Frank Drake in 1961 tackled a most puzzling question:
Does there exists any intelligent extra-terrestrial life in our Milky Way galaxy?
Drake came up with a method to estimate how many alien civilizations can actually communicate with us. It is known as the Drake Equation and it multiplies together 7 numbers. Each number is denoted by a symbol.
Multiply all these numbers together, you will have a prediction of the number of alien civilization in our galaxy that can communicate with us.
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Try putting in some numbers in the Drake Equation Calculator and see what you predict for the number of alien civilizations.