Superluminal

A superluminal phenomenon (also called hyperluminal ) refers to the propagation of information or matter at a speed greater than c (speed of the light). Sometimes the term is also applied to cases where there is no actual propagation of information or material, but there is an apparent propagation of either at speeds greater than light, which are also discussed in this article.

The theory of special relativity consistently describes particles that never exceed the speed of light, although the formalism of the theory raises questions about what hypothetical particles, called tachyons, whose speed exceeds that of light would look like or what effects they would have. The existence of tachyons would violate physical causality and imply the possibility of time travel. However, no tachyon particles have been observed and the scientific consensus is that tachyons do not exist. On the other hand, it depends on the hypothesis that extraordinarily distorted regions of space and time could allow matter to reach distant locations in less time than light would travel "undistorted"; the space-time. Apparently some types of superluminal phenomena are not excluded within the framework of the theory of general relativity; however, these are not produced under the usual conditions that we have been able to observe in space-time formed by ordinary matter. Some examples of superluminal phenomena are the phenomena associated with the Alcubierre metric, the Krasnikov tube, traversable wormholes, or some forms of tunneling in quantum mechanics. Note that within material media it is possible that subatomic particles do travel faster than the speed of light in the middle, but never faster than light in a vacuum.

Supralight journey of non-information

In the context for this article, superluminal propagation is considered to be the transmission of information or matter at a speed greater than that of light in a vacuum, which is c = 299,792,458 m /. Some processes seem to propagate faster than c, but they can't seem to assume the transmission of useful information. Examples of these phenomena are given in the following sections.

In addition, in some material media, the wavefront of light propagates at an effective velocity c/n < c (where n is the refractive index of the medium), although other particles can go faster than c/n, but faster than c (even so, individual photons always propagate at c because they are massless particles). In these circumstances the so-called Cherenkov radiation occurs. None of these phenomena violate special relativity or create problems with chance, and they do not qualify as superluminal transmission.

Possibility of realization

Superluminal travel or communication is problematic in a universe consistent with Einstein's theory of relativity. In a hypothetical Newtonian universe where Newton's laws and Galileo's Transformations are exact, the following would be true:

• The laws of Physics would be the same in any reference system, although some laws would include terminology that involves the speed of that reference framework.
• The measured amounts in different frames of reference are related by Galileo's transformations, although for some quantities the transformation will be more complicated than for other
• Speeds are added linearly.
• In a frame of reference, a point x corresponds to the trajectory x-vtwhere the frame moves at a relative speed (relative to the original reference frame) called v.
• There is nothing fundamental about the speed of light.
• All observers agree on their time measures, in other words, there is absolute time.
• Simultaneity is a well-defined concept, in which all observers would agree on whether any two events are simultaneous.

However, according to special relativity, what we measure as the speed of light in a vacuum is actually the physical constant c. This means that all observers, regardless of their acceleration or relative velocity, will always see zero-mass particles (such as photons or gravitons) traveling at speed c. This means that the measurements of time and speed in different frames are no longer identical for all observers (space and time relative to the observer), but by Poincaré Transformations, which in turn implies that:

• To accelerate a non-zero mass object until it stores to c infinite time would be needed with finite acceleration, or infinite acceleration with finite time.
• Either way, such acceleration requires infinite energy. Going beyond light in a homogeneous space would require more than infinite energy, so it lacks physical and mathematical sense.
• Traveling faster than light in an inertial reference frame would be tantamount to travelling backwards (or later depending on the sense) in time if observed from a different reference frame, but equally valid

Therefore, it seems that there are only a limited number of reasons to justify superluminal behavior.

Option A: Ignore Special Relativity

This is the simplest solution, and is particularly popular in science fiction. Physical phenomena such as time dilation, gravitational lensing or the existence of gravitational waves have been empirically verified, from which it follows that the universe can be reasonably described by Einstein's relativistic mechanics, and not by Newtonian mechanics. However, at present we lack a completely satisfactory theory to describe all physical phenomena, so general relativity and quantum mechanics are two incompatible theories, although both are well-founded and very precise, there are situations in which they do not describe reality exactly, and that is the reason why a "theory of everything" that would generalize both theories consistently. Special relativity has been consistently integrated with quantum mechanics, in the so-called quantum field theory (which does not include gravitational phenomena or strongly accelerated reference frames). In fact, the current quantum field theory is only applicable to a flat universe. However, precise measurements show that space ceases to be flat around large gravitational masses.

