3
$\begingroup$

I was reading Brian Cox's "The Quantum Universe" and at one point he was taking the analogy of the ball resting at the bottom of a valley to explain the finite potential well or "electron in a box" situation.

He said how kicking the ball uphill but not with enough energy causes the friction between the ball and the surface of the hill to slow its motion, and he went on to say how this friction can be explained through Quantum Electrodynamics (QED). I'm curious as to how QED explains friction in such a situation.

$\endgroup$
  • $\begingroup$ I think that's just a throwaway comment - QED explains everything in your everyday experience except for gravity (since you really don't encounter the strong or weak forces much at all). $\endgroup$ – jacob1729 Aug 20 '18 at 22:00
  • 2
    $\begingroup$ Friction is derived from electromagnetic interactions, which are explained by QED. You'd have to put a huge amount of effort to actually use first-principles QED for this, though. $\endgroup$ – probably_someone Aug 21 '18 at 0:23
  • $\begingroup$ It's analagous to using statistical mechanics to explain the interactions of many molecules. Technically you just need the more "fundamental" Newton's laws (or Maxwell's equations, etc. as well) to describe the motion of each particle, but this would be tedious if not computationally impossible. The same thing is true here. We use simplified friction models since they do a fairly good job. Trying to actually use QED to describe friction, while technically more fundamental, would be tedious if not computationally impossible. $\endgroup$ – Aaron Stevens Aug 21 '18 at 1:14
2
$\begingroup$

In a strict sense, the forces of friction are just a macroscopic aproximation of electromagnetic repulsion forces between the atoms of two surfaces, which depends on the microscopic arrangement of these and the elements involved. Therefore, friction is just another expression of electromagnetic interactions in everyday life.

Since QED is the (relativistic) quantum description of electromagnetism (through the language of quantum field theory), you could theoretically use it to explain with more precision the electromagnetic interactions between atoms and thus friction (obviously this isn't too useful in practice). So I guess he was refering to this fact as a throwaway comment.

$\endgroup$
2
$\begingroup$

I agree with others in the comment in that the comment was more like a "throwaway" comment; using QED to explain friction you observe in everyday is like using a cannon to kill a fly.

But there is a notion called quantum friction, and for this you really need to use QED. Theorists predict (in this paper) that if you have a rotating metallic sphere near a surface in a vacuum, you get friction force from quantum fluctuation between the sphere and the surface. Basically, friction here arises from the Camisir force between two nearby surfaces.

As an aside, apparently this idea was first suggested by Pendry (in this paper), and he is more well-known in the field of metamaterials for prediction of perfect lensing with negative refractive index, and invisibility cloak.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.