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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.

A heatwave in May set monthly temperature records across Europe; a heatwave in June became the hottest ever observed in western Europe. Now, in July, yet another heatwave is developing. Just 50 years ago, the June heatwave would have been virtually impossible. But global warming is making heatwaves more frequent, longer and more intense.

Worldwide, heat is the deadliest type of weather, killing more than each year. The number will increase, since even if we reached net zero tomorrow, the carbon dioxide we have already emitted will keep raising temperatures.

“This is just the start,” says at University College London. “Things are unfolding in a very, very major way now, because this isn’t just about it [being] too hot in London, and the long-term effects are going to be savage.”

Outside the tropics, the time of the year in which temperatures above 32°C occur has lengthened by 12 days in the past half-century. In Europe, the fastest-warming continent, the season of strong heat stress now starts on average in June and continues until almost September. Sometimes, like this year, it starts in May.

That increases people’s exposure to hot days and heatwaves. Parts of North America, Europe, South America and Africa now experience up to 50 more days of strong heat stress compared with the 1970s.

“If you’ve got heatwaves that last longer, and then you’ve got more heatwaves, people are going to be in that raised physiological state for longer,” says Neil Maxwell at the University of Brighton, UK. “That can lead to greater inflammatory marker responses, and that ultimately puts a greater stress upon individuals.”

Strong heat stress almost never occurred at night before 1998. But now, nighttime temperatures in western Europe and other places are increasing at of global warming as a whole.

A drop in body temperature triggers sleep. If the environment is too hot, it is harder to fall asleep, as well as to enter a state of deep sleep. And loss of sleep over several nights in a row can hinder reaction time and boost anxiety and stress.

“If you don’t get cooling periods at night, which we define in this country as less than 20°C at night, sustained temperatures without cooling have worse impacts,” says Montgomery.

The hottest summer ever seen led to apocalyptic scenes in Europe in 2022. Wildfires broke out in France, Portugal and Spain. Italy’s longest river, the Po, ran dry in places, and wrecks of Nazi ships full of explosives were discovered as the Danube fell to record lows. In the UK, temperatures exceeded 40°C (104°F) for the first time.

More than 60,000 people died because of these baking temperatures. The highest mortality rates were in Mediterranean countries, which had some of the biggest temperature anomalies, with temperatures reaching higher than 40°C in Italy, Greece and Spain. These countries also have some of the , whose bodies aren’t as resilient to heat and who are more likely to have chronic illnesses.

“You also get inflammatory responses from heat, so heat exposure in itself triggers all sorts of bad biology in your body, basically, that is directly harmful… and in particular in people with diseases,” says Montgomery.

The frequency of a heat stress day followed by a tropical night of at least 20°C has increased 73 per cent in Europe since the 1970s. These are called “compound events” because the body isn’t able to cool down and recover at night, compounding the heat stress.

Europe has also seen prolonged periods of heat stress become more common. And Africa is now almost three times more likely to suffer hot spells lasting three-quarters of the year or more.

Leaders like US President Donald Trump have made pledges to plant millions of trees while increasing CO2 emissions. But in the case of urban heat, trees can make a big difference. They create areas of shade, and they also draw moisture from the soil, which then evaporates from their leaves, cooling the environment. Neighbourhoods with tree canopies can be as much as than similar places.

But although many cities have started planting trees to deal with heat, a recent found that many still have swathes of territory below the 30 per cent canopy cover that can reduce dangerous heat island effects. More than 90 per cent of the buildings in Paris and London fall below this threshold.