A decade before we discovered that the universe was expanding, we knew that galaxies could change color. When The Flash moves faster than the Milky Way, he turns purple.

In 1912, astronomer Vesto Slipher was measuring how the light waves washing in from distant galaxies bunched up and spread out. Like how an ambulance’s sirens appear to get higher or lower pitched if the vehicle is racing towards you or speeding away, light can appear at a higher or lower frequency if the source is moving towards or away from you. This is the Doppler Effect.

Slipher found that galaxies rapidly approaching our Milky Way had bluer light – a blueshift — while galaxies moving away from us appeared redder – a redshift. Building on Silpher’s critical work, in 1929 Edwin Hubble published a paper documenting the redshifts of as many galaxies as he and his collaborators could find. That paper would eventually prove that the universe is expanding.

You would see the same red and blueshifts that changed our conception of a static universe if you observed The Flash like Vesto and Hubble.

It’s not that The Flash is simply quick or speedy – the superhero is the fastest thing that moves with mass. According to comic book lore, he can run at or very near the speed of light (which means he lives closer to the present moment than any one of us, oddly enough). And when something moves this fast, the Doppler Effect starts to make a real difference. Depending on how fast The Flash moves and in what direction, he is either going to be purple or invisible.

Like how Vesto and Hubble used the apparent color change in a galaxy’s light to estimate how fast that galaxy was moving, we can use the same equations to work backwards and find out what happens to The Flash’s color as he races around fighting crime. First, let’s assume that The Flash emits red light (the light reflecting off of his suit), or light with a wavelength of 620-750 nanometers (depending on the hue):

Now imagine The Flash is running towards you at 15 percent the speed of light to save you from a falling piano (the situation is pretty serious). Like how a galaxy’s light seems to bunch up and get bluer, The Flash’s red suit is going to appear as though its reflected wavelengths are getting shorter (if you could perceive them). This is what he’d look like:

But The Flash can famously sprint close to light-speed, so let’s make that falling piano even closer to your head. If you could perceive The Flash running to save you at 20 percent of light-speed, his suit’s color would blueshift even further:

The piano is a few centimeters from your cranium. Sprinting at 30 percent the speed of light, The Flash goes green:

At 40 percent:

At 50 percent of light-speed:

Flashy.

This is where the rainbow ends. Even if you could see The Flash running towards you at all (at 50 percent the speed of light or 335,000,000 miles per hour, our brains don’t compute fast enough to see him move from one spot to another), beyond 50 percent the speed of light The Flash’s bright red suit would blueshift beyond what we can see and into ultraviolet.

And because the color of The Flash’s suit starts at the low end of the visible spectrum, watching him run away from you – like watching a distant galaxy’s light undergo a redshift – would be pretty anti-climactic. At just 10 percent the speed of light, The Flash redshifts into infrared, and becomes invisible.

If you live in Metropolis, you might confuse your speedy savior with a bird or a plane, but if you live in Central City, you won’t even see your hero coming.

—

For more geeky calculations about The Flash, check out my first episode of Because Science!

Kyle Hill is the Science Editor of Nerdist Industries. Follow @Sci_Phile.

IMAGES: Body images The Flash #1 by Francis Manapul and Brian Buccellato, coloring by Nicole Enriquez