Tachyons are hypothetical particles that always travel faster than the speed of light. Einstein showed that such particles would allow communication back in time, which opens up all sorts of problems with the fundamental law of the universe. Although physicists have not proven that tachyons cannot exist, there is good reason to believe that they do not.
The barrier that nothing with matter can travel at the speed of light is not simply an expression of a limitation of technology or a representation of a failure of imagination. It is baked into the very laws of the universe as expressed by Einstein’s theory of special relativity.
Let’s say you want to start traveling faster than the speed of light. You start to relax and give yourself a little nudge. Because you have mass, your nudge has to overcome a bit of inertia to get you going, but you’ll eventually get going. For example, you light a rocket and launch it.
But once you’re off the launch pad, you don’t stop. You have some super advanced engine that allows you to keep pushing, which causes you to keep accelerating. At speeds much lower than the speed of light, it all makes sense: For every second you start the engines, you get the same acceleration and the same increase in speed.
But as you approach the speed of light, something funny starts to happen. The same amount of power put into your engines starts to give you less and less acceleration, so you get less speed bang for your buck. Despite your engines working to the limit, you find yourself approaching the speed of light, but never reaching it. At some point you realize that to reach the speed of light you need to put an infinite amount of energy into your engines – which you don’t have.
The problem is that energy is mass as given by E = mc^2. The faster you move, the more kinetic energy you have, meaning you literally get heavier the faster you go. As you approach the speed of light, your mass goes to infinity, so it takes an infinite amount of rocket power to reach the speed of light.
Tachyon solution
But these rules apply to objects with masses starting below the speed of light. Matterless objects, like light itself, automatically move at the speed of light, never slowing down or speeding up. In 1967, based on work dating back to the past, physicist Gerald Feinberg proposed a new class of particles: objects with “imaginary mass”. (“Imaginary” here refers to the mathematical expression for the square root of -1.) These particles, which he called tachyons, would never move slower than the speed of light. In fact, they would be forced to always exceed the speed of light, and would have the same difficulty slowing down to the speed of light as we do trying to accelerate to it.
Feinberg wasn’t the first to consider faster-than-light particles, but he was the one who coined our word for them. Einstein toyed with the idea, but found that such particles violated a central rule of the universe: causality.
Causality is so fundamental that it underlies everything we understand about the workings of the universe. Simply put, causality says that causes must come before effects. I have to text you before your phone beeps, I have to put a piece of cheese in my mouth before I can eat it, and so on.
Cause trouble
But tachyons are capable of disrupting causality. To see how, let’s set up a little thought experiment. I’m sitting on Earth while you’re having some great adventure in space. I want to send you a signal with tachyons, so I turn on my tachyon transmitter and broadcast a message. From my perspective, the tachyons are moving away from me faster than the speed of light in your direction. So far so good.
If you stand perfectly still, then the tachyon will eventually reach you in less time than it would take light to get there. You wouldn’t be able to see the tachyon coming until it had already passed you, which is still not a big deal. If you pointed the scope at me, you’d get the tachyon before you’d see the image I hit the send button on. Interesting, but still no big deal.
The problem arises if you start moving. In relativity, from your perspective, you are standing still while the Earth appears to be moving away. This represents time dilation: From your point of view, everything in the universe—including the action of me pressing the button—slows down. In fact, if you travel fast enough, you can receive my tachyon and send a reply before I even press the button; you can send a signal back in time.
Once you enable sending signals back in time, you can play many fun games that create contradictions. You can have a message sent to you to prevent your grandparents from meeting, which means you would never exist – but you have to exist to go back in time to prevent your grandparents from meeting. You can set off an explosion that destroys the tachyon emitter before it receives your message. You can even destroy yourself in your own past. And since we don’t live in a universe where these contradictions and violations of causality occur, it seems unlikely that tachyons exist.
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