Quantum mechanics is both the most powerful theory physicists have ever devised and the most baffling. On the one hand, countless experiments have confirmed its predictions; the theory undergirds modern technology and enables the electronic devices we use every day. On the other hand, quantum mechanics describes an underlying reality that is utterly at odds with the world we perceive. In the quantum realm, a single particle exists in many places at once—at least while no one is looking at it. The theory also allows for inexplicable connectedness: a pair of atoms, no matter how widely separated, can be “entangled,” such that whatever happens to one atom instantaneously affects the other. Albert Einstein called the phenomenon “spooky action at a distance.”
These paradoxes have defined—or plagued—the theory since its inception more than a century ago. To this day, physicists still don’t agree on what quantum mechanics is telling us about the nature of reality. Are there multiple universes? Do things come into existence only when they’re observed? Is consciousness somehow central to the laws of physics? And what if all these mysteries could have been resolved right at the birth of quantum mechanics?
. That’s the case that physicist Antony Valentini, a physicist at Imperial College London, makes in his new book Beyond the Quantum: A Quest for the Origin and Hidden Meaning of Quantum Mechanics (Oxford University Press, 2026).
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Valentini argues that Louis de Broglie, a French physicist and Nobel laureate, developed a framework for quantum mechanics that eliminated its paradoxes around 100 years ago. In pilot wave theory, as de Broglie’s brainchild is known, particles are guided by attendant waves. The particles themselves are always in one position and one position only; it is the spatially extended pilot wave that creates the impression that a particle is at once here and there. There’s no need for an observer to conjure that particle into being. Even though de Broglie’s conjecture in 1924 about the wavelike nature of matter was quickly confirmed by experiment and became integral to quantum theory, the physics community discounted or misrepresented the larger ideas from which he derived his key insights.
Valentini has spent his entire career championing and extending de Broglie’s views. He recently spoke to Scientific American about his lonely path and why de Broglie might have been on to something.
[An edited transcript of the interview follows.]
In the history of science, has there ever been another situation like this, where there have been such wildly divergent views about what a theory means?
I’m not sure there has. If you go back to the time of [Isaac] Newton, he thought that space was empty and that there was a direct gravitational action at a distance. And on the continent, there were the Cartesians [followers of mathematician and philosopher René Descartes], who thought, “Oh no, space is full of this material medium, and that explains gravitational attraction.” But [the debate] didn’t last all that long. Certainly in the quantum case, the sheer variety of interpretations that say such completely different things about the world—I think it’s a pretty safe bet that there’s no analogue in the history of science.
One of the most striking things about modern physics is the stark divide between the macroscopic and quantum worlds, each of which seems to be governed by entirely different physical laws. You liken this to the way medieval astronomers split the cosmos into earthly and celestial regions.
I think it’s a useful and valid parallel, this idea that there was a heavenly realm that we couldn’t understand; anything above the moon and beyond was eternal and unchanging, completely different from the sublunar world, which was made of ordinary, imperfect matter that was always changing. It’s a distinction that goes back to Aristotle. The parallel with quantum mechanics is extraordinary, that the quantum system is something that our mind can’t understand. We can only understand the macroscopic one.
Austrian physicist Erwin Schrödinger developed quantum theory’s wave equation, which describes quantum systems as waves that evolve with time. What role did this equation play inthe so-called measurement problem: If a particle exists in different places at once, why do measurements find any given particle in only a single location?
Schrödinger created the measurement problem by removing the particles from de Broglie’s theory. Mathematically, a [quantum wave] is a superposition of many different positions: a particle can be here and here and here; it can be anywhere. You can have a superposition of a live cat and a dead cat, or a superposition of different energies. They’re all just different variations on the same theme. The wave equation contains all possible positions.How can you then explain that we see this little pointlike object if the only reality is an extended wave?
And this conundrum was recognized early in the development of quantum theory.
Here’s Wolfgang Pauli writing to Niels Bohr in 1927: “In the last issue of the Journal de Physique, a paper by de Broglie has appeared…. It is very rich in ideas and very sharp, and on a much higher level than the childish papers by Schrödinger, who even today still thinks he may … abolish material points.”And becauseSchrödinger removed [particles from his equation], we’ve ended up with decades of confusion.
Why do you think de Broglie’s theory was set aside and neglected?
I’m not sure there is one simple answer. It’s maybe a mix of reasons.
In 1923 de Broglie had developed a new theory of motion. It was a complete break, very different from Newtonian or even Einsteinian physics. And yet this completely passed people by. The only thing that entered the collective consciousness of physicists was that de Broglie had shown that a particle can behave like a wave.
Word of de Broglie’s thesis spread, though hardly anyone actually read it. Einstein did. It was Einstein who really alerted people that de Broglie had done something very important. He encouraged Schrödinger to read it—and he read it. Most other people, it seems, never read de Broglie’s thesis.
And then there is the sociological point that de Broglie was quite isolated in Paris. De Broglie was a bit of a loner; he worked essentially by himself. At that time, in the 1920s, France was really a backwater in theoretical physics. It was strong in experimental physics, strong in mathematics but not in theoretical physics.
Has your pursuit of pilot wave theory been a lonely one? Rewarding? Frustrating?
The short answer is all of that and more. Has it been lonely? It’s been this peculiar situation. I’ve really tried to get the key points across to physicists. And it just seems to fall on deaf ears. It’s as if people are stuck on repeat—the same wrong arguments, the same historical misconceptions just go around and around and around.
When I first came across pilot wave theory, it seemed to me so obvious. Oh my god, pilot wave theory in principle is a wider physics; quantum theory is a special case of something bigger. Pilot wave theory has exciting new physics, and maybe we can find evidence for it.
In your book, you describe how pilot wave theory’s predictions about the physics of matter differ in some cases from the predictions of accepted quantum mechanics. In particular, you mention how the cosmic microwave background (CMB)—the radiation created during the big bang that now permeates the universe—might support some of the predictions of pilot wave theory.
The CMB is an excellent and promising avenue, and I’ve done a lot of work on that with various collaborators. There are reported anomalies in the CMB that qualitatively match the kind of anomalies that pilot wave theory would predict. There are some tantalizing hints, but the data are just too noisy to draw any firm conclusions. This probably won’t be settled for another 10 years or so.
Is pilot wave theory true? Is it an accurate theory of the world? If I knew it was true, I wouldn’t be researching it. There’s always, in the back of my mind, the thought that this could all be completely wrong! Or it could be that it’s sort of partly right. In the late 19th century Ludwig Boltzmann modeled [gas molecules] as little billiard balls—little hard spheres that are bouncing around. It turns out that molecules are much more complicated than that. But still, his model contained a lot of truth. It might be that pilot wave theory is a bit like that, an approximate model.
