To most of us light is something that we can see. But there is light that cannot be seen. This light is called invisible light.
    Most of us are familiar with ultra-violet light—the light that tans our skin. Ultra-violet light cannot be seen and we are only aware of it because of the effect it has on our skin. Another invisible light that most of us are familiar with is infra-red light.  i guess the most common use for infra-red light is to operate a television set using a remote control. Then there are X-rays. X-rays are another kind of invisible light that are capable of penetrating solid objects.
    We are all familiar with radio waves but I doubt that many of us are aware that they are a form of light—invisible light.
    Another kind of invisible light is the high-energy gamma rays. They are able to pass through most solid objects.
    All light, visible and invisible, has a place on what is called, the electro-magnetic spectrum—each having its own frequencies.
    Various kinds of telescopes have been built to detect visible and invisible light. Large "arrays" of radio telescopes search for radio signals from outer space. There are also telescopes designed to detect X-rays, and others that can detect infra-red and gamma rays coming from different areas of the universe.
    More kinds of invisible light will probably be discovered eventually. If and when they are discoverd engineers will be called upon again to design telescopes that are able to detect them.
    My source for this article was Neil DeGrasse Tyson’s article "Unfit for Vision" which appears in the latest issue of Natural History magazine. Neil DeGrasse Tyson is the director of The Hayden Planetarium at the American Museum of Natural History. He is an astrophysicist.
  Bob

Tyson on light that is "Unfit for Vision"

To most of us light is something that we can see. But there is light that cannot be seen. This light is called invisible light.
    Most of us are familiar with ultra-violet light—the light that tans our skin. Ultra-violet light cannot be seen and we are only aware of it because of the effect it has on our skin. Another invisible light that most of us are familiar with is infra-red light.  i guess the most common use for infra-red light is to operate a television set using a remote control. Then there are X-rays. X-rays are another kind of invisible light that are capable of penetrating solid objects.
    We are all familiar with radio waves but I doubt that many of us are aware that they are a form of light—invisible light.
    Another kind of invisible light is the high-energy gamma rays. They are able to pass through most solid objects.
    All light, visible and invisible, has a place on what is called, the electro-magnetic spectrum—each having its own frequencies.
    Various kinds of telescopes have been built to detect visible and invisible light. Large “arrays” of radio telescopes search for radio signals from outer space. There are also telescopes designed to detect X-rays, and others that can detect infra-red and gamma rays coming from different areas of the universe.
    More kinds of invisible light will probably be discovered eventually. If and when they are discoverd engineers will be called upon again to design telescopes that are able to detect them.
    My source for this article was Neil DeGrasse Tyson’s article “Unfit for Vision” which appears in the latest issue of Natural History magazine. Neil DeGrasse Tyson is the director of The Hayden Planetarium at the American Museum of Natural History. He is an astrophysicist.
  Bob

Hey Jim,

I’m not sure that Bob was after SETI in particular; Tyson’s article was about the electromagnetic spectrum generally. But you raise some interesting issues.

[quote author=“jimmiekeyes”]SETI is assumed by many of those who have heard of it to be looking for messages from outer space. In point of fact it is looking for any electromagnetic emissions of the kind you mention. The sort we are pumping out in huge quantities with our radio and television broadcasts.

A discovery of that sort is far more likely than a message aimed at us. The time travel restriction due to the distances involved makes the prospect of a message quite dim.
Jim

Yes, the problem with this is that our TV broadcasts are comparatively weak. I recall going to a talk many years ago with someone in the SETI program who said that basically our radio telescopes could only hope to pick up equivalent TV/radio traffic from very nearby stars ... (and even then it would be overwhelmed by our own TV/radio “noise” so it would be impossible to hear, unless we could build a radio telescope on the far side of the moon).

So I think our present SETI aim is to assume that the distant civilizations are beaming the messages in our precise direction with some sort of high-powered focused signal, like lasers or focused radio beams.

But is it really so likely that an alien civilization would beam out focused messages towards us? Remember, we have to assume they would be far enough away that our TV/radio traffic would have dissipated before reaching them ... or wouldn’t have had time to reach them at all.

Basically we have to assume they are sending out these beacons in all directions in the hopes of getting a response back.

Doesn’t sound too likely to me ...

:(

Yes Doug,
SETI is listening for a very unlikely event simply put by you in connection with the non-specific radio and television emissions and the specific beamed emissions somehow assumed to be going on somewhere.

Why we assume that is the case is beyond me - we’re not doing it why assume some other society is?

However I understood we (SETI) had time on the best of the radio telescopes so the weak signals might arrive and later be sorted out of the noisy background by software designed for the purpose.

I support the effort nonetheless.
Jim

As I understand it, they aren’t looking in the same part of the electromagnetic spectrum as earth-bound TV and Radio, since those are just too noisy and would swamp the listening apparatus. They are looking around the so-called “hydrogen water hole” I think ...

Anyhow for more see here . Especially this which is very interesting:

“For us to detect an alien civilization 100 light years away that is broadcasting “omnidirectionally”, that is, in all directions, the aliens would have to be using a transmitter power equivalent to several thousand times the entire current power-generating capacity of the entire Earth.”

... so we basically have to assume they aren’t broadcasting omnidirectionally, but rather with a focused beam ...

Similarly, we are broadcasting “omnidirectionally” with TV and Radio. So this implies that our TV/radio broadcasts would only be detectable at well under 100 light years’ distance.

Agree with you, Jim, that SETI is a worthwhile effort. Just wish it was more likely to be successful ...