Evrim Yazgin
Cosmos science journalist
For decades, physicists have tried to unify gravity with the other fundamental forces of nature. This “holy grail” of theoretical physics may finally be within reach thanks to new research.
There are 4 fundamental forces of nature: gravity, electromagnetism, the weak nuclear force (responsible for radioactive decay) and the strong nuclear force (responsible for holding the atomic nucleus together).
Physicists have had a good run of unifying forces over the centuries.
Electricity and magnetism were unified into electromagnetism by James Clerk Maxwell in the 19th century. Electromagnetism and the weak nuclear force were brought together in the electroweak interaction in the 1950s and ‘60s. Electromagnetism and the nuclear weak and strong forces are all unified in the standard model of particle physics.
Gravity itself is considered the outcome of the “first great unification” when in the 17th century Isaac Newton merged the phenomena of gravity on Earth with the movements of celestial bodies in space.
It stands to reason, many physicists argue, that all 4 fundamental forces should be unified in a “grand theory of everything” which will lead to a deeper understanding of the nature of the universe.
But gravity doesn’t play nice with the other 3 fundamental forces.
The problem with gravity
The weak nuclear force has “weak” in its name, but it is really gravity which is the weakling among the forces.
The nuclear strong force is the strongest – more than 100 times stronger than electromagnetism and a million times stronger than the nuclear weak force. Gravity is a hundred million trillion trillion trillion (1 with 41 zeroes after it) times weaker than the strong nuclear force.
Consider the Earth as a way to picture how weak gravity is.
The gravitational pull of an object is related to its mass. Earth is about 6 million billion billion kg. But we can very easily counteract its gravitational pull by picking something up. The muscles in your arm overcome the gravitational pull of the entire Earth every time you pick up your phone or a glass of water.
Why is gravity so weak? No one knows.
Meanwhile, gravity – best described by Einstein’s General Theory of Relativity – doesn’t gel with quantum mechanics and vice versa. This is most clear in regions of extreme gravity like black holes or in trying to understand the singularity that existed at the birth of the universe.
And all 3 other forces have “carrier particles” which transfer the force. Electromagnetism is transmitted by photons – light particles. The strong nuclear force is transferred by gluons and the weak nuclear force relies on W+, W– and Z bosons.
Some physicists predict there is a “graviton”, but no such force carrier particle for gravity has been discovered.
Quantum gravity on the horizon
The main research area into unifying gravity with the other forces is a proposed theory of “quantum gravity”.
A new paper published in Reports on Progress in Physics may have brought such a theory a step closer.
“If this turns out to lead to a complete quantum field theory of gravity, then eventually it will give answers to the very difficult problems of understanding singularities in black holes and the Big Bang,” says lead author Mikko Partanen from Aalto University in Finland.
A unified theory could also solve some of the universe’s greatest mysteries, according to Partanen.
“Some fundamental questions of physics still remain unanswered,” he says. “For example, the present theories do not yet explain why there is more matter than antimatter in the observable universe.”
Gauging interest
The force carrier particles for the non-gravitational forces all generate “fields” with which other particles interact. These fields are described through complex mathematics known as a gauge theory. If you can find a suitable gauge theory to describe gravity, you’re on your way to a quantum gravity theory and unification.
“The most familiar gauge field is the electromagnetic field,” says co-author Jukka Tulkki, also from Aalto University. “When electrically charged particles interact with each other, they interact through the electromagnetic field.”
Finding a gravitational gauge field has eluded physicists for decades.
“The main idea is to have a gravity gauge theory with a symmetry that is similar to the standard model symmetries, instead of basing the theory on the very different kind of spacetime symmetry of general relativity,” says Partanen.
Partanen and Tulkki discovered a new symmetry-based approach which has shown initial promise. But there is more mathematical work to be done to show that the symmetry doesn’t break down further along in the calculations. And, with the initial results from Partanen and Tulkki will help other physicists to test the maths.
“We still have to make a complete proof, but we believe it’s very likely we’ll succeed,” says Tulkki.
“I can’t say when, but I can say we’ll know much more about that in a few years,” says Partanen. “Like quantum mechanics and the theory of relativity before it, we hope our theory will open countless avenues for scientists to explore.”
