The universe is a string-net liquid
15 March 2007, Zeeya Merali
Is this a new kind of matter (Image: Elmar Lackner/Mindat)
In
1998, just after he won a share of the Nobel prize for
physics, Robert Laughlin of Stanford University in California was asked how his
discovery of "particles" with fractional charge, now called
quasi-particles, would affect the lives of ordinary people. "It probably
won't," he said, "unless people are concerned about how the universe
works."
Well,
people were. Xiao-Gang Wen at the Massachusetts
Institute of Technology and Michael Levin at
The
first hint that a new type of matter may exist came in 1983. "Twenty five
years ago we thought we understood everything about how matter changes
phase," says Wen. "Then along came an
experiment that opened up a whole new world."
In
the experiment, electrons moving in the interface between two semiconductors
behaved as though they were made up of particles with only a fraction of the
electron's charge. This so-called
fractional quantum hall effect (FQHE) suggested that electrons may not be
elementary particles after all. However, it soon became clear that electrons
under certain conditions can congregate in a way that gives them the illusion
of having fractional charge - an explanation that earned Laughlin, Horst Störmer and Daniel Tsui the
Nobel prize (New Scientist, 31 January 1998, p
36).
Wen suspected that the effect could be
an example of a new type of matter. Different phases of matter are characterised by the way their atoms are organised. In a liquid, for instance, atoms are randomly
distributed, whereas atoms in a solid are rigidly positioned in a lattice. FQHE
systems are different. "If you take a snapshot of the position of
electrons in an FQHE system they appear random and you think you have a
liquid," says Wen. But step back,
and you see that, unlike in a liquid, the electrons dance around each other in
well-defined steps.
“The position of the electrons in this material appears random like in a
liquid, but they also move in well-defined steps”
It
is as if the electrons are entangled. Today, physicists use the term to
describe a property in quantum mechanics in which particles can be linked
despite being separated by great distances. Wen
speculated that FQHE systems represented a state of matter in which
entanglement was an intrinsic property, with particles tied to each other in a
complicated manner across the entire material.
This
led Wen and Levin to the idea that there may be a
different way of thinking about matter. What if electrons were not really
elementary, but were formed at the ends of long "strings" of other,
fundamental particles? They formulated a model in which such strings are free
to move "like noodles in a soup" and weave together into huge
"string-nets".
“What if electrons were not elementary, but were formed at the ends of
long strings of other, fundamental particles?”
Light and matter unified
The
pair ran simulations to see if their string-nets could give rise to
conventional particles and fractionally charged quasi-particles. They did. They
also found something even more surprising. As the net of strings vibrated, it
produced a wave that behaved according to a very familiar set of laws -
Maxwell's equations, which describe the behaviour of
light. "A hundred and fifty years after Maxwell wrote them down, here they
emerged by accident," says Wen.
That
wasn't all. They found that their model naturally gave rise to other elementary
particles, such as quarks, which make up protons and neutrons, and the
particles responsible for some of the fundamental forces, such as gluons and
the W and Z bosons.
From
this, the researchers made another leap. Could the entire universe be modelled in a similar way? "Suddenly we realised, maybe the vacuum of our whole universe is a
string-net liquid," says Wen. "It would
provide a unified explanation of how both light and matter arise." So in
their theory elementary particles are not the fundamental building blocks of
matter. Instead, they emerge from the deeper structure of the non-empty vacuum
of space-time.
"Wen and Levin's theory is really beautiful stuff,"
says Michael Freedman, 1986 winner of the Fields medal, the highest prize in
mathematics, and a quantum computing specialist at Microsoft Station Q at the
Other
theories that try to explain the same phenomena abound, of course; Wen and Levin realise that the
burden of proof is on them. It may not be far off. Their model predicts
specific arrangements of atoms in the new state of matter, which they dub the
"string-net liquid", and Joel Helton's group at MIT might have found
it.
Helton
was aware of Wen's work and decided to look for such
materials. Trawling through geology journals, his team spotted a candidate - a dark
green crystal that geologists stumbled across in the mountains of
Herbertsmithite (pictured) is unusual
because its electrons are arranged in a triangular lattice. Normally, electrons
prefer to line up so that their spins are in the opposite direction to that of
their immediate neighbours, but in a triangle this is
impossible - there will always be neighbouring
electrons spinning in the same direction. Wen and
Levin's model shows that such a system would be a string-net liquid.
Although
herbertsmithite exists in nature, the mineral
contains impurities that disrupt any string-net signatures, says Lee. So
Helton's team made a pure sample in the lab. "It was painstaking,"
says Lee. "It took us a full year to prepare it and another year to analyse it."
The
team measured the degree of magnetisation in the
material, in response to an applied magnetic field. If herbertsmithite
behaves like ordinary matter, they argue, then below about 26 °C the spins of
its electrons should stop fluctuating - a condition called magnetic order. But
the team found no such transition, even down to just a fraction above absolute
zero.
They
measured other properties, too, such as heat conduction. In conventional
solids, the relationship between their temperature and their ability to conduct
heat changes below a certain temperature, because the structure of the material
changes. The team found no sign of such a transition in herbertsmithite,
suggesting that, unlike other types of matter, its lowest energy state has no
discernible order. "We could have created something in the lab that nobody
has seen before," says Lee.
The
team plans further tests to visualise the position of
individual electrons, looking for long-range entanglement by firing neutrons at
the crystal and observing how they scatter. "We want to see the dynamics
of the spin," says Lee. "If we tweak one [electron], we can see how
the others are affected."
This intrigues Paul Fendley, a quantum computing specialist at the
Even
if herbertsmithite is not a new state of matter, we
shouldn't be surprised if one is found soon, as many teams are hunting for
them, says Freedman. He says people wrongly assume that particle accelerators
are the only places where big discoveries about matter can be made.
"Accelerators are just recreating conditions after the big bang and
repeating experiments that are old hat for the universe," he says.
"But in labs people are creating [conditions] that are colder than
anywhere that has ever existed in the universe. We are bound to stumble on
something the universe has never seen before."
From issue 2595 of New Scientist magazine,
Silicon for a quantum age
Herbertsmithite could be the new silicon - the building block for quantum computers.
In theory, quantum
computers are far superior to classical computers. In practice, they are
difficult to construct because quantum bits, or qubits,
are extremely fragile. Even a slight knock can destroy stored information.
In the late 1980s,
mathematician Michael Freedman, then at
Physicists told Freedman
that the material he needed simply didn't exist, but Joel Helton's group at MIT
might just prove them wrong. The material would be a string-net liquid with
elementary and quasi-particles at the end of each string. Physicists could
manipulate quasi-particles with electric fields, braiding them around each
other, encoding information in the number of times the strings twist and knot,
says Freedman. A disturbance might knock the whole braid, but it won't change
the number of twists - protecting the information.
"The hardware
itself would correct any errors," says Miguel Angel Martin-Delgado of
New Scientist
magazine