To put it simply, the unification of electric power and magnetic force is roughly as follows: under Lorentz transformation, the electrostatic field in one inertial system looks like an electrostatic field with changed size and direction plus the magnetic field in another inertial system-the magnetic field that was not there originally appeared in the transformation! Static magnetic field can also transform electric field. The unified second-order antisymmetric tensor of four-dimensional electromagnetic field *** 16 components, but only six independent components, namely three components of electric field and magnetic field. The "size" of this tensor remains the same in the Lorentz transformation, but it changes its "direction" (so its component will change).
Magnetism is often understood to be derived from electricity, which is correct in a certain sense, but it should be noted that the derivation is mutual, not unidirectional. It is better to regard the two as one thing from the perspective of four-dimensional time and space. Of course, for people who are used to three-dimensional things, it is very difficult. It requires a good imagination and a high level of mathematics, and a deep understanding of physics is naturally indispensable.
Magnetic fields can be generated by moving charges, changing electric fields or static magnetic monopoles. Although no magnetic monopole has been found so far, no theory can completely deny its existence. On the contrary, many theoretical frameworks contain magnetic monopoles. There is a theory that in the early days of BIGBANG, magnetic monopoles can be produced at the edge of the horizon of each space-time region, but due to the rapid expansion during the inflation period, there are few horizons, so there are few magnetic monopoles.
In fact, the magnetic field is the inevitable result of space-time characteristics. That is to say, as long as Einstein's special theory of relativity describes that the effect of "the foot shrinks and the clock slows down" is established in time and space (a large number of facts prove it to be true), then the existence of electricity is bound to be accompanied by the existence of magnetic force.
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For example, at time 0, two particles with the same mass M and the same electric quantity Q fly at (0,0) and (0, 1) respectively in the xy coordinates, and their velocities are both V and their directions are along the positive direction of the X axis. Let the relativistic factor be r, r = (1-vv/cc) (- 1/2). The following quantities with apostrophes are all measured in a dynamic system in which two particles are relatively static, and the quantities without apostrophes are measured in a static system in which the particles are relatively static with the ground. In the dynamic system, the velocity along the Y-axis generated by a particle at the origin under the action of electric power is u'=dy'/dt', and the acceleration is a'=du'/dt'=d(dy'/dt')/dt'. In the static system, "scale contraction" only occurs in the X axis direction, not in the Y axis direction, so dy = dy'; And "slow clock" has nothing to do with the direction, so dt = RDT '；; Therefore, u=dy/dt=dy'/rdt'=u'/r, a =...= a'/RR. In short, from the point of view of pure relativistic space-time kinematics, the particle acceleration A measured in the static system is only 1/rr of A' in the dynamic system. If v=0.943c, then r=3 and a=a'/9.
Looking at the same problem from the dynamic point of view, if there is only electric power F without magnetic force F, then it will definitely come to a conclusion that contradicts the previous kinematics results. First of all, we should know the relationship between f' in dynamic system and f in static system, and we can get qualitative results from the analogy of "scale effect" of objects. A sphere in a dynamic system looks like an ellipsoid flattened in the X direction in a static system. Similarly, in the static system, the isotropic power line distribution in the dynamic system seems to be sparse in the X direction and dense in the plane direction perpendicular to the X direction-in the static system, the electric field of two charges in the connection direction becomes stronger, and the quantitative analysis gives: F = rF'. The mass will increase with the speed -m = RM', so a=F/m=(rF')/(rm')=F'/m'=a'. This is obviously contradictory to a=a'/rr in the previous paragraph! With the magnetic force F, the magnetic force between two particles attracts each other, which can just weaken the force to ensure a=a'/rr. The results of quantitative analysis are: (F-f)=(F'-f')/r, a = (f-f)/m = [(f'-f')/r]/(RM') = (f'-f')/RRM' = a'/RR. This is consistent with the conclusion of kinematics.
To sum up, it can be said that the space-time view of relativity requires that magnetic force must coexist with electricity. On the other hand, it can be said that the universal magnetic force in the macroscopic low-speed world is a powerful proof of the correctness of the relativistic view of time and space! It is generally believed that in the macroscopic low-speed world, the influence of relativity is too small to be ignored, but why is magnetism so common? One of the most fundamental reasons is that electricity is actually extremely huge. As the relativistic effect of electricity, although the relative proportion is still small, the absolute value is not small, so that we can feel it in our daily life.
If q= 1C and the distance R= 1m, the electric power between two particles is f = 9 *109n-9 billion Newton! If v= 10m/s (Liu Xiang-like velocity), the magnetic force between two particles is f = 10-5n- one thousandth of Newton! If v= 1km/s (bullet-like speed), then the magnetic force between two particles is F = 0. 1N ... The reason why we have not experienced huge electric power is mainly because there are two kinds of charges, and the general object is always very close to the state of electric neutrality.
As a space-time effect, any other force has a similar "magnetic force", but just as magnetic force is often much weaker than electric power, the already weak gravity (it is about 40 times weaker than electric power10! The corresponding "magnetic force" is even more minimal. But under extreme conditions, such as near a high-speed rotating black hole, you can feel a force similar to "magnetism" and different from ordinary gravity! Whether the black hole rotates or not, as long as the mass is the same and the distance is the same, the so-called gravity will be the same, and the direction of force always passes through the center of the black hole. However, the general theory of relativity points out that there is also a force perpendicular to the centripetal gravity near the rotating black hole, which is often said that "the space-time near the rotating black hole is dragged and rotated". This force is similar to "magnetism"-instead of pulling you to the center of the black hole, it pulls you to rotate with the black hole, although there is no friction between you and the black hole like that between you and the surface of the earth. The satellite launched by the United States two years ago to verify the general theory of relativity measures the "magnetic" part of the earth's gravity.