great near…vacuum between the stars。 We observe this routinely with pulsars。
Imagine a radio signal that's a little below the strength that we could otherwise detect on Earth。 Occasionally the signal will by chance be temporarily focused； amplified； and brought within the detectability range of our radio telescopes。 The interesting thing is that the lifetimes of such brightening； predicted from the physics of the interstellar gas； are a few minutes—and the chance of reacquiring the signal is small。 We should really be pointing steadily at these coordinates in the sky； watching them for months。
Despite the fact that none of these signals repeats； there's an additional fact about them that； every time I think about it； sends a chill down my spine： 8 of the 11 best candidate signals lie in or near the plane of the Milky Way Galaxy。 The five strongest are in the constellations Cassiopeia； Monoceros； Hydra； and two in Sagittarius—in the approximate direction of the center of the Galaxy。 The Milky Way is a flat； wheel…like collection of gas and dust and stars。 Its flatness is why we see it as a band of diffuse light across the night sky。 That's where almost all the stars in our galaxy are。 If our candidate signals really were radio interference from Earth or some undetected glitch in the detection electronics； we shouldn't see them preferentially when we're pointing at the Milky Way。
But maybe we had an especially unlucky and misleading run of statistics。 The probability that this correlation with the galactic plane is due merely to chance is less than half a percent。 Imagine a wall…size map of the sky； ranging from the North Star at the top to the fainter stars toward which the Earth's south pole points at the bottom。 Snaking across this wall map are the irregular boundaries of the Milky Way。 Now suppose that you were blindfolded and asked to throw five darts at random at the map （with much of the southern sky； inaccessible from Massachusetts； declared off limits）。 You'd have to throw the set of five darts more than 200 times before； by accident； you got them to fall as closely within the precincts of the Milky Way as the five strongest META signals did。 Without repeatable signals； though； there's no way we can conclude that we've actually found extraterrestrial intelligence。
Or maybe the events we've found are caused by some new kind of astrophysical phenomenon； something that nobody has thought of yet； by which not civilizations； but stars or gas clouds （or something） that do lie in the plane of the Milky Way emit strong signals in bafflingly narrow frequency bands。
Let's permit ourselves； though； a moment of extravagant speculation。 Let's imagine that all our surviving events are in fact due to radio beacons of other civilizations。 Then we can estimate—from how little time we've spent watching each piece of sky—how many such transmitters there are in the entire Milky Way。 The answer is something approaching a million。 If randomly strewn through space； the nearest of them would be a few hundred light years away； too far for them to have picked up our own TV or radar signals yet。 They would not know for another few centuries that a technical civilization has emerged on Earth。 The Galaxy would be pulsing with life and intelligence； but—unless they're busily exploring huge numbers of obscure star systems—wholly oblivious of what has been happening down here lately。 A few centuries from now； after they do hear from us； things might get very interesting。 Fortunately； we'd have many generations to prepare。
If； on the other hand； none of our candidate signals is an authentic alien radio beacon； then we're forced to the conclusion that very few civilizations are broadcasting； maybe none； at least at our magic frequencies and strongly enough for us to hear：
Consider a civilization like our own； but which dedicated all its available power （about 10 trillion watts） to broadcasting a beacon signal at one of our magic frequencies and to all directions in space。 The META results would then imply that there are no such civilizations out to 25 light…years—a volume that enpasses perhaps a dozen Sun…like stars。 This is not a very stringent limit。 If； in contrast； that civilization were broadcasting directly at our position in space； using an antenna no more advanced than the Arecibo Observatory； then if META has found nothing； it follows that there are no such civilizations anywhere in the Milky Way Galaxy—out of 400 billion stars； not one。 But even assuming they would want to； how would they know to transmit in our direction？
Now consider； at the opposite technological extreme； a very advanced civilization omnidirectionally and extravagantly broadcasting at a power level 10 trillion times greater （1026 watts； the entire energy output of a star like the Sun）。 Then； if the META results are negative； we can conclude not only that there are no such civilizations in the Milky Way； but none out to 70 million light…years—none in M31； the nearest galaxy like our own； none in M33； or the Fornax system； or M81； or the Whirlpool Nebula； or Centaurus A； or the Virgo cluster of galaxies； or the nearest Seyfert galaxies； none among any of the hundred trillion stars in thousands of nearby galaxies。 Stake through its heart or not； the geocentric conceit stirs again。
