The limits these people broke were those of established knowledge. You don't have to put your life in danger to challenge authority. Just sustain a theory not in line with the mainstream, and you are going to face strong opposition from the ones who have been preaching the official lines for years and feel their authority shaken.
Let's take Einstein's case. He wasn't even a professor. He didn't start from an academic strong point. When he published his first papers on his relativity theory, he was working as a clerk in a patent office in Switzerland. Before anyone acknowledged his view, he found in front of him a wall of opposition from all the physicists who wanted to stick to Newton's officially accepted theories. Isn't this challenging authority?
True, it isn't always that dramatic. Sometimes you have just to defy common knowledge to come up with your solutions. Archimedes wasn't challenging any authorities, but Galileo definitely was. He was very well aware that, by defending his theory of earth motion, he was contradicting the Church official line, and as such he could be charged with blasphemy and heresy. He was tried for that, actually, and he had to make a public denial of his theory, under threat of death penalty. The important thing to note here was that many scientists of his period didn't even take into account the possibility of contradicting the Bible. Galileo's first challenge wasn't against the Church but against fear of challenging what in those days was believed to be common sense.
Anyway, I may have used too strong words to explain my point, but the bottom line is that scientific curiosity is going to challenge something established, be it a tangible authority or a widespread belief. The rule being broken can be as strong as a prohibition to say something contrary to the government or as trivial as going against common sense.
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