Option B: The Casimir vacuum

The Newtonian vacuum resembles the quantum vacuum predicted by quantum field theory. The quantum vacuum has an energy associated with it, called the vacuum energy, and this can vary depending on some circumstances^{[citation needed]}. When it decreases, light can reach a value greater than c. Said vacuum can be produced by joining (up to separations on an atomic scale) 2 perfectly sanded metal plates. This is called the Casimir Vacuum, and from the calculations^[which?] it follows that light will exceed c in such an environment. However, this could not be verified experimentally due to current technological limitations^{[citation required]}.

Einstein's equations about special relativity implicitly assume the concept of homogeneity. Space is the same (homogeneous) everywhere. In the case of the Casimir Vacuum, this is clearly violated, since the value of c inside the vacuum is different from that of the rest of the universe, which alters the equations of special relativity^{[citation required]}. However, considering that there are two frames of reference (the vacuum is one, the rest of the universe is the other), the equations of special relativity no longer apply since it can no longer be assumed that there is homogeneity in the universe ^{[citation required]}.

In other words, the Casimir Effect divides space into different homogeneous sectors, each of which follows the rules of general relativity in its own way.

While the above is, technically speaking, going faster than light, it is only true when compared to regions of space dissociated from the Casimir phenomenon. It is not clear whether the Casimir vacuum is stable under the laws of quantum mechanics, and whether communication can be established between the region of space under Casimir effects, and other regions.

Option C: Dismiss causation

Another approach would be to accept special relativity, but admit that some mechanisms of general relativity, such as wormholes, would allow traveling between 2 given points without going through the intervening space.

While this solves the need for infinite acceleration, it still carries the problem of violating causality and generating closed time curves. Causality is not needed in special or general relativity^{[citation needed]}, but it is considered a basic property of the universe, which cannot be ignored. This is why many ^{[how many?]}scientists ^[who?] hope (and wish) that quantum gravity can solve this pothole. An alternative is to assume that if time travel were possible, it would never lead to paradoxes. This is called the Novikov self-consistency principle.

Option D: Ditch absolute relativity

Due to the strong support of empirical findings for special relativity, any modifications to it must be very subtle and difficult to measure. The best known attempt is doubly special relativity, which posits that the Planck length is the same in any frame of reference. This concept is associated with the work of Giovanni Amelino-Camelia and João Magueijo.

A consequence of this theory is to have a variable speed of light, where the speed of photons changes according to energy, and even some zero-mass particles could exceed c. While recent evidence ^[what?]casts serious doubts about this theory, some scientists^[who?] they still consider it viable. However, even if true, this theory remains unclear as to whether it would allow information to exceed c, and it does not seem to allow particles with non-zero mass to travel faster anyway. than light.

There are speculative theories that inertia is produced by the combined mass of the universe (Mach's Principle, for example), which implies that the universe stands still (as opposed to the inertial motion of everything else in it).) is "preferred" to carry out common measurements of the laws of nature (in other words, that the laws appear to be the way they are because we measure them in the context of the chosen frame of reference, in this case, the universe).

If this is confirmed, it would imply that special relativity is an approximation to a more general theory, but since by definition this confirmation would only occur outside the observable universe, it is hard (to put it somehow) to imagine, let alone more difficult to construct experiments that test this hypothesis.

Option E: Go to a place where special relativity doesn't apply

A very popular option in science fiction movies, games, series and novels is to assume the existence of some other "place" (usually called hyperspace), which can be accessed from our universe, and in which the laws of physics and relativity are different, can be distorted, manipulated or even do not exist, which facilitates rapid transport between distant points of the universe without the need to use a lot of energy or momentum for that purpose.

To achieve this trip, it is often assumed that in hyperspace special relativity does not affect, or that what in our universe are 2 very distant places, in this other place they can perfectly well be very close places.

Unfortunately, this approach has not yet been seriously proposed by any branch of science, although on the other hand its existence could not be ruled out in a theoretically conclusive way^{[citation required ]}.

