Ryan Wills
Sometimes, you work tirelessly on a problem, only to realise you have been going about it all backwards. Imagine trying to fit a massive antique piano through a tiny doorway. You have tried everything – rotating it, removing the legs, forceful shoving – but you just can’t get it to fit. Eventually, you realise it is easier to construct a room to house the piano where it already sits.
Now, some physicistsare grappling witha similarrethink.For decades, theacceptedroute toan ultimate theory of everythinghas involvedtaking our best theory of gravity andsqueezingit intotheframeof quantum mechanics. Given that quantum theory is wildly successfulindescribingthe otherthree ofthefour fundamental forces of nature, itis an understandableapproach.Yet,almost acentury later,scientists stillhaven’tmanaged tomakegravityfit.
That’swhy a few mavericks havechampioned analternativestrategy.They suggestthattweaking the equations of quantum mechanics–constructinga new room forgravity–helps explain howthe strange world of particlesgives riseto our everyday reality.
Various experimental avenues are opening up to probe this approach, involving everything from levitating diamonds and glowing metals to swinging pendulums and ticking clocks. The tests promise to shine a light on how the quantum world operates and guide the search for a more complete understanding of the universe. “This is like going into the open ocean: we have no clue where to go,” says Angelo Bassi, a physicist at the University of Trieste in Italy. “But maybe … by going in the wrong direction, we’ll discover the right thing.”
The world as we know it is definite. Your books rest solidly on their shelf, your clock ticks steadily forward and your cat is demonstrably alive. In the realm of atoms, however, nothing is certain. Quantum mechanics allows us to describe certain properties of particles, like their position, only in terms of likelihood. You can predict – with great success – the odds of finding a particle in one of many places, but where it will be observed in a given test is completely unknowable. Before that measurement happens, the object exists in a wave-like blur of all those possibilities at once, which we describe mathematically with something called a wave function.
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This leaves us with two big conundrums that lie at the heart of quantum theory. For one, it is unclear how and when the fuzzy quantum world gives rise to classical concreteness. The other problem is that this probabilistic description clashes with Albert Einstein’s classical understanding of gravity. Efforts to recast Einstein’s work on gravity into the language of forces and particles have resulted in constructions such as string theory that are cumbersome and practically untestable.
A long-standing assumption has been that, deep down, everything is quantum. But a century after theinceptionof quantum mechanics,physicists arestill struggling to makea cohesive story out ofit.“There must be something else going on, and we have to understand what,”says. “The important step is to push quantum mechanics to its limits.”
One route to finding theselimitsinvolvesone ofthe many oddities of quantum mechanics:the principle of superposition.Scientists today routinely put a single particle into a mixed state of being in twodistinct locations,a trick they can verify with interferencepatternsfromthoseinteractingpossibilities. But oncetheymeasure where the particle is, it collapses intoonedefinitivestate:either left or right, say.
There are manypossible explanationsof what happens when a measurement occurs – asevidencedby thevariety ofinterpretations of quantum mechanics. The many-worlds interpretation says that each possible scenario plays out in a different branch of reality, while the Copenhagen interpretation says, essentially, to trust the maths.
Some physicists want to adapt quantum mechanics to include the classical force of gravity Hans Berggren/Getty Images
Anothergroup of explanationssearches for a physical solution.In the 1980s,physicists Giancarlo Ghirardi, Alberto Rimini and Tullio Weberproposedthat someinvisibleprocesswastamperingwith quantum waves, causing them tosuddenlycollapse. In the following years,physicist at the Wigner Research Centre for Physics in Hungary and University of Oxford mathematician Roger Penrose proposed that gravity could be a culprit for this mysterious process. Essentially, the Diósi-Penrose model argues that, in the battle between quantum and gravity, quantum cracks first. The basic premise the pair set out was that putting a large mass into a superposition would force space-time to curve in two different ways – something it cannot permit. They proposed that the integrity of space-time prevails and causes the quantum waves to collapse.
