Light, a mirror and ice

Brain cells talk to each other with tiny electrical signals. In the cell’s membrane there are proteins that work like doors; when a door opens, charged particles rush in and the cell “fires”.

For years the problem was this: among billions of cells, how do we activate only the ones we want? An electrode stimulates every cell around it at once, and a drug is slow.

The answer came from a wholly unexpected place: a single-celled green alga living in pond water. This alga swims towards light because its membrane carries a door that opens when light touches it. Hegemann and Nagel found this door and showed how it works. Deisseroth and his team then placed its gene into nerve cells: the blue light came on, and the cell fired.

Look at your hands: they are mirror images of each other, yet a right-hand glove will not fit the left hand. Many molecules also come in two such forms, “right-handed” and “left-handed”. The same atoms, the same bonds; the only difference is the mirror image.

The body notices the difference at once: one hand of the same molecule smells of spearmint, the other of caraway. It is the same with medicines. More than half of the medicines we use are “handed” molecules like this, and often only one hand does the job.

Here is the strange part: in the laboratory, an ordinary reaction makes equal amounts of both hands. Living things, however, always use just one; the building blocks of our proteins are left-handed, and the sugars in our DNA are right-handed. How did this preference begin? The question had gone unanswered for more than a hundred years.

Neutrinos are particles that interact with almost nothing. Right now, trillions of them pass through your body every second and you feel nothing. They pass straight through stars, and even through the whole Earth, without stopping.

That makes them unique messengers: they set out from places light cannot escape and reach us in a straight line. For the same reason they are very hard to catch.

Halzen’s idea was this: very rarely, a neutrino does hit an atom and gives off a small flash of blue light. To see that light you need a medium that is very large, very clear and very dark. The ice of the South Pole is exactly that. Holes reaching about 2.5 kilometres down were melted into the ice with hot water, and thousands of light sensors were lowered into them. A cubic kilometre of ice became a telescope.