Option F: Go faster without speeding up

It is often implicitly assumed, that in order to speed something past c, it must first go through c (sort of like saying that to go to 100km/h, first you have to go to 99km/h), finding the problem of needing infinite energy. The energy required to accelerate becomes an asymptote as it approaches the speed of light.

Similar to the idea of wormholes, there may be a way to change speed instantly (ie without speeding up). So an object going more than c might only need energy comparable to an object going less than c. The problem is how to "convince" the particles (and the human being who "pilots" them) to move faster than light without speed up.

Option G: Space-Time Fabric

Contrary to popular belief, Einstein never said that it was impossible to exceed the speed of light, but this was inferred from his equations. However, he had no objection accepting that the fabric of space-time can go faster than light.

It is hypothesized that when the universe was created, the fabric of space-time traveled faster than light. Therefore, if we could manipulate such tissue, we could exceed the speed of light. Miguel Alcubierre with his

Alcubierre drivel

metric theorizes that it is possible to "warp" space-time by shrinking it in front of oneself, and expanding it behind oneself. Unfortunately, such a warp would require the emission of negative energy (see vacuum energy)^{[citation needed]} which has not yet been discovered or created.

Option H: Time Warp Travel

You can reach far parts of the universe without going faster than light^{[citation needed]}. The concept is simple, if we distort the time we travel through, we can speed it up or slow it down at the same time^{[citation needed]}.

Example: A spaceship travels from Galaxy A to Galaxy B which is 150 light years away. The ship travels at the speed of light. But when we observe the ship make the journey, we see that it arrived in only 1 year.

What happened was the following, the ship traveled all the way accelerating time 150 times around it, so that it really took 150 years to reach Galaxy B, but since it only accelerated time on its way, for the rest of the universe was as if it had traveled 150 times the speed of light.

While traveling by speeding up time seems unhelpful, it is extremely important if within the same ship time was slowed 150 times to match the rest of the universe. In short, for the pilot and the universe it took 1 year to reach Galaxy B, but for the ship it was 150 years.

Tachyons

In special relativity, although it is impossible to accelerate an object to the speed of light, or for objects with non-zero mass to be able to travel at such a speed, it is not impossible that an object always travels faster than light. These hypothetical particles are called tachyons, and their existence has not been proven or disproved.

Although such particles have never been observed, they are present in numerous theories of physics:

• They appear in the standard model of interaction in particle physics

• In the Theory of Bosonic Strings

• And even in superbody theory

In each of these examples, one sees that tachyons may not be thought of so much as a particle, but as a "destabilization" of the theory.

General relativity

General relativity was developed after the special theory of relativity to include concepts such as gravity. It maintains, such as this, the impossibility of objects to accelerate to the speed of light within the reference frame of any local observer. However, it allows for distortions in space-time such that they would allow an object to move faster than the speed of light, from the point of view of a distant observer. The Alcubierre engine takes advantage of one of these distortions, producing a wave-shaped rupture in space-time, allowing the particle to surf it, that is, move with it and take advantage of its speed, without need to accelerate itself to the speed of light. Another theoretical way to take advantage of this type of distortion is by using a wormhole, which would connect two distant points in space in such a way that they would be connected by a shortcut. Both ways would require the creation of extreme curvature in a very specific region of space-time, making the gravitational field generated at such a location immeasurable, generating tidal forces of such magnitude that they would destroy any object close enough. To counteract the unstable nature of such fields and prevent the distortions from collapsing under their own 'weight', it would be necessary to introduce exotic matter or negative energy into them.

General relativity speculates that any technique used to travel faster than light would also allow time travel. And as a consequence, it would be possible, albeit theoretically, to violate the principle of causality. Many physicists claim that the phenomena described above are, in fact, impossible, and that future theories of gravity (see GUT or Grand Unification Theory), would forbid such violations. One theory concludes that the existence of stable wormholes is possible, although any attempt to use a network of them to violate the principle of causality would result in their collapse. In string theory or superstring theory, Eric Gimon and Petr Hořava discuss whether in a five-dimensional supersymmetric Gödel universe, quantum corrections to the general theory of relativity effectively separate from spacetime those regions that contain violative temporal curvatures. of the causality principle. Particularly for quantum theory, there is an imperfect supertube that cuts the known space-time in such a way that it prevents the existence of a closed curve inside it.

