Faster-than-light (superluminal) travel and communication are hypothetical ways for matter or information to move faster than the speed of light in a vacuum (c). According to Einstein's special theory of relativity, only particles with no rest mass, such as photons, can travel at the speed of light. Nothing else can reach or exceed this speed.

Scientists have proposed hypothetical particles called tachyons that might move faster than light. However, if tachyons existed, they could break the rules of cause and effect and allow time travel. Most scientists believe tachyons do not exist.

Based on all observations and current scientific theories, all matter moves slower than the speed of light (subluminal) relative to the space around it. Some speculative ideas suggest faster-than-light movement, such as the Alcubierre drive, Krasnikov tubes, traversable wormholes, and quantum tunneling. These ideas sometimes suggest ways to bypass general relativity, like stretching or shrinking space to make an object appear to move faster than light. However, these proposals are still considered impossible because they break current rules about cause and effect and require unlikely mechanisms, such as exotic matter.

Superluminal travel of non-information

In this article, "faster-than-light" refers to the movement of information or matter at speeds greater than c, a constant equal to the speed of light in a vacuum, which is 299,792,458 meters per second (or about 186,282 miles per second). This is not the same as traveling faster than light, because:

• Some processes move faster than c, but they do not carry information (examples are discussed later).
• In certain materials, light travels slower than c (at c/n, where n is the refractive index), and some particles can move faster than c/n (but still slower than c). This creates a type of radiation called Cherenkov radiation (explained further below).

These examples do not break the rules of special relativity or cause problems with cause and effect, so they are not considered "faster-than-light" in the way described here.

In some cases, things may seem to move faster than light, but they do not carry energy or information faster than light. These examples do not break the rules of special relativity. For example:

• From Earth, objects in the sky appear to complete one full circle around Earth every day. Proxima Centauri, the closest star outside our solar system, is about 4.5 light-years away. In this view, Proxima Centauri seems to move in a circle with a radius of 4 light-years. Its apparent speed could be much greater than c, but this is only because of the large distance and the way motion is calculated for circular paths. Similarly, comets can appear to move faster or slower than light depending on their distance from Earth, even though their actual speed is not faster than light.

• If a laser beam is swept across a distant object, the spot of light on the object may appear to move faster than c. Similarly, a shadow on a distant object can seem to move faster than c. However, the light itself does not travel faster than c, and no information is sent faster than light. In these cases, nothing is actually moving faster than light. For example, if a garden hose is swung side to side, the water does not instantly follow the direction of the hose.

The speed at which two objects move toward each other in a single frame of reference is called their closing speed. This can be close to twice the speed of light, such as when two particles move toward each other near the speed of light in opposite directions.

Special relativity does not prevent this. It explains that using simple rules (like Galilean relativity) to calculate speed is incorrect when dealing with objects moving near the speed of light. Instead, special relativity provides the correct formula for calculating relative speeds.

For example, if two particles move at speeds v and −v in a frame of reference (toward each other), their closing speed is 2v. If v is close to c, the closing speed may be greater than c. However, this does not mean the particles themselves are moving faster than light in their own frames.

If a spaceship travels to a planet 1 light-year away (as measured from Earth) at high speed, the time it takes to reach the planet may be less than one year from the traveler’s perspective. However, Earth-based clocks will always measure the journey as taking more than one year. The speed calculated by dividing Earth’s distance by the traveler’s time is called proper speed. Proper speed has no upper limit because it is not measured in a single frame of reference. A light signal sent at the same time as the traveler would always reach the destination before the traveler does.

When light travels through a material, its phase velocity (the speed of a specific wave component) can sometimes be faster than c. This happens in some materials at X-ray frequencies. However, phase velocity does not carry information, so it does not break relativity.

The group velocity (the speed at which the overall shape of a wave pulse moves) can also exceed c in some cases. However, this does not mean information is moving faster than light. Even though the peak of a pulse might appear to move faster than c, the actual information about the pulse arrives no faster than c. In some cases, like with a Gaussian beam in a vacuum, the peak of a pulse can move faster than c due to diffraction, but the overall energy does not.

According to Hubble’s law, the expansion of the universe causes distant galaxies to appear to move away from us faster than light. However, this is not a real speed in the way relativity defines velocity. In general relativity, velocity is only meaningful locally, and there is no single way to define the speed of a distant galaxy. The apparent faster-than-light motion is a result of how space itself expands, not actual movement of the galaxies.

Many galaxies observed in telescopes have redshifts of 1.4 or higher, meaning they are moving away from us faster than light. Because the universe’s expansion is slowing over time, some galaxies that appear to move faster than light may still send signals that eventually reach us.

