The experiment reported by Nature magazine is to realize this idea. But this is not the so-called superluminal violation of causality (or relativity) as people think. To illustrate this problem, let's take a look at this experiment made by China scientist Wang Lijun.
Optical pulses are wave packets composed of light waves with different frequencies, amplitudes and phases. The velocity of each component of optical pulse is called phase velocity, and the peak velocity of wave packet is called group velocity. In a vacuum, they are the same, but in a medium, we know that there are the following group velocities and media.
Relationship of refractive index:
vg = c / ng，ng = n + ω(dn/dω)
Obviously, in some cases (such as a medium with strong anomalous dispersion), negative group velocity can occur. At this time, the propagation time of optical pulse in the medium is shorter than that in vacuum. When the difference δ t = (L/V)-(L/C) reaches an absolute value, the phenomenon of "superluminal" can be observed, that is, "before the optical pulse peak enters the medium, there is already a pulse peak on the other side" (.
So is this superluminal violation of the law of cause and effect? When we carefully examine Wang's experiment, we will find that the emergent light pulse appears before the peak of the incident pulse enters the medium, and the leading edge of the incident pulse has already entered the medium before this (as shown in the figure), so the emergent pulse can be regarded as the interaction between the leading edge of the incident pulse and the medium. In fact, the significance of Wang's experiment lies in realizing the observable phenomenon of negative group velocity, rather than discovering the "superluminal" hyped by the media. Negative group velocity cannot be understood here as the speed of light, nor is it the speed of energy transfer. Of course, this experiment itself shows that our human 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 contains the effect of quantum interference and involves the understanding of the nature of light. Unveiling the superluminal experiment is still the goal of scientists.
After understanding this experiment, many people will wonder whether this superluminal effect can be used to transmit information. In Wang's experiment, superluminal pulses can't carry useful information, so superluminal transmission of information is out of the question, and superluminal transmission of the same energy is also impossible.
Just as sensational as the superluminal experiment, there is another superluminal phenomenon-quantum teleportation, that is, quantum hyperspace transmission (or quantum teleportation). This wonderful phenomenon has attracted people's attention because of its close connection with quantum information transmission and quantum computer realization. The so-called hyperspace means that the transmission of quantum state is not carried out in our usual space, so it will not be limited by the speed limit, and the quantum state will be instantaneously transmitted from A to B (in fact, the quantum state information of particles in A is extracted and instantly copied to particles in B). This kind of quantum information transmission does not need time, and it is really superluminal (which can also be understood as the function of distance). In the process of quantum hyperspace transmission, particles A and B are related by quantum entangled state, and the quantum state is determined by quantum measurement. So when the quantum state of particle A is detected, particle A and particle B instantly collapse to their own eigenstates, and then the state of particle B contains the information of particle A. This information transmission is "superluminal".
However, if an observer wants to know what the transmitted information is immediately, it is impossible, because particle B is still in the state of quantum superposition at this time, and the measurement of it can not get complete information. We must know what we have measured particle A, so we must tell the measurer of particle A at this time through realistic information transmission methods (such as telephone and network). In the end, we can't get information faster than the speed of light! The experiment of quantum hyperspace transmission has been realized in 1997 (see Nature, 390,575.38+0997). On July 20 1 1 year, it was confirmed that neutrinos were 7 kilometers faster than the speed of light (the experimental data was finally confirmed to be wrong).
The above two superluminal schemes are still in the stage of theoretical discussion and experiment, which is far from practical application. Moreover, both of them involve the nature of physics, and experimental phenomena and their explanations are under debate.
cherenkov effect
The speed of light is slower in medium than in vacuum. The propagation speed of particles in the medium may exceed the speed of light. In this case, radiation will occur, and this is cherenkov effect. This is not superluminal in the true sense, but superluminal in the true sense refers to exceeding the speed of light in a vacuum.
Third observer
If A moves eastward at a speed relative to C 0.6c and B moves westward at a speed relative to C0.6c. For C, the distance between A and B increases at a speed of 1.2c, and this "speed"-the speed of two moving objects relative to the third observer-can exceed the speed of light. But the relative motion speed of two objects will not exceed the speed of light. In this example, in the coordinate system of A, the speed of B is 0.88c. In the coordinate system of B, the speed of A is also 0.88c.
Shadows and spots
Wave your hand under the light, and you will find that the shadow is faster than the hand. The ratio of the speed of shadow and hand shaking is equal to the ratio of their distance to the lamp. If you shake the flashlight at the moon, it is easy to make the light spot falling on the moon move faster than the speed of light. Unfortunately, information cannot travel faster than light in this way.
