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Physics is littered with the remains of theories that once seemed destined to explain everything. A theory can dominate for decades, even centuries, only to be wiped out when new experimental results arrive or a fitter idea comes around. Before long, they’re relegated to little more than fossils in the scientific record.

Thermodynamics, though, is like the crocodile of physics – an unchanging ancient beast that has survived the field’s great mass-extinction events: the arrival of quantum mechanics, the fusion of space and time into one fabric, the discovery of the expanding universe. Thermodynamics has persisted through them all. Even Albert Einstein believed it would outlast quantum theory and general relativity, two fields he helped establish.

Now, it may finally have met its real test: an extremal black hole, a theoretical variety of cosmic behemoth that sits at the limits of the theory of general relativity. One of the basic laws of black hole thermodynamics suggests that nature could never make such a beast. But recent work has shown that nature may have found a loophole.

To see why that would be such a big deal, we need to return to the early 1970s, when our modern understanding of black holes was only beginning to take shape. Jacob Bekenstein, then a graduate student at Princeton University, pointed out that matter falling into a black hole seemed to disappear from the observable universe, taking its entropy with it.

Entropy tells us how many microscopic arrangements can produce the same observable state. Imagine a glass of water sitting on a table. To us, it has an obvious temperature, volume and pressure, but the molecules within can be arranged and moving in an immense number of different ways while the glass, overall, looks exactly the same. It’s worth mentioning that entropy is a very powerful tool – counting these arrangements can help us unpick what the microscopic components of a system are. According to the second law of thermodynamics, the total entropy of an isolated system always increases over time. So what happens when something falls into a black hole, crossing a threshold beyond which nothing can ever return? Is it just gone from the account of entropy in the cosmos? Certainly not without breaking the laws of physics. So, Bekenstein proposed a radical solution: black holes must possess entropy of their own.

Physicist Stephen Hawking initially objected. If a black hole had entropy, thermodynamics implied that it must also have a temperature – and anything with a temperature should radiate. That was awkward, given that the defining feature of a black hole was that nothing escaped it. But then physicists, including Hawking, crunched the numbers and saw that once they included quantum mechanics, black holes did emit a faint glow, now known as Hawking radiation. They had a temperature after all.

And when doing the maths, physicists noticed a striking resemblance between the laws that governed black holes and the laws of thermodynamics, which concern the transformation of work, entropy and energy in engines. In fact, there were rock-solid correspondences between the behaviour of black holes and the first two laws of thermodynamics, which state that energy cannot be created or destroyed and that entropy can never decrease in a closed system.

Physicists had little reason to think the third law would be any different. In ordinary thermodynamics, the third law says you can never cool a system all the way to absolute zero. The colder it gets, the harder it becomes to remove the last scraps of heat, until reaching zero would take an impossible amount of time or effort.

Black holes were thought to play by the same rule. A black hole is characterised by just three numbers: its mass, electric charge and spin. From those, you can work out the area of its event horizon, its temperature and its entropy. And, in line with the third law, it seemed that a black hole could never quite reach zero temperature, or stop emitting Hawking radiation altogether.

But in 2024, at the Massachusetts Institute of Technology and at the University of California, Berkeley, , as they’re known, could indeed exist. So, the third law of black hole physics may not be much of a law after all.

“Extremal black holes were thought of as an idealised, unattainable limit: something you could write down as a solution, but which could never be reached [through any real physical] process, but we proved that wrong,” says Kehle.

The broken third law

We didn’t think this third law for black holes was true just because it mirrored thermodynamics. It also seemed to keep the rest of physics safe.

Charged and rotating black holes can have two horizons. The outer one is the familiar event horizon, the point beyond which nothing can return. Deeper inside lies the Cauchy horizon. Cross that and general relativity stops telling you what happens next: even perfect knowledge of the past would no longer be enough to predict the future.

Mathematical physicist Roger Penrose thought the Cauchy horizon would always stay safely inside the event horizon, keeping the singularity at the core of the black hole hidden from the rest of the universe. He called this idea cosmic censorship: a kind of built-in protection mechanism that stops us witnessing a place where the laws of physics have broken down – a so-called naked singularity.

But the black hole’s gravitational pull is counteracted by its charge and rotation. Increase either of these properties and the Cauchy horizon expands while the event horizon contracts. Eventually, the two coincide at a limit known as extremality. The gravitational intensity at the event horizon – a quantity known as its surface gravity – then falls to zero. Because a black hole’s Hawking temperature is proportional to this surface gravity, an extremal black hole has zero temperature and emits no thermal Hawking radiation.

Extremal black holes ride on the edge of this cosmic danger zone – right as the Cauchy horizon threatens to eclipse the event horizon and reveal a naked singularity – and were thought to be impossible. But Kehle and Unger found two ways around this.

Their models showed that an extremal black hole could form if you physically added charged matter to a black hole gradually, or fired a beam of charged particles into a region of empty space and caused it to collapse into an extremal black hole directly. Crucially, these mechanisms didn’t risk creating a naked singularity.

