Extract from ABC News
There's a problem with gravity.
It's a problem that Albert Einstein identified more than 100 years ago.
It's a problem we could be on the cusp of solving, or that could take another century to untangle.
And the problem is that we don't fully understand how gravity works.
"People have been hitting this scientific problem with sledgehammers for a long time," says astrophysicist Geraint Lewis from the University of Sydney.
"And it just refuses to yield."
We know what gravity does. It keeps us on our feet. It keeps the Earth orbiting around the Sun.
And thanks to Einstein's theory of gravity, we know enough to map the path of orbiting planets and colliding galaxies.
His theory of general relativity has been unsurpassable since it was published in 1915.
"But there are places where relativity breaks down," Professor Lewis says.
The equations stop giving us the answers we need.
So for more than 100 years, physicists have been searching for a new theory to explain what Einstein could not.
It's both a serious quest to advance fundamental physics ... and a not-so-serious wager on the nature of space-time.
Einstein's incomplete theory of gravity
The theory of general relativity tells us that gravity is the curvature of space-time.
Massive objects, like the Sun, warp the space and time around them. This causes objects like the Earth to move in a particular orbit.
This was a departure from Isaac Newton's view of gravity, which dominated physics for 200 years.
Newton saw gravity as a force separate from space and time — while Einstein saw gravity as a product of space-time.
For mathematician Robyn Arianrhod, an affiliate of Monash University, general relativity is "one of the most extraordinary creations of the human mind".
The theory's predictions keep coming true — even when Einstein himself doubted they would.
From the first observations of light bent by the Sun during a solar eclipse in 1919, to the detection of tiny ripples in the fabric of space-time known as gravitational waves in 2015.
"I mean, how powerful is that?" Dr Arianrhod says. "And the equations are a handful of symbols long."
General relativity has been heralded for its beauty and simplicity. But even Einstein knew his theory could not explain everything.
"That's why he was always searching for the underlying principles that would incorporate everything together," Dr Arianrhod says.
But try as he might, Einstein couldn't find the answer.
As he wrote in a letter in 1938:
"Most of my intellectual children, at a very young age, end up in the graveyard of disappointed hopes."
There are a few reasons we know general relativity isn't complete.
At the heart of black holes, and when we try to make sense of the start of the universe, the mathematics goes wonky.
"What it's telling us is that there's a point where relativity as we know it breaks down," Professor Lewis says.
"And whatever is going to replace it comes into play."
General relativity also breaks down at the very small scale.
This is where quantum theory takes over — an entirely different set of rules that tells us how subatomic particles, such as photons and electrons, behave.
But this means we have two sets of rules to explain the universe which are fundamentally incompatible.
A quantum conundrum
The simplest way to understand this chasm between general relativity and quantum mechanics, is to compare how both theories envision the world.
While general relativity sees space-time as continuous, quantum theory sees the universe as discrete parts that make up a bigger whole.
They can't both be true.
"Does gravity have to yield? Does quantum mechanics have to yield? Do they have to meet somewhere in the middle?" Professor Lewis asks.
It has long been assumed that gravity is the problem.
According to the Standard Model of Physics there are four fundamental forces, each of which are carried by different types of particles.
For the electromagnetic force, it's the photon. For the strong force it's the gluon. And bosons carry the weak force.
So what carries the gravitational force?
So far we haven't been able to find the discrete parts that make gravity possible, in part because it's so weak.
Gravity is weaker than even the weak force — and therefore incredibly difficult to experiment with.
Still, there are many theories with many passionate champions. After all, the prize for those who come out on top is hefty.
"The physicist that finds [the answer] knows they'll be on their way to Sweden to pick up a Nobel Prize," Professor Lewis says.
"If you crack this nut, that's the path that you're going down. Because you will change physics."
Understanding the true nature of gravity could have "staggering implications," says physicist Susan Scott of the Australian National University (ANU).
"It could tell us where the laws of nature come from, whether the cosmos is built on uncertainty or whether it's deterministic," Professor Scott says.
It could also give us new insight into black holes, and closure on the enigma that is the start of the universe.
