Scientists identifiy the first evidence of ‘virtual particles’ hopping in and out of existence, which were first predicted in the 1930s

This diagram represents the the light of a neutron star passing through an area of space where vacuum birefringence, a theory of quantum electrodynamics, is occurring. The light’s magnetic and electric fields (red and blue arrows) are altered and aligned as they pass through the empty space near a neutron star, suggesting the intense magnetic field there creates virtual particles that affect the light.

About 400 light-years from here, in the area surrounding a neutron star, the electromagnetic field of this unbelievably dense object appears to be creating an area where matter spontaneously appears and then vanishes.

Quantum electrodynamics (QED) describes the relationships between particles of light, or photons, and electrically charged particles such as electrons and protons. The theories of QED suggest that the universe is full of “virtual particles,” which are not really particles at all. They are fluctuations in quantum fields that have most of the same properties as particles, except they appear and vanish all the time. Scientists predicted the existence of virtual particles some 80 years ago, but we have never had experimental evidence of this process until now.


How can we possibly see such a thing? One of the properties virtual particles have in common with actual particles is that they both affect light. In addition, intense magnetic fields are thought to excite the activity of virtual particles, affecting any light that passes through that space more dramatically.

So a team of astronomers pointed our most advanced ground-based telescope, the European Southern Observatory’s Very Large Telescope (VLT), at one of the densest objects we know of: a neutron star.

Neutron stars have magnetic fields that are billions of times stronger than our sun’s. Using the VLT, Roberto Mignani from the Italian National Institute for Astrophysics (INAF) and his team observed visible light around the neutron star RX J1856.5-3754 and detected linear polarization—or the alignment of light waves according to external electromagnetic influences—in the empty space around the star. This is rather odd, because conventional relativity says that light should pass freely through a vacuum, such as space, without being altered. The linear polarization was to such a degree (16 degrees to be precise) that the only known explanations are theories of QED and the influence of virtual particles.

“According to QED, a highly magnetized vacuum behaves as a prism for the propagation of light, an effect known as vacuum birefringence,” Mignani says. “The high linear polarization that we measured with the VLT can’t be easily explained by our models unless the vacuum birefringence effects predicted by QED are included.”


Vacuum birefringence was first predicted in the 1930s by Werner Heisenberg and Hans Heinrich Euler. It was an exciting time for the development of quantum mechanics, when many of the advanced theories still studied today were developed.

In the quantum realm, matter behaves very strangely to say the least. It violates both Newton’s classical laws of physics and Einstein’s theories of relativity and gravity. Matter can exist in two separate places at once. Entangled particles, separated by miles, can influence each other instantaneously. As far as we can tell, the smallest building blocks of matter exist with multiple, or even infinite properties, known as quantum states, until they are observed or measured.

Fortunately, we can model and even predict some quantum phenomena, and we do this using wave functions. A wave, such as a sine curve, is represented by an equation that has multiple correct values to make it a true mathematical statement. This same basic principle can be applied to physical models of particles that exist in different locations, or with different properties, or sometimes don’t exist at all. When the particles are measured, the wave function collapses, and the matter only exists with one set of properties like you would expect. The researchers were able to measure the virtual particles around a neutron star indirectly, by measuring the light that passes through them.

These concepts are so profound that Einstein and Niels Bohr famously debated, at length, whether the universe even exists as a tangible smattering of matter across the void, or if it is a fluid conglomerate of infinite possible realities until we observe it. The first experimental evidence of vacuum birefringence—absurdly strong electromagnetic forces tugging at the very foundations of matter—reminds us that this is still an open-ended question.

Artificial intelligence replaces physicists

Physicists are putting themselves out of a job, using artificial intelligence to run a complex experiment. The experiment created an extremely cold gas trapped in a laser beam, known as a Bose-Einstein condensate, replicating the experiment that won the 2001 Nobel Prize.

Physicists are putting themselves out of a job, using artificial intelligence to run a complex experiment.

The experiment, developed by physicists from The Australian National University (ANU) and UNSW ADFA, created an extremely cold gas trapped in a laser beam, known as a Bose-Einstein condensate, replicating the experiment that won the 2001 Nobel Prize.

“I didn’t expect the machine could learn to do the experiment itself, from scratch, in under an hour,” said co-lead researcher Paul Wigley from the ANU Research School of Physics and Engineering.

