These theoretical particles ignore the basic structure of cause and effect. If scientists ever prove they’re real, tachyons would shatter our understanding of reality.
Tachyons Are the Cosmic Engines of Time Travel. We Just Need to Prove They Exist
Einstein’s special theory of relativity teaches us that nothing can accelerate past the speed of light. But what if, when you were born, you were already moving faster than light? What would that look like? Turns out, the type of particle that could enable such a strange circumstance already has a name: the tachyon, and its existence wouldn’t just break our understanding of physics, but our grip on reality as we know it.
The Rules for Traveling Faster Than Light
The speed of light is a real beast of a limit. As objects move faster, they gain more energy. But relativity tells us that mass and energy are the same thing (remember the famous E=mc2 formula). So as objects move faster, they also become more massive. This isn’t really noticeable until you get close to the speed of light, in which case your energy (and mass) balloon out of control. An object approaching the speed of light gets picks up an infinite amount of mass, which would require an infinite amount of energy to push—therefore, all massive objects are forever restricted to speeds below that of light. The only things that can go light speed are massless particles, like photons, the carriers of light.
But relativity also tells us that there is a strange mirror image to these familiar laws. If a special kind of particle were to appear on the scene, already traveling faster than light, then it would be forced to always move at light speed—forever. Slowing down such a particle would also require an infinite amount of energy.
All the known particles in the universe travel at or below the speed of light. However, the hypothetical tachyon particle could beat light speed. One problem: its existence would rewrite an aspect of our physical universe so fundamental that we don’t usually even think about it.
Tachyons Don’t Follow Cause-and-Effect Logic …
The underlying issue here is causality. Put simply, causality is the principle that everything has a cause, that every cause leads to an effect, and that every effect comes from a cause. Causality underpins everything we experience in our everyday lives. For example, if I want to put coffee in my mug (an effect), I have to pour the coffee from the pot into the mug (a cause). If I see a broken window, I don’t assume it just magically appeared that way; instead, I start looking for the baseball my kid was just throwing in the backyard.
Causality provides the logical ordering for all the events in our lives. It’s how we make sense of the flow of time. Causes lead to effects, which in turn become new causes that lead to new effects, and so on, from past to future. The past contains all the causes that have led to the current reality, and the future is full of many possible effects based on the present causes. This unbroken chain of causes and effects forms the basic structure of experience.
Yet, tachyons break that chain.
… Or the Speed of Light
Special relativity doesn’t just tell us about speed limits in the universe. It also tells us about the relationship between cause and effect in the universe. Because the speed of light is the ultimate speed limit in the cosmos, it’s also the fastest possible way for one cause to lead to an effect—because that’s the fastest that anything can move.
If I want to tap your shoulder, I have to walk over to you and reach out my hand. The fastest I can do that is at the speed of light. If you’re nearby, I can tap your shoulder relatively quickly. If you’re far away, I have to travel to you first. In other words, the speed of light limits what effects I can have in the universe; the further away other objects are, the longer it will take me to have an effect.
If tachyons did exist, they would completely shatter our understanding of reality.
The limit on the speed of moving objects also imposes a limit on the speed of causality. If something is too far away and I don’t have enough time to reach it, then I can never influence it, because the speed of light boxes me in. For instance, if you were to move to the Andromeda galaxy, 2.5 million light years away, and I wanted to tap your shoulder but only had an hour and a half to get there, I’m out of luck. There’s no possible way that I can tap your shoulder—no way my cause can lead to that effect.
With tachyons, however, it’s possible to violate those rules.
The Time-Traveling Particle
Because tachyons travel faster than light, it’s possible to set up scenarios in which cause and effect don’t bear their normal, sensible ordering. For example, let’s say I’m sitting on Earth and you’re in a spaceship, somewhere far away. We each have a special transmitter that can send and receive faster-than-light tachyons (just like any sci-fi show).
I send you a message, and it reaches you faster than light. That means that if you have a telescope pointed back at me, you’ll receive the message before you see me sending it, because the light from me reaching your telescope is limited to, well, the speed of light! Then, you can compose a return message and send it back to me; again, I would receive the message before you ever saw me send the original message—meaning you’re effectively sending your reply back in time, a special trick available only through faster-than-light tachyon particles.
This is still bizarre, but it’s not causality-breaking—yet. So, let’s start messing with reality. Within this imaginary setup, let’s pretend that my tachyon transmitter has a self-destruct mechanism, but only if it receives a secret coded message. You happen to know this secret message. When you receive my transmission and reply, you send the self-destruct signal to my transmitter.
Because tachyons can travel faster than light, your response to my initial message actually arrives before I sent anything. But your response instructed my transmitter to self-destruct, making it impossible for me to send my message in the first place. But the only way to get the self-destruct signal was for me to send my message ... and for you to respond.
Which is it? Did I send my message, or did my transmitter self-destruct? It’s a paradox caused by the presence of those faster-than-light tachyons, breaking the normal ordering of cause and effect. Because tachyons can travel into the past, they can trigger effects to happen before their causes, which leads to all sorts of ridiculous paradoxical scenarios.
Can Tachyons Exist?
We have no proof that tachyons don’t exist. There is nothing in the theory of relativity itself that rules out their possibility. But if they did exist, they would completely shatter our understanding of reality. Physics would break down, since physics is the story of cause and effect at a fundamental level. But physics wouldn’t be alone. Our entire understanding of reality and existence are predicated on causality, on cause-and-effect relationships. Tachyons destroy those relationships; with tachyons in the picture, effects could no longer be traced back to causes, and causes can lead to effects both in the future and in the past. The logical flow of time would become a tangled, nonsensical mess.
Sure, our fundamental understanding of reality could be wrong, and the possibility of the existence of tachyons might be the first step in rewriting our basic assumptions. Or maybe, just maybe, we live in a rational, orderly universe, and we’re going to have to do everything slower than the speed of light—whether we like it or not.