Square them first 3 prime numbers
1x4x9
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Book 2 of the Three Body Problem trilogy is called The Dark Forest, and it offers a solid reason we have heard no other signs of intelligence elsewhere.
Goes like this. If a presumption exists that there are other civilizations in communications distance then you are faced with The Dark Forest.
In a dark forest, you are sitting there and you hear a sound. Do you rush towards it with open arms? No. You wonder if it is a predator. You shut the hell up.
The next step is you don’t even have to hear that sound. You reasonably presume anyone nearby could be a predator and you shut the hell up . . . pre-emptively.
The theory goes a step further. If you hear that sound, and chose to shut up, then centuries pass and at some point in those centuries a leader arises who says, . . . that sound is a threat that we don’t have to endure. Let’s try to force their sun to go nova or something. Whatever the approach, the safest, wisest course, is erase that potential threat. Over centuries, filled with people who are not inclined to do that, will have one or two who are, and it only takes one.
And so . . . shut the hell up.
If one was a prime number than it would be the only prime number.