A strange area around black holes called the 鈥減lunging region鈥� has been spotted for the first time. This area, where matter stops circling a black hole and instead falls straight in, was predicted by Albert Einstein鈥檚 general theory of relativity, but it has never been observed before. Studying plunging regions could teach us about how black holes form and evolve, as well as reveal new information about the fundamental nature of space-time.

When any matter gets too close to a black hole, it rips apart and forms an orbiting ring around it called an accretion disc. General relativity predicts there should be an inner boundary to the accretion disc past which nothing can orbit the black hole 鈥� instead, it should plunge straight in, rapidly accelerating to near the speed of light as it falls.

Merging black holes may create bubbles that could swallow the universe

The area between a pair of large black holes on the verge of colliding could provide the conditions to create dangerous bubbles of "true vacuum"

鈥淚t鈥檚 like a river turning into a waterfall, and until now we鈥檝e only been looking at the river,鈥� says at the University of Oxford. 鈥淚f Einstein was wrong, then it would be stable all the way down 鈥� there would only be a river.鈥� Now we鈥檝e gotten our first peek at the waterfall, suggesting Einstein was correct.

Mummery and his colleagues spotted evidence of the plunging region around a black hole in a binary system called MAXI J1820+070, which is about 10,000 light years from Earth. They used data from the Nuclear Spectroscopic Telescope Array (NuSTAR), a space-based X-ray telescope, to build models of the light from the black hole鈥檚 accretion disc.

They found the models only fit the data when they included the light emitted by matter in the plunging region in addition to light from the accretion disc. 鈥淏efore, we sort of thought that anything that crosses this boundary would have no time to really radiate appreciably before it plunges into the black hole鈥�, so researchers wouldn鈥檛 see anything, says at Los Alamos National Laboratory in New Mexico, who was not involved with this work. 鈥淏ut it turns out that this plunging region gives you extra light that you wouldn鈥檛 have expected.鈥�

This extra light could solve a long-standing problem in X-ray astronomy, in which black holes appear to be spinning faster than theory predicts. The spin of a black hole and the brightness of the area around it are connected, so adding some extra light could bring the spins back in line with predictions. 鈥淏lack hole spins tell us about all kinds of things, so if we could measure it better, we could answer loads of questions in astrophysics,鈥� says Salvesen.

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That includes questions about the nature of gravity and space-time itself, because plunging regions are some of the most extreme regions of space we can observe. The plunging region is just outside the event horizon, beyond which the gravitational forces are so strong, no matter or even light can escape.

鈥淭echnically, if the matter had a rocket it could escape the plunging region, but it鈥檚 doomed 鈥� its orbit has become unstable and it鈥檚 rapidly accelerating toward the speed of light,鈥� says Mummery. 鈥淭his stuff has about as much chance of coming back as water off the edge of a waterfall.鈥� The researchers are now trying to make more observations of these strange cosmic waterfalls to illuminate the conditions in these extraordinary areas.

We finally know why Stephen Hawking鈥檚 black hole equation works

Stephen Hawking and Jacob Bekenstein calculated the entropy of a black hole in the 1970s, but it took physicists until now to figure out the quantum effects that make the formula work

A pair of black holes have been seen wobbling at a rate of three times per second as they merged, in an extreme example of a prediction made by Albert Einstein鈥檚听general theory of relativity that has been seen clearly for the first time.

This wobbling, known as precession, occurs when the orbit or rotation of an object slowly changes with time 鈥� a common example is when a spinning top begins to spin at a different angle as it slows down. Gravity-induced orbital precession, a consequence of general relativity鈥檚 prediction that heavy objects bend space-time, sees the shape of such an object鈥檚 orbit change over time.

This effect had been observed very weakly in neutron stars orbiting one another, but was so subtle that the orbits only wobbled, or precessed, at a rate of a few times a year.

Now, at Cardiff University, UK, and his colleagues have seen a much more extreme effect in a pair of black holes moving at a fifth of the speed of light, caused by one of them spinning at a 90-degree angle to its orbital motion. As they merged, the black holes released a gravitational wave, known as GW200129, that carried the signature of precession at a rate of three times a second.

鈥淚t鈥檚 10 billion times faster than what was found in earlier measurements, so it really is the most extreme regime of Einstein鈥檚 theory where space and time are warped and distorted in completely crazy ways,鈥� says Hannam.

