For a demonstration that overturned the great Isaac Newton’s ideas about the nature of light, it was staggeringly simple. It “may be repeated with great ease, wherever the sun shines,” the English physicist Thomas Young told the members of the Royal Society in London in November 1803, describing what is now known as a double-slit experiment, and Young wasn’t being overly melodramatic. He had come up with an elegant and decidedly homespun experiment to show light’s wavelike nature, and in doing so refuted Newton’s theory that light is made of corpuscles, or particles.
But the birth of quantum physics in the early 1900s made it clear that light is made of tiny, indivisible units, or quanta, of energy, which we call photons. Young’s experiment, when done with single photons or even single particles of matter, such as electrons and neutrons, is a conundrum to behold, raising fundamental questions about the very nature of reality. Some have even used it to argue that the quantum world is influenced by human consciousness, giving our minds an agency and a place in the ontology of the universe. But does the simple experiment really make such a case?
In the modern quantum form, Young’s experiment involves beaming individual particles of light or matter at two slits or openings cut into an otherwise opaque barrier. On the other side of the barrier is a screen that records the arrival of the particles (say, a photographic plate in the case of photons). Common sense leads us to expect that photons should go through one slit or the other and pile up behind each slit.
They don’t. Rather, they go to certain parts of the screen and avoid others, creating alternating bands of light and dark. These so-called interference fringes, the kind you get when two sets of waves overlap. When the crests of one wave line up with the crests of another, you get constructive interference (bright bands), and when the crests align with troughs you get destructive interference (darkness).
But there’s only one photon going through the apparatus at any one time. It’s as if each photon is going through both slits at once and interfering with itself. This doesn’t make classical sense.
Mathematically speaking, however, what goes through both slits is not a physical particle or a physical wave but something called a wave function—an abstract mathematical function that represents the photon’s state (in this case its position). The wave function behaves like a wave. It hits the two slits, and new waves emanate from each slit on the other side, spread and eventually interfere with each other. The combined wave function can be used to work out the probabilities of where one might find the photon.
The photon has a high probability of being found where the two wave functions constructively interfere and is unlikely to be found in regions of destructive interference. The measurement—in this case the interaction of the wave function with the photographic plate—is said to “collapse” the wave function. It goes from being spread out before measurement to peaking at one of those places where the photon materializes upon measurement.
This apparent measurement-induced collapse of the wave function is the source of many conceptual difficulties in quantum mechanics. Before the collapse, there’s no way to tell with certainty where the photon will land; it can appear at any one of the places of non-zero probability. There’s no way to chart the photon’s trajectory from the source to the detector. The photon is not real in the sense that a plane flying from San Francisco to New York is real.
Werner Heisenberg, among others, interpreted the mathematics to mean that reality doesn’t exist until observed. “The idea of an objective real world whose smallest parts exist objectively in the same sense as stones or trees exist, independently of whether or not we observe them ... is impossible,” he wrote. John Wheeler, too, used a variant of the double-slit experiment to argue that “no elementary quantum phenomenon is a phenomenon until it is a registered (‘observed,’ ‘indelibly recorded’) phenomenon.”
But quantum theory is entirely unclear about what constitutes a “measurement.” It simply postulates that the measuring device must be classical, without defining where such a boundary between the classical and quantum lies, thus leaving the door open for those who think that human consciousness needs to be invoked for collapse. Last May, Henry Stapp and colleagues argued, in this forum, that the double-slit experiment and its modern variants provide evidence that “a conscious observer may be indispensable” to make sense of the quantum realm and that a transpersonal mind underlies the material world.
But these experiments don’t constitute empirical evidence for such claims. In the double-slit experiment done with single photons, all one can do is verify the probabilistic predictions of the mathematics. If the probabilities are borne out over the course of sending tens of thousands of identical photons through the double slit, the theory claims that each photon’s wave function collapsed—thanks to an ill-defined process called measurement. That’s all.
Also, there are other ways of interpreting the double-slit experiment. Take the de Broglie-Bohm theory, which says that reality is both wave and particle. A photon heads towards the double slit with a definite position at all times and goes through one slit or the other; so each photon has a trajectory. It’s riding a pilot wave, which goes through both slits, interferes and then guides the photon to a location of constructive interference.
In 1979, Chris Dewdney and colleagues at Birkbeck College, London, simulated the theory’s prediction for the trajectories of particles going through the double slit. In the past decade, experimentalists have verified that such trajectories exist, albeit by using a controversial technique called weak measurements. The controversy notwithstanding, the experiments show that the de Broglie-Bohm theory remains in the running as an explanation for the behavior of the quantum world.
Crucially, the theory does not need observers or measurements or a non-material consciousness.
Neither do so-called collapse theories, which argue that wavefunctions collapse randomly: the more the number of particles in the quantum system, the more likely the collapse. Observers merely discover the outcome. Markus Arndt’s team at the University of Vienna in Austria has been testing these theories by sending larger and larger molecules through the double slit. Collapse theories predict that when particles of matter become more massive than some threshold, they cannot remain in a quantum superposition of going through both slits at once, and this will destroy the interference pattern. Arndt’s team has sent a molecule with more than 800 atoms through the double slit, and they still see interference. The search for the threshold continues.
Roger Penrose has his own version of a collapse theory, in which the more massive the mass of the object in superposition, the faster it’ll collapse to one state or the other, because of gravitational instabilities. Again, it’s an observer-independent theory. No consciousness needed. Dirk Bouwmeester at the University of California, Santa Barbara, is testing Penrose’s idea with a version of the double-slit experiment.
Conceptually, the idea is to not just put a photon into a superposition of going through two slits at once, but to also put one of the slits in a superposition of being in two locations at once. According to Penrose, the displaced slit will either stay in superposition or collapse while the photon is in flight, leading to different types of interference patterns. The collapse will depend on the mass of the slits. Bouwmeester has been at work on this experiment for a decade and may soon be able to verify or refute Penrose’s claims.
If nothing else, these experiments are showing that we cannot yet make any claims about the nature of reality, even if the claims are well-motivated mathematically or philosophically. And given that neuroscientists and philosophers of mind don’t agree on the nature of consciousness, claims that it collapses wave functions are premature at best and misleading and wrong at worst.