A single atom in a cold, closed, vacuum system.

[nukapprentice]

Let us assume there is a single atom in a vacuum chamber which is kept at near absolute zero. Now assume the system is closed and that the chamber is large enough such that the atom cannot diffuse far enough to reach the walls. Neglecting other stochastic effects, what do you think the atom's motion will be as it cools down?

 

[DrClaude]

what do you think the atom's motion will be as it cools down?

What cools it down?

 

[mfb]

A single atom does not show diffusion, it will just fall down freely in a parabolic path. Without gravity, it will travel in a straight line until it hits the chamber walls.

 

[nukapprentice]

Ah, you're right mfb about diffusion, so thank you. I guess my next question would be, could we apply an external magnetic field such that the atom is kept from hitting the container walls? In terms of how it cools down, I am just assuming that the system is already at that low temperature. Here the atom is dissipating heat to its surroundings.

 

[DrClaude]

I guess my next question would be, could we apply an external magnetic field such that the atom is kept from hitting the container walls?

Yes, you could build a magnetic trap.

In terms of how it cools down, I am just assuming that the system is already at that low temperature. Here the atom is dissipating heat to its surroundings.

I repeat my question: how can it dissipate heat? If the atom is in the middle of the chamber, there is no mechanism for it to lose energy.

 

[nukapprentice]

Heat cannot transfer from a hotter body(the atom) to the colder environment (the cooled chamber) by irradiation?

 

[DrClaude]

Heat cannot transfer from a hotter body(the atom) to the colder environment (the cooled chamber) by irradiation?

Unless the atom is electronically excited, no. There is no way for the atom to lose kinetic energy except by collisions (this includes scattering of photons, for which you would need a laser of the proper wavelength to observe any significant cooling).

 

[nukapprentice]

Ah, thank you so much DrClaude, makes sense. However, how do you explain something ( a larger body) cooling off in the vacuum of outer space then?

 

[f95toli]

Ah, thank you so much DrClaude, makes sense. However, how do you explain something ( a larger body) cooling off in the vacuum of outer space then?

You can't think of solids as just a collection of atoms. The properties are very different, te.g. because the electrons are delocalized. This is why solids have continuous spectra, whereas spectra from atoms tend to show discrete lines.
Hence, the radiation/emission characheristics are very different for a solid, and are reasonably well described as black body radiation.

Hence, a solid at a temperature T>0K will always radiate.

 

[mfb]

"Temperature" is an effective parameter for a bunch of atoms. A single atom does not have a temperature at all.
You can slow it down with laser cooling (it has this name as the usual application is to cool a gas of many atoms).

 

[nukapprentice]

Thanks for the great info mfb and f95. I guess there is some type of "transition" between microscopic and macroscopic worlds in terms of thermodynamics. This is very interesting I suppose, since based on this discussion an atom could just remain in a very cold (~2 K), magnetically confined vacuum environment and maintain the same total energy. Having said that, does anyone know of things which experience an internally damping mechanism instead of an external one i.e. friction?

 

[mfb]

Energy is conserved. To reduce the energy content of a system, you need some way to get rid of the energy.
In addition, entropy cannot get reduced - and heat is the "worst" type of energy, you cannot convert it to other forms without some external heat sink.