Stephen Hawking, Given Two Years to Live in 1963, Is Going To Space Over 50 Years Later.


Article Image
Dr. Stephen Hawking delivers a speech entitled ‘Why we should go into space’ on April 21, 2008, at George Washington University’s Morton Auditorium in Washington, DC.

Stephen Hawking gave an interview to Piers Morgan on “Good Morning Britain”, where he confirmed that he’ll be going to space on Richard Branson’s Virgin Galactic spaceship. Branson actually offered him the trip in 2015 for free, and Hawking says “since that day, I have never changed my mind.”

When the flight will be we don’t yet know. Virgin Galactic’s SpaceShipTwowas previously slated to launch at the end of 2017, but no hard date has been announced yet.

Hawking’s spaceflight will be an amazing feat for the 75-year-old scientist, known for his work in physics and cosmology, adding another chapter to an already remarkable life. When he was only 21, he was diagnosed with amyotrophic lateral sclerosis (ALS) also known as Lou Gehrig’s disease. This rare neurogenerative disease is deadly and Hawking was told he had 2 years to live

50+ years later, Hawking is still going strong (and going to space). Paralyzed and confined to a wheelchair, speaking through a specially-designed computer system since 1985, the scientist has achieved more than most do in a lifetime, not letting the debilitating disease slow him down.

Hawking has done groundbreaking work on black holes, discovering (along with James Bardeen and Brandon Carter) four laws of black holes mechanics.

His 1974 “Hawking radiation” theory that black holes are slowly evaporating due to particles robbing them of energy can still land him a Nobel Prize as recent research appears to prove it.

He has also done outstanding work on gravitational singularities, one-dimensional points that have infinite mass in infinitely tiny spaces. Cooperating with mathematician Roger Penrose, Hawking proved the existence of singularities and proposed key theorems on their origins. 

hawking nasa

Stephen Hawking, date unconfirmed but likely in 1990s. 

His other scientific achievements include pioneering work on cosmic inflation and the early state of the universe (which Hawking proposed had no time or beginning). 

Hawking is also famous for being one of the world’s most popular science educators, writing numerous books like the bestseller “A Brief History of Time,” which sold more than 10 million copies. 

How did Hawking, who also has been a professor of mathematics at University of Cambridge for the 30 years, thrive despite the illness? In an interview with Scientific American, ALS expert and professor of neurology Leo McCluskey, called Hawking “an outlier”. His case is exceptional and probably represents just a few percent of ALS patients. If Hawking developed the disease while still a teenager, it could be a “juvenile-onset” variant that progresses very slowly. He has also had great care.

How will Hawking fare in space? We don’t know the details of Virgin Galactic flight yet but Hawking seems quite enthusiastic:

“I can tell you what will make me happy, to travel in space. I thought no one would take me but Richard Branson has offered me a seat on Virgin Galactic and I said yes immediately.”

Source:http://bigthink.com

Physicists Made a ‘Black Hole’ in a Lab That May Finally Prove Hawking Radiation Exists


IN BRIEF

Scientists may have found signs that phonons, the very small packets of energy that make up sound waves, were leaking out of sonic black holes, just as Hawking’s equations predicted.

SURVIVING A BLACK HOLE

Some 42 years ago, renowned theoretical physicist Stephen Hawking proposed that not everything that comes in contact with a black hole succumbs to its unfathomable nothingness. Tiny particles of light (photons) are sometimes ejected back out, robbing the black hole of an infinitesimal amount of energy, and this gradual loss of mass over time means every black hole eventually evaporates out of existence.

Known as Hawking radiation, these escaping particles help us make sense of one of the greatest enigmas in the known Universe, but after more than four decades, no one’s been able to actually prove they exist, and Hawking’s proposal remained firmly in hypothesis territory.

But all that could be about to change, with two independent groups of researchers reporting that they’ve found evidence to back up Hawking’s claims, and it could see one of the greatest living physicists finally win a Nobel Prize.

