In astronomy, an explicit motion that exceeds the speed of light
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The superluminal movement is an explicit movement that exceeds the speed of light in astronomy. It appears in some radio galaxiesa quasarRecently, it was also found thatMicroquasarThe source of galactic radiation.These sources are believed to contain black holes at the center, thus causing mass bodies to shoot out at high speeds.The superluminal motion was first discovered in the early 1970s, which was initially regarded as an evidence against the theory that quasars have cosmological scale distances.Although some astrophysicists still defend this argument, most people believe that the apparent speed greater than the speed of light is an optical illusion, which does not include anySpecial relativityContrary physics.
Chinese name
Superluminal motion
Foreign name
Superluminal motion
Nature
An explicit object moving faster than the speed of light
The explanation of this phenomenon is quite simple and direct, that is, "light travel time effect".Imagine a small mass of matter fromGalaxyThe center sets out and moves towards you very quickly, almost head-on.
When this lump is still in the center of the Milky Way, it emits some light towards you.After it moves towards you (and slightly to the side), and shines again, this time the light will take a relatively short time to move towards you, so that it is closer to you.If you ignore this fact, you will underestimate the real time interval (in terms of your inertial reference system), so you will overestimate the speed.
In other words, if you want to calculate how fast the lump moves, but assume that its moving direction is perpendicular to the connection line between you and the Milky Way, then you will underestimate the time interval, because you ignore the fact that it also moves towards you, and get several times the speed of lightrate。
This phenomenon is common in two reverseJet, both far away from us and close to us.If these two radiation sources are observedDoppler shift, the speed and distance can be determined and are not affected by other observation items.[1]
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Superluminal motion
In 1966,Martin Rees (Martin Rees) predicted (Nature 211, page 468) that an objectRelativistic velocityAnd when moving in the tangential direction, it may appear to the remote observer that there is a lateral velocity far greater than the speed of light.
A few years later (in 1970), such radiation sources were really discovered in the form of astronomical radio radiation sources at a very far distance, such as radio galaxies and quasars.They are called "superluminal radiation sources".This discovery is an amazing result of a new technology, called the Very Long Baseline Interferometry, which allows position determination of less than millisecond and can be used in the determination of position change in the sky;This change is called proper motion and usually lasts for several years.The explicit speed is obtained by multiplying the observed self motion by the distance, which can be up to 6 times the speed of light.After that, scientists observed the expansion of quasars, measured their distance from the earth through spectra, and calculated that their expansion speed also exceeded the speed of light.
In 1994, while obtaining a record of the rate of the Milky Way, he discoveredSuperluminal radiation source——Cosmic X-ray source GRS1915+105.The expansion time of the lump is much shorter.Many individual lumps have been detected (I.F. Mirabel and L.F. Rodriguez in Nature 371, page 48). The superluminal radiation ("A superluminal source in the Galaxy") in the Milky Way expands in pairs, often up to 0.5 angular second in a week.Because of the analogy with quasars, such radiation sources are called micro quasars.[2]
quantum
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In the world of quantum theory, physical quantities are "paired".In other words, you cannot measure two related physical quantities accurately at the same time (called "uncertainty principle").Speed is paired with position.Therefore, as long as the position of the particle is accurately measured, its speed will not be completely determined, and the same particle may have multiple positions at the same time, with multiple existence.It is also possible that the same event has multiple different histories, (such as Schrodinger Cat). In theory, space and time are not absolutely continuous and smooth. When the time is less than 10 ^ - 31s, space cannot continue to be divided. When the time is less than 10 ^ - 33m, space cannot continue to be divided.There is no absolute space-time in quantum space, so it is possible to exceed the speed of light.Scientists have observed this phenomenon in the laboratory, and named the particles that exceed the speed of light as "hyperons".In a black hole, particles "evaporate" out of the black hole by virtue of this uncertainty principle.[3]
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The speed of light (FTL or superluminacy) will become a topic of discussion, which is derived from the inference that local objects cannot exceed the speed of light c in vacuum in relativity. The speed of light has become the upper limit of speed in many occasions.Before that, Newtonian mechanics did not limit the speed of superlight.In the theory of relativity, the speed of motion is closely related to other properties of the object, such as the mass and even the passage of time in its reference system. If an object whose speed is lower than the speed of light (in vacuum) wants to accelerate to the speed of light, its mass will grow to infinity and therefore require infinity of energy, and the passage of time it feels will even stop,So theoretically it is impossible to reach or exceed the speed of light (as for photons, that is because they are always at the speed of light, rather than increasing from below to the speed of light).But it also makes physicists (as well as the general public) particularly interested in some "seemingly" super light speed physical phenomena.
