entire scientific enterprise rests.
The rub – which Smolin readily admits – is that there is no evidence that quantum mechanics is wrong. On the contrary, experiments designed to expose a flaw in the theory have instead demonstrated that the universe is a lot weirder than we thought. Weirdest of all is the effect called “entanglement,” which links the state of particles that have become widely separated. This phenomenon has been verified in the laboratory many times over and is the basis for efforts to develop commercially useful quantum computers.
Ironically, the practical question of how quantum computer systems work has created a new wave of writing and thinking about what quantum mechanics really means – a discipline known as “quantum foundations” that physicists of Smolin’s generation were once taught to avoid if they valued career success. Readers looking to sample the academic conversation can dip into Emergent Quantum Mechanics, a new, open-access collection of papers drawn from a 2017 symposium marking David Bohm’s centenary. Those looking for a more plain-language and even-handed take should consult Beyond Weird, by Philip Ball, a master explainer whose forward-looking perspective captures the growth of quantum information theory and its role in advancing the foundations of quantum theory.
A more narrative exploration can be found in What Is Real?, Adam Becker’s superbly written history of the Copenhagen interpretation and its challengers. Here, the embattled Bohm comes to life as he struggles to advance his “pilot wave” alternative to the Copenhagen juggernaut. So, too, does Hugh Everett, a hard-drinking and philandering genius who made his fortune optimizing how the U.S. military allocated its resources during the Cold War. While still a PhD student, Everett developed the “many worlds” interpretation of quantum mechanics, still an area of active research, which requires the universe to branch into separate but equally real versions of itself every time a particle is faced with doing one thing or another. Other forgotten characters also receive their due, including Grete Hermann, a German mathematician who demonstrated in 1935 that an influential “proof” that the Copenhagen interpretation had to be correct was simply wrong. Her contribution when unnoticed, Becker writes, adding, “It’s hard to imagine that her gender had nothing to do with the reception of her work.”
Becker also exposes another form of intellectual chauvinism when he examines the philosophical backdrop against which quantum mechanics emerged, and suggests that one reason the Copenhagen interpretation has persisted this long is that scientists have now stopped paying attention to, and benefiting from, the work of philosophers.
As the latest entry into the conversation, Smolin’s book feels the most immediate and personal.
Here is no detached narrator, but an active participant in the fray who perceives the debate over the nature of reality in personal terms. While discussing Everett’s many-worlds theory, for example, he recounts how he narrowly missed being a passenger on Swissair Flight 111, which crashed into the ocean off Nova Scotia 21 years ago. If Everett is correct, then Smolin’s life in a parallel universe ended with a deadly plunge into the Atlantic. The implication, he notes, presents a challenge to the concept of morality. What is the point of a good act when that act automatically generates another universe in which we choose not to do good?
Smolin’s intuition about what is going on is different from both Everett’s and Bohm’s. But he firmly rejects the Copenhagen interpretation and insists, instead, on a reality that exists independently from us and obeys the bedrock principle of cause and effect. For Smolin, entanglement is not proof that the Copenhagen crowd got it right in 1927, but rather an important clue that resolving the quantum impasse will require a radically new understanding of the nature of space and time.
“Realism, in any version, has a price,” he writes. “The question is what price we have to pay to get a new theory that makes complete sense and describes nature correctly and completely.”
Certainly, Bohm, who died in 1992, paid a price for his resistance in the form of professional isolation. After he spoke, I remember raising my hand and asking, with skepticism, why anyone should believe in his metaphorical dancers behind the curtain instead of in the cleaner, simpler idea that the universe is at it appears – in a state of perpetual contingency. The look on his face betrayed the burden of one used to being discounted by lesser minds.
While the way forward remains elusive, Smolin and others who seek to illuminate how physics got to where it is today are at least making the quest for answers a bit less costly.
Quantum Weirdness – A Pocket Guide
Quantum mechanics is a highly successful theory that accounts for the behaviour of matter and light with extraordinary precision. At the same time, fundamental features of the theory paint a view of reality that is sharply at odds with everyday experience – something that physicists have struggled with for nearly a century.
At small scales, particles of matter exhibit wave-like properties. In the standard (Copenhagen) view of quantum mechanics, the wavelike nature of a particle is linked to its “probability function,” a mathematical description of where the particle could be at a given time. Until it’s observed, the particle is considered to be in a mix of all possible places, like a ghost that is present everywhere but nowhere.
The probability function has strange implications, as illustrated by the famous thought experiment known as Schrodinger’s Cat. The scenario involves one particle of a radioactive substance in a sealed box. If the particle decays, it triggers a detector that releases a poison gas that kills a cat which is also hidden inside the box. Since quantum mechanics cannot say exactly when the particle will decay before it is measured, it remains both decayed and not-decayed – with the cat both alive and dead – until the box is opened.
THE UNCERTAINTY PRINCIPLE
Unlike classical physics, in quantum mechanics, certain properties – such as position and momentum – are strangely linked in such a way that improving knowledge about one produces a loss of information about the other. This manifests in phenomena such as “quantum tunnelling,” widely used in electronics, in which the fundamental uncertainty in a particle’s position allows it to pass through what would otherwise be an insurmountable barrier.
Quantum mechanics predicts (and experiments confirm) that two particles can be linked in such a way that making a measurement of one instantaneously determines a key property of the other, even after the particles have become widely separated. Einstein dubbed the idea “spooky action at a distance.” Entanglement has since become crucial to efforts to develop commercial quantum computers.
The double-slit experiment is a classic demonstration of quantum mechanics. It involves firing electrons (electrically charged particles) one at a time at a barrier with two slits and then seeing where each electron ends up. The pattern that emerges is identical to what would happen if each electron were a wave that can pass through both slits at the same time. The pattern disappears when a device is included to spot which slit each electron passes through. Different formulations of quantum mechanics view what is really happening in this real-world experiment very differently.
COPENHAGEN INTERPRETATION (Niels Bohr, Werner Heisenberg)
The electron takes all possible pathways at the same time. The pathways interfere with each other to produce the wave pattern. This requires accepting that a particle can be in two places at once unless it is being watched.
PILOT WAVE THEORY (Louis de Broglie, David Bohm)
The electron only passes through one of the slits but it is guided by an undetected entity called a pilot wave. This requires accepting that there are many hidden features of the universe, including empty “ghost waves” with no particles to guide to account for all possible outcomes.
MANY-WORLDS THEORY (Hugh Everett)
The experiment forces the universe to split into parallel universes, each of which accounts for one of the possible paths the electron might take. In this theory, there are no hidden structures or particles that are in multiple places at once – but it requires belief that at every moment countless new parallel universes are being born.