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zation； knowing nothing about us； might nevertheless conclude we're listening to。
Hydrogen is by far the most abundant kind of atom in the Universe。 It's distributed in clouds and as diffuse gas throughout interstellar space。 When it acquires energy； it releases some of it by giving off radio waves at a precise frequency of 1420。405751768 megahertz。 （One hertz means the crest and trough of a wave arriving at your detection instrument each second。 So 1420 megahertz means 1。420 billion waves entering your detector every second。 Since the wavelength of light is just the speed of light divided by the frequency of the wave； 1420 megahertz corresponds to a wavelength of 21 centimeters。） Radio astronomers anywhere in the Galaxy will be studying the Universe at 1420 megahertz and can anticipate that other radio astronomers； no matter how different they may look； will do the same。
It's as if someone told you that there's only one station on your home radio set's frequency band； but that no one knows its frequency。 Oh yes； one other thing： Your set's frequency dial； kith its thin marker you adjust by turning a knob； happens to reach from the Earth to the Moon。 To search systematically through this vast radio spectrum； patiently turning the knob； is going to be very time…consuming。 Your problem is to set the dial correctly from the beginning； to choose the right frequency。 If you can correctly guess what frequencies that extraterrestrials are broadcasting to us on—the 〃magic〃 frequencies—then you can save yourself much time and trouble。 These are the sorts of reasons that we first listened； as Drake did； at frequencies near 1420 megahertz； the hydrogen 〃magic〃 frequency。
Horowitz and I have published detailed results from five years of full…time searching with Project META and two years of follow…up。 We can't report that we found a signal from alien beings。 But we did find something puzzling； something that for me in quiet moments； every now and then； raises goose bumps：
Of course； there's a background level of radio noise from Earth—radio and television stations； aircraft； portable telephones； nearby and more distant spacecraft。 Also； as with all radio receivers； the longer you wait； the more likely it is that there'll be some random fluctuation in the electronics so strong that it generates a spurious signal。 So we ignore anything that isn't much louder than the background。
Any strong narrow…band signal that remains in a single channel we take very seriously。 As it logs in the data； META automatically tells the human operators to pay attention to certain signals。 Over five years we made some 60 trillion observations at various frequencies； while examining the entire accessible sky。 A few dozen signals survive the culling。 These are subjected to further scrutiny； and almost all of them are rejected…for example； because an error has been found by fault…detection microprocessors that examine the signal…detection microprocessors。
What's left—the strongest candidate signals after three surveys of the sky—are 11 〃events。〃 They satisfy all but one of our criteria for a genuine alien signal。 But the one failed criterion is supremely important： Verifiability。 We've never been able to find any of them again。 We look back at that part of the sky three minutes later and there's nothing there。 We look again the following day： nothing。 Examine it a year later； or seven years later； and still there's nothing。
It seems unlikely that every signal we get from alien civilizations would turn itself off a couple of minutes after we begin listening； and never repeat。 （How would they know we're paying attention？） But； just possibly； this is the effect of twinkling。 Stars twinkle because parcels of turbulent air are moving across the line of sight between the star and us。 Sometimes these air parcels act as a lens and cause the light rays from a given star to converge a little； making it momentarily brighter。 Similarly； astronomical radio sources may also twinkle—owing to clouds of electrically charged （or 〃ionized〃） gas in the 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； n