Supermassive blackholes (QSOs, to be specific) are the foundation of our highly accurate GPS system.

You see, the Earth “jiggles” in its orbit from the perspective of the satellites. By using QSOs as a static reference frame, the GPS satellites can quantify Earth’s movement and apply corrections.

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» Chapter 218 «

Many people talk about how General Relativity solved the issue of Mercury’s orbital precession.
The story is much more amazing than you think.

You see, Mercury’s precession has been known for hundreds of years, and in that time Mercury precessed 574″ (” is 1/360 of a degree) per century. Many precise calculations were done to account for this – the sun’s lack of symmetry, the other planets, etc. however, 43″ were left unaccounted for, and it was known that those 43″ were not due to error.

When Einstein did the math for calculating Mercury’s precession he used a simplified model – ignoring the asymmetry of the sun, the other planets, etc. and found the precession to be 43″ – exactly what was missing from previous models.

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» Chapter 217 «

You think of Pythagorean’s theorem as the relationship between the areas of squares whose sides lie on the side of a triangle.
I mention this, as it works with more than just squares – any regular polygon works.

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» Chapter 216 «