Is Space Pixelated? The Quest for Quantum Gravity

The search for signatures of quantum gravity forges ahead.

Sand dunes seen from afar seem smooth and unwrinkled, like silk sheets spread across the desert. But a closer inspection reveals much more. As you approach the dunes, you may notice ripples in the sand. Touch the surface and you would find individual grains. The same is true for digital images: zoom far enough into an apparently perfect portrait and you will discover the distinct pixels that make the picture.

The universe itself may be similarly pixelated. Scientists such as Rana Adhikari, professor of physics at Caltech, think the space we live in may not be perfectly smooth but rather made of incredibly small discrete units. “A spacetime pixel is so small that if you were to enlarge things so that it becomes the size of a grain of sand, then atoms would be as large as galaxies,” he says.

“Gravity is a hologram.” — Monica Jinwoo Kang

Adhikari and scientists around the world are on the hunt for this pixelation because it is a prediction of quantum gravity, one of the deepest physics mysteries of our time. Quantum gravity refers to a set of theories, including string theory, that seeks to unify the macroscopic world of gravity, governed by general relativity, with the microscopic world of quantum physics. At the core of the mystery is the question of whether gravity, and the spacetime it inhabits, can be “quantized,” or broken down into individual components, a hallmark of the quantum world.

“Sometimes there is a misinterpretation in science communication that implies quantum mechanics and gravity are irreconcilable,” says Cliff Cheung, Caltech professor of theoretical physics. “But we know from experiments that we can do quantum mechanics on this planet, which has gravity, so clearly they are consistent. The problems come up when you ask subtle questions about black holes or try to merge the theories at very short distance scales.”

Because of the incredibly small scales in question, some scientists have deemed finding evidence of quantum gravity in the foreseeable future to be an impossible task. Although researchers have come up with ideas for how they might find clues to its existence—around black holes; in the early universe; or even using LIGO, the National Science Foundation-funded observatories that detect gravitational waves—no one has yet turned up any hints of quantum gravity in nature.

Professor of Theoretical Physics Kathryn Zurek would like to change that. She recently formed a new multi-institutional collaboration, funded by the Heising-Simons Foundation, to think about how to observe signatures of quantum gravity. The project, called Quantum gRavity and Its Observational Signatures (QuRIOS), unites string theorists, who are familiar with the formal tools of quantum gravity but have little practice designing experiments, with particle theorists and model-builders who are experienced with experiments but not working with quantum gravity.

“The idea that you might be able to look for observable features of quantum gravity is very far from the mainstream,” she says. “But we’ll be lost in the desert if we don’t start focusing on ways to link quantum gravity with the natural world that we live in. Having observational signatures to think about tethers us theorists together and helps us make progress on new kinds of questions.”

Rana Adhikari, left, and Kathryn Zurek, right. Credit: Lance Hayashida/Caltech

As part of Zurek’s collaboration, she will work with Adhikari, an experimentalist, to develop a new experiment that uses tabletop instruments. The proposed experiment, called Gravity from Quantum Entanglement of Space-Time (GQuEST), will be able to detect not individual spacetime pixels themselves, but rather connections between the pixels that give rise to observable signatures. Adhikari compares the search to tuning old television sets.

“When I was growing up, we could not get NBC, and we would try to tune around to get it. But most of the time, we would see the pixelated snow. Some of that snow we know is coming from the cosmic microwave background, or the birth of the universe, but if you tuned just off the peak of that, you could find snow from solar storms and other signals. That’s what we are trying to do: to carefully tune in to the snow, or fluctuations of spacetime. We will be looking to see if the snow fluctuates in ways that align with our models of quantum gravity. Our idea could be bogus, but we have to try.”

A new blueprint for the universe

Cracking the problem of quantum gravity would be one of the greatest achievements of physics, on par with the two theories that researchers want to merge. Albert Einstein’s general theory of relativity reshaped the view of the universe, showing that space and time can be thought of as one continuous unit, spacetime, which curves in response to matter. Gravity, the theory explains, is nothing more than the curvature of spacetime.

The second theory, quantum mechanics, describes the three other known forces in the universe aside from gravity: electromagnetism, the weak nuclear force, and the strong nuclear force. A defining feature of quantum mechanics is that these forces can be quantized down to discrete packets, or particles. For example, the quantization of the electromagnetic force results in a particle known as the photon, which makes up light. The photon works behind the scenes at microscopic scales to transmit the force of electromagnetism. Though the electromagnetic field appears continuous at the large scales we are used to, it becomes “bumpy” with photons when you zoom in. The central question of quantum gravity, then, is this: does spacetime also become a frothy sea of particles at the smallest scales, or does it remain smooth like the surface of an unbroken lake? Scientists generally believe that gravity should be bumpy at the smallest scales; the bumps are hypothetical particles called gravitons. But when physicists use mathematical tools to describe how gravity might arise from gravitons at very tiny scales, things break down.

