Chapter 9: Free Fallin’ With Einstein

This is a continuation of part 1. Title inspired by Tom Petty’s song.

Einstein’s happiest thought (1907): “For an observer falling freely from
the roof of a house, the gravitational field does not exist” (left).
Conversely (right), an observer in a closed box — such as an elevator or
spaceship — cannot tell whether his weight is due to gravity or acceleration.

Newtonian model of gravity satisfactorily explained the observable phenomena on earth. But it did not explain everything, especially things at a large scale. Einstein knew we needed new theories to cover the gaps. Ultimately, he successfully changed the fundamental explanation of gravity and its effects on our space and time.

After proposing special relativity in 1905, Einstein was struggling with the fact that it didn’t include acceleration or gravity. He knew something was amiss, but it would take him another decade to generalize his theory.

As with special relativity, Einstein’s thinking evolved with thought experiments. Finally, one became the happiest thought of his life“what would it be like for someone to be in a free fall?” Einstein realized for the falling person, its like there is no gravity, as if they’re weightless and accelerating down towards Earth. Any loose change from their pockets would also just float along. Something Galileo confirmed long time ago.

Similarly, if one is in an enclosed elevator accelerating up through space, the effect would be indistinguishable from experience of gravity. So gravity was not like a force in Newton’s model and there wasn’t any action at a distance.


Gravity became a result of curvature of spacetime that is like a fabric that can be stretched or pulled in presence of matter. It is the outcome of spacetime that is not flat, like in Newton’s model. Einstein’s equations explained the interplay between matter, energy and motion. John Wheeler best summarized it as “matter tells spacetime how to curve, and curved spacetime tells matter how to move”.


But that didn’t mean Newtonian model was wrong, it was a subset of Einstein’s model. On earth, Newton’s model was perfectly sufficient. So for Einstein to prove his theories, there were no earthly experiments that could be performed. There were no space programs with inter-planetary missions or satellites roaming around the earth at that time either. Though his model explained Mercury’s orbit, Einstein had to do more ‘thought experiments’ to come up with scenarios that would further support his theories.

He postulated that one of the effects of general relativity would be apparent bending of starlight as it moved through the curved space around a massive object — gravitational lensing. Eager to find a way, Einstein would bounce around ideas with his colleagues. Like “what if we saw starlight during dusk or dawn”.

After plenty of back and forth, Einstein hit upon the idea of measuring the apparent bending of starlight during a solar eclipse. We need the solar eclipse as Sun’s own light would not let us see any light behind it. The light from distant stars travels along the space that has been curved by Sun’s mass. The distant star does not actually move, but to us on Earth, it would appear as if it has slightly shifted positions.

Actual photographic plates from the 1919 Eddington Expedition, showing (with lines) the positions of the identified stars that would be used for measuring the light deflection due to the Sun’s presence. This was the first direct, experimental confirmation of Einstein’s General Relativity. Source

As World War I was raging at this time, it was difficult for Einstein to get help from international scientific community. Fortunately for Einstein, he found an unlikely ally in Sir Arthur Eddington, who was looking to increase scientific collaboration. Eddington, with support from Frank Watson Dyson, avoided military conscription by building support for testing Einstein’s theory during a solar eclipse.

In May 1919, Eddington finally mounted an expedition to Príncipe(Africa) to validate, even though inaccurately, Einstein’s general theory of relativity. This confirmation gave Einstein instant fame and worldwide recognition. A Nobel prize soon followed in 1921, but surprisingly it cited his 1905 paper on photoelectric effect. Also, as part of their divorce agreement, the prize money was collected by his first wife and collaborator — Mileva Maric.

Many more accurate measurements have been done since to confirm Einstein’s general theory of relativity. Specifically, for ‘gravitational lensing’, measurements done by Gravity Probe B are probably the most accurate.

Ultimately, Einstein’s theories are not just mathematical triumphs, but a necessity of our daily lives. Consider the case of GPS satellites that are used in your car, airplanes and cruise ships. These satellites are at an altitude of 20,000 km and are moving at an orbital speed of about 14,000 km/hour. As per Special and General relativity, the clocks on ground and on the satellites will be different. Our space infrastructure has to account for this fact and pre-correct for it. Without this correction, our navigation will be off by several blocks.

Einstein and Charlie Chaplin at the Hollywood premiere of City Lights, January 1931. Credit

When hundred authors against Einstein was published in 1931, Einstein quipped only one would been enough to disprove relativity. With the rise of hatred under Nazi flag, Einstein eventually left Germany in 1932 and never returned.

Einstein was a life long non-conformist, a pacifist, an imaginative thinker. The only time his moral compass shook was during World War II, when he advocated for the development of Nuclear weapons. A necessary decision at the time, but one that would haunt him for the rest of his days.

Einstein’s theories cover everything from the infinitesimal to the infinite, from the emission of photons to the expansion of the cosmos. The full range of today’s technologies can be traced back to him. Even a century later, we are living in his universe and still proving his theories correct, no matter how counterintuitive or astonishing they might be.

The two pillars of physics he created, continue to occupy our best minds, yielding several Nobel prizes, and awaiting for a genius who can combine them into one “Theory of Everything”. Not to forget, they also provide plenty of fodder to science fiction movies with time travel, wormholes, black holes and even the Ant-Man.

One could argue his true genius was to show how we can use the power of our imagination to understand the universe around us. Thus, making science more popular and approachable, and proving that support for science is a trait of a progressive society. As he once declared, “imagination is more important than knowledge”, he believed in keeping his childlike curiosity alive. Einstein clearly remains a public inspiration a century later, as I am sure he will be a century from now.

Let’s take Einstein’s example to day dream, question convention, embrace simplicity and our humanity — I am sure we all will be grateful for it!

For those who made it to end — Einstein explaining mass-energy equivalence in his own words



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