The ratio of the shaking speed of the shadow and the hand is indeed equal to the ratio of the distance between them and the lamp, but the fastest speed of the shadow will not exceed the speed of light. The same is true of light spots. Suppose there is a slope with an elevation angle of 60 degrees, and the object moves horizontally at a speed of 0.6C, then the theoretical projection speed on the slope is1.2c. In fact, the maximum speed of the shadow is c, and the phenomenon is that the shadow does not appear in the vertical projection direction of the object, but lags behind.
rigid body
Knock on one end of a stick, will the vibration immediately spread to the other end? Doesn't this provide a way of superluminal communication? Unfortunately, the ideal rigid body does not exist. Vibration propagates in a rod at the speed of sound, which is ultimately the result of electromagnetic action and cannot exceed the speed of light. An interesting question is, when you hold the upper end of a stick vertically and suddenly release it, does the upper end of the stick start falling first or the lower end of the stick start falling first? The answer is the upper end. )
phase velocity
The phase velocity of light in medium can exceed the speed of light in vacuum in some frequency bands. Phase velocity refers to the "propagation velocity" corresponding to the phase lag of a continuous sine wave (assuming that the signal propagates for a long time and reaches a stable state) after propagating in a medium for a certain distance. Obviously, a simple sine wave cannot convey information. In order to transmit information, it is necessary to modulate the slowly varying wave packet on the sine wave. The propagation speed of this wave packet is called group velocity, which is less than the speed of light. (Translator's Note: Sommerfeld and Brillouin's research on pulse propagation in the medium proves that the propagation speed of a signal with an initial time of [zero before a certain moment] in the medium cannot exceed the speed of light. )
Superluminal galaxy
The apparent speed of galaxies moving towards us may exceed the speed of light. This is an illusion, because the time reduction from galaxies to us is not corrected.
For example, if we measure a galaxy light-years away from us 10, its speed is 2/3 c and it is moving towards the earth.
The distance measured now is 30 light years, because the galaxy just reaches 10 light years when it emits light.
Three years later, when the galaxy reaches 8 light years, the apparent distance is three times that of 8 light years, or 24 light years.
As a result, in three years, the apparent distance has been reduced by 6 light years. ...
Relativistic rocket
People on the earth saw the rocket moving away at a speed of 0.8c, and the clock on the rocket was slower than that on the earth, which was 0.6 times that of the earth. If you divide the distance traveled by the rocket by the time spent on the rocket, you will get the "speed" of 4/3 C (there is a problem here. First of all, with the rocket as the reference frame, the speed of the rocket is 0. At this time, only the speed of the earth away from the rocket can be calculated. The time of the rocket reference system is 0.6 times that of the earth, and the speed of the earth away from the rocket =0.6*0.8c/0.6=0.8c). Therefore, the people on the rocket are moving at a speed "equivalent to" superluminal speed. For people on the rocket, time has not slowed down, but the distance between galaxies has shrunk to 0.6 times, so they also feel that they are moving at a speed equivalent to 4/3 C. 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.
The speed of gravity propagation
The speed of gravity propagation is the speed of light. In the theory of relativity, the existence of gravity is only the result of the bending of time and space, not the actual force, and this "consequence" propagates at the speed of light.
EPR paradox
1935, Einstein, Podolski and Rosen published a thought experiment, which showed the incompleteness of quantum mechanics. They believe that there is an obvious distance effect when measuring two separated particles in an entangled state. Ebhard proved that it is impossible to use this effect to transmit any information, so superluminal communication does not exist. But the EPR paradox is still controversial.
Virtual particle
In quantum field theory, force is transmitted through virtual particles. Because of Heisenberg's uncertainty, these virtual particles can travel at superluminal speed, but they are only mathematical symbols, and superluminal travel or communication still does not exist.
Quantum tunnel
Quantum tunneling is the effect of particles escaping from the barrier higher than their own energy, which is impossible in classical physics. Calculate the time for particles to pass through the tunnel, and you will find that the speed of particles exceeds the speed of light.
References: T.E. Hartman, Journal of Applied Physics, 33, 3427 (1962).
A group of physicists made an experiment of superluminal communication by using quantum tunneling effect: they claimed that Mozart's 40th symphony was transmitted through a barrier with a width of 1 1.4cm at a speed of 4.7c, which, of course, caused great controversy. Most physicists believe that because of Heisenberg's uncertainty, it is impossible to use this quantum effect to transmit information faster than light. If this effect holds, it is possible to use similar devices to transmit information to the past in a high-speed moving coordinate system.
Ref: W. Heitmann and G. Nimtz, Phys A 196,154 (1994); A. Enders and G. Nimtz, Phys Rev E48,632 (1993)
Tao Zhexuan believes that the above experiment is not convincing. It takes less than 0.4 nanosecond for the signal to travel through the distance of 1 1.4cm at the speed of light, but the acoustic signal of 1000 nanosecond can be predicted by simple extrapolation. Therefore, it is necessary to carry out experiments on superluminal communication or high-frequency random signals at a longer distance.
Ghasemi effect
When the distance between two uncharged conductor plates is very close, a very weak but still measurable force will be generated between them, which is the Casimir effect. Casimir effect is caused by vacuum energy. Scharnhorst's calculation shows that the speed of photon moving laterally between two metal plates must be slightly higher than the speed of light (for a gap of one nanometer, this speed is higher than the speed of light 10-24). Under some cosmological conditions (such as near cosmicstring [if it exists]), this effect will be much more significant. However, further theoretical research shows that it is impossible to use this effect for superluminal communication.
