What do we do if SETI is successful?

As a pragmatic opportunist

- Setup a massive array of antennas in space for reception only

- Try to decode their radio traffic and understand how they are exchanging information

- Steal their their knowledge and use it to advance human race forward.

- Reduce all our electromagnetic emissions to minimum to deny them the same advantage. Forbid anyone from sending signal towards them so we have time to technologically catch up to them without them noticing.

Any kind of contact will ends up in abysmal disaster as we have seen in the past, when advanced civilization shown up on shores of less advanced one.

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We could give them everything what we know and they could give us back a relativistic kill missile. No reason to try to conquer a planet if you can just extinguish a protentional threat, which luckily was naive to be useful before extinguishing.