Daily news

17 February 2010

What happens at absolute zero?

The Boomerang Nebula is the coldest natural object known in the universe, seen here by the Hubble Space Telescope

The curious things that happen at low temperatures keep on throwing up surprises. Last week, scientists reported that molecules in an ultra-cold gas can chemically react at distances up to 100 times greater than they can at room temperature.

In experiments closer to room temperature, chemical reactions tend to slow down as the temperature decreases. But scientists found that molecules at frigid temperatures just a few hundred billionths of a degree above absolute zero (−273.15°C or 0 kelvin) can still exchange atoms, forging new chemical bonds in the process, thanks to weird quantum effects that extend their reach at low temperatures.

“It’s perfectly reasonable to expect that when you go to the ultra-cold regime there would be no chemistry to speak of,” says Deborah Jin from the University of Colorado in Boulder, whose team reported the finding in Science (DOI: 10.1126/science.1184121). “This paper says no, there’s a lot of chemistry going on.”

New Scientist takes a look at the weird and wonderful realm of the ultra-cold.

Why is absolute zero (0 kelvin or −273.15°C) an impossible goal?

Practically, the work needed to remove heat from a gas increases the colder you get, and an infinite amount of work would be needed to cool something to absolute zero. In quantum terms, you can blame Heisenberg’s uncertainty principle, which says the more precisely we know a particle’s speed, the less we know about its position, and vice versa. If you know your atoms are inside your experiment, there must be some uncertainty in their momentum keeping them above absolute zero – unless your experiment is the size of the whole universe.

What is the coldest place in the solar system?

The lowest temperature ever measured in the solar system was on the Moon. Last year, NASA’s Lunar Reconnaissance Orbiter measured temperatures as low as −240°C in permanently shadowed craters near the lunar south pole. That’s around 10 degrees colder than temperatures measured on Pluto so far. Brrrrrrrrr.

What is the coldest natural object in the universe?

The coldest known place in the universe is the Boomerang Nebula, 5,000 light years away from us in the constellation Centaurus. Scientists reported in 1997 that gases blowing out from a central dying star have expanded and rapidly cooled to 1 kelvin, only one degree warmer than absolute zero. Usually, gas clouds in space have been warmed to at least 2.7 kelvin by the cosmic microwave background, the relic radiation left over from the big bang. But the Boomerang Nebula’s expansion creates a kind of cosmic refrigerator, allowing the gases to maintain their unusual cool.

What is the coldest object in space?

If you count artificial satellites, things get chillier still. Some instruments on the European Space Agency’s Planck space observatory, launched in May 2009, are frozen down to 0.1 kelvin, to suppress microwave noise that would otherwise fog the satellite’s vision. The space environment, combined with mechanical and cryogenic refrigeration systems using hydrogen and helium, chill the coldest instruments to 0.1 kelvin in four sequential steps.

What is the lowest temperature ever achieved in the laboratory?

The lowest temperature ever recorded was back here on Earth in a laboratory. In September 2003, scientists at the Massachusetts Institute of Technology announced that they’d chilled a cloud of sodium atoms to a record-breaking 0.45 nanokelvin. Earlier, scientists at the Helsinki University of Technology in Finland achieved a temperature of 0.1 nanokelvin in a piece of rhodium metal in 1999. However, this was the temperature for just one particular type of motion – a quantum property called nuclear spin – not the overall temperature for all possible motions.

What weird behaviour can gases display near absolute zero?

In everyday solids, liquids and gases, heat or thermal energy arises from the motion of atoms and molecules as they zing around and bounce off each other. But at very low temperatures, the odd rules of quantum mechanics reign. Molecules don’t collide in the conventional sense; instead, their quantum mechanical waves stretch and overlap. When they overlap like this, they sometimes form a so-called Bose-Einstein condensate, in which all the atoms act identically like a single “super-atom”. The first pure Bose-Einstein condensate was created in Colorado in 1995 using a cloud of rubidium atoms cooled to less than 170 nanokelvin.