If signals from extraterrestrial civilizations ever reach Earth, the aliens themselves will more likely be already non-existent.

Attempting to update the so-called Drake equation from 1961, which is our best formula for predicting how many detectable civilizations should exist in the Milky Way galaxy, physicist Claudio Grimaldi made estimates about which parts of the galaxy should be filled with alien signals at a concrete point in time, ScienceNews magazine wrote.

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It was earlier postulated that technologically savvy civilizations are born and die at a regular pace. When a civilization ceases to exist and stops broadcasting, the signals still linger there like "ghosts," which can be still be found in some parts of our galaxy, researchers say.

Most notably, if the civilization lived for less than 100,000 years, which is the time it takes light to cross the galaxy, then Earth stands a really miniscule chance of intercepting signals from that civilization while it's still broadcasting, the researchers found. That makes a reply difficult — they may be dead by the time our signal is returned to them.

Surprisingly, the research group also calculated that the average number of extraterrestrial signals reaching Earth at a given time should match the number of civilizations that are currently emitting signals.