In quantum mechanics

In quantum mechanics, a set of events occurs that make critical the assumption of c as absolute and insurmountable maximum velocity; certain phenomena give the impression of implying instantaneous propagation.

Hartmann Effect

A photon or an electron tunneling through a quantum barrier can manifest a shorter travel time than that required by light in an equivalent distance, these times are evaluated by observing the peak of the corresponding wave packet, before and after the barrier. Taking into account the thickness of the tunnel barrier, the peak of the wave packet is reduced and seems to achieve superluminal speed. This phenomenon is called the Hartman effect or the Hartman-Fletcher effect.

Delayed Choice Quantum Eraser

Main article: Delayed-choice quantum eraser

The delayed-choice quantum eraser is a version of the EPR paradox in which the observation (or not) of interference after the passage of a photon through a double-slit experiment depends on the conditions of observation of a second photon tangled with the first. The characteristic of this experiment is that the observation of the second photon can take place at a time after the observation of the first photon, which can give the impression that the measurement of the subsequent photons "retroactively" determines what happens. whether or not the above photons show interference, although the interference pattern can only be seen by correlating measurements from both members of each pair and thus cannot be observed until both photons have been measured, ensuring that an observing experimenter only the photons that pass through the slit do not get information about the other photons in an FTL or backwards in time fashion

Casimir effect

Representation of the Casimir effect and its strength

The Casimir effect is a phenomenon observable on a very small scale, however it is measurable by its pressure on conductive plates, such pressure on these conductive plates is exerted by the so-called quantum vacuum (see: vacuum energy) located between such plates; the pressure can be positive or negative depending on the geometry of the device. In quantum field theory, the quantum vacuum is supposed to be the place of creation and annihilation of numerous virtual particles. The existence of basically different conditions for the vacuum outside and inside the plates then implies a difference in energy between the two, which is the cause of the differences in the pressure measured on the plates.

Virtual particles are by definition external to their mass bed, which means they do not satisfy anymore E2=p2c2+m2c4{displaystyle E^{2}=p^{2}c^{2}+m^{2}c^{4}}and are by definition unobservable individually even if its collective effect is measurable as it happens in the almostmir effect and in all quantum corrections to the classic observable of quantum fields.

EPR Paradox

It is also possible to cite the experience hypothesized by Einstein, Podolsky and Rosen (EPR paradox) that seems to have been confirmed experimentally by Alain Aspect in 1981 and 1982. In this case, the measurement of the state of one of the entangled quantum systems of a pair of them imposes on the other system (of another measure) a complementary state. From this, what has been called a "quantum teleportation" works. Among the most important advances in this matter are those of the team led by the Austrian Rainer Blatt at the University of Innsbruck and the American David Wineland of the National Institute of Standards and Technology, in Boulder, Colorado), they would have carried out teleportation or quantum teleportation of a complete atom of baryonic matter (calcium ions in the first of the experiments and beryl in the second). This would allow many applications in quantum computing concerning the EPR paradox. For its part, the "Sciences" of the city of Geneva was given by his findings to Professor Nicolas Gisin in November 2006 for his work on the matter (Gisin claims to have exceeded speed c ), although such a claim is still doubtful.

The Marlan Scully Experience

The Marlan Orvil Scully experience, also carried out by B.G.Englent and H.Walther, which is why it is also called the ESW Experience, is a variant of the EPR paradox in which the observation, or not, of a pattern of interference after the passage of a photon through a Young's slit depends on the conditions of observation of a second correlative photon to the first. The particularity of this experience is that the observation of the second photon can take place in the "distant" in relation to the observation of the first photon which gives the impression that the observation of the first photon "informs" about an event that takes place in the future.

Apparently faster than light

Relative movement

An observer may incorrectly conclude that two objects are moving faster than the speed of light, if they mistakenly add the two speeds together according to the postulates of Newtonian physics.

