Cosmic Calendar 15 billion BC Universe began (BIG BANG) 10 billion BC Our galaxy formed 5 billion BC Solar system (sun, earth and other planets) formed 2 million BC Homo sapiens emerged 5000 BC Writing invented 1888 AD Hertz produced radio waves 1903 AD Letter "S" sent by radio waves across Atlantic Ocean by Marconi 1959 AD Cocconi and Morrison proposed SETI 1960 AD First attempt to detect extraterrestrial civilizations by Drake 1979 AD First issue of COSMIC SEARCH |
We note that our advanced technology (even mankind for that matter!) has only been around for a very short time. It is apparent that we are a very young, emerging civilization.
Light, radio or other electromagnetic waves travel 300,000 kilometers per second in empty space. This is the top speed at which anything can travel. We can express astronomical distances using this velocity. |
Distances in light travel time (approx.) Earth to moon 1 second Earth to sun 500 seconds (8 min.) Sun to Mars 12.5 minutes Sun to Jupiter 40 minutes Sun to Pluto 5.5 hours Solar system diameter (at orbit of Pluto) 11 hours Sun to nearest star 4 years Sun to center of galaxy 30,000 years Diameter of galaxy 100,000 years Distance of Andromeda galaxy 2 million years Distance to "edge" of universe 15 billion years |
Thus, the distance to the moon in light travel time is about one second, to the nearest star (other than the sun) about 4 years and to the limit of our universe about 15,000 million years.
The waves (light or radio) from some galaxy 2 million light years distant (same as 2 million years light-travel time) are 2 million years old when they get here. This means we see or observe the galaxy as it was 2 million years ago. Like cosmic archeologists, we are looking back in time 2 million years. The farther out we look the farther back in time we see. Thus, in a very real sense time and distance are closely inter-related. |
Age of universe (T) = 15 billion years Radius of universe (R) = 15 billion light years R = cT where c = velocity of light = 300,000 kilometers per second |
Summary:
Numbers and the SETI Probability Game. How likely is it that there are other civilizations in our galaxy capable of communication? If you consider all of the factors you can write a simple relation, as done by Frank Drake, for estimating the probability. I use the word "estimate" intentionally because our knowledge of most of the factors is so poor that we are really only guessing. Your guesses may be as good (or bad) as the next person's so why not play the probability game and see what number you come up with? The relation, known as Drake's Equation, involves 7 factors as follows: |
|Number of | |Rate | |Fraction| |Number | |Fraction| |civilizations| |of | |of stars| |of | |of | |in our galaxy| |star | |with | |planets | |planets | |capable of | = |formation| X |planets | X |per star| X |on which| |communiaction| |(per | |with | |life | |now | |year) | |suitable| |appears | |environ-| |ment | |Fraction| |Fraction | |Longevity | |of life | |of | |of each | |bearing | |intelli- | |technology | x |planets | X |gent | X |in | |on which| |societies| |communi- | |intelli-| |which | |cative | |gence | |develop | |mode (years)| |emerges | |communi- | |cation | |ability | |
Worked Example: |
|Number of | |1 | |civilizations| |per | |in our galaxy| = |year| X 1/5 X 2 X 1 X 1 X 1/10 X 1000 = 40 |capable of | |communication| |now | |
Suppose you guessed that stars in our galaxy form at the rate of one per year (probably not a bad estimate), that 1/5 of the stars have planets (no one knows), that there are 2 planets with stable environments (a guess), that life appears on each (fraction = 1), that intelligence emerges on each of these (fraction= 1), that 1/10 of these develop communication capability and that these remain in this state for 1000 years. Then, it works out that the number is 40 (as above).
According to this estimate, there would be 40 civilizations in our galaxy capable of communication, or about one per trillion (10^11 or 10 to the eleventh power) cubic light years so that the nearest one might be something like 10,000 light years away. However, if it is in a communicative mode for only 1000 years, as assumed, it will be extinct long before its signals are received at the earth. The longevity time (last factor) is very uncertain. Is it 10 years or one million? In this example we have assumed 1000 years. Now, you play the SETI game by writing down your numbers. Who knows? Your result may be the right one. Summary:
The Wavelength Picture. Even if we believe that there may be other civilizations, in which direction should we look and, if we use radio, on what wavelength (or frequency) should we listen? Let us answer the last part of this question now. The most abundant element in the universe is hydrogen (H). It emits a natural radio signal, called the hydrogen line, at a wavelength of 21 centimeters (or frequency of 1420 megahertz). An advanced extra-terrestrial civilization would certainly be aware of this hydrogen radiation and if they chose to attract attention to themselves with a beacon signal they might choose to use this wavelength or one close to it. Also 21 centimeters is near a minimum of the background noise from the galaxy (and beyond) and is relatively free of absorption by the interstellar medium and by our atmosphere and the probable atmosphere of other planets (see Sky Noise Diagram). Therefore, a beacon should be detectable at a greater distance at 21 centimeters or thereabouts. This is the general line of reasoning used by Cocconi and Morrison in 1959 (see lead article of this issue). There is also natural emission from the hydroxyl (OH) radical at 18 centimeters. Now if you combine hydrogen (H) with the hydroxyl radical (OH) you get water (H2O), so the wavelength region between 18 and 21 centimeters is often referred to as the "waterhole", in further allusion to the fact that galactic civilizations might (like radio amateurs) gather around this "waterhole" to chat like different species of animals gather around an African waterhole to drink. Summary:
Notes: Waterhole wavelengths:
Copyright © 1996-2005 Ohio State University Radio Observatory and North American AstroPhysical Observatory. Originally designed by Point & Click Software, Inc.
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