Albert Einstein wasn’t a famous scientist in early 1919. His theory that massive objects such as the sun can affect space  was unproven. Most people believed, as Isaac Newton had stated, that 

space was unchangeable. You know, heavenly. And with one way to observe it.

In 1905, Albert Einterin had proposed his Theory of Special Relativity, with acknowledged assistance from Dutch physicist Hendrick Lorentz.

This Theory dealt with light. Einstein proposed that light has the same speed no matter how it is observed. This is different than for example, a bullet shot from a moving train. To someone watching the train pass, the bullet would travel at its speed plus that of the train. Light however, doesn’t change speed. It won’t move faster if you shine a flashlight from a train (He came up with this theory while riding a train.) Thus, Einstein said, nothing can be accelerated to go faster than the speed of light. This theory also describes the famous E=mc2 equation showing that energy and mass are interchangeable. This equation is the basis of nuclear chemistry and nuclear bombs. Bombs either involve loss of mass due to fusion (creating a heavier element from lighter ones as powers the sun) or fission (busting an atomic nucleus into smaller pieces). In either case, mass is converted to energy in the process.

One consequence of Special Relativity is that time and space are not constant but can change relative to the observer. If a clock is moving along, as if on a train, time is “normal” to a person on the train. But to an observer, it appears to be moving slower…it contracts. If the train can reach the speed of light, time will appear to stop. Likewise, as the train moves away, it appears shorter in length (but not height or width). Near the speed of light, it will contract to have almost no length.

As for mass, which as we know is related to energy, it will get larger and larger as the train gains speed until the mass will be almost infinite. Another way to think of this is that the train will be so massive it will become unstoppable. The contraction in length and in time and increase in mass are called the Lorentz factor and can be calculated with an equation.

In 1916, Einstein proposed the theory of General Relativity: gravity is caused by curves in space and time. That’s right, attraction is not caused by masses acting on each other as Newton thought, but because objects bend space-time.

His ideas make sense–the sun can make a dent in space like a bowling ball or a heavy partner on a mattress. The dent can be thought of as pulling planets towards the sun as if they are circling a drain.

Gravity isn’t the force Newton thought it was. The bodies aren’t acting on each other. They are reacting to the dents made in space. Each plant makes its own little dent.

Here’s the Earth making a little dent in space circling around the Sun’s bigger dent. From https://theconversation.com/how-einsteins-general-theory-of-relativity-killed-off-common-sense-physics-50042

Instead of saying that the Earth and the Sun attract each other in a fixed stage-like realm, Einstein said that each warps the space and time surrounding the other. This is what Einstein called General Relativity. He took his inspiration from the orbit of Mercury.

One prediction of General Relativity was that light from the stars would be bent towards the sun as they passed it. (This was also predicted by Classical Gravity but not to the same extent.) He calculated that the shift of light from the Hyades cluster would be bent one two thousandths of a degree. Of course, the sun was too bright to be able to observe this, until the eclipse of 1919. Several British and Dutch astronomers saw this as a great time to observe the light as it passed by the sun. They devised an experiment where they would measure the light bending as it passed the sun during the eclipse.

“To pinpoint the position of the Hyades in the sky, (British astronomer) Eddington first took a picture at night from Oxford. Then, on 29 May 1919, he photographed the Hyades as they lay almost directly behind the sun during the total eclipse… experienced that day. Comparing the two measurements, Eddington was able to show that the shift was as Einstein had predicted and too large to be explained by Newton’s theory.”

Another phenomenon predicted by Einstein that held true is that light will lose energy and change color (become more red) as it is affected by gravity.

Another key idea of General Relativity is the equivalence principle. Gravity pulling in one direction is the same to as acceleration in the opposite direction. A train accelerating forwards feels just like sideways gravity pushing you back against your seat. An elevator accelerating upwards feels just like gravity pushing you into the floor. (Elevators became popular in the 1880s.) Two difference forces might feel the same: this is sometimes called Einstein’s Elevator.

Today’s GPS devices rely on the precise measurements made by the theory of relativity.

The 1919 eclipse made Einstein famous. Most people didn’t really understand any of the science but they seized on it as a blow to the absolute world, including moral taboos and “absolute” roles for women they’d been raised with. It was particularly popular with the young who saw the universe as giving them permission to seek their own truths.

At the time, Albert was divorcing his first wife, a physicist, in order to marry his cousin, who he felt would take better care of him. Wisely, his ex had put in the divorce settlement that if he won the Nobel prize, she’d get half the money. He won the prize and never did publish anything as grand as those two theories developed while collaborating with his first wife, Meleva Maric. People, including their son, to speculated that the work was as much hers as his. She did get half of the prize money, but struggled financially for the rest of her life. And Albert was an absolute jerk to her. Read here for more.