What is a tachyon?

kieronconway
Sep 14, 2025
3 min read

Updated: 1 day ago

Tachyons are hypothetical particles that, if they exist, can only move faster than the speed of light. In theory, a tachyon can never slow down to the speed of light — just as a normal particle with real mass can never be accelerated up to, or beyond, light speed.

Fundamental Differences from Normal Particles

The differences between real particles and hypothetical tachyons are profound:

a) Rest mass

Normal particles have a real, invariant rest mass: the same in all inertial frames of reference.
Tachyons, by definition, would always be moving faster than light and could never be at rest. Because of this, they have no rest frame, and their “mass” is usually described as imaginary in the mathematical sense, involving √–1.

b) Energy behaviour

For a normal particle, as it is accelerated, its energy increases without bounds as it approaches the speed of light, which it can never reach.
For tachyons: If you apply a force opposite to its motion, instead of slowing it down, it actually speeds up — pushing against its direction of flight takes away energy, which makes it go faster!
Conversely, if you apply a force in the same direction as its motion, you would slow it down, meaning it approaches c from above but never drops below it. Pushing with it, adds energy, which slows it down towards, but never reaching c!

Graphical Representations

Normal Particles: If you plot energy versus velocity:

A normal particle starts at rest, then accelerates as energy is added.
As it approaches the speed of light, the energy required to accelerate it further rises without limit. To reach exactly c would require infinite energy, which is impossible.

Tachyons: On the other side of the graph, a hypothetical tachyon might start, say, at three times light speed.

As it slows towards the speed of light, its energy would increase without bounds.
It could never slow down to, or below the velocity of light in a vacuum, meaning it must always remain faster than light.

Where Do Tachyons Come From?

Tachyons don’t appear in classical general relativity solutions. Instead, they emerge as mathematical oddities:

In special relativity, if you allow for “imaginary mass” (using √–1), the equations can describe trajectories that are always faster than light.
In mathematics, just as the square root of 9 has two solutions (+3 and –3), equations in physics often have both real and imaginary solutions. For example: a real solution in an equation might correspond to a normal particle (with mass > 0, speed < c) and an “imaginary mass” solution corresponds to a tachyon (mass involving √–1, speed > c).

In physics, imaginary solutions sometimes correspond to real, physical things (e.g., electrons and positrons as two valid solutions). But in the case of tachyons, the imaginary-mass solution appears to describe something that cannot exist in reality — it violates causality (the relationship between cause and effect) and the known structure of space-time.

Violating causality is a big no-no in physics, which is why tachyons are not taken seriously by physicists. In theory, a tachyon could be used to send messages faster than light speed and this has the effect that some observers in space-time would receive the message before it was sent, breaking the link between cause and effect!

Faster Than Light Travel

Einstein’s special relativity makes one thing clear:

Nothing with real, positive mass can be accelerated to the speed of light in a vacuum, or beyond — the energy required becomes infinite.
Tachyons don’t solve this, because their existence would break causality and the consistency of physics itself.

General relativity, however, allows spacetime itself to behave in ways that look like faster-than-light travel:

Cosmic inflation — the early universe expanded faster than light, not because matter moved faster than light, but because space itself expanded.
Warp drive — a hypothetical bubble of warped space-time could carry a ship faster than light relative to distant observers.
Wormholes — shortcuts through space-time that could, in theory, connect distant regions more quickly than light could travel the long way around.

All three ideas remain speculative and face enormous practical barriers — especially the need for exotic matter or negative energy on a scale far beyond our capabilities.

Conclusion

Tachyons are fascinating mathematical curiosities that highlight the limits of relativity. But there’s no experimental evidence that they exist, and their presence would break fundamental principles of physics like causality.

For now, both tachyons and faster-than-light travel remain firmly in the realm of theoretical speculation and science fiction.

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