Image caption Flashes of light can be generated by collapsing bubbles in a fluid

Students and enthusiasts attending a recording for BBC Radio 4 have probably seen a new state of matter only recently discovered, an expert says.

The state of matter is a plasma like those in conventional nuclear fusion tests, but at higher densities.

And far from needing expensive apparatus, the conditions can be achieved in a simple glass tube containing a routine liquid.

The professor behind the demonstration says it can be achieved for a mere £10.

The audience was attending a demonstration lecture by chemist Prof Andrea Sella being recorded at University College London for Spooklights on Radio 4.

During the lecture, Prof Sella demonstrated a phenomenon called sonoluminescence - flashes of light created by collapsing bubbles in a fluid. The flashes are extraordinarily faint, but in the darkened auditorium, those attending could see the evanescent sparks quite clearly.

As the name suggests, sonoluminescence is traditionally created by intense sound waves - rapid pressure oscillations - focused into a liquid. In the low-pressure regions of the sound waves, fluid is ripped apart to create tiny bubbles, the source of the light.

Prof Sella's demonstration is far simpler, involving a simple sealed glass tube part filled with phosphoric acid and traces of the inert gas xenon. Then all that's needed is a gentle shaking of the tube. As the acid hits the tube's bottom, there's a distinct metallic clink, as if a heavy ball bearing is striking the glass wall.

Hotter than the Sun

In fact, it's just a water-hammer effect, an impact that shatters the liquid column, creating a trail of bubbles that are clearly visible in daylight.

With the lights off, what's seen is a trail of blue sparks - the sonoluminescence.

"When the bubbles collapse," Prof Sella explains, "they generate incredibly high temperatures - 10,000 degrees. That's twice the temperature of the surface of the Sun."

Seeking more information on what goes on inside that bubble, Prof Sella contacted a world authority on the effect, physicist Seth Putterman of the University of California, Los Angeles (UCLA). And he learned far more than he bargained for.

Prof Putterman has also long been trying to understand the precise source of the light. Judging from its intensity and characteristics, the light demands a source containing billions upon billions of free electrons.

But although 10,000 degrees sounds extreme by human experience, it's nowhere near enough to strip the electrons from the molecules and atoms in the sonoluminescence.

Dense plasma

What Prof Putterman realised earlier this year is that under these peculiar circumstances a kind of electrical cascade can take place. If a few electrons escape the embrace of their home atoms, their field makes it easier for further electrons to escape, and so on until the entire bubble interior has become ionised.

"Not only is it creating a plasma," Prof Putterman explains, "we believe it's a new state of matter because it's an extremely dense plasma - the density is hundreds to 10,000 times the density they achieve inside nuclear fusion experiments."

According to Prof Putterman's experiments, the plasma goes through a phase transition - analogous to the melting of ice to water. Which is why he feels justified in describing the plasma as an entirely new state.

He also confirmed that the conditions in Andrea Sella's "plink tube" demonstration are precisely those needed to create this new state.

Not that that means nuclear fusion is occurring inside the tubes. Claims of nuclear fusion inside fluid bubbles have been extremely controversial.

Prof Putterman is emphatic: "We have not yet succeeded - no-one has yet succeeded - in generating nuclear fusion inside these bubbles. However, we're looking around for that trick that could boost our parameters by a factor of 10, to get it to the region of fusion."

Professor Sella, meanwhile, is delighted that his simple demonstration should reveal to onlookers a state of matter that has only just been discovered.

"I can't wait to tell my nuclear physicist friends, that for a cost of around £10, I'm up in the region that they do for the cost of hundreds of millions of pounds. It's very exciting."