In July 2000, the British journal Nature (2000406:277) published a paper on the "superluminal" experiment, which caused people to discuss whether superluminal bottoming exists.In fact, scientists have long studied how to make the group velocity of light pulse in the medium exceed the speed of light in vacuum, and this experiment reported in Nature realized this idea.But this is not the so-called superluminal speed that violates the law of cause and effect (or the theory of relativity). To illustrate this problem, let's take a look at the experiment conducted by Wang Lijun, a Chinese scientist.
An optical pulse is a wave packet composed of light waves with different frequencies, amplitudes and phases. The velocity of each component of an optical pulse is called phase velocity, and the velocity of the peak of the wave packet is called group velocity.In vacuum, the two are the same, but in the medium, as we know, there are the following group velocities and the medium.
Relation of refractive index: vg=c/ng, ng=n+ω (dn/d ω)
Obviously, under certain circumstances (such as media with strong anomalous dispersion), a negative group velocity can occur. At this time, the optical pulse takes less time to travel in the medium than in the true air. When the difference Δ T=(L/v) - (L/c) reaches an absolute value large enough, the phenomenon of "superluminal velocity" can be observed, that is, "before the peak of the optical pulse enters the medium, there is a pulse peak on the other side".
So does this superluminal speed violate causality?After careful examination of Wang's experiment, we will find that although the outgoing light pulse appears before the peak of the incoming pulse enters the medium, the front edge of the incoming pulse has already entered the medium before this, so the outgoing pulse can be regarded as generated by the interaction between the front edge of the incoming pulse and the medium.In fact, the significance of Wang's experiment lies in the realization of the phenomenon of observable negative group velocity, rather than the discovery of "superluminal velocity" as hyped by the media. Negative group velocity here cannot be understood as the speed of light, nor is it the speed of energy transmission.Of course, this experiment itself shows that our understanding of light has taken another step forward.It is far from enough to explain this experiment only by the formula of the relationship between refractive index and group velocity, which includes the effect of quantum interference, involves the understanding of the nature of light, and it is still the goal of scientists to open the veil of "superluminal experiment".
Many people will think about whether this "superluminal" effect can be used to transmit information after understanding this experiment. In Wang's experiment, the "superluminal" pulse cannot carry useful information, so there is no way to talk about the superluminal transmission of information, nor is the superluminal transmission of energy.
The same sensational effect as the superluminal experiment is another "superluminal" phenomenon, quantum teleportation (or quantum teleportation), which attracts people's attention because of its close connection with quantum information transmission and the realization of quantum computers.The so-called hyperspace means that the transmission of quantum states is not carried out in our usual space, so it will not be restricted by the speed limit of light, and the quantum states will be transiently transmitted from A to B (in fact, the quantum state information of A particles is extracted and transiently reproduced on B particles). This transmission of quantum information does not need time,It is the real superluminal speed (also can be understood as the action of super distance).In the process of quantum hyperspace transmission, the quantum non cloning law is followed. A and B particles are correlated through quantum entanglement. The determination of quantum state is carried out through quantum measurement. Therefore, when the quantum state of A particle is detected, A and B particles collapse to their respective eigenstates instantaneously, and then the state of B particle contains the information of A particle.The transmission of this information is "faster than light".
However, if an observer wants to know immediately what the information transmitted is, it is impossible, because particle B is still in the quantum superposition state at this time, and its measurement cannot obtain complete information. We must know what measurement has been taken for particle A, so we must not fail to use real information transmission methods (such as telephone, network, etc.)Tell the surveyor in second place the state of particle A at this time.In the end, we can't get information faster than the speed of light!The experiment of quantum hyperspace transmission was realized in 1997.
At present, the above two superluminal schemes are only in the stage of theoretical discussion and experiment, which is far from practical use. Moreover, these two issues involve the essence of physics, and experimental phenomena and their explanations are in dispute.