To identify the precession, the team reanalysed data first collected in 2020 by three gravitational wave detectors, based in the US and Italy. A previous analysis was inconclusive, but using a more advanced model of the gravitational wave signal, Hannam and his team found that the best way to explain the signal was with one of the black holes, spinning at almost the upper limit allowed by general relativity, causing the orbit of the system to precess.

鈥淭he astrophysical implications of the detection are quite significant,鈥� says at Cardiff University, who wasn鈥檛 involved with the work. The extreme spin, and misalignment with its orbit, isn鈥檛 predicted by current ideas of black hole formation, which involve imploding stars, and needs another explanation, he says.

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Einstein鈥檚 famous theory of general relativity has passed its most stringent test to date. After 16 years of observing a pair of pulsars 鈥� highly compact neutron stars that emit beams of radio waves from their poles 鈥� researchers saw relativistic effects that have previously only been predicted by the theory.

General relativity describes the effect of gravity on space-time. 鈥淚f we have something that鈥檚 really massive, it鈥檒l warp the space-time around it to a greater extent than something that鈥檚 less massive,鈥� says at the University of East Anglia in the UK.

This means for extremely massive bodies, such as neutron stars, general relativity predicts that light will be bent markedly around them as photons follow the warped path of space-time. What鈥檚 more, when neutron stars accelerate, which can happen if two of them are spiralling around one another, they will emit gravitational waves 鈥� ripples in space-time 鈥� that will cause their orbits to shrink as they lose energy. This is known as orbital decay.

Now, Ferdman and his colleagues have observed both of these theoretical predictions on two pulsars, known as PSR J0737鈭�3039A/B. These orbit each other in just 147 minutes at speeds of up to 1 million kilometres per hour. As they orbit, one of the pulsars is simultaneously rotating around its axis about 44 times every second, while its companion rotates once every 2.8 seconds. Every time each pulsar rotates, we receive a blast of radio beams on Earth.

鈥淭hose pulses will arrive extremely regularly unless there鈥檚 something in the way or some astrophysical phenomenon that鈥檚 causing a delay in those arrivals,鈥� says Ferdman.

Using seven radio telescopes in Australia, the US and Europe, the researchers monitored the double pulsar system between its discovery in 2003 until 2019.

They found that the radio pulses consistently arrived later than expected, and calculated this was because they were being deflected at an angle of 0.04 degrees due to the strong space-time curvature around the two stars. This is the first experimental evidence of such a high curvature, according to Ferdman鈥檚 team.

The researchers also found that the pulsars underwent orbital decay due to the emission of gravitational waves. They could detect the energy carried by these waves with a precision 1000 times better than is possible with direct gravitational wave detectors on Earth, such as the LIGO detectors in the US.

鈥淭his is the most stringent test to date of Einstein鈥檚 theory, and it sets the bar to which future experiments must [operate] in terms of precision in order to put general relativity to the test with any significance,鈥� says Ferdman.

鈥淸General relativity] is the best theory we have of gravity, but we know because of the incompatibility with quantum mechanics and the standard model [of particle physics], it鈥檚 not the final word,鈥� he says.

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Hello, and welcome to November鈥檚 Lost in Space-Time, the monthly physics newsletter that unpicks the fabric of the universe and attempts to stitch it back together in a slightly different way. To receive this free, monthly newsletter in your inbox, sign up here.

Einstein鈥檚 forgotten twisted universe

There鈥檚 a kind of inevitability about the fact that, if you write a regular newsletter about fundamental physics, you鈥檒l regularly find yourself banging on about Albert Einstein. As much as it comes with the job, I also make no apology for it: he is a towering figure in the history of not just fundamental physics, but science generally.

A point that historians of science sometimes make about his most monumental achievement, the general theory of relativity, is that, pretty much uniquely, it was a theory that didn鈥檛 have to be. When you look at the origins of something like Charles Darwin鈥檚 theory of evolution by natural selection, for example 鈥� not to diminish his magisterial accomplishment in any way 鈥� you鈥檒l find that other people had been scratching around similar ideas surrounding the origin and change of species for some time as a response to the burgeoning fossil record, among other discoveries.

