[ print ]

Senior Astronomer, SETI Institute; Author, Confessions of an Alien Hunter
The Danger From Aliens

The recent reset of the long-count Maya calendar didn't end the world. But there are serious scientists who worry that Armageddon could soon be headed our way, although from a different quarter—an attack by malevolent, extraterrestrial beings.

The concern is that future radio broadcasts to the stars, intended to put us in touch with putative aliens, might carelessly betray our presence to a warlike society, and jeopardize the safety of Earth. The well-known physicist Stephen Hawking has weighed in on this dreadful possibility, suggesting that we should be careful about sending signals that could trigger an aggressive reaction from some highly advanced race of extraterrestrials.

It all sounds like shabby science fiction, but even if the probability of disaster is low, the stakes are high. Consequently, some cautious researchers argue that it's best to play safe and keep our broadcasts to ourselves. Indeed, they urge a world-wide policy of restraint and relative quiet. They would forbid the targeting of other star systems with transmissions of greater intensity than the routine radio and television that inevitably leak off our planet.

That sounds like a harmless precaution, and who would quibble about inexpensive insurance against the possible obliteration of our world. But this is one worry I don't share. Even more, I believe the cure is more deadly than the disease. To fret about the danger of transmissions to the sky is both too late and too little. Worse, it will endlessly hamstring our descendants.

Ever since the Second World War, we've been broadcasting high-frequency signals that can easily penetrate Earth's ionosphere and seep into space. Many are television, FM radio, and radar. And despite the fact that the most intense of these transmissions sport power levels of hundreds of thousands of watts or more, they dwindle to feeble static at distances measured in light-years. Detecting them requires a very sensitive receiving setup.

As an example of the difficulty, consider an alien society that wields an antenna comparable to the Arecibo telescope in Puerto Rico—a thousand feet in diameter, and the largest single-element radio telescope on Earth. This antenna would be unable to pick up our television broadcasts even from the distance of Alpha Centauri which, at 4.4 light-years, is our nearest stellar neighbor. And frankly, it's improbable that we have cosmic confreres this close, or even 10 or 20 times farther. Astronomers such as Frank Drake and the late Carl Sagan have estimated that the nearest Klingons (or whatever their species) are at least a few hundred light-years away. Our leakage signals—when they eventually reach that far—will be orders of magnitude weaker than any our best antennas could detect.

Such arguments might appear to justify the suggestion by the self-appointed defenders of Earth that we need not fear our current broadcasts. They will be undetectably weak. But they claim that we should concern ourselves with deliberate, highly targeted (and therefore highly intense) transmissions. We can continue to enjoy our sitcoms and shopping channels, but we should forbid anyone from shouting in the galactic jungle.

There's a serious flaw in this apparently plausible reasoning. Any society able to do us harm from the depths of space is not at our technological level. We can confidently assume that a culture able to project force to someone else's star system is at least several centuries in advance of us. This statement is independent of whether you believe that such sophisticated beings would be interested in wreaking havoc and destruction. We speak only of capability, not motivation.

Therefore, it's clearly reasonable to expect that any such advanced beings, fitted out for interstellar warfare, will have antenna systems far larger than our own. In the second half of the twentieth century, the biggest of the antennas constructed by earthly radio astronomers increased by a factor of ten thousand in collecting area. It hardly strains credulity to assume that Klingons hundreds or thousands of years further down the technological road will possess equipment fully adequate to pick up our leakage. Consequently, the signals that we send willy-nilly into the cosmos—most especially our strongest radars—are hardly guaranteed to be "safe."

There's more. Von Eshleman, a Stanford University engineer, pointed out decades ago that by using a star as a gravitational lens you can achieve the ultimate in telescope technology. This idea is a straightforward application of Einstein's theory of General Relativity, which predicts that mass will bend space and affect the path of light beams. The prediction is both true and useful: Gravitational lensing has become a favored technique for astronomers who study extremely distant galaxies and dark matter.

However, there's an aspect of this lensing effect that's relevant to interstellar communication: Imagine putting a telescope, radio or optical, onto a rocket and sending it to the Sun's gravitational focus—roughly twenty times the distance of Pluto. When aimed back at the Sun, the telescope's sensitivity will be increased by thousands or millions of times, depending on wavelength. Such an instrument would be capable of detecting even low-power signals (far weaker than your local top-forty FM station) from a thousand light-years. At the wavelengths of visible light, this setup would be able to find the street lighting of New York or Tokyo from a similar remove.

Consequently, it's indisputable that any extraterrestrials with the hardware necessary to engage in interstellar warfare will have the capability to heft telescopes to the comparatively piddling distance of their home star's gravitational focus.

The conclusion is simple: It's too late to worry about alerting the aliens to our presence. That information is already en route at the speed of light, and alien societies only slightly more accomplished than our own will easily notice it. By the twenty-third century, these alerts to our existence will have washed across a million star systems. There's no point in fretting about telling the aliens we're here. The deed's been done, and the letter's in the mail.

But what about a policy to limit our future leakage? What about simply calming the cacophony so we don't continue to blatantly advertise our presence? Maybe our transmissions of the past half-century will somehow sneak by the aliens.

Forget it. Silencing ourselves is both impossible and inadvisable. The prodigious capability of a gravitational lens telescope means that even the sort of low-power transmissions that are ubiquitous in our modern society could be detectable. And would you really want to turn off the radar sets down at the airport, or switch off city streetlights? Forever?

In addition, our near-term future will surely include many technological developments that will unavoidably be visible to other societies. Consider powersats—large arrays of solar cells in orbit around the Earth that could provide us with nearly unlimited energy, sans the noxious emissions or environmental damage. Even in the best cases, such devices would back-scatter hundreds or thousands of watts of radio noise into space. Do we want to forbid such beneficial technologies until the end of time?

Yes, some people are worried about being noticed by other galactic inhabitants – beings that might threaten our lifestyle or even our world. But that's a worry without a practical cure, and the precautions that some urge us to take promise more harm than good. I, for one, have let this worry go.