February 2, 2005
Limitations of Radio Searches for Extraterrestrials
In Part Three in the series on stellar and terrestrial evolution, Neil deGrasse Tyson, Director of the Hayden Planetarium and host of the PBS/NOVA Series "Origins", discusses the limits of radio searches for extraterrestrial life.
Astrobiology Magazine -- Neil deGrasse Tyson is the Director of the Hayden Planetarium at the American Museum of Natural History in New York, and also a Visiting Research Scientist at Princeton University's Department of Astrophysics. He writes a monthly column called "Universe" for Natural History magazine, and is the author of several books, including "One Universe: At Home in the Cosmos" and "The Sky is Not the Limit: Adventures in an Urban Environment".
In this interview with Astrobiology Magazine editor Leslie Mullen , Tyson discusses the limits of radio searches for alien life.
Astrobiology Magazine (AM): You say in the Origins companion book that the Earth's radio emission is now comparable to or stronger than the sun's. So for aliens looking in the radio frequency, we should be the brightest spot in the solar system. Do you think this indicates that SETI is futile - that we are bright enough in radio that we should be a beacon to any aliens looking in our direction, and they should have contacted us by now?
Neil deGrasse Tyson (NT): We're broadcasting in very specific frequencies. So if you have a tunable receiver on another planet, we would be the loudest source in any of those various frequencies. But all collected together, the sun is a much brighter source than the Earth.
It depends on how you measure. There's a total energy, and then there's what we call a specific intensity, which is in a given band. And in any given band, Earth dominates.
AM: So for the aliens, as they look toward us, the sun would still be brighter to them because they're not looking in a particular band?
NT: Well, we're looking in particular bands. That's what SETI is. So why wouldn't they?
AM: Ok, well if we're so bright in every band, doesn't that argue that if they are trying to contact us, they should have by now?
NT: Possibly, however the frontier of those broadcasts is only 60 or 70 light years away. Most stars are farther away than that. So if they are looking in our direction, we would be radio quiet until our radio broadcasts turn on in time for them.
AM: What do you think of the idea that more advanced civilizations will be radio silent, just as we are becoming more radio silent due to satellite transmission and fiber optics and that sort of thing?
NT: That's correct. That's a scary, very realistic notion. Not only that, but our TV waves aren't escaping Earth anymore because a growing number are receiving their signals via cable. So the total broadcast universe is shrinking.
Also, if you communicate in a scrambled way, if you send a radio signal that is decrypted, the more successfully encrypted it is, the less noticeable it is, the less it rises above the background noise. And so if a civilization is so advanced that it encrypts all of its radio transmissions, then while we were observing that civilization, it would seem to just shut itself off.
AM: I had this idea that maybe they heard all our radio noise, but they said, "Oh, they're still back there in the radio age, we've got to wait awhile before we contact them."
NT: Yeah! They're just waiting for us to get more advanced before they even deign to have a conversation with us.
There's a lot of interesting speculation that you can come up with about why they haven't contacted us. But it presupposes that they exist at the same time that we do. Most of the time that anyone would have contacted Earth, we wouldn't have had the technology to know we were being contacted.
Suppose ancient Rome received radio signals from space. They would've concluded there was no intelligent life on Earth. The technology has just come so late in the history of civilization.
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