Just over a week ago, European physicists announced they had measured the strength of gravity on the smallest scale ever.

In a clever tabletop experiment, researchers at Leiden University in the Netherlands, the University of Southampton in the UK, and the Institute for Photonics and Nanotechnologies in Italy measured a force of around 30 attonewtons on a particle with just under half a milligram of mass. An attonewton is a billionth of a billionth of a newton, the standard unit of force.

The researchers say the work could “unlock more secrets about the universe’s very fabric” and may be an important step toward the next big revolution in physics.

But why is that? It’s not just the result: it’s the method, and what it says about a path forward for a branch of science critics say may be trapped in a loop of rising costs and diminishing returns.

Gravity

From a physicist’s point of view, gravity is an extremely weak force. This might seem like an odd thing to say. It doesn’t feel weak when you’re trying to get out of bed in the morning!

Still, compared with the other forces that we know about – such as the electromagnetic force that is responsible for binding atoms together and for generating light, and the strong nuclear force that binds the cores of atoms – gravity exerts a relatively weak attraction between objects.

And on smaller scales, the effects of gravity get weaker and weaker.

It’s easy to see the effects of gravity for objects the size of a star or planet, but it is much harder to detect gravitational effects for small, light objects.

The need to test gravity

Despite the difficulty, physicists really want to test gravity at small scales. This is because it could help resolve a century-old mystery in current physics.

Physics is dominated by two extremely successful theories.

The first is general relativity, which describes gravity and spacetime at large scales. The second is quantum mechanics, which is a theory of particles and fields – the basic building blocks of matter – at small scales.

These two theories are in some ways contradictory, and physicists don’t understand what happens in situations where both should apply. One goal of modern physics is to combine general relativity and quantum mechanics into a theory of “quantum gravity”.

One example of a situation where quantum gravity is needed is to fully understand black holes. These are predicted by general relativity – and we have observed huge ones in space – but tiny black holes may also arise at the quantum scale.

At present, however, we don’t know how to bring general relativity and quantum mechanics together to give an account of how gravity, and thus black holes, work in the quantum realm.

New theories and new data

A number of approaches to a potential theory of quantum gravity have been developed, including string theory, loop quantum gravity and causal set theory.

However, these approaches are entirely theoretical. We currently don’t have any way to test them via experiments.

To empirically test these theories, we’d need a way to measure gravity at very small scales where quantum effects dominate.

Until recently, performing such tests was out of reach. It seemed we would need very large pieces of equipment: even bigger than the world’s largest particle accelerator, the Large Hadron Collider, which sends high-energy particles zooming around a 27-kilometre loop before smashing them together.

Tabletop experiments

This is why the recent small-scale measurement of gravity is so important.

The experiment conducted jointly between the Netherlands and the UK is a “tabletop” experiment. It didn’t require massive machinery.

The experiment works by floating a particle in a magnetic field and then swinging a weight past it to see how it “wiggles” in response.

This is analogous to the way one planet “wiggles” when it swings past another.

By levitating the particle with magnets, it can be isolated from many of the influences that make detecting weak gravitational influences so hard.

The beauty of tabletop experiments like this is they don’t cost billions of dollars, which removes one of the main barriers to conducting small-scale gravity experiments, and potentially to making progress in physics. (The latest proposal for a bigger successor to the Large Hadron Collider would cost US$17 billion.)

Work to do

Tabletop experiments are very promising, but there is still work to do.

The recent experiment comes close to the quantum domain, but doesn’t quite get there. The masses and forces involved will need to be even smaller, to find out how gravity acts at this scale.

We also need to be prepared for the possibility that it may not be possible to push tabletop experiments this far.

There may yet be some technological limitation that prevents us from conducting experiments of gravity at quantum scales, pushing us back toward building bigger colliders.

Back to the theories

It’s also worth noting some of the theories of quantum gravity that might be tested using tabletop experiments are very radical.

Some theories, such as loop quantum gravity, suggest space and time may disappear at very small scales or high energies. If that’s right, it may not be possible to carry out experiments at these scales.

After all, experiments as we know them are the kind of thing that happen at a particular place, across a particular interval of time. If theories like this are correct, we may need to rethink the very nature of experimentation so we can make sense of it in situations where space and time are absent.

