Here is a little more info:
We are quite aware just how far it is "out there". With the speed of light being the fundamental limit for baryonic matter, it will be next to impossible to travel between the stars (at least under our current level of physics knowledge). So the universe may be populated with little isolated bits of intelligence all wondering if any other species are out there.
Just in the past few years we have advanced far enough in our technical prowess to both "announce" to the universe we are here (radio waves) and to receive the same from another species. With that in mind and the speed of light being a constant in a vacuum, the expanding sphere of radio noise heralding our presence has only gone about 60 light years or so. So there may be an entire galactic community out there but our "knock" hasn't yet hit the door so to speak.
(Note: Astronomers use another term for stellar distance that may be not so familiar called the Parsec. A Parsec (parallax-arcsecond) is the distance needed for one astronomical unit (AU) to subtend one second of arc. However for this discussion I will revert to the more familiar “Light Year”)
This also applies in the opposite direction. If there was a radio producing species only 70 light years away and they have had radio only for 50 years, we would have no possible way of detecting them for another 20 years. Expand that out to approximately 100,000 light years (diameter of just our little galaxy) and you begin to see the problem.
While SETI is indeed a long shot, we do have one example of a species that sends signals out into interstellar space: ourselves. This means that intelligent life in the universe is possible and proven. Further, it is possibly detectable if that intelligence uses any form of EM radiation to communicate as we have for years. It is therefore not without merit.
Why EM and not some other means I hear you ask?
There appears to be only four fundamental forces in all of nature; Strong Force, Weak Force, Gravity, and Electromagnetism EM. Both the strong force and Weak force are confined to the nucleus of the atom. Gravity requires prodigious amounts of energy to manipulate, so the only one that appears practical for long distance communication is EM. In an extremely short period of time, we are using EM across the entire spectrum from basically DC to light.
I personally believe for a race to become technologically advanced, it must eventually realize the need for the ability to store and convey information over long distances. Since radio waves (I am including any EM in this such as RADAR, TV, microwave, etc.) are still the best method for accomplishing this, any other race would use/do the same. For about the past 60 years we have been isotropically radiating EM across a huge RF spectrum into outer space. What many SETI systems are looking for is another species that is doing the exact same thing we are; unintentional radiation of EM into outer space.
Even the nearest star (Proxima Centauri), actually a member of a ternary star system, is 4.22 light years away. This is why radio and/or optics (I know some people who are contemplating optical SETI) are the best and at the moment the only way to find other civilizations. The advancing sphere of radio noise radiating from our own planet now encompasses an area more than 120 light years across. There are literally thousands of stars within that volume of space. As we measure the heavens with more precise instrumentation we are finding a plethora of planets orbiting other stars.
A few years ago there were many people who were quite skeptical about the possibility of extra solar planetary systems. Statements were made like "just because there is one known planetary system (ours), it doesn’t mean there are others". It appears now that planetary systems are the norm instead of the exception. I am now hearing the same argument against the possibility of ET. I personally do not "believe" ET is out there. I suspect ET is out there. We are finding the building blocks of life through out the heavens. I also think life is far more tenacious than most people believe. We even find life in the sulfur volcanic vents on this planet.
The other question that periodically comes up is “what about von Neumann probes”?
Two things. First, there may me thousands of races out there, but they are "stuck" just like we are by general relativity. Sigh! Second, the prodigious amount of time it would take to cross the galaxy at sub light speed would possibly prohibit even the most ambitious race from basically spending the money and wasting the precious resources of a local solar system to attempt that feat. Also how long can a machine function and replicate without error entering the picture. Not unlike evolution. :)
And just where in the heck do we look and at what frequency?
Lets start with a bit of a background:
Radio astronomers use temperature to describe the strength of detected radiation. Any body with a temperature above -273 deg C (approximately absolute 0) emits electromagnetic radiation (EM). This thermal radiation isn’t just in the infrared but is exhibited across the entire electromagnetic spectrum. (Note: it will have a greater intensity (peak) at a specific area of the EM spectrum depending on its temperature). For example, bodies at 2000 K (Kelvin), the radiation is primarily in the infrared region and at 10000 K, the radiation is primarily in the visible light region. There is also a direct correlation between temperature and the amount of energy emitted, which is described by Planck’s law.
When the temperature of a body is lowered, two things happen. First, the peak shifts in the direction towards the longer wavelengths and second, it emits less radiation at all wavelengths.
