"Why Does E = mc²?" by Brian Cox & Jeff Forshaw

Why Does E=mc2? (And Why Should We Care?) was another recommendation from Dan Satterfield.  I happened to read it after the two Frank Close books, which was serendipitous for me -- learning stuff in the first two seemed to help me ease through this one just fine.  I learned the simple basics of the E=mc2 equation years ago, even though I wasn't taught why it meant what it did, much less the many other implications behind the equation and the research that led to it.

I had a hard time really internalizing "spacetime" as a measuring unit. First, I had to understand the thought experiment about two people watching the faster moving of the two, and considering their own perspectives about who is moving and how quickly.  Next, I had to accept that no one person "standing still" is truly still, given how the Earth moves, inside the Solar System that moves, inside the Milky Way that moves.  Last, that light travels one speed only.  At this point, we're forced to figure out what other aspect has to change in order for the original two-people-moving scenario to add up.

But getting the point of spacetime is key to understanding that Einstein proposition about clocks running slower as they themselves move faster, as best exemplified by the Twins Paradox.  Also, seeing things as measured by spacetime is what makes something like a "warping" of it to shorten travel times theoretically possible.

The most serious sit-up-and-notice moment I had was reaching this point:  "... the earth is doing nothing more than falling in a straight line around the sun.  It is just that the straight line is in a curved spacetime, which manifests itself as a (nearly) circular orbit in space".  I actually had to put the book down at this point and ponder things for several minutes.  I had to put it into my own words for my brain to process it:  "The earth is falling straight at the sun, but since the sun itself is moving, the earth's fall path gets pulled along in the direction that the sun itself is falling (toward the center of the Milky Way).  But if you only consider things from the perspective of the sun being still, the earth's movement seems to be the circular orbit".  I had heard before about the light-bending experiment using an eclipse, but this explanation really made me grasp it.