On the other hand, the very fact we can perform straightforward experiments involving gravity at small scales may suggest that space and time are present after all.

Which will prove true? The best way to find out is to keep going with tabletop experiments, and to push them as far as they can go.

A general view of the LHC experiment during a media visit at CERN near Geneva, Switzerland, July 23, 2014.
| Photo Credit: Reuters

Scientists at the world’s largest atom smasher expressed confidence Monday about moving forward with a multibillion-euro project to build a larger and more powerful particle collider that could help unlock more mysteries of the universe.

Leaders of the European Organization for Nuclear Research, or CERN, said planning is on track for its envisioned Future Circular Collider, which is estimated to cost 15 billion Swiss francs (about 16 billion euros or $17.2 billion) and is hoped to start operating in a first phase by 2040.

But nothing is certain yet, aside from the interest from mostly European and Western countries that bankroll CERN, which is home to the Large Hadron Collider. That project is perhaps best known for helping confirm the subatomic Higgs boson in 2012 after a decades-long quest for what was described as “the missing cornerstone of physics.”

“The Future Circular Collider is a possible facility. I’m saying ‘possible’ because we are today at the level of feasibility study. It’s not yet an approved project,” said Fabiola Gianotti, the CERN director-general. She said review committees had not turned up any “technical show-stoppers” for the project so far.

She touted the proposed collider as “a wonderful instrument to improve our understanding of fundamental physics” and a driver of innovation in areas like cryogenics, superconducting magnets, vacuum technologies and detector-instrumentation technologies that could offer socioeconomic benefits for society.

However, the science that the future collider could generate remains largely unknown. “It’s true that at the moment we do not have a clear theoretical guidance on what we should look for,” Gianotti said.

The laboratory that already houses what CERN leaders call the world’s biggest machine uses a network of magnets to accelerate particles through a 27-kilometer (17-mile) underground loop along the French-Swiss border and slam them together, capturing and interpreting the results of the collisions to help explain how fundamental physics works.

Monday’s briefing on the new collider included some proposed changes to the original plan announced in 2019. Among them: The loop will be 91 kilometers instead of the 100-kilometer circuit first envisioned.

But they still aim to boost energy levels of the particle collisions to 100 TeV, or 100 trillion electron volts, about eight times more powerful than the Large Hadron Collider’s 13TeV.

One way physicists seek clues to unravel the mysteries of the universe is by smashing matter together and inspecting the debris. But these types of destructive experiments, while incredibly informative, have limits.

We are two scientists who study nuclear and particle physics using CERN’s Large Hadron Collider near Geneva, Switzerland. Working with an international group of nuclear and particle physicists, our team realised that hidden in the data from previous studies was a remarkable and innovative experiment.

In a new paper published in Physical Review Letters, we developed a new method with our colleagues for measuring how fast a particle called the tau wobbles.

Our novel approach looks at the times incoming particles in the accelerator whiz by each other rather than the times they smash together in head-on collisions. Surprisingly, this approach enables far more accurate measurements of the tau particle’s wobble than previous techniques. This is the first time in nearly 20 years scientists have measured this wobble, known as the tau magnetic moment, and it may help illuminate tantalising cracks emerging in the known laws of physics.

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Why measure a wobble?

Electrons, the building blocks of atoms, have two heavier cousins called the muon and the tau. Taus are the heaviest in this family of three and the most mysterious, as they exist only for minuscule amounts of time.

Interestingly, when you place an electron, muon or tau inside a magnetic field, these particles wobble in a manner similar to how a spinning top wobbles on a table. This wobble is called a particle’s magnetic moment. It is possible to predict how fast these particles should wobble using the Standard Model of particle physics – scientists’ best theory of how particles interact.

Since the 1940s, physicists have been interested in measuring magnetic moments to reveal intriguing effects in the quantum world. According to quantum physics, clouds of particles and antiparticles are constantly popping in and out of existence. These fleeting fluctuations slightly alter how fast electrons, muons and taus wobble inside a magnetic field. By measuring this wobble very precisely, physicists can peer into this cloud to uncover possible hints of undiscovered particles.

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Testing electrons, muons and taus

In 1948, theoretical physicist Julian Schwinger first calculated how the quantum cloud alters the electron’s magnetic moment. Since then, experimental physicists have measured the speed of the electron’s wobble to an extraordinary 13 decimal places.