If this is the case, superpositions would have a lifetime that is inversely proportional to the square of their mass. Quantum objects could live in a superposition for very long periods of time, but the larger the object was, the faster it would collapse. This would explain why we never see larger objects in superposition – because their substantial gravitational tug would instantly force a collapse. It also tackles the thorny problem of measurement, because any device large enough to detect and relay information about a quantum system would become part of that system and disturb it gravitationally. This idea moved the discussion away from merely interpreting quantum theory and instead towards revising it.
Ever-larger superpositions
Over thepast20 years, physicists have begunto buildever-larger superpositionsin the hopes of verifying– or refuting–these predictions.Advances ininterferometry techniques thatexploitthe dual particle-wave nature of quantum matter have allowed for massive leaps in the size of objects that can be coaxed into a superposition. Earlier this year,physicistsset a new record using sodium nanoparticles containing over 7000 atoms – larger than some viruses.
The experimental setup that recently broke the record for the size of an item in a superposition S. Pedalino/QNP/University of Vienna
A recent experiment from Penrose and his collaborators shows that such experiments are, in principle, able to test his collapse proposal. In a paper yet to be peer-reviewed, posted online in December 2025, a team led by atBen-GurionUniversity of the Negev in Israelputarubidium atom into a superposition of two states:one levitating in place and the other ingravitationalfreefall.Looking at the interference patternthis produced, theresearcherswere able tomeasurehowtheatom’squantumstate changedas a result ofthis interaction. The signature they found,confirming that–at this microscopic scale, at least–the superposition principle iscompatible with general relativity.
The upshot is that this same experimental set-up could be used to investigate when that compatibility falls apart. Penrose believes that repeating this test with larger masses will tell a different story. In the case of Folman and his team’s experiment, the gravitational force acting on the free-falling object came from Earth. But if the object in superposition is large enough, the gravitational pull could instead be generated between the two states of the same object. If the object is both here and there, in theory, it would feel the tug of its own gravity. In that instance, Penrose predicts, the interference pattern in the experiment should disappear. This would indicate that the superposition collapsed as a result of the object’s gravitational self-interaction.
, a physicist at the National Institute for Nuclear Physics in Frascati, Italy, is impressed by the technological mastery demonstrated in the experiment. “It’s absolutely fascinating,” she says. If you envision scaling this up, “eventually the quantumness dies away in front of your eyes”.
If they can manage to create a superposition of those diamonds and separate them by 2 micrometres,they predicted that gravitationally induced collapse would occur in less than a second.
Others are less optimistic about the timeline. “Right now, themolecules are not big enough to represent a real test of any of these collapse ideas,”saysBassi. “The day will come, but it will be a long journey.”
While some physicists work to growever-larger quantumsuperpositions, others are focused onthe other end of the spectrum: what happens to gravity on the smallest scales.
Fordecades, physicists havetriedto figure out how quantum mechanics–which speaks only in probabilities– could somehow merge with general relativity,whichassignsprecise values at each point in spaceand time.Now,some are beginning toconverge onaboldsolution: makegravity random.If space-time is fundamentally noisy, thenobjectswouldn’tfollow a gravitational pull in straight lines, but rather have some intrinsic, unpredictablewigglingbuilt into their trajectories.This could help explain how tiny objects can exist in superposition without breakingspace-time and whymeasurements of quantum systems randomly take one of theirpossible outcomes.
Random gravity
In 2023,at University College Londonsolidified this idea inwhat he calls a, which is a hybrid framework that allows the microscopic and macroscopic scales to function differently but still interact. “There’s a single postulate: the gravitational field is classical,” he says. “Everything else follows.”
ճٳǰfrom Diósiand at PSL University in France in 2016, which showed a mathematically consistent way for gravity to be random. Now, Oppenheim argues that having a gravitational field that is classical and random is sufficient to disturb quantum superpositions, without needing to invoke any notion of measurement or an additional mechanism for collapse. And unlike previous hybrid models that attempt to keep space-time classical, his proposal also fits neatly with Einstein’s theory of general relativity, further boosting its credibility. Oppenheim and his colleagues alsototest these ideas by very preciselymonitoringthe mass of an object subject to gravity.
