The Thought That Led to General Relativity

Imagine two completely different scenarios. In the first, an astronaut is floating out in the middle of space away from any gravitational fields. In this state, the astronaut is completely weightless and simply floats within the vacuum of space. In the second, a person is falling from the roof of a building and quickly and accelerates towards the ground. Though in the second scenario the individual is accelerating at 9.8 m/s^2 while in the other they are completely stationary, both individuals feel completely weightless. Einstein reasoned that not only are these two scenarios similar, but that they are in fact indistinguishable, a realization he called the happiest thought of his life.

To better understand how these two scenarios are equivalent, imagine that in both scenarios, the person holds a ball in their hand then releases it without applying any additional force. In the first scenario (in space) the ball simply floats in place relative to the observer, who is also at rest. The same thing would also happen if the astronaut was moving, as the ball would simply move at the same velocity as the astronaut, thereby appearing at rest.

In the second scenario (the falling person), assuming there is no air resistance, the ball continues accelerating at the same speed the person does, meaning that it also appears at rest, as if floating alongside the person. Furthermore, just like the person, the ball also “feels” weightless.

Taking this thought experiment a step further, as Einstein did, we can reason that if in both scenarios all connections with the environment are cut off, there is no way for the astronaut and falling man to experimentally distinguish whether they are falling or floating in space.

At first, this might just seem like an interesting coincidence, but Einstein quickly realized that this had far greater implications about gravity and space and time. To understand these implications, let’s imagine a different scenario, but with the same fundamental idea.

continued https://open.substack.com/pub/theuniversewithphysics/p/the-thought-that-led-to-special-relativity