The Gravity of the Situation: Einstein vs. Newton

Take a look at this paragraph from the book, “The Beginning of Infinity by David Deutsch”, I am currently reading:

“The fundamental theories of modern physics explain the world in jarringly counter-intuitive ways. For example, most non-physicists consider it self-evident that when you hold your arm out horizontally you can feel the force of gravity pulling it downwards. But you cannot. The existence of a force of gravity is, astonishingly, denied by Einstein’s general theory of relativity, one of the two deepest theories of physics. This says that the only force on your arm in that situation is that which you yourself are exerting, upwards, to keep it constantly accelerating.”

While it would make me disappointed if you don’t consider this mind-bending as I did when I came across this concept in secondary school, I would understand, especially if you are not a science student. But shulai try my best to explain?

Okay, in physics, one of the first things you get introduced to is Isaac Newton’s law of gravity. It tells us that gravity is an invisible force that pulls objects toward massive bodies (e.g., Earth). So, when you hold your arm out horizontally, it is the force of gravity that is pulling it downward. Newton put forward this law in 1687 and it was used to explain the observed motions of the planets and their moons. If you throw a ball up, it is the force of gravity that forces it to come down. For centuries, that’s all we knew about gravity

Then Albert Einstein came in 1915, more than 200 years later, with his own theory of gravity, also known as General Relativity or The General Theory of Relativity. He said that you can’t actually feel gravity pulling your arm down. Really? Yes!

Yes, the idea is strange, but it’s one of the deepest and most mind-bending theories of modern physics.

Einstein said that gravity isn’t really a force at all, in the way we usually understand forces. Instead, he argued that gravity is the result of the way space and time itself are curved around massive objects like the Earth. This bending of space-time affects how objects move, but not because of some invisible pulling force.

But let’s not get ahead of ourselves. To really understand this, we have to understand what spacetime is.

Think of spacetime like the fabric of the universe where every other object (earth, moon, sun, etc) lie. Scientists say that as the universe expanded after the Big Bang, spacetime stretched out along with it. Spacetime is woven into the very fabric of the cosmos, and it responds to mass and energy. Everything travels through spacetime. Even though spacetime is not something you can touch or see, it’s all around us.

With that background, let’s go back to Einstein.

According to Einstein’s theory of general relativity, when holding your arm out straight in front of you, what’s really happening is that your arm is accelerating upwards. Not because of gravity pulling it, but because you’re using muscles to push it up against gravity.

Not clear, abi? I told you it’s mind-bending.

Let’s imagine you have a trampoline, and you place a heavy ball, say, a bowling ball, right in the center of it. What happens? The trampoline curves and dips in the middle, right? Now, if you roll a smaller ball, like one of those marbles used in construction sites, onto the trampoline, it will roll towards the heavier ball. The marble is rolling towards the bigger ball not because the heavy ball is pulling it with an invisible force, but because the trampoline’s surface is curved. The marble follows the curved path created by the bowling ball’s weight on the trampoline.

Now, imagine the trampoline as space-time. Massive objects like the Earth curve space-time, creating a kind of “dip” or “well” in the fabric. Objects, like the marble (or even light), move along these curves not because something is pulling them, but because the space-time around them is shaped in a way that influences their motion.

This is similar to how gravity works according to Einstein: It’s not a force pulling objects, but rather the curved space-time that causes objects to move in a certain way.

And since 1915, Einstein’s theory has been proven right. If you are a physics enthusiast, I encourage you to look up these experiments: Michelson–Morley experiment, the Kennedy–Thorndike experiment, and the Ives–Stilwell experiment.

So, is Newton’s concept of gravity a lie? Not really. It’s just a simplified model. In fact, rockets, satellites, and space travel all rely on Newton’s equations because they are so effective in the conditions we deal with in our daily lives.

So, it’s not wrong, it’s just not complete. When you get into conditions of very high speeds (close to the speed of light), very massive objects (like black holes) and very high gravitational fields (like near a star), then what Newton proposed would no longer be sufficient; it would break down. That’s when the genius of Einstein shines forth.

That’s it. I was too excited reading that passage not to share.

Back to reading…