Last night I stood in the chilly air watching the Eastern horizon waiting to watch a rocket launch from Wallops Island. The sky was mostly clear, but the moon was almost full and bright enough that I wondered if I would see very much.
My mind wandered to a lunch in the late 70s. I was just back from watching a total solar eclipse and had newly developed prints to prove it. Ray Davis was a regular at our lunch table and I thought he'd appreciate it more than most - after all - he had forced a massive rethinking of the physics deep within the Sun and possibly even deeper.1 As he shuffled through the deck of images he said "isn't it wonderful that we can detect the Moon and deduce what it is..."
I've thought about that over the years. Our senses only detect a tiny bit of our universe - our separation from much of Nature is enormous. We're so much larger than the microscopic and so much smaller than the astronomical. Our perception of time removes us from the fast and slow. The range of our senses is incomplete - vision and hearing are primary senses, but occupy a small portion of what exists. Evolution has provided our brain with filters that give us an imperfect view of what we can sense.
Despite all of these shortcomings we also have a powerful tool - our brain. We are naturally curious and have figured out good ways to ask questions. Part of this process has involved making instruments that augment our senses.
It looked like a big tank buried about two miles underground in an abandoned South Dakota mine, but it was the world's first serious neutrino telescope made to detect neutrinos coming from the Sun. Neutrinos are nearly ephemeral particles that interact very weakly with matter. If one encounters a chunk of lead a light-year thick it only has about a fifty-fifty chance of interacting along the way. But neutrinos are very common - when four protons fuse to become a helium nucleus in the Sun's core two neutrinos are produced in the process.2 Point your palm towards the Sun - an average sized palm will have about seven trillion neutrinos pass through it every second. It also turns out that if fusion in the Sun's core suddenly stopped, it would take thousands of years to notice it visually as light takes many thousands of years to make the journey from the core to the surface. Neutrinos fly out directly, so we'd notice it in about 500 seconds (the time it takes a neutrino to travel from the solar core to the Earth).
Since Ray's solar telescope neutrino detectors have grown and improved. The newest and best is the IceCube facility at the South Pole. It is designed to look for neutrino bursts from extremely energetic events - exploding stars and beyond. The biggest problem with neutrino detection is beating down the noise from other sources. A good way to do this is a thick shield - IceCube uses a lot of ice. it is relatively easy to sink holes in the ice and then drop strings of detectors down the holes. The result is a cube shaped array that is mostly embedded in one and a half to two kilometers of ice. The detectors sense the passage of muons - particles created from an interaction between a type of neutrino and the ice.3
So what does this have to do with the moon? Hang on .. I'm getting there ...
Part of the practice of building a particle detector is calibrating it (this is true of any detectors - a point I fear is lost in some early attempt of the Internet of Things). Muon and neutrino tracks are reconstructed by examining the time and position signals are produced in the cube. There is also a huge amount of noise that needs to be rejected. It would be really nice to have a strong muon source at a known location in the sky to use as a baseline. Unfortunately there isn't one, but there is a very good anti-source called the Moon.
The largest source of muons at the Earth's surface are cosmic rays. They strike atoms in the atmosphere and create muons in the process. A muon view of the sky would show a mostly constant glow as cosmic rays come in straight lines from all directions. The Moon is a cosmic ray shield, so if you could only see muons there would be something of a hole where the Moon lurks.
That's exactly what the IceCube guys did - a map of the muon shadow of the Moon.
There are other neat ways to detect the Moon outside the visual. Here is one done with gamma rays. Processes in the Sun aren't energetic enough to produce a large flux of gamma rays, but cosmic rays (which are really high energy charged particles rather than electromagnetic radiation) can produce them when they strike a surface - the Moon for example. The Moon is much brighter than the Sun in the gamma ray portion of the spectrum and shows up rather nicely.
Oh - the launch was lovely...
1 I wrote a bit about this in an earlier post. Through extremely difficult and careful experimentation and observation of the Sun he had determined either some fundamental particle physics or our understanding of the nuclear physics in stars was incomplete or possibly wrong. It turned out the problem was with particle physics.
2 The actual process is a bit too involved to note here, but basically two protons are converted to two neutrons and two neutrinos.
3 There are three basic types of neutrinos. It turns out muon neutrinos are the most practical to detect. Also the detectors detect light from the rapidly moving muons. As they move through the very clear ice they give of Cherenkov radiation. Lots of beautiful physics going on:-) A proposal to build a similar cube in water was made in the late 70s - the Deep Underwater Muon and Neutrino detector. It was to live in deep water off Hawaii. It turned out to be impractical.
This one was inspired by Yotam Ottolenghi's wonderful Jerusalem cookbook. The trick to a cookbook is to find a bit of inspiration and then improvise. It will find its way to a Thanksgiving table.
Figs and Sweet Potatoes
° 2 large sweet potatoes (about 1 kg or 2.2 lb total) washed
° 5 tbl olive oil
° 3 tbl balsamic vinegar
° 1.5 tbl white cane sugar
° 12 scallions cut in half lengthwise and then cut into inch long segments
° 1 red chili thinly sliced
° 6 ripe figs cut into quarters
° a finishing sea salt (I love Maldon) and fresh ground black pepper
° something to sprinkle over the top ... I used toasted crushed pecans, if you are into cheese I think crumbled soft goat cheese should work well.
° preheat oven to 475°F
° halve the sweet potatoes lengthwise and cut into 3 or 4 long wedges (I usually peel them, but not here - the skin is tasty when roasted). Mix with the olive oil, about 2 tsp salt and some pepper
° spread the sweet potatoes out on a baking sheet with the skin side down. Bake for about a half hour or until soft. cool
° make a balsamic reduction by putting the vinegar and sugar in a small saucepan. Bring to a boil, cut the heat and let it simmer until the mixture thickens. Remove the pan from the heat - you don't want it to get too viscous (less than honey for me).
° arrange the potatoes on a plate
° heat 2 tbl olive oil in a saucepan over a medium heat and add the scallions and chili slices. Cook for about 5 minutes stiring frequently - don't let the chili burn. Spoon over the potatoes. Drizzle the reduction over everything - if it is too thin add a few drops of water. Sprinkle the pecans or goat cheese over the top and serve.