In 1967, Columbia University physicist Gerald Feinberg published a paper in Physical Review proposing particles that always travel faster than light. He called them tachyons, from the Greek tachys meaning “swift.” Nearly six decades later, no experiment has detected one, but the theory keeps reshaping how physicists think about causality, energy, and the fabric of spacetime.

What Exactly Is a Tachyon?

A tachyon is a hypothetical particle that exists only at speeds exceeding the speed of light (c = 299,792,458 m/s). Unlike ordinary matter (called bradyons or tardyons), which can never reach light speed, tachyons can never slow down to it. The light speed barrier works both ways.

The key physics: tachyons carry imaginary rest mass. In the relativistic energy equation $E = \frac{m_0 c^2}{\sqrt{1 - v^2/c^2}}$, plugging in $v > c$ makes the denominator imaginary. But if $m_0$ is also imaginary (written as $m_0 = i\mu$ where $\mu$ is real), the two imaginary terms cancel, producing real, positive energy and momentum.

This means tachyons have an inverted energy-speed relationship. Normal particles gain energy as they speed up. Tachyons lose energy as they accelerate. A tachyon with zero energy would theoretically travel at infinite speed, a state physicists call “transcendent.”

How Do Tachyons Challenge Special Relativity?

Einstein’s special relativity doesn’t actually forbid faster-than-light particles. It forbids accelerating through the light barrier. Tachyons sidestep this by never being below c in the first place.

The real problem is causality. According to special relativity, FTL signals could arrive before they’re sent in certain reference frames. Physicist Gregory Benford (UC Irvine) and colleagues formalized this as the tachyonic antitelephone problem in 1970: if you could send tachyon signals, you could theoretically communicate with your own past.

Most physicists consider this a strong argument against tachyons existing as real, detectable particles. The Stanford Encyclopedia of Philosophy notes that tachyon-based causal loops would violate every known formulation of causality in physics.

What Have Experiments Found?

Physicists have searched for tachyons since the 1960s with consistent null results:

  • Alvager and Erman (1966) examined beta decay from thulium-170, looking for anomalous particles among 10,000 decays. Found nothing above background.
  • Baltay et al. (1970) searched for tachyon production in proton collisions at Brookhaven National Lab. Set an upper limit of $10^{-3}$ on tachyon production branching ratios.
  • Clay and Crouch (1974) analyzed cosmic ray air showers for precursor signals (particles arriving before the light-speed shower front). No tachyon candidates detected.
  • OPERA experiment (2011) at CERN’s Gran Sasso lab initially reported neutrinos traveling faster than light. The result generated global excitement but was traced to a loose fiber optic cable causing a 73-nanosecond timing error.

The Particle Data Group at Lawrence Berkeley National Lab maintains comprehensive limits. No credible evidence for tachyons has emerged from any accelerator experiment, cosmic ray survey, or astrophysical observation.

Tachyons in Quantum Field Theory

In modern physics, “tachyon” has taken on a different meaning. A tachyonic field has a negative mass-squared term in its Lagrangian: $(\square + m^2)\phi = 0$ with $m^2 < 0$. This doesn’t mean FTL particles exist. It signals that the vacuum state is unstable and wants to decay to a lower-energy configuration.

This is actually important physics. The Higgs mechanism, which gives particles mass, involves a tachyonic instability. Before electroweak symmetry breaks, the Higgs field has a tachyonic mode. The field “rolls” to its true vacuum, and the tachyon disappears. As the CERN Higgs FAQ explains, this is how particles acquire mass in the Standard Model.

In string theory, open bosonic strings also have tachyonic modes, indicating the underlying vacuum is unstable. Ashoke Sen’s work on tachyon condensation showed these modes drive the string vacuum to a stable state, a key result in string field theory.

