The idea of particles traveling faster than light has tantalized physicists since the very dawn of relativity. While often dismissed as an impossibility, the mathematical rigor behind superluminal particles was developed over several decades by some of the 20th century's brightest minds.
Pre-Relativistic Era: Sommerfeld and Thomson
Before Albert Einstein published his theory of Special Relativity in 1905, the absolute limit of the speed of light was not established physics. As early as 1904, physicist Arnold Sommerfeld mathematically investigated the hypothetical properties of bodies moving faster than light. Similarly, J.J. Thomson (who discovered the electron) explored electromagnetic fields generated by superluminal point charges. However, these investigations were rendered seemingly obsolete with the advent of Einstein's framework.
1917: Tolman's Paradox
In 1917, physicist Richard Tolman firmly established why special relativity seemed to forbid faster-than-light travel. He demonstrated that if a particle could carry information faster than $c$, one could construct a scenario where an effect preceded its cause. This became known as Tolman's paradox, or the Tachyonic Antitelephone. For nearly 50 years, this causality violation was considered sufficient proof that superluminal particles could not exist.
1962: Sudarshan, Bilaniuk, and Deshpande
The modern theoretical framework for tachyons began in 1962. Physicists George Sudarshan, O.M.P. Bilaniuk, and V.K. Deshpande published a paper titled "Meta-Relativity" in the American Journal of Physics. They proposed that while special relativity prevents a particle from accelerating past the speed of light, it does not mathematically forbid the existence of a particle that was created already traveling faster than light.
They divided the universe into three mass classifications: bradyons (slower than light), luxons (speed of light), and a third class of superluminal particles with imaginary mass. To resolve Tolman's paradox, Sudarshan introduced the Reinterpretation Principle, arguing that a negative-energy particle traveling backward in time is physically equivalent to a positive-energy antiparticle traveling forward in time.
1967: Gerald Feinberg Coins "Tachyon"
Despite Sudarshan's work, the concept did not gain widespread attention until 1967. Gerald Feinberg, a physicist at Columbia University, published a seminal paper in Physical Review titled "Possibility of Faster-Than-Light Particles." Feinberg independently developed the quantum field theory of imaginary mass particles and formally coined the term tachyon (from the Greek tachys, meaning "swift").
Feinberg's paper was highly influential because it provided a robust quantum field theoretic framework for tachyons. He established the foundational understanding that a tachyon's energy decreases as its velocity increases, approaching infinite speed at zero energy.
1970s-1990s: Bosonic String Theory and Tachyon Condensation
During the development of Bosonic String Theory in the 1970s, theorists realized that the mathematical equations only worked in 26 dimensions, and worse, the lowest vibrational state (the ground state) of the string was a tachyon. This was seen as a catastrophic flaw, as it implied the vacuum of string theory was unstable.
It wasn't until the late 1990s that physicist Ashoke Sen demonstrated that this was not a bug, but a feature. He showed that the tachyon in open string theory represented the instability of D-branes, and "tachyon condensation" described the physical process of these branes decaying into the stable closed-string vacuum. This fundamentally shifted the definition of "tachyon" in modern physics from a sci-fi superluminal particle to a mathematical indicator of vacuum instability.
2011: The OPERA Neutrino Anomaly
The most dramatic moment in modern tachyon history occurred in September 2011. The OPERA collaboration at the Gran Sasso laboratory in Italy announced a shocking result: muon neutrinos fired from CERN in Switzerland appeared to arrive 60 nanoseconds earlier than a beam of light would have in a vacuum.
The physics community was thrown into an uproar. If true, this meant neutrinos were tachyons, and a century of special relativity would need to be rewritten. Hundreds of theoretical papers were published attempting to explain the superluminal neutrinos.
However, the excitement was short-lived. In 2012, the OPERA team discovered two equipment flaws that caused the anomaly: a fiber optic cable attached improperly to a GPS receiver, and a clock oscillator ticking slightly too fast. Once corrected, the neutrinos were confirmed to travel at speeds consistent with $c$. The tachyon remained elusive.