“The math become impossible and produces absurd answers such as infinity where we should get finite numbers as answers. It implies something is amiss,” says Hirosi Ooguri, the Fred Kavli Professor of Theoretical Physics and Mathematics and director of the Walter Burke Institute for Theoretical Physics. “It is not well appreciated how hard it is to build a consistent theoretical framework, to unify general relativity and quantum mechanics. “It would seem to be impossible, but then we have string theory.”

Strings at the bottom

Many scientists would agree that string theory is the most complete and probable theory of quantum gravity to date. It describes a universe with 10 dimensions, six of which are squirreled away unseen while the remaining four make up space and time. True to its name, the theory postulates that all matter in the universe is, at the most fundamental level, made of teeny strings. Like a violin, the strings resonate at different frequencies or notes, with each note corresponding to a unique particle such as an electron or photon. One of these notes is thought to correspond to the graviton.

John Schwarz, the Harold Brown Professor of Theoretical Physics, Emeritus, was one of the first people to realize the power of string theory to bridge the gap between the quantum world and gravity. In the 1970s, he and his colleague Joël Scherk struggled to use the mathematical tools of string theory to describe the strong nuclear force. However, they realized the theory’s disadvantages could be turned into advantages if they changed course.

Hiroshi Ooguri. Credit: Brandon Hook/Caltech

“Instead of insisting on constructing a theory of the strong nuclear force, we took this beautiful theory and asked what it was good for,” Schwarz said in a 2018 interview. “It turned out it was good for gravity. Neither of us had worked on gravity. It wasn’t something we were especially interested in, but we realized that this theory, which was having trouble describing the strong nuclear force, gives rise to gravity. Once we realized this, I knew what I would be doing for the rest of my career.”

It turns out that, compared with the other forces, gravity is an oddball. “Gravity is the weakest force we know of,” explains Ooguri. “I’m standing here on the fourth floor of the Lauritsen building, and the reason gravity is not pulling me through the floor is that, inside the concrete, there are electrons and nuclei that are supporting me. So, the electric field is winning over the gravitational force.”

However, while the strong nuclear force weakens at shorter and shorter distances, gravity becomes stronger. “The strings help soften this high-energy behavior,” Ooguri says. “The energy gets spread out in a string.”

Tabletop tests of quantum gravity

The challenge with string theory lies not only in making it consistent with our everyday, low-energy world, but also in testing it. To see what occurs at the minuscule scales where spacetime is theorized to become grainy, experiments would need to probe distances on the order of what is known as the Planck length, or 10–35 meters. To reach such extreme scales, scientists would have to build an equally extreme detector. “One way to go is to make something the size of the solar system and look for signatures of quantum gravity that way,” says Adhikari. “But that’s really expensive and would take hundreds of years!” Instead, Zurek says, researchers can investigate aspects of quantum gravity using much smaller experiments. “For the lower-energy experiments we are proposing, we don’t need the whole machinery of string theory,” she says. “Theoretical developments associated with string theory have provided us with some tools and a quantitative grasp on what we expect to be true in quantum gravity.”

The experiments proposed by Zurek, Adhikari, and their colleagues focus on effects of quantum gravity that could be observed at more manageable scales of 10–18 meters. That is still very small, but potentially doable using very precise laboratory instruments.

“A spacetime pixel is so small that if you were to enlarge things so that it becomes the size of a grain of sand, then atoms would be as large as galaxies,”

Rana Adhikari

These tabletop experiments would be like mini LIGOs: L-shaped interferometers that shoot two laser beams in perpendicular directions. The lasers bounce off mirrors and meet back in their place of origin. In LIGO’s case, gravitational waves stretch and squeeze space, which affects the timing of when the lasers meet. The quantum gravity experiment would look for a different kind of spacetime fluctuation consisting of gravitons that pop in and out of existence in what some call the quantum, or spacetime, foam. (Photons and other quantum particles also pop in and out of existence due to quantum fluctuations.) Rather than look for the gravitons individually, the researchers seek “long-range correlations” between complicated collections of the hypothetical particles, which result in observable signatures. Zurek explains that these long-range connections are like larger ripples in the sea of spacetime as opposed to the frothy foam where individual particles reside.

“We think there are spacetime fluctuations that may perturb the light beams,” she says. “We want to design an apparatus where spacetime fluctuations kick a photon out of the beam of the interferometer, and then we would use single-photon detectors to read out that spacetime perturbation.”

