Alright! Welcome back to my short series on finally getting Relativity! In the first installment, I addressed the background to Einstein’s revolutionary breakthrough, which covered Galileo, Newton, and the birth of Classic Physics (aka. Newtonian Physics). In the second installment, I addressed how the problems of reconciling electromagnetism with established theories of motion led Einstein to propose his groundbreaking Special Theory of Relativity.

Hopefully, I did them justice while also presenting them in a way that met Einstein’s challenge (“If you can’t explain it to a six-year-old…”). This brings us to the last lap, wherein I attempt to explain how Einstein generalized his theory to account for gravity and thereby made sense of the Universe (well, not quite, but he definitely pushed that ball farther downfield!)

Towards a General Theory

Between 1905 and 1915, Einstein sought to tackle the problem presented by gravity. In short, Newton’s Universal Gravitation was running into theoretical problems, where it also didn’t line up with observations. When it came to the then-known Solar bodies, Newton’s equations explained the orbits of all the planets, save one: Mercury. While its orbit seemed to conform to what Newtonian gravity predicted in the short term, there were long-term changes that could not be accounted for.

To explain this briefly, Mercury has a highly eccentric orbit around the Sun. If you could visualize its orbit on a 2-D plane, you’d see that its x-axis is about 20% longer than its y-axis. Basically, its orbit is in the shape of an oval. The thing is, over time, this ovoid-shaped orbit also rotates around the Sun. This is known as a “precession of perihelion,” where the farthest point in a planet’s orbit moves around the larger body in a circle.

Aside from Mercury’s orbit, there were also several outstanding issues regarding how Special Relativity (SR) applied to the Universe at large. The first issue was the idea of instantaneous communication. As Einstein previously demonstrated with SR, information is not communicated instantly across spacetime. If a star explodes 1 billion light-years away, we would not know about it until 1 billion after the fact.

As such, it was impossible to interpret gravity as an instantaneous force of attraction between point objects that grew stronger the closer you got to it. In keeping with the laws of electromagnetism, Einstein hypothesized that gravity behaved as a field. Within this field, objects would experience the force of attraction and be pulled towards each other.

This raised another issue for Einstein, which was acceleration. From a theoretical standpoint, scientists could not explain how it was any different from gravity. Einstein illustrated this with another clever metaphor: an elevator. A person standing in an elevator that is moving at a constant speed might not know they were moving at all. Cut the cable, though, and that person will immediately experience weightlessness as the car plummets (aka. freefall). Basically, the car’s acceleration towards the ground counteracts the feeling of gravity.

The same is true of a spacecraft. When floating in space, the crew would feel weightless. But the moment they fired up their thrusters, they would feel it in their bones! If the spacecraft interior were designed so that people stood upright parallel to the ship’s long axis, this acceleration would make them feel like they were standing on solid ground. If the ship were accelerating at 9.8 m/s2, the crew would feel the equivalent of Earth-normal gravity.

Now apply that to a pinwheel or rotating cylinder in space. If the object is rotating fast enough, the centripetal force will pull any objects inside outwards. For people standing inside, this would feel like gravity. At sufficient speeds (depending on the size of the pinwheel or cylinder), Earth-normal gravity could be simulated. For over a century, many noted scientists (from Tsiolkovsky to O’Neill) have proposed that such facilities could be the key to exploring and settling the Solar System.

On top of that, the matter of time dilation also comes up in SR. If objects experiencing acceleration are subject to time dilation, then gravity itself affects spacetime. From this, Einstein’s new interpretation of gravitation was born!

Behold: General Relativity!

As Einstein would describe it in a series of papers, gravitational forces are not really an attraction between point sources with mass. Instead, Einstein postulated, gravity is a field generated by massive objects that alters the curvature of spacetime around the object. This causes nearby objects to trace the curvature of spacetime around the object and experience acceleration and time dilation – the level of which depends on the object’s mass.

Much like SR, Einstein’s General Theory of Relativity (GR) would have a number of consequences for theoretical physics. For starters, if what Einstein was saying was true, it meant that the curvature of spacetime would alter the pathway of everything, including light! Interestingly enough, this was how GR would be proven correct. In 1919, Arthur Eddington and his colleagues conducted an experiment during a solar eclipse (the Eddington Experiment).

One of Eddington’s photographs of the 1919 solar eclipse experiment. Credit: F. W. Dyson (et al.)/Wikipedia Commons

For this experiment, Eddington and his colleagues conducted observations simultaneously from two equatorial observatories. One was located on the northeast coast of Brazil, the other on the island of Sao Tome and Principe off the coast of West Africa.

Specifically, Eddington’s team was looking for stars that would be passing behind the Sun at the time. If Einstein’s theory of GR was correct, the light from these stars would follow the spacetime curvature caused by the Sun’s gravity. This effect would make it appear like the stars were shining beside it. Because the Sun was totally eclipsed, the stars’ light wouldn’t be drowned out by sunlight.

Lo and behold, Einstein was correct! Not only did they see these stars, but their positions in the night sky were precisely where GR predicted they would be! The story made the front page of newspapers worldwide and made Einstein and General Relativity household names! Alas, this “light-bending” phenomenon was just the tip of the iceberg!

But that feels like something for another time. In fact, I imagine this series is going to suffer from multiple addendums in the near future. Possibly some meaty corrections, too! In the meantime, I hope this has proven to be a comprehensible and (dare I say it) accurate synopsis of what is arguably the greatest scientific breakthrough in human history.

4 thoughts on “I Finally Get Relativity – Part the Last!

  1. Multiple addendums would be most welcome! And I really like the way you explained the precession of the perihelion of Mercury. I’ve known about that for a long time and that it was part of the evidence for general relativity, but I never fully grasped what, exactly, it meant.

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