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
ٰԴDz.”
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.
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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.”