Re: [Phys-L] zero-point motion at the introductory level

[John Denker <jsd@av8n.com>]

On 02/18/2014 01:06 PM, Chuck Britton wrote:

> As Far as I Know - SuperFluid flows as well as Superconducting
> currents would continue at Absolute Zero.

They would.

> I am aware of no evidence to the contrary - but the actual experiment
> has not been performed.

Actually, relevant experiments have been performed.  People
have set up persistent currents in a superconductor and
observed no decay over a period of years.  Ditto for the flow
of superfluid helium in a tube, flowing around and around
in a closed loop.  All evidence is that the flow is exceedingly
stable, and only gets more stable as the temperature goes down.

> But perhaps these don’t constitute ‘motion’.

Of course they constitute motion.

OTOH these examples do not demonstrate zero-point motion.
They are not thermal equilibrium states, so AFAICT they are 
not entirely responsive to the original question that was 
asked.  They are good answers to a slightly different question.

I think the original question had more to do with what thermal
/equilibrium/ looks like at low temperatures, where the classical
approximation is no longer valid.

On 02/18/2014 11:25 AM, Bill Nettles wrote:

> I don't know what absolute zero for a neutron star would mean, but
> they are fermions, so that has to provide a clue.

The neutron star is already cold enough to be in the low-temperature
regime, the non-classical regime, the degenerate regime.  Ditto 
for the conduction electrons in a metal, and the electrons within 
a single atom at ordinary temperatures.

The relevant criterion is the actual temperature divided by the
Fermi temperature.  For metals, the Fermi temperature is on the
order of thousands of degrees, so the conduction electrons are
highly degenerate even at room temperature.  Changing the temperature
10% either way does not change the pressure of the electron gas,
because all you are seeing is the zero-point fluctuations.

For a neutron star, the Fermi temperature is something like 10^13
kelvin, so even though the actual temperature is thousands of
degrees, the neutrons are verrrry highly degenerate.  A few
thousand degrees is essentially absolute zero, on the scale of
things.