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The superluminal speed that people are interested in generally refers to the superluminal speed to transmit energy or information.According to special relativity, superluminal travel and superluminal communication in this sense are generally impossible.At present, the argument about superluminal speed is that some things can indeed exceed the speed of light, but they cannot be used to transmit energy or information.However, the existing theory does not completely exclude the possibility of superluminal speed in the true sense.First, we will discuss the first case: it is not really superluminal speed.
Not really superluminal speed
Cherenkov effect
Superluminal motion
The speed of light in a medium is smaller than that in a vacuum.The propagation speed of particles in the medium may exceed the speed of light in the medium.In this case, radiation will occur, which is called Cherenkov effect.This is not the true superluminal speed. The true superluminal speed means that it exceeds the speed of light in vacuum.
Third Observer
If A moves eastward at 0.6c relative to C, B moves westward at 0.6c relative to C.For C, the distance between A and B increases at a rate of 1.2c.This "speed" -- the speed between two moving objects relative to a third observer -- can exceed the speed of light.But the speed of two objects relative to each other does not exceed the speed of light.In this example, the speed of B in the coordinate system of A is 0.88c.In the coordinate system of B, the speed of A is also 0.88c.
Shadow and flare
Shake your hand under the lamp, and you will find that the shadow speed is faster than the hand speed.The ratio of the speed of the shadow and the hand shaking is equal to the ratio of their distance to the lamp.If you shake your flashlight towards the moon, you can easily make the light spot falling on the moon move faster than the speed of light.Unfortunately, information cannot be transmitted faster than light in this way.
rigid body
If you hit one end of a stick, will the vibration immediately spread to the other end?Doesn't this provide a super light speed communication mode?Unfortunately, the ideal rigid body does not exist, and the propagation of vibration in the stick is carried out at the speed of sound, which in the final analysis is the result of electromagnetic action, so it is impossible to exceed the speed of light.
Phase velocity
The phase velocity of light in the medium can exceed that in vacuum in some frequency bands.The phase velocity refers to the "propagation velocity" corresponding to the phase lag of a continuous sine wave (assuming that the signal has been propagating for a long enough time to reach a stable state) after propagating for a certain distance in the medium.Obviously, a simple sine wave cannot transmit information.To transmit information, it is necessary to modulate the slow changing wave packet on the sine wave. The propagation speed of such wave packet is called group velocity, which is less than the speed of light.
Superluminal galaxy
Superluminal motion
The apparent speed of galaxies moving towards us may exceed the speed of light.This is an illusion, because there is no correction for the reduction of time from the galaxy to us.For example, if we measure a galaxy 10 light years away from us, its velocity is 2/3c.Now, the measured distance is 30 light-years, because when the light it emitted arrived, the galaxy reached 10 light-years;Three years later, when the galaxy reached 8 light years, its apparent distance was three times that of 8 light years, that is, 24 light years.As a result, the visual distance decreased by 6 light years in 3 years.
Relativistic rocket
People on the earth saw the rocket moving away at a speed of 0.8c, and the clock on the rocket slowed down relative to people on the earth, 0.6 times that of the earth clock.If you divide the distance the rocket moves by the time on the rocket, you will get a "speed" of 4/3 c.Therefore, the people on the rocket move at a speed "equivalent to" the speed of superlight.For the people on the rocket, time has not slowed down, but the distance between galaxies has shrunk to 0.6 times of the original, so they also feel that they are moving at a speed equivalent to 4/3c.The problem here is that the number obtained by dividing the distance in one coordinate system by the time in another coordinate system is not the real speed.
Velocity of gravitational propagation
Some people think that gravity travels faster than light.In fact, gravity travels at the speed of light.
EPR paradox
In 1935, Einstein, Podolski and Rosen published a thought experiment to show the incompleteness of quantum mechanics.They believe that there is an obvious over distance effect when measuring two separated particles in the entangled state.Ebhard proved that it is impossible to use this effect to transmit any information, so superluminal communication does not exist.But there is still controversy about the EPR paradox.
Virtual particle
In quantum field theory, force is transmitted through virtual particles.Because of Heisenberg uncertainty, these virtual particles can travel at superluminal speed, but virtual particles are only mathematical symbols, and superluminal travel or communication still does not exist.