For example, if we take two accelerated particles each located at one end of a circular particle accelerator or synchrotron, they would appear to an observer immobile with respect to it, as well as to anyone who adds the velocities of those according to the postulates of Newtonian physics, as moving just below twice the speed of light. However, if the observer knows the special theory of relativity and composes the velocities according to it, he will correctly conclude that:

for two particles moving to β β {displaystyle beta } and − − β β {displaystyle} respectively, where

β β =v/c{displaystyle beta =v/c,!}

and

− − β β =− − v/c{displaystyle -beta =-v/c,!},

then from the observer's point of view, the relative speed Δβ (using the speed of light c as unit) becomes

Δ Δ β β =β β − − (− − β β )1+β β 2=2β β 1+β β 2{displaystyle Delta beta ={beta -(-beta) over 1+beta ^{2}={2beta over 1+beta ^{2}}}}}},

which is less than the speed of light.

Phase velocity greater than c

The group speed of a wave can routinely exceed the speed of light in a vacuum[1]. However, this does not imply that the signal or some physical entity propagates faster than c. In most optical media, the refractive index is greater than the sum of all the wavelengths, thus keeping the group velocity below the speed of light. In these media the individual photons keep moving at the speed of light which coincides with the phase speed.

Daily movement of the sky

For an observer on Earth, objects in the sky move around the Earth in the course of a day. The closest star to the solar system, Proxima Centauri, is 4 light years from the solar system, it moves with a circular trajectory that could be described with a speed greater than c, since an object that moves with a circular movement has a speed that is can be expressed as the product of the radius and the angular velocity.

Closing distances

If a ship travels one light-year away (as measured by the current position of the earth) away from the earth at a high speed, the time it would take to reach that distance could be less of a light year being measured by the traveler's clock (although this time on earth will always be greater). The value is determined by the distance, measured by the earth at the beginning, by the time taken, measured by the traveler's watch, it is known as its own speed or speed in relativity, of which there is no limit in this speed thanks to the fact that the speed it is not a measure of speed in an inert image. A light signal that leaves the earth at the same time as the traveler could always reach the destination before the traveler.

Further distance from Earth

Taking into account that you cannot travel faster than light, it can be concluded that a human being does not have the capacity to go beyond 40 light-years (time measured from your reference system), if the traveler it is functional between 20 and 60 years, so only the very few stellar systems that are between 20 and 40 light years from Earth will be reached. This idea can be considered wrong: if our traveler has the ability to accelerate his ship to a speed slightly less than that of light, time dilation would extend his life by a few thousand Earth years, seen from the point from the point of view of someone in our solar system and despite the fact that from the point of view of our subject time continues to run in the same way. On the other hand, if he for some reason decided to return to earth, he would find himself on a planet in which thousands of years have already passed after his departure. The speed of the ship was not observed as superluminal, nor would our traveler have felt this speed, if he did not feel a contradiction in the distance from the point of view of the direction of his travel. By the time he circled back to earth he would observe that his target would feel the passage of time more than our traveler. So our traveler would not exceed speed c, but his own speed or his best of him, the distance traveled from the earth's point of view divided by his own time, can get a speed greater than c.

Spots of light and shadows

If you aim a laser beam at a distant point and move it, it should be easy to make the point move faster than light. Similar to a shadow cast on a distant object, it's possible for the shadow to be cast. move through the object faster than the speed of light.

Group speed greater than c

On the other hand, the group speed of a wave (for example a ray of light), can exceed the speed of light under certain special circumstances. In these cases, where there is typically a rapid decay of intensity, the maximum of the envelope of a pulse can travel at a speed greater than c. However, even this situation does not imply signal propagation above c, even though one is tempted to associate maximum pulse with signal. This association has proven to be misleading, basically because the information received when a pulse arrives can be obtained before the maximum pulse arrives.

In 2000 the journal Nature published a summary of the results of an experiment, by Lijun J. Wang and his team at the NEC Research Institute in Princeton, New Jersey, which showed that the speed of light in the form of packets or pulses can, under very special conditions, exceed 310 times its limit phase speed of 300,000 kilometers per second, established in Einstein's theory of special relativity. It should be clarified that this result does not violate physical causality, since the speed of group does not correspond to the real propagation speed of the photons, which always move at a speed equal to c:

«Indeed, it can be made that our luminous impulses travel at a speed greater than c. This is a special property of light itself, which is different from a known object, like a brick, as the light is a wave without mass»

Lijun J. Wang en El País20/07/2000

According to the explanation of the team that carried out the experiment, superluminal pulses are the result of classical interference mechanisms due to the wave nature of light and no information (signal) is transmitted at a speed greater than c.