h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

It's always struck me that C and P refer to properties of the system under observation whereas T refers to a reference frame in which the system is being observed. As such I've always doubted the rationale for discussing them in a way that implies they are connected.

Sorry, thats not a statement about your article as all the stuff I've ever seen talks about them as a group. I just wonder if there is something relevant here?

Anyone interested in T symmetry might like to read "Time's Arrow and Archimedes' Point" by Huw Price.

Personally, I don't get his point, because gravity blatantly violates T symmetry (I think); and quantum theorists just ignore gravity.

Ask me in about three years, when I've learned a lot more Physics, and I'll have a much more informed opinion.

I don't know how relevant this is to your post, but I thought it wouldn't hurt to mention it.

I think gravity must be T invariant. It works like this

The T reversed version of an apple falling to the ground is an apple flying away from the ground, thus we think there's some sort of gravitational repulsion.

But a falling apple is accelerating towards the Earth, so the T reversed apple is decelerating away from it. Repulsion will cause the apple to accelerate away from earth, but the decellerating apple obbeys newtons laws perfectly, exactly as an apple would if you threw it up in the air. It decellerates to zero speed, and is then caught by the tree (or falls back).

Think of the solar system, the earth orbits the sun, the moon orbits the earth, comits follows their highly elliptical paths and nearly everything spins around; all driven by gravitational attraction. T reverse this, and everything just spins or orbits backwards, but still driven by gravitational attraction. It gravaity was different the motion of the planets would be quite different.

The reason why the apple flying up into the air seems silly is due to probability. It's very probable for a twig to snap, an apple to fall, and hit the ground dissipating it's energy as thermal vibrations in the soil. It is extremely improbable (but not totally impossible) for random thermal vibrations to suddenly bunch together to drive an apple from the ground, which is then caught by the tree as it fuses itself back to the branch. The probability of this happening is so small it would take longer than the age of the universe to even calculate it. But it is allowed by the laws of physics.

However one thing that presumably does violate T is a black hole. Anything that passes the event horizon can't come out. This may be example of a incident of T violation required to allow CP violation and CPT invariance.