After the last post, someone asked about laser cooling. I'll try it without images.. just draw them in your mind.
First off what is hot and cold?
Think of heat as the energy of motion of the atoms and molecules in something. They can be moving through space, rotating or vibrating. Temperature is just a measure of the average kinetic energy of the molecules of something. "hotter" means a higher kinetic energy. Cold is just the absence of heat. . you don't add cold to something.. you have to remove heat.
Light has both energy and, even though it doesn't have mass, momentum. We don't notice the effect at our scale, but if a molecule absorbs a photon of light it will feel a force.
But isn't that adding energy and isn't using a laser something like using a flame thrower to blow out a candle?
Imagine a very simple system. A single atom that can only move to the left or right. Its temperature is directly related to its kinetic energy.. half its mass times its velocity squared. Put a laser on the left side and point it at the atom. Now the fun.
Laser light is monochromatic - one color or a single frequency. Lasers are perfect, but in a gedanken experiment like this it can be. Thanks to quantum mechanics it turns out atoms and molecules have very specific absorption frequencies. If the laser light frequency isn't spot on, the atom completely ignores the light. Now pick a special kind of laser that allows you to tune the frequency of the light.
If the atom was just sitting there and light at its absorption frequency strikes it, it will take the momentum of the light and move to the right. Not exactly what we want. We'll invoke the Doppler effect -- the fact the pitch of a siren rises as it approaches and lowers as it leaves. The same is true for light. Dial in a frequency just below the absorption frequency of your atom. If the atom is moving to the left the frequency of light it "sees" is a bit higher than the frequency the laser is putting out. If you've made the right adjustment the molecule absorbs the light. The force is opposite the atom's leftward motion and the atom slows down a bit. If the atom was moving to the right the frequency of the light at the atom would be lower and no absorption takes place.
To handle motion to the right, put another laser on the right pointed to the left. Now whichever way the atom moves it will slow down. Since there is less kinetic energy its temperature has dropped. You've cooled it.
To go to the real world imagine a cavity filled with a gas of these atoms. There are three directions so use six lasers (or three with mirrors) up-down, left-right, in-out. Any motion of the atom can be described as a combination of the three directions so you are cooling the random motions of the atoms of the gas. A refrigerator that uses light..
There are a number of tricks to achieve very low temperatures. The record is a few millionths of a degree above absolute zero for a very tiny sample and something like half a degree above absolute zero for a sample large enough to see.
The technique turns out to be very useful for investigating quantum mechanics.. particularly of macroscopic objects like the guts of quantum computers. It's also used in some of the most accurate atomic clocks. The next generation atomic clocks in GPS satellites will use little laser cooled samples for much better accuracy which leads to more accurate navigation.
Imagine - a constellation of laser cooled atomic clocks 12,600 miles up so you can find your way around better with your smartphone.
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