Looking up at the sky is one of those pleasures that leads to the sort of questions kids ask. What are clouds and why do they float? Why are rain clouds so dark? Why are the edges of clouds so well defined? What makes a sunrise or sunset red? When did people start noticing the sky is blue? Why is the sky blue?
The wonderful online comic strip xkcd reminded me of this the other day. Kids pepper their parents with questions and this one is likely to come up. The simple answer is given in the strip, but that doesn't get at what scattering is and why Rayleigh scattering works with air molecules. Why it does and why it scatters as wavelength-1/4 is something that usually happens in the first or second year of a college level physics course, but the follow-on question can stump the adult who hasn't considered a few more things. It turns out our distorted perception of the world is centrally involved which leads to another herd of questions.1
If perception is involved one begins to question if there are any cultural components to color perception. It turns out there are. Now one begins to question the nature of what color is. Is it fundamental or is there a mapping between wavelength and what we perceive as - say - blue, red or yellow?2
childlike questions frequently aren't!
Some people use the analogy that science is like pealing an onion - you describe something but find, upon closer inspection, that there is a deeper explanation and beneath that yet another. I'm afraid it doesn't work for me. A better analogy is throwing a stone into a pond. There is a splash and then a circular wave that expands outwards. The wave laps up against things that you didn't know existed. The edge of it can be thought of as the boundary between what we know and what we can know if we work at what has been uncovered. We are newly ignorant and that is a wonderful thing. The beauty is that our ignorance is expanding much more rapidly than what we know. It is like diamond filled beaches that no one has seen are being constantly discovered. Mining the old beaches can lead to questions that will lead you to entirely new shores. That is why people do science - it isn't the discovery of an answer, but rather the discovery of a question.
Michael Faraday, in addition to being a brilliant physicist, was the Carl Sagan and Neil deGrasse Tyson of his day. He had the habit of delivering public science lectures on subjects like candles. Alan Alda (yes - that Alan Alda) is passionate about the communication of science and is an associate professor and one of the founders of the Center for Communicating Science at SUNY Stony Brook. He felt there is a huge problem communicating science to the public and wanted to do something about it invoking Faraday as an inspiration.
Alda proposed a competition to simply describe how a flame works - a question that drove him when he was young. There were hundreds of serious entries and they were judged for clarity by an army of volunteer 11 year olds.
This year the challenge will be to explain time. This may turn out to be interesting, Science is pretty good at explaining how a flame works at a fundamental level and, for a gifted explainer, the gist of it can be communicated to an 11 year old. Time is deeper - it is fundamental at the level where we really don't know what it is. That doesn't stop people from trying to learn about it by observation and experiment. Physics is beginning to take cautious baby steps - time once was only for philosophers, but now some science is being done. Somehow the process of discovery needs to be communicated. Figuring out how to communicate this to an 11 year old seems like an impossible task. I'm very curious to see if there are some clever ways to communicate something basic about science. At least it gets away from the fallacy that science is about assembling "facts"...
These things are fine and good, but there is still much to be discovered just staring at the sky.
If you are in a spectacularly dark area - the sort of place you sometimes get in places where large telescopes tend to get built - the Milky Way becomes impossibly bright and rich on moonless nights. As your eyes adjust to the dark, you begin to notice a faint glow far from the times when you have remaining hints of sunrise or sunset. The glow is strongest at the horizon and so faint it is colorless. If you have some tools available you discover it is mostly green, but there are some other even fainter colors - far too weak for your cones to let your brain in on the show. This is the airglow.
Recently I came across this wonderful image taken from the International Space Station. It happens to be the airglow from space. If you look very closely you'll see a faint bluish band that hugs the Earth.3 That happens to be the dense part of the atmosphere - the piece up to about four miles up that sustains life. Most of our weather takes place here and we call it home and tend to think of it as large and expansive.
It isn't and we need to take care of it...
But that aside the atmosphere continues for some distance. The brightest region - the outer green band- is about 100 km above the surface. This is where a bit of physics begins to unravel what is going on.
If you break light from this area into its spectral components, you get a few pronounced lines that were mostly described by atomic (not nuclear!) physics more than 100 years ago. The bright green line has a wavelength of 558nm and is given off when atomic oxygen is struck by ultraviolet light. It also has a fainter red line from a region at a higher altitude. The yellow light is from sodium atoms and there is red light from excited OH radicals at somewhat lower altitudes. But without getting deeply into spectroscopy and worrying about each line there are a few other questions that come up.
