[camus_absurd]

A lot of physics experiments are very indirect, and you're not actually measuring the 'thing' you're looking for. As an example, in particle collision experiments, you're often looking at how the resulting collisions behave, and then applying our current understanding and models to reason about why the resulting collisions behave the way they do. Basically, it's like saying, 'This particle X had Y behavior, which is explained by our current model that says there must be a Z interacting with X to produce Y.' This is, of course, an extreme simplification, but it illustrates why we can claim with a high level of confidence that certain 'unmeasurable' things must exist.

[godelski]

  > A lot of physics experiments are very indirect, and you're not actually measuring the 'thing' you're looking for. 

This is actually true for any experiment. Just at a macro scale the proxy is often less complicated. A simple example of this might be measuring how big a piece of paper is. You get a ruler and measure it, right? Your measurement is an approximation of the ruler's measurement, which is an approximation based on the standard meter. I have a bunch of rulers and I can tell you that none of them measure exactly the same. Usually it doesn't matter though because the difference is much smaller than the level of uncertainty and we're usually measuring with enough accuracy that we don't care.

What physicists do is a generalization of this same thing. Usually much more accurately than your paper experiment. But yes, in high energy physics (HEP) you're usually measuring very indirectly and based on theory. This is a big part of the Von Neumann's Elephant thing. Fitting data is easy, explaining it isn't. The casual nature is the critical aspect

[XorNot]

An important way to think about this is what actually happens when you do an experiment: i.e. the measurement is ultimately happening at your eye-ball, and is a chain of linked effects we believe we understand all the way down to the item under test (which is part of the reason metrology itself is fascinating).

[exe34]

the nice thing about particle physics though is that usually the existing theory predicts the numbers to ~18 decimal places.