Optical SETI offers a much higher chance of finding a real signal because of the simple fact that there are no known natural sources of nanosecond pulses (or shorter). Any 10 nanosecond pulse of light can only be from intelligent life (or of course a shocking new natural phenomenon). Recently a researcher in Australia has detected a pulse. Unfortunately the details of the signal are not available. Thanks to Sentient Developments for pointing out this article.

The details of optical SETI are a lot more complex than radio based SETI. Radio SETI is fairly clear, just listen for a signal which shouldn't be there. But optical SETI requires some understanding of light pulses. For example, every day our own sun puts out flares, which would look like pulses from a few light years away. Supernovae, gamma-ray bursters, novae, and variable stars all put out more light at some point than normal. So what makes an optical pulse natural versus synthetic? The answer is time.

There are no natural phenomena which generate nanosecond light pulses**. A nanosecond pulse means that a burst of photons is detected, and all the photons arrive within a few nanoseconds of each other, then no more arrive until the next burst. In general this means the light was generated with a laser. Lasers are a little easier to understand, all the photons are the same, same color (or wavelength), and same phase. There are continuous lasers which have no pulses, they emit a steady stream of photons.

From a few light years away it is plausible to measure the photon color and determine that a light source is a laser, but unlikely. For example, amateur astronmers know that a planetary nebula emits OIII lines at 501nm. All these photons are the same color but they are absolutely not from a laser. With extremely precise spectrometry it might be possible to determine that a certain set of photons came from a laser. But there would be a lot of arguing.

However, if the laser is pulsed, and the pulse width is less than 100 nanoseconds, then this signal came from a laser built by an intelligent species. It's not even necessary to measure the color of the photons. If they are nanosecond pulses then we are not alone. All that is needed is that the pulse be bright compared with the background. For example, point an 8" telescope at a magnitude 6 star. The telescope collects some number of photons/second, let's say it's a million. So on average about once every microsecond a photon hits the detector. Now suddenly a pulse of 5 photons arrives, all within 10 nanoseconds. Even if that pulse is not repeated it stands out like a beacon, 5 random photons never arrive on top of each other. A 5 photon pulse means we are not alone. This works until the average photon rate is around a 100 million per second, so really bright stars will drown out possible optical signals.

With a photomultiplier tube, boxcar integrator, and a decent oscilliscope we amateur astronomers could build our own optical SETI equipment and discover intelligent life.

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** There is of course always the chance that a shocking new natural phenomenon will be discovered which emits nanosecond pulses. If you discover that you might have to settle for a Nobel instead of aliens.