 And the answer to that is that big things, things like pens and people and pebbles and so forth have a very very small wavelength. It turns out if you do these measurements that the wavelength is inversely proportional to the momentum of the object. So our wavelength is going to be measured in meters. The momentum of our object is going to be measured in kilograms meters per second. And Planck's constant in SI units is absolutely tiny. And so for a classical normal size object that we might expect, we're going to have an absolutely tiny wavelength, which means any kind of interference pattern is going to be too small to see. We would never see those effects. Now you might wonder what happens when that momentum goes really small so you can have something at rest. Well actually it's really hard to have something at rest. Even the tiny thermal fluctuations of a baseball give it enough momentum jiggling to give it a seriously small wavelength. And so in order to see an interference pattern, you have to very carefully control everything about your system. And it helps if it's very very light as well, which is why we can do it fairly easily for photons or for electrons, but it's considerably harder to do for something like a large molecule. So this relationship between the wavelength and the momentum was developed in the mid-1920s by de Bois. And a couple of years later, Erwin Schrodinger developed the full mathematical description of how quantum mechanical systems change in time. And at that point, after a couple of decades of confusion and work trying to explain a rafter phenomena that couldn't be explained without quantum mechanics, quantum mechanics was finally in a form where it was really quantitatively explaining things that people had been trying to figure out for some time.