# Q & A: -273 Celsius

Q:
Why does all matter stop moving at 0 Kelvin (-459.4 Fahrenheit)(-273 Celsius)? Why is it is it at that temperature?
- Greg (age 16)
Emmanuel College, Perth, WA, Australia
A:

There’s a natural absolute temperature scale , in which temperature is the measure of how likely things are to be found with extra energy above the minimum possible energy, i.e. in states other than the ground state. Formally, the temperature is a measure of how much extra energy is needed to increase the number of available states by a certain factor. The Kelvin scale is an example of such an absolute temperature scale. 0 K means that everything has settled into the lowest possible energy state.

Everyday temperature scales have no fundamental significance. They were just devised to have some easily reproducible reference points in familiar ranges. For Celsius, that’s just the freezing point (0°C) and boiilng point (100°C) of water at some standard pressure. It’s basically an accident that true absolute zero lands at -273.15 on that arbitrarily chosen scale.

Mike W.

Motion hasn’t all ceased at 0 degrees Kelvin, it just means that there are no lower energy states and no probability for the system to be found in a state that’s not the lowest energy state (no more energy can be extracted from the system). Electrons will still move in their orbits, and nucleons will still orbit in the nuclei.

Tom

(published on 10/22/2007)

## Follow-Up #1: energy at low temperature

Q:
If zero K represents the lowest energy level, than what is the next higher energy level than zero K. I mean to say how much energy must be provided to a system at 0 K to raise its energy level to the next higher state? If that is so, does that mean heat/energy transfer is also a discreet quanta, no less and no higher either?
- Indrajit Kuri
New Delhi, India
A:
The next higher energy level depends entirely on the particular system. For a particle of mass m that can rattle around in a rectangular box with longest side L, there's an energy level that's up by (3/8)h2/mL2 from the ground state energy, where h is Planck's constant.

Temperatures T>0 are not characterized by saying specifically which state a system is in, or even what energy the system has. A temperature corresponds to a set of probabilities for the system being in different states. When the system is big, those probabilities cluster in a collection of states with a relatively narrow range of energies. Only at T=0 does that collapse to a single state. (Or two states, if the system has half-integer spin.)

Mike W.

(published on 07/04/2011)