Why does the SETI project search for radio signals? (Intermediate)

How does sampling many single narrow band frequencies in a radio spectrum not used on Earth (except for some radar) help in the search for life? Why does SETI ignore Earth band signals which are the most likely to be used by an alien culture for the same reason we use those signals? UHF, VHF signal sources use their frequencies not because of luck but because they present the most signal capacity for the buck. Why does it make the assumption that some altruistic taxpayer on planet X is going to spend tons of money for unknown years to send a useless signal into space? We send huge signals into space by TV and radar at no extra expense to the taxpayer.

First, even if a civilization did develop technology the same way we did, it would probably change technologies rapidly. Radiating large amounts of power (i.e. something remotely detectable) into space is very wasteful, so over time civilizations like ours would probably try to find methods that don't involve broadcasting signals everywhere. Another astronomer here, Shami Chatterjee, uses TV signals as an example of this. Broadcast TV was going strong from the 60s until recently, but now everything is cable, which can't be detected from a distance. So to detect our TV transmissions, aliens would have to be specifically looking our way in the right 40 year time period, or they would pretty much miss our "strong tv signal" period of evolution. There are millions of stars for us to look at, and so searching for another civilization's random broadcasts doesn't seem too efficient. This random radiation really isn't "free" to a civilization either, it's part of the cost of the service.

Also, higher frequency radiation doesn't travel well for long distances. It's easily scattered by small particles, for example interstellar dust. The SETI people look at long wavelength radio signals for a couple reasons. First, radio waves travel straight through the large amounts of dust in our galaxy, and second, hydrogen (the most abundant element) radiates at around 1420 MHz. Any technologically advanced civilization that wants to study the galaxy will be looking at this wavelength and understand its importance, so if you want to contact another civilization, sending signals at this wavelength could attract attention. (Or you could send signals at a multiple of this wavelength.) Cell phone signals from across the galaxy would never make it to us, plus, once again, they might only be using this technology for a very short time. What's economical changes very quickly!

Another problem is that signals have to be strong for us to detect them. Most emissions from Earth (the exception is some radar transmissions) would not be detectable with our current systems at the distance of the nearest star. And there is a lot of "noise" from signals on Earth which makes it difficult to detect faint signals. SETI routinly finds "signals" that are produced on Earth or in Earth orbit. In fact, there are only a few frequency bands dedicated to any type of astronomy, and outside these frequency ranges (sometimes even inside them) there is so much interference from our communications that detecting faint signals is impossible.

Finally, to address the "taxpayer" issue, I'd first like to point out that taxpayers are not currently paying for SETI! This funding was eliminated many years ago, and the enterprise is now funded by private donors. And we on Earth have not tried very hard to send signals out into space. You are right that it's entirely possible that civilizations might decide not to waste power and energy in an effort to contact others. However, a dedicated broadcast will be the easiest thing to pick up! As technology develops we will probably have a better chance of detecting random signals. And someday if we are able to closely view extrasolar planets we might be able to tell if they have excess emission at parts of the spectrum, but for now people are trying to do the smartest thing that technology and money will allow. And there's also no reason to believe that there isn't a civilization that would want to try to contact others across the galaxy.

This page last updated June 27, 2015.

About the Author

Lynn Carter

Lynn uses radar astronomy to study the planets, especially Venus. She got her PhD in Astronomy from Cornell in Summer 2004 and is now working at the Smithsonian in Washington D.C. on the Mars Express radar.

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