- Physicists say understanding gravity requires a quantum mechanical explanation.
- However, there is no direct evidence for hypothetical quantum gravity particles called gravitons.
- Experimenters hope to discover the effects of gravitons within ten years.

To our knowledge, our physical world is governed by four fundamental forces: electromagnetism, weak and strong nuclear forces, and gravity. Besides playing with bar magnets or marveling at the light of a rainbow, gravity is what we know best here on earth. Yet, it’s actually the least understood strength of the group.

Our understanding of gravity has undergone a number of renovations over the past hundred years – from Newton’s interpretation of the motions of planets and apples to Einstein’s theory of general relativity and spacetime. But for physicists like __Catherine Zurek__a professor of theoretical physics at Caltech whose work focuses on dark matter as well as the observational signals of quantum gravity, that’s still not enough.

She’s not the only one. Theorists and experimentalists around the world have been working for decades to write a so-called “theory of everything” that would unify quantum explanations of the very small with classical physics of the very large (like humans and planets). A testable theory of quantum gravity is at the heart of this quest for a single theory that explains everything in our universe.

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“For many reasons, we believe that the fundamental understanding of gravity should be quantum mechanical in nature,” says Zurek *Popular mechanics*. “So we have to figure out how to make these basic principles of quantum mechanics [work for] heaviness. It’s quantum gravity – classical gravity proven by quantum mechanics.

Zurek is part of a Caltech and Fermilab joint __crew__ who is currently developing a new kind 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 pretty confident that a quantum explanation of gravity *should* exist, but finding a theory to support that belief — let alone prove it to be correct — has been much more difficult, Zurek says.

In the standard model of particle physics, a model that explains all fundamental forces except gravity, the forces are carried by specialized particles. For example, the electromagnetic force is transmitted by photons, which can be felt as light. Following this logic, physicists suggested that gravity should also have its own particle, which physicists dubbed the “graviton.” However, trying to fit a graviton into the picture using existing math has led scientists into a tangle of impossible math, such as B. Equations ending in infinity.

Physicists are considering a number of theories to solve this problem, but Zurek says string theory remains the best description so far.

Physicists originally proposed string theory in the late 1960s, and it can take on many different flavors. The general idea is that the universe is made up of ten dimensions (or sometimes more) – only four of which make up space and time as we know them. The remaining dimensions are a kind of invisible frame. In this multidimensional model, very small objects called “strings” replace the particles. These strings, like plucked guitar strings, vibrate at different frequencies based on different fundamental particles. Scientists suggest that such a frequency should be attributed to the theoretical graviton.

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One of the most startling conclusions we can draw from string theory is that gravity may not even be strictly “real.”

In other words, gravity – and even spacetime – can only be emergent properties produced by the quantum entanglement of particles. __Netta Engelhardt__theoretical physicist at the Massachusetts Institute of Technology, __said __* Espace.com* that this phenomenon is similar to the sensation of heat, which is in fact only the experience of our body of the speed of the air molecules which surround us.

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All this is purely theoretical for the moment. Although string theory has proven itself in many ways, including providing an integrated and elegant description of gravity, there are still many questions it doesn’t answer, Zurek says. For example, string theory cannot yet incorporate an existing understanding of the Standard Model.

“It is believed that if we understand string theory well enough, we will understand how to integrate Standard Model matter into this theoretical structure of quantum gravity, but it is unclear how to do this. [yet].”

**Find physical evidence for quantum gravity**

Xuanyu Han//Getty Images

Zurek’s work does not attempt to confirm or refute string theory, but it does *East* looking for ways to bring the quest for quantum gravity into the physical world. The basic design of GQuEST is a desktop version of the Laser Interferometer Gravitational-wave Observatory (LIGO) gravitational wave detector.

With incredibly precise measurements, the researchers look for small fluctuations in the path of photons as they pass between mirrors. These disturbances can be the effect of gravitons. Researchers hope to observe such effects within the next five to ten years.

“We believe that with this type of measurement, we may be able to see the quantum nature of gravity in this type of experiment for the first time,” Zurek says. “From this perspective, it will be a big step forward in our understanding of how quantum mechanics and gravity come together.”

Sarah is a Boston-based science and technology writer 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 *the opposite*.

Source: www.popularmechanics.com

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