Monday, December 10, 2012

Why go to the South Pole?

I'm currently stuck in Sydney airport on a 7 hour layover.  Australian immegration law bars me from leaving the airport (it's a boot-able offense) and my attempts to video the rotational handedness of a toilet flushing ended poorly (these toilets have jets that direct water radially inward!).  So as I sip my Victorian Bitter (how is that the Aussies are know for their beer?), I figured I'd answer a question that several friends have asked: "why do we need to go to the South Pole?"  Why not do this somewhere closer, let's say Mt Palomar near San Diego or on Mt Wilson above LA?

The answers are that:
1) it's really dry at the South Pole and
2) the air is really still there over the winter

We're trying to look at microwaves (technically millimeter waves) left over from the Big Bang.  We're stuck looking at microwaves because our expanding universe has stretched the wavelengths of the Big-Bang's afterglow from infrared into the microwave range.  Wayne Hu from University of Chicago is well know for his animated gifs to explain cosmology, and here's his explanation of how our universe's expansions not only drags the galaxies apart but also stretches wavelengths of light into the red end of the spectrum.

Most people experience microwaves at home with their microwave ovens.  Those work because water in your food absorbs microwaves.  However, the water vapor in our atmosphere also absorbs the microwaves coming in from the heavens, so in our business it pays to set up shop somewhere very dry.  The center of Antarctica, technically a desert,  is one of the driest places in the world because all of the water freezes out of the air (The Atacama Desert in Chile is also very good).  There's nothing particularly special about the South Pole, except that it happens to be in the middle of Antarctica and has lots of support infrastructure courtesy of the NSF and US Navy.

The stable air is just as important.  Generations of kids have grown up learning about twinkling stars from nurse rhymes, but it's a real pain for astronomy.  Stars and distant city lights "twinkle" because of atmospheric turbulence, and it can make images look fuzzy.  My friend Christoph Baranec designs telescopes with "adaptive optics" that can correct for turbulence with deformable mirrors.  Below are pictures he took of Jupiter with his Robo-AO system on and off:
That can work well for visible and infrared light, but those techniques are not as useful for microwaves.  Our images are much coarser, so turbulence doesn't appreciably distort our images-  it just contributes unwanted noise that can make the measurement take (much) longer.  The beauty of Antarctica is that the sun sets once a year - in March- and doesn't rise until September.  As a result, there's no daily heating and cooling of water or land-masses that might generate high altitude winds.  So we get 6 months of pristine uninterrupted viewing.

1 comment:

  1. Roger, thanks for keeping us updated - look forward to future posts.