The demonstration that turned the great Isaac Newton's ideas about the nature of light was incredibly simple. It "can be repeated with great ease wherever the sun shines," the English physicist Thomas Young told the Royal Society in London in November 1803, describing what is now called the double slit experiment. And Young was not an enthusiastic youth. He came up with an elegant and elaborate experiment demonstrating the wave nature of light, and thereby refuted Newton's theory that light is composed of corpuscles, that is, particles.
But the birth of quantum physics in the early 1900s made it clear that light is made up of tiny indivisible units - or quanta - of energy that we call photons. Young's experiment with single photons, or even with individual particles of matter such as electrons and neurons, is a mystery that makes you wonder about the very nature of reality. Some have even used it to assert that the quantum world is influenced by human consciousness. But can a simple experiment really demonstrate this?
Can consciousness define reality?
In its modern quantum form, Young's experiment involves firing individual particles of light or matter through two slits or holes cut in an opaque barrier. On one side of the barrier is a screen that records the arrival of particles (say, a photographic plate in the case of photons). Common sense makes us expect photons to pass through either one or the other slit and accumulate behind the corresponding passage.
But no. The photons hit certain parts of the screen and avoid others, creating alternating streaks of light and darkness. These so-called fringes resemble a picture of two waves meeting. When the crests of one wave align with the crests of another, you get constructive interference (bright streaks), and when the crests align with troughs, you get destructive interference (darkness).
But only one photon passes through the device at a time. It looks like the photon goes through both slits at once and interferes with itself. This is contrary to common (classical) sense.
Mathematically speaking, it is not a physical particle or a physical wave that passes through both slits, but the so-called wave function - an abstract mathematical function representing the state of a photon (in this case, position). The wave function behaves like a wave. It hits two slits, and new waves come out on the other side of the slits, propagate and interfere with each other. The combined wave function calculates the probability of where the photon might be.
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The photon has a high probability of being where the two wave functions constructively interfere, and low - where the interference is destructive. Measurements - in this case, the interaction of the wave function with the photographic plate - leads to the "collapse" of the wave function, to its collapse. As a result, it points to one of the places in which the photon materializes after measurement.
This apparently measurement-induced collapse of the wave function has become the source of many conceptual difficulties in quantum mechanics. Before the collapse, there is no way to tell for sure where the photon will end up; it can be anywhere with nonzero probability. There is no way to trace the trajectory of a photon from source to detector. The photon is unreal in the sense that an airplane flying from San Francisco to New York is real.
Werner Heisenberg, among others, interpreted this mathematics in such a way that reality does not exist until it is observed. “The idea of an objective real world, the smallest particles of which exist objectively in the same sense that stones or trees exist, regardless of whether we observe them or not, is impossible,” he wrote. John Wheeler also used a variant of the double-slit experiment to state that "no elementary quantum phenomenon will be a phenomenon until it becomes a registered ('observed', 'definitely recorded') phenomenon."
But quantum theory gives absolutely no clue as to what counts as "measurement." She simply postulates that the measuring device must be classical, without defining where this line between the classical and the quantum lies, and leaving the door open for those who believe that collapse is causing human consciousness. Last May, Henry Stapp and his colleagues said the double-slit experiment and its current versions suggest that "a conscious observer may be necessary" to give meaning to the quantum realm, and that transpersonal intelligence is at the core of the material world.
But these experiments are not empirical evidence for such claims. In a double-slit experiment performed with single photons, one can only test the probabilistic predictions of mathematics. If probabilities pop up as tens of thousands of identical photons are sent through the double slit, the theory says that the wavefunction of each photon collapsed - thanks to a fuzzy process called measurement. That's all.
In addition, there are other interpretations of the double slit experiment. Take, for example, the de Broglie-Bohm theory, which says that reality is both a wave and a particle. The photon is directed to the double slit at a certain position at any moment and passes through one slit or the other; therefore, each photon has a trajectory. It travels through a pilot wave that penetrates both slits, interferes and then directs the photon to the site of constructive interference.
In 1979, Chris Dewdney and colleagues at Brickbeck College London modeled this theory's prediction of the paths of particles that would travel through a double slit. Over the past ten years, experimenters have confirmed that such trajectories exist, although they have used the controversial technique of so-called weak measurements. Despite the controversy, experiments have shown that de Broglie-Bohm theory is still able to explain the behavior of the quantum world.
More importantly, this theory does not need observers or measurements or intangible consciousness.
Neither are they needed by the so-called collapse theories, from which it follows that wave functions collapse randomly: the larger the number of particles in a quantum system, the more likely the collapse is. Observers simply record the result. Markus Arndt's team at the University of Vienna in Austria tested these theories by sending larger and larger molecules through a double slit. Collapse theories predict that when particles of matter become more massive than a certain threshold, they can no longer remain in a quantum superposition and pass through both slits at the same time, and this destroys the interference pattern. Arndt's team sent a molecule of 800 atoms through the double slit and still saw interference. The search for the threshold continues.
Roger Penrose had his own version of the theory of collapse, in which the higher the mass of an object in superposition, the faster it collapses to one state or another due to gravitational instabilities. Again, this theory does not require an observer or any kind of consciousness. Dirk Boumeester of the University of California, Santa Barbara is testing Penrose's idea with a version of the double slit experiment.
Conceptually, the idea is not only to put a photon in a superposition of passing through two slits at the same time, but also to put one of the slits in superposition and make it be in two places at the same time. According to Penrose, the replaced slit will either remain in superposition or collapse with a photon on the fly, which will lead to different interference patterns. This collapse will depend on the mass of the slits. Boumeester has been working on this experiment for ten years and may soon confirm or deny Penrose's claims.
In any case, these experiments show that we cannot yet make any claims about the nature of reality, even if these claims are well supported mathematically or philosophically. And given that neuroscientists and philosophers of mind cannot agree on the nature of consciousness, the claim that it leads to the collapse of wave functions would be premature at best and wrong at worst.
Ilya Khel
