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Dr. Christopher S. Baird

Why is the lowest temperature called absolute zero if it's relative to the frame of reference?

Category: Physics      Published: May 31, 2013

icicles on equipment
Temperatures are the same in all reference frames, otherwise icicles such as these would disappear depending on your frame of reference. Public Domain Image, source: NASA.

Temperatures such as absolute zero are the same in all reference frames. It's true that an object's overall motion depends on what reference frame you are in (a book on a train will be motionless in the train's frame, but speeding along in the ground frame). And it's true that temperature involves motion. But the motion that is associated with an object's temperature is a different kind of motion from the kind that is affected by a choice of reference frame.

"Object motion" describes the movement of the entire object relative to a certain frame of reference. Ultimately, every object is made out of atoms which are moving (oscillating) randomly about, so an object's overall motion is really an average over the directional motion of all of its atoms. When this average is taken, all of the random motion of the atoms cancels out, leaving just the overall forward motion of the entire object (if the object is in the process of being shattered to bits the picture gets more complicated). In contrast to object motion, "thermal motion" describes the movement of the atoms in the object relative to each other, and not relative to the reference frame. As a result, thermal motion is independent of the frame of reference. Thermal motion is the driving mechanism behind temperature, so that temperature is also independent of frame. While the speed of a baseball will be different depending on whether you measure it from the ground or from a passing train, the temperature of the baseball will be the same in all frames. You can think of thermal motion as perfectly unordered atomic motion and object motion as perfectly ordered atomic motion. There is a third type of motion which an object can have, which is somewhere in between the order of object motion and randomness of thermal motion. This third type of motion is ordered vibrations. For instance, a ringing metal bell on a train has thermal motion (its temperature), ordered vibrational motion (its ringing), and object motion (its movement down the track). The word "ordered" is important because completely random vibrations are just part of thermal motion.

Because the temperature of an object is just an average over the motion energy (kinetic energy) of its atoms relative to each other, there is an absolute temperature minimum beyond which an object can no longer be cooled. This point, where all the atoms have been completely stopped  relative to each other, is known as "absolute zero" and corresponds to the number zero on the Kelvin temperature scale. An object cannot be cooled below this point because there is no atomic thermal motion left to stop. If you take a basketball and cool it down to absolute zero, and then throw it, the basketball will still be at absolute zero (you would have to throw it carefully so as to not give it heat in the moment of throwing, and also get rid of the surrounding air which would heat it up due to air resistance). All of the atoms in the basketball will be moving in unison away from you, so there will be non-zero object motion. But all the atoms will not be moving relative to each other, so there will still be zero thermal motion, and therefore zero temperature. Absolute zero is "absolute" both in the sense that no object can get colder, and in the sense that it is the same in all frames.

Topics: absolute zero, bulk motion, motion, temperature, thermal motion, thermodynamics, vibrations