[Kamog]

The author says, "As we all know, perpetual motion machines are impossible,..." and gives the example that if you drop an object dropped into a tunnel through the Earth it would oscillate back and forth endlessly, saying that this result violates the laws of physics.

It does not. Perpetual motion machines are, in fact, possible if all friction is eliminated. Dropping an object into a tunnel through the earth will oscillate endlessly if there were no air inside the tunnel to create friction. If there is some air in the tunnel, the object's energy would be gradually lost as heat and the oscillation would decay and eventually stop, as expected.

It does not require the expenditure of energy to hold an object still in one place! You don't need a constant energy source to keep a fridge magnet stuck to the fridge or have a heavy object resting on a table. The object doesn't move because all the forces on the object are in balance and cancel each other out.

A light beam shining through glass will slow down and it does speed up again once it exits the glass. Why does it speed up again when it exits the glass? That's a good question, and I don't know why. I'm very curious as to what the reason is. But I think the reason a block of glass will heat up when light is shined though it is because the glass is imperfect and not completely transparent. Part of the light is absorbed and turned to heat. So not all the light emerges from the other side.

Regarding the Twin Paradox Thought Experiment: Hmm, I never thought of that paradox! I would very much like to know how to resolve this paradox.