Scientists might be on the verge of cracking one of the biggest problems in physics, allowing them to finally create a grand Theory of Everything.
Currently, two separate theories are used to explain different aspects of the Universe around us – quantum mechanics and gravity. Though many have attempted to bring the two together into a single idea, no one has yet managed to create a convincing theory.
“Combining gravity and quantum theory into a unified framework is one of the central goals of modern theoretical physics,” Dr Mikko Partanen, lead author of a study published in Reports on Progress in Physics detailing a new approach to the problem, told BBC Science Focus. “One might say that it is the Holy Grail of physics.”
The difficulty in creating a theory of ‘quantum gravity’ is that the two ideas look at the Universe at completely different scales.
Quantum mechanics looks at the smallest scales of the particles that make up atoms. Physicists have used it to create the Standard Model, which gels together three of the fundamental forces that govern our Universe – electromagnetism, the strong force (which holds protons and neutrons together) and the weak force (responsible for radioactive decay).
The fourth fundamental force is gravity, as laid down by the laws of General Relativity written by Albert Einstein. This thinks of gravity as the warping of space-time – large masses and high-energy objects distort space-time as they move through it, and this distortion affects the objects around them. It governs the Universe on the scales of planets, stars and galaxies. And it stubbornly refuses to play nicely with the laws of quantum mechanics.
A tale of two theories
One of the biggest problems is that gravity is a ‘deterministic classical’ theory, meaning its laws state with certainty what the consequences of an action will be. Drop a ball, gravity means it will definitely fall to the ground.
Quantum theory, however, is probabilistic in nature. It doesn’t predict the precise outcome of a situation, only the likelihood of it happening.
“These are challenging to combine,” Partanen said. “Attempts to apply quantum theory in the presence of gravitational interaction have led to many nonsensible results.”
For instance, when quantum physicists try to measure an electron’s mass, their equations spin off into infinity. Conversely, when gravity is applied to extreme gravitational situations, such as at the edge of a black hole, Einstein’s equations stop making sense.
“Some appealing approaches to solving the problem, such as string theory [which replaces particles with vibrating strings of energy], do not currently make unique, testable predictions that could distinguish these theories from the Standard Model or General Relativity,” Partanen said.
Rather than devising an entirely new theory to connect the two, Partanen and his colleague Prof Jukka Tulkki looked at gravity in the same way they think about quantum mechanics. They did this by reformatting the equations of gravity so that they use fields.
Fields are how quantum theory describes how a physical quantity changes over space and time. You might already be familiar with electric and magnetic fields.
Using these new ideas, the pair were able to recreate the rules of General Relativity, only now in a format that is much easier to combine with quantum mechanics.
Testing the Theory of Everything
One of the most exciting parts of the theory is that it doesn’t introduce any strange new particles to discover or physical laws to work out. This means that physicists should already have the tools needed to test this new theory of quantum gravity.
“We’ve already succeeded in showing that – without introducing a single new parameter to be determined experimentally – our theory explains known classical effects of gravitational interaction,” Partanen said.
According to Partanen, the new theory creates equations which recreate phenomena such as the bending of light as it passes by a massive galaxy or redshift – the stretching of light to longer, redder wavelengths as objects move away from us in the expanding Universe.
But that just proves the theory is valid, not that it’s right.
To do that, experimenters need to look at those extreme gravitational situations where General Relativity stops being accurate.
If quantum gravity can better predict what’s seen under these circumstances, that would be the first step in confirming their new theory of quantum gravity and proving Einstein wrong (or at least showing he didn’t have the whole picture).
Practically, though, this will prove difficult as the difference between the two theories is incredibly small.
For instance, if they were to look at how much the mass of the Sun bends the light from a distant star, the difference the two theories predict would be different by 0.0001 per cent. Astronomers can’t currently make measurements that accurate.
Fortunately, there are more massive objects out there where the difference is much more noticeable.
“In the case of neutron stars, where the gravitational fields are stronger, the relative difference can become as large as a few per cent,” Partanen said. While there aren’t currently any observatories capable of making this observation, existing technology could build one.
The theory is still in its early stages. The team will now begin working up a complete mathematical proof, checking that the theory doesn’t start spouting infinities or other mathematical headaches.
If things still look promising at this point, the team will begin to apply their theory to extreme situations, such as black hole singularities.
“Since our theory represents a completely new kind of attempt to unify all four fundamental forces of nature within a single framework, its closer examination may reveal phenomena that we cannot yet even imagine,” Partanen said.
Read more
- Schrödinger’s cat just got warmer, and quantum physics may never be the same again
- This bizarre quantum theory could finally explain how our Universe began
- Dead and alive: why it's time to rethink quantum physics
About our expert
Mikko Partanen is a postdoctoral researcher at Aalto University in Espoo, Finland, in the Department of Physics and Nanoengineering, focusing on the study of light and its quantum properties. His research has been published in journals such as Annals of Physics, New Journal of Physics and Scientific Reports.
