of Einstein the 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 of the universe. Nothing can travel faster than it. Every single moving object in the universe is bounded by this fundamental limitation.
Speed and mass
This is nothing like that the speed of sound. Early scientists wondered if we would ever be able to move faster than that speed, not because of any fundamental rule of the universe, but because we didn’t know if our engineering and material capabilities could withstand the extreme turbulence generated by moving at such speeds. But everyday objects already surpass the speed of sound. For example, whiplash is caused by the tip creating a sonic boom as it travels faster than the speed of sound.
Read more: Is there a particle that can travel back in time?
The problem with trying to beat the speed of light is that the faster you go, the more kinetic energy you have. But relativity tells us that energy equals mass, so the faster you move, the more massive you become (and yes, that means a moving baseball has more mass than a stationary one, but that’s a small effect) .
As you approach the speed of light, your mass increases to infinity. The closer you get to the speed of light, the more your mass spirals out of control. With higher masses, you have to push yourself harder to accelerate, and you quickly find yourself in a position where you’ll need an infinite amount of energy to overcome the speed of light.
Exploring the speed of light
It’s not just a matter of clever engineering or coming up with some trick – it’s built into the fabric of the universe.
However, there are proposals to design specialized devices that could supposedly overcome this limitation without directly violating relativity. These concepts work because special relativity is a local law of physics: it tells you that you can never measure nearby motion that moves faster than the speed of light.
For example, a wormhole it can send you to a distant destination faster than you could normally travel through space, although you can travel down the wormhole as slowly as you like. “Alcubiere driving” is essentially a warp drive, warping the spacetime near you so that it can pull you forward as fast as you want—even faster than light—without even firing a missile.
Read more: Black holes are accelerating the expansion of the universe, cosmologists say
But these fantastic ideas suffer from two flaws. For one thing, in order to function, they each require the use of some exotic form of matter, namely matter that has negative mass. Negative mass is really weird. If you drop a ball with negative mass, it will fly up. If you kick the ball, it will roll in the opposite direction, and so on. This would completely destroy everything we know about motion 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 warp drive.
Second, the ability to travel faster than light automatically allows for time travel. As soon as you come up with methods to cheat the speed of light, you can set up scenarios where the signals (or the spaceships) can return to their starting point before taking off. This creates all kinds of messy situations and causality-breaking paradoxes, like dropping a bomb to destroy your spaceship before it takes off – but unless it takes off, the bomb is never delivered.
What is possible
Since causation seems quite important in our universe, with the past staying safely in the past and always causing antecedent effects, violating this aspect of reality also seems like a no-go.
The limit to the speed of light fits into the most fundamental relationship in the universe: the relationship between space and time, expressed through special relativity. Whenever we test this theory, we also test every other aspect of the theory, including its limits on the speed of light. And special relativity is perhaps one of the most well-tested theories in all of science. It has stood strong for more than a century.
This is not to say that someday in the distant future humanity could not come up with some new theory of physics that completely rewrites our understanding of speed, space, time and causality. Which makes it impossible to completely rule out faster-than-light travel. But for now, it seems that we and everything else in the universe are permanently stuck in the slow lane.