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This article comes from the official account of Wechat: ID:fanpu2019, author: Whiteny Clavin, translator: Zhang Yi

Is time and space pixel-like? Just like looking at a digital picture, when it is magnified many times, we can see a piece of pixels. If spacetime is quantized on a very small scale-a sea of bubbly particles, gravity and other particles emerge from it, we may be able to find ripples on the sea surface-observable features of quantum gravity. This is a scheme put forward by theoretical physicists to solve quantum gravity, and is ready to be tested by experiments.

Progress has been made in the study of the characteristics of quantum gravity the sand dunes seen from a distance look smooth and wrinkle-free, like silk sheets in the desert. But the closer you look, the more details you reveal. As you approach the dunes, you may notice ripples in the sand; when you touch the surface, you will find grains of sand. The same is true of digital images: a seemingly perfect portrait is large enough to find countless different pixels that make up the image.

The universe itself may also be pixel-like. Rana Adhikari, a physics professor at the California Institute of Technology, and others believe that our space may not be completely smooth, but made up of very small discrete units. "Space-time pixels are so small that if you enlarge it to the size of a grain of sand, the atom will be as big as a galaxy." He said.

Adhikari and physicists around the world are looking for this pixel because it is a prophecy of quantum gravity, one of the most esoteric physics puzzles of our time. Quantum gravity refers to a set of theories, including string theory, which aims to unify the macro world of gravity controlled by general relativity with the micro world of quantum physics. At the heart of this puzzle is whether gravity and its space-time can be "quantized" or broken down into individual components, which is the hallmark of the quantum world.

"sometimes there is a misunderstanding in science communication that quantum mechanics and gravity are irreconcilable." "but experiments show that we can do quantum mechanics on this gravitational planet, and it's clear that they are consistent," said Clifford Cheung, a professor of theoretical physics at the California Institute of Technology. "the trouble is the subtle questions about black holes, or trying to unify them on a very short distance scale."

"Gravity is a hologram."

-Monica Kang (Monica Jinwoo Kang)

Because of the extremely small scale involved, some physicists believe that finding evidence of quantum gravity in the foreseeable future is an impossible task. Although researchers have come up with various ideas about how to find clues to its existence-around black holes; in the early universe; and even using LIGO, the National Science Foundation-funded observatory for detecting gravitational waves-no one has found any traces of quantum gravity in nature.

Katherine Zurick (Kathryn Zurek), a professor of theoretical physics, wants to change that. She recently set up a new multi-agency team funded by the Helsing-Simmons (Heising-Simons) Foundation to study how to observe the signals of quantum gravity. The project, called Quantum Gravity and its observed signals (Quantum Gravity and Its Observational Signatures,QuRIOS), brings together string theorists and particle theorists who are familiar with the formal tools of quantum gravity but have little experience in experimental design, as well as model builders who are good at experiments but do not work on quantum gravity.

"the idea that it is possible to find observable features of quantum gravity is a far cry from the mainstream." "but if we don't start to focus on ways to connect quantum gravity with the nature in which we live, we will be lost in the desert," she said. thinking from the perspective of observational characteristics can connect us theorists and help us make progress on new issues. "

Lana Adhikari (left) and Catherine Zurick (right). Photo Source: Lance Hayashida / Caltech as part of the collaboration, Zuric will work with experimenter Adhikari to design a new experiment using desktop instruments. The experiment, called Gravity from Quantum entanglement of Space-time (Gravity from Quantum Entanglement of Space-Time, GQuEST), will be able to detect connections not between individual space-time pixels themselves, but between pixels that produce observable features. Adhikari likens the search to tuning old televisions.