Emergent spacetime

“Gravity is a hologram,” says Monica Jinwoo Kang, a Sherman Fairchild Postdoctoral Fellow in Theoretical Physics at Caltech, when explaining the holographic principle, a key tenet of Zurek’s model. This principle, which was realized using string theory in the 1990s, implies that phenomena in three dimensions, such as gravity, can emerge out of a flat two-dimensional surface. “The holographic principle means that all the information in a volume of something is encoded on the surface,” Kang explains.

More specifically, gravity and spacetime are thought to emerge from the entanglement of particles taking place on the 2-D surface. Entanglement occurs when subatomic particles are connected across space; the particles act as a single entity without being in direct contact with each other, somewhat like a flock of starlings. “Modern perspectives on quantum gravity inspired by string theory suggest that spacetime and gravity materialize out of networks of entanglement. In this way of thinking, spacetime itself is defined by how much something is entangled,” says Kang.

“We’ll be lost in the desert if we don’t start focusing on ways to link quantum gravity with the natural world that we live in.”

Kathryn Zurek

In Zurek and Adhikari’s proposed experiment, the idea would be to probe this 2-D surface, or what they call the “quantum horizon,” for graviton fluctuations. Gravity and spacetime, they explain, emerge out of the quantum horizon. “Our experiment would measure the fuzziness of this surface,” says Zurek.

That fuzziness would represent the pixelation of spacetime. If the experiment succeeds, it will help redefine our concept of gravity and space at the most fundamental, deepest levels.

“If I drop my coffee mug and it falls, I’d like to think that’s gravity,” says Adhikari. “But, in the same way that temperature is not ‘real’ but describes how a bunch of molecules are vibrating, spacetime might not be a real thing. We see flocks of birds and schools of fish undertake coherent motion in groups, but they are really made up of individual animals. We say that the group behavior is emergent. It may be that something that arises out of the pixelation of spacetime has just been given the name gravity because we don’t yet understand what the guts of spacetime are.”

AstrophysicsCalifornia Institute of TechnologyGravityPopular
Comments ( 17 )
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  • Daniel Izzo

    Is Gravity the answer to the universe’s missing Monopole Problem ? I think gravity as an attractive force is our missing monopole problem. I was just watching the Youtube video: ” Why Is The Universe The Same Everywhere?” and it says: ” On a variety of scales there appear to be no monopoles at all in the observable universe, a monopole is hypothesized to be just half a traditional magnet …this is a problem because there is no fundamental reason why there shouldn’t be any monopoles, Not only that but our current understanding of the universe’s earliest moments suggests that the cosmos should be in fact flooded by them. ”

    I wrote this earlier as it relates:

    Why Is There Something Rather Than Nothing?
    A: ) Because Magnetism is an attractive force, not a repulsive force ?
    That’s a guess, gravity is also an attractive force.
    Isaac Asimov in a book thought it was because there were more protons than electrons in the universe ( thus forming matter )

  • Hamilton Kulchetscki

    This article is very long n extensive to read,moreover it is very complicated to understand, why do not state simple facts unto simple wordings? Besides that, make up your mind about final conclusions…for its context is full of doubts like yet,still maybe’s,perhaps,n the rest like that

  • Silvio

    Check out spyroe theory for a new concept for quantum gravity.

  • Wal Forester

    I think the people that come up wih this crsp are pixelated.

    • Irina Novitskaya

      Very true. Our bodies are pixelated, but they are not who we are:) We just see ours and others bodies as in movies.

  • Keith kucera

    Time is conduit that connects to all time before space it’s space that is emergent

  • Krishna moni Borah

    When these pixels(say A & B) overlapped, it creates mass and when and another new pixel(C) overlap with already overlapped pixels(A, B) , it will increase the mass of whole system(A, B + C) and if new pixel(C) overlapped and passed out thought(A, B) instantly, we call this (A, B) travel in time in forward direction(normal time taken). There are many things I want to say but I can’t explain here.

    Belongs to infinite stability

  • Brenda tait

    There sre many people from sll walks of life who have seen througj what yous sre saying figured it out long time ago I did tv programs public access local town yesrs ago to find out same questions im starting up again in Scotland I would like to film yous over zoom if I could thank you BrenDa


    … where is that Plank guy, when one needs him. …

  • Irina Novitskaya

    Everything is pixelated including your thoughts about pixels:) Everything is an illusion that you as Reality see. You are not your body or your thoughts, this is the first postulate that you need to understand. Everything including space and time (pixels?) is inside the Reality (‘you are’).

  • bob mcbobberton

    shut up

  • Corey P

    When I take magic mushrooms, I see the pixels, we are in a digital simulation

  • Alexander E Gibson

    You should check out Klee Irwin’s Quantum Gravity Research group, they are making amazing strides in this field, This article is just a taste really, the technological advancements this will allow are beyond imagination.

