No experiment has ever detected a tachyon, and mainstream physics treats them as hypothetical particles that almost certainly do not exist as real, freely propagating objects. The idea dates to the 1960s, when a handful of physicists explored what special relativity would predict for particles whose rest mass is an imaginary number, meaning they would always travel faster than light. But the word “tachyon” has taken on a second, quite different life in modern theoretical physics, where tachyonic fields appear routinely in string theory and cosmology as mathematical signals that a system is unstable. These two meanings get tangled together constantly, and sorting them out is the key to understanding the state of the question.
Where the Idea Came From
Before Einstein published his 1905 theory of special relativity, physicists like Arnold Sommerfeld openly discussed whether electrons could exceed the speed of light. Once relativity became established, the question seemed closed: accelerating any ordinary particle to light speed would require infinite energy, so faster-than-light travel looked impossible. But a small group of theorists in the 1960s noticed a loophole. Relativity forbids crossing the light-speed barrier, yet it does not explicitly rule out particles that were born on the other side of it, always traveling faster than light. These hypothetical particles were named “tachyons,” from the Greek word for swift. From roughly 1965 to 1985, the tachyon hypothesis was seriously entertained by a minority of physicists.
1The European Physical Journal H. Tachyons before tachyons: Lev Strum (1890–1936) and superluminal velocitiesThe mathematics works like a mirror image of ordinary matter. For a normal particle, adding energy makes it go faster. For a tachyon, adding energy would slow it down, bringing it closer to light speed but never reaching it from above. Losing energy would make a tachyon speed up without limit. A tachyon at rest is nonsensical in the same way a normal particle at infinite speed is nonsensical: both would require crossing the light-speed barrier. The price of this odd behavior is that the particle’s rest mass must be an imaginary number, a quantity whose square is negative. That sounds bizarre, but it is mathematically consistent within special relativity. Whether nature actually contains such objects is an entirely different question.
The Causality Problem
The deepest objection to tachyons is not about energy or mass. It is about cause and effect. In special relativity, if a particle can travel faster than light, certain observers in different reference frames would see it arrive before it was sent. This is not a minor technicality. It opens the door to causal paradoxes, situations where an effect precedes its cause, or where you could in principle send a message to your own past.
The oldest and most famous version of this problem is the Tolman antitelephone paradox, first sketched out by the physicist Richard Tolman in 1917. The setup involves two observers moving relative to each other who exchange tachyonic signals. Because of the way relativity handles simultaneity, a round-trip tachyonic message could arrive back at the sender before it was ever sent. Some theorists have proposed ways around this paradox, arguing that a careful treatment of tachyon mechanics can preserve causality by imposing constraints on how tachyons interact with ordinary matter.
2arXiv. The Tolman “Antitelephone” Paradox: Its Solution by Tachyon MechanicsThese proposed fixes have never gained wide acceptance. Most physicists regard the causality problem as a strong theoretical reason to doubt that tachyons exist as real particles. Faster-than-light signaling would undermine the entire causal structure that underpins both relativity and quantum mechanics, and no one has demonstrated a consistent, paradox-free framework in which tachyonic particles interact with the rest of physics.
What Experiments Have Found
If tachyons existed and interacted with ordinary matter, they would leave detectable traces. Various experimental groups have looked for those traces, and every search has come up empty.
One approach involved hunting for tachyons associated with cosmic rays. If high-energy collisions in the atmosphere could produce tachyons, those tachyons would arrive at ground-level detectors before the ordinary shower particles because they would be traveling faster than light. A dedicated search scanned more than 200,000 cosmic ray air showers over 223 days, examining the time window before each shower for any sign of a faster-than-light precursor particle. The experiment identified over 1,500 candidate events, but after careful analysis, none could be convincingly attributed to tachyons.
3Canadian Journal of Physics. Search for tachyons preceding cosmic ray extensive air showers of energyOther searches have looked at particle collisions in accelerators, at astrophysical signals, and at any anomaly that might hint at faster-than-light propagation. None have produced positive results. The experimental record is consistent and unambiguous: no tachyon has ever been observed.
The OPERA Neutrino Episode
The closest the physics world came to a tachyon scare happened in 2011, when the OPERA experiment in Italy announced that neutrinos sent from CERN in Switzerland appeared to arrive about 60 nanoseconds earlier than light would have. If confirmed, the finding would have implied that neutrinos travel faster than light, which would have been the first evidence for a tachyonic particle in nature.
The result set off a frenzy of speculation, but suspicion was high from the start. Within months, the collaboration discovered that a loose fiber-optic cable and a malfunctioning oscillator had introduced timing errors. A dedicated follow-up measurement in 2012 found that the neutrino arrival time was consistent with the speed of light to within a few nanoseconds. The measured difference was so small it was indistinguishable from zero, and the corresponding limit on any deviation of the neutrino velocity from the speed of light was less than a few parts per million.
