I’m not sure if this is really a hard tweet. My editor, who never gets any of my math or physics jokes, actually got this one. And it’s doing a little better in the stars than my usual hard tweets. But I think it’s worthy of explanation anyway, because it’s such a cool topic.
You are all familiar with Doppler Shift. That’s when the ambulance coming toward you sounds like, “nee-ner, nee-ner, nee-ner” and after it runs you over, it sounds like “noo-ner, noo-ner, noo-ner.” That is, higher pitch coming toward you, and lower pitch going away from you. The reason this happens is because sound travels in waves. Think of those waves like a spring. When the sound source is coming toward you the spring gets squished. And when the sound source is going away, the spring gets stretched out.
Squishing waves brings them closer together. And that means that the number hitting over a period of time is going to be higher. You will be hit with waves more frequently. So we say the wave has a higher frequency. Cool, right? Higher frequency gives higher pitch. So that’s why the sound is higher when it comes at you.
The opposite happens when it moves away. The waves get stretched out, so you are hit at a lower frequency, and hence lower pitch.
But you knew all that, right? Well, the same thing happens with light! If you think about how fast light moves (really fucking fast) relative to the light source (not really all that fast, here on earth), the amount of that Doppler shift in the light isn’t going to be much at all. But if the object is moving close to the speed of light, it can make a big difference.
When you increase the frequency of light, it moves toward the blue range. So bluer is the light equivalent of higher pitch. When you reduce the frequency of light, it moves toward the red range, so redder is the light equivalent of lower pitch.
So if an ambulance was coming toward you near the speed of light, it would look bluish. And then after it passed you, it would look reddish.
Of course, you wouldn’t get to see any of this because it would happen too fast, and ambulances traveling near the speed of light leave a horrific wake of devastation in their path. So you’ll just have to trust me on this.
So, knowing all this, we can return to our tweet. Two otherwise identical cars, blue in front, and red behind. As they approach, it looks like a blue car, after it passes, it looks like a red car. It appears as though it was just a gray car that passed you at nearly the speed of light.
Homework: Step out of the way the next time an ambulance comes toward you at a speed significantly less than the speed of light (no point worrying about those near-speed-of-light ones).