"when I was growing up, when we couldn't get NBC, we would try to adjust the channel. But most of the time, we only see snowflakes in pixels. We know that some of those snowflakes come from the cosmic microwave background (cosmic microwave background), or when the universe was born, but if you just hit the peak, you can see the snowflakes of solar storms and other signals. This is what we are trying to do: listen carefully to the snowflakes or the fluctuations of time and space. We will study whether the ups and downs of snowflakes are consistent with our quantum gravity model. This idea may be wrong, but we must try. "

A new blueprint for the universe the solution to the quantum gravity puzzle will be one of physics's greatest achievements, comparable to the two theories that researchers want to unify. Einstein's general theory of relativity reshaped our view of the universe, showing that space and time can be thought of as a continuum-spacetime, which bends in response to matter. According to general relativity, gravity is nothing but the curvature of space-time.

The second theory, quantum mechanics, describes three known forces in the universe besides gravity: electromagnetic force, weak nuclear force and strong nuclear force. A typical feature of quantum mechanics is that these forces can be quantized as discrete wave packets or particles. For example, the quantization of electromagnetic forces leads to a particle called photon, which forms light. Photons work behind the scenes on a microscopic scale to transmit electromagnetic forces. Although the electromagnetic field looks continuous on a large scale that we are used to, when you zoom in, it becomes "bumpy" by photons. So the core question of quantum gravity is: will space-time also become a sea of bubbly particles on the smallest scale, or will it be as smooth as a mirror like a stagnant lake? Physicists generally believe that gravity should also be bumpy on the smallest scale; these stumbles are hypothetical particles called graviton. But when physicists use mathematical tools to describe how gravity is generated from gravitons on a very small scale, it falls apart.

"it has become mathematically impossible, producing absurd answers such as infinity rather than the finite number it should be. that means something must be wrong." "people don't quite understand how difficult it is to establish a consistent theoretical framework to unify general relativity and quantum mechanics," said Hirosi Ooguri, a Fred Caffrey professor of theoretical physics and mathematics and director of the Walter Burke Institute for theoretical Physics. "it seems impossible, but we have string theory."

Oli Bosi (Hiroshi Ooguri) (Photo Source: Brandon Hook / Caltech string at the bottom

Many physicists would agree that string theory is by far the most complete and probable theory of quantum gravity. It describes a 10-dimensional universe in which six dimensions are curled up and the other four make up space and time. As its name suggests, the theory assumes that at the most basic level, all matter in the universe is made up of tiny strings. Like a violin, the strings resonate at different frequencies or notes, and each note corresponds to a unique particle, such as an electron or photon. One of the notes corresponds to the graviton.

John Schwarz, professor emeritus of theoretical physics at Harold Brown, was one of the first to realize the power of string theory to bridge the gap between the quantum world and gravity. In the 1970s, he and his colleague Jo ë l Scherk used the mathematical tools of string theory to describe strength. However, they realize that if they change direction, the weakness of the theory may become an advantage.

"instead of insisting on building a strong theory, we accepted this beautiful theory and asked ourselves what the benefits were." "it turns out to be very good for gravity," Schwartz said in an interview in 2018. Neither of us has studied gravity. It's not something we're particularly interested in, but we find it difficult to describe force and cause gravity. Once I realize that, I know what I'm going to do for the rest of my career. "

It turns out that gravity is a strange thing compared with other forces. "Gravity is the weakest force we know." "standing on the fourth floor of the Lauritsen building, gravity didn't pull me across the floor because I was supported by electrons and nuclei inside the concrete, so the electric field prevailed over gravity," Okuri explained.

However, although the force weakens at shorter and shorter distances, gravity becomes stronger and stronger. "these strings help soften this high-energy behavior," Ohli said. "the energy is scattered in one string."

