- Physicists say that understanding gravity requires a quantum mechanical explanation.
- But no direct evidence of hypothetical quantum gravity particles, called gravitons, exists.
- Experimenters hope to find the effects of gravitons within ten years.
As far as we know, our physical world is governed by four fundamental forces: electromagnetism, weak and strong nuclear forces, and gravity. Apart from playing with bar magnets or marveling at the light of a rainbow, it’s gravity that we’re most familiar with here on Earth. Yet, it’s actually the least understood force of the bunch.
Our understanding of gravity has undergone a number of face lifts in the past several hundred years—from Newton’s take on the movements of planets and apples to Einstein’s theory of general relativity and spacetime. However, for physicists like Kathryn Zurek, a theoretical physics professor at Caltech whose work focuses on dark matter as well as observational signals of quantum gravity, that still isn’t good enough.
She’s not the only one. Theorists and experimentalists around the world have toiled for decades to compose a so-called “theory of everything” that would unite quantum explanations of the very small with the classical physics of the very large (such as humans and planets). A verifiable theory of quantum gravity is at the center of this quest to offer a single theory that explains everything in our universe.
“For many reasons, we believe that the fundamental understanding of gravity needs to be quantum mechanical in nature,” Zurek tells Popular Mechanics. “So we need to figure out how to make these foundational principles of quantum mechanics [work for] gravity. That is quantum gravity—it’s classical gravity, which has been quantum mechanically proved.”
Zurek is part of a joint Caltech and Fermilab team that is currently developing a new type of experiment called Gravity from Quantum Entanglement of Space-Time (GQuEST) that will look for gravity-like fluctuations by looking for observable effects on photons.
What Is Quantum Gravity?
Scientists are fairly confident that a quantum explanation of gravity should exist, but finding a theory to support this belief—let alone proof that it’s correct—has been much more difficult, Zurek says.
In the standard model of particle physics, a model that explains all fundamental forces except gravity, forces are carried by specialized particles. For example, the electromagnetic force is ferried by photons, which can be experienced as light. Following this logic, physicists have proposed that gravity should have its own particle as well, which physicists have dubbed the “graviton.” However, trying to incorporate a graviton into the picture with existing math has led scientists into a tangle of impossible math, such as equations ending in infinities.
Physicists are mulling over a number of theories to solve this problem, but Zurek says that string theory remains the best description to date.
Physicists originally proposed string theory in the late 1960s, and it can come in many different flavors. The general idea is that that the universe is made up of ten (or sometimes more) dimensions—only four of which make up space and time as we know it. The remaining dimensions are a type of unseen scaffolding. In this multi-dimensional model, very small objects, called “strings, replace particles.” These strings resonate like plucked guitar strings at different frequencies, in accordance with different fundamental particles. Scientists theorize that one such frequency should map to the theoretical graviton.
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One of the more mind-boggling conclusions we can draw from string theory is that gravity might not strictly even be “real.”
That is, gravity—and even spacetime—may just be emergent properties created by the quantum entanglement of particles. Netta Engelhardt, a theoretical physicist at the Massachusetts Institute of Technology, tells Space.com that this phenomenon is similar to the feeling of heat actually being just our bodies’ experience of the speed of air molecules around us.
For now, this is all purely theoretical. Even though string theory has proven itself in a number of ways, including providing an embedded and elegant description of gravity, there are still many questions it doesn’t answer, says Zurek. For example, string theory can’t yet incorporate an existing understanding of the standard model.
“It’s believed that when we understand string theory well enough… that we’ll understand how to add the matter of the standard model into that theoretical structure of quantum gravity, but it’s not known how to do that [yet].”
Finding Physical Evidence of Quantum Gravity
Zurek’s work isn’t looking to confirm or refute string theory, but it is looking for ways to bring the search for quantum gravity into the physical world. The basic design of GQuEST is a table-top version of the Laser Interferometer Gravitational-wave Observatory (LIGO)’s gravity wave detector.
Using incredibly precise measurements, the researchers will look for small fluctuations in the path of photons as they pass between mirrors. These perturbations may be the effect of gravitons. Researchers hope to observe these kinds of effects within the next five to ten years.
“We think that we may be able to see for the first time the quantum nature of gravity in this type of experiment with this type of measurement,” Zurek says. “From that perspective, it’ll be a major step forward in our understanding of how quantum mechanics and gravity come together.”
Sarah is a science and technology journalist based in Boston interested in how innovation and research intersect with our daily lives. She has written for a number of national publications and covers innovation news at Inverse.
