the season to be jolly, though after a year in which the
much-anticipated Large Hadron Collider at CERN has been delayed, you
might think particle physicists don't have much to be jolly about. Two
very different developments, however, have made 2008 particularly
exciting for them.
first shows that with a good theory and hard work, we can unlock
nature's deepest mysteries. The story began in the 1960s, when it was
proposed that protons and neutrons are made up of constituents called
quarks. Experiments then demonstrated that protons did contain
point-like components. These bounced around inside protons as if they
were free. So why didn't they get knocked out of protons when
physicists smashed them about in accelerators?
An answer came in the 1970s, when a fascinating property of quantum chromodynamics,
the theory of the strong interaction between quarks, was discovered.
According to QCD, the attraction between quarks gets weaker the closer
they are, and vice versa. Experiments confirmed the idea over very
small distances, but at the scale of protons, the forces became too
strong to calculate with available mathematics. This was frustrating
because in principle QCD should explain all the properties of protons
and neutrons, including their masses.
Physicists have been working for more than three decades to describe the theory on a finite lattice of points in space and time, so that it could be crunched by a supercomputer. Now, a European collaboration using this method has reported a remarkable result.
found that QCD predicts the masses of the lightest strongly interacting
particles, with an accuracy of 1 to 2 per cent. This may not seem
spectacular until you realise that quarks only contribute a very small
fraction of the total mass of the particles they make up. Most of the
mass comes from the quantum fields holding the quarks together. A
theory drawn up to explain an obscure puzzle about elementary particles
has now allowed us to calculate from first principles the properties of
a whole range of other particles, including the neutron, which is
responsible for more of the mass of the Earth than any other particle.
The second reason to be jolly is a set of puzzling satellite results.
While looking at the cosmic rays zooming through our galaxy, one
satellite saw more high-energy positrons (anti-electrons) than
expected, while another saw an excess of high-energy electrons.
explanation is that particles and antiparticles of dark matter (thought
to make up most of the mass of our galaxy, but yet to be discovered)
collide and annihilate each other, throwing out these positrons and
electrons. Unfortunately, the abundance and energies of the positrons
and electrons don't match what would be expected from the favoured dark
matter candidate - a particle called the neutralino. Undaunted,
theorists have come up with alternative dark matter ideas for these
excess particles ranging from the baroque to the bizarre, excited that
they could finally represent direct evidence for a new world of physics.
dark matter explanation for the excess particles smells wrong to me,
not least because there's a simpler explanation - a nearby pulsar could
be spewing them out. But what makes these observations worth
celebrating is that young theorists can now experience the thrill of
ambulance chasing: creating new wild theories to explain puzzling data.
I haven't done it since the 1980s - the last time particle physics had
enough data to be regularly puzzling.
Both developments show the promise of cutting-edge physics. Most scientific theories we come up with are wrong - if they weren't, there would be no challenge. But if we persevere when we get a theory that works, the sky is the limit.
- From issue 2687 of New Scientist magazine, page 74.