The air around us is a chaotic superhighway of molecules whizzing through space and constantly colliding with each other at speeds of hundreds of miles per hour. Such erratic molecular behavior is normal at ambient temperatures.
But scientists have long suspected that if temperatures were to plunge to near absolute zero, molecules would come to a screeching halt, ceasing their individual chaotic motion and behaving as one collective body. This more orderly molecular behavior would begin to form very strange, exotic states of matter — states that have never been observed in the physical world.
Now experimental physicists at MIT have successfully cooled molecules in a gas of sodium potassium (NaK) to a temperature of 500 nanokelvins — just a hair above absolute zero, and over a million times colder than interstellar space.
Martin Zwierlein, professor of physics at MIT and a principal investigator in MIT’s Research Laboratory of Electronics, says that while molecules are normally full of energy, vibrating and rotating and moving through space at a frenetic pace, the group’s ultracold molecules have been effectively stilled — cooled to average speeds of centimeters per second and prepared in their absolute lowest vibrational and rotational states. Zwierlein said:
We are very close to the temperature at which quantum mechanics plays a big role in the motion of molecules. So these molecules would no longer run around like billiard balls, but move as quantum mechanical matter waves. And with ultracold molecules, you can get a huge variety of different states of matter, like superfluid crystals, which are crystalline, yet feel no friction, which is totally bizarre. This has not been observed so far, but predicted. We might not be far from seeing these effects, so we’re all excited.
The researchers published their results in the journal Physical Review Letters in May, 2015.
Every molecule is composed of individual atoms that are bonded together to form a molecular structure. The simplest molecule, resembling a dumbbell, is made up of two atoms connected by electromagnetic forces. The scientists sought to create ultracold molecules of sodium potassium, each consisting of a single sodium and potassium atom.
However, due to their many degrees of freedom — translation, vibration, and rotation — cooling molecules directly is very difficult. Atoms, with their much simpler structure, are much easier to chill. As a first step, the MIT team used lasers and evaporative cooling to cool clouds of individual sodium and potassium atoms to near absolute zero. They then essentially glued the atoms together to form ultracold molecules
By first cooling atoms to ultralow temperatures and only then forming molecules, the group succeeded in creating an ultracold gas of molecules, measuring one thousand times colder than what can be achieved by direct cooling techniques.
To begin to see exotic states of matter, Zwierlein says molecules will have to be cooled still a bit further, to all but freeze them in place. He said:
Now we’re at 500 nanokelvins, which is already fantastic, we love it. A factor of 10 colder or so, and the music starts playing.
Bottom line: Experimental physicists at MIT have successfully cooled molecules in a gas of sodium potassium (NaK) to a temperature of 500 nanokelvins — just a hair above absolute zero, and over a million times colder than interstellar space.