Gravity is a nuisance, don’t you think? The gravity of our home planet makes it hard to climb a mountain.
It can cause a broken leg if you fall down the stairs. It makes you very much aware of your body mass. And it’s the main reason why pigs – or people – don’t fly.
More thought experiments
Wouldn’t it be great to experience no gravity?
To effortlessly float around in any direction you wish, like astronauts on board the International Space Station?
You wouldn’t need stairs anymore, or elevators, for that matter.
Then again, as every astronaut can tell you, drinking a cup of coffee would become quite a challenge.
But it’s an interesting thought experiment.
What would the world, and indeed the Universe, look like if there were no gravity, if we could shut it off completely, or if it had never existed at all?
Earth without gravity
For one, life on our planet would have been completely different. In a world with no gravity, there’s no need for the evolution of wings.
Legs aren’t really necessary either. And animals could be much larger than blue whales or dinosaurs, as weight no longer puts constraints on body size.
But wait a moment. Life would have a very difficult time existing in the first place, as Earth would have no atmosphere.
Only through gravity is our planet capable of retaining that thin layer of breathable gases.
Without gravity, the atmosphere – and the oceans, for that matter – would escape into space, and the third rock from the Sun would be as barren as low-mass worlds like the Moon and Mercury.
And there’s another disastrous consequence of shutting off gravity.
It’s gravitational attraction that keeps Earth orbiting the Sun, completing one 940-million-km (584-million-mile) loop in one year.
If someone were to suddenly flip the G-switch, Earth would fly off into space along a straight line, with its instantaneous velocity of 29.8 km/s (66,660mph). Not a good prospect.
There probably are planets wandering through empty space, having been kicked out of their planetary systems one way or the other, but without the cherishing light and heat of a nearby Sun, it’s hard to see how such rogue worlds could sustain life.
Planets and stars without gravity
In fact, the whole concept of orbits would cease to exist if there were no gravity.
No moons or ring systems orbiting planets. No planets orbiting stars.
No stars orbiting the supermassive black hole in the centre of our Milky Way Galaxy.
Worse: no stars to orbit around in galaxies at all. Each and every celestial body would be on its own, flying around haphazardly.
Come to think of it, the whole architecture of the Universe is shaped by gravity.
Every system and structure, from the smallest binary stars to the largest supercluster of galaxies, is held together by this single force of nature.
Which is quite remarkable, as gravity is by far the weakest of the four fundamental forces.
The reason the other forces are less relevant for the large-scale structure of the cosmos is that two of them – the strong and weak nuclear force – only act across subatomic distances.
While the third – electromagnetism – can both be attractive and repulsive, which means that it averages out over large distances.
Gravity, in contrast, is a long-range force that can only be attractive.
Sure enough, nuclear forces are important to keep atoms together.
Magnetic fields play an important role in the motions of charged particles, and without electromagnetism there would be no light.
But gravity, the feeblest of forces, is the great architect of the cosmos.
So would protons and neutrons still cling together to form atomic nuclei in a Universe with no gravity?
Yes – their behaviour is governed by the strong nuclear force. Would molecules – from simple carbon monoxide to complex DNA – still be able to exist?
Absolutely – they are held together by electromagnetic forces.
But would the Andromeda Galaxy or the Virgo Cluster remain intact without the force of gravity? No way.
But there’s more. Not only does gravity allow the existence of structure in the Universe at almost every conceivable scale, it also governs the formation of each and every part of that structure.
Every solid object in space larger than, say, a small pebble owes its existence to gravitational attraction.
Take the Moon, for example. According to the popular Big Splash theory, the Moon formed when fragments created from the collision between the newly born Earth and a small protoplanet clumped together.
But without gravity, nothing would ever clump together, and Earth wouldn’t have a Moon in its night sky.
Earth itself would also not exist, by the way.
The planets in our Solar System accreted from gas and dust particles in the protoplanetary disc orbiting the young Sun.
