Is There a Particle That Can Travel Back in Time?

Yes, there is a hypothetical particle, called the tachyon, that could travel back in time. One catch: It almost certainly doesn’t exist.

Before we start talking about time travel, we first must talk about the speed of light. All objects in our universe are constrained to go no faster than the speed of light. The only particles capable of achieving light speed are massless particles, like light itself. Anything with even a tiny amount of mass will find it impossible to achieve light speed. That’s because the faster you go the more massive you become, and at light speed your mass becomes infinite, which would take an infinite amount of energy to accelerate.

But the speed of light isn’t just an expression of how fast objects can travel. It’s an expression of how fast objects can influence each other. Every single interaction in the universe, whether it’s your sibling hitting you or a supernova’s shock wave blasting through a gas cloud, is limited to the speed of light. The speed of light is actually the speed of causality – the fastest possible way that one cause can create an effect, and the fastest possible way that events can influence each other.

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Going faster than light means that you could go faster than causality. Said another way, going faster than light means going faster than time itself, meaning that faster-than-light travel automatically allows for time travel into the past.

There is a hypothetical class of particles that always travel faster than light. Einstein himself played around with the idea, calling them “meta-particles,” but today we call them tachyons, a word coined in 1967 by physicist Gerald Feinberg from the Greek word meaning “swift.”

Tachyons would be strange. Just as we massive objects could never ever exceed the speed of light, tachyons could never dip below light speed – they would be equally constrained on the other side of that invisible boundary. For tachyons, slowing down means increasing mass, and slowing down all the way to light speed would require an infinite amount of energy. To make this work, the mass of the tachyon has to be imaginary, but in the mathematical sense: Its mass would be multiplied by a factor of the square root of negative one.

At first glance, tachyons wouldn’t cause much trouble. You could fly out in a rocket ship, and on Earth, I could beam tachyon messages to you. If you were looking back at me through a telescope, those tachyons would reach you before the photons carrying the image of me sending the message arrived in your telescope. That’s a little weird but doesn’t necessarily violate anything about physics.

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The problem is that with tachyons, you could start to construct some truly weird scenarios. For example, you could move in a certain direction with a certain speed and send a tachyon signal back to me. If you construct things just right in the rocket ship, that signal can arrive back to me before I sent the original one out.

Suppose that signal back to me contained instructions to destroy my transmitter. The only way to destroy the transmitter is through the reception of your signal, but the only way to get your message is for me to first send mine. If I get my signal out, then my transmitter was destroyed in the past. But if the transmitter was destroyed, I can’t get the signal out.

This is just one of many common time-travel paradoxes brought about by traveling faster than light. This doesn’t rule out the existence of tachyons explicitly, but it does signal that they likely don’t exist. It seems impossible for us to travel backwards into the past or send signals into our own past: Everything in our universe not only travels no faster than the speed light, but also always in the direction of the future.

Physicists have proposed the “causality protection conjecture,” which says that faster than light travel (and travel into the past) is outright impossible. As of now this conjecture is merely a, well, conjecture, and not proven. We do not currently understand why travel into the past is forbidden, but we hope that someday we can construct a law of physics that tells us why.