What is the coldest temperature ever created?

For several decades, it was thought that absolute zero was the coldest temperature that could ever theoretically be achieved.  However, research within the last 20 years has led to several exciting breakthroughs in the science of extreme temperatures. Scientists have been able to not only cool particles to temperatures below that of the Bose-Einstein Condensate (BEC), they have been able to go below absolute zero!

Absolute zero is the coolest - or is it?

In 1995, the an American research team cooled gas particles to an extremely low temperature in order to create the BEC.  This temperature was one microkelvin (one millionth of a degree) above absolute zero.  After that, scientist regularly reached temperatures in the nanokelvins, which is one billionth of a degree above absolute zero. Still, science continued to push further, attempting to reach even lower temperatures.

In 2003, an researchers at MIT (led by Dr. Wolfgang Ketterle, one of the co-discovers of the BEC), were able to cool gas particles to a record-low temperature of 500 picokelvin.  They used a process very similar to the one used to create the BEC (see the page on the BEC for more information).  Particles had to be kept within a special magnetic field, called a "gravito-magnetic trap", so they could actually be cooled.  At the time, reaching this temperature was seen as the extreme of particle cooling. How could you possibly get any colder, when it was impossible to reach absolute zero?

Well, it turns out that it is possible to reach temperatures below absolute zero.  Here, we enter into the mysterious world of negative temperatures - where matter behaves in very strange ways.  

Basic physics states that within positive temperatures, atoms are more likely to occupy low energy states than high energy states.  In negative temperatures, they are more likely to occupy the highest energy states.  This means that although negative temperatures are below absolute zero, they are actually hotter than at positive temperatures, because the particles possess more energy.  They've gone beyond infinite temperatures, into the area of negative temperatures.  

At negative temperatures, gas particles exhibit some very bizarre behaviour.  Within normal temperatures, heat flows from hot to cold, or from the particles with the highest temperature to the particles with the lowest temperature.  At the same time, energy always flows from negative temperatures to positive temperatures.  So, despite these conflicting ideas, particles at negative temperatures are still hotter than those at positive temperatures.  Entropy is also affected.  Positive temperatures release entropy when they give off energy; negative temperatures can actually absorb entropy.  

In order to achieve negative temperatures, scientists had to limit the amount of energy that an atom could possess, as well as isolate the particles from outside influence.  Magnetic fields and laser were also used to control the particles.  Temperature is also related to pressure, so scientists had to make sure that the gas had negative pressure, so it would have a negative temperature.

Research into negative temperatures provides promising indications in a number of areas.  These include dark matter, new states of matter, and super-efficient engines.  The future of low and negative temperature science looks optimistic, and many new things will be discovered in the years to come!