This turns out to be extremely useful. When a radio astronomer looks at a particular point of the sky and says that it has a noise temperature of 1500 K, he/she isn’t declaring how hot the body (nebulae, etc) really is, but is providing a measurement of the strength of the radiation from the source at the observed frequency. For example, radiation from an extra solar body may be heated from a nearby source such as a star. If this body is radiating at a temperature of 500 K, it exhibits the same emissions across all frequencies that a local test source does. The calculated noise figure will be the same across all frequencies. (Note: this does not take into account other sources of radiation such as synchrotron radiation).
So, here’s the rub. Not only does the source that is of interest to the radio astronomer emit thermal radiation but also both the local environment (ground, atmosphere, etc) and the equipment (antenna, amplifiers, cables, receiver, etc) being used to make the measurements. To accurately observe and measure the distant sources, the radio astronomer must subtract all of the local environment and detection equipment noise additions.
In 1963, Arno Penzias and Robert Wilson were working with a horn antenna trying to make it work with as high efficiency as possible for the Telstar project. This antenna was also going to be used for radio astronomy at a later date. They pointed it to a quiet part of the sky and took measurements. When they subtracted all of the known sources of noise, they found approximately 3 K left over. They worked very diligently to eliminate/describe this noise source and were unable to. This mysterious source of noise seemed to be there no matter where they pointed the antenna. What they had discovered was the microwave background produced from the Big Bang. This 3 (closer to 2.7) K microwave background originated approximately 300,000 years after the Big Bang itself had occurred. It has been determined that when these signals originated, the universe had already cooled down to around 3000 K.
So are frequencies radio astronomers use affective for SETI?
There are two real sources of noise that limits the radio astronomer's ability to search for very weak signals. The galactic noise halo interferes with us below 1 GHz and noise due to earth's atmosphere interferes with us above about 10 GHz. This pretty much keeps all SETI searches (at least radio ones) between 1 and 10 GHz. Inside these two frequencies, from about 1.4 to 7 GHz the noise level drops off even further to near the 2.7 Kelvin Cosmic Microwave Background (CMB) that permeates all space. Hydrogen (H) molecules, the most abundant element in the universe, excite and emit (masers) at around the 1.4 GHz frequency (21 cm band) and the hydroxyl (OH) emits at around 1.65 GHz. This is where much of our radio astronomy and SETI research is concentrated. Since H + OH is water, the frequency gap between these two is often called the “Water Hole”.
This is one of the coveted frequencies of radio astronomers; thusly we have international treaties to not broadcast at these frequencies since they would interfere with radio astronomers. So here we are looking for signs of a narrowband signal heralding the fact that intelligent life is not wholly constrained to our tiny little planet at these very frequencies.
Just imagine another tool building species that ends up developing radio and radio astronomy that may also recognizes the importance of this 21cm band. And they also may instigate a SETI search using these same bands such as we do. So here is the question. Would they hear us at those frequencies? They are the very ones that we are not transmitting on at all. I could just see 500 races all looking for each other at the very frequency band that none of them are transmitting on due to the very nature of its importance for the exploration of the universe.
BTTT
The "Flying Spaghetti Monster" knows all; sees all!
"Thou shalt have no main course before Him!"
"In His Noodly Appendage we trust!"
The Flying Spaghetti Monster emits and receives on all wavelegths! All sentient transmission in the Universe are known to Him via his Very Large Noodly Appendage Array. His Harmonic Semolina Oscillations are embedded in every EM signal in the Universe! He is omnipresent!
An entire solar system, most of it not really inhabitable, contains a prodigious amount of resources. The star at it's heart can supply massive amounts of energy too.
Just imagine another tool building species that ends up developing radio and radio astronomy that may also recognizes the importance of this 21cm band. And they also may instigate a SETI search using these same bands such as we do. So here is the question. Would they hear us at those frequencies? They are the very ones that we are not transmitting on at all. I could just see 500 races all looking for each other at the very frequency band that none of them are transmitting on due to the very nature of its importance for the exploration of the universe.
That's part of the reason I think we need to look at additional frequencies to try to detect any "leakage" from their normal communications channels. That is more challenging of course, since they may not be using, probably aren't in fact, nice narrow band waveforms.
But, as you say, if everyone is listening, and no one is transmitting, at least on the "water hole" band, then it's going to be bloody hard to hear find other. :)
Hi, RA! Thanks for the comments. I've been offline and just got back on this morning... Will have some time tonight, I hope, and will return to read everyone's comments and follow up.
Very nice tutorial, RA, thanks!