The challenge of string theory is not only to make it consistent with our daily low-energy world, but also how to test it. In order to see what happens on a tiny spatio-temporal scale, when it is theoretically granular, the experiment goes deep into the so-called Planck length, an order of magnitude of 10-35 meters. In order to reach such an extreme scale, scientists must build an equally extreme detector. "one way is to make something the size of the solar system to find the characteristics of quantum gravity," Adhikari said. but it's too expensive, and it takes centuries! " Instead, Zurick says, researchers can use much smaller experiments to study all aspects of quantum gravity. "for our proposed low-energy experiment, we don't need the whole mechanism of string theory." "the theoretical development related to string theory provides us with some tools and quantitative understanding that is expected to be true in quantum gravity," she said. "

The experimental scheme proposed by Zuric, Adhikari and their colleagues focuses on the effects of quantum gravity, which can be observed on a more manageable scale such as 10-18 meters. This is still very small, but it is potentially feasible to use extremely accurate laboratory instruments.

"Spatio-temporal pixels are so small that if you zoom in to the size of a grain of sand, the atom will be as big as a galaxy."

Lana Adhikari (Rana Adhikari)

These desktop experiments are like mini LIGO: an L-shaped interferometer that fires two laser beams in a vertical direction. The two lasers are reflected from the mirror and meet at the starting point. In the case of LIGO, gravitational waves stretch and compress space, affecting the time at which lasers meet. The quantum gravity experiment will look for a different type of space-time fluctuation, which consists of gravitons, which suddenly appear and disappear in what some people call quantum or spacetime bubbles (because of quantum fluctuations, photons and other quantum particles also suddenly appear and disappear). Instead of looking for individual gravitons, the researchers searched for observable features caused by "long-range correlations" (long-range correlations) between complex sets of hypothetical particles. Zuric explains that these long-range connections are like the larger ripples in the ocean of space-time, rather than the porous bubbles in which individual particles reside.

"We think that the existence of spatio-temporal fluctuations may disturb the [laser] beam." "We want to design a device in which spatio-temporal fluctuations kick a photon out of the interferometer beam, and then we use a single-photon detector to read the spatio-temporal disturbance," she said. "

Emerging space-time "Gravity is a hologram (hologram)." Monica Jinwoo Kang, a postdoctoral fellow in theoretical physics at the California Institute of Technology (Sherman Fairchild), explained the holographic principle (holographic principle). Holographic principle is a key principle of Zuric model. This principle was realized in the 1990s using string theory, which means that three-dimensional phenomena such as gravity can emerge from a flat two-dimensional surface (emerge). Kang explained, "the holographic principle means that all the information in the volume of something is encoded on the surface."

More specifically, gravity and spacetime emerge from particle entanglement (entanglement) on a two-dimensional surface. Subatomic particles are entangled across space; particles, as a single entity, have no direct contact with each other, a bit like a flock of starlings. "the modern view of quantum gravity inspired by string theory shows that space-time and gravity are born out of a network of quantum entanglement (networks of entanglement). Along this line of thinking, space-time itself is defined by the degree of entanglement of something." Kang said.

"if we don't start to pay attention to ways to connect quantum gravity with the nature in which we live, we will be lost in the desert." -Catherine Zurick.

In the experiment proposed by Zurick and Adhikari, their idea was to detect the two-dimensional surface, or what they call the "quantum event horizon" (quantum horizon), to find the fluctuations of the graviton. Gravity and spacetime emerge from the quantum horizon, they explained. "our experiment will measure the fuzziness of this surface." Zuric said.

This fuzziness will represent the pixelation of spacetime of time and space. If the experiment is successful, it will help to redefine our concepts of gravity and space at the most basic and deepest level.

"if I release the coffee cup, it will fall. I think that's gravity." Adhikari said, "but just as temperature is not 'real' but describes how a bunch of molecules vibrate, space-time may not be a real thing. We see flocks of birds and fish moving in unison, but they are actually made up of individual animals. We call group behavior emerging. Similarly, something that emerges from the pigmentalization of space-time happens to be given the name gravity, because we do not yet understand what the core of space-time is. "

This article is translated from Quantum Gravity: The Quest for the Pixelation of Space-Caltech Magazine

Original link: https://magazine.caltech.edu/ post / quantum-gravity

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