Two English physicists try to explain Einstein’s famous equation and much more, including relativity and quantum mechanics. I didn’t understand quite a bit and didn’t try to do the math (which is relatively limited), but found their explanations reasonably helpful. For example, they explain that the speed of light is an upper limit because photons have no mass. It isn’t anything to do with light per se. Any particle with no mass travels at the speed of light and no faster. Gluons don’t have mass and, if they exist, neither do gravitons. So we might just as well call it “the speed of particles with no mass”. 

They also explain that mass and energy are constantly being exchanged in accordance with Einstein’s equation. Atomic weapons are just the most spectacular example of a process that is universal to nature, and occurs, for example, every time heat is generated or there is some other chemical reaction.

I’m still confused by the Twin Paradox. Why would someone in a spaceship moving close to the speed of light age more slowly than someone staying on Earth, if all motion is relative? Why not say that the person moving near the speed of light is standing still and the person who stayed at home is moving near the speed of light? The answer is that the person in the spaceship is accelerating and decelerating, and that’s why we can properly say that he or she is moving faster than the person on Earth and why he or she ages more slowly. There are formulas that explain this, but it still sounds fishy. 

I’m also bothered by the idea that the Big Bang had no location. If the universe is expanding in all directions, why can’t we say where the Big Bang occurred? And maybe put a monument there with a gift shop?  (9/8/11)