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Tachyons are hypothetical particles, and their movement speed is always faster than the speed of light. Einstein showed that such particles could allow events to go back in time, but this also raised various questions about the basic rules of the universe. While physicists haven't proven that tachyons don't exist, there's good reason to believe they don't exist.
The impediment to the inability of anything of quality to travel at the speed of light is not only a symptom of engineering limitations but also a failure of imagination. It is integrated into the laws of the universe, as expressed in Einstein's Special Theory of Relativity .
Suppose you want to start traveling faster than the speed of light, and you start with a break and give yourself a little push. Because you have mass, your push has to overcome a bit of inertia to get you forward, but you will eventually move forward. For example, you light a rocket and launch it into space.
But once you leave the launch pad, you don't stop. You have some super advanced engines that keep you pushing, that keep you accelerating. At speeds well below the speed of light, it all makes sense: every second you fire your engine, you get the same acceleration and the same speed boost.
But as you approach the speed of light, some interesting things start to happen.
The same amount of energy put into the engine starts giving you less and less acceleration, so you gain less and less speed. Despite running your engines to maximum speed, you still find yourself inching closer to, but never reaching, the speed of light. At some point, you realize that to reach the speed of light, you have to put an infinite amount of energy into your engine—and you don't.
The problem here is that energy is mass, given by E = mc^2. The faster you move, the more kinetic energy you have, which means the faster you are, the heavier you are. As you approach the speed of light, your mass becomes infinite, so it takes infinite rocket power to reach the speed of light.
But these rules apply to objects whose mass starts below the speed of light. Massless objects, such as light itself, automatically travel at the speed of light, never slowing down or speeding up. In 1967, building on work decades earlier, physicist Gerald Feinberg proposed a new class of particles: objects with "virtual mass." (The "imaginary number" here refers to the mathematical term for the square root of -1.) These particles, which he calls tachyons, never travel faster than the speed of light. In fact, they will be forced to always exceed the speed of light, and it will be just as difficult to slow down to the speed of light as it is for us to try to accelerate to the speed of light.
Feinberg wasn't the first to think about tachyons , but he was the one who coined the term for us. Einstein considered the idea, but discovered that these particles violated a core rule of the universe: cause and effect.
Causation is so fundamental that it underlies all our understanding of the workings of the universe. Simply put, causation states that a cause must precede an effect. I have to text you before your phone rings, I have to put a piece of cheese in my mouth before I can eat it, etc.
But tachyons can violate causality. To understand how, let's conduct a small thought experiment. I sit here on Earth while you go on some great adventures in the universe. I wanted to send you a signal using tachyon, so I started my tachyon transmitter and sent a message.
From my perspective, tachyons are hurtling towards you at faster than the speed of light. So far, so good.
If you stand perfectly still, eventually the tachyons will take less time to reach you than it takes for light to get there. You can't see a tachyon until it passes by you, which is still not a big deal. If you had a telescope pointed at me, you would receive the tachyons before you saw me push the button to send an image of it.Curious, but still no big deal.
The problem comes if you start moving. In relativity, from your perspective, you stand still while the Earth appears to be receding. This introduces time dilation to : from your perspective, everything in the universe - including my action of pressing the button - slows down. In fact, if you travel fast enough, you can receive my tachyon and send a reply before I even hit the button; you can also send the signal in time.
Once you allow signals to be sent in time, you can play a lot of fun games to create conflicts. You can send a message back to prevent your grandparents from meeting, which means you will never exist - but you need to exist to go back in time to prevent your grandparents from meeting. You can trigger an explosion to destroy the tachyon emitter before it can receive your message. You can even destroy yourself in your own past.
And because we don't live in a universe where these contradictions and violations of causation occur, it seems unlikely that tachyons exist.
