Physicists studying the strange events that occur when one particle beam
smashes into another watch the action with sensors called vertex detectors. Made
from concentric cylinders of silicon, they record the energy and direction of
the charged particles formed during the collision. Although vertex detectors are
very good at spotting weak signals, they have an Achilles heel—they are
damaged by the very radiation they measure.

When a charged particle whizzes through silicon, it leaves a trail of free
electrons and positively charged holes that tell physicists about the particle
that created them. However, the particles sometimes bash into atoms of silicon,
dislodging them and creating a “trap” that pulls in both the electrons and the
holes. As more and more traps form, the detector slowly loses its sensitivity
and eventually becomes useless.

What particle physicists needed was a way to fill in the traps. One possible
answer emerged a year ago when Vittorio Palmieri, a physicist at CERN, suggested
that cooling the detectors might do the trick.

Now an international research collaboration has shown that Palmieri’s idea
works. In silicon cooled to 77 kelvin, electrons that fall into a trap lack the
energy to escape and the trap quickly fills. With the traps out of action, the
detector regains its sensitivity. “We got some dead detectors, dipped them in
liquid nitrogen and brought them back to life,” says Palmieri. He and his
colleagues have named this process the Lazarus effect.

With detectors costing around ÂŁ20 million, the cost of installing a
cooling system is insignificant.

Kurt Ehlers, of the California State University at Monterey Bay, and his
colleagues first realised that a notoriously difficult equation for describing
fluid motion could be simplified and applied to the cyanobacterium
Synechococcus, which swims in seawater at around 25 micrometres per second
without any visible hair-like cilia.

Then, two researchers at Harvard University, biophysicist
Aravinthan Samuel and Howard Stone, an expert in fluid mechanics, went a step
further and developed a simpler mathematical model for Synechococcus
locomotion. The model suggests that the cyanobacterium achieves its swimming
prowess by passing rhythmic compression pulses along the length of its outer
surface up to 1000 times a second (Physical Review Letters, vol 77, p
4102).

The compression waves, expanding and contracting like a line of
connected springs, push the surrounding seawater back and forth. However, on
balance, the water is pushed backwards, propelling the cyanobacterium
forward.

Tom Pitta and Howard Berg, biologists at Harvard, want to test the idea. They
are attempting to attach a small bead to the surface of the cyanobacterium and
watch it for any tell-tale back-and-forth surface motion. “If the surface itself
does not move, then it is a very good question indeed how this organism swims,”
says Stone.

When an air bubble a few tens of micrometres across created by ultrasound
collapses, it releases a single pulse of light lasting less than 10 billionths
of a second. Physicists have noted that the spectrum of emitted light is similar
to that given off by objects heated to tens of thousands of degrees (see
“Bubbles hotter than the Sun”, żěè¶ĚĘÓƵ, 29 April 1995, p 36).

Physicist Claudia Eberlein has now poured cold water on the idea that the
collapsing bubbles are so incredibly hot. If they were, she argues, they would
break down the surrounding water into its constituent atoms. The “hot bubble”
theory cannot explain why the flashes are so short, she adds.

Eberlein’s new theory, which will be published in Physical Review
Letters next week, looks to quantum physics for an explanation. According
to quantum theory, empty space is not as empty as it seems: if we could examine
a vacuum at the “Planck scale”—a resolution of 10^-35 łľ±đłŮ°ů±đ˛ő—w±đ
would see a seething mass of virtual particles, including photons, flitting in
and out of existence.

These particles are so short-lived that there is no hope of detecting them
directly. But physicists calculate that an accelerating block of glass, or any
other material that alters the velocity of light, can cause a sufficient
imbalance in the buffeting it receives from the virtual photons to emit light.
For a block of glass, however, the intensity of the light emitted by even the
most rapid feasible acceleration would be below the threshold of detection.

When Eberlein applied the same calculations to the accelerating front of
water in a collapsing air bubble, she found that the characteristics of the
emitted light would closely match those seen in sonoluminescence. She was
inspired by the late Nobel prizewinning physicist Julian Schwinger, who
suggested in 1992 that sonoluminescence has something to do with quantum vacuum
effects.

Because virtual particles always arise in pairs, Eberlein says it would be
possible in principle to test her theory by analysing the distribution over time
of the photons given off by collapsing bubbles. “The crucial experiment would be
to show that the radiation has nonclassical photon statistics,” she says.

Theoretical physicists always found the idea of ultrahot bubbles “slightly
alarming”, says Peter Knight of Imperial College, London. “This would come as a
bit of a relief.”