Even Einstein鈥檚 special relativity, the precursor to general relativity that first introduced the idea of warping space and time, responded to a clear need (first distinctly identified with the advent of James Clerk Maxwell鈥檚 laws of electromagnetism in the 1860s) to explain why the speed of light appeared to be an absolute constant.

When Einstein presented general relativity to the world in 1915, there was nothing like that. We had a perfectly good working theory of gravity, the one developed by Isaac Newton more than two centuries earlier. True, there was a tiny problem in that it couldn鈥檛 explain some small wobbles in the orbit of Mercury, but they weren鈥檛 of the size that demanded we tear up our whole understanding of space, time, matter and the relationship between them. But pretty much everything we know (and don鈥檛 know) about the wider universe today stems from general relativity: the expanding big bang universe and the standard model of cosmology, dark matter and energy, black holes, gravitational waves, you name it.

So why听am听I banging on about this? Principally because, boy, do we need a new idea in cosmology now 鈥� and in a weird twist of history, it might just be Einstein who supplies it. I鈥檓 talking about an听intriguing feature by astrophysicist Paul M. Sutter in the magazine last month听. It deals with perhaps general relativity鈥檚 greatest (perceived, at least) weakness 鈥� the way it doesn鈥檛 mesh with other bits of physics, which are all explained by quantum theory these days. The mismatch exercised Einstein a great deal, and he spent much of his later years engaged in a fruitless quest to unify all of physics.

Perhaps his most promising attempt came with a twist 鈥� literally 鈥� on general relativity that Einstein played about with early on. By developing a mathematical language not just for how space-time bends (which is the basis of how gravity is created within relativity) but for how it twists, he hoped to create a theory that also explained the electromagnetic force. He succeeded in the first bit, creating a description of how massive, charged objects might twist space-time into mini-cyclones around them. But it didn鈥檛 create a convincing description of electromagnetism, and Einstein quietly dropped the theory.

Well, the really exciting bit, as Sutter describes, is that this 鈥渢eleparallel gravity鈥� seems to be back in a big way. Many cosmologists now think it could be a silver bullet to explain away some of the most mysterious features of today鈥檚 universe, such as the听nature of dark matter听and dark energy and the听troublesome period of faster-than-light inflation听right at the moment of the big bang that is invoked to explain features of today鈥檚 universe, such as its extraordinary smoothness. Not only that, but there could be a way to test the theory soon. I鈥檇 recommend听reading the feature听to get all the details, but in the meantime, it鈥檚 about as exciting a development as you鈥檒l get in cosmology these days.

Is the universe fine-tuned?

Let鈥檚 take just a quick dip into the physics arXiv preprint server, where the latest research is put up. One paper that caught my eye recently has the inviting title . It鈥檚 by Zhi-Wei Wang at the College of Physics in China and Samuel L. Braunstein at the University of York in the UK, and it deals with a question that鈥檚 been bugging a lot of physicists and cosmologists ever since we started making detailed measurements of the universe and developing cogent theories to explain what we see: why does everything in the universe (the strengths of the various forces, the masses of fundamental particles, etc.) seem so perfectly tuned to allow the existence of observers like us to ask the question?

This has tended to take cosmologists and physicists down one of two avenues. The first says things are how they are because that鈥檚 how they鈥檙e made. For some, that sails very close to an argument via intelligent design, aka the existence of god. The other avenue tends to be some form of multiverse argument: our universe is as it is because we are here to observe it (we could hardly be here to observe it if it weren鈥檛), but it is one of a random subset of many possible universes that happen to be conducive to intelligent life arising.

This paper examines more closely a hypothesis from British physicist Dennis Sciama (doctoral supervisor to the stars: among his students in the 1960s and 1970s were听) that if ours were a random universe, there would be a statistical pattern in its fundamental parameters that would give us evidence of that. In this paper, the researchers argue that the logic is actually reversed. In their words: 鈥淲ere our universe random, it could give the false impression of being intelligently designed, with the fundamental constants appearing to be fine-tuned to a strong probability for life to emerge and be maintained.鈥�

Full disclosure 鈥� I鈥檓 writing something on this very subject for New 女生小视频鈥檚 65th-anniversary issue, due out on 20 November. Read more there!