While molecular forces may create millimetre-sized structures, any larger clump of matter only forms through gravitational interactions.
And what about stars? Well, these huge balls of incandescent gas form out of tenuous nebulae that contract under their own gravity.
Without gravity, a cloud of hydrogen and helium gas would never collapse in on itself.
And without gravity, pressures and temperatures would never rise high enough to ignite nuclear fusion.
No nucleur fusion means no energy production, no light and no heat.
A Universe without gravity wouldn’t contain stars at all.
And without stars, the Universe would also lack red giants, planetary nebulae and white dwarfs (the terminal stages of low-mass stars like our own Sun), as well as supernovae, neutron stars and black holes (the end products of the evolution of massive stars).
Even galaxies, the most majestic objects in the cosmos, owe their existence to the world-building qualities of gravity.
What if there had never been gravity?
Minute density fluctuations in the primordial soup, just a few hundred million years after the Big Bang, formed the seeds for the formation of the first galaxies and the first supermassive black holes in their cores – another example of gravitational collapse.
If gravity had never switched on, there would be no Whirlpool, no Pinwheel and no Sombrero.
It’s a thought-provoking insight: if there were no gravity, our Universe would be cold and dark, without form or structure.
And, more importantly, it would be without much chemical variety, since the fusion reactions in the first three minutes after the Big Bang led to a Universe that, early in its history, consisted almost exclusively of hydrogen and helium, the two simplest and lightest elements.
All heavier atoms in the world around you, including the carbon in your muscles, the calcium in your bones and the iron in your blood, are produced in stellar interiors, in supernova explosions or in the catastrophic collisions of neutron stars.
No gravity effectively means no chemistry, let alone life.
Universe without gravity
And what about the Universe itself?
Nobody knows how the Universe was born. We don’t know precisely what role gravity played in the creation of the cosmos, if any.
What we do know is that the evolution of the Universe at large is a tug-of-war between the expansion of empty space (which is actually accelerating and has been for about five billion years or so) and the mutual gravitational attraction of all cosmic matter.
Without gravity, nothing would put the brakes on the expansion of the Universe, and it would live forever, growing colder, darker and emptier over time.
Then again, our real Universe, with gravity, probably awaits a more or less similar fate. So in the very end, there may not be that much difference.
The funny thing is: a Universe without gravity is actually impossible.
As a thought experiment, it’s an interesting concept to consider, but according to Albert Einstein’s general theory of relativity, gravity is an inevitable property of our world.
That’s because there is an intimate connection between space, time and gravity, the three of which are all intertwined through the constancy of the speed of light.
Just as well. Yes, it would be wonderful to be able to fly.
And yes, a world in which no one ever breaks a leg by falling down the stairs sounds great.
But if the feeblest of the forces of nature didn’t exist, our Universe would be no fun at all, and we wouldn’t be here to wonder about its absence.
That’s the gravity of the matter, isn’t it?
Gravity's effect on time
Gravity is a weird force. It not only acts on mass – every single particle that has mass exerts a tiny amount of gravitational attraction on every other particle – but also on space and time.
In fact, according to Albert Einstein’s general theory of relativity, gravity is nothing else than the slight curvature and deformation of four-dimensional spacetime, as a result of the presence of mass or energy.
One consequence of the theory (which has been confirmed over and over again) is that gravity influences the flow of time.
In a strong gravitational field, time slows down, at least for an outside observer. At the edge of a black hole – its event horizon – time even comes to a complete standstill.
So if you want to live longer, you’d better buy an apartment at the top level of a 30-storey building, or build your house in the Himalayas, where the gravitational field of Earth is just a tiny bit weaker than at sea level.
The differences are extremely small, of course, but atomic clocks on board GPS satellites need to be corrected for these effects, lest your navigation software sends you into a ditch.
This article appeared in the February 2025 issue of BBC Sky at Night Magazine
