"Hello? Are you still there?"
Anybody who has experienced a momentary disruption in a cell phone conversation knows that the first task is to verify that the other party is still on the line. Researchers in the pursuit of evidence for extraterrestrial intelligence (SETI) have found themselves on the tantalizing brink of making what appears to be first contact on a number of occasions, only to be unable to verify that they have found a real signal--that is, someone at the other end of the line. (For more on the importance of data in estimating ET civilizations, see "Quantifying ET." For more on other SETI efforts, see "Searching for Life around Other Sunlike Stars." )
Recently the Planetary Society's Megachannel Extraterrestrial Assay (META) found 11 apparent transmissions. Intriguingly, these 11 candidate transmissions tended to be located in or near the plane of the Milky Way's spiral disk, just as one might expect if the transmissions originated from stars in our galaxy [see "Refugees for Life in a Hostile Universe," by Guillermo Gonzalez, Donald Brownlee and Peter D. Ward; Scientific American, October 2001]. In addition, in 1977, as part of its SETI program, the Ohio State University telescope affectionately known as the "Big Ear" picked up what appeared to be a boomingly loud transmission. The operator on duty was so excited, he wrote "Wow!" on the paper record of the observation.
Unfortunately, all attempts to verify these candidates--and others from various SETI programs--have failed. As Carl Sagan wrote, extraordinary claims require extraordinary evidence. Without more evidence than a single detection, SETI researchers are unwilling to cite any candidate as evidence of ET transmissions. Before dismissing these examples too hastily, though, many astronomers--including myself--have suggested that we should also consider whether these might represent real ET transmissions that were not verified because they were corrupted or modified during their journey to Earth.
Anybody who has watched the flickering of celestial bodies in the night sky knows that a star can appear to brighten, fade and then brighten again. Suppose that there were ways in which ET transmissions also could brighten and fade. If a possible source of an ET transmission was observed initially when it was bright but then the verification attempts occurred when the signal had faded, one might conclude mistakenly that the ET transmission was not real. Are there ways in which a real ET transmission might brighten and fade?
In fact there are (at least) two ways an ET transmission could brighten and fade. The first is "twinkling." Almost all SETI programs have been conducted in the radio portion of the spectrum. Distant radio sources can twinkle, just like the light from a star seen from Earth's surface. Unlike stars, which brighten and fade because their light passes through Earth's variable and obscuring atmosphere, an ET transmission would change because it would pass through the gas clouds that fill the space between the stars. The typical size of one of these gas clouds is roughly the same as the distance between Earth and the sun. Such twinkling is observed routinely in observations of pulsars. (Pulsars are rapidly rotating neutron stars, the remnants of a supernova explosion of a massive star--stars having masses roughly between 8 and 25 or 50 times the mass of our sun). Also, whereas the twinkling of a star is fairly mild, the twinkling of a distant radio source can be violent. A radio source could double in apparent brightness, then fade to essentially zero and remain faint for a considerable amount of time--hours or even days.
A second possible explanation for ET transmission variability is gravitational microlensing. According to Einstein's theory of general relativity, mass can deflect light. Indeed, one of the early triumphs of general relativity was the observation of the deflection of light by the sun's mass. More recently, stars in the inner part of the Milky Way galaxy have been observed to brighten then fade as massive objects--another star, a white dwarf, a neutron star, a black hole, etc.--pass in front of the background stars, temporarily focusing the light from those background stars.