efyu_lemonardo
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That's not how relativity works.Look up length contraction and consider relativity.
The real reason is the study assumes a huge number of earth like civilizations scattered across the Galaxy, broadcasting simultaneously.
All together their broadcasts would have reached approximately half the Galaxy within 1500 years, according to the estimate.
See: www.gizmodo.com.au/2016/06/new-calculation-shows-well-make-contact-with-aliens-in-about-1500-years/
Edit: I should note that "huge number" is an exaggeration, as the article only assumes several hundred such civilizations over the 1500 year span.