Quantum Gravity: An Ambitious Quest to Unravel Physics’ Most Complex Puzzle

Translation Introduction

Since humans realized the existence of an infinite space above them, they have been striving to understand the secret of gravity that binds them to the earth and the universe. While engrossed in this pursuit, questions lingered in his mind: What keeps the stars suspended in the void, and organizes the universe in a harmonious dance? That hidden force that Newton called “gravity” was the first key to understanding the cosmic order, then “Einstein” came to reveal that it is not a force in the traditional sense, but a curvature in the fabric of spacetime itself.

However, after more than a century of equations and research, gravity still stands alone in the world of physics, refusing to join the quantum rhythm that governs atoms and particles. Could gravity be quantized like others, or does it speak a language too deep to be measured?

This translated article from “New Scientist” magazine sheds light on a bold attempt by scientists to answer this question, not on paper this time, but in the laboratory, where thought blends with the microscope, and ambiguity turns into a tangible possibility.

Translation Text

We all realize how complex physics is. For example, if you wanted to capture a ripple in the fabric of spacetime, you would need a detector capable of detecting a change in length smaller than one part in a million of the size of an atom. And if you’re looking for the “Higgs boson,” all you need is seven billion dollars, 14 years of work, and a team of 6,000 scientists.

However, there is a test that surpasses all of that in its audacity, which makes even the most optimistic physicists hesitate before taking it: putting gravity itself under the microscope of the quantum world.

The theory of quantum gravity is the greatest dream pursued by modern physics, and the bridge that may link two fundamental pillars in our description of the universe: general relativity, which is our theory of gravity on large cosmic scales, and quantum mechanics, which describes the behavior of other forces in nature on microscopic scales.

Each of the two theories has been tested separately with extreme precision, and they have succeeded in all experiments without exception. But as soon as we try to combine them, the equations collapse and the language falters in the face of contradiction.

If we can prove that gravity is subject to quantum laws (perhaps by observing the particle that carries it), it will be the key that solves a century of bewilderment.

But the problem is that the most sensitive detectors are still unable to reach the enormous energies needed to detect the effects of particles known as gravitons (hypothetical massless elementary particles believed to carry the force of gravity).

Just a few years ago, the late theoretical physicist Freeman Dyson expressed the frustration of many physicists by saying that the idea of quantum gravity may remain a distant dream. But recently, some scientists have begun to question this judgment, believing that the door has not yet been closed.

If they are right, we may be about to see the first signs that reveal how general relativity converges with quantum mechanics after a century of divergence. As Oxford University theoretical physicist Vlatko Vedral says: “It seems that the time is more technically right now than ever before.”

The Journey of Unifying Physics

The two pillars of modern physics have never harmonized naturally at all. On the one hand, general relativity declares that existence is a single continuous fabric of spacetime, and that gravity is only the result of the curvature of this fabric around massive objects. On the other hand, quantum mechanics whispers a different truth; that the universe is composed of tiny, indivisible particles, carrying a strange mystery, which is their inability to accurately determine their position.

For decades, physicists have walked while balancing the two theories together despite their contradiction. In general, general relativity proves its effectiveness in explaining major phenomena, where gravity dominates, while quantum mechanics takes over the description of the microscopic world, where other forces take control.

However, the “two truths” cannot be correct forever, so how can nature be connected and fragmented at the same time, or be at the same time disciplined in its law (i.e., predictable), and random in its behavior? This contradiction reaches its peak at the Big Bang, when the entire universe shrank to an infinitesimal point with infinite gravity. As for those who aspire to understand that first moment, they will find only one path before them, which is to try to reconcile the two theories.

Attempting to subject gravity to the framework of quantum mechanics, or what is called “quantization of gravity,” is the greatest dream that has haunted physicists for more than half a century. During these decades, many ideas have emerged that claim to be able to solve this dilemma, most notably string theory, which imagines that the building blocks of the world are not particles, but vibrating strings.

Since strings by their nature extend across higher spatial dimensions that we do not see, the Big Bang in this framework was not a single point of infinite density as general relativity says, but a more diffuse and flexible state. In this way, the contradiction between general relativity and quantum mechanics is alleviated when we try to describe the universe in its first moments, where the scales were infinitesimal.

The other approach to reconciling the two theories is loop quantum gravity, which attempts to build the fabric of spacetime itself from indivisible quantum units.

Both approaches produce a quantum version of gravity, and both are extremely difficult mathematically, but what makes the matter more complicated is the absence of any practical means to test them experimentally. Since 1957, theoretical physicist Richard Feynman pointed out that “the only serious difficulty” lies in the absence of experiments that can reveal the fingerprints of quantum gravity, adding: “And even more than that, we will not get such experiments anytime soon.”

As we see, behind his saying lies a logic that is simple in its essence, but deep in its significance. We know that all other forces that are subject to quantum mechanics are transmitted by indivisible elementary particles. Photons transmit the electromagnetic force responsible for light and basic chemical interactions in matter, and gluons transmit the strong nuclear force that binds protons and neutrons inside the nucleus of the atom. As for “W” and “Z” bosons, they carry the weak nuclear force that enables some particles to decay radioactively.

