Inquiring Minds

Questions About Physics

Lowest speed

Dear fermi lab:

I know very well that there are upper limit of real objects speed in nature (which is the speed of light), but I wonder a lot if there are a lower speed limit in our universe.

ahmed kamal

Dear Ahmed,
Yes, as stated most clearly in Einstein's Special Theory of Relativity, the RELATIVE motion of an object as seen by an observer must be less than the speed of light. Clearly, if some solid object is bolted firmly to the observer's velocity measuring device, then the RELATIVE motion should read exactly zero.

Now we might want consider more closely the motions of the atoms that make up the object and the velocity measuring device that have this AVERAGE RELATIVE velocity of exactly zero. At any temperature above absolute zero, the atoms will be vibrating about their average positions in the solid (this is what we mean by temperature - the average vibrational energy of atoms in the solid). Thus, although the objects have an average relative motion of exactly zero, their constituent atoms don't! As a further complication, even if we could cool the solid down to a temperature of absolute zero, the constituent atoms would still retain a small amount of vibration that is referred to as 'zero point energy', a consequence of the essential quantum mechanical nature of atoms. In this sense, it can be said that atoms are always in motion and cannot be slowed down to a speed of zero.

These, and similar apparent enigmas, are fascinating aspects of 20th century physics. The Feynman Lectures on Physics, or related physics textbooks found in most public libraries, are a good source of further information on the amusing aspects of relativity and quantum mechanics that your question probes.

Good luck, keep wondering!

Chuck Brown
Staff Scientist, Fermilab.

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last modified 5/21/1998