4Journal of High Energy Physics. Measurement of the neutrino velocity with the OPERA detector in the CNGS beam using the 2012 dedicated dataThe OPERA episode is often remembered as an embarrassment, but it also demonstrated that the physics community treats faster-than-light claims with appropriate skepticism. The anomaly was checked, rechecked, and ultimately traced to equipment error. Neutrinos, despite being strange in many ways, travel at or extremely close to the speed of light.
Tachyons in String Theory Mean Something Different
Here is where the conversation gets confusing for anyone following modern theoretical physics. The word “tachyon” appears constantly in string theory papers, but it refers to something quite different from a real faster-than-light particle zipping through space.
In the simplest versions of string theory, certain string vibration modes have a mathematical property equivalent to an imaginary mass. These modes are called tachyons, and they indicate that the particular version of the theory is unstable, like a ball sitting on top of a hill rather than resting in a valley. The system wants to “roll down” to a lower energy state. This rolling process is called tachyon condensation. In open string field theory, for example, an analytic solution for tachyon condensation has been used to prove a conjecture by the physicist Ashoke Sen about what happens when unstable objects in string theory (called D-branes) decay.
5Advances in Theoretical and Mathematical Physics. Analytic solution for tachyon condensation in open string field theoryThe tachyonic string mode is not a particle you could detect flying through a laboratory. It is a sign that the theoretical vacuum you are expanding around is not the true ground state of the theory. Once the tachyon condenses, which means the field rolls to its true minimum, the instability goes away. In the mature versions of superstring theory, a mathematical procedure called the GSO projection removes the tachyonic mode entirely, leaving a stable, tachyon-free theory. The projection simultaneously ensures that the theory has supersymmetry, a deep symmetry between bosons and fermions.
6arXiv. Spontaneously Broken Space-Time Supersymmetry in Open String Theory without GSO ProjectionWork on string field theory around the tachyon vacuum has explored what the theory looks like after the tachyon has fully condensed. These investigations have found that the resulting state has no open string dynamics at all, and that the tensions of various brane-like solutions in this vacuum match the known properties of D-branes perfectly.
7arXiv. String Field Theory Around the Tachyon VacuumSo when a string theorist talks about tachyons, they are not claiming that faster-than-light particles exist. They are discussing the mathematical structure of a theory and its stability properties. The tachyon is a symptom of a fixable problem in the formalism, not a prediction that something exotic is flying around the universe.
Tachyonic Fields in Ordinary Physics
The confusion deepens because tachyonic fields also show up in standard quantum field theory and condensed matter physics, again without implying that any particle is traveling faster than light. A tachyonic field is simply one where the mass-squared term in the equations is negative. This does not produce real superluminal particles. Instead, it signals an instability: the field is sitting at a local maximum of its potential energy and will roll toward a minimum.
The most famous example is the Higgs field before electroweak symmetry breaking. In the unbroken phase of the Standard Model, the Higgs has a negative mass-squared parameter. The field “condenses” to a nonzero value, giving mass to other particles in the process. Nobody calls the Higgs a tachyon in popular science, but mathematically it is a tachyonic field that underwent condensation. The same kind of instability appears in other contexts. In Bose-Einstein condensates, researchers have shown theoretically that annihilating domain walls can trigger tachyon condensation accompanied by spontaneous symmetry breaking, with the resulting dynamics following well-known scaling laws of phase-ordering kinetics.
8Physical Review D. Tachyon condensation due to domain-wall annihilation in Bose-Einstein condensatesThe pattern is the same across these examples: a negative mass-squared in the equations does not mean something is going faster than light. It means the system is unstable and will evolve toward a new equilibrium. Once it gets there, the tachyonic mode disappears. These are real, physically meaningful phenomena, but they have nothing to do with superluminal travel.
A related point of confusion involves superluminal group velocities in optics. In parametrically amplifying media, wave packets can propagate with group velocities that exceed the speed of light. Analysis of these “optical tachyons” has shown that while the peak of a pulse can arrive earlier than a light-speed signal would predict, the quantum information carried by the pulses remains causal.
9Optics and Spectroscopy. Optical Tachyons in Parametric Amplifiers: How Fast Can Quantum Information Travel?In other words, nothing you could use to send a message actually breaks the speed limit. The “superluminal” label applies to a mathematical feature of the wave, not to the transmission of information.
Tachyonic Fields in Cosmology
Tachyonic fields have also been explored as possible explanations for dark energy, the mysterious force accelerating the expansion of the universe. The idea, sometimes called the rolling tachyon model, comes from string-theory-inspired effective field theories. A tachyon field rolling down toward its ground state can produce an evolving equation of state that mimics dark energy. Coupling this rolling tachyon to Einstein’s gravity shows that the universe expands as the field rolls, potentially accelerating at first and then settling into a non-accelerating final state depending on the initial conditions and the geometry of the universe.
10Physics Letters B. Cosmological evolution of the rolling tachyonThese models have been tested against observational data, including supernovae, the cosmic microwave background, and the baryon acoustic oscillation scale. Generalized tachyon field models can be constrained to fit the data, yielding estimates for the matter density, model parameters, and the redshift at which the universe transitioned from decelerating to accelerating expansion. In some versions, the tachyon field acts as a unified description of both dark energy and dark matter, behaving like dark matter at early times and like dark energy later.