“A simple computer program would have taken longer than the age of the Universe to run through all the combinations and work this out.”

Bose-Einstein condensates are some of the coldest places in the Universe, far colder than outer space, typically less than a billionth of a degree above absolute zero.

They could be used for mineral exploration or navigation systems as they are extremely sensitive to external disturbances, which allows them to make very precise measurements such as tiny changes in the Earth’s magnetic field or gravity.

The artificial intelligence system’s ability to set itself up quickly every morning and compensate for any overnight fluctuations would make this fragile technology much more useful for field measurements, said co-lead researcher Dr Michael Hush from UNSW ADFA.

“You could make a working device to measure gravity that you could take in the back of a car, and the artificial intelligence would recalibrate and fix itself no matter what,” he said.

“It’s cheaper than taking a physicist everywhere with you.”

The team cooled the gas to around 1 microkelvin, and then handed control of the three laser beams over to the artificial intelligence to cool the trapped gas down to nanokelvin.

Researchers were surprised by the methods the system came up with to ramp down the power of the lasers.

“It did things a person wouldn’t guess, such as changing one laser’s power up and down, and compensating with another,” said Mr Wigley.

“It may be able to come up with complicated ways humans haven’t thought of to get experiments colder and make measurements more precise.

The new technique will lead to bigger and better experiments, said Dr Hush.

“Next we plan to employ the artificial intelligence to build an even larger Bose-Einstein condensate faster than we’ve seen ever before,” he said.

The research is published in the Nature group journal Scientific Reports.

Dead or Alive, Schrödinger’s Cat Can Be in 2 Boxes at Once, New Research Using Light Particles Reveals


By Tia Ghose

Bizarrely behaving light particles show that the famous Schrödinger’s cat thought experiment, meant to reveal the strange nature of subatomic particles, can get even weirder than physicists thought.

Not only can the quantum cat be alive and dead at the same time — but it can also be in two places at once, new research shows.

“We are showing an analogy to Schrödinger’s cat that is made out of an electromagnetic field that is confined in two cavities,” said study lead author Chen Wang, a physicist at Yale University. “The interesting thing here is the cat is in two boxes at once.”

The findings could have implications for cracking unsolvable mathematicalproblems using quantum computing, which relies on the ability of subatomic particles to be in multiple states at once, Wang said.

Cat experiment

The famous paradox was laid out by physicist Erwin Schrödinger in 1935 to elucidate the notion of quantum superposition, the phenomenon in which tiny subatomic particles can be in multiple states at once.

In the paradox, a cat is trapped in a box with a deadly radioactive atom. If the radioactive atom decayed, the cat was a goner, but if it had not yet decayed, the cat was still alive. Because, according to the dominant interpretation of quantum mechanics, particles can exist in multiple states until they are measured, logic dictated that the cat would be both alive and dead at the same time until the radioactive atom was measured.

Cat in two boxes

The setup for the new study was deceptively simple: The team created two aluminum cavities about 1 inch (2.5 centimeters) across, and then used a sapphire chip to produce a standing wave of light in those cavities. They used a special electronic element, called a Josephson Junction, to superimpose a standing wave of two separate wavelengths of light in each cavity. The end result was that the cat, or the group of about 80 photons in the cavities, was oscillating at two different wavelengths at once — in two different places. Figuring out whether the cat is dead or alive, so to speak, requires opening both boxes.

Though conceptually simple, the physical setup required ultrapure aluminum and highly precise chips and electromagnetic devices to ensure that the photons were as isolated from the environment as possible, Wang said.

That’s because at large scales, quantum superposition tends to disappear almost instantaneously, as soon as these superimposed subatomic particles whose fates are linked interact with the environment. Most of the time, this so-called decoherence would happen so quickly that researchers would have no time to observe the superposition, Wang said. So devices that keep coherence (or keep the particles in superposition) for long periods of time, known as the quality factor, is extremely important, Wang added.

“The quality of these things determines once you put a single excitation into the system, how long does it live, or does it die away,” Wang told Live Science.

If the excitation of the system — the production of the electromagnetic standing wave — is similar to the swing of a pendulum, then “our pendulum swings essentially tens of billions of times before it stops.”

The new findings could make for easier error correction in quantum computing, Wang said. In quantum computing, bits of information are encoded in the fragile superposition states of particles, and once that superposition is lost or corrupted, the data is also corrupted. So most quantum computing concepts involve a lot of redundancy.

