Gravity is an idea that has been driving physicists crazy for as long as physicists have been around.
You might think that we have it all worked out by now. Einstein worked out the spacetime thing right? Wrong!
Gravity looks like it may be the final boss of physics. Like any good boss battle, it takes various forms.
First, a quick overview of what it actually is. Gravity is one of the four fundamental interactions and is a force generated by objects with mass. They pull all known forms of energy towards themselves, like marbles rolling down to the bottom of a sink.
It’s responsible for the large-scale structures of the universe, from the shape of galaxies to the spin of solar systems. It keeps our feet on the ground, gives objects weight, keeps the moon orbiting the Earth, the Earth orbiting the sun, and the sun rotating around the Milky Way.
Well into the 18th Century, scientists believed in what was called the mechanistic universe. That is, they believed that the universe operated like a grand machine, like a perfectly constructed, incredibly intricate clock.
Part of this philosophy was that there could be no interaction without contact.
When it came to gravity, people held onto ideas that predated even the mechanistic universe. They believed in the ideas of Aristotle from the 3rd Century BC: That every object has a ‘natural place’ which they all move towards. The Earth is the heaviest object, and its natural place is below everything else. Water sits on top of it, and air rises above both, which is why bubbles rise in water. This natural place was considered a fundamental law of the universe, and contributed to the long-held belief of the Earth sitting at its centre.
Isaac Newton shattered this worldview.
To understand how he did this, we have to look at Newton on a personal level. By all accounts, he was an unusual guy. On the one hand, he held one of the most prestigious academic posts in the world, the Lucasian Professor of Mathematics of Cambridge University. On the other, he would spend his nights researching and practising alchemy and the occult. One of his goals was turning base metals into gold, and discovering hidden clues in the Book of Revelations to uncover the date of the apocalypse (apparently it’s happening in 2060).
When he proposed the idea of gravity, an invisible force that permeates the Earth that can’t be seen or touched, other scientists considered him out of his mind and accused him of bringing the occult into science. The idea of ‘action at a distance’ was preposterous to their mechanistic worldview.
Newton was unable to discover the physical cause of gravity. He called the idea “inconceivable” and “absurd”, but he was happy to concede that science did not yet understand the phenomena of the material world, and leave the question open for the next generation of physicists.
One of his most famous quotes was “I frame no hypothesis.”
To his credit, Newton’s gravity was just a mathematical description of the motion of the planets that he saw through his telescope. 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 physical cause of gravity was unknown, it worked and we’d all just better get on with it. This represented a major change in the scientific community by moving away from the mechanical paradigm.
Hundreds of years later in the early 20th Century, Albert Einstein was a young German patent clerk. He was travelling home from work on a streetcar and was staring out his window, daydreaming.
The nearby clock tower struck 6pm, and a thought came to him. If he looked out the window, he’d see the clock face say 6 pm. He can see it because light waves are carrying the image to him.
But if his streetcar was travelling at the same speed, the speed of light, then every time he looked out the window the clock would still say 6pm. Time, at least from the perspective of everyone else, would be frozen.
This was the beginning of the idea of spacetime. The idea goes like this:
We imagine space (not outer space. Geometric space, as in up and down, left and right, forward and back) 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.
Gravity causes a ‘gravity well’ to form within the underlying ‘fabric’ of spacetime, like a football resting on a suspended bedsheet. The depression 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). 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.
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.