Based on this, if gravity belongs to the same family of forces, shouldn’t it have its own quantum messenger? That imagined particle called the graviton.

Perhaps the graviton is not an independent entity in itself as much as it is an echo of something deeper, as a vibration in a string as string theory says, or a vibration in the fabric of spacetime itself as described by loop quantum gravity. But whatever the details, it is almost certain that something resembling the graviton must appear in the picture.

The problem is that the particles that transmit forces become increasingly rare in their interaction with others the weaker the force is. Since gravity is the weakest force in nature, it is about 10 trillion trillion times weaker than the force known in physics as the “weak force.”

It may seem to us that gravity is strong, as we see it drop the apple from its branch to the ground, but the force of attraction between only two apples is almost non-existent. Therefore, the probability of observing the gravity particle, or what is known as the graviton, has always been unimaginably small.

According to traditional theory, the effects of quantum gravity can only appear at imaginary energies that exceed the limits of any human laboratory. Even the Large Hadron Collider, the largest particle accelerator on Earth, responsible for the discovery of the Higgs boson, remains about a billion million times weaker than the level required to see quantum gravity in action.

At the beginning of the new millennium, the late physicist Freeman Dyson said that building detectors capable of detecting the graviton may require them to be so enormous that they collapse under their own weight, turning into a black hole.

When the Graviton Whispers

Therefore, it was not strange that the news caused widespread surprise during the year 2024, when a research team led by “Igor Pikowski” from Stockholm University announced that what was thought impossible may become possible inside the laboratory. The experimental plan is based on a microscopic metal rod that resembles a very small tuning fork.

This rod is first cooled to a temperature close to absolute zero, so that all its atoms enter a uniform behavioral state. After that, a laser beam is shone on it that slightly stimulates this collective state, to become in a quantum way blurred in both vibrating and non-vibrating states at the same time. Once prepared in this way, the researchers say that the rod will respond to the slightest whisper in the universe, and even to the faint “hum” of only one graviton.

It sounds wonderfully suspicious to some physicists. Several months before Pikowski’s team published their research, theoretical physicist Daniel Carney, from the National Laboratory in Lawrence Berkeley, California, with his colleagues, re-examined the old question of whether gravitons can be observed using technologies originally developed to detect gravitational waves.

Those waves are nothing but ripples in the fabric of spacetime, sometimes arising from distant catastrophic cosmic events, such as the merging of black holes. Like gravitons, these waves remained for decades inhabiting the world of the impossible, beyond the reach of our tools. The first evidence of them was only observed in 2015, when the Laser Interferometer Gravitational-Wave Observatory “LIGO” in the United States was able to record a very small disturbance in spacetime that does not exceed one part in a million of the width of a single atom.

On the other hand, Carney and his colleagues posed a logical question: if gravitational waves can be observed, why can’t gravitons also be observed? In principle, a gravitational wave is a group of gravitons, and the researchers believed that the technologies developed during the last decade have reached a level of sensitivity that makes them capable of capturing infinitesimal ripples that may only be the trace of a single graviton.

But here lies the dilemma, as any such signal would be identical to that issued by a classical non-quantum gravitational wave, which happens to be very small. Carney confirms that this same ambiguity will hinder any signal recorded in the metal rod experiment that resembles a tuning fork like the one proposed by Pikowski’s team, saying: “When you apply energy to it, it starts to vibrate, but that doesn’t tell you whether the gravitational field itself is subject to quantum laws or not.”

On the other hand, a heated debate rages among scientists about what this resonance experiment can really reveal. Some of them believe – like Carney – that it will not provide any conclusive evidence regarding the quantum nature of gravity, while others – like Pikowski and Vlatko Vedral – believe that it may strongly indicate that gravity is indeed quantum, even if not conclusively.

However, most physicists agree that observing a clear and undeniable signal of a single graviton remains a distant dream. As Pikowski says: “We may have to wait 100 years or more to get conclusive evidence that gravity is quantum.”

So were the skeptics right in the end? Not exactly.. Quantum mechanics is not limited to the concept of quantization alone, as it possesses other features that distinguish it. One of these features is quantum entanglement, which means that whenever two quantum particles interact, some of their properties become instantly correlated, no matter how far apart they are.

In 2017, both Vlatko Vedral and Chiara Marletto, from the University of Oxford, and independently another team led by Sugato Bose from University College London; presented a proposal to take advantage of this phenomenon.

The idea is summarized in preparing two material masses in a quantum state so that their positions are uncertain, then isolating them from all other forces, and simply waiting. If it is later observed that the positions of the two masses begin to entangle or connect with each other at the same moment, it means that they are entangled by gravity itself. Therefore, gravity must be a quantum phenomenon.

But it is not as simple as it seems at first glance, decades ago Richard Feynman thought about a similar experiment, then dismissed it describing it as “extremely difficult to the point of incomprehensibility”. The larger the mass, the more complex it is to bring it into a quantum state, and the smaller it is, the gravity between them fades to the point where it is impossible to measure.

Worse, our world today is full of photons – from light, heat and radio waves – that flood everything, as a cosmic noise that threatens to obliterate any subtle quantum whisper we are trying to observe. However, technology has come a long way since Feynman’s time, when quantum experiments only included a few atoms.

