If Max Planck hadn’t ignored some bad advice, he would never have started a revolution. The pivotal moment happened in 1878, when young Planck asked one of his professors whether to continue pursuing a career in physics. Herr Professor Philipp von Jolly told Planck to find another line of work.
All the important discoveries in physics had already been made, the professor assured his young protégé. As Planck later recalled, von Jolly told him, “[Physics] may yet keep going in one corner or another, scrutinizing or putting in order a jot here and a tittle there, but the system as a whole is secured, and theoretical physics is noticeably approaching its completion.”
Putting one of those jots in order, it turned out, eventually won Planck a Nobel Prize — and led to the birth of quantum mechanics. The troublesome trifle concerned a very ordinary phenomenon: Why do objects glow the way they do when heated? All materials, no matter what they’re made of, behave the same way with increasing temperature: turning red, then yellow, then white. Yet no physicist in the 19th century could explain this seemingly simple process.
The problem came to be called the ultraviolet catastrophe, because the best theorem of the day predicted that objects heated to very high temperatures should spew infinite amounts of short-wavelength energy. Since we know a strong current doesn’t turn light bulbs and toasters into energy-spewing death rays, 19th century physics clearly wasn’t the last word.
Planck found an answer in 1900 with what amounted to a modern-day hack. He proposed (guessed, really) that energy could only be absorbed or emitted in discrete packets, or quanta. It was a radical departure from so-called classical physics, which held that energy flowed in smooth, continuous streams. At the time, Planck had no theoretical justification — but it turned out to work anyway. His quanta effectively capped the amount of energy that heated objects could release at any temperature. No more death rays.
So began the quantum revolution. It would take decades of incandescent theoretical work by Albert Einstein, Werner Heisenberg, Niels Bohr and other titans to transform Planck’s inspiration into a full theory, but it all started because no one understood what happened to things when they get hot.
The resulting theory, quantum mechanics, deals with particles and blips of energy in the realm of the ultra-small, divorced from our everyday experience, and all but invisible to our clumsy mammalian sensory apparatuses. Well, not completely invisible. Some quantum effects are hiding in plain sight, blatantly and beautifully obvious, like the sun’s rays and the twinkling of the stars — something else that couldn’t be fully explained before the advent of quantum mechanics.
How much of the quantum world can we experience in our daily lives? And what sort of information can our senses glean about the true nature of reality? After all, as the origin of the theory itself makes clear, quantum phenomena can lie just under our noses. In fact, they may be taking place right inside our noses.
The Quantum Schnozz
What’s going on in your nose when you wake up and smell the coffee, or the slice of bread browning in your non-lethal toaster? For such an in-your-face sensory organ, the nose is poorly understood. No less a luminary than Enrico Fermi, who built the world’s first nuclear reactor, once remarked to a friend while frying onions that it would be nice to understand how our sense of smell works.
So you’re lying in bed, and someone has thoughtfully brewed some freshly ground Sumatran dark roast. Molecules from the elixir waft through the air. Your inhalations draw some of these molecules into a cavity between your eyes just above the roof of your mouth. The molecules stick to a layer of mucus on the upper surface of the cavity, embedded with olfactory neurons. Dangling from the brain like the tentacles of a jellyfish, olfactory neurons are the only part of the central nervous system constantly exposed to the outside world.
What happens next isn’t quite clear. We know the molecules bind to some of the 400 different receptors on the surface of the olfactory neurons; we don’t know exactly how that contact creates our sense of smell. Why is smell such a difficult sense to understand?
“In part, it’s the difficulty of setting up experiments to probe what’s going on inside the olfactory receptors of the nose,” says Andrew Horsfield, a materials scientist at Imperial College London.
The conventional explanation for how smell works seems straightforward: The receptors accept very specific shapes of molecules. They’re like locks, which can be opened only by the right keys. Each of the molecules escaping from your cup of joe, according to this model, fits into a particular set of receptors in your nose. The brain interprets the unique combination of receptors activated by their bound molecules as the smell of coffee. In other words, we smell the shapes of molecules.
But there’s a fundamental problem with the lock-and-key model: “You can have molecules of wildly different shape and composition, which all give you the same odor perception,” says Horsfield. It seems that something more than shape must be involved, but what?
A controversial alternative to the lock-and-key model suggests our sense of smell arises not just from the shape of molecules, but also from the manner in which those molecules vibrate. All molecules constantly jiggle with distinct tempos, based on their structure. Could our noses somehow detect differences in those vibrational frequencies? Luca Turin, a biophysicist at the Alexander Fleming Biomedical Sciences Research Center in Greece, believes they can.
