A lone researcher says he can cut an electron in two. If he's
right, quantum physics is dead. Marcus Chown investigates
AT THE TIME no one even realised it had happened. More than thirty
years ago, researchers in Minnesota did the unthinkable and broke the
"indivisible" electron into fragments. This, at least, is the contention of
British physicist Humphrey Maris, and no one has yet been able to prove
him wrong. "Electron fragments behave to all intents and purposes like
entirely separate particles," says Maris, who is based at Brown University
in Rhode Island. "I call them electrinos."
Pause a moment to consider what Maris is saying. The electron is the
lightest subatomic particle and the one with the greatest claim to being
absolutely fundamental. In fact, in the 103 years since its discovery,
there has been no other evidence whatsoever that the electron is
divisible. It is the modern incarnation of Democritus's "uncuttable" atom.
The claim that electrons are divisible is therefore nothing short of a
bombshell dropped into the world of physics. "If Humphrey is correct, it
means a Nobel Prize," says Gary Ihas of the University of Florida. Nobel
prizewinner Philip Anderson of Princeton University thinks Maris must be
wrong. "But it's not obvious why," he admits.
Maris does not have definitive proof of his hypothesis. But earlier this
year he published a paper that put it on a firm theoretical basis, and
marshalled supporting evidence from past experiments. Now he is doing
his own experiments, trying to break up the electron.
Whether Maris succeeds or not, he may have found a large crack in one
of the foundation stones of modern physics. "Humphrey has succeeded
in exposing a fundamental flaw in the framework of quantum theory,"
says Peter McClintock of the University of Lancaster.
This astonishing heresy is centred around the electron's wave function,
the mathematical entity that, according to quantum theory, encapsulates
everything about the electron that it is possible for us to know. Among
other things, an electron's wave function describes the probability of
finding it at any particular location. The wave function of an electron
confined to, say, a spherical cavity is the three-dimensional description
of how the electron's location is "smeared out" over the space.
In its lowest energy state, the wave function is spherical. The next
highest energy level gives the wave function a dumb-bell shape. "It was
while thinking about this state I was led to the conclusion that an electron
might split in two," says Maris. If the dumb-bell could be stretched and
pinched, he reasoned, might it simply divide?
Maris is expert in liquid helium, a substance that gives physicists the
perfect opportunity to test this idea because electrons can exist
independently and autonomously within it. When electrons from a
radioactive source are fired into a vat of helium, repeated interactions
with the electrons of the helium atoms slow them down until, finally, they
grind to a halt. The intruding electrons do not attach themselves to
helium atoms as a third electron, however. The Pauli exclusion principle
makes sure of that, because it forbids more than two electrons from
sharing the same quantum state. Faced with helium atoms whose
electrons have bagged the lowest energy state--the ground state--an
interloper with no spare energy has no choice but to lodge in the space
between atoms. There it clears a bubble of space around itself--an
electron bubble.
Electron bubbles form only in certain types of liquid--those in which the
van der Waals force of attraction between atoms is weak enough to allow
an electron to push them apart. In fact only two substances fit the bill:
liquid helium and liquid hydrogen. At very low temperatures in helium,
electron bubbles displace more than 700 helium atoms, creating a cavity
around 38 angstroms (3.8 nanometres) across. Inside this cavity
quantum mechanics rules, ensuring that the electron can occupy only a
limited set of energy states.
Light touch
Maris worked out that an electron in a bubble could be put into the
dumb-bell-shaped excited state by illuminating the helium with light that
had a wavelength of about 10 micrometres, which is easily supplied by a
carbon dioxide laser. In this state, Maris calculated, the electron imparts
most of its force to the ends of the dumb-bell; this force is enough, he
realised, to make the bubble wall wobble violently. "I found that the
force exerted by the electron was enough to elongate the bubble until it
formed a thin neck," he says. "If the pressure in the liquid was great
enough, there was the possibility of it pinching off the neck so that the
bubble might actually split in two."
