There is no now

While some things about computers are "virtual," they still must operate in the physical world and cannot ignore the challenges of that world. Rear Admiral Grace Hopper (one of the most important pioneers in our field, whose achievements include creating the first compiler) used to illustrate this point by giving each of her students a piece of wire 11.8 inches long, the maximum distance that electricity can travel in one nanosecond. This physical representation of the relationship between information, time, and distance served as a tool for explaining why signals (like my metaphorical sign above) must always and unavoidably take time to arrive at their destinations. Given these delays, it can be difficult to reason about exactly what "now" means in computer systems.