Gravity is a concept that has been driving physicists crazy for as long as there have been physicists.
You might think that we have it all worked out by now. Einstein worked out the ‘spacetime’ thing right?
Well, kind of. But he 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, a really quick overview of what it’s trying to explain.
Things are drawn towards heavier things. Apples are drawn towards the center of the Earth. So are asteroids.
If a thing is moving fast enough and at the right angle, it can ‘orbit’ the heavier thing without falling in.
The Moon orbits the Earth. The Earth orbits the Sun. And the Sun orbits the supermassive black hole in the centre of our Milky Way Galaxy.
It’s influence shapes the biggest objects in the universe, from the galaxies to the spin of solar systems.
Part One. Natural Place
For a very long time in the West, scientists and philosophers believed that this stuff was 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 as at the centre of everything else.
Water sits in a layer on top of it, and air is above both, which is why bubbles rise 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 began to believe in what was called the ‘mechanistic universe’. The universe apparently operated like a grand machine, like a perfectly constructed, incredibly intricate, and throughly 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 out of the water.
Part Two. Gravity
By all accounts, Isaac Newton was an unorthodox guy. On one hand, he held (perhaps) the most prestigious academic post in the world, the Lucasian Professor of Mathematics of Cambridge University.
On the other, he spent his nights researching alchemy and practising the occult.
One of his goals 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 motion of the planets that he saw through his self-made telescope using mathematical language.
While it was never said outright, his equations implied an invisible force that permeates all things including the Earth and that can’t be seen or touched, which 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.
During his life Newton was unable to discover the cause of gravity. 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 his force was unusual, it did make accurate predictions. It was 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 Three. 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 struck 6pm, and an unusual thought came to him. If he looked out the window, he’d see the clock face say 6pm. The reason why he can see it is because light waves are carrying the image to him.
But if his streetcar was travelling away from the clock tower at the same speed as the light waves were, then every time he looked out the window, he would always see the same light waves. The clock would always say 6pm.
Time, at least from the perspective of everyone else, 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 just parts of the same thing. It distorts both space and time simultaneously. Spacetime is a concept built on this discovery. If they act in this way, perhaps our intuitions are wrong and space and time are actually the same thing; a fabric.
The explanation of gravity – why things are attracted to heavier things – is that heavy things create depression in the underlying ‘fabric’ of spacetime, like a football resting on a suspended bedsheet.
The ‘gravity well’ that an object, like the Sun, causes within this underlying fabric causes other objects that aren’t as big to fall towards it, like an asteroid (or to orbit it, like the Earth).
What led everyone to adopting Einstein’s space time theory came from an experiment.
During a solar eclipse, when the Sun’s light was dim, stars could appear in the daytime. Stars that were right next to the Sun were visible, but they 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 in a difference place.
Even light, a particle without mass, is affected by gravity because they it travel through spacetime. Some massive astronomical objects like galaxies or galaxy clusters have a lensing effect, magnifying the light from galaxies that lie far beyond, giving us a window into the distant universe.
Once again it seemed like the question of gravity was put to rest.
Part Four. Quantum Mechanics
But a few decades after Einstein, a new field of physics entered the scene. Quantum mechanics. When physicists combined Einstein’s general relativity and quantum mechanics in their equations, instead of fitting together neatly, the equations produced impossible results. The two theories were utterly incompatible.
At the moment, physicists use quantum mechanics when dealing with things that are small, like within the atom, and general relativity when dealing with things that are large, like putting a satellite in orbit around the Earth.
But they know that this fundamental disconnect exists, just like how Newton could not find the physical cause of gravity. This split within physics is one of the most significant unsolved problems in science.
The other fundamental forces have already been incorporated into quantum mechanics, and it’s gravity that is next on the list. The problem is that compared to the other forces, it is exceptionally weak and therefore hard to detect at the sub-atomic scale of quantum mechanics, and it may require more powerful instruments than what we currently have access to.
The effort to create a unified framework of the fundamental forces is the driving force behind the push to develop a theory of everything.