The Future Is Here

# What Happens If You're Traveling At The Speed Of Light And Turn On Your Headlights?

One of the most popular questions from our "ask a physicist" feature was, "What happens if you're driving at the speed of light and you turn on your headlights?" The simple answer: You can't. So quit trying.

At least ten of you asked some variant of this question, but Turael asked it first and arguably most succinctly. So Turael, feel free to email me so we can send you a free copy of A User's Guide to the Universe.

Sadly, physics teaches that we are forever confined to sub-light speeds.

There were several comments, in response to the original call for questions, that took umbrage at this certainty on the part of physicists. Are we just being closed-minded? There are always people who are skeptical of equations, regardless of other evidence. For example, long after the advent of nuclear weapons and power have vindicated Einstein, I still get detailed manuscripts from people every few weeks claiming that E=mc2 is wrong.

So no lengthy derivations, but I will give you a few observations, and hopefully that will be enough. Special relativity predicts that if you take a massive particle and keep applying forces on it, it goes faster and faster, slowly approaching the speed of light, but never quite reaching it. Right now, for example, the Large Hadron Collider has protons flying around it at a whopping 3.5 TeV. This means that the protons are traveling 99.999994% the speed of light, and when the LHC gets up to full power (at about twice the energy), the protons will go even faster, but even then, less than the speed of light. At these speeds, the difference between "at" and "a tiny bit below" the speed of light may seem academic, but it makes a world of difference.

We always need to accept the possibility that we could be wrong, but in this case, there's just so much evidence that we're right! The particle colliders wouldn't actually work if relativity were wrong. For that matter, neither would GPS devices. Michelson and Morley found in 1887 that light travels at the same speed to all moving observers, a result that doesn't make sense unless special relativity is correct. All of modern physics (and technology) is built on an edifice of special relativity, and so far, it's proven ridiculously accurate. In other words, you've got a very big barrier to overcome if you want to prove Einstein wrong.

Part of the reason that people are so confused about this aspect of relativity is that it flies in the face of everyday experience. If I'm in a boxcar moving 60 mph and throw a 90 mph fastball, someone standing by the side of the tracks will see the ball moving at 150 mph. It seems like the same logic should work with light. Except that it doesn't.

Strange things happen when you get close to the speed of light, and they become stranger still when you realize that your high school physics teachers (perhaps inadvertently) lied to you. Lots of you are sci-fi nerds, so I'm guessing that at least once in your lives you learned Newton's force equation, F=ma. Deciphering the symbols, it means that if you apply a constant force to a particle, it should experience a constant acceleration. Taken to its natural extremes, if I apply a force for long enough and the particle keeps accelerating, eventually it should exceed the speed of light. Voila! Newton's force equation (at least in the form it's normally written) is wrong.

But then what happens when you get close to the speed of light and turn on your headlights? From your perspective: nothing, or at least nothing special. If you held a mirror in front of you, you'd look exactly as you always have. In fact, one of the surprising things about special relativity is that if you weren't looking at all of the scenery passing you, you couldn't tell that you are moving at all.

But from the perspective of people standing on the sidelines, things look really cool. Stationary observers would notice that your entire ship (or racecar, or whatever you're driving at 99% the speed of light) is compressed along your direction of motion. If you're standing the right way, it'll look like you've lost weight and that your body has been flattened under a giant stone.