The Lake County Astronomical Society

Communicating with Martians

Gary Smith

I came across a neat story in the book Strange Beauty. It is a story used by Richard Feynman to point something out to his students. The physics that brought about the story involves a law called "reflection symmetry", which can be put metaphorically: A particle and its mirror image should act precisely the same. Whether something is right-handed or left-handed, reflected or not, is arbitrary - the result of accident or decree. My intent here is not to get into the physics that led up to the story, as that would take a long time, but to give you something to think about. How to communicate with another being on another planet via radio only.

Suppose we are trying to communicate with a Martian entirely through the use of radio signals. We want to tell him that our hearts are on the left side of the body, or that we drive our screws into wood by twisting them to the right. How would we convey this information?

On earth, we might explain, if we stand facing north and stick out our arms, the hand pointing in the direction where the sun sets is left. The Martian says, "O.K. We are also bilaterally symmetrical and have two arms and two legs. And our planet orbits the sun in the same direction as does earth. But we're confused by this idea of north. Which way should we face so that the hand pointing to the sunset is what you call left? If Mars had a magnetic field like Earth's, we could tell the Martian to take a magnetized needle and suspend it on a pivot. It would line up along the field in a north-south direction. "Fine," he says. "But which end of the needle is north and which is south?" We look at our own compasses and say, "The end of the needle that is painted red points north." "Great," the Martian says. "So which end do we paint red?" "Well, the one that's pointing north." As the story shows, when stripped from their local context, the words "left" and "right" or "north" and "south" seem to be meaningless conventions. There appears to be no innate phenomenon called handedness.

Remember that all communication in this story must be over the radio, and for good reason. We could conceivably send the Martian one of our screws with instruction to drive it into a solid object and note the direction it turns. That is what we call right. Or we might somehow send a light beam polarized so that all the photons are spinning either left or right. But that would be cheating, merely establishing our arbitrary convention on another planet like some kind of infection. We could have the Martian look at the Big Dipper: Viewed right side up, the four stars forming the bowl are to the right of the handle. But that also violates the spirit of the game. The point is that there is no way to derive leftness or rightness purely through natural law. Or so it was widely believed.

Well, how would you do it? After much discussion in the book it all comes down to the smallest, least massive and least-interacting thing in the universe, the neutrino. A photon, the carrier of electromagnetism, can spin either clockwise or counterclockwise. Moving through space, it can be thought of as tracing out a right-hand or left-hand spiral. But a neutrino, intimately related to the weak force, will always trace a left-hand spiral. Nature, at this level, is lopsided.

Now we can tell the Martians which side our hearts are on.