Reading the “Time Frames” comic about depicting time in comics made me think about Brian Greene’s The Fabric of the Cosmos in which he uses several metaphors and visualizations to help explain the nature of this thing we call time. We tend to think we know what “now” is, and think of it as a snapshot of the current state of the world. That model suffices in our day-to-day lives quite nicely, but it isn’t a very good model of the concept of time on a universe-sized scale. Einstein’s famous Theory of relativity states that time and space are closely related, and that perception of both time and space is relative to the observer. The concept of “now” is also relative. Greene uses the metaphor of a loaf of bread, the long axis representing time, and a “slice” of the loaf representing an instant in time across a 2D universe. The angle at which the bread is sliced depends on an observer’s relative motion, with a maximum angle of 45 degrees corresponding to a maximum velocity of the speed of light. Two observers, Bob and Alice at different relative velocities would have slices at different angles, and so their “now” slices would intersect at some line in space. In Bob’s “now” some events in Alice’s “now” haven’t happened yet, they are in Bob’s “future”, while others are in Bob’s “past”. Time is an elusive concept, just when we think we know what we’re talking about we get hit with something like “my ‘now’ isn’t the same as your ‘now'”. It’s no wonder there are so many ways to depict its passage in the comic medium!
On a slightly related tangent, the medium used can have some interesting affects on our perception of time and motion. In this video, recorded at 25 frames per second, a stream of falling water appears to freeze in time, or even flow backwards, when it interacts with sound waves at or near 25Hz. It’s not really an optical illusion, more of a media illusion.