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Thursday, January 14, 1999 Published at 14:27 GMT


The coldest place in the Universe

A cloud of Rubidium atoms - the coldest anywhere

By BBC News Online Science Editor Dr David Whitehouse

As a famous coastal resort, Brighton, south of London, might not be too keen to be known as the coldest place not just on earth but in the entire universe.

But that is just what a tiny part of this area of Sussex can claim to be.

A sample of gas inside a glass cell in a corner of a physics laboratory at the University of Sussex, just along the coast from Brighton, is the coldest place anywhere.

The gas in the cell has been cooled so that its temperature is only a few hundred billionths of a degree above so-called absolute zero, the coldest possible temperature.

[ image: Dr Malcolm Boshier]
Dr Malcolm Boshier
Dr Malcolm Boshier, working with Aidan Arnold and Calum MacCormick, have been investigating a theory of Albert Einstein's, developed with Satyendra Bose, that atoms cooled close to absolute zero would conglomerate into a 'superatom.'

This state of matter, first produced in 1995, is known as a Bose-Einstein Condensate, or Bec, can be used for precise measurements of previously unknown quantities.

Record temperature

According to Dr Boshier, "A Bec represents the tightest control you can have over atoms. We should be able to build devices that will be extremely sensitive to anything that affects an atom's energy levels, and that includes gravity.

Dr Bolshier explains how the experiment works
"Achieving Bec here at Sussex is going to do for atoms what lasers did for light," he added.

The record low temperature, achieved with a cloud of rubidium atoms, is reached firstly by pre-cooling the atoms by bouncing lasers off them.

[ image: Trapped by lasers, cooled by magnetic fields]
Trapped by lasers, cooled by magnetic fields
The atoms, which are held in a high quality vacuum, are then trapped in a strong magnetic field so that only the hottest atoms can escape. Those that remain are cold enough to form a Bec.

Dr Boshier explains that this record low temperature can be imagined by visualising a thermometer as long as the UK, where the top mark stands for room temperature.

The supercold temperature reached in his laboratory represents 1% of the width of a hair right at the bottom of this scale.

As well as entering the record books, the Bec may have some practical spin-offs producing better clocks and new ways to prospect for oil using detectors that are ultra-sensitive to minute changes in gravity.

About temperature

The lowest temperature is absolute zero at -273.15 C or 0 Kelvin or K. The absolute temperature scale is named after the physicist Lord Kelvin.

Dr Bolshier on temperature
The lowest temperature ever recorded on the Earth was -89 C in Antarctica, certainly chilly, but not very cold in absolute terms at 184 K.

The coldest place in the Solar System is probably Triton, a moon of the distant planet Neptune, some 2,800 million miles away.

[ image: Atom temperature distribution: the narrower the peak the lower the temperature]
Atom temperature distribution: the narrower the peak the lower the temperature
At the temperature of Triton (-235 C or 38 K) and at low pressure, nitrogen freezes like water ice.

Colder still is space itself. The vacuum of space is not empty but contains radiation that has a certain temperature.

Arno Penzias and Robert Wilson won the Nobel Prize for the discovery, in 1964, that outer space is filled with radiation at a temperature of 2.73 K. This radiation is the remnant of the Big Bang.

This is the lowest natural temperature but is available only in deep space.

The temperature of the Bec in Sussex is a million times colder than the cold of outer space.

Colder in the lab

In the laboratory on Earth lower temperature experiments normally start with the cryogenic liquids, nitrogen and helium.

Helium was first liquefied in 1908 and normally boils at 4.2 K.

Liquid helium can be cooled further by evaporation by reducing the pressure over it with a powerful pump. Using a rare isotope, 3He (helium-three), it is possible to get the temperature down to 0.3 K.

Using a mixture of Helium isotopes and a device called a dilution refrigerator first developed in Manchester and in Moscow, temperatures down to 0.002 K (2 mK or 'millikelvin') can be achieved.

Even lower temperatures can be obtained using the magnetic properties of the nuclei of atoms such as copper.

Liquid helium has been cooled to 90 microK (a 'microkelvin' is a millionth of a K) at the University of Lancaster.

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