“The fact [that this work proves] extremal black holes can form classically tells us that these objects are not just mathematical limits. They are allowed within classical gravity,” says at the Free University of Brussels in Belgium. “Whether quantum mechanics allows for their formation remains, however, an open issue.”

What is extremality good for?

That matters because these strange objects may provide a bridge between black holes in the messy, evolving universe and the highly idealised ones that have long appeared in string theory and quantum mechanics.

For decades, physicists have wanted to know what black holes are made of. Their entropy suggests that there must be many possible arrangements of microstates, corresponding to the same black hole. But what, exactly, is being rearranged?

That question is one reason theoretical physicists such as Toldo have been interested in extremal black holes for some time. Much of their appeal comes from their zero temperature, which would usually mean zero entropy. But despite having zero temperature, extremal black holes can still possess entropy. That makes them unusually clean models for exploring what a black hole’s microstates might actually be.

“[String theory] gives you powerful techniques with which you can count the states inside a black hole,” says Toldo. “You can try to start figuring out what the states inside the black hole are” and what they’re made of. But until now, much of this work dealt with extremal black holes as idealised mathematical models. Showing that one can form dynamically brings those ideas a step closer to the kinds of objects that might genuinely arise in our universe.

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There may even be a way to spot one. As matter tumbles towards a black hole, it can be squeezed and heated so violently that it gives off a burst of radiation. This is nothing like Hawking radiation: it comes from the infalling material itself. Around an ordinary black hole, the resulting disturbance soon dies away. But an extremal horizon lacks the usual damping effect, allowing part of it to linger and even grow. Last year, researchers should leave a distinctive fingerprint in the faint tail of radiation travelling out to distant observers.

But actually detecting an extremal black hole remains a distant prospect. For now, the more immediate implications are theoretical. If the third law of black hole physics can be broken, then the analogy between black holes and thermodynamic systems may not be as exact as we thought.

Kehle and Unger’s work has exposed more than a gap in our understanding of black holes. It raises the more unsettling possibility that we don’t fully understand thermodynamics either.

“This work made me realise that I understand black holes better than I understand classical thermodynamics,” says Kehle. “For example, when the third law says you can’t cool a system to absolute zero with a ‘finite sequence of operations’, what actually counts as an operation? There could be something more fundamental lurking there that we don’t know about.”

“We’ve disproved the third law as it was written, but we don’t yet understand this problem in any sense. There’s still a lot to be done,” says Unger.

Quantum mechanics and general relativity don’t fit together, and a big part of the issue comes down to gravity. For decades, the accepted route to an ultimate theory of everything has involved taking our best theory of gravity and squeezing it into the frame of quantum mechanics. Yet, almost a century later, scientists still haven’t managed to make gravity fit. Ivette Fuentes is a professor of quantum mechanics who conducts experiments at the scales where quantum theory and general relativity interplay. Fuentes sat down with New Scientist features editor Thomas Lewton to discuss the issues and fascinating theories that pop out when we try to fit classical and quantum mechanics together.

Read more: The experiments that could finally explain gravity

What really happens in the quantum world? In this conversation, physicist Sean Carroll explores some of the deepest mysteries in quantum mechanics: the famous double-slit experiment, wave function collapse, the Many Worlds interpretation, entropy and the arrow of time. Speaking to New Scientist reporter Jacklin Kwan, Carroll discusses why electrons appear to behave like waves, how observation seems to affect reality and whether the universe constantly branches into countless parallel worlds. Carroll also explains the measurement problem, the challenges of interpreting quantum theory and why physicists still debate what quantum mechanics is actually telling us about the nature of reality. Carroll is a theoretical physicist, cosmologist and author whose work focuses on the foundations of physics, quantum mechanics, cosmology and the nature of time.

Read more: Why quantum physics says there’s a multiverse

In December 2019, a bad tooth almost killed me. A terrible toothache turned into the biggest health crisis of my life, ultimately landing me in an intensive care unit for a week. Once I recovered, I had to make sense of why this happened to me. Personal negligence? Terrible luck? A fault of the US healthcare system? Rattled and unsure of how to feel, I turned to a place where I had long found answers to existential questions – quantum physics.

Physics is often considered to be humanity’s oldest science, getting its start with early astronomers. Much of our understanding of the world is built on physics as a solid, rigorous, objective foundation. It is a science that breaks the world into pieces, analyses each of them, then reassembles them all into a whole that we understand better. This process, based on empiricism and mathematics, doesn’t care about feelings. Physics isn’t personal: for example, regardless of who you are, you cannot escape a black hole. Yet I have always taken physics extremely personally.

In my book, , I invite the reader to do the same and, by showing how this benefited me, argue that making the objective subjective can be life-changing.

