This is some
cool as
hell physics I just feel like
sharing for its
sheer cleverness value.
The
premises are all
high school /
university / secondary school (in
england) level, so not to hard to understand.
Ok, so how do you cool a gas, lets say a
monatomic gas (where each "
molecule" of
gas is a single
atom, e.g.
Argon)?. One way to do it, is to place 6
lasers as if each was in the centre of one face of a
cube pointing in, and the gas was in the centre of the
imaginary cube occupying some
volume. That is to say we have two lasers on the X axis, two on Y, two on Z, all facing to the centre.
Now the lasers fire light of a single frequency, and the energy associated with each photon of light is directly proportional to the frequency (E = h x Freq.). Now atoms absorb light differently depending on its frequency, they actually have an absorption resonance frequency. i.e. If you were to change the frequency of the light gradually you would see the absorption of the gas go up, peak at the resonant frequency, and drop off again. At the resonant frequency it is absorbing the max. amount of photons, and therefore it is having the max. amount of momentum transferred to it (when the photon hits and gets absorbed, its Energy gets transferred to the gas molecule as momentum in the direction of travel of the light (momentum is a vector quantity) and the photon ceases to exist ).
So now comes the really clever bit. The frequency of the lasers is set to just BELOW that of the resonant absorption frequency of the gas. Either side of this resonant freq, absorption drops off fairly quickly. Now due to the Doppler Effect, gas molecules that are moving TOWARDS the lasers, will experience that the frequency of the incoming light has INCREASED (think how frequency of the waves hitting you at the beach might increase if you were paddling out towards them), and has therefore been shifted nearer or to the resonant absorption freq. of the gas, and hence the gas experiences a change of momentum --> a force, and is slowed down. Gas molecules travelling away from a laser find the light frequency has shifted further away from the resonant absorption frequency, and thus experience little or no change of momentum, and are largely unaffected.
What this boils down to then is that a gas molecule slows down when travelling towards a laser, and is unaffected travelling away from one. Now imagine the setup I describe earlier, all lasers pointing in, each at the centre of one face of a cube. As velocity/momentum are vectors (have magnitude and direction) we can treat the x, y and z axis' separately, to allow for the possibility of gas molecules travelling diagonally etc. If you think about it, the gas molecules are all slowed down no matter how they're travelling unless they are already travelling very slowly, which is the object of the exercise. Like magic you have a super cooled gas (because "heat" is actually the random vibrational motion of atoms).
Hopefully I didn't mess explaining this up too much, but I really dig this idea due to its ingenuity. It combines 3-4 areas of physics to create such an intrinsically clever device that finds a reasonable amount of use in different areas of research.