Save
Schrödinger's Cat
Nobody
knows how the universe jumps from quantum mystery to the world as we experience
it. Here, three scientists straddle the quantum borderland with bold, new
research.
If I were to die and go to T-shirt heaven, the shirt I'd wear for the rest
of eternity would be one I saw at a science conference years ago: It bore the
slogan Save Schrödinger's Cat! The poor cat of Erwin Schrödinger's
famous thought experiment in quantum mechanics has been in bad shape since he
was first invented in 1935. He has spent his life in a box with a tube of
cyanide, and we don't know if he broke it and died, or is still alive-and we
won't know until we open the box and look. Until then, by the strange lights of
quantum theory, he's both dead and alive, and being a quantum cat, he's both at
once. This cat needs rescuing, like rainforests and Chinese alligators. But so
far it's been impossible. Even Schrödinger himself often lamented ever meeting
his famous feline.
Schrödinger's cat illuminates something sticky at the heart of quantum
mechanics. The most successful physical theory of all time, it has been used to
construct or understand everything from lasers to snowflakes. But pure
contradiction lies at its heart, because in the quantum world particles can be
in multiple locations simultaneously, and doing incompatible things, and in
multiple states, and acting in ways that make you wish you were not a quantum
theorist. This is called "superposition"-and superposition persists
until somebody looks into the cat box, atwhich point the possibilities collapse
into something real and classical-something we can hear, touch, feel, see, and
maybe even understand.
According to quantum theory, superposition should be possible at any scale,
but in the real world, things like keys, cats, and cars don't bilocate. If
quantum theory holds true at the tiny scale of photons and electrons, when does
it stop working and why?
We have a few answers, but no real solutions to one of the biggest mysteries
of modern physics. Superposition has been discovered at surprisingly
"big" scales-physicist Anton Zeilinger of the University of Vienna
fired a buckyball-otherwise known as a fullerene, a beau-tifully shaped
molecule that contains sixty carbon atoms-through two slits at once. But that's
still a lot smaller than a fabulous feline. If we could define the quantum
"borderland," we might be on our way to creating a new physics-one
that would begin to unify relativity theory and quantum mechanics. That
unification has been a holy grail of physics for some time now but most of the
faithful have abandoned the search.
Mathematical physicist Roger Penrose wants to keep looking. Working with
colleague Dik Bouwmeester and other researchers at the University of
California, Santa Barbara, Penrose has conceived an experiment to test the
quantum/classical borderline. "I would like to see whether or not I am
right about this new physics," he says.
"Our experiment is aimed at building a 'Schrödinger's cat' consisting
of a tiny mirror, roughly one-tenth of the thickness of a human hair, rather
than an actual cat. The mirror would be placed in a superposition of two very
slightly different locations." In a nod to Schrödinger, he calls the
experiment FELIX, an acronym for the ponderous sounding Free-Orbit Experiment
with Laser-Interferometry X-Rays. "According to my own ideas, this
superposition would spontaneously degenerate to one location or the other in
seconds. My prediction is at variance with what standard quantum mechanics
predicts." And, says Penrose, "It probes the very boundary of
application of our present theory of quantum mechanics." The grail, or at
least a well-behaved cat, just might be hiding at this boundary.
The experiment is a risky one. "I could be proved wrong by the
experiment, but not proved right by it," admits Penrose. "But that is
the nature of science!" In other words, if the experiment shows no effect,
it may just mean they looked in the wrong place-and have to keep looking.
Bouwmeester is not the only scientist inspired by Penrose; Andreas Mershin,
a physicist at Texas A&M University, wants to test an even bolder proposal
of Penrose's-that quantum effects can take place in the brain and may underlie
consciousness. Penrose, together with anesthesiologist Stuart Hameroff, has
proposed that microtubules in the neurons of the brain can actually operate in
a quantum fashion. (See, "Playing with Penrose" in the March-April
issue.) Mershin plans to test this in an experiment with tubulin and surface
plasmons-collective waves of electrons displaced on the surface of a metal.
"Plasmons are likea bunch of electrons getting together and moving
together in coherence, so that they seem like a particle," says Mershin.
Physics has already proved that a photon can be changed to a plasmon and back
to a photon-a relatively barbaric process-and stay, in the odd vocabulary of
quantum theory, entangled.
"Entanglement is a purely quantum property," notes Mershin.
"The trick we're proposing is to put tubulin on the surface of the metal
where plasmons are being excited. And the electric field from these electrons
should affect what is known as the electric constant of the tubulin. If we can
show that tubulin is capable of sustaining this quantum entanglement, Penrose
and Hameroff will seem a lot more credible."
Physicist Jack Tuszynski, of the University of Calgary, is exploring the
quantum border as well-hunting for the grail in the brain itself. A former
condensed matter physicist, he moved into biophysics and says, "I'm having
the time of my life. I was always interested in biology but there was not
enough hard data to sink your teeth into as a physicist. That changed about a
decade ago with nanotechnology." He recalls when he first heard the
Hameroff-Penrose hypothesis: "I didn't see how it could work. Now I have
less and less difficulty accepting that some interactions in the microtubules
are quantum. At the moment I'm looking at how different molecules bind to
microtubules."
One such molecule is the famous anti-cancer drug taxol. It binds to
microtubules, which are responsible for cell division, and makes them rigid,
stopping the process of cell division.
"I'm trying to develop a variant that would just bind in cancer cells
and not in normal cells," says Tuszynski. "You can zoom in to a tiny
piece of tubulin and see how taxol gets into the site. It sits there and then
an amino acid arm latches on and locks into the molecule. That in itself
doesn't tell you anything about quantum effects. But we find that when
biochemists play with the structure of taxol molecules, they find that two
rings are necessary to bind. And when you look at these rings they're only 2.5
angstroms in diameter. Yet they're crucial in this molecular reaction. How can
2.5 angstroms be anything other than quantum? That is the dimension of
individual atoms."
Tuszynski thinks these rings are actually docking devices. "At this level there is a lot of thermal noise." Thermal noise is a key issue, because most scientists believe that the warmth of a human brain destroys all quantum coherence. "Somehow, even through this noise," says Tuszynski, "a quantum interaction may be surviving and playing a major role."
Schrödinger's cat may be on the endangered species list, but he's still got a chance.
Originally published in Science & Spirit magazine, January, 2004.
