First their was the “Wow” signal, and then it seems about once a year someone claims that SETI has discovered an extraterrestrial civilization. These have been real signals detected by SETI but discarded as not originating from an alien civilization. Arguably, the SETI protocol makes this impossible and explains why no confirmed detections have occurred to date.
SETI can only look for carrier waves, it can not detect any signal modulation if that modulation occurs at rates that would be useful for carrying information. If SETI detects a signal from space, determines that it is in fact arriving from deep space, SETI has no effective way to distinguish between a natural signal such as a star atmosphere MASER, and a signal from a distant civilization.
When a signal is detected by SETI, the protocol calls for first pointing the antenna off of it’s present direction. If the signal remains, then it is a signal bleeding in from some earthly origin.
If the signal goes away when the antenna is pointed away, the antenna is then pointed back to it’s original position. If the signal is again present, then at least you know it’s coming from that direction.
You do not yet know if the signal is from deep space or an orbiting satellite. So then you get your buddies elsewhere around the planet to point their antennas at it. If the antennas both receive the signal pointed in exactly the same direction, then you have a signal originating from deep space.
At this point SETI requires that the signal continue to be present the next day. This is test that all of the potential signals that have survived earlier tests, including the “Wow” signal, have failed.
A little radio background explains why it is nearly impossible for a legitimate signal from an alien civilization to pass this test.
In order for a radio signal to arrive from a distant star system and be detectable here on earth, even with huge dish antennas and extremely sensitive receivers, that signal would need to be transmitted with a huge effective radiated power. Practical limits on transmitter power make it essentially impossible for an intelligent civilization to generate enough power without the aid of a high gain antenna.
A high gain antenna increases the power transmitted in a particular direction by not radiating power in other directions. The higher the antenna gain, the narrower the transmitted beam. In order to achieve the necessary gain to transmit a signal to a distant planet, a very high gain narrow beam width antenna must be employed.
In order for a signal to be received here on the Earth, the transmitting antenna on the distant world would have to be aimed precisely at our earth.
The earth rotates and presumably so would the planet of the transmitting civilization. The earth further is orbiting around our Sun, and presumably home planet of the aliens would be orbiting their star. The earth isn’t orbiting our star alone but is part of the earth-moon system and the earth-moon systems’ center of gravity orbits the sun, making the earths motion around the Sun even more complex. The Sun then orbits in our galaxy and bobs up and down in the galactic plain, presumably the same would be true of the distant civilizations’ Sun.
Consequently, while it is possible that rarely the transmitting antenna of an alien civilization and our receiving antenna may point at each other, the odds of that recurring a day later, as required by the SETI protocol, are infinitesimally small. It is my believe that the protocol is therefore flawed.
The only signals from an alien civilization that SETI could hope to detect and confirm, under the requirements of the existing protocols, would be from a civilization intentionally trying to signal us and constantly re-aiming their transmitting antenna to follow the motion of our planet relative to theirs.
I can see one plausible scenario where this could potentially occur but it would seem unlikely. In a decade or two, it is expected that we will have the technology to directly image planets orbiting close star systems and even analyze the spectrum of the light reflected from these distant planets to determine if compounds that tend to indicate the presence of life exist in their atmospheres.
There may be alien civilizations around relatively nearby stars with this technological capability and they may have imaged our planet, performed a spectroscopic analysis and determined that there is likely to be life on this planet. In order to determine whether or not a technological civilization exists here they may have decided to intentionally transmit a signal that a technological civilization would understand was intentionally directed at them and then await a reply from us. That’s about the only plausible scenario under which SETI could receive a signal that it could confirm under it’s own protocol.
I believe it is likely that many of these alien candidate signals that failed the repeatability test were in fact alien signals that we received as the result of a chance alignment of their transmitting antenna and our receiving antenna.
So I think that it is very probable that the “Wow” signal, the signal now being reported by Steven Greer of the Disclosure Project, another signal received in 2004, I think it is probable that these are in fact of alien origin. But with current technology we will not be able to find out, unless one of our respective civilizations had developed faster than light capability and visits.