There’s no doubt General Relativity is an important theory, but I had no idea people were planning events to celebrate it. Ah well, I should join in the fun.
- Not many people know this, but Einstein wrote a book to explain GR aimed at the general reader. Since it was published a hundred years ago, it’s long since out of copyright and easy to find online.
- If you’d prefer the same info in video form, I recommend this series from PBS Space Time. There’s no math, but it tries hard not to simplify anything. No rubber sheets are invoked, for instance.
- GR makes a number of predictions, above and beyond Newtonian Physics. Here’s a good article on six of them, each of which are remarkably accurate. Gravity waves are the only gray area; we have very strong indirect evidence but no direct observation.
- Einstein tacked on a “cosmological constant” as a fudge factor. The consensus at the time was that the universe was static and eternal, yet GR predicted such a universe would collapse down to a singularity. So he added a negative pressure to spacetime, acting as a balance. Two years later we discovered space was expanding, which eventually led Einstein to dismiss the cosmological constant as “his greatest blunder.” It really was a blunder, but by fluke we found something that behaves very much like a cosmological constant, dark energy.
- Einstein was very skeptical of Black Holes and Quantum Mechanics, and it turns out he was right to worry: our best challenges to GR come from either. The latest thinking in physics is leaning towards the existence of a firewall at the event horizon of a black hole, contradicting GR’s prediction of no special boundary. On a more fundamental level, GR proposes a view of the universe that’s local (can’t get from A to B without passing through every point in between) and smooth, while QM breaks locality on the small scale and asserts the existence of a quantum foam. As of now, the evidence is leaning to the QM side.