What's a Simulator?

My students, and students in other labs, frequently do simulation work, but don't always have a clear idea in their heads about the purpose of simulation work. So, for the record, a quick definition:

A simulator is a microscope (letting you see details invisible with ordinary measurements), a telescope (letting you see things far away or at a larger scale than in the real world), a time machine (letting you look into the future), or an X-ray machine (letting you see the insides of things you can't otherwise look into).

But to be valuable, a simulation has to be believable. That means you need to validate it in some way. The ideal case is for equations, experiments, and simulation to all agree, but typically we simulate things because they are are hard to model analytically or to build and test at sufficient scale (in space, time, or dollars). So, a good way to do things is to simulate at small scale, and compare that to equations or experiments. If you can do that, and make a good argument for why the simulator continues to work as you scale up, then confidence in your simulation increases.