How about a beam of neutrinos? The advantage of neutrinos is that they barely interact with ordinary matter, so a neutrino beam could travel through the entire galaxy without being absorbed. The disadvantage of neutrinos is that they barely interact with ordinary matter, making their detection a monumental task. Neutrino detectors typically consist of huge quantities of "stuff" sunk deep underground to avoid cosmic rays, and scientists then detect neutrinos that slam into this "stuff." (For instance, the IceCube detector uses a cubic kilometer of the ice at the South Pole). Even these huge detectors typically see only a handful of events. And no messages from aliens so far.
|IceCube -- looks like a fun place to work, doesn't it?|
These methods of communication may seem outlandish, but even more bizarre ideas have been proposed. Many of these fall into the "message in a bottle" category -- some sort of message left behind for us to discover. How about a message encoded in DNA and passed down through the generations? This idea was the key plot element in an episode of Star Trek: The Next Generation. But a pair of Kazakh scientists claims to have found evidence for such a message in our own DNA, presumably embedded there eons ago by an alien intelligence. What would such a message say: Instructions to find the creators of the message? Warnings of pitfalls to avoid? Or more likely, “Congratulations, you have just won the intergalactic lottery. Please transmit your bank account number and password to...”
Surely, DNA wins the prize for the most far-out way to send a message to us. But wait, there’s an even more profound possibility. Physicists Steve Hsu and Tony Zee suggested that a sufficiently advanced race might have left a message encoded in the cosmic microwave background, the radiation left over from the Big Bang. Of course, this would require an intelligence so powerful that it could have intervened in the evolution of the early universe 13 billion years ago. It’s hard to imagine anything more extraordinary. But Carl Sagan did. In his novel Contact (but not in the movie!), the heroine discovers a secret message embedded in the digits of π. But π is a mathematical constant, not something that can be physically manipulated. The only one who could insert a message into π would be the creator of the universe itself. I don’t dare speculate about what that message might say.
Pedantic footnote: Yes, it's likely that π contains all possible sequences of digits, so you could find all of the works of Shakespeare, your grocery list from last week, etc., embedded in it. Sagan's novel assumes that the message is relatively close to the first few digits.
Update: Sagan's idea of embedding a message in the digits of π might have originated with my former postdoctoral advisor, Bill Press, at Harvard. Press mentioned it in a lunchtime conversation with Cornell physicist Saul Teukolsky, who carried the idea back to Carl Sagan.