Einstein’s special theory of relativity governs our understanding of both the flow of time and the speed at which objects can move. In special relativity, the speed of light is the ultimate speed limit to the universe. Nothing can travel faster than it. Every single moving object in the universe is constrained by that fundamental limit.
Speed of Light and Sound
This isn’t something like the speed of sound. Early scientists wondered if we could ever go faster than that speed, not because of some fundamental rule of the universe, but because we didn’t know if our engineering and materials science capabilities could withstand the extreme turbulence generated by moving at such speeds. But everyday objects already surpass the speed of sound. For example, the crack of a whip is caused by the tip creating a sonic boom as it travels faster than the sound speed.
The problem with trying to surpass the speed of light is that as you go faster, the more kinetic energy you have. But relativity tells us that energy is the same as mass, so the faster you go the more massive you become (and yes, this means that a moving baseball has more mass than one standing still, but that’s a tiny effect).
As you approach the speed of light, your mass balloons up to infinity. The closer you get to the speed of light, the more out of control your mass becomes. With higher masses, you must push yourself harder to accelerate, and you quickly find yourself in a position where it would take an infinite amount of energy to overcome light speed.
Exploring Light Speed
This isn’t just a matter of clever engineering or figuring out some trick – this is built into the fabric of the universe.
That said, there are proposals out there for designing specialized devices that could supposedly overcome this limit without outright breaking relativity. These concepts work because special relativity is a law of local physics: It tells you that you can never measure nearby motion going faster than light speed.
For example, a wormhole could send you to a distant destination faster than you could go traveling normally through space, even though you could travel down the wormhole as slowly as you want. The “Alcubierre drive” is essentially a warp drive, deforming spacetime near you so that it can pull you forward as fast as you want to go – even faster than light – without once even firing a rocket.
Negative Mass and Backward Time Travel
But these fanciful ideas suffer from two drawbacks. For one, to function they each require the use of some exotic form of matter, namely matter that has negative mass. Negative mass is truly weird. If you were to drop a ball of negative mass, it would fly up. If you kicked the ball, it would roll in the opposite direction, and so on. It would completely break everything we know about movement and momentum, and we have absolutely no examples of negative mass existing in our universe. Without negative mass, you can’t build a wormhole or a warp drive.
Second, the ability to go faster than light automatically permits backward time travel. As soon as you concoct methods to cheat the speed of light, you can set up scenarios where signals (or spaceships) can return to their origin point before they departed. This sets up all sorts of messy causality-violating situations and paradoxes, like dropping off a bomb to destroy your spaceship before it’s departed – but unless it departs, the bomb never gets delivered.
Is Faster Than Light Travel Possible?
Since causality seems kind of important in our universe, with the past staying safely in the past and causes always preceding effects, violating that aspect of reality also seems like a no-go.
The speed of light limit is baked into the most fundamental relationship in the universe: the relationship between space and time as expressed through special relativity. Every single time we test that theory, we are also testing every other aspect of the theory, including its limitations of light speed. And special relativity is perhaps one of the most well-tested theories in all of science. For over a century, it has stood strong.
This isn’t to say that someday in the far future, humanity couldn’t concoct some new theory of physics that completely rewrites our understanding of speed, space, time and causality. Which makes it impossible to outright rule out faster-than-light travel. But for now it seems like we, and everything else in the universe, is permanently stuck in the slow lane.
Read More: Why Can't We Reverse the Arrow of Time?