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In particle physics, everything is based on symmetry. Symmetry magazine is the main industry newsletter for particle physics. All of the fundamental laws of physics can be expressed as symmetries. Other than symmetry, the only rule is basically "anything that can happen, will happen".
There are two general categories of symmetries in particle physics, internal symmetries and spacetime symmetries. I'm only going to discuss spacetime symmetries here.
Within the category of spacetime symmetries there are continuous symmetries like rotational symmetry (responsible for the conservation of angular momentum), translational symmetry (responsible for regular conservation of momentum), time translation (responsible for conservation of energy), and Lorentz boosts (responsible for Einstein's theory of relativity).
But then there is also another kind of spacetime symmetry--discrete symmetries. There are 2 important discrete spacetime symmetries and they are pretty simple to explain. The first is called time reversal symmetry, usually denoted by the symbol T. As an operator, T represents the operation of flipping the direction of time from forwards to backwards--basically, hitting the rewind button. Parts of physics are symmetric with respect to T and other parts are not. The other important one is P (parity), which flips space instead of time--it's basically what you see when you look in the mirror and left and right are reversed, everything is backwards.
Here is a video of me doing a cartwheel, an every day process which by itself would appear to break both P and T. The animation shows the forward-in-time process first which is a right-handed cartwheel, followed by the time reverse which then looks like a left-handed cartwheel. Because applying T in this case accomplishes exactly the same thing as P (if you ignore the background), this means that this process breaks both P symmetry and T symmetry, but it preserves the combination of the 2, PT:
And now for the front handspring. Unlike the cartwheel, this process respects P symmetry. If you flip left and right, it still looks the same. However, if you time reverse it, it looks like a back handspring instead of a front handspring! So the handspring respects P symmetry but not T symmetry.
Of the 4 fundamental forces of nature--gravity, electromagnetism, the strong force, and the weak force--the first 3 respect time-reversal symmetry while the fourth, the weak force, does not. Because the other 3 are symmetric, it was assumed for a long time (until the 1960's) that all laws of physics had to be symmetric under T. Only in 1964 did the first indirect evidence that the weak force does not respect T symmetry emerge, and more direct proof came in the late 90's and still more interesting examples have piled on within the past decade.
( Lots more explanation behind the cut! )
There are two general categories of symmetries in particle physics, internal symmetries and spacetime symmetries. I'm only going to discuss spacetime symmetries here.
Within the category of spacetime symmetries there are continuous symmetries like rotational symmetry (responsible for the conservation of angular momentum), translational symmetry (responsible for regular conservation of momentum), time translation (responsible for conservation of energy), and Lorentz boosts (responsible for Einstein's theory of relativity).
But then there is also another kind of spacetime symmetry--discrete symmetries. There are 2 important discrete spacetime symmetries and they are pretty simple to explain. The first is called time reversal symmetry, usually denoted by the symbol T. As an operator, T represents the operation of flipping the direction of time from forwards to backwards--basically, hitting the rewind button. Parts of physics are symmetric with respect to T and other parts are not. The other important one is P (parity), which flips space instead of time--it's basically what you see when you look in the mirror and left and right are reversed, everything is backwards.
Here is a video of me doing a cartwheel, an every day process which by itself would appear to break both P and T. The animation shows the forward-in-time process first which is a right-handed cartwheel, followed by the time reverse which then looks like a left-handed cartwheel. Because applying T in this case accomplishes exactly the same thing as P (if you ignore the background), this means that this process breaks both P symmetry and T symmetry, but it preserves the combination of the 2, PT:
And now for the front handspring. Unlike the cartwheel, this process respects P symmetry. If you flip left and right, it still looks the same. However, if you time reverse it, it looks like a back handspring instead of a front handspring! So the handspring respects P symmetry but not T symmetry.
Of the 4 fundamental forces of nature--gravity, electromagnetism, the strong force, and the weak force--the first 3 respect time-reversal symmetry while the fourth, the weak force, does not. Because the other 3 are symmetric, it was assumed for a long time (until the 1960's) that all laws of physics had to be symmetric under T. Only in 1964 did the first indirect evidence that the weak force does not respect T symmetry emerge, and more direct proof came in the late 90's and still more interesting examples have piled on within the past decade.
( Lots more explanation behind the cut! )