Quantum tunnel
Superluminal motion
Quantum tunneling is the effect of a particle escaping from a potential barrier higher than its own energy, which is impossible in classical physics.Calculate the time when particles pass through the tunnel, and it will be found that the particle speed exceeds the speed of light.A group of physicists made an experiment of superluminal communication using quantum tunneling effect: they claimed to transmit Mozart's 40th Symphony through an 11.4cm wide barrier at a speed of 4.7c.Of course, this has caused great controversy.Most physicists believe that because of Heisenberg uncertainty, it is impossible to use this quantum effect to transmit information at superluminal speed.If this effect is true, it is possible to use similar devices to transmit information to the past in a high-speed coordinate system.
Terence Tao believes that the above experiment is not convincing.It takes less than 0.4 nanoseconds for a signal to travel 11.4 cm at the speed of light, but it can predict an acoustic signal up to 1000 nanoseconds through simple extrapolation.Therefore, it is necessary to conduct experiments on faster than light communication over a longer distance or on high-frequency random signals.
Casimir effect
When the distance between two uncharged conductor plates is very close, there will be very weak but still measurable force between them. This isCasimir effect。Casimir effect is caused by vacuum energy.Scharnhorst's calculation shows that the speed of photons moving laterally between two metal plates must be slightly higher than the speed of light (for a gap of one nanometer, this speed is 10-24 higher than the speed of light).Under certain cosmological conditions (such as near the cosmic string [if they exist]), this effect will be much more significant.However, further theoretical research shows that it is impossible to use this effect for superluminal communication.
Cosmic expansion
Hubble's theorem says that galaxies at distance D separate at the speed of HD.H is a constant independent of galaxies, called the Hubble constant.Galaxies far enough away may separate from each other at a speed faster than the speed of light, but this is relative to the separation speed of a third observer.
The moon rotates at a speed faster than light
Superluminal motion
When the moon is on the horizon, suppose we circle at the speed of half a cycle per second, because the moon is 385000 kilometers away from us, and the speed of the moon relative to us is 1.21 million kilometers per second, about four times the speed of light!This sounds quite ridiculous, because in fact we are rotating ourselves, but we say that the moon is circling us.But according to the general theory of relativity, any coordinate system, including the rotating coordinate system, is available. Isn't it that the moon is moving at superluminal speed?
The problem is that in general relativity, the speed of different places cannot be directly compared.The speed of the moon can only be compared with other objects in its local inertial system.In fact, the concept of speed is not very useful in general relativity, and it is difficult to define what is "superluminal speed" in general relativity.In general relativity, even "the speed of light is constant" needs to be explained.Einstein himself said on page 76 of Relativity: Special and General Theory that "the speed of light is constant" is not always correct.When there is no absolute definition of time and distance, how to determine the speed is not so clear.
Nevertheless, modern physics believes that the speed of light in general relativity is still constant.When the unit of distance and time are linked by the speed of light, the invariance of the speed of light is defined as an axiom that goes without saying.In the previous example, the speed of the moon is still less than the speed of light, because at any time, it is within the future cone of light emitted from its current position.
Clarify the definition of superluminal speed
The various specious examples of "superluminal speed" listed in the first part show the difficulty in defining "superluminal speed".For example, the "superluminal speed" of shadows and light spots is not the real superluminal speed, so what is the real superluminal speed?In the theory of relativity, "world line" is an important concept. We can use "world line" to give a clear definition of "superluminal speed".
What is "World Line"?We know that all objects are composed of particles. If we can describe the position of particles at any time, we will describe the whole "history" of objects.Imagine a four-dimensional space composed of three dimensions of space and one dimension of time.Since a particle can only be in a specific position at any time, its entire "history" is a continuous curve in this four-dimensional space, which is called "world line".The world lines of an object are a collection of the world lines of all the particles constituting it.
Not only the history of particles can form world lines, but also the history of some artificially defined "things" can form world lines, such as shadows and light spots.The shadow can be defined by the points on its boundary.These points are not real particles, but their positions can be moved, so their "history" also constitutes a world line.