However, because the universe’s expansion is accelerating, most galaxies will eventually move beyond a point called a cosmological event horizon. Once past this point, any light they emit will never reach us. The current distance to this event horizon is about 16 billion light-years. This means a signal from an event happening now would eventually reach us if it was within 16 billion light-years, but signals from events farther away would never reach us.

Superluminal communication

Faster-than-light communication, according to the theory of relativity, is similar to time travel. The speed of light in a vacuum, or near a vacuum, is a fundamental constant called "c." This means that all observers, whether moving at a constant speed or accelerating, will always measure massless particles like photons traveling at speed "c" in a vacuum. This result shows that time and speed measurements in different frames of reference are not connected by simple changes but instead by mathematical rules called Poincaré transformations. These rules have important effects:

• The momentum of a massive object increases as its speed increases. At the speed of light, an object would have infinite momentum.
• To accelerate an object with mass to the speed of light would require infinite energy, either through infinite time with any acceleration or infinite acceleration in a short time.
• In any case, achieving faster-than-light speed would need infinite energy.
• Observers moving at less than light speed might disagree about the order of two events if the events are separated by a large distance. Faster-than-light travel could appear to move backward in time from some viewpoints or require unproven ideas like possible changes to Lorentz symmetry at extremely small scales, such as the Planck scale. Any theory allowing true faster-than-light travel must address time travel and its problems or assume that Lorentz symmetry is not a perfect rule but one that might break at scales we cannot observe.
• In special relativity, the speed of light is guaranteed to be "c" only for observers moving at constant speed (inertial frames). For observers accelerating or changing direction (non-inertial frames), the measured speed of light might differ from "c." In general relativity, no large region of curved spacetime has a perfectly inertial frame, so it is possible to use a global coordinate system where objects appear to move faster than light. However, near any point in curved spacetime, a local inertial frame can be defined, where the speed of light is always "c," and all massive objects move slower than light in that local frame.

Justifications

Special relativity states that the speed of light in a vacuum remains the same in all inertial frames. This means the speed of light does not change, even if the observer is moving at a constant speed. The equations of special relativity do not assign a specific value to the speed of light, which is determined through experiments using a fixed unit of length. Since 1983, the meter has been officially defined using the speed of light.

Experiments to measure the speed of light have been conducted in a vacuum. However, the vacuum we know is not the only type of vacuum that can exist. A vacuum has energy, called vacuum energy, which might change under certain conditions. If vacuum energy decreases, light may travel faster than the standard speed of light, a phenomenon called the Scharnhorst effect. This type of vacuum can be created by placing two perfectly smooth metal plates very close together, forming a Casimir vacuum. Calculations suggest that light would travel slightly faster in this vacuum. For example, a photon traveling between plates spaced 1 micrometer apart might increase its speed by about one part in 10. However, this effect has not yet been confirmed by experiments. Some studies suggest that the Scharnhorst effect cannot be used to send information backward in time with a single set of plates because the plates’ rest frame would define a preferred frame for faster-than-light signaling. Other studies note that multiple plates in motion might allow for potential causality violations, but these ideas remain unproven and require a theory of quantum gravity to fully analyze. Some scientists argue that the original calculations about the Scharnhorst effect may have used incorrect approximations, making it unclear if the effect actually increases signal speed.

Eckle et al. claimed that particle tunneling occurs in zero real time. Their experiments involved electrons tunneling through a barrier, where a relativistic prediction suggested a tunneling time of 500–600 attoseconds (one quintillionth of a second). However, their measurements only detected 24 attoseconds, the limit of their test’s accuracy. Other physicists believe that experiments showing particles spending very short times inside barriers are still consistent with relativity, though there is debate about whether the explanation involves changes in the wave packet or other effects.

Special relativity is strongly supported by experimental evidence, so any changes to it must be very subtle and hard to measure. One proposed modification is doubly special relativity, which suggests the Planck length is the same in all reference frames. This idea is associated with scientists Giovanni Amelino-Camelia and João Magueijo. Other theories suggest that inertia arises from the combined mass of the universe, as in Mach’s principle. If true, this would imply that the universe’s rest frame might be preferred by natural laws. However, testing this idea is difficult because the relevant comparisons would lie outside the observable universe. Despite this, some experiments have been proposed to investigate these theories.