Not everybody likes the idea of making gravity random, though.at the University of Southampton,UK,a close collaborator of Penrose, thinks that positing a fluctuating gravitational field without explaining where the randomness comes from ishiding the problem.“Although I disagree with what he does, I really like it,” she says. “He finds an alternative way and proposes an experimentto test it.”
Furthermore, post-quantum gravityisnowhelpingto probe.Recently, physicists haveexplored the consequences of a. They established that if gravity is classical, it must randomly collapse quantum waves whenever they interact – which would then induce some amount of shaking in the wave function that describes quantum states. In the past year, separate studies led by Bassi and Daniel Carney at Lawrence Berkeley National Laboratory in California calculated the . Theiranalysespropopennewwindowsfor testingthesemodels.
New experiments
Overthepast few years, three main channelsof experimenthaveemergedin the search forsigns of.
Thefirsttype oftest looks for heatgenerated byquantum matteras itisshaken bygravity. Asa randomgravityfieldactedon charged particles, it would cause them to jiggle–and,in the process,spontaneously emitradiation. Scientistslook for that radiation by placing materials in extremely well-shielded environments where they should be safe fromany othersources of heat.
Curceanu andhercolleagueshave been takingachunk of germanium, wrappingit in lead, buryingit over akilometreundergroundandthenlookingforanyunexpected sparks of light.from her teamhave yet to spot anysignificantanomalous radiation, tightening the constraints on theseideasand,in some cases,. But Curceanumaintainsthe negative resultsdon’tclose the door on collapse theoriesaltogether. “When you eliminate the simplest models,” she says,“the real work can start.”
Artist’s impression of LISA Pathfinder, which has provided some of the tightest constraints yet on gravitational randomness ESA/ATG medialab
Another channel focuses on oscillating pendulums, looking for subtle swerves in their movement caused by gravitational randomness. Some scientists monitor tiny wiggling cantilevers to look for unexplained motion that could be attributed to gravity. Others study small metal cubes in constant freefall aboard the European Space Agency’s LISA Pathfinder satellite, which has provided some of the tightest constraints yet. Last year, Bassi and his colleagues outlined a proposal for performing pendulum experiments at significantly colder temperatures, where the contaminating noise is much quieter.
Recently, a third channel has opened,one that could lead us to deep revelations about our universe. A team led by at Sapienza University of Rome showed that all collapse models that invoke gravity also have important consequences for time itself. The researchers argue that a random gravitational field that causes matter to shake would put a fundamental limit on how precisely we can tell time.
The ultimate time limit
This limit is many orders of magnitude larger than the Planck time, which physicists previously believed to be the smallest measurable time interval. “The ultimate fuzziness of time may not require extreme quantum gravity, but can arise from more accessible physics,” says Curceanu, who co-authored the paper.
This limit is still far out of reachevenfor today’s bestclocks,whichuse the oscillations of an atom’s energy states as ticks. Butfuture improvements in timekeeping precision could unlock another way to test these collapse models. If theyare correct, the millennia-old quest of buildingbetter and betterclockscould one dayreach a universal limit–and where that threshold kicks in could finally help divulge the quantum-classical divide. Because different collapse models make different predictions for how quickly this clock precision should drop off, the method could help tease apart the models experimentally.
“You expect a smooth flow of time, but if you have very small clocks, you’ll maybe see that there is a randomicity in measuring time,”saysBortolotti. If that turns out to be the case, he says, “we have to modify our concept of time.”
Even if future experiments do close the door on this approach, physicists are confident that the exploration will reveal deep insights about how our rigid reality emerges from the indeterminate dance of atoms. “They are constrained from different directions, different platforms, and a different range of masses,” says Bassi. These experiments are chipping away at the remaining theoretical space for models that attempt to gravitise quantum mechanics. “Either they together shrink it to zero, and that’s the end. Or they will find something.”
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