When you take a step, you’re anchored to the Earth by a force. You can jump to overcome it for a moment, but you are always pulled back down. Of course this force is gravity, but it’s a concept that has been driving physicists crazy for as long as there have been physicists.

You might have thought we had it worked out by now. Einstein solved it when he discovered ‘spacetime’ right? Well, kind of. He unintentionally kicked a ton of problems down the road that generations of physicists have been struggling with since.

Gravity looks like it may be the final boss of physics. Like any good boss battle, it has taken various forms.

First, let’s give a quick overview of what gravity is trying to explain:

Things in the universe are drawn towards things with mass. The Earth has a much greater mass than you do, so you are pulled towards it. Everything from apples to asteroids that are near the Earth will feel the same force.

If a thing is moving fast enough and at the right angle, it can ‘orbit’ the more massive thing without falling in. This is how the Moon orbits the Earth, the Earth orbits the Sun, and the Sun orbits the supermassive black hole in the center of our Milky Way Galaxy.

Gravity has an incredibly long range, and determines the shape of the biggest objects in the universe from the spin of solar systems to the whirlpool shape of galaxies, and beyond.

Part I. Natural Place

For a long time in the West, philosophers believed that things in motion were explained by the ideas of Aristotle from the 3rd Century BC. They believed that every object has a ‘natural place’ which they all move towards. The Earth has its natural place at the centre of everything. Water sits on top of it, and the air sits above both. This was why, they reasoned, bubbles rose in water. The Moon, other planets, and stars had their natural place above the air.

After the scientific revolution between the 14th-17th Centuries, Aristotle’s ideas still held sway but our big-picture ideas changed a little. Scientists instead began to believe in what was called the ‘mechanistic universe’. The universe apparently operated like a grand machine, like a perfectly constructed, incredibly intricate giant clock. Part of this philosophy was that there could be no interaction between two things without them directly contacting each other.

Isaac Newton blew this worldview out of the water.

Part II. Gravity

By all accounts, Isaac Newton was an unusual guy. On the one hand, he held the most prestigious academic post in the world, the Lucasian Professor of Mathematics at Cambridge University.

On the other, he spent his nights researching alchemy and practising the occult. One of his obsessions was turning base metals into gold, and to discover hidden clues in the Bible’s Book of Revelations to uncover the date of the apocalypse. Apparently it’s happening in 2060.

Newton published a book that described the movement of the planets that he saw through his self-made telescope using maths instead of abstract theory. While it was never said outright, his precise equations implied an invisible force that permeates all things including the Earth. The force can’t be seen or touched, and was called ‘gravity’.

Other scientists considered him out of his mind. It was like he proposed it happened by magic. The idea of ‘action at a distance’ was preposterous to their mechanistic worldview.

But Newton only published the equations that describe the movement of the planets. He was unable to discover the cause of his invisible force. Regarding this, one of his most famous quotes was;

“I frame no hypothesis.”

He called his own idea “inconceivable” and “absurd”. But he was happy to concede that science did not yet understand everything, and left the question open for the next generation of physicists.

While the implications of his equations were startling, they did make accurate predictions. They were used by later astronomers to discover the planet Neptune.

Eventually the scientific community came around to his ideas, including his position that while the cause of gravity was unknown, it worked and we’d all just better get on with it.

Part III. Spacetime

Hundreds of years later in the early 20th Century, Albert Einstein was a young German patent clerk. He was travelling home from work in a streetcar, and was staring out his window daydreaming.

The nearby clock tower chimed, and an unusual thought came to him. If he looked out the window, he’d see the clock face say ‘6pm’. He knew that that the reason he could see it was that light waves were carrying the image to him.

But, he thought, imagine if his streetcar was travelling away from the clock tower at the same speed as light. Every time he looked out the window, he would see the same image; the clock would always say 6pm. Time, at least from his perspective, would be frozen.

That was the beginning of the idea of spacetime, and honestly, it’s way more out there than Newton’s idea of gravity. The idea goes like this:

We imagine space (as in the physical space of the universe – its height, width, and depth) and time to be fully separate things.

But some things, like light and gravity, act as if space and time were both parts of the same thing. They are distorted by both space and time simultaneously. If they act in this way, perhaps our intuitions are wrong and space and time are actually parts of the same thing; a spacetime ‘fabric’. The explanation of gravity – why things are attracted to more massive things – is that heavy things create a depression in the underlying fabric of spacetime, like a basketball resting on a suspended bedsheet.

The ‘gravity well’ that an object, like the Earth, causes within this underlying fabric causes other objects to fall towards it, like a person or an asteroid.

Just like Newton, Einstein published the maths of his theory of ‘General Relativity’ first. The proof came from a later experiment.

During a solar eclipse, when the Moon hid the Sun’s light, stars would appear in the daytime. But stars that were right next to the Sun appeared in slightly different spots than what they would do at night. The Sun’s gravity well had curved the path of light coming from these stars, making them appear to be in a different place.

Even more impressive is that massive astronomical objects like galactic clusters have a lensing effect. They can magnify the light from galaxies that lie far beyond, giving us a window into the very distant universe.

At this point, it seemed like the question of gravity was put to rest.

Part IV. Finish him!

Around the same time that Einstein proposed his theories, a new field of physics was being created; Quantum mechanics. It described the tiny world of electrons and quarks inside of the atom.

If both General Relativity and Quantum Mechanics theories accurately described reality, we’d be able to seamlessly combine them. But instead of fitting together neatly, the two theories proved utterly incompatible and their equations produced impossible results.

It’s a sign that the spacetime explanation of gravity isn’t the full story. At the moment, physicists use the two theories independently and avoid areas where they cross paths. But everyone knows that this fundamental disconnect exists, lingering in the background.

This split within physics is one of the most significant unsolved problems in science.

The major problem is that gravity is easy to detect when you’re looking at very large objects like planets, stars, and galaxies. It follows that even tiny objects that have mass, like an electron or quark, must have gravity too.

Connecting the two theories requires understanding how gravity works for these tiny objects, but it’s force at this level is absolutely minuscule and hard to detect. Measuring it experimentally requires vastly more powerful instruments than we have invented.

Getting these measurements, and the theory of ‘quantum gravity’ that will follow, is a scientific Holy Grail that will unify physics, and for that reason it is known as the theory of everything.