Finding a theory of everything – one that explains all the forces and particles in the universe – is undoubtedly the holy grail of physics. While each of its leading theories works extraordinarily well, they also clash with each other, leaving physicists searching for a deeper, more fundamental theory.
But do we really need a theory of everything? And are we close to reaching one? This is what we discuss in the sixth and final episode of our Great Mysteries of Physics podcast, hosted by me, Miriam Frankel, Science Editor of The Conversation and supported by FQxI, the Foundational Questions Institute.
Our two best theories of nature are quantum mechanics and general relativity, which describe the smallest and largest scales of the universe, respectively. Each one is hugely successful and has been tested experimentally over and over again. The problem is that they are incompatible with each other in many ways, including mathematically.
“General relativity is about geometry. This is how space is curved and how space-time – this unified entity that contains three dimensions of space and one dimension of time – is also curved,” explains Vlatko Vedral, professor of physics at the University of Oxford in the UK United. “Quantum physics is actually all about algebra.”
Physicists have already managed to combine quantum theory with Einstein’s other great theory: special relativity (which explains how speed affects mass, time and space). Together, these form a framework called “quantum field theory,” which is the basis for the Standard Model of Particle Physics, our best framework for describing the most basic building blocks of the universe.
The Standard Model describes three of the four fundamental forces of the universe – electromagnetism and the “strong” and “weak” forces that govern the atomic nucleus – excluding gravity.
While the Standard Model explains most of what we see in particle physics experiments, there are some gaps. To bridge these, an extension called “supersymmetry” has been proposed, suggesting that particles are connected through a deep relationship. Supersymmetry suggests that every particle has a “super partner” with the same mass but opposite spin. Unfortunately, particle accelerators like the Large Hadron Collider (LHC) at Cern in Switzerland have failed to find evidence of supersymmetry, despite being explicitly designed to do so.
On the other hand, there are recent hints from both the LHC and Fermilab in the US that suggest there may be a fifth force of nature. If these results could be replicated and confirmed as actual discoveries, this would have implications for uniting quantum mechanics and gravity.
“I think [the discovery of a new force] that would be great,” says Vedral. “It would challenge this thing that has been around for over half a century now that there are four fundamental forces.”
Vedral argues that the first thing to do if we discover a fifth force would be to establish whether it can be described by quantum mechanics.
If it could, it would indicate that quantum theory may ultimately be more fundamental than general relativity, accounting for four out of five forces, suggesting that general relativity may eventually need to be modified. If not, this would upset physics, suggesting that we may need to change quantum mechanics as well.
And the other mysterious properties?
But what should a theory of everything include? Would it be enough to combine gravity and quantum mechanics? And what about other mysterious properties like dark energy, which causes the universe to expand at an accelerated rate, or dark matter, an invisible substance that makes up most of the matter in the universe?
As Chanda Prescod-Weinstein, assistant professor of physics and astronomy at the University of New Hampshire in the United States explains, physicists prefer to use the term “quantum theory of gravity” rather than “theory of everything”.
“Dark matter and dark energy make up most of the energy content of matter in the universe. So it’s not really a theory of everything if it doesn’t account for most of the energy content of matter in the universe,” he argues. “This is why I’m glad we don’t actually use ‘theory of everything’ in our work.”
While they may not explain everything, there are several proposed theories of quantum gravity. One is string theory, which suggests that the universe is ultimately made up of tiny vibrating strings. Another is loop quantum gravity, which suggests that Einstein’s spacetime arises from quantum effects.
“One of the strengths that people will point to with string theory is that string theory is based on quantum field theory,” Prescod-Weinstein explains. “It carries with it the entire Standard Model, whose loop quantum gravity doesn’t work the same way.” But string theory also has its weaknesses, she argues, such as requiring extra dimensions for which we’ve never seen any evidence.
Theories are difficult to test experimentally, requiring much more energy than we can produce in any laboratory. Vedral argues that while we ultimately can’t directly probe the tiny scales needed to find evidence for theories of quantum gravity, it may be possible to amplify those effects so that we can indirectly observe them at larger scales with tabletop experiments.
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This article is republished from The Conversation under a Creative Commons license. Read the original article.
Vlatko Vedral has received funding from the Templeton and Moore Foundations. Chanda Prescod-Weinstein has received funding from NSF, DoE, NASA, FQxI and the Heising-Simons Foundation. She is a member of the American Physical Society, American Astronomical Society, FQxI, NASEM Elementary Particle Physics: Progress and Promise Committee