A point in four-dimensional space-time represents an "event", that is, three spatial coordinates plus a time coordinate.Space time distance can be defined between any two "events". It is the square of the space distance between two events minus the square of the product of the time interval and the speed of light and then the root sign.The narrow relative argument shows that this space-time distance is independent of the coordinate system, so it is of physical significance.
Space time distance can be divided into three categories: time like distance: space interval is less than the product of time interval and speed of light Light like distance: space interval is equal to the product of time interval and speed of light Space like distance: space interval is greater than the product of time interval and speed of light.Next, we need to introduce the concept of "local".A smooth curve, viewed "locally", is very similar to a straight line.Similarly, four-dimensional space-time is flat locally, and the world line is similar to a straight line locally, that is, it can be described by uniform motion, which is the instantaneous speed of particles.On the world line of photons, locally, the distance between adjacent events is light like.In this sense, we can say that the world line of photons is light like.
The world line of any particle moving at a speed lower than the speed of light. Locally, the distance between adjacent events is timelike.In this sense, we can say that this kind of world line is timelike.The world line of particles or artificially defined "points" moving at superluminal speed is like empty.The word "world line" here is spacelike, which means that the space-time distance of adjacent events is spacelike when viewed locally.Because there may be curved space-time, there may be such a world line: locally, the distance between adjacent events is time like, and particles do not move faster than light speed;But there are two events that are far apart, and their space-time distance is empty.Is this a superluminal situation?
The significance of this problem is that it can define both local "superluminal speed" and global "superluminal speed".Even if local superluminal speed is impossible, the possibility of global superluminal speed is not ruled out.Global superluminal speed is also worth discussing.In a word, "superluminal speed" can be defined by the spacelike world line. The advantage of this definition is that it excludes the situation that two objects move at "superluminal speed" relative to the third observer.
Now let's consider what is the "thing" we want to transmit at superluminal speedThe main purpose is to eliminate useless things such as "shadow" and "light spot".Particles, energy, charge, spin and information are what we want to transmit.One question is: How do we know whether the thing being transmitted is the original thing?This problem is relatively easy. For a particle, we observe its world line. If the world line is continuous and no other particles are separated from this particle, we can generally think that this particle is still the original particle.
Obviously, transmitting the whole object is technically more difficult than transmitting information.Now we can transmit information at the speed of light without any difficulty.Essentially, we just put information on the time series of photons and retrieve human readable information from the time series of photons, and the speed of photons is naturally the speed of light.Similarly, if tachyons really exist, we only need to find a technology that can control the direction of their generation and emission to achieve superluminal communication.
It is very likely that the cost of transporting different particles is very different, and the more economical way is to use replication technology.If we can get all the information about an object, and we have mastered the technology of copying the original object from this information, then superluminal communication and superluminal travel are equivalent.Science fiction has had this idea for a long time, which is called remote fax.To put it simply, it is like faxing a copy of the original and then destroying the original, which is equivalent to sending the person over.Of course, the question is whether a conscious complex object like human can be duplicated.
Unlimited energy
E = mc^2/sqrt（1 - v^2/c^2）
The above formula is the energy possessed by a particle with a static mass of m moving at the speed of v.Obviously, the higher the speed, the greater the energy.Therefore, work must be done to accelerate the particle, and the work done is equal to the increase of particle energy.Note that when v approaches c, the energy tends to infinity, so it is impossible to accelerate particles to the speed of light in the usual way, let alone exceed the speed of light.
But this does not rule out the possibility of making particles faster than light in other ways.Particles can decay into other particles, including photons moving at the speed of light (the rest mass of photons is zero, so although they move at the speed of light, their energy can also be finite, and the above formula is invalid for photons).The details of the decay process cannot be described by classical physics, so we cannot deny the possibility of producing superluminal particles through decay (?).
Another possibility is particles whose speed is always higher than the speed of light.Since there are photons always moving at the speed of light and particles always moving at a speed lower than the speed of light, why can't there be particles always moving at a speed higher than the speed of light?
The problem is that if v>c in the above formula, either the energy is imaginary or the mass is imaginary.If there are such particles, does the imaginary energy and mass have any physical significance?How should they be interpreted?Can we deduce observable predictions?As long as we find the evidence of the existence of such particles, find the method to detect such particles, and find the method to deflect the movement of such particles, we can achieve superluminal communication.
