John Bauer [Public domain], via Wikimedia Commons

Why can’t anything go faster than light? What if I was going 99.9999% the speed of light and I accelerated?

Short answer: The speed of light is simply the limit for massive objects. If you accelerated, you could only asymptotically approach the speed of light.

Long answer: Let me give you an analogy.

Imagine we’re trying to cross a troll bridge. The troll says that we’re allowed to cross half the distance between where we are standing and the other side, but every time we do, we owe the troll a dollar. Getting halfway across the bridge is cheap, as it only costs us a dollar. Getting 75% of the way across is still pretty cheap, and only cost us $2 to get there. And so forth and so forth. Eventually, no matter how close we get to the other side, we’ll eventually run out of dollars before we cross the bridge.

Accelerating a massive object is sort of like that. At first, you can increase that object’s velocity a lot by pumping energy into it (by accelerating it). But eventually, once you’re going 99% the speed of light, it starts getting really hard. If you triple the kinetic energy of a particle already going 99% the speed of light, you’ll only get it going 99.9% the speed of light. You can keep increasing the energy of this object, but you’ll only succeed in tacking on more 9s to the end of that number. Any acceleration will only allow the object to approach the speed of light asymptotically. There’s simply no ‘breaking through.’


By Supersonic0714 (Own work) [CC BY-SA 3.0 ( or GFDL (], via Wikimedia Commons

“Yeah, okay, but what if I-“

Another common question similar to the one above asks what would happen if you pushed on the end a long solid bar, perhaps a few light years long. Would the other end move instantaneously? The answer is a resounding no. When you push the bar, you’re actually just pushing on the atoms adjacent to your hand. Those atoms in turn push on the next atoms, and so forth down the line. The result is a wave of atoms bumping into other atoms moving down the length of the bar. This is, quite literally, a sound wave. Of course, the speed of sound in solids is much higher than air and is generally several miles per second, but compared to the speed of light this is still almost nothing. For example, the speed of sound in aluminum is 3.2 miles/second (5100 meters/s) so if you pushed the end of an aluminum rod that was one light year long you would be waiting 59,000 years to see the other end twitch.

There’s plenty more questions and puzzles from special relativity, so maybe we’ll save them for another time.





asked by /u/cazbot
image credit: Wikimedia Commons




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