ice laser

[dragonstar57]

i heard someone say that an ice laser/beam was imposable, something about thermodynamics. but could someone explain the scientific principle that makes it imposable?

[insane_alien]

what do you mean by 'ice laser'? using ice as the lasing medium?

[dragonstar57]

no I mean a beam that freezes stuff

like mr. freeze from those old batman movies

[insane_alien]

ah right. well it has to do with the way lasers transmit energy and the basics of heat.

 

heat is the energy containied in the vibrational modes of the molecules the object is composed of (basically, the faster they wiggle, the more heat they contain).

 

Lasers send out a beam of photons. When the photons hit these wiggling molecules they are absorbed and they change how they wiggle because of the additional energy they now have. this makes them wiggle faster(and hence stuff heats up).

 

in order to have anything resembling a freezeray you'd need to have a beam that extracts energy and the only way this can happen is if there is such a thing as negative energy but there has never been ANY observation that this exists and it may infact be impossible with current theories.

 

well, thats the way its portrayed in movies at least.

 

if we go to reality, there is a way to cool down stuff with lasers but it only works on stuff thats already very cold, in a gas state and very very low pressure. this works by adding energy in the right direction at the right time so it acts against the direction of motion. it seems pretty tricky and i don't understand it fully myself. i think its either swansont or klaynos that works with this technology and they could explain it better.

 

but its definitely not anything like the movie freezerays (and klaynos is nothing like big arnie)

[dragonstar57]

so IF negative energy exists it would be possible?

[swansont]

If you mean laser cooling, it's very possible and works like this: an atom will absorb light if it is close to a resonance, and each absorption and emission will give a momentum change of E/c, the momentum of a photon. So you shine a laser on it. All of the absorptions come from one direction, but the emissions are randomly distributed, and contribute no net momentum — this means there is a net force on the atom. Now, tune the laser slightly below the resonance. If the atom is moving toward the laser, the light will be Doppler-shifted closer to resonance and scatter more photons per unit time, so there is a slowing force on the atom. If laser light is coming from all six cartesian directions, there is a slowing force exerted on the atoms no matter which direction they are moving. This dissipates the atom's kinetic energy (the accelerations can be hundreds of g's; the force is quite viscous and known as optical molasses) and if you have an ensemble of atoms, they will be cooled.

 

We routinely get the atoms in our lab to a few microKelvins.

[dragonstar57]

no I meant a laser that could turn all of Gotham into a giant glacier!!!