Hypothetically, if one person is contacted/meets extraterrestrial life, they are said to be crazy or lying. Nobody believes them. Even though many people in the past claim to have met aliens, the majority of the world discounts their experiences.
So, the question is, how could the world be convinced of alien life in one event?
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If I was abducted by aliens and given super powers and a giant alien space Cadillac, then I would believe. But in a more serious note I would say that no one would believe until they saw it in person. If someone famous was killed by an alien and a HD quality video was released to the public no one would believe. They would say that some sick person made the video and that everyone who believed it was retarded. Or it would be a government conspiracy, etc. It would probably take an alien literally being seen in person by everyone on earth. And they would have to look completely different from us or someone would just say that they were mutated humans. People are cynical. Especially of something that... Well, alien.