  • Bernt GRANNAS

    Time is only a mathematical construction to be able to compare speeds with each other. For example, if we were to take the planet Venus’ orbital period around the sun to 1 year, our heart rate would beat by maybe 40 beats per minute. It’s wrong to look at the clock and say “Oh what time goes by”, There is no time that goes by but only the clock hand goes by.

    Bernt Grannas

    The speed x Time = The distance.

    If the time is zero, the distance is also zero.

    This means that Feinman’s road integral, which means that the pilot wave takes all roads through the universe, does not have to take any road at all because everything is in the same place.

    Einstains gravity theory : Space – Time

    New Quantum Gravity Theory : Space – Without Time

    Since time stands still in the dark energy (in empty space), the pilot waves are preserved forever.

    Roger Penrose believes that the brain can come into contact with the quantum fluctuation in empty space, which means that psychic people can see both the future and the past.

    If the speed of light were the fastest, space would not even know that black holes exist


    The curvature of space-time space is propagated immeddiately and not at the speed of light (Because time is zero in the dark energy in empty space)

    When rockets have been sent far trough the space, you have noticed that Einstein’s formulas need a correction term.

    If you assume that the curvature of the space occurs immediately and not at the speed of light the correction term is not needed to get an exactly calculating result.

    See the theory below.

    TIME ARROW IN EMPTY SPACE (in the dark energy)

    In empty space particles are moved due the quantum fluctuations in the dark energy. Quantum fluctuations are caused by matter and anti-matter appearing in equal amounts thereby propelling particles.

    It is considered that time moves backwards in anti-matter (Feynmans diagram). Therefor in empty space no time exists since particles move the same amount forward and backwards in time and the sum of times arrows are zero.

    This explains why the value of the probability calculations are exact right if particles are taking every possible way between two points, (by Feynman’s path-integral) this also explains why particles seem to know that they are going to be measured and react accordingly.

    Past, present and future exists all at the same time since time doesn’t move in empty space and the entropy in dark energi is constant. ( Quantum fluctuations in the dark energy mean maximum disorder which means that time disappears )

    de Broglie’s-Bohm’s Pilot waves are quantumfluctuations in the dark energy.

    Pilot wave theory – Wikipedia

    In theoretical physics, the pilot wave theory, also known as Bohmian mechanics, was the first known example of a hidden-variable theory, presented by Louis de Broglie in 1927. Its more modern version, the de Broglie–Bohm theory, interprets quantum mechanics as a deterministic theory, avoiding troublesome notions such as wave–particle duality, instantaneous wave function collapse, and the …


    The dark energy consists of quantum fluctuations with both matter and antimatter, both react to the gravity space-time curvature acceleration. The galaxies are therefore attracting the dark energy which also are subject to the acceleration at same time.

    This means additional mass which also slows down time according to Einstein’s formulas for acceleration and gravity.

    All this would explain why galaxies seem to spin too fast according to the visible mass. Then no dark matter is needed (ounly dark energy) as an explanation for the too faast galaxiesspin.


    The 4 th dimension of space-time curvature is the acceleration (gravity) that acts on the quantum fluctuations in the dark energy in empty space. It is the effect of gravity on the quantum fluctuations in the dark energy in empty space that constitutes what we call the space-time curvature in the 4th dimension.

    Ps. The theory is easy to test in reality.It means that space-time curvature occurs immediatly and not at the speed of light

    Because no time exist in the dark energy, this open the possibility of communication immediately across the whole universe using quantum fluctuations in the dark energy.(pilot waves)

    Bernt Grannas (




    … if I look at my wrist watch there is a now, and if I look away then forget about that now, then I look again there is another now… yeah, it is my now, which can be different from somebody else time, but it is a still time… the only thing I can agree with no time stuff, is that it could be derived from something else, and then it is not really needed, because it could be explained by something else… it is all those new digital clocks, that make people confused, with old ones one would hear ticking…

  • Quantum Yoyo

    Daniele Oriti: Cosmology as quantum gravity hydrodynamics


    Every instant for existence has time di-synchrony between past to future – vice versa… time abstracted anent space


    meeting/the doom spacetime

    (Time in all the microcosm is dis-uniform, hence, statistical variables and sets.)

    Can’t imagagine how to derive a harmonic oscillation at minuscule scale of zero to the infinitely . . . below Cantorian sets. Google Images

    Unless “Avi” can explain this:


    A Source Book

    Wolfgang Mückenheim

    5 Feb 2022

    1.1 Cantor’s original German terminology on infinite sets

    tower of transfinite mathematics

    In other words, how can the infinitely large be made out of or from the infinitely small for evermore?