Number of the Month: Minus 273.15 degrees Celsius

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work at the lowest possible temperatures is conducted at this drop tower in Bremen

Where is the coldest place in the universe? Not in Siberia, nor in the Arctic, and not even in space. It may be hard to believe, but in fact the coldest place in the universe is to be found in special laboratories. On earth, in other words. Here, researchers have achieved something incredible: they are able to generate a temperature of minus 273.15 degrees Celsius, give or take the odd millionth of a degree. Minus 273.15 degrees Celsius: this is the temperature described by physicists as absolute zero – the theoretically lowest possible temperature.

How are such freezing temperatures generated?

Several dozen laboratories in Germany are able to achieve temperatures very close to absolute zero. A special helium mixture is used to generate this icy cold. One of the laboratories is to be found in the drop tower at the Center of Applied Space Technology and Microgravity (ZARM) in Bremen.

What are such low temperatures needed for?

The scientists in Bremen for example cool a cloud of several thousand individual atoms down to the point at which a “giant atom” of one millimetre is generated, a so-called Bose-Einstein condensate (BEC). In the drop tower experiment, the giant atom is created in a state of weightlessness – and its behaviour studied. These giant atoms could prove very useful in space: BECs can be used to build sensors for the positioning or rotation of satellites which are much more precise than today’s sensors.

Work at such low temperatures is also conducted at the Walther Meißner Institute for Low Temperature Research in Garching near Munich: researchers here study how metals such as aluminium in this state become superconductive, that is to say they have zero electrical resistance. Superconductors are thus a perfect conductor of electricity and allow the unhindered flow of electrons. These days some areas of research would be hard to imagine without superconductivity.

Low temperatures also benefit medicine

Numerous doctor’s surgeries and hospitals use temperatures which are only marginally higher: magnetic resonance imaging scanners work with superconductive magnets – at a temperature of minus 269 degrees Celsius. Luckily the scanners are sufficiently well-insulated to ensure that the patient will not feel the cold at all.

Helmholtz Extreme

In the current edition of Helmholtz Extreme, the Helmholtz Association reports on other freezing-cold places in the universe. The article can be found here:

www.helmholtz.de > HELMHOLTZ Extreme