The speed of light in vacuum, $c = 299,792,458$ meters per second, is the most fundamental speed limit in physics. Einstein's special relativity established that no object with mass can be accelerated to the speed of light. But relativity says something subtler than most people realize: it does not categorically forbid particles that were always superluminal. The search for such particles has spanned over a century.
1. The Speed of Light as a Barrier, Not a Wall
Special relativity divides the universe into three kinematic sectors. Ordinary matter (bradyons) always travels below $c$. Massless particles like photons (luxons) always travel at exactly $c$. The third sector, occupied by hypothetical superluminal particles (tachyons), describes entities that always travel above $c$.
The crucial insight is that the speed of light functions as a barrier, not a wall. Bradyons cannot be pushed through $c$ from below, and tachyons cannot be slowed through $c$ from above. Each class is permanently confined to its own side of the divide. The mathematics of special relativity is fully consistent within each sector. Nothing in the Lorentz transformation equations produces a logical contradiction when applied to $v > c$, as long as the particle was never at or below $c$ to begin with.
2. Historical Proposals for FTL Particles
The idea that particles might exceed the speed of light predates Einstein. In 1904, the German physicist Arnold Sommerfeld analyzed the electromagnetic radiation pattern of a charged particle moving faster than light, discovering it would produce a cone of radiation analogous to the sonic boom of a supersonic aircraft. This "Cherenkov-like" radiation for superluminal charges was a purely theoretical exercise at the time.
The modern theoretical foundation was laid in 1962 by Olexa-Myron Bilaniuk, V. K. Deshpande, and E. C. George Sudarshan at Syracuse University. Their paper "Meta Relativity" demonstrated that faster-than-light particles are fully compatible with the postulates of special relativity. They showed that such particles would have imaginary rest mass, real energy and momentum, and the counterintuitive property of speeding up as they lose energy.
In 1967, Gerald Feinberg at Columbia University published "Possibility of Faster-Than-Light Particles" in Physical Review, coining the term tachyon. Feinberg went further than his predecessors by attempting to construct a quantum field theory of tachyons, analyzing their emission and absorption properties, and proposing experimental signatures that could be searched for. His paper remains the foundational reference for the field. For a full treatment of tachyon physics, see our comprehensive guide.
3. Tachyons: The Primary FTL Candidate
Tachyons remain the only theoretically well-defined candidate for a fundamental faster-than-light particle. Their properties are fully determined by special relativity and the assumption of imaginary mass ($m² < 0$):
- Speed range: From just above $c$ (at high energy) to infinite velocity (at zero energy). A tachyon can never decelerate to $c$ or below.
- Energy-speed inversion: Unlike ordinary particles, tachyons accelerate as they radiate energy. The lowest energy state corresponds to infinite speed.
- Real observables: Despite imaginary mass, a tachyon's energy, momentum, and velocity are all real, measurable quantities.
- Cherenkov radiation: A charged tachyon moving through vacuum would emit electromagnetic Cherenkov radiation, analogous to the blue glow emitted by particles exceeding the speed of light in a medium like water.
Experimental searches for tachyons have focused on looking for this vacuum Cherenkov radiation and on anomalies in particle decay kinematics. No positive detection has been made. For details on experimental efforts, see our page on tachyon detection methods.
4. Apparent FTL Phenomena That Are Not Really FTL
Several well-known physical phenomena appear to involve faster-than-light propagation but, on careful analysis, do not transmit information superluminally. Understanding these cases is essential for distinguishing genuine FTL from illusion.
Phase Velocity and Group Velocity
A monochromatic wave's phase velocity (the speed at which a crest moves) can exceed $c$ in many media. In X-ray propagation through glass, the phase velocity is superluminal. Similarly, the group velocity of a wave packet can exceed $c$ in regions of anomalous dispersion, as demonstrated by Lijun Wang at Princeton in 2000, who sent a light pulse through cesium gas at a group velocity of $-c/310$ (meaning the peak exited before it entered). Neither phase velocity nor group velocity carries information. The signal velocity, defined by the wavefront, remains at or below $c$.
Quantum Tunneling
When a particle tunnels through a potential barrier, the traversal time can be extremely short, leading to an apparent superluminal crossing speed. Gunter Nimtz at the University of Cologne claimed in the 1990s to have transmitted Mozart's 40th Symphony at 4.7 times $c$ through a microwave waveguide below cutoff. However, the consensus view is that tunneling involves reshaping of the wave packet rather than genuine faster-than-light propagation of a signal. The leading edge of the wave packet, which carries the information, is never truly superluminal.