In 2019, a team led by Markus Arndt, from the University of Vienna, succeeded in bringing 2000 atoms into a single quantum state, an achievement that is considered an important milestone towards the dream of gravitational entanglement that looms on the horizon. As Vlatko Vedral says in this context: “Today, three or four research groups are racing to achieve that.”

But the road is still long for scientists, as the masses used today in laboratories are a million times smaller than the gravity between them would have a measurable effect.

However, Markus Aspelmeyer from the University of Vienna pins his hopes on carrying out a real experiment within 15 years, although he admits that this date is only a promise he made to himself before retirement forces him to stop. But he also realizes that the ideal experiment requires the masses to be placed very far apart from each other, so that light rays or any external influence do not tamper with their delicate threads.

If the distance is not sufficient, it would not be possible to ascertain that what is happening between them is the action of quantum gravity alone, without violating the laws of relativity. But here lies the paradox: the further the masses are apart, the weaker the force of gravity between them becomes, to the point where it is almost zero. This is why Aspelmeyer explicitly acknowledges: “We may accomplish it before the coming meteorite falls to end everything.”

So there is no way to capture the graviton directly, and any trace of entanglement generated by gravity may remain out of reach for 15 years or more, and may remain ambiguous even after that. The conclusion seems faint, as if the long journey ended in a mirage. However, the door has not been closed yet, as there is a third path that looms on the horizon, a path that begins with a completely different question: What if gravity from the beginning is not subject to quantum laws at all!?

There are good reasons to believe that gravity is fundamentally different from other forces. It is primarily the only force that affects everything without exception. According to general relativity, this is because gravity is not a force in the traditional sense, but an expression of the curvature of spacetime itself, meaning that it does not attract objects towards each other, but rather creates a slope in the fabric of spacetime that makes objects slide into it as if they were walking on a slope. Thus, it can be said that gravity is not a force like other forces, but more like a geometric illusion resulting from the nature of spacetime itself.

A Hum That Does Not Subside

In order not to tamper with the consistency of the picture drawn by relativity, some physicists have sought to find a way out that allows keeping spacetime as a classical entity, so that gravity is treated as the only fundamental force not subject to quantum laws.

According to some hypotheses, gravity may be responsible for the collapse of the quantum behavior of objects when their mass exceeds a certain limit, which may explain why our daily world appears classical.

Despite the diversity of visions within this trend known as the “semi-classical approach”, a study conducted by Jonathan Oppenheim’s team from University College London in 2023 revealed a common denominator among them all: that gravity in its depths, must involve a degree of hidden chaos, a random whisper that scientists call “gravitational noise.”

To understand the idea, imagine an object that – according to quantum mechanics – can exist in two positions at the same time. Then the question arises: Where does its gravity come from: from the first position or the second? And if gravity is not a quantum force but an inherent feature of the fabric of spacetime, how can it emanate from two different positions, and there is only one spacetime! ? In a world where mass is scattered between probabilities, while spacetime insists on its certainty, it has no choice but to speculate where it should bend.

Oppenheim says that the issue is not how gravity does its action, but in the inevitability that it does so as long as it is an inherent property in the fabric of spacetime itself.

If it is true that gravity is not quantum in its origin, then any experiment that seeks to measure mass with absolute precision will collide with a limit that cannot be exceeded, as a degree of noise or randomness will always be present. Oppenheim adds: “This applies to any theory in which spacetime is assumed to be classical in its essence, so the principle is very broad.”

The most beautiful thing about this idea is that it does not need to wait a century or a technical miracle to be tested, unlike the graviton or quantum entanglement experiments, meaning that this idea can be tested now, and some attempts have already begun. In 2021, Aspelmeyer’s team conducted a unique experiment, during which they made a few small beads of gold, each with a diameter of only a few millimeters, and fixed two beads on the ends of a toothpick-sized rod to look like a miniature dumbbell. Then they suspended the rod horizontally on a spring.

After that, they made a third bead oscillate nearby to see if its faint gravity would have an effect. Indeed, the team observed an acceleration of about one part in 100 billion of the Earth’s gravitational acceleration, which is the weakest source of gravity they have ever recorded.

These measurements have not yet reached enough precision, according to Oppenheim, we need to improve them 1000 times before they tell us something meaningful about the nature of gravity.

However, it seems that we are approaching rapidly, as Aspelmeyer’s team is now working on an improved experiment, using masses that are 10,000 times smaller, manufactured using microcomputer chip technologies. Once the results of these experiments are integrated with what is revealed by ongoing research on gravitational entanglement, it will be possible to narrow the margin of cosmic noise itself, and see whether gravity will remain loyal to its classical world, or whether it is about to touch the borders of quantum?

Here you may wonder: how long will all this take? In fact, no one knows for sure. But what once seemed like a distant dream (i.e., testing whether gravity is actually a quantum phenomenon, instead of just thinking about it theoretically), is now within reach. In the end, Oppenheim concludes his speech by saying: “The matter is now a matter for the experimentalists, nature does not care what the theorists think.”