Physics in a Flash


I saw the first episode of the new CW show “The Flash” last night. Like most shows on the network, it was mindless fun. It has the CW stamp of lightweight actors, cardboard characters, bubblegum philosophy and idiotic science, but it was still enjoyable. The show, of course, centers around the eponymous superhero who can move at speeds impossible for the mundane human, and how he uses this new power to right wrongs and fight evil. It’s very formulaic; one could guess within the first few minutes who fit where into the grand scheme of things: who’s evil, who’s good, who the love interest is, who will eventually betray whom, and so on. But beyond the surface entertainment, the “science” of the show provides even more fun.

I’m a physicist, and like to think about physics in this world and in the magical world of TV land. The Flash provides a good venue for some entertaining ideas: first of all, how fast does the Flash move? It’s a bit hard to tell just by watching the show, but the writers/directors provided an indirect clue when they showed him moving against a graphic of city streets as seen from above. He seemed to move about 10 blocks in about a second. In New York, 10 city blocks is 1 mile, so 1 mile per second seems a pretty reasonable guess; in metric terms, about 1,600 meters per second. This is about 1,000 times faster than the average walking speed of an adult mundane. Hmmm… Well, if he’s going from 0 to 1600 in about a tenth of a second flat, he’s got an acceleration of nearly 1600 g’s! (I’m using the fact that acceleration is change in velocity over change in time, and that 1 g is about 10 meters per second per second in metric units.) He’s got to be made of stern stuff, as even experienced jet pilots will black out at 9 g’s, and get pretty dead if they’re exposed to accelerations of better than 20 g’s for any length of time. Another way to put it is that this acceleration is something like falling from the stratosphere and hitting the ground suddenly. Oww…

This leads to some other things: if he suddenly sped up and launched himself off of a ramp into the air, he could go a distance of about 160 mile before hitting the ground again! (I’m ignoring air resistance here, and the curvature of the Earth, and Coriolis force, and a bunch of other things as well. However, the show itself is hardly realistic, so neener, neener.) What if we don’t ignore air resistance? The force due to drag of the air as he runs around is something like 800 times his own weight; think about lying on a matress with 800 people sitting on top of you. This is what he feels whenever he runs around! The power which it takes to get him to run around like this is something like 10 million times the average metabolic rate of a normal human being, or the total power used in all of the daily activities by all of the residents of a large town!

The moral: thinking about Physics makes shows like this much more fun.

A quick note:  One thing which I had forgotten (shame on me!) is that if my estimate is correct, he’s running at several times the speed of sound.  My estimates for the forces acting on him are low.  I was assuming that he wasn’t compressing the air ahead of him to any significant extent.  Running this fast,  he’s going to generate a shock wave because the compression of the air in front of him can’t get out of the way fast enough.  Someone watching him as he runs by will see him first, then hear the noise of his passage, as he is outrunning the sound waves he generates…  It’s like watching a supersonic jet pass overhead.