11Journal of Cosmology and Astroparticle Physics. Constraints on the generalized tachyon field models from latest observational dataThese cosmological tachyon models are taken seriously as mathematical frameworks, but they do not predict that any physical particle exceeds the speed of light. The “tachyon” in these models is a scalar field with certain mathematical properties, not a superluminal projectile. Whether the rolling tachyon is the correct explanation for dark energy remains an open question; it is one of many competing models, and current observational data do not clearly favor it over simpler alternatives like a cosmological constant.
What Would a Tachyon Gas Look Like
Theoretical physicists have tried to work out what would happen if you had a collection of tachyons behaving as a gas, using the standard tools of statistical mechanics. The exercise is illuminating because it shows how strange the physics would get.
The energy spectrum of a tachyon is unusual: instead of starting from zero and going up, it has a gap determined by the tachyon’s imaginary mass parameter, and the relationship between momentum and energy is inverted compared to ordinary particles. Early analyses found that an ideal gas of bosonic tachyons would exhibit Bose-Einstein condensation, a striking prediction given how different the underlying particles would be from normal bosons. However, the same analysis found that such a gas would violate the third law of thermodynamics, which states that entropy approaches a well-defined minimum as temperature approaches absolute zero.
12Il Nuovo Cimento B. The ideal gases of tachyonsMore careful calculations have grappled with the question of how to handle the peculiar energy spectrum. One approach that integrates over all momenta, using an extended distribution function, avoids imaginary quantities and produces an entropy that behaves sensibly, but only with specific choices about how to treat the low-momentum region where the tachyon energy spectrum becomes zero.
13arXiv. Careful calculation of thermodynamical functions of tachyon gasThese results suggest that even in a purely theoretical setting, tachyonic matter is pathological. The thermodynamics does not behave the way it should unless you make special assumptions, and even then, it remains unclear whether the resulting system is physically meaningful. This is not proof that tachyons cannot exist, but it adds to the pile of theoretical reasons why they probably do not.
Tachyons and Black Holes
If tachyons existed, their behavior near a black hole would be dramatically different from that of ordinary matter. An analysis of tachyon motion in the gravitational field of a non-rotating black hole, using the relativistic equations for a particle with imaginary rest mass, found that there are no stable circular orbits for tachyons in the Schwarzschild spacetime. All unstable circular orbits lie in a narrow region between the event horizon and 1.5 times the gravitational radius. Perhaps most striking, the calculations indicate that tachyons could escape from within the event horizon, the boundary from which not even light can return.
14arXiv. Tachyon motion in a black hole gravitational fieldThis is a direct consequence of moving faster than light. The event horizon is defined as the surface where the escape velocity equals the speed of light. Something already exceeding that speed would not be trapped. The analysis also found that low-energy tachyons near a black hole could gain energy through gravitational interactions, potentially producing observable effects if such particles existed. No such effects have been observed, which is consistent with the absence of tachyons in nature but represents yet another way that future observations could in principle test the hypothesis.
Why the Word Persists
Given that no tachyon has ever been detected and the theoretical problems are severe, it might seem odd that the term keeps showing up in research papers. The reason is that “tachyon” has become shorthand for a particular mathematical structure: a field with a negative mass-squared, signaling instability. That structure is genuinely important in physics. It appears in the Higgs mechanism, in string theory, in cosmological models, and in condensed matter systems. Physicists keep writing about tachyons not because they believe in faster-than-light particles, but because tachyonic instabilities are a recurring and physically meaningful feature of quantum field theories.
The gap between the popular understanding and the technical usage is wide. When a news headline says “physicists study tachyons,” a general reader pictures a glowing particle streaking through space backward in time. A physicist reading the same headline pictures a scalar field rolling down a potential energy curve. Both are using the same word, but they are talking about fundamentally different things. The original 1960s question, whether faster-than-light particles zoom around the universe, has been answered about as firmly as physics can answer negative questions: every experiment says no, every theoretical framework says the idea creates more problems than it solves, and the concept survives only in a heavily transformed mathematical sense that has little to do with actual superluminal travel.
In quantum field theory, the appearance of a tachyonic mode is more like a warning light on a dashboard. It tells you something about the state of the theory, not about a particle you should expect to find. When a field has vacuum instability indicated by tachyon poles in its propagators, that typically reflects a breakdown of the perturbative expansion rather than the existence of real faster-than-light excitations.
15Physics Letters B. “Vacuum instability” and tachyon poles in non-asymptotically free field theoriesThe honest answer to “do tachyons exist” depends on which meaning of the word you have in mind. Faster-than-light particles that zoom through the cosmos? Almost certainly not, and every experimental search has failed to find them. Tachyonic fields, mathematical objects with negative mass-squared that signal instability and drive phase transitions? Those are real and well-established features of modern physics, but they are not what most people mean when they ask the question.