Just a thought experiment, let’s say the earth is flat, looks like many professionals would know, including
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Pilots/ATC/sea captain
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Land surveyor and civil engineers
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Astronomers and Geologists, at least the one dealing with experimental data
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Militaries, at least radar operator and artillery
Just these categories are already a lot of persons, I assume I miss a lot of others. So how many person does it make in total ? 10 % of the population ?
This is a bit extreme, but maintaining a conspiracy with lots of “insiders” isn’t impossible. Just think of how many people worked on the Manhattan project.
First you compartmentalize, so that only a core few know the whole picture. Then you threaten everyone involved with serious jail time for informing. (This is why we know Bob Lazar is full of shit, where he telling the truth, he’d be in prison)
It’s true that secrets can’t be kept forever, but they can be kept for a while.
The flaw in flat earth is not the number of conspirators, but the purpose of the conspiracy. It all falls apart when we ask why the government wants to keep flat earth from the public.
To sell more globes, obviously.
Manhattan leaked before they even started work.
From memory, a sci-fi magazine seller figured it out, just based on address changes. He even figured out it was likely an attempt to make an atomic bomb. The USSR also had it completely compromised. The first soviet bomb was basically a direct copy of the American one.
That was also with most/all the workers in 1 location. They also had a very good reason to maintain security (grunts on the ground, not just leadership).