UNDERSTANDING THE THEORY

Let’s go back to 1974, when all of this began. Hawking had gotten into an argument with Princeton University graduate student, Jacob Bekenstein, who suggested in his PhD thesis that a black hole’s entropy – the ‘disorder’ of a system, related to its volume, energy, pressure, and temperature – was proportional to the area of its event horizon.

As Dennis Overbye explains for The New York Times, this was a problem, because according to the accepted understanding of physical laws at the time – including Hawking’s own work – the entropy and the volume of a black hole could never decrease.

Hawking investigated the claims, and soon enough, realised that he had been proven wrong. “[D]r Hawking did a prodigious calculation including quantum theory, the strange rules that govern the subatomic world, and was shocked to find particles coming away from the black hole, indicating that it was not so black after all,” Overbye writes.

Hawking proposed that the Universe is filled with ‘virtual particles’ that, according to what we know about how quantum mechanics works, blink in and out of existence and annihilate each other as soon as they come in contact – except if they happen to appear on either side of a black hole’s event horizon. Basically, one particle gets swallowed up by the black hole, and the otherradiates away into space.

The existence of Hawking radiation has answered a lot of questions about how black holes actually work, but in the process, raised a bunch of problems that physicists are still trying to reconcile.

“No result in theoretical physics has been more fundamental or influential than his discovery that black holes have entropy proportional to their surface area,”says Lee Smolin, a theoretical physicist from the Perimeter Institute for Theoretical Physics in Canada.

While Bekenstein received the Wolf Prize in 2012 and the American Physical Society’s Einstein prize in 2015 for his work, which The New York Times says are often precursors to the Nobel Prize, neither scientist has been awarded the most prestigious prize in science for the discovery. Bekenstein passed away last year, but Hawking is now closer than ever to seeing his hypothesis proven.

The problem? Remember when I said the escaping photons were stealing an  infinitesimal amount of energy from a black hole every time they escaped? Well, unfortunately for Hawking, this radiation is so delicate, it’s practically impossible to detect it from thousands of light-years away.

A WAY FORWARD?

The measured thermal spectrum of the Hawking radiation. The solid curve is the measurement. The dashed curve is the theoretical thermal spectrum.
The measured thermal spectrum of the Hawking radiation. The solid curve is the measurement. The dashed curve is the theoretical thermal spectrum.

But physicist Jeff Steinhauer from Technion University in Haifa, Israel, thinks he’s come up with a solution – if we can’t detect Hawking radiation in actual black holes thousands of light-years away from our best instruments, why not bring the black holes to our best instruments?

As Oliver Moody reports for The Times, Steinhauer has managed to created a lab-sized ‘black hole’ made from sound, and when he kicked it into gear, he witnessed particles steal energy from its fringes.

Reporting his experiment in a paper posted to the physics pre-press website,arXiv.org, Steinhauer says he cooled helium to just above absolute zero, then churned it up so fast, it formed a ‘barrier’ through which sound should not be able to pass.

“Steinhauer said he had found signs that phonons, the very small packets of energy that make up sound waves, were leaking out of his sonic black hole just as Hawking’s equations predict they should,” Moody reports.

To be clear, the results of this experiment have not yet been peer-reviewed – that’s the point of putting everything up for the public to see on arXiv.org. They’re now being mulled over by physicists around the world, and they’re already proving controversial, but worthy of further investigation.

“The experiments are beautiful,” physicist Silke Weinfurtner from the University of Nottingham in the UK, who is running her own Earth-based experiments to try and detect Hawking radiation, told The Telegraph. “Jeff has done an amazing job, but some of the claims he makes are open to debate. This is worth discussing.”

Meanwhile, a paper published in Physical Review Letters last month has found another way to strengthen the case for Hawking radiation. Physicists Chris Adami and Kamil Bradler from the University of Ottawa describe a new technique that allows them to follow a black hole’s life over time.

