Radio silence

36,400,000. That is the expected number of intelligent civilizations in our galaxy, according to Drake’s famous equation. For the last 78 years, we had been broadcasting everything about us – our radio, our television, our history, our greatest discoveries – to the rest of the galaxy. We had been shouting our existence at the top of our lungs to the rest of the universe, wondering if we were alone. 36 million civilizations, yet in almost a century of listening, we hadn’t heard a thing. We were alone.

That was, until about 5 minutes ago.

The transmission came on every transcendental multiple of hydrogen’s frequency that were listening to. Transcendental harmonics – things like hydrogen’s frequency times pi – don’t appear in nature, so I knew it had to be artificial. The signal pulsed on and off very quickly with incredibly uniform amplitudes; my initial reaction was that this was some sort of binary transmission. I measured 1679 pulses in the one minute that the transmission was active. After that, the silence resumed.

The numbers didn’t make any sense at first. They just seemed to be a random jumble of noise. But the pulses were so perfectly uniform, and on a frequency that was always so silent; they had to come from an artificial source. I looked over the transmission again, and my heart skipped a beat. 1679 – that was the exact length of the Arecibo message sent out 40 years ago. I excitedly started arranging the bits in the original 73x23 rectangle. I didn’t get more than halfway through before my hopes were confirmed. This was the exact same message. The numbers in binary, from 1 to 10. The atomic numbers of the elements that make up life. The formulas for our DNA nucleotides. Someone had been listening to us, and wanted us to know they were there.

Then it came to me – this original message was transmitted only 40 years ago. This means that life must be at most 20 lightyears away. A civilization within talking distance? This would revolutionize every field I have ever worked in – astrophysics, astrobiology, astro-

The signal is beeping again.

This time, it is slow. Deliberate, even. It lasts just under 5 minutes, with a new bit coming in once per second. Though the computers are of course recording it, I start writing them down. 0. 1. 0. 1. 0. 1. 0. 0... I knew immediately this wasn’t the same message as before. My mind races through the possibilities of what this could be. The transmission ends, having transmitted 248 bits. Surely this is too small for a meaningful message. What great message to another civilization can you possibly send with only 248 bits of information? On a computer, the only files that small would be limited to…

Text.

Was it possible? Were they really sending a message to us in our own language? Come to think of it, it’s not that out of the question – we had been transmitting pretty much every language on earth for the last 70 years… I begin to decipher with the first encoding scheme I could think of – ASCII. 0. 1. 0. 1. 0. 1. 0. 0. That’s B... 0. 1. 1 0. 0. 1. 0. 1. E…

As I finish piecing together the message, my stomach sinks like an anchor. The words before me answer everything.

“BE QUIET OR THEY WILL HEAR YOU”