If You Drove a Car at Light Speed, Would the Headlights Work?


Few physics questions are more frequently asked than this one—the great comedian Stephen Wright even mulled it over during his first HBO special. But, at the end of the day, there’s really no definitive answer.

Driving any sort of vehicle at light speed (299,792,458 meters per second—a rate also known as “c”) looks to be flat-out impossible. As objects travel faster, they gain more mass. Accelerating faster and faster demands even more energy as the mass of the object increases (at least from an outside observer’s perspective; in the vehicle even weirder things are happening, but more on that in a second). And anything that possesses mass would literally need an infinite amount of energy in order to hit light speed. Given these limitations, scientists at the Large Hadron Collider—earth’s most powerful particle accelerator—have only ever been able to push subatomic particles like protons around at 99.9999991% of c. Close, but no cigar.

However, photons—the particles with which visible light is constructed—are massless, so the rules don't apply. In fact, particles that lack mass must always travel at c.

Now let’s speculate for a moment. If you reached c in, say, Mrs. Frizzle’s magic school bus, what would happen? For starters, the little hands on your wristwatch won’t budge. When in motion, clocks slow down, and once something arrives at the speed of light, time stops entirely. Under those circumstances, you’d be unable to flick on Frizzle’s high beams or, indeed, do anything else.

Okay, forget about the original question. If you were driving at just below light speed, would the headlights work? Absolutely. You’d still have two rays that were traveling at c, making them fast enough to race ahead of the automobile.

This brings us to an interesting phenomenon. Imagine that, out of sheer boredom, you decide to fire a bullet towards the windshield of your parked truck and measure the projectile’s speed. You then learn that it was going exactly 1,700 miles per hour. Afterwards, you repeat this experiment while driving at 10 mph. From your perspective, the second bullet’s speed will still be 1,700 mph. Yet, someone standing outside of the car would clock it at 1,710 mph.

Light doesn’t work that way. If, after speeding back up to 10 mph, you shined a light onto your windshield, you’d measure its speed at c. Meanwhile, the outside observer wouldn’t record it as having gone c + 10 mph. Instead, that person would concur with you and say that it was traveling at c. This doesn’t sound possible, but Einstein’s Theory of Relativity holds that light speed is constant. Regardless of one’s frame of reference, it supposedly never changes.

We’ve long understood that light travels a touch more slowly through such mediums as water. And its speed may be still more variable. This past winter, a team of optical physicists published an exciting new paper. Led by University of Glasgow professor Miles Padgett, the group changed the shapes of a few photons and raced them against some unaltered specimens. Consistently, the touched-up models moved at slightly slower speeds, even while passing through vacuums.

These stragglers only fell a few millionths of a meter behind. Still, it’s clear that c really represents light’s top speed and not its uniform pace. As Einstein would be the first to admit, this whole subject could always use more illumination.