Not all PhDs push boundaries of knowledge in quite that way. Often what you do was already known, but no one really figured out how to correctly apply it. Alternatively, it's often combining different things that were already well known, but no one had combined.
It's novel work that makes a PhD. Novel work is distinct from pushing the boundary of knowledge. Often what you do doesn't change what "humanity knows" in any way.
Both my wife and I have PhDs. Neither of us did things that look like the figures there. It's not a good mental model for what all PhDs mean, though it is a good model for some.
I combined fields and reinterpreted a ton of things that had already been done to draw very different fundamental conclusions about what was going on in a particular location. I put out alternative hypotheses for observations that had already been collected. It's not new knowledge at all and I didn't add to what we "know". I just added an additional hypothesis to the set of multiple working hypotheses that will hopefully be tested decades from now.
My wife worked on how to actually apply well-known methods in other fields to our field. Her work was half engineering, half field experiments. Lots of folks had been working with fiber optic strain gauges for decades. However, no one was using them to measure in-situ strain in rock masses yet (which has since become common). The application was broadly "known", but actually doing it and demonstrating that is novel.
It's novel work that makes a PhD. Novel work is distinct from pushing the boundary of knowledge. Often what you do doesn't change what "humanity knows" in any way.
Both my wife and I have PhDs. Neither of us did things that look like the figures there. It's not a good mental model for what all PhDs mean, though it is a good model for some.
I combined fields and reinterpreted a ton of things that had already been done to draw very different fundamental conclusions about what was going on in a particular location. I put out alternative hypotheses for observations that had already been collected. It's not new knowledge at all and I didn't add to what we "know". I just added an additional hypothesis to the set of multiple working hypotheses that will hopefully be tested decades from now.
My wife worked on how to actually apply well-known methods in other fields to our field. Her work was half engineering, half field experiments. Lots of folks had been working with fiber optic strain gauges for decades. However, no one was using them to measure in-situ strain in rock masses yet (which has since become common). The application was broadly "known", but actually doing it and demonstrating that is novel.