We've had a long-running series here at UT on potential solutions Fermi paradox—why aren't we able to detect any alien life out there in the Universe?
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But more possible solutions are being developed all the time. Now, another paper adds some additional theory to one of the more popular solutions—that aliens are just too busy to care about us.
The paper, released on arXiv, was written by Amri Wandel of the Racah Institute of Physics at the Hebrew University of Jerusalem. It makes two basic assumptions. First, aliens don't really care about planets with life on them. Second, they would care if they could detect intelligent life on one.
"A new solution to the Fermi Paradox is presented: probes or visits from putative alien civilizations have a very low probability until a civilization reaches a certain age called the Contact Era after the onset of radio communications.
"If biotic planets are common, putative advanced civilizations may preferentially send probes to planets with technosignatures, such as radio broadcastings.
"The contact probability is defined as the chance to find a nearby civilization located close enough so that it could have detected the earliest radio emissions (the radiosphere) and sent a probe that would reach the Solar System at present.
"It is found that the current contact probability for Earth is very low unless civilizations are extremely abundant.
"Since the radiosphere expands with time, so does the contact probability.
"The Contact Era is defined as the time (since the onset of radio transmissions) at which the contact probability becomes of order unity.
"At that time alien probes (or messages) become more likely. Unless civilizations are highly abundant, the Contact Era is shown to be of the order of a few hundred to a few thousand years and may be applied not only to physical probes but also to transmissions (i.e. SETI).
"Consequently, it is shown that civilizations are unlikely to be able to inter-communicate unless their communicative lifetime is at least a few thousand years." ■