# Einstein Brownian Motion Imagine trying to prove that molecules exist in the early 1900s. How would you do it? Ellenberg writes: "Invisible demons are unpredictable; there's no mathematical model for what those rascals will do next. Molecules, on the other hand, are subject to the laws of probability. If a particle is struck by a tiny water molecule moving in a random direction, the particle is moved by the impact to travel a tiny distance in that direction. If there are a trillion such impacts every second, then the pollen moves a small fixed distance in a randomly chosen direction every one-trillionth of a second. What does the pollen do in the long term? That might be predictable, even if the individual impacts can't be seen. Einstein worked out mathematically [from a theoretical vantage point] how particles would behave under a sequence of motions in random directions. This made the molecular theory something that could be tested experimentally!". > Einstein came up with a *model* that made a *prediction*, and that could be compared to actual experimental results. Einstein’s theory demonstrated how Brownian motion offered experimentalists the possibility to prove that molecules existed, despite the fact that molecules themselves were too small to be seen directly. --- Date: 20210821 Links to: [Best Scientific Experiments and Thought](Best%20Scientific%20Experiments%20and%20Thought.md) Tags: #review References: * Shape, Page 83 * [Good article](https://physicsworld.com/a/einsteins-random-walk/)