In 1997 James Cordes, Carl Sagan and I published the first possibility of radio twinkling. We found that the previous attempts to verify the 11 META candidates had not taken sufficient account of this possibility. That is, radio twinkling could have caused a real ET transmission to fade to the point of being undetectable. The META candidates might have represented real transmissions. If that were the case, the original META analysis had shown that--taking into account the fraction of the sky observed in META, the possibility of more distant transmitters, etc.--there could be more than 10,000 ET transmitters in the Milky Way galaxy.
Between 1997 and 1998 nine of the 11 candidates were reobserved as part of the SETI Institute's Project Phoenix. These were far more sensitive verification attempts than the original META attempts. Jill Tarter, Peter Backus and I have now shown that these new verification attempts were sufficiently sensitive to rule out, effectively, the possibility that the META candidates were transmissions from ETs. More precisely, we were able to show that, even taking radio twinkling into account, we had at least a 97.8 percent chance of detecting the META candidates and probably better than a 99.9 percent chance of detecting them if they were real, intrinsically steady transmitters.
We also considered the gravitational microlensing possibility for the first time. Even at the most optimistic the META candidates imply a galactic ET population not too much larger than 10,000. However, the existing gravitational microlensing observations of stars show that microlensing is extremely unlikely to occur. There were only a small number of META candidates. Yet to explain even this small number as gravitationally lensed ET transmitters requires that the number of transmitters in the galaxy be vast, literally billions and billions.
So what were the META candidates? The more mundane, and therefore probably more likely, possibility is that they represent terrestrial transmitters. The META program went to great lengths to try to filter out human-generated transmissions (as have all other radio SETI programs). These efforts may not have been 100 percent successful and some terrestrial transmissions may have slipped through. (Indeed, in the META results contain a similar number of what were thought initially to be ET candidates but later recognized to be terrestrial interference.) In this respect, it is worth noting that the total number of signals analyzed by the META program was in excess of 50 trillion. Only a quite small fraction of terrestrial transmissions would have to slip through the META filters to explain the candidates.
The other, far more speculative possibility is that the META candidates represent real, but intrinsically transient ET transmissions. A terrestrial analog of an intrinsically transient signal is a scientific radar like the one at Arecibo Observatory. As the radar is transmitting to another planet, moon or asteroid, it is sitting on the surface of a rotating planet and moving to track the object being studied. Moreover, the portion of the sky illuminated by the radar is much larger than the object being studied. Any beings in the path of the radar might someday see a brief pulse as the radar beam sweeps past their planet. Similarly, if there are other civilizations with powerful radars, we might occasionally be in the path of their radars. Unfortunately, it is exceedingly difficult to verify that we have detected an ET radar pulse as opposed to a human-generated transmission. Perhaps future radio telescopes will have this capability, though. A couple on the drawing board include the SETI Institute's Allen Telescope Array and the Low Frequency Array project. More distant (perhaps in the next decade) is the Square Kilometer Array.
For now, we must be conservative and conclude that at least nine, and possibly all 11, of the META candidates were not ET transmissions. The original META analysis concluded that if the 11 candidates were real ET transmissions, then there are at least 10,000 ET transmitters in the Milky Way galaxy. Conversely, if none of the ET candidates is real, we must conclude that there can be no more than 10,000 other civilizations (broadcasting on the META frequencies) in the Milky Way galaxy.
Joseph Lazio is a radio astronomer at the Naval Research Laboratory in Washington, D.C. In addition to SETI, his research interests include pulsars and the propagation of radio waves through the interstellar medium. The views expressed here are not necessarily endorsed by the Department of the Navy or of the Department of Defense.