“It’s well understood that 99 percent of computation or more will be done to correct for errors, rather than computation itself,” Wang said.

Their system could conceivably get around this problem by encoding the redundancy in the size of the cavity itself rather than in separate, calculated bits, Wang said.

“Demonstrating this cat in a ‘two boxes state’ is basically the first step in our architecture,” Wang said.

See more at:

Watch A Bowling Ball And Feather Falling In A Vacuum

Here is the perfect example of how any two objects will fall at the same rate in a vacuum, brought to us by physicist Brian Cox. He checked out NASA’s Space Simulation Chamber located at the Space Power Facility in Ohio. With a volume of 22,653 cubic meters, it’s the largest vacuum chamber in the world.

In this clip from the BBC, Cox drops a bowling ball and a feather together, first in normal conditions, and then after virtually all the air has been sucked out of the chamber. We know what happens, but that doesn’t stop it from being awesome, especially with the team’s ecstatic faces.

Thanks to Kebmodee for bringing this to the attention of the It’s Interesting community.

Scientists propose existence and interaction of parallel worlds: Many Interacting Worlds theory challenges foundations of quantum science

Academics are challenging the foundations of quantum science with a radical new theory on parallel universes. Scientists now propose that parallel universes really exist, and that they interact. They show that such an interaction could explain everything that is bizarre about quantum mechanics.

Griffith University academics are challenging the foundations of quantum science with a radical new theory based on the existence of, and interactions between, parallel universes.

In a paper published in the journal Physical Review X, Professor Howard Wiseman and Dr Michael Hall from Griffith’s Centre for Quantum Dynamics, and Dr Dirk-Andre Deckert from the University of California, take interacting parallel worlds out of the realm of science fiction and into that of hard science.
The team proposes that parallel universes really exist, and that they interact. That is, rather than evolving independently, nearby worlds influence one another by a subtle force of repulsion. They show that such an interaction could explain everything that is bizarre about quantum mechanics.

Quantum theory is needed to explain how the universe works at the microscopic scale, and is believed to apply to all matter. But it is notoriously difficult to fathom, exhibiting weird phenomena which seem to violate the laws of cause and effect.

As the eminent American theoretical physicist Richard Feynman once noted: “I think I can safely say that nobody understands quantum mechanics.”

However, the “Many-Interacting Worlds” approach developed at Griffith University provides a new and daring perspective on this baffling field.

“The idea of parallel universes in quantum mechanics has been around since 1957,” says Professor Wiseman.

“In the well-known “Many-Worlds Interpretation,” each universe branches into a bunch of new universes every time a quantum measurement is made. All possibilities are therefore realised — in some universes the dinosaur-killing asteroid missed Earth. In others, Australia was colonised by the Portuguese.

“But critics question the reality of these other universes, since they do not influence our universe at all. On this score, our “Many Interacting Worlds” approach is completely different, as its name implies.”

Professor Wiseman and his colleagues propose that:

•The universe we experience is just one of a gigantic number of worlds. Some are almost identical to ours while most are very different;
•All of these worlds are equally real, exist continuously through time, and possess precisely defined properties;
•All quantum phenomena arise from a universal force of repulsion between ‘nearby’ (i.e. similar) worlds which tends to make them more dissimilar.
Dr Hall says the “Many-Interacting Worlds” theory may even create the extraordinary possibility of testing for the existence of other worlds.

“The beauty of our approach is that if there is just one world our theory reduces to Newtonian mechanics, while if there is a gigantic number of worlds it reproduces quantum mechanics,” he says.

“In between it predicts something new that is neither Newton’s theory nor quantum theory.

“We also believe that, in providing a new mental picture of quantum effects, it will be useful in planning experiments to test and exploit quantum phenomena.”

The ability to approximate quantum evolution using a finite number of worlds could have significant ramifications in molecular dynamics, which is important for understanding chemical reactions and the action of drugs.

Professor Bill Poirier, Distinguished Professor of Chemistry at Texas Tech University, has observed: “These are great ideas, not only conceptually, but also with regard to the new numerical breakthroughs they are almost certain to engender.”

