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Ask Dr. SETI ®

Chapter 3: Philosophy

Why Not Use Light?

Dear Dr. SETI:
Why does SETI use radio signals instead of light signals to search/communicate? To me, light is the most obvious source for navigation, communication, and other purposes. Does our species have different sciences from the extraterrestrials?

Mark B., via email

The Doctor Responds:
That is an excellent question, Mark. Before I answer it directly, let's clarify some terminology.

When you say "light" you probably mean visible light, electromagnetic radiation at wavelengths to which our human eyes are sensitive. But, to the physicist, "light" includes all electromagnetic waves (since they all travel at the same speed, and follow the same laws of nature). Most such "light" is invisible to our human eyes. We see only a very narrow sliver of the spectrum, extending from red to violet (in terms of wavelength, that's 750 to 400 nm; expressed as frequencies, we see 400 to 750 THz). Of course, we have no way of knowing the range to which alien eyes are adapted, if indeed they have eyes at all.

A number of different types of electromagnetic radiation are included in the category of light to which our human eyes are not sensitive. Radio waves, microwaves, X-rays, gamma rays, infra-red and ultra-violet rays are all light that is "invisible" to our eyes (but not to our instruments). And all can travel through interstellar space.

So, which is best for the SETI enterprise? Those "colors" of light that technology can generate at high power levels, which travel relatively unimpeded through the interstellar medium (and through planetary atmospheres), and for which little or no natural interference exists. A difficult but vital task for SETI scientists is to select the regions of the electromagnetic spectrum in which to concentrate their efforts.

"Searching for Interstellar Communications" was the very first modern SETI article, published in Nature magazine in 1959. In it, Professors Phil Morrison and Giuseppi Cocconi contemplated that very question. Their research at the time was focused on gamma rays, so that's where they started. The two scholars quickly realized that another form of light, in the microwave part of the radio spectrum, was much more likely to succeed. We on Earth could already generate prodigious amounts of microwave radiation. We already had very sensitive microwave receivers (our early radio telescopes). The sky was quiet at these frequencies. And we were already studying the Universe in the microwaves, in search of natural astrophysical phenomena, so it made sense to search the same space for artificial signals as well. We still do much of our SETI research in the part of the invisible light spectrum that Cocconi and Morrison suggested.

But, we don't stop there. Today, we have technology to generate visible and infra-red light at very high power levels indeed (using lasers, something that hadn't even been invented yet when SETI science was born). And, we have sensitive receivers for detecting laser flashes. So, we now practice optical SETI, looking for what you call "light." In other words, we are following your worthy suggestion!

Are there other kinds of "light" we should be exploiting in the SETI enterprise, perhaps as yet undiscovered on Earth? Probably. Prof. Morrison used to talk about "zeta waves," mysterious communications media that we have not yet discovered on Earth, but that extraterrestrial species exploit. You can be sure that, once we discover them, we will certainly start conducting zeta wave SETI! Until then, we use what we have.

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