Of course； it might be a token not of intelligence but of stupidity to pour so much energy into interstellar （and intergalactic） munication。 Perhaps they have good reasons not to hail all ers。 Or perhaps they don't care about civilizations as backward as we are。 But still—not one civilization in a hundred trillion stars broadcasting with such power on such a frequency？ If the META results are negative； we have set an instructive limit—but whether on the abundance of very advanced civilizations or their munications strategy we have no way of knowing。 Even if META has found nothing； a broad middle range remains open—of abundant civilizations； more advanced than we and broadcasting omnidirectionally at magic frequencies。 We would not have heard from them yet。
ON OCTOBER 12； 1992—auspiciously or otherwise the 500th anniversary of the 〃discovery〃 of America by Christopher Columbus—NASA turned on its new SETI program。 At a radio telescope in the Mojave Desert； a search was initiated intended to cover the entire sky systematically—like META； making no guesses about which stars are more likely； but greatly expanding the frequency coverage。 At the Arecibo Observatory； an even more sensitive NASA study began that concentrated on promising nearby star systems。 When fully operational； the NASA searches would have been able to detect much fainter signals than META； and look for kinds of signals that META could not。
The META experience reveals a thicket of background static and radio interference。 Quick reobservation and confirmation of the signal—specially at other； independent radio telescopes—is the key to being sure。 Horowitz and I gave NASA scientists the coordinates of our fleeting and enigmatic events。 Perhaps they would be able to confirm and clarify our results。 The NASA program was also developing new technology； stimulating ideas； and exciting schoolchildren。 In the eyes of many it was well worth the 10 million a year being spent on it。 But almost exactly a year after authorizing it； Congress pulled the plug on NASA's SETI program。 It cost too much； they said。 The post…Cold War U。S。 defense budget is some 30；000 times larger。
The chief argument of the principal opponent of the NASA SETI program—Senator Richard Bryan of Nevada—was this ＇from the Congressional Record for September 22； 1993＇：
So far； the NASA SETI Program has found nothing。 In fact； all the decades of SETI research have found no confirmable signs of extraterrestrial life。
Even with the current NASA version of SETI； I do not think many of its scientists would be willing to guarantee that we are likely to see any tangible results in the ＇foreseeable＇ future 。 。 。
Scientific research rarely； if ever； offers guarantees of success—and I understand that—and the full benefits of such research are often unknown until very late in the process。 And I accept that； as well。
In the case of SETI； however； the chances of success are so remote； and the likely benefits of the program are so limited； that there is little justification for 12 million taxpayer dollars to be expended for this program。
But how； before we have found extraterrestrial intelligence； can we 〃guarantee〃 that we will find it？ How； on the other hand； can we know that the chances of success are 〃remote〃？ And if we find extraterrestrial intelligence； are the benefits really likely to be 〃so limited〃？ As in all great exploratory ventures； we do not know what we will find and we don't know the probability of finding it。 If we did； we would not have to look。
SETI is one of those search programs irritating to those who want well…defined cost/benefit ratios。 Whether ETI can be found； how long it would take to find it； and what it would cost to do so are all unknown。 The benefits might be enormous； but we can't really be sure of that either。 It would of course be foolish to spend a major fraction of the national treasure on such ventures； but I wonder if civilizations cannot be calibrated by whether they pay some attention to trying to solve the great problems。
Despite these setbacks； a dedicated band of scientists and engineers； centered at the SETI Institute in Palo Alto； California； has decided to go ahead； government or no government。 NASA has given them permission to use the equipment already paid for； captains of the electronics industry have donated a few million dollars； at least one appropriate radio telescope is available； and the initial stages of this grandest of all SETI programs is on track。 If it can demonstrate that a useful sky survey is possible without being swamped by background noise—and especially if； as is very likely from the META experience； there are unexplained candidate signals…perhaps Congress will change its mind once more and fund the project。
Meanwhile； Paul Horowitz has e up with a new program—different from META； different from what NASA was doing—called BETA。 BETA stands for 〃Billion…channel ExtraTerrestrial Assay。〃 It bines narrow…band sensitivity； wide frequency coverage； and a clever way to verify signals as they're detected。 If The Planetary Society can find the additional support； this system—much cheaper than the former NASA program—should be on the air soon。
WOULD I LIKE TO BELIEVE that with META we've detected transmissions from other civilizations out there in the dark； sprinkled through the vast Milky Way Galaxy？ You bet。 After decades of wondering and studying this problem； of course I would。 To me； such a discovery would be thrilling。 It would change everything。 We would be hearing from other beings； independently evolved over billions of years； viewing the Universe perhaps very differently； probably much smarter； certainly not human。 How much do they know that we don't？
For me； no signals； no one calling out to us is a depressing prospect。 〃plete silence；〃 said Jean…Jacques Rousseau in a different context； 〃induces melancholy； it is an image of death。〃 But I'm with