About Measuring in Reflection

Welcome to Measuring in Reflection! Here I will blog about various topics in physics, with a focus on the tools, experimental techniques, and concepts that physicists use to understand the world. Reflections on measurement, so to speak.

There are many excellent blogs about physics for a general audience, but most focus on the subjects of inquiry rather than the methods of inquiry. The subjects of inquiry are very interesting — that’s why we do physics! But the whole point of science is that it’s comprehensible. All too often physics in particular is treated as this mysterious thing beyond the grasp of most people. The goal of this blog is to push back against that tendency.

Different posts will likely end up assuming somewhat different levels of familiarity with concepts in physics, but I intend for this to be a blog that can be read and understood by non-physicists. I also reserve the right to write about other somewhat related subjects, including the culture and practice of science and the uses and abuses of quantitative reasoning applied to the world at large.

About the name: “measuring in reflection,” which I’ll admit is syntactically awkward in everyday English, is itself an experimental technique. The term comes from microwave and optical engineering, but has been adopted by various subfields of physics formulated in the language of quantum optics, and I would argue the basic concept is much more broadly applicable to experimental science.

Measuring in reflection means throwing some kind of “probe” at the system you want to study, and inferring things about the system by observing the probe when it bounces back at you. For example, let’s say you throw a bouncy ball into a dark room. The time elapsed before it bounces back tells you something about the distance to the far wall. The speed at which it bounces back might tell you whether the wall is hard or padded with some kind of squishy material capable of absorbing much of the ball’s kinetic energy. If the ball comes back coated in wet paint, you will probably conclude that the wall has been freshly painted, and perhaps also that you’ve ruined a perfectly good shirt. In the context of the experiments in my field, a typical system is a resonant electrical circuit, and the probe is a small oscillating voltage applied to the end of a cable connected to that circuit.

Measuring in reflection can also be used in a broader sense to describe a process where you throw something at the system and see what else comes back out at you (hopefully not an irate human being you just hit in the head with a bouncy ball). It is distinct from measuring in transmission, which means observing what happens to a probe that goes out “the other side” of the system (whatever “the other side” means in the context of a particular experiment). Really the only other thing you can do is study what comes out of the system when you don’t perturb it.

To stretch the concept perhaps slightly too far, I’ve repeatedly flung myself at experimental physics for about a decade now, and hopefully learned something each time I bounced back. Now I’d like to share those things with you.