Fine... if lasers IMPART energy upon impact, just how the hell did they figure out you can slow/stop atoms with them? it seems so counterintuitive.


As a side note:
Dr. Heisenburg gets pulled over by a police officer. The police officer says, "Sir, do you have ANY idea how fast you were just going?" Dr. Heisenburg thinks a moment and answers, "No, Officer... but I know EXACTLY where I was!"

BWAAahahaa! That's a good one! :) I'll have to tell that to my geek friends. :)

NPR's Science Friday for April 1st: Two Scientists Walk Into A Bar...

Hehe, good one.

As for the laser cooling, it's actually not too difficult to at least entertain the idea that an energy input might result in cooling. After all, it takes energy to cool something down---your fridge doesn't on its own.

The mechanism that actually makes it work, though, is a kind of selective photon absorption. It works like this: atoms have electrons buzzing around them. For each atom, these electrons can have only discrete levels of energy---that is, their energy states are quantized. This comes about because of wave-particle duality. Electrons have energy due to their mass alone, and thus they also have an intrinsic wavelength. When they're buzzing around an atom they actually have to occupy a particular wave state called a "standing wave" so that their orbits remain stable. But the circumference of this standing wave orbit can only be an integer number of wavelengths long---if not, after multiple cycles the wave cancels itself out and therefore can't exist. So you end up with a limited number of "acceptable" orbits, which are the quantized energy states.

These states are not completely stable, though. At some random time a high-energy state will "decay" to a low-energy state, and when it does it releases a "pulse" of light energy---a photon. This is called spontaneous emission. The reverse process can happen, too, where an atom absorbs a photon and goes into a higher energy state. But, to do so, the photon needs to have the right amount of energy, the so called "transition" energy. If not, the atom is unlikely to absorb it.

This is where laser cooling starts. What they do is to intentionally detune the energy of the photons (their wavelength or colour, which all amounts to the same thing) so that they are below this transition energy. Every time an atom goes to absorb the photon, it finds that the photon is, one might say, undernourished. The atom's not very likely to absorb it like that---it's no good.

Bring in relativity. When you're moving fast in one direction you see the world differently than if you were to move fast in the other direction. What ends up happening is an effect, like the Doppler effect for sound (think what happens to the sound of an ambulance as it rushes past you), where an atom that is moving towards the source of the photons actually sees them as if they had higher energy (were better nourished) than an atom that is moving away from the source of the photons. What this means is that the electrons of the atoms are more likely to absorb photons if the atoms are moving towards the laser.

I should also mention at this point that photons also have momentum (despite having no mass---it's another relativity thing). Momentum must be conserved---it's one pretty fundamental law of physics. So when an atom absorbs a photon, not only does it gain energy, it also gets a "kick" in the direction that the photon was travelling.

Now consider adding a second laser, at the opposite side and facing the opposite direction to the first, so the two lasers are shining at each other. Now what you have is atoms in the middle: those with little motion not absorbing photons because they don't have enough energy, and those with motion in either direction absorbing photons because, to them, the photons seem good. When absorption happen, the atoms going left get a kick to the right, and the atoms going right get a kick to the left. In both cases this slows them down. That is, it cools them.

There is one little extra complication, though: spontaneous emission. An atom with high energy will, at some random time, emit a photon and go into a low energy state, and when they do they will get a kick in the direction opposite to where the photon goes, because photons have momentum, and momentum must be conserved. But, for spontaneous emission, the direction is entirely random---it's just as likely that the atom will get a kick in a direction that cools it down more as it is that the atom will get a kick in a direction that makes it hotter. So, overall, there is no effect on the cooling, except that spontaneous emission puts a limit on just how cold you can go with this technique.

Thanks for asking. I had to get my head around the process again, but it was worthwhile. I probably dumbed it down a little below what you already know, but I figured it'd be good practise and maybe informative for others.

Consider me informed. I kinda sorta knew how it worked, but would not have been able to state the process anywhere near that well. Nicely done. :)

Thank you.

No worries. Some of it I knew, a lot I didn;t. Thank you!


...I'm guessing this isn;t exactly the type of question you were expecting.

Well... I figured enough people know what I do that at least someone would ask something sciencey. I don't mind at all---I like spreading the gospel.

I am now informed... Moar quantum mechanics plz :3

What happens if you cross the streams? :D

Don't cross the streams. It would be bad. 8-|

But what if I DO! :D

It depends on the wavelengths of the streams... you might get lucky and get WHITE!

Have you ever had your balls itch so bad but you were in a crowd of people at the mall? And then a group of midgets walks by and you laugh so hard that milk comes out your nose? And then the escalator blew up?

Awww man. Happens every Saturday I swear.

Oh man, funnily enough-- wait, milk? Oh... Sorry, no.

Where do you live?

Roughly an hour's west of Toronto.

Canadian or Michigander? o3o

Australian! (Ex-pat in Canada. I'm a loooong way from home...)

How much wood could a woodchuck chuck if a woodchuck could chuck wood?

None. Lazy fuckers.

On the surface it's pretty simple, but the details get a bit complicated. Basically, fire is a rapid oxidization process. What's happening is that some fuel substance is combining with oxygen (typically from the air), bonding with it and in the process releasing lots of energy. Oxygen is very good at this because, due to it's structure, it has a relatively high electron affinity, which means it's difficult to take electrons away from it, once it's gotten attached to them. That is, it takes more energy to pull things apart from oxygen. So when the reverse process happens, and oxygen combines with something, energy is released. In a fire this energy is released as heat and light. The flame that you see is the light energy that is released as the oxygen and fuel combine.

If you're wondering what stops fires from starting spontaneously, the reason is that there is a minimum amount of heat required to get the oxygen and the fuel to bond. Without this heat, the oxygen forms O₂ molecules in states that are "incompatible" for combining with fuel. (The term is "forbidden transition", and explaining what this means and how it comes about would actually take a few paragraphs.) But this incompatible state of oxygen is where it stays when it has low energy. If you give it enough energy (as heat, for example) the oxygen might go into a different state, one that is compatible with bonding with the fuel. When it does it begins a chain reaction, where the combination releases energy in light and heat. Heat which can cause other oxygen molecules to go into the compatible state and then combine with the fuel...

Interestingly, rust is also an oxidization process, where iron combines with oxygen. It's even exothermic---that is, it releases energy---like a fire. The only reason it's not considered "fire" is because it's so slow.

I'll confess I don't know this topic as well as others, but I believe this is about right. Hope that answers your question.

Science is SRS BSNS.

Nah, I just didn't realize. Besides, fire is fascinating, complicated process.

Does she really have to sell sea shells by the sea shore?

No. In fact, the sea shore is probably the worst place to be selling sea shells by, because there is generally an abundant supply of free sea shells adjacent to her that anyone interested in obtaining sea shells could just as easily take. Few people realize that in the original story, after pouring her life-savings into this doomed venture, she eventually ran out of money and actually ended up having to file for chapter 11 bankruptcy. Originally, it was meant to be a cautionary tale about the danger of marketing to the wrong crowd, but the silly tongue-twister part caught on while the actual moral behind it didn't.*

Are you moving on up to the east side?

I feel I am sufficiently both up and east for the moment (see earlier question).