Journal Reference:

1.Michael J. W. Hall, Dirk-André Deckert, Howard M. Wiseman. Quantum Phenomena Modeled by Interactions between Many Classical Worlds. Physical Review X, 2014; 4 (4) DOI: 10.1103/PhysRevX.4.041013

Physicists discover the Majorna Particle, originally predicted in 1937, which is simultaneously matter and anti-matter

Since the 1930s scientists have been searching for particles that are simultaneously matter and antimatter. Now physicists have found strong evidence for one such entity inside a superconducting material. The discovery could represent the first so-called Majorana particle, and may help researchers encode information for quantum computers.

Physicists think that every particle of matter has an antimatter counterpart with equal mass but opposite charge. When matter meets its antimatter equivalent, the two annihilate one another. But some particles might be their own antimatter partners, according to a 1937 prediction by Italian physicist Ettore Majorana. For the first time researchers say they have imaged one of these Majorana particles, and report their findings in the October 3 Science.

The new Majorana particle showed up inside a superconductor, a material in which the free movement of electrons allows electricity to flow without resistance. The research team, led by Ali Yazdani of Princeton University, placed a long chain of iron atoms, which are magnetic, on top of a superconductor made of lead. Normally, magnetism disrupts superconductors, which depend on a lack of magnetic fields for their electrons to flow unimpeded. But in this case the magnetic chain turned into a special type of superconductor in which electrons next to one another in the chain coordinated their spins to simultaneously satisfy the requirements of magnetism and superconductivity. Each of these pairs can be thought of as an electron and an antielectron, with a negative and a positive charge, respectively. That arrangement, however, leaves one electron at each end of the chain without a neighbor to pair with, causing them to take on the properties of both electrons and antielectrons—in other words, Majorana particles.

As opposed to particles found in a vacuum, unattached to other matter, these Majoranas are what’s called “emergent particles.” They emerge from the collective properties of the surrounding matter and could not exist outside the superconductor.

The new study shows a convincing signature of Majorana particles, says Leo Kouwenhoven of the Delft University of Technology in the Netherlands who was not involved in the research but previously found signs of Majorana particles in a different superconductor arrangement. “But to really speak of full proof, unambiguous evidence, I think you have to do a DNA test.” Such a test, he says, must show the particles do not obey the normal laws of the two known classes of particles in nature—fermions (protons, electrons and most other particles we are familiar with) and bosons (photons and other force-carrying particles, including the Higgs boson). “The great thing about Majoranas is that they are potentially a new class of particle,” Kouwenhoven adds. “If you find a new class of particles, that really would add a new chapter to physics.”

Physicist Jason Alicea of California Institute of Technology, who also did not participate in the research, said the study offers “compelling evidence” for Majorana particles but that “we should keep in mind possible alternative explanations—even if there are no immediately obvious candidates.” He praised the experimental setup for its apparent ability to easily produce the elusive Majoranas. “One of the great virtues of their platform relative to earlier works is that it allowed the researchers to apply a new type of microscope to probe the detailed anatomy of the physics.”

The discovery could have implications for searches for free Majorana particles outside of superconducting materials. Many physicists suspect neutrinos—very lightweight particles with the strange ability to alter their identities, or flavors—are Majorana particles, and experiments are ongoing to investigate whether this is the case. Now that we know Majorana particles can exist inside superconductors, it might not be surprising to find them in nature, Yazdani says. “Once you find the concept to be correct, it’s very likely that it shows up in another layer of physics. That’s what’s exciting.”

The finding could also be useful for constructing quantum computers that harness the laws of quantum mechanics to make calculations many times faster than conventional computers. One of the main issues in building a quantum computer is the susceptibility of quantum properties such as entanglement (a connection between two particles such that an action on one affects the other) to collapse due to outside interference. A particle chain with Majoranas capping each end would be somewhat immune to this danger, because damage would have to be done to both ends simultaneously to destroy any information encoded there. “You could build a quantum bit based on these Majoranas,” Yazdani says. ”The idea is that such a bit would be much more robust to the environment than the types of bits people have tried to make so far.”

New invisibility technology

Doctoral student Joseph Choi demonstrates a multidirectional ‘perfect paraxial’ cloak using 4 lenses.

Choi uses his hand to further demonstrate his device.

A laser shows the paths that light rays travel through the system, showing regions that can be used for cloaking an object.

Scientists at the University of Rochester have discovered a way to hide large objects from sight using inexpensive and readily available lenses.

Cloaking is the process by which an object becomes hidden from view, while everything else around the cloaked object appears undisturbed.