Reference: K. scharnhorst, Physics Bulletin B236,354 (1990) S. Ben Menahem, Physics Bulletin B236, 133 (1990). Advanced learning). IAS SNS-AST-90-25 Barton & amp； Scharnhorst, J Phys A26, 2037 (1993) We still can't understand the theory of superluminal, but do intelligent human beings see the mystery?
First of all, we must understand our limitations as human beings. In the static inertial system, our understanding of time is accumulated by the displacement of the object in the same time period, but we can't start from the smallest unit time Δ t. But because we are human beings, not gods, there must be a time and space corresponding to our thinking. For example, Einstein put forward the principle that the speed of light is constant. We can imagine that light becomes a second hand and passes through a square every second, that is, the speed of light is constant. In fact, as an observer in the reference frame, our perception of the time interval Δ t is constant, and the constant speed of light is based on the constant frequency of this perception.
Our observation of self-feeling is continuous, in fact, with frequency. According to Einstein's equation of mass and energy e = MC 2, we know that the disappearance of static mass does not mean vacuum, but is reflected in other forms, so we need to redefine the meaning of numbers. When an object reaches the speed of light (though theoretically impossible), its horizon is tangent to us. According to the principle that the speed of light is constant, that is, the propagation speed of electromagnetic waves is constant, we know that the observation frequency in each reference frame is dominated by observers. Although observers are constantly observing, they also know things through discrimination. We can regard the so-called continuous observation slice as discrimination time, that is, the discrimination time interval is synchronized with the speed of light.
When the object reaches the speed of light, the time slice that has been agreed with us reaches the category of 1: 1. At this time, the observer in our reference system can neither identify the object at the speed of light nor respond to the observer. Although the horizon is separated at this time, we don't regard it as 0, but as 1 at this time. That is, the observer and the object move in parallel along the absolute space-time view at the ratio of 1, but the object is parallel to our observation time slice, so we can't get any relevant information, but it doesn't mean that the object doesn't exist, so we regard it as 1 at this time. When an object continues to accelerate, will time go backwards? The answer is obviously no, when we take 1 as the point, the function of distance, that is, exceeding the speed of light, is moved to the denominator, that is, it becomes the mode of 1/X (because I don't have a relative calculation method of the transformation number, I hope scientists can discuss the relevant data), and at this time we still can't directly observe or identify the object. At this time, our time observation slice is 1, and the motion slice of the object may be 2, which means we will lose a frame of observation. However, we cleverly reduced the frequency, that is, I took the motion slice of the object and sampled it every two times 1 time. This means that we have observed that objects are discontinuous. Maybe objects suddenly appear and disappear. We live in unknown so, but in fact, our observation just can't keep up with the space-time transformation of objects.
We usually think that our logic is that matter will not be produced out of thin air, nor will it disappear out of thin air. Matter is continuous, and the function of distance will make these come true, but time will not go back. If we regard it as 1 point, it is easy to understand.
Superlight speed leads to many phenomena, such as pulsars, which run faster than light speed (non-rotation). We will observe it as a pulse form. From our reference system, the higher the frequency of a pulsar, the smaller its speed beyond the speed of light, and the lower its frequency, the greater its speed beyond the speed of light. A black hole is a star whose speed is just equal to the speed of light. Humans have not fully understood and explained the basic theoretical mechanism, especially the study of Newton's three laws-motion mechanics. Our knowledge has always been addicted to macro research. In the right direction, what should be developed is to completely explain the microscopic essential roots of Newton's three laws. But many people don't realize the laws of science and the details in life.
At the beginning of our research, it only came from a beam of light-a beam of light passing through the glass. Then uncover the essence of material movement and "speed". Light passing through glass is not only the result of refraction and momentum conservation, but also the process of speed change. And this process, is inadvertently let us spy on the micro-nature of the "speed" of movement. External causes can always change the state of things through internal causes. Guided by this idea, we break through the microscopic essence of material movement. The root of the "speed" of material movement must have internal factors, and then we can see the macroscopic external phenomenon-the phenomenon that Newton's three laws can explain and see. Of course, there may be a broader "extreme" phenomenon that Newton's three laws can't see.
As a rigorous scientific research, it is impossible for us to demonstrate our theory only by the phenomenon that light passes through glass. For this reason, we have been looking for it for many years, and we have seen many phenomena that can verify the results in basic experiments. Finally, we found that the contact of positive and negative electrons instantly becomes high-energy photons. The annihilation process of positrons and positrons is a very obvious phenomenon process, which is caused by the destruction and reorganization of a certain structure inside material particles. This is also the most favorable evidence that "internal causes change external causes".
Therefore, to some extent, we have explained the mystery of the "speed" of matter movement, and on the basis of this basic theoretical framework, we have deduced the "superluminal principle", explained the reasons for the momentum non-conservation and the million-degree high temperature of the corona in extreme cases, explained the universe before the Big Bang, explained the "cosmic space" and the redshift phenomenon of the expanding objects in the universe, and even deduced a very feasible "time-space shuttle" method.
Recommended bibliography: stargate-the superluminal principle of spacecraft