Closing the quantum loopholes

While I鈥檓 banging on about Einstein, I stumbled across one of my favourite features I鈥檝e worked on while at the magazine the other day, and thought it was worth sharing. Called 鈥淩eality check: Closing the quantum loopholes鈥�, it鈥檚 from 2011, a full 10 years ago, but the idea it deals with stretches back way before that 鈥� and is still a very live one.

The basic question is: is quantum theory a true description of reality, or are its various weirdnesses 鈥� not least the 鈥渆ntanglement鈥� of quantum objects over vast distances 鈥� indications of goings-on in an underlying layer of reality not described by quantum theory (or indeed any other theory to date)? I talked about entanglement quite a bit in last month鈥檚 newsletter, so I won鈥檛 go into its workings here.

The alternative idea of 鈥渉idden variables鈥� explaining the workings of the quantum world goes back to a famous paper published by Einstein and two collaborators, Nathan Rosen and Boris Podolsky, back in 1935. It led Einstein into a long-drawn-out debate about the nature of quantum theory with another of its pioneers, Niels Bohr, that continued decorously right until Einstein鈥檚 death in 1955. It wasn鈥檛 until the 1980s that we began to have the theoretical and experimental capabilities to actually pit the two pictures against one another.

I love the story not just for this rich history, but also for the way that, after each iteration of the experiments 鈥� every time showing that quantum theory, and entanglement, are the 鈥渞ight鈥� explanation for what is going on, whatever they might mean 鈥� the physicists found another loophole in the experiments that might allow Einstein鈥檚 hidden variable idea back into the frame again.

That led them to some pretty impressive feats of experimental derring-do to close the loopholes again 鈥� the feature opens with a group of modern physicists shooting single photons between observatories on Tenerife and La Palma in the Canary Islands. In an update to the story that we published in 2018 (with the rather explicit title听鈥淓instein was wrong: Why 鈥榥ormal鈥� physics can鈥檛 explain reality鈥澨�), they even reproduced the result with photons coming at us from galaxies billions of light years away 鈥� proving that, if not the whole universe, then a goodly proportion of it follows quantum rules. You can鈥檛 win 鈥榚m all, Einstein.

Coming up

One reason I鈥檝e been thinking particularly frequently about Einstein and his work lately is that I鈥檝e been putting together the latest听听called 鈥淓instein鈥檚 Universe鈥�. It鈥檚 a survey of his theories of relativity and all those things that came out of it: the big bang universe and the standard model of cosmology, dark matter and energy, gravitational waves, black holes and, of course,听the search for that elusive unifying theory of physics. I鈥檝e just putting the finishing touches to the听Essential Guide听with my left hand as I type this, and I think it鈥檚 a fair expectation that you鈥檒l find me banging on about that (and Einstein) a lot more next month.

Also in New 女生小视频

1. Talking of fine-tuned universes, if you haven鈥檛 done so already, you can still catch up with Brian Clegg鈥檚 New 女生小视频 Event talk,听, from last month, available on demand.

2. If you鈥檙e fan of big ideas (I hope that鈥檚 why you鈥檙e here) and like casting your net a little wider than just physics, then a ticket to our听Big Thinkers series of live events gives you access to 10 talks from top researchers from across the board, including Harvard astronomer Avi Loeb on the search for extraterrestrial life and Michelle Simmons and John Martinis on quantum computing.

3. It happened just after my last newsletter, but it would be remiss not to mention the awarding of this year鈥檚 Nobel prize to three researchers who played a leading role in advancing our understanding of chaotic systems 鈥� notably the climate. You can find out more about what they did听here.

From the big bang onwards you might think we know a lot about the universe鈥檚 first fraction of a second. But that just isn鈥檛 true. Why, for example, didn鈥檛 antimatter annihilate matter? Why haven鈥檛 we observed any dark matter particles yet? How does dark energy accelerate universe expansion? And why do theories of cosmic inflation lead inevitably to the conclusion that there should be an infinite or nearly infinite number of universes in existence?

For Dan Hooper, head of theoretical astrophysics at the Fermi National Accelerator Laboratory, Chicago, solving these questions involves radically rethinking what we think we know about the universe鈥檚 very early history.

Interviewed by Richard Webb, Executive Editor, New 女生小视频 at the Royal Institution, London in Feb 2020.