Why is it so bright? Of course you have to consider the camera, but it turns out the airglow is about 1000 times bright on the side of the Earth facing the Sun (duh), but it is washed out during the day (duh again). From orbit we can see a slice of the atmosphere that is still illuminated by the Sun while we are on Earth's dark side. Nothing like an ideal location!
Why are the colors confined to such narrow bands? This gets really interesting. It turns out that bright green band comes from the atomic Oxygen excited state - it takes a very long time (by the standards of atomic physics) for this to happen - about a second. At lower altitudes there are so many other air molecules that most of the just excited Oxygens get whacked by some other molecule that collides with them. This process changes their energy and they usually don't give off that particular light. As you go up in altitude the air becomes less dense and there is a point where the average collisions are rare enough - over a second between them - that the process can occur. There are similar arguments for the other layers, but it is all about putting different bits of information together from fields that, on the surface, seem different.
It is common in science to find a new bit of ignorance in the ordinary world. With the wonderful work going on at CERN it is easy to forget that quite a bit of real ignorance lurks on tabletops with very simple apparatus and, in some fields, there are wonders in a local pond (8 minute mp3).
The whole trick is to be curious like an 11 year old and learn to ask questions and be open to something you don't expect. I've had surprising and sometimes excellent results being curious and linking to outlying fields while doing work in fields far from my training and I suspect the same is true for of many of you.
Science has a formal method for doing this, but the same principles apply in many other fields. Drilling in and asking questions, being open to answers you don't expect and even finding there are richer new questions, and connecting the dots with other fields that are very different from your own.
Oh - and if you have never seen a very dark sky you owe it to yourself to do so! You can find some fairly dark areas in the lower 48 states - mostly in the desert Southwest, but there are a few good areas in other regions. I'm about 250 miles from a pretty good area in Pennsylvania and have been known to drive for watching a meteor shower (not tonight though) or just getting a fix and remembering what a dark sky is like.
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1
It turns out the answer has a couple of pieces. The intensity of shortwave length visible light from the Sun decreases rapidly as wavelength decreases, but more important is perceptual. We have three cones that register what we call color. The cone associated with blue (the S cone) is very insensitive to shorter wavelengths. We simple don't perceive most of the violet that is in the sky.
It turns out many birds and insects are sensitive into the ultraviolet. They would see a much more violet sky - even a somewhat ultraviolet sky - than we do.
It is wonderful when kids ask great followup questions. This one is deep, but obvious in retrospect - if you see an eight year old do that you may want to encourage them to keep asking.
2 It turns out color is very artificial - you can convince yourself of this easily by overlapping a very pure "red" light with a very pure "green" light. Where they add we register something we call "yellow." Neither of the original lights had any yellow wavelength in them. It turns out to be a construction of your mind.
The wonderful NPR show Radio lab had a great segment on color perception that will leave you with some wonderful shores to explore from.
highly recommended!
3 Also look closely and you'll see the constellation Orion.
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Recipe Corner
Echoing the theme we end with peeling apples rather than onions.
This one is simple and good - maple roasted apples that almost give the flavor of an apple pie without the crust. Great as a dessert by themselves - perhaps with a little topping like crushed walnuts. I've tried it with a bit of cinnamon and nutmeg mixed in, but prefer them straight. You'll want to use a rich grade B maple syrup (the grade has nothing to do with quality - grade B is darker and richer than A .. which has its own delights).
Feel free to scale the recipe - there won't be any problems as long as your tray is large enough. It is terrific served over slightly softened vanilla ice cream or (better yet) a vanilla gelato (try Talenti)
Mapled Apples
Ingredients
° 4 large baking apples peeled and cored
° 100g high quality grade B maple syrup
° 55g butter (a half stick)
° pinch of salt
° 1/2 tsp ground cinnamon (optional)
° 1/4 tsp ground nutmeg (optional)
Technique
° preheat the oven to 375° F
° cut the apples into about a dozen equally sized wedges each
° melt the butter in a microwave oven in a bowl big enough to hold the apple slices
° mix the maple syrup, salt and optional spices in with the butter and toss in the apples and make sure they get coated with the mixture on both sides
° spread the pieces out on a greased cooking sheet and bake until soft and just caramelizing at the edges - about 40 minutes in my oven. Turn them over about 20 minutes into the bake.
I drove my family nuts as a kid asking questions like, "why is the sky blue?" So lovely as an adult to have someone to ask these questions with (and not get made fun of). Thank you for being you Steve.
Posted by: Jean Russell | 12/13/2012 at 09:32 AM