But there's more than Nobel Prizes at stake here.
The theorist that comes out on top could not only book a trip to Sweden, but find themselves with a lifetime supply of potato chips.
More on that later ... first we need to meet the contestants ...
String theory
String theory is perhaps one of the most intimidating concepts out there.
But in the simplest terms possible, all it proposes is that particles are made from tiny, vibrating strings.
"So just like in music, where different combinations of strings produce different notes, tiny vibrating strings produce different particles," Professor Scott says.
That would include the proposed particle for gravity — the graviton.
String theory is not just a theory of quantum gravity. It also endeavours to be a theory of everything.
The number of dimensions required to make string theory work differs depending on the mathematical interpretation.
M-theory, for example, requires 11 dimensions.
"Which is seven more than we have in Einstein's theory of space-time," Professor Scott says.
Superstring theory requires 10 and bosonic string theory requires a whopping 26 dimensions.
"As yet, there's not a shred of evidence that these extra dimensions exist," Professor Scott says.
Loop quantum gravity
Loop quantum gravity theorises that space-time isn't continuous, instead it's made up of a network of tiny, interwoven loops.
Italian physicist Carlo Rovelli, one of the founders of the theory, says the idea is akin to a t-shirt: the fabric might seem continuous, but if you look closely you can see the threads.
"But the threads are not in space, because they are space themselves," Professor Rovelli says.
The potential to quantise space-time — not just gravity — is what sets quantum loop theory apart from string theory.
However, like all current theories of quantum gravity, this is difficult to test.
The loops in loop quantum gravity would be inconceivably small — about 0.000000000000000000000000000000000016 metres.
"This would be impossible to test in any particle accelerator on Earth," Professor Scott says. And so new experiments need to be devised.
Betting on space-time
The fact these theories have been around for decades and yet remain untested might be a bad omen — at least according to Jonathan Oppenheim, a physicist at University College London.
"It's possible the reason it's become so difficult is because we've gone off in the wrong direction," Professor Oppenheim says.
"Perhaps the idea of quantising gravity has been the wrong approach."
Professor Oppenheim is working on a controversial hybrid theory that does not quantise gravity, and also modifies some of Einstein's equations.
"I think probably 99 per cent of my colleagues thought I was a crackpot," he says.
"Most people think we should quantise gravity."
Carlo Rovelli is one of those people.
When he heard Professor Oppenheim speak at a conference in 2020 he was so riled up, he agreed to a wager on whose theory would come out on top.
They settled on one-to-5,000 odds, meaning if Professor Rovelli wins he gets a single item (like a packet of crisps) but if Professor Oppenheim wins he gets 5,000.
However the point of this bet isn't to earn a lifetime supply of crisps. It's to establish what it would take for the scientists to change their minds.
Finding proof
One of the ways physicists are trying to prove gravity is a quantum phenomenon, is by catching it in the act.
We know what quantum behaviour looks like. An example is entanglement, where two particles interact and then remain connected over vast distances.
If those particles became entangled, having only ever interacted gravitationally, it would imply that gravity is a quantum phenomenon.
Professor Oppenheim says emerging techniques in the lab bring new hope to the field.
"It has always been assumed that we had to go to incredibly high energies in order to test quantum gravity," he says. "Like we almost have to find black holes."
"What we've learned recently is that's not the case. There are low-energy experiments we can perform."
One example came out of the University of Southampton earlier this year, where scientists used magnets to detect the gravitational pull on a particle.
And here in Australia, where ANU scientists have proposed quantum gravity experiments using sensitive lasers.
Professor Scott says these tabletop techniques give us more opportunities to uncover gravity's secrets.
"This is probably the most exciting time to be involved in gravity research since Einstein presented his theory in 1915."
There isn't a guaranteed timeline, though. The answer has alluded us for 100 years, and could allude us for 100 more.
"It's possible that in five years, we're much more ahead," Dr Rovelli says. "As it is possible that in 20 years we're still confused."
Want to know where the next Einstein might come from? Listen to 'Einstein revolutionised physics, now the field is hunting for a vital shake-up of his theories' on RN's The Science Show.
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