Built on Facts

Horribly Cold Lasers

Posted by Matt Springer on December 1, 2008
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Well, my Thanksgiving posting break lasted longer than I thought. Real life is a more fun place than the internet though, and I hope you were having lots of fun and food and were not on the internet to notice my absence.

Among the things I did this Thanksgiving was watch Dr. Horrible’s Sing-Along Blog for the first time. It is, as you know, a thing of surpassing brilliance. It warrants a few posts about its unique self-financed studio-conglomerate-free creation, because that model would probably work well in other arenas of creativity, not the least of which is science. That I’ll save for later. For now, if you haven’t watched it there’s places you can see it for free online, though I highly recommend ordering the DVD if you like it, because it will encourage other such productions as well as reward the cast and crew for a job well done.

Anyway, one of the things that Dr. Horrible is trying to create is a freeze ray. Really his version freezes time, but that’s a little outside of our current grasp. Maybe we’ll do better if our freeze ray only freezes things by cooling them down. Microwave ovens do the same thing in reverse, so how hard can it be? Pretty hard, of course. A ray will pretty much by definition pump energy into things and so more energy will mean more temperature in most cases. But there’s some cases where sufficient cleverness can save the day, Captain Hammer style.

Get a diffuse gas of atoms and shine lasers at it from all directions. The atoms will all have a particular set of frequencies that they like to absorb, while they’re effectively transparent to other frequencies. So tune your laser to shine light of a frequency just below that frequency the atoms like to absorb. The Doppler effect will come into play since these atoms are moving: atoms moving toward the source of the laser will see the light shifted up in frequency. This means they’ll absorb the light and be gradually slowed down just as though it were a pool ball being slowed by blowing on it opposite its direction of motion. But atoms moving more slowly don’t see the light shifted as strongly and aren’t affected by the light. By adjusting the light appropriately you can cool the atoms down gradually to very low temperatures.

Here’s a neat little Wikipedia diagram of it being done:

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It’s a freeze ray! Well, sort of. There’s a lot of limitations. The atoms already have to be very cold and diffuse. Each laser tuning will only work for one particular atomic transition and so only one kind of atom at a time can be affected anyway. And the sample has to be hit from all sides to be effective. But it’s cooling, and it’s done with beams of light. Call it a baby step.

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Comments

  1. #1 Kobra
    December 1, 2008

    That’s damn clever. Is that how they get atoms cooled to near absolute zero?

  2. #2 Eric Lund
    December 1, 2008

    Kobra: Yes, that’s one of the ways.

    You can also use magnetic cooling, which can get you into the microkelvin range. I’ve seen one such device. That technique also requires you to get the atoms pretty cold to begin with. Basically, you cool to liquid He temperatures by conventional means, then use one of these techniques (there may be others, too) to get colder.

  3. #3 Tom
    December 1, 2008

    The optical molasses described above can get you into the microKelvin range. Subsequent evaporative cooling in magnetic traps is one of the steps used in creating Bose-Einstein condensates that get you down into the nanoKelvin regime.

    To the atomic physicists who do this kind of work, liquid Helium is considered very hot.

  4. #4 Nick
    December 1, 2008

    This was one of the most understandable explanations of laser cooling I’ve ever read, great post!

    And since we’re on the topic of coldness… I was in the market the other day thinking about how I was simultaneously being warmed by the infrared photons from the lights and cooled by the air escaping the freezer section. That got me to thinking… why is the vacuum of space cold? If there aren’t any atoms around to slow down the vibrations of other atoms like there are in a air-filled situation then how is cold “transferred” (even though I think that’s the wrong way to think about it)? Do you simply radiate heat faster than you can you replenish it?

  5. #5 Chris
    December 1, 2008

    That’s so cool!.

  6. #6 Anonymous
    December 1, 2008

    at nick

    space isn’t cold for exactly that reason, the cold of space is just something sci fi authors invented

  7. #7 Tercel
    December 2, 2008

    Many of the magneto optical traps like this use an optical molasses to cool the atoms, and a magnetic field with a very high second derivative (divergence) to move the trapped atoms to a cluster at the center of the trap. These traps act on the dipole moment of neutral atoms. I worked in a lab that used traps like this, it’s a very interesting field of research. As an added bonus, the day to day lab activity is quite fun, and the people in this field are (in my experience) great to work with.

    Nick, Anon:

    Space is cold. But vacuum is also a very good insulator. Without any heat source (like sunlight, for example) any object will cool by blackbody radiation until it is in thermal equilibrium with the ambient radiation in space. This is the cosmic microwave radiation background, and it has a temperature of about 4 K, although I could be wrong about that number. However, with a heat source objects will warm up rapidly because radiative cooling is nowhere near as effective as exchanging heat with another substance (like the air around us here on earth). This is why equipment like satellites must be built to handle extreme temperature changes, as they get very hot in the sun, and very cold in the shade.

    You can think of it like being in an ice cold room, but trapped under bedsheets that are way too thick. Without your body heat, the bed still gets very cold, but there is so much insulation that it still can get uncomfortably hot given a heat source.

  8. #8 Eric Lund
    December 2, 2008

    In space, “temperature” is a concept you have to generalize, because things are often not in thermodynamic equilibrium. For example, the plasma in the solar corona has a temperature of ~106 K (compared with 6000 K on the surface of the sun). Some of the plasma in the Earth’s magnetosphere is even hotter, with notional temperatures of up to ~108 K. But there is so little of it that heating by conduction is essentially nonexistent. Nick is correct that there are very few collisions to transfer energy between particles. But macroscopic objects do radiate a black body spectrum, as Tercel said (the cosmic microwave background temperature is 2.7 K). Satellite builders have to design radiators for any electronics exposed to sunlight (because increasing surface area increases radiation rate) as well as thermal blankets for instruments that are routinely exposed to long periods of cold. For earth orbiting satellites, especially beyond LEO and for highly inclined orbits, more attention is paid to heat issues because most of the satellite orbit is in sunlight and relatively little time is spent in full eclipse.

  9. #9 Tercel
    December 4, 2008

    Yes! 2.7K sounds more like it, thank you. I don’t know where I got 4K from; is that the boiling point of liquid helium?