Nearly a century after its founding, physicists and
philosophers still don’t know—but they’re working on it
- By Anil Ananthaswamy on September 3, 2018
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 ifeach 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
wave functions 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.
ABOUT THE AUTHOR(S)
Anil Ananthaswamy
Anil Ananthaswamy is the author of The Edge of Physics, The Man Who Wasn't There and, most recently, Through Two Doors at Once: The
Elegant Experiment That Captures the Enigma of Our Quantum Reality.
SOURCE:
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7 SEPT. 2018