Expanding Universe
Distant galaxies recede from us at speeds exceeding $c$ due to the expansion of space itself. Galaxies beyond the Hubble sphere have recession velocities greater than $c$, and we can observe some of them because their light was emitted when they were closer. This is not a violation of special relativity because the speed limit applies to objects moving through space, not to the expansion of the spatial metric itself. No information is transmitted faster than light.
Quantum Entanglement
Measuring one entangled particle instantaneously determines the state of its partner, regardless of distance. Einstein called this "spooky action at a distance." However, the no-communication theorem proves rigorously that entanglement cannot be used to transmit information. The measurement outcomes appear random to each observer individually; the correlations only become apparent when the observers compare notes through a classical (subluminal) channel.
5. The Scharnhorst Effect
One of the most intriguing theoretical predictions of genuine superluminal propagation comes from Klaus Scharnhorst and Gabriel Barton. In 1990, they calculated that photons traveling between two Casimir plates (closely spaced conducting surfaces that suppress quantum vacuum fluctuations) should travel slightly faster than $c$. The suppressed vacuum reduces the effective "drag" on virtual electron-positron pairs that briefly appear during photon propagation.
The predicted speed increase is extraordinarily small: roughly one part in $10^36$ for plates separated by one micrometer. This is far beyond current measurement capability. Nonetheless, it represents a case where standard quantum electrodynamics itself predicts $v > c$ for photons in a modified vacuum. For a detailed treatment, see our page on the Casimir effect and tachyonic physics.
6. The OPERA Incident
In September 2011, the OPERA experiment at Gran Sasso National Laboratory in Italy reported that muon neutrinos sent from CERN (730 km away) arrived 60.7 nanoseconds earlier than expected for light-speed travel. If correct, this would have constituted the first direct observation of faster-than-light particles.
The announcement triggered enormous scientific and media attention. Thousands of theoretical papers attempted to explain or accommodate the result. However, the OPERA collaboration identified two equipment problems in February 2012: a faulty optical fiber connection in the GPS synchronization system (which made the neutrinos appear to arrive early) and an oscillator clock running slightly too fast. When corrected, the neutrino travel time was consistent with the speed of light.
Lessons from OPERA
The OPERA episode demonstrated both the rigor and the self-correcting nature of physics. The collaboration was transparent about its anomalous result, invited scrutiny, and ultimately identified the systematic error. Four independent experiments (ICARUS, LVD, Borexino, and OPERA itself after repairs) subsequently confirmed that neutrinos travel at a speed consistent with $c$ to within experimental precision.
7. Why FTL Particles Would Break Causality
The deepest objection to faster-than-light particles is not energy or momentum but causality. In special relativity, if a signal can travel faster than light in one reference frame, then there exist other reference frames (related by a standard Lorentz boost) in which that signal travels backward in time. If two such signals can be exchanged between two observers in relative motion, a closed causal loop is created: a message can be sent into the sender's own past.
This construction, first described by Albert Einstein and later formalized as the tachyonic antitelephone, would allow genuine time-travel paradoxes. You could, in principle, send a message to yourself before you decided to send it, creating a logical contradiction.
Proposed resolutions include the reinterpretation principle (originally by Bilaniuk, Sudarshan, and Feinberg), which reinterprets a negative-energy tachyon traveling backward in time as a positive-energy tachyon traveling forward in time in the opposite direction. Whether this fully resolves the paradox remains debated. Some physicists argue that a consistent quantum field theory of tachyons would require abandoning the principle of Lorentz-invariant causality altogether.
8. Current Status of the Search
As of the mid-2020s, no faster-than-light particle has been experimentally detected. The constraints are strong:
- Neutrino speed measurements: Post-OPERA experiments have confirmed neutrinos travel at $c$ to within a few parts per billion. The 2017 multimessenger observation of neutron star merger GW170817 constrained the speed of gravitational waves relative to light to within one part in $10^15$.
- Vacuum Cherenkov searches: High-energy cosmic ray observations place stringent limits on the existence of charged tachyons. If they existed with appreciable coupling to the electromagnetic field, their Cherenkov radiation would have been detected.
- Collider experiments: No anomalous missing energy signatures consistent with tachyon production have been observed at the LHC or previous colliders.
- Tachyonic fields in theory: While tachyonic fields ($m² < 0$) are essential in the Standard Model (the Higgs field before symmetry breaking) and string theory (open string tachyons on unstable branes), they describe vacuum instabilities, not detectable superluminal particles.
The theoretical landscape has shifted. Most physicists now view tachyonic fields not as sources of literal faster-than-light particles but as signals of vacuum instability that resolve through condensation. Nevertheless, the question of whether special relativity's third kinematic sector is physically realized remains open. Superluminal particles are not logically forbidden, and the absence of evidence is not evidence of absence. The search continues through precision neutrino experiments, cosmic ray observatories, and theoretical work on Lorentz invariance violation.