Quantum field theory
So far, all physical phenomena except gravity conform to the standard model of particle physics.The Standard Model is a relativistic quantum field theory, which can describe three basic interactions including electromagnetic interaction, weak interaction and strong interaction as well as all observed particles.According to this theory, any pair of operators responding to physical observables at space time events which are separated by a space like interval commute.In principle, this means that no action can travel faster than the speed of light.
However, no one can prove that the standard model is self consistent.It is very likely that it is actually not self consistent.In any case, it cannot guarantee that particles or interactions that it cannot describe will not be found in the future.No one has extended it to include general relativity and gravity.Many people who study quantum gravity doubt whether such simple statements about causality and locality can be generalized in this way.In a word, in the future more perfect theories, there is no guarantee that the speed of light is still the upper limit of speed.
The possibility of undetermined superluminal speed
Grandfather paradox
(Causality): The best evidence against superluminal speed is probably the grandfather paradox.According to special relativity, particles moving faster than light in one reference system may return to the past in another coordinate system.Therefore, superluminal travel and superluminal communication also mean returning to the past or transmitting information to the past.If time travel is possible, you can go back and kill your own grandfather.This is a strong rebuttal to superluminal speed.But it cannot rule out the possibility that we may travel at a limited superluminal speed but cannot go back to the past.Another possibility is that when we travel faster than light speed, causality is destroyed in a consistent way.In a word, time travel and superluminal travel are not the same but related.If we can go back to the past, we can also travel faster than light in general.
Tachyon
Tachyons are particles predicted theoretically.It has a local velocity (instantaneous velocity) that exceeds the speed of light.Its mass is imaginary, but its energy and momentum are real.Some people think that such particles cannot be detected, but this is not necessarily the case.The examples of shadows and light spots show that things that exceed the speed of light can also be observed.At present, there is no experimental evidence for the existence of tachyons, and most people suspect their existence.It is claimed that there is evidence that these neutrinos are tachyons in the experiment of measuring the mass of neutrinos emitted by the Tritium Beta decay.This is very doubtful, but it cannot be completely ruled out.
The problem of tachyon theory is that it violates causality, and the existence of tachyons makes vacuum unstable.The latter can be avoided in theory, but then we cannot achieve the superluminal communication we want.In fact, most physicists believe that tachyons are the manifestation of the pathological behavior of field theory, and the public's interest in tachyons is mostly due to their many appearances in science fiction.
Wormhole
A well-known suggestion for global faster than light travel is to use wormholes.Wormhole is a shortcut to connect two places in curved space-time. It takes less time for light to travel from A to B through the wormhole than it takes for light to travel from A to B along the normal path.Wormholes are the corollary of classical general relativity, but creating a wormhole requires changing the topological structure of space-time.This is possible in quantum gravity theory.Opening a wormhole requires a negative energy area. Misner and Thorn suggest that the Casimir effect be used to generate a negative energy area on a large scale.Visser recommends using cosmic strings.These suggestions are almost unrealistic.Strange materials with negative energy may not exist in the form they require.
Thorn found that if wormholes can be created, they can be used to construct closed timelike world lines in space-time, so as to realize time travel.Some people think that the explanation of the multiplicity of quantum mechanics can be used to eliminate the causal paradox, that is, if you go back to the past, history will occur in a different way from the original.Hawking believes that wormholes are unstable and therefore useless.But wormholes are still a fruitful area for thought experiments, which can be used to clarify what is possible and what is impossible under the known and proposed physical laws.
Curvilinear propulsion
Curved phase propulsion is to bend space-time in a specific way, so that objects move faster than light speed.Miguel Alcubierre is famous for proposing a space-time geometric structure that can realize curved phase propulsion.The curvature of space-time enables objects to travel at the speed of superlight while maintaining a timelike world line.Like wormholes, curvilinear propulsion requires strange materials with negative energy density.Even if such substances exist, it is unclear how they should be arranged to achieve curved phase propulsion.
Vacuum material state
By establishing two assumptions of the energy state of vacuum matter, and based on the equivalent Binet equation, the mass and energy equations of matter particles moving at the speed of light and superlight speed are given, which are combined with the relevant conclusions of Einstein's special relativity;As corollaries, the possible corresponding relations between these equations and dark matter and dark energy are preliminarily discussed.[4]
