Absolute Zero

Absolute zero is the theoretical temperature at which the atoms in a substance would stop moving and have the lowest possible energy. It corresponds to 0 Kelvin, -273.15 degrees Celsius, and -459.67 degrees Fahrenheit. While scientists have gotten extremely close to absolute zero, they have never reached it.

At absolute zero, molecules would have no motion.

But even though the molecules in ice are relatively fixed and unable to move around, they and their atoms still vibrate, for even ice has some heat. It can be cooled still more, and the vibrations are lessened. Finally, if they stopped altogether, no heat would be left, and the temperature would be at absolute zero. This is nearly 460° below zero on our usual thermometer scale.

At this point, scientists think that the nuclei of the atoms still have a kind of motion called "spin."

In general, things expand or get bigger as they become hotter and decrease in size as they are cooled, and this makes it possible for the most common instrument for measuring temperature, the thermometer.

Strange things that happen near absolute zero

At temperatures near absolute zero, matter behaves very differently from the way it does in our ordinary world. Helium that is just cold enough to become liquid behaves much as any other liquid, but as it is cooled to about 3° above absolute zero, it changes to another form that has some very strange properties. If an open container of this sort of helium is hung by a string, drops start falling from the bottom, and soon the container is empty, even though there is no leak. The helium flows up the side of the container in a very thin film, goes over the lip and down the outside. It seems to want to get warm; it always flows toward a warmer region.

Another curious effect of low temperature is that of superconductivity. The power to conduct heat and electricity is enormously increased. Lead at room temperatures is a rather poor conductor, but when immersed in liquid helium, a lead wire offers practically no resistance to the flow of electric current. The current thus started in a ring of lead will keep flowing around almost indefinitely.