That’s exciting stuff, because it means that whatever information or matter that passes over the event horizon doesn’t ‘disappear’ but is slowly leaking back out during the later stages of the black hole’s evaporation.

“To perform this calculation, we had to guess how a black hole interacts with the Hawking radiation field that surrounds it,” Adami said in a press release. “This is because there currently is no theory of quantum gravity that could suggest such an interaction. However, it appears we made a well-educated guess because our model is equivalent to Hawking’s theory in the limit of fixed, unchanging black holes.”

Both results will now need to be confirmed, but they suggest that we’re inching closer to figuring out a solution for how we can confirm or disprove the existence of Hawking radiation, and that’s good news for its namesake.

As Moody points out, Peter Higgs, who predicted the existence of the Higgs boson, had to wait 49 years for his Nobel prize, we’ll have to wait and see if Hawking ends up with his own.

Physicists Made a ‘Black Hole’ in a Lab That May Finally Prove Hawking Radiation Exists


IN BRIEF

Scientists may have found signs that phonons, the very small packets of energy that make up sound waves, were leaking out of sonic black holes, just as Hawking’s equations predicted.

SURVIVING A BLACK HOLE

Some 42 years ago, renowned theoretical physicist Stephen Hawking proposed that not everything that comes in contact with a black hole succumbs to its unfathomable nothingness. Tiny particles of light (photons) are sometimes ejected back out, robbing the black hole of an infinitesimal amount of energy, and this gradual loss of mass over time means every black hole eventually evaporates out of existence.

Known as Hawking radiation, these escaping particles help us make sense of one of the greatest enigmas in the known Universe, but after more than four decades, no one’s been able to actually prove they exist, and Hawking’s proposal remained firmly in hypothesis territory.

But all that could be about to change, with two independent groups of researchers reporting that they’ve found evidence to back up Hawking’s claims, and it could see one of the greatest living physicists finally win a Nobel Prize.

UNDERSTANDING THE THEORY

Let’s go back to 1974, when all of this began. Hawking had gotten into an argument with Princeton University graduate student, Jacob Bekenstein, who suggested in his PhD thesis that a black hole’s entropy – the ‘disorder’ of a system, related to its volume, energy, pressure, and temperature – was proportional to the area of its event horizon.

As Dennis Overbye explains for The New York Times, this was a problem, because according to the accepted understanding of physical laws at the time – including Hawking’s own work – the entropy and the volume of a black hole could never decrease.

Hawking investigated the claims, and soon enough, realised that he had been proven wrong. “[D]r Hawking did a prodigious calculation including quantum theory, the strange rules that govern the subatomic world, and was shocked to find particles coming away from the black hole, indicating that it was not so black after all,” Overbye writes.

Hawking proposed that the Universe is filled with ‘virtual particles’ that, according to what we know about how quantum mechanics works, blink in and out of existence and annihilate each other as soon as they come in contact – except if they happen to appear on either side of a black hole’s event horizon. Basically, one particle gets swallowed up by the black hole, and the otherradiates away into space.

The existence of Hawking radiation has answered a lot of questions about how black holes actually work, but in the process, raised a bunch of problems that physicists are still trying to reconcile.

“No result in theoretical physics has been more fundamental or influential than his discovery that black holes have entropy proportional to their surface area,”says Lee Smolin, a theoretical physicist from the Perimeter Institute for Theoretical Physics in Canada.

While Bekenstein received the Wolf Prize in 2012 and the American Physical Society’s Einstein prize in 2015 for his work, which The New York Times says are often precursors to the Nobel Prize, neither scientist has been awarded the most prestigious prize in science for the discovery. Bekenstein passed away last year, but Hawking is now closer than ever to seeing his hypothesis proven.