“A lot of people have worked on a lot of different aspects of optical cloaking for years,” John Howell, a professor of physics at the upstate New York school, said on Friday.

The so-called Rochester Cloak is not really a tangible cloak at all. Rather the device looks like equipment used by an optometrist. When an object is placed behind the layered lenses it seems to disappear.

Previous cloaking methods have been complicated, expensive, and not able to hide objects in three dimensions when viewed at varying angles, they say.

“From what, we know this is the first cloaking device that provides three-dimensional, continuously multidirectional cloaking,” said Joseph Choi, a graduate student who helped develop the method at Rochester, which is renowned for its optical research.

In their tests, the researchers have cloaked a hand, a face, and a ruler – making each object appear “invisible” while the image behind the hidden object remains in view. The implications for the discovery are endless, they say.

“I imagine this could be used to cloak a trailer on the back of a semi-truck so the driver can see directly behind him,” Choi said. “It can be used for surgery, in the military, in interior design, art.”

Howell said the Rochester Cloak, like the fictitious cloak described in the pages of the Harry Potter series, causes no distortion of the background object.

Building the device does not break the bank either. It cost Howell and Choi a little over $US1000 ($1140) in materials to create it and they believe it can be done even cheaper.

Although a patent is pending, they have released simple instructions on how to create a Rochester Cloak at home for under $US100 (114).

There is also a one-minute video about the project on YouTube.

NASA unveils model of warp-drive starship, which is currently impossible.

NASA’s Harold White has been working since 2010 to develop a warp drive that will allow spacecraft to travel at speeds faster than light — 186,000 miles per second.

White, who heads NASA’s Advanced Propulsion Team, spoke about his conceptual starship at a conference last fall. But interest in his project reached a new level this week when he unveiled images of what the craft might look like.

Created by artist Mark Rademaker, who based them on White’s designs, the images show a technologically detailed spacecraft that wouldn’t look out of place in a “Star Trek” movie. Rademaker says creating them took more than 1,600 hours.

For now, warp speed is only possible in TV and movies, with both “Star Trek” and “Star Wars” referencing an idea that was completely speculative at the time. White has fittingly named the concept spacecraft IXS Enterprise, for the starship famously piloted by Captain James T. Kirk in the “Star Trek” TV series and movies.

At the SpaceVision 2013 Space Conference last November in Phoenix, White talked about his design, the concepts behind it and the progress that’s been made in warp-drive development over the decades. He discussed the idea of a “space warp,” a loophole in the theory of general relativity that would allow for massive distances to be traveled very quickly, reducing travel times from thousands of years to days.

In his speech, White described space warps as faraway galaxies that can bend light around them. They work on the principle of bending space both in front of and behind a spacecraft. This would essentially allow for the empty space behind the craft to expand, both pushing and pulling it forward at the same time. The concept is similar to that of an escalator or moving walkway.

“There’s no speed limit on the expansion and contraction of space,” White said at the conference. “You can actually find a way to get around what I like to call the 11th commandment: Thou shall not exceed the speed of light.”

It’s the idea of space warps that inspired physicist Miguel Alcubierre in 1994 to first theorize a mathematical model of a warp drive that would be able to bend space and time. While studying Alcubierre’s equations, White decided to design his own retooled version of the Alcubierre Drive. His recently unveiled design has much less empty space than the first concept model, increasing its efficiency.

The warp drive that White’s team has been working on would literally transcend space, shortening the distance between two points and allowing the craft to break the speed of light. This would be a spaceship with no speed limit.

Because travel into space has been extremely limited due to existing means of propulsion, such a technology could blow open the possibilities of space exploration. It could allow for study of the farthest reaches of space, parts that scientists once considered unimaginable.

Although the technology to create the spacecraft or the warp drive doesn’t yet exist, the artistic renderings Rademaker created could potentially be a model of what’s to come — the first spacecraft to break the speed-of-light barrier and journey beyond our solar system.

In his design, White says he drew from Matthew Jeffries’ 1965 sketches of the Enterprise from “Star Trek,” saying parts of that ship were mathematically correct. He worked with Rademaker and graphic designer Mike Okuda to update the math and produce what he believes to be a viable spacecraft.

According to NASA, there hasn’t been any proof that a warp drive can exist, but the agency is experimenting nonetheless. Although the concept doesn’t violate the laws of physics, that doesn’t guarantee that it will work.

“We’re starting to talk about what the next chapter for human space exploration going to be,” White said at SpaceVision.