The heavier the particle, the more its wobble will change because of undiscovered new particles lurking in its quantum cloud. Since electrons are so light, this limits their sensitivity to new particles.

Muons and taus are much heavier but also far shorter-lived than electrons. While muons exist only for mere microseconds, scientists at Fermilab near Chicago measured the muon’s magnetic moment to 10 decimal places in 2021. They found that muons wobbled noticeably faster than Standard Model predictions, suggesting unknown particles may be appearing in the muon’s quantum cloud.

Taus are the heaviest particle of the family – 17 times more massive than a muon and 3,500 times heavier than an electron. This makes them much more sensitive to potentially undiscovered particles in the quantum clouds. But taus are also the hardest to see, since they live for just a millionth of the time a muon exists.

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To date, the best measurement of the tau’s magnetic moment was made in 2004 using a now-retired electron collider at CERN. Though an incredible scientific feat, after multiple years of collecting data that experiment could measure the speed of the tau’s wobble to only two decimal places. Unfortunately, to test the Standard Model, physicists would need a measurement 10 times as precise.

Lead ions for near-miss physics

Since the 2004 measurement of the tau’s magenetic moment, physicists have been seeking new ways to measure the tau wobble.

The Large Hadron Collider usually smashes the nuclei of two atoms together – that is why it is called a collider. These head-on collisions create a fireworks display of debris that can include taus, but the noisy conditions preclude careful measurements of the tau’s magnetic moment.

From 2015 to 2018, there was an experiment at CERN that was designed primarily to allow nuclear physicists to study exotic hot matter created in head-on collisions. The particles used in this experiment were lead nuclei that had been stripped of their electrons – called lead ions. Lead ions are electrically charged and produce strong electromagnetic fields.

The electromagnetic fields of lead ions contain particles of light called photons. When two lead ions collide, their photons can also collide and convert all their energy into a single pair of particles. It was these photon collisions that scientists used to measure muons.

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These lead ion experiments ended in 2018, but it wasn’t until 2019 that one of us, Jesse Liu, teamed up with particle physicist Lydia Beresford in Oxford, England, and realised the data from the same lead ion experiments could potentially be used to do something new: measure the tau’s magnetic moment.

This discovery was a total surprise. It goes like this: Lead ions are so small that they often miss each other in collision experiments. But occasionally, the ions pass very close to each other without touching. When this happens, their accompanying photons can still smash together while the ions continue flying on their merry way.

These photon collisions can create a variety of particles – like the muons in the previous experiment, and also taus. But without the chaotic fireworks produced by head-on collisions, these near-miss events are far quieter and ideal for measuring traits of the elusive tau.

Much to our excitement, when the team looked back at data from 2018, indeed these lead ion near misses were creating tau particles. There was a new experiment hidden in plain sight!

First measurement of tau wobble in two decades

In April 2022, the CERN team announced that we had found direct evidence of tau particles created during lead ion near misses. Using that data, the team was also able to measure the tau magnetic moment – the first time such a measurement had been done since 2004. The final results were published on Oct. 12, 2023.

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This landmark result measured the tau wobble to two decimal places. Much to our astonishment, this method tied the previous best measurement using only one month of data recorded in 2018.

After no experimental progress for nearly 20 years, this result opens an entirely new and important path toward the tenfold improvement in precision needed to test Standard Model predictions. Excitingly, more data is on the horizon.

The Large Hadron Collider just restarted lead ion data collection on Sept. 28, 2023, after routine maintenance and upgrades. Our team plans to quadruple the sample size of lead ion near-miss data by 2025. This increase in data will double the accuracy of the measurement of the tau magnetic moment, and improvements to analysis methods may go even further.

Tau particles are one of physicists’ best windows to the enigmatic quantum world, and we are excited for surprises that upcoming results may reveal about the fundamental nature of the universe.

