Wednesday, January 22, 2014

The Case for the Wow! Signal

I recommend Robert Gray's book The Elusive Wow! if you want detailed background on the Wow! event of August 15th 1977. The book also describes the author's efforts to replicate the signal. To date, no one has reported a reliable second detection of the Wow! Signal, although efforts to find it have not been persistent.

The Wow! Signal was a single 72 second event detected by the Big Ear radio telescope operated by Ohio State University, which was sweeping past the constellation Sagittarius at the time. This telescope was designed to conduct a survey of the radio sky, not to study individual sources in detail. The survey was successful and a number of new radio sources were discovered. After that, some of the science team thought that a SETI search would be a good use of the telescope.

I think the case against the Wow! Signal as an ET beacon is well known.

We are all well aware of the fallacy of the argument from ignorance. Just because the Wow! Signal has not been proven to be from a known source doesn't mean it's from ET. It's possible that the Wow! Signal was some sort of strange problem with the Big Ear's receiver that only occurred once, or that it was an extraordinarily elaborate and strenuous hoax. For these reasons, you would need to see independent confirmation, and we haven't; Robert Gray's single-handed and largely self-funded efforts to do so have been far from comprehensive.

Since the Wow! Signal was discovered after the fact, when Jerry Ehman went through a stack of printouts, it was too late to get another radio telescope to break off what it was doing, swing over and confirm the signal.  The signal was never confirmed, and no similar signal has been found in that region of space.

The are multiple reasons we think that the Wow! Signal may have been an ET beacon.

Let's describe a simple model of an ET beacon, which essentially is a one bit signal. It's purpose is to announce ET's existence (and possibly the interest in conducting a slow but meaningful conversation), and you want any pretentious primates out there to have the best chance to see it and not confuse it for a natural source. You almost certainly do not know that we humans are here, since we have only transmitting microwaves for about 37 years in 1977, and the sphere defined by that round trip light time does not contain many stars. You would most likely have a list of candidate stars to transmit your beacon to, and spend some time transmitting to each one before moving on to the next. you might go through this list many times, but the amount of time each candidate star would see a signal might be limited.

How long you would be willing to maintain the signal directed at one star would depend on such factors as the rotation rate of your planet, the length of your year, how long your list is, and your strategy for sending such a powerful signal. One such strategy might use gravitational lensing from a massive star, and the time duration for directing the signal to us would necessarily be limited.

To make your radio beacon detectable and not mistaken for a natural source, you make it as narrow band as you can, and give it as much power as you could afford. Also, you would want it to be at a frequency well known to radio astronomers at which they might be listening. There is a well-known neutral Hydrogen microwave line at a wavelength of 21 cm that would be known to even beginning radio astronomers, and is a logical place to listen for a beacon.

The thing that impresses me most about the Wow! signal is not its strength, but its narrow band. The entire
The Wow! Signal Printout
signal fell within a 10 kHz band, and may have been narrower - that was simply how the receiver was tuned at the Big Ear. During the entire 72 seconds it stayed in that band. That, combined with the receive beamwidth and scan rate of the Big Ear, and the frequency at which it was detected, means that a man made source is extremely unlikely. All the natural sources of that strength are well catalogued and are not to be found where the Big Ear was pointing at the time. Furthermore, natural sources are not narrow band, and this is one reason SETI researchers look for such signals, since they stand out against the natural background.

Almost equally impressive is the frequency of the signal. It is very nearly on top of the 21 cm Hydrogen line. The small difference between the observed frequency and the 21 cm line is easily explainable as Doppler shift due to relative velocity between the transmitter and Earth. Another key feature of this frequency is that it is not used by satellites to transmit to Earth.

The duration and profile of the signal is also important, but that is exactly how long you would expect a signal to rise, peak and fall if it were very far distant from Earth - essentially at a fixed location on the celestial sphere - in this case in the constellation Sagittarius. We can't identify a specific star that the signal came from because of ambiguity in the direction of the source equal to about half a degree - a huge angle in astronomical terms.

So, there are three independent lines of evidence that point to the Wow! Signal being an ET beacon. With that going on, you would expect humanity to make it a priority to repeat the detection. This has by and large not happened.

What do you think we should do?

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1 comment:

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