Take my bad tooth. Once the crisis was over, the issue that kept me up at night was essentially one of cause and effect. The effect was that I nearly died in the ICU, that much is clear, but what was the cause? In trying to process the event, I came up with several conflicting options. It was completely my fault because I dislike going to the dentist. It wasn’t my fault at all because I was a graduate student and couldn’t afford the dentist anyway. Trying to reconcile these two sequences of cause and effect only disturbed me further.

Relief unexpectedly came to me through chatting with physicists who study causality in the quantum realm. Reporting for New Scientist, I learned about the “quantum switch”, a procedure that allows a system to exhibit indefinite causality, where different sequences of cause and effect could exist at the same time through the quantum phenomenon of superposition. The idea is not without its critics, but experiments with particles of light have added credence to it. Some researchers have taken it as far as suggesting that the quantum switch should be built into emerging quantum technologies like quantum computers and batteries to make them work better.

As a physicist, I understand that I have very little in common with a��particle of light. Being macroscopic and warm, I am unaffected by the laws of quantum physics, while the photon can’t escape its quantum nature. Yet, thinking about the photon in the quantum switch, with its behaviour simultaneously dictated by “A causes B” and “B causes A” in a way that seems forbidden in every other arena, lessened my tooth conundrum.

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Maybe here, too, several conflicting ideas could be true at the same time. This brought me some peace and informed my future decision-making. I go to the dentist more now, and I believe that improving the working conditions of graduate students in the US, for example by providing them with dental insurance, is urgent and crucial.

In Entangled States, I describe a dozen examples like this, instances of quantum physics offering me guidance for something I couldn’t understand about my life and the world, or at least nudging me into thinking about it differently. I write about reckoning with my queerness, my experience of being a young immigrant, the way I build relationships, the way I used to teach high school students and much more, all in conversation with what I have learned about quantum physics both as a scholar and a reporter.

Being immersed in cutting-edge science, reporting from the border where human knowledge touches the unknown – which is exactly where quantum physics shines – undoubtedly changed me. Embracing its influence in an idiosyncratic and emotional way that complements the objective rigour of the science itself has improved my life and made me a better person. I highly recommend it. Instead of thinking about all things quantum as absolutely abstract and odd, do consider sometimes taking them personally.

On Q-Day, your privacy will be at stake. This is the moment when quantum computers break the encryption protecting the modern world, bank transactions become readable, private messages get exposed and even state secrets become vulnerable.

For years it sounded like sci-fi, something that was decades away from happening, if it happened at all. But now, research suggests that we may be hurtling towards Q-Day at a rapid speed.

In this video, New Scientist uncovers why many experts think the countdown to Q-Day may already have begun, and explains how quantum computers work and why these machines could both threaten the security of the modern world and unlock breakthroughs that could change our lives. Special thanks to Quantum Motion for letting us film at its facilities.

A large, cold molecule that resembles a butterfly, with “wings” made from electrons, has been made for the first time, completing the search for a “zoo” of similar molecules. The result could provide a gateway to completely new parts of the quantum realm.

at RPTU University Kaiserslautern-Landau in Germany and his colleagues made the molecule by cooling rubidium atoms to a few millionths of a degree above absolute zero by using lasers and electromagnetic forces. The researchers then used lasers again to make some atoms very large by pushing their outermost electron very far from their nuclei. The quantum properties of atoms that have been cooled and enlarged in this way can be precisely manipulated with lasers, which the team leveraged to move a giant atom’s electron towards a normal-sized rubidium atom, binding them together to create a new type of molecule with extreme properties.

Each molecule was about 25 nanometres in size – bigger than the diameter of a DNA strand that contains billions of atoms –��and thousands of times more responsive to electric fields than most molecules. The shape of the new molecule was determined by its electrons, with the outermost being spread out in space in a shape that resembles the wings of a butterfly.

Ott says that tuning the lasers just right to get molecules with this exact configuration was tricky – it took weeks of tweaking before the team successfully created them in the lab. Ott compares the process to searching for an object on a road by inspecting 1 millimetre at a time while standing a kilometre away.

Team member at Purdue University in Indiana says the work is a culmination of several past mathematical and experimental studies that helped narrow down this search. Based on mathematical models, researchers have spent 20 years searching for a “zoo” of giant ultracold molecules ought to exist, and the new butterfly is the last one to be discovered, he says.

At the same time, this experiment opens a path towards creating more exotic and elusive ultracold molecules, including those that could be very heavy in addition to being very large and charged, says at the University of Warsaw in Poland. at the University of Nottingham in the UK says the butterfly molecule was difficult to create but could now be used as precursor for making something even more difficult – ultracold atoms with negative charge, or anions. If made ultracold, anions could be used in tests of fundamental laws of particle physics or antimatter studies, but standard cooling methods have failed to chill them so far.

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Eiles and Ott have already mathematically investigated exactly how to use the butterfly molecule to create ultracold anions and hope to see the first signs of them within just a few years. “The theory is already written,” says Eiles.