This handout illustration released by U.S. National Science Foundation on September 27, 2023, shows an anti-matter gravity experiment ALPHA-g at an undisclosed location. For the first time, scientists have observed antimatter particles — the mysteriously absent twins of the visible matter all around us — falling downwards due to the effect of gravity, Europe’s physics lab CERN announced on September 27, 2023. Though most physicists anticipated this result, the experiment was hailed as “huge milestone” that definitively rules out that gravity repels antimatter upwards — a finding that would have upended our most fundamental understanding of the universe.
| Photo Credit: AFP

For the first time, scientists have observed antimatter particles — the mysterious twins of the visible matter all around us — falling downwards due to the effect of gravity, Europe’s physics lab CERN announced on Wednesday.

The experiment was hailed as “huge milestone”, though most physicists anticipated the result, and it had been predicted by Einstein’s 1915 theory of relativity.

It definitively rules out that gravity repels antimatter upwards — a finding that would have upended our fundamental understanding of the universe.

Around 13.8 billion years ago, the Big Bang is believed to have produced an equal amount of matter — what everything you can see is made out of — and antimatter, its equal yet opposite counterpart.

However there is virtually no antimatter in the universe, which prompted one of the greatest mysteries of physics: what happened to all the antimatter?

“Half the universe is missing,” said Jeffrey Hangst, a member of CERN’s ALPHA collaboration in Geneva which conducted the new experiment.

“In principle, we could build a universe — everything that we know about — with only antimatter, and it would work in exactly the same way,” he told AFP.

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Physicists believe that matter and antimatter did meet and almost entirely destroyed each other after the Big Bang.

Yet matter now makes up nearly five per cent of the universe — the rest is even less understood dark matter and dark energy — while antimatter vanished.

Newton’s apple flying up?

One of the key outstanding questions about antimatter was whether gravity caused it to fall in the same way as normal matter.

While most physicists believed that it did, a few had speculated otherwise.

A falling apple famously inspired Isaac Newton’s work on gravity — but if that apple was made of antimatter, would it have shot up into the sky?

And if gravity did in fact repel antimatter, it could have meant that impossibilities such as a perpetual motion machine were possible.

“So why not drop some and see what happens?” Hangst said.

He compared the experiment to Galileo’s famous — though likely apocryphal — 16th-century demonstration that two balls of different mass dropped from the Leaning Tower of Pisa would fall at the same rate.

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But this experiment — the result of 30 years of work on antimatter at CERN — was “a little bit more involved” than Galileo’s, Hangst said.

One problem was that antimatter barely exists outside of rare, short-lived particles in outer space.

However in 1996, CERN scientists produced the first atoms of antimatter — antihydrogen.

Another challenge was that, because matter and antimatter have an opposite electrical charge, the moment they meet they destroy each other in a violent flash of energy scientists call annihilation.

A magnetic trap

To study gravity’s effect on antimatter, the ALPHA team constructed a 25-centimetre-long (10-inch) bottle placed on its end, with magnets at the top and bottom.

Late last year, the scientists placed around 100 very cold antihydrogen atoms into this “magnetic trap” called ALPHA-g.

As they turned down the strength of both magnets, the antihydrogen particles — which were bouncing around at 100 metres a second — were able to escape out either end of the bottle.

The scientists then simply counted how much antimatter was annihilated at each end of the bottle.

Around 80% of the antihydrogen went out of the bottom, which is a similar rate to how regular bouncing hydrogen atoms would behave if they were in the bottle.

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This result, published in the journal Nature, shows that gravity causes antimatter to fall downwards, as predicted by Einstein’s 1915 theory of relativity.

In more than a dozen experiments, the CERN scientists varied the strength of the magnets, observing gravity’s effect on antimatter at different rates.

While the experiment rules out that gravity makes antihydrogen go upwards, Hangst emphasised it did not prove that antimatter behaves in exactly the same way as normal matter.

“That’s our next task,” he said.

Marco Gersabeck, a physicist who works at CERN but was not involved in the ALPHA research, said it was “a huge milestone”.

But it marks “only the start of an era” of more precise measurements of gravity’s effect on antimatter, he told AFP.

Other attempts to better understand antimatter include using CERN’s Large Hadron Collider to investigate strange particles called beauty quarks.

And there is an experiment onboard the International Space Station trying to catch antimatter in cosmic rays.

But for now, exactly why the universe is awash with matter but devoid of antimatter “remains a mystery,” said physicist Harry Cliff.

Since both should have annihilated each other completely in the early universe, “the fact that we exist suggests there is something we don’t understand” going on, he added.