Turin, who also happens to be one of the world’s leading perfume experts, was inspired by a vibrational theory of smell first proposed by chemist Malcolm Dyson in 1938. After Turin first caught scent of Dyson’s idea in the 1990s, he started looking for molecules that would allow him to test the theory. He hit upon sulfur compounds, which have a unique odor and a characteristic molecular vibration. Turin then needed to identify a completely unrelated compound — one with a different molecular shape than sulfur but possessing the same vibrational frequency — to see if it would smell anything like sulfur. Eventually, he found one, a molecule containing boron. And sure enough, it reeked of sulfur. “That’s when the penny dropped,” he says. “I thought, ‘This cannot be a coincidence.’ ”
Since that odoriferous eureka moment, Turin has been gathering experimental evidence to support the idea, collaborating with Horsfield to work out the theoretical details. Five years ago, when Turin and colleagues designed an experiment in which some of the hydrogen molecules in a musk-scented fragrance were replaced with deuterium — a variety of hydrogen containing an extra neutron — they found that people could smell the difference. Since hydrogen and deuterium have identical shapes but different vibrational frequencies, the results again suggested that our noses could indeed detect vibrations. Similar experiments with fruit flies complemented those results.
Turin’s idea remains contentious — his experimental data have divided the interdisciplinary community of olfactory researchers. But if he is right, and we do smell vibrations in addition to shapes, how do our noses manage the feat? Turin speculates that a quantum effect called tunneling might be involved.
In quantum mechanics, electrons and all other particles possess a dual nature; each is both a particle and a wave. This sometimes allows electrons to spread out and travel, or tunnel, through materials in ways that would be forbidden to particles under the rules of classical physics. The molecular vibrations of a scent molecule might provide the right jump down in energy that electrons need to tunnel from one part of an odor receptor to another. The tunneling rate would change with different molecules, triggering nerve impulses that create the perceptions of different smells in the brain.
Tucked away in our noses, then, might be a sophisticated electronic detector. How could our noses have evolved to take advantage of such quantum strangeness? “I think we underestimate the technology, so to speak, of life by a couple of orders of magnitude,” says Turin. “Four billion years of R&D with unlimited funding is a long time. And I don’t think this is the most amazing thing that life does.”
OK, so you’re quaffing your coffee, nearly awake. Your eyelids are gearing up for daytime mode, blinking, letting in a bit of the light that’s streaming through the window. As you sip your brew, ponder this: The particles of light warming your face and entering your eyes originated a million years ago in the center of the sun, around the time our not-quite-human ancestors started to use fire. The sun wouldn’t even be sending out those particles, named photons, if not for the same phenomenon that might underlie our sense of smell — quantum tunneling.
Some 93 million miles separate the sun and Earth, and it takes photons just over eight minutes to cover that distance. But the bulk of their journey occurs inside the sun, where a typical photon spends a million years trying to escape. Matter is so tightly packed at the center of our star — the hydrogen there is about 13 times denser than lead — that photons can travel only an infinitesimal fraction of a second before being absorbed by a hydrogen ion, which then spits the photon out for another soon-to-be-interrupted journey, ad infinitum. After about a billion trillion such interactions, a photon finally emerges from the surface of the sun, having zigged and zagged randomly for a thousand millennia.
But the photons never would have been born, and the sun wouldn’t shine, were it not for quantum tunneling. The sun and all other stars generate light by nuclear fusion, smashing hydrogen ions together to form helium, a process that releases energy. Every second, the sun converts about 4 million tons of matter into energy. But hydrogen ions, single protons, have positive electric charges and naturally repel each other. So how can they possibly fuse?
With quantum tunneling, the wave nature of protons allows them to overlap ever so slightly, like ripples merging on the surface of a pond. That overlap brings the proton waves close enough so that another force — the strong nuclear force, which kicks in only at extremely small distances — can overcome the particles’ electrical repulsion. The protons fuse and release a single photon.
Our eyes have evolved to be exquisitely sensitive to these photons. Some recent experiments have shown that we can even detect single photons, which raises an intriguing possibility: Could humans be used to test some of the weird features of quantum mechanics? That is, could a person — like a photon or an electron or Schrödinger’s hapless cat, dead and alive at the same time — directly engage with the quantum world? What might such an experience be like?
“We don’t know because no one has tried it,” says Rebecca Holmes, a physicist at Los Alamos National Laboratory in New Mexico. Three years ago, when she was a graduate student at the University of Illinois at Urbana-Champaign, Holmes was part of a team led by Paul Kwiat that showed people could detect short bursts of light consisting of just three photons. In 2016, a competing group of researchers, led by physicist Alipasha Vaziri at Rockefeller University in New York, found that humans can indeed see single photons. Seeing, though, might not accurately describe the experience. Vaziri, who tried out the photon-glimpsing himself, told the journal Nature, “It’s not like seeing light. It’s almost a feeling, at the threshold of imagination.”