Physicists discover a surprisingly straightforward way to turn light into matter

By Jonathan Webb

The design, published in Nature Photonics, adapts technology used in fusion research.

Several locations could now enter a race to convert photons into positrons and electrons for the very first time.

This would prove an 80-year-old theory by Breit and Wheeler, who themselves thought physical proof was impossible.

Now, according to researchers from Imperial College London, that proof is within reach.

Prof Steven Rose and his PhD student, Oliver Pike, told the BBC it could happen within a year.

“With a good experimental team, it should be quite doable,” said Mr Pike.

If the experiment comes to fruition, it will be the final piece in a puzzle that began in 1905, when Einstein accounted for the photoelectric effect with his model of light as a particle.

Several other basic interactions between matter and light have been described and subsequently proved by experiment, including Dirac’s 1930 proposal that an electron and its antimatter counterpart, a positron, could be annihilated upon collision to produce two photons.

Breit and Wheeler’s theoretical prediction of the reverse – that two photons could crash together and produce matter (a positron and an electron) – has been difficult to observe.

“The reason this is very hard to see in the lab is that you need to throw an awful lot of photons together – because the probability of any two of them interconverting is very low,” Prof Rose explained.

His team proposes gathering that vast number of very high-energy photons by firing an intense beam of gamma-rays into a further cloud of photons, created within a tiny, gold-lined cylinder.

That cylinder is called a “hohlraum”, German for “hollow space”, because it contains a vacuum, and it is usually used in nuclear fusion research. The cloud of photons inside it is made from extraordinarily intense X-rays and is about as hot as the Sun.

Hitting this very dense cloud of photons with the powerful gamma-ray beam raises the probability of collisions that will make matter – and history.

“It’s pretty amazing really,” said Mr Pike. He says it took some time to realise the value of the scheme, which he and two colleagues initially jotted down on scrap paper over several cups of coffee.

“For the first 12 hours or so, we didn’t quite appreciate its magnitude.”

But their subsequent calculations showed that the design, theoretically at least, has more than enough power to crack the challenge set by Breit and Wheeler in the 1930s.

“All the ingredients are there,” agrees Sir Peter Knight, an emeritus professor at Imperial College who was not involved in the research but describes it as a “really clever idea”.

“I think people will seriously start to have a crack at this,” Prof Knight told BBC News, though he cautioned that there were a lot of things to get right when putting the design into practice.

“If it’s done in a year, then they’ve done bloody well! I think it might take a bit longer.”

Some healthy scientific competition may speed up the process.

There are at least three facilities with the necessary equipment to test out the new proposal, including the Atomic Weapons Establishment in Oldham.

“The race to carry out and complete the experiment is on,” said Mr Pike.

Thanks to Da Brayn for bringing this to the attention of the It’s Interesting community.

Stephen Hawking: ‘There are no black holes’

Stephen Hawking's black hole theory
Notion of an ‘event horizon’, from which nothing can escape, is incompatible with quantum theory, physicist claims.

by Zeeya Merali

Most physicists foolhardy enough to write a paper claiming that “there are no black holes” — at least not in the sense we usually imagine — would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape.

In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form.

“There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.”

Hawking posted his paper on the arXiv preprint server on 22 January1. He titled it, whimsically, ‘Information preservation and weather forecasting for black holes’, and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013.

Hawking’s new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues.

In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein’s general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards — stretched out along the way, like spaghetti — and eventually crushed at the ‘singularity’, the black hole’s hypothetical infinitely dense core.

But on analysing the situation in detail, Polchinski’s team came to the startling realization that the laws of quantum mechanics, which govern particles on small scales, change the situation completely. Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or ‘firewall’, that would burn the astronaut to a crisp.

This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe — whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.

Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.

In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.

Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out ‘Hawking radiation’. In that case, the event horizon would, in theory, become smaller than the apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real boundary. “The absence of event horizons means that there are no black holes — in the sense of regimes from which light can’t escape to infinity,” Hawking writes.

“The picture Hawking gives sounds reasonable,” says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “You could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon.”

Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon.

Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).

If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were.

“It would be worse than trying to reconstruct a book that you burned from its ashes,” says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy.

Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “In Einstein’s gravity, the black-hole horizon is not so different from any other part of space,” says Polchinski. “We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales.”

Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking’s, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls,” he says. “But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance.”