A Computational Approach to Tachyon Dispersion

If you want to visualize tachyon physics, you can plot the dispersion relation. For a tachyon with imaginary mass $i\mu$, the energy-momentum relationship is $E^2 = p^2c^2 - \mu^2c^4$. Here’s a quick Python visualization:

import numpy as np
import matplotlib.pyplot as plt

c = 1  # natural units
mu = 0.5  # tachyon mass parameter

p = np.linspace(mu * c, 5, 500)  # momentum range
E_tachyon = np.sqrt(p**2 * c**2 - mu**2 * c**4)
E_normal = np.sqrt(p**2 * c**2 + mu**2 * c**4)  # normal particle

plt.figure(figsize=(8, 5))
plt.plot(p, E_tachyon, 'r-', label='Tachyon ($m^2 < 0$)', linewidth=2)
plt.plot(p, E_normal, 'b-', label='Bradyon ($m^2 > 0$)', linewidth=2)
plt.plot(p, p * c, 'k--', label='Photon ($m = 0$)', alpha=0.5)
plt.xlabel('Momentum (p)')
plt.ylabel('Energy (E)')
plt.title('Dispersion Relations: Tachyon vs Normal Matter')
plt.legend()
plt.grid(True, alpha=0.3)
plt.show()

Notice the tachyon curve starts at a minimum momentum of $p = \mu c$. Below that, energy becomes imaginary, meaning tachyons cannot exist below a threshold momentum. This is the mathematical signature that separates them from ordinary particles.

For more on how data analysis intersects with physics research, see our analysis of AI’s role in space operations and exoplanet detection dashboards.

Why Most Physicists Don’t Believe Tachyons Are Real

The physics community’s consensus, summarized well by Sean Carroll (Caltech/Johns Hopkins), is that tachyons as detectable FTL particles almost certainly don’t exist. The reasons:

  1. Causality violation is experimentally unsupported. No backward-in-time signal has ever been observed.
  2. Vacuum instability from tachyonic modes resolves itself through condensation (as in the Higgs mechanism), eliminating the tachyon.
  3. Every experimental search since 1966 has returned null results with increasingly tight bounds.
  4. Quantum field theory reinterprets tachyonic modes as instabilities, not physical particles.

That said, tachyons remain useful as theoretical tools. They probe the boundaries of relativity, inform string theory, and help physicists understand phase transitions and symmetry breaking.

Sources & Further Reading

Frequently Asked Questions

Are tachyons real or purely theoretical?

Tachyons remain purely theoretical. Despite systematic searches since the 1960s across particle accelerators, cosmic ray detectors, and astrophysical observations, no experiment has ever detected a tachyon. The Particle Data Group at Berkeley Lab tracks these limits. In modern quantum field theory, “tachyon” typically refers to an unstable field mode rather than a physical faster-than-light particle.

Could tachyons enable time travel or backward communication?

Theoretically, FTL signals could arrive before being sent in certain reference frames, which is the tachyonic antitelephone paradox described by Benford, Book, and Newcomb in 1970. However, this paradox is widely considered evidence against tachyons existing, since no violation of causality has ever been observed experimentally. The consensus view is that tachyonic antitelephones are logically possible but physically unrealizable.

What happened with the OPERA “faster than light” neutrinos?

In September 2011, the OPERA experiment at CERN reported neutrinos arriving 60 nanoseconds faster than light over 730 km. The finding made global headlines. By February 2012, CERN confirmed the anomaly was caused by a loose fiber optic connector and a clock oscillator running fast. Corrected measurements showed neutrinos traveling at exactly light speed, consistent with standard physics.

How are tachyons used in string theory?

In bosonic string theory, open strings have a tachyonic ground state, indicating vacuum instability. Ashoke Sen’s 1999 work showed this tachyon “condenses” as the unstable D-brane decays to a stable vacuum, where the tachyon disappears. This tachyon condensation was a major result confirming that tachyonic modes in string theory don’t represent physical FTL particles but rather signal phase transitions between vacuum states.