In the near future, Holmes and Vaziri expect experiments that will test what people perceive when photons are put into strange quantum states. For example, physicists can coax a single photon into what they call a superposition, where it exists in two different places simultaneously. Holmes and her colleagues have proposed an experiment involving two scenarios to test whether people might directly perceive a superposition of photons. In the first, ho-hum scenario, a single photon would go into either the left or right side of a person’s retina, and the person would note on which side of the retina they sensed the photon. But in the other scenario, a photon would be placed in a quantum superposition that would allow it to do the seemingly impossible: travel to both the right and left sides of the retina simultaneously.
Would the person then sense light on both sides of the retina? Or would the interaction of the photon with the eye cause the superposition to “collapse,” as physicists say, into one position or the other — and if so, would it happen equally on the right and left side, as theory suggests?
“Based on standard quantum mechanics, the photon in the superposition probably wouldn’t look any different to them than actually randomly sending a photon to the left or the right,” says Holmes. If it turns out that someone participating in the experiment did indeed perceive the photon in both places simultaneously, would that mean the person herself was in a quantum state? “You could say the observer was in a quantum superposition for some vanishingly small amount of time,” says Holmes. “But no one has tried this, so truly, we don’t know. That’s reason enough to do the experiment.”
Feeling Your Way
Now back to that cup of coffee. The cup feels substantial, a solid chunk of matter firmly in contact with the skin of your hand. But that’s an illusion: We never really touch anything, at least not in the sense of two solid slabs of matter coming together. More than 99.9999999999 percent of an atom consists of empty space, with nearly all its stuff concentrated in the nucleus.
When you exert pressure against the cup with your hand, the seeming solidity comes from the resistance of electrons in the cup. Electrons themselves don’t have any volume at all — they’re just fleeting, zero-dimensional flecks of negative electric charge that surround atoms and molecules like clouds. And the laws of quantum mechanics limit them to specific energy levels around atoms and molecules. As your hand grasps the cup, it forces electrons from one level to another, and that requires energy from the hand’s muscles, which the brain interprets as touching something solid.
Our sense of touch, then, arises from an exceedingly complex interaction between electrons around the molecules of our bodies and those of the objects we encounter. From that information, our brain creates the illusion that we possess solid bodies moving through a world filled with other solid objects. Touch doesn’t give us an accurate sense of reality. And it may be that none of our perceptions match what’s really out there. Donald Hoffman, a cognitive neuroscientist at the University of California, Irvine, believes that our senses and brain evolved to hide the true nature of reality, not to reveal it.
“My idea is that reality, whatever it is, is too complicated and would take us too much time and energy [to process],” he says.
Hoffman likens the picture our brain constructs of the world to the graphical interface on a computer screen. All the colorful icons on the screen — the trash can, the mouse pointer, the file folders — bear no resemblance at all to what’s really going on inside the computer. They’re abstractions, simplifications that allow us to interact with complex electronics.
In Hoffman’s view, evolution has shaped our brains to operate in much the same way, as a graphical interface that doesn’t reproduce the world with any sort of fidelity. Evolution doesn’t favor the development of accurate perceptions; it rewards ones that enhance survival. Or as Hoffman puts it, “Fitness beats truth.”
Hoffman and his graduate students have run hundreds of thousands of computer models in recent years to test his ideas. In the simulations, artificial life-forms compete for limited resources. And in every case, the organisms programmed to emphasize fitness outcompete the various ones primed for accurate perceptions. For example, if one organism is tuned to accurately perceive, say, the total amount of water present in an environment, it will lose out to an organism that’s tuned to perceive something simpler: the optimal amount of water needed to stay alive.
So while one organism might construct a more accurate representation of reality, that representation doesn’t enhance its survivability. Hoffman’s studies have led him to a remarkable conclusion: “To the extent that we’re tuned to fitness, we will not be tuned to reality. You can’t do both.”
His ideas align with what some physicists believe to be a central message of quantum theory: Reality is not completely objective — we cannot separate ourselves from the world we observe. Hoffman fully embraces that view. “Space is just a data structure,” he says, “and physical objects are themselves also data structures that we create on the fly. When I look at that hill over there, I create that data structure. Then I look away and I’ve trashed that data structure because I don’t need it anymore.”
As Hoffman’s work shows, we haven’t yet come to grips with the full meaning of quantum theory and what it says about the nature of reality. Planck himself struggled for most of his life to understand the theory he helped launch, and always believed in an objective universe that exists independently of us. He once wrote about why he decided to go into physics against the advice of his mentor: “The outside world is something independent from man, something absolute, and the quest for the laws which apply to this absolute appeared to me as the most sublime scientific pursuit in life.” Maybe it will take another century, and another revolution, to prove whether he was right, or as mistaken as Professor von Jolly.