When you're having fun, time flies. Waiting in a traffic jam, not so much. Your birthday was last month, and your mortgage payment is due in a few days. The fact that we perceive time is certainly no illusion. But is it really there, or is it something we invented?

Early on in human history, we decided to start measuring the days and weeks, and eventually hours, minutes and seconds. Time was useful in organizing society, planting crops and getting ready for dates. Things worked extremely well until scientists started muddling it all up.

In the 17th century, English scientist Isaac Newton was pretty sure time existed as a universal constant. But in 1908, Hermann Minkowski, expanding on one of Einstein's ideas on the relationship between space and time, suggested a space-time continuum. This theory held that space and time were inextricably mixed, with all events occurring along the same timeline. Einstein presented his theory of general relativity not long after this and proposed that time is but an illusion.

Around the same time (if you believe in time, that is), the field of quantum mechanics grew out of an effort to explain the relationship between matter and energy. This presented a little problem for scientists trying to create a single, unified theory to account for the universe and its component parts. Quantum mechanics requires the existence of time to work. General relativity does not. List of all episodes here: Through The Wormhole.