Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: quantum fluctuations?

Learn more physics!

Most recent answer: 07/29/2017
Q:
Textbooks give only a single value for the bond energy between neighboring atoms in a solid. This appears to be only an approximation, which is valid only at a coarser level of the time scale. However, if one considers the valence electrons in the electron clouds around several nuclei in a solid, it stands to reason that the bond energy should fluctuate on a finer time scale as the electrons come closer and move further away from each and the nuclei in some complex manner. Is this reasoning correct? If so, would it explain why fracture, melting and other phenomena are stochastic?
- S.R. (age 64)
Cleveland, OH, USA
A:

The fluctuations you describe sound like a natural implication of quantum mechanics as it's often described, including by professional physicists. Nevertheless, they do not occur. At low temperature, the crystal settles into a unique lowest-energy state. Nothing fluctuates. The positions of electrons etc. do not take on sharply defined values. They're spread out, but the spread is static, not fluctuating.

The stochastic nature of the various phenomena you mention requires the presence of some thermal energy. The deep origin of the unpredictability of thermal physics remains unclear. Perhaps I should say, instead, that it seems unsurprising because nature can be in a huge number of states and there's no practical way to tell in detail which one we're in. The mystery is why the past is more predictable, less stochastic, than the future, or, as people put it technically, why the past is low entropy.

Mike W.


(published on 07/29/2017)

Follow-up on this answer.