Einstein once called the speed of light “The Universe’s speed limit”. He claimed that traveling faster than the speed of light would violate the causality principle.
For the layman, that means cause and effect. An example of this would be a bullet hitting a target before the trigger was even pulled.
Accelerating to light speed or exceeding it would also violate certain fundamental energy conditions. It could even allow for time travel.
So why can’t anything go faster than the speed of light?
Before we can dive into that, we have to know what the speed of light actually is, what it means, and clear up some common misconceptions regarding this “universal speed limit“.
The speed of light, (or the speed of a photon) in a near-perfect vacuum is exactly 186,282 miles per second. We perceive photons (light) traveling at this speed because they are massless, or have no ‘weight’ (but they do have kinetic energy, more on that in a bit).
Every particle in our universe (including photons) move or ‘swim’ through what scientists call “the Higgs field”. As a result of this interaction, particles acquire their mass. Different particles interact with the Higgs field with different strengths, which is why some particles are heavier (have more mass) than others. Photons move through, but do not interact at all with the Higgs field.
What does that mean?
Since photons don’t interact with the Higgs field, it means they aren’t bound by any speed limit. They’re free to move at the fastest possible speed – their own “light” speed. Why isn’t the speed of light slower or faster than 186,282 miles per second? It’s because that exact speed is simply fundamental constant of our universe.
Wondering why light doesn’t travel at a different speed is like wondering why gravity isn’t reversed or what it would be like if our universe only had 2 spatial dimensions instead of 3 (4 if you include time). Those constants, along with the speed of light, were set in place when our universe was created at the moment of the big bang.
Universal Speed Limit
Particles that have mass require energy to accelerate them. The closer to the speed of light you get a particle, the more energy is required to go faster. This is because the particles themselves get more massive in proportion to the increased velocity. In short, the faster you go, the heavier you get.
Thanks to this inconvenient truth, if you wanted to accelerate a single electron to ‘light speed’, you would need an infinite amount energy due to the electron becoming infinitely heavy. There isn’t enough energy in the entire universe to propel just a single electron to the speed of light.
From A Photon’s Perspective
One of the methods Einstein used to help formulate his theory of special relativity was to visualize what the universe would look like from the perspective of a photon. Einstein saw that life as a photon would be quite bizarre. For instance, if you were a photon, time would have no meaning to you. Everything would appear to happen instantaneously.
Imagine for a moment that you are a happy little photon created by a star in another galaxy some 4 billion light years away. From my perspective here on Earth, it took you exactly 4 billion years to travel from that star till you reached my retina. From your perspective, one instant you were created and then the next, you are bouncing off my eyeball. You experienced no passage of time. Your birth and death happened instantaneously.
This is because time slows for you as your get closer to light speed, and at it, it completely stops. This is also another reason why nothing can go faster than light. It would be like slowing down a car to a stop, and then trying to go slower than completely stopped.
One should think of the speed of light as ‘infinite speed’. A common misconception is thinking the speed of light is just like any other finite speed. The speed of light is only finite from the perspective of the outside observer; from the perspective of a photon, it’s infinite. If you move at exactly the speed of light you could go anywhere, no matter how far, in exactly zero seconds.
Can light be slowed down?
There is at least one real world example of superluminal (faster than light) travel. It’s cheating a bit, but it occurs when light passes through water.
Remember, nothing can go faster than 186,282 miles per second, but the photons that normally travel at that speed can be slowed down. An example of this can be seen in water where light is slowed to 3/4th of its normal speed.
In nuclear reactors, the charged particles emitted off radioactive rods through the water they are submerged in exceed this reduced speed.
Because the particles contain an electric charge, they emit energy called Cherenkov radiation. Any particles they bump into become radioactive, giving the water an eerie, mysterious blue glow.
So what’s the speed of gravity?
It’s an interesting question that many people may not think about. Does gravity also have a speed and if so, what is it?
Gravity does in fact have a speed. The speed of gravity is technically the same as the speed of light. The reason for this speed is the same reason why light travels at the speed it does. It’s simply a fundamental property of spacetime itself. If the Sun were to disappear right this instant, the earth would continue its orbit for 8 minutes (the time it takes light to reach Earth from the Sun) before the orbit would be perturbed by the Sun’s absence.
So to sum up, nothing can travel faster than light because the speed of light can be thought of as infinite speed. To match or exceed it would be to go infinity miles per second/hour. A loop-hole does in fact exist, however. The loop-hole requires a control or warping of the fabric of spacetime itself which we’ve touched on in a previous article. (See: “Why aren’t we exploring the galaxy yet?” and “What is a warp drive?“)
Photos are available under a Creative Commons Attribution license by Wikicommons.