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Quote by: The Second Law  @Auto: Regarding #1, why forty light years? Where did that number come from? |
The first radio transmission was made by Marconi in 1901. Radio was not widely used, however, until after 1920, when the first commercial radio station (KDKA) was opened in Pittsburgh, Pennsylvania.
I'm assuming that an advanced civilization would be able to detect signals broadcast from 1920 onwards (at the most). The wavefront of Earth's manmade radio broadcasts, from that point on, form a sphere about 86 light-years in radius and expanding outward at the speed of light. So a sufficiently advanced species would have to be within that area of space in order to detect any signals from Earth.
Now, let's assume that there is such a species at the edge of that sphere. They have begun to intercept our radio broadcasts. If they cannot exceed the speed of light, then any reply of theirs would take at least ~86 years to get here. (By "reply" I mean either a return signal or a spacecraft.) As a result, we would not hear from them until around 2092 at the earliest.
On the other hand, let's say that these hypothetical aliens live about half of that distance away, around 43 light-years from Earth. Their reply, then, would take at least 43 years to get here. But also they would have started detecting our broadcasts 43 years ago, in 1963. If they have sent a reply, we should be receiving it any day now. The fact that we have not means that they're not that close or they lack the ability to detect us.
- Rob