This sounds harmless enough, but the implications are staggering. If the
bubble split, half of the electron's wave function would be trapped in
each of the two daughter bubbles (see Diagram). As the wave function is
the essence of an electron, the electron would be split into two. The
indivisible would have been divided.
Maris planned to test his idea in the
laboratory but first decided to search
back through the literature to see
whether anyone had done the kind of
experiments he had in mind. He soon
found what he was looking for. In the
late 1960s, Jan Northby and Mike
Sanders at the University of Minnesota
studied the speed of electron bubbles
moving in an electric field in liquid
helium. They measured the electric
current that flowed as the bubbles
moved, and then illuminated the helium
with light. The researchers expected this to increase the current. They
reasoned that light would eject some of the electrons from the bubbles,
and that these would whiz through the helium, boosting the current--and
that is exactly what they observed.
But as physicists have since realised, this reasoning was flawed. "We
now know that knocked-out electrons form new electron bubbles," says
Maris. "The current should not have increased." Inexplicably, however, it
did. In 1990 and 1992, researchers at Bell Labs in New Jersey ran a
similar experiment, with the same result. No explanation has ever been
found--until now, perhaps.
Maris suggests that, instead of ejecting the electrons, the light boosted
them from the ground state to the dumb-bell-shaped excited state, and
the electron bubbles split. "There were more bubbles, and being smaller
they were more mobile," says Maris. Although the total charge in the
system remained the same, the smaller bubbles felt less drag in the
helium, and thus moved faster. "Consequently, the current went up,"
Maris explains.
Maris believes he has further evidence to support his explanation.
Northby and Sanders saw the increased current only below 1.7 kelvin,
exactly the temperature at which Maris's theory says the effect should
take hold. According to his calculations, electron bubbles should split
apart only below a critical temperature of 1.7 K. The crucial factor is
viscosity. If it is too great, says Maris, the liquid will behave like treacle,
resisting the elongation of the bubble and squeezing it back to a sphere.
Below 2.19K liquid helium becomes a superfluid: as you cool it, its
viscosity starts to disappear. By 1.7 K, Maris calculated, the liquid would
be so slippery that it couldn't stop the bubbles dividing.
Other experimenters have studied the mobility of electrons in a more
precise way. They include Gary Ihas and Mike Sanders at the University
of Michigan in 1971 and Van Eden and McClintock of the University of
Lancaster in 1984. These physicists created a short burst of about a
million bubbles which they carefully timed as they moved through liquid
helium in an electric field. Since the bubbles were created together, they
should have crossed the finishing line together. To the surprise of the
experimenters, most of the bubbles arrived in three separate clumps.
Maris's explanation is again simple. Unlike the electrons in the Minnesota
experiment, these electrons had been created in an electrical
discharge--a miniature bolt of lightning. This produced light, and Maris
says that some of this light boosted electrons within the bubbles to the
excited state, causing them to split, and split again. Hence the spread of
arrival times, with whole, half and quarter charges making up most of
the current.
McClintock is not yet convinced by Maris. But he
admits that nobody else has come up with a
plausible explanation. "The electrino idea offers
a possible way out," he concedes.
Maris has long realised the furore his ideas
would cause. He spent several years working out
the details of electron bubble fission and gathering experimental
evidence without ever telling anyone what he was thinking. "It took time
to get used to the idea and pluck up the courage to announce it," he
admits. Finally, in June this year, he decided to go public. He presented
his work at a Minneapolis conference on quantum fluids and solids, and
then published it in a paper in the Journal of Low Temperature
Physics (vol 120, p 173).
The conference organisers thought Maris's work important enough to
give him an extra two-hour session. At the end, more than 100 physicists
questioned every aspect of the theory. "My first reaction was extreme
scepticism, like everyone else," McClintock says. Maris, though, had an
answer for everything. "He'd obviously thought long and hard about the
whole thing," McClintock concedes.
