The Arrow of Time

There is a concept in physics called the arrow of time. We experience the physical world as proceeding in a time direction we call "forward." The laws of physics, though, work equally well either forward or backward in time. What explains our sense that time has a direction? Look at the first animated GIF below, of a bouncing rubber ball. You might find it difficult to determine if the animation is playing forward or backward. For the brief moment of the animation, time seems to have no particular direction.

Of course, if the animation were allowed to play for a longer period of time, showing several bounces of the ball, you would, in fact, be able to tell whether it was playing forward or backward. Still, the laws of physics would work either way, forward or backward. Even the law of the conservation of energy (AKA the first law of thermodynamics) would hold whether time moved forward or backward.

Now look at the second animation below. The animation is clearly playing in reverse. But how do you know? What is different from the animation above (which is also, by the way, playing in reverse)?

What you cannot see is what is happening at the microscopic level, the level where heat is revealed to be a transfer of energy from molecule to molecule. A given collection of molecules can be thought of as having a certain amount of disorder, or randomness. In general, interacting collections of molecules will either maintain their randomness or increase it. The arrow of time follows from this observation, which is called the second law of thermodynamics. A broken egg could possibly reassemble itself and leap back up onto the counter, but it would be extraordinarily unlikely. Hence you can tell the direction of time - time's arrow.

I took advantage of Brian Bennett's animated GIF assignment (Turn a GIF into a FIG) at ds106. I pulled this together very quickly, recording video with a Surface tablet, trimming and resizing the videos with VirtualDub and then exporting the video clips into separate folders as images. I renumbered the images so that they would import into Photoshop in reverse, making a backward animation.* I slowed the animations down and then saved them as animated GIF's. Both animations use the same number of frames.

*h/t to Mariana Funes for pointing out that I could have imported the frames, selected all of them, and then selected the tool "Reverse Frames."