Is this reddit comment accurate: 'I wrote a description of electricity and magnetism as gauge forces here. It is actually similar to gravity warping space, but it warps something else called the U(1) gauge group. That's a fancy way of saying that there is some kind of circle attached to every point in spacetime (a point with x,y,z,t coordinates), and the points on each circle are connected to the points on the neighboring circles. The simplest case is just having each point on the circle connected to the same points on the neighboring circles, but that's kind of boring. Instead you can twist these connections so that the points are connected to points slightly clockwise or counterclockwise as you follow the connections to nearby circles. Since there is a circle at every neighboring place in spacetime, you can choose a different way of connecting to nearby circles for every direction in spacetime. The connections can also be twisted in a more interesting way. If you choose a point on the circle at one place, and trace a path through points on the neighboring circles, you can choose points on those circles that were connected to the previous points in an unbroken line. You can even trace a loop in spacetime that starts and ends in the same place. But surprise! You don't always end up at the same point you started with on the circle. This is called curvature, in analogy to the curved surface of the earth. If you drive 1 mile east, 1 mile north, 1 mile west, then 1 mile south, you won't end up exactly where you started because the earth is curved (take that flat earthers!). The amount of curvature (how far off your point was from the point you started from) depends on which direction in spacetime you traced the loop. And if your loop goes a bit in the x direction, then a bit in the y direction, then in the -x direction, then in the -y direction, the amount of curvature you get is called the magnetic field in the z direction (as we can see, it would make more sense to say that it's the magnetic field associated with the x-y plane). And similarly, the electric field in the z direction is the curvature in the z-t plane (the t direction is the future, -t direction is the past). So now we see why there are 6 components of the electromagnetic field. There's 6 independent ways of choosing a loop to go around, by choosing a direction x,y,z,t to start the loop from and a direction to end it with. Tracing the connections between the circles at the places along the loop brings you back to a different point on the circle, and you can use the curvature of these connections along these basic directions to figure out how different your point is for any loop you choose. The curvatures in the y-z, z-x, x-y directions give you the x,y,z components of the magnetic field, and the x-t,y-t,z-t curvature gives you the x,y,z electric field.
So what is a gauge symmetry in this context? It's a way of relabeling the circles by rotating them by some amount (after all, what does the left or the top of the circle even mean here? What really changes if the labels on the circle get rotated?). A global symmetry would be rotating them all by the same amount, a local symmetry is rotating each one by a different angle (though it has to be continuous, if the circles are nearby then they have to be rotated by nearly the same angle). The interesting part is that the connection between circles doesn't get changed by this symmetry transformation, since you can figure out an equivalent connection for the new circles by unrotating the new circle, following the old connection to another old circle, and rotating it back. This combined procedure defines a new connection for the new circles that contains all the same information as the old one, even though all the labels on it have been changed by the gauge transformation. The electric and magnetic fields (the curvature of the connection) don't change either, so everything still works the same with the new labeling, and it's called "gauge equivalent". Charges are a way of applying this gauge transformation to things other than the electric and magnetic field, so that even though they are interacting through these gauge forces, the gauge transformation keeps everything equivalent. To accomplish this, each charge has a "phase" that keeps track of where on the circle it is, and when the rotations get applied they get applied once to something with charge +1, twice to something with charge +2, or the opposite rotation gets applied if the charge is negative. This is how it works for the circle U(1), the gauge group of the electromagnetic field. The Strong Nuclear force is the exact same thing with the group SU(3), though the results are more complicated because it matters in which order the transformations get applied. That complication is the reason why gluons have color charge, as opposed to photons which don't have electric charge. The kinds of charge that can exist are determined by how many different directions the group can move in (the number of generators of the Lie Group). For U(1) that's just 1 (positive and negative rotations about a single axis), but other groups can have more'.
No comments:
Post a Comment