Special relativity states that no object can move faster than the speed of light. However, general relativity allows the space between distant objects to expand, creating a "recession velocity" that can exceed the speed of light. Galaxies more than 14 billion light-years away from Earth are thought to have this kind of recession velocity. Miguel Alcubierre proposed a "warp drive," where a spaceship would be enclosed in a bubble of space that contracts in front and expands behind, allowing the bubble to move faster than light without violating relativity. However, practical use of this idea faces significant challenges. Another possibility from general relativity is a "traversable wormhole," which could create a shortcut between distant points in space. Travelers moving through a wormhole would not locally move faster than light but could reach their destination faster than light traveling through normal space.

Gerald Cleaver and Richard Obousy, a professor and student from Baylor University, theorized that manipulating extra spatial dimensions in string theory with a large amount of energy could create a "bubble" allowing a spaceship to travel faster than light. They suggest that altering the 10th spatial dimension could change dark energy in the three large spatial dimensions (height, width, and length). Cleaver noted that positive dark energy currently causes the universe’s expansion to accelerate.

The possibility that Lorentz symmetry might be broken has been studied extensively, especially after the development of the Standard-Model Extension, a theory that describes this potential violation. This framework has enabled experiments in gravity, particles, and cosmic rays to search for evidence of Lorentz symmetry breaking. Breaking this symmetry introduces direction dependence and unconventional energy dependence, leading to effects like Lorentz-violating neutrino oscillations and changes in particle dispersion relations, which could allow particles to move faster than light.

In some models, Lorentz symmetry is still part of fundamental physical laws but may have been broken shortly after the Big Bang, leaving a "relic field" that affects particle behavior based on their velocity relative to the field. Other models suggest that Lorentz symmetry might be broken in a more fundamental way. If Lorentz symmetry is not a fundamental symmetry at the Planck scale or another fundamental scale, particles with speeds different from the speed of light could be the basic building blocks of matter.

In current models of Lorentz symmetry violation, the parameters describing these effects are expected to depend on energy. Therefore, low-energy experiments cannot confirm high-energy phenomena. However, many studies using the Standard-Model Extension have searched for evidence of Lorentz violation at high energies.

FTL neutrino flight results

Very accurate measurements from the MINOS collaboration showed that 3 GeV neutrinos traveled at a speed equal to the speed of light, with a difference of one part in a million.

On September 22, 2011, a paper shared before official publication by the OPERA Collaboration reported that 17 and 28 GeV muon neutrinos, sent 730 kilometers (454 miles) from CERN near Geneva, Switzerland, to the Gran Sasso National Laboratory in Italy, traveled faster than light by a difference of about 1 in 40,000. This result had a 6.0-sigma significance, meaning it was very unlikely to occur by chance. On November 17, 2011, OPERA scientists repeated the experiment and confirmed the same results. However, scientists questioned the reliability of these findings. In March 2012, the ICARUS collaboration could not repeat the OPERA results using their equipment, finding that neutrino travel time matched the speed of light. Later, the OPERA team identified two errors in their setup: a fiber-optic cable attached incorrectly, which caused the faster-than-light measurements, and a clock oscillator that was ticking too fast.

Tachyons

In special relativity, it is not possible to speed up an object until it reaches the speed of light, or for a heavy object to travel at the speed of light. However, some scientists think there might be objects that always move faster than light. These imaginary particles that scientists believe might exist are called tachyons or tachyonic particles. Studies trying to understand tachyons using quantum mechanics did not create particles that move faster than light. Instead, these studies showed that tachyons could cause problems in physical systems.

Some scientists have proposed that neutrinos might act like tachyons, but other scientists disagree with this idea.

General relativity

General relativity was created after special relativity to explain gravity and other related ideas. It follows the rule that no object can reach the speed of light in the frame of reference of any observer who is moving alongside it. However, it allows for changes in spacetime that could make an object seem to move faster than light to someone far away. One example is the Alcubierre drive, which works by creating a wave-like movement in spacetime that carries an object along with it. Another possibility is a wormhole, which connects two distant points as if through a shortcut. Both of these ideas require extreme bending of spacetime in a small area, and their gravity would be extremely strong. To stop these structures from collapsing under their own gravity, they would need to use a type of material that is not yet proven to exist, called exotic matter or negative energy.

General relativity also suggests that faster-than-light travel could allow for time travel, which could cause problems with the order of cause and effect. Many scientists believe these ideas are not possible and that future theories about gravity may prevent them. One theory claims that stable wormholes might exist, but using them to break the order of events would cause them to collapse. In string theory, researchers have proposed that in a specific type of universe, changes in general relativity at the quantum level might block areas of spacetime where time travel could happen. Specifically, a special structure called a supertube could divide spacetime in a way that stops closed loops of time from forming completely, even though such loops might exist in the full spacetime.