Maris was encouraged by the response --or lack of it--from his peers. "I
was nervous someone would find a hole," he admits. "But to my relief
nobody dismissed the idea out of hand."
Experts in quantum theory are not so accommodating, though. "The idea
of an electron splitting into fractionally charged fragments is totally
incompatible with quantum field theory," says Anthony Leggett of the
University of Illinois at Urbana-Champaign. He admits that there could be
something wrong with quantum field theory. "However, given its
overwhelming success in explaining the world, this is highly unlikely," he
says.
According to quantum theory, it is possible to have strange
"superposition states", where the whole electron exists in both bubbles
until a measurement forces it to be in one or the other. "But we cannot
consider states which have half an electron on each," Leggett insists. It is
impossible to solve the equations of quantum mechanics with anything
other than a whole-charge electron. The formulations of quantum
electrodynamics, the area of physics that deals with the behaviour and
properties of electrons, don't allow for half electrons, or any other
fraction.
"If the electron splits and you can measure a fractional charge, this flies
in the face of standard quantum mechanics as well as high-energy
physics," agrees David Pritchard of the Massachusetts Institute of
Technology. "The idea that the electron is a point particle without
structure is established up to very high energies."
Half measures
Like Leggett and Pritchard, most physicists are convinced that Maris's
claim falls at the first fence, though they cannot pinpoint why. Their
scepticism is understandable. If Maris is right then quantum theory is
wrong--and nobody has the slightest idea what they would use to replace
it.
Maris being right would have some positive practical consequences,
however. He speculates about building a device which introduces a
partition into a cavity to divide the wave function of an electron. This
could lead to circuits which exploit the properties of fractionally charged
particles, he says. Half-mass, half-charge electrons might give
electronics a whole new dimension. Then there's the possibility of a new
kind of chemistry. Maybe you could take an electron bubble out of the
liquid, attach the electron fragment to an atom and do novel chemistry
with fractional electrons. Could this really happen? Maris says he doesn't
know.
The electron fragments, having once been part of the same electron,
might even be "entangled", sharing a strange telepathic link. Quantum
physicists have already managed to achieve this with photons, and used
these entangled particles of light to perform astonishing feats such as
teleportation and elementary quantum computing. Fractional charge
might add a new string to their bow.
The most profound consequences of splitting the electron, though, would
be on theoretical physics. Maris's only concrete claim is that an electron's
wave function can be split and mimic a fractional electron. He has no
idea of the full consequences of this--and neither has anyone else.
Maris's hypothesis seems to throw everything we know about quantum
theory into confusion. At the very least, he believes, his work challenges
physicists to be specific about what they mean by the fuzzy entity that
describes quantum systems. "People are going to have to hone their
ideas of the wave function," he says. "Most importantly, they are going
to have to address the question: what is a wave function? Is it a real
thing, or just a mathematical convenience?"
Physicists have always been content to think of the wave function as a
mathematical device with observable consequences. But Maris believes
the time has come for the idea to be grounded in reality. For the electron
bubbles in helium, he says, the size of the bubble is determined by how
much of the wave function is trapped inside the bubble. If there is no
part of the wave function inside the bubble, the bubble will collapse. "This
makes the wave function seem to be a tangible object," he argues.
Maris remains an experimentalist at heart, though. Since the theorists
have nothing to say about the myriad questions he has raised, he
believes answers won't be found until there is some more evidence to go
on--and that means doing more experiments. Maris and others, he
believes, are now looking for that evidence. "Already, the results of my
experiments are encouraging," he says.
But Maris also insists that he won't be upset if his idea is eventually
disproved. Having lobbed in his bombshell, he seems to have decided to
sit on the sidelines, enjoying the ensuing chaos. "What I have come up
with is an intriguing puzzle," he says. "I want people to think. I would be
happy if I was completely wrong but made a lot of people think.
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