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Let me see, our nearest star system is Alpha Centauri. That's 4.5 light years away. So if they could travel at the speed of light it would take them 4.5 years to get here. However we're pretty sure there's no radio activity in that system, hence no space traveling inhabitants, so they would have to come from even further away. So more years to get here, and that's if they can travel at 300,000mps. We can only do a fraction of LS. I reckon the numbers are just too great to even consider that we have been visited.
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