The problem? Remember when I said the escaping photons were stealing an  infinitesimal amount of energy from a black hole every time they escaped? Well, unfortunately for Hawking, this radiation is so delicate, it’s practically impossible to detect it from thousands of light-years away.

A WAY FORWARD?

The measured thermal spectrum of the Hawking radiation. The solid curve is the measurement. The dashed curve is the theoretical thermal spectrum.
The measured thermal spectrum of the Hawking radiation. The solid curve is the measurement. The dashed curve is the theoretical thermal spectrum.

But physicist Jeff Steinhauer from Technion University in Haifa, Israel, thinks he’s come up with a solution – if we can’t detect Hawking radiation in actual black holes thousands of light-years away from our best instruments, why not bring the black holes to our best instruments?

As Oliver Moody reports for The Times, Steinhauer has managed to created a lab-sized ‘black hole’ made from sound, and when he kicked it into gear, he witnessed particles steal energy from its fringes.

Reporting his experiment in a paper posted to the physics pre-press website,arXiv.org, Steinhauer says he cooled helium to just above absolute zero, then churned it up so fast, it formed a ‘barrier’ through which sound should not be able to pass.

“Steinhauer said he had found signs that phonons, the very small packets of energy that make up sound waves, were leaking out of his sonic black hole just as Hawking’s equations predict they should,” Moody reports.

To be clear, the results of this experiment have not yet been peer-reviewed – that’s the point of putting everything up for the public to see on arXiv.org. They’re now being mulled over by physicists around the world, and they’re already proving controversial, but worthy of further investigation.

“The experiments are beautiful,” physicist Silke Weinfurtner from the University of Nottingham in the UK, who is running her own Earth-based experiments to try and detect Hawking radiation, told The Telegraph. “Jeff has done an amazing job, but some of the claims he makes are open to debate. This is worth discussing.”

Meanwhile, a paper published in Physical Review Letters last month has found another way to strengthen the case for Hawking radiation. Physicists Chris Adami and Kamil Bradler from the University of Ottawa describe a new technique that allows them to follow a black hole’s life over time.

That’s exciting stuff, because it means that whatever information or matter that passes over the event horizon doesn’t ‘disappear’ but is slowly leaking back out during the later stages of the black hole’s evaporation.

“To perform this calculation, we had to guess how a black hole interacts with the Hawking radiation field that surrounds it,” Adami said in a press release. “This is because there currently is no theory of quantum gravity that could suggest such an interaction. However, it appears we made a well-educated guess because our model is equivalent to Hawking’s theory in the limit of fixed, unchanging black holes.”

Both results will now need to be confirmed, but they suggest that we’re inching closer to figuring out a solution for how we can confirm or disprove the existence of Hawking radiation, and that’s good news for its namesake.

As Moody points out, Peter Higgs, who predicted the existence of the Higgs boson, had to wait 49 years for his Nobel prize, we’ll have to wait and see if Hawking ends up with his own.

Physicists have created a ‘black hole’ in the lab that could finally prove Hawking radiation exists


Will Stephen Hawking get his Nobel prize?

Some 42 years ago, renowned theoretical physicist Stephen Hawking proposed that not everything that comes in contact with a black hole succumbs to its unfathomable nothingness. Tiny particles of light (photons) are sometimes ejected back out, robbing the black hole of an infinitesimal amount of energy, and this gradual loss of mass over time means every black hole eventually evaporates out of existence.

Known as Hawking radiation, these escaping particles help us make sense of one of the greatest enigmas in the known Universe, but after more than four decades, no one’s been able to actually prove they exist, and Hawking’s proposal remained firmly in hypothesis territory.

But all that could be about to change, with two independent groups of researchers reporting that they’ve found evidence to back up Hawking’s claims, and it could see one of the greatest living physicists finally win a Nobel Prize.

So let’s go back to 1974, when all of this began. Hawking had gotten into an argument with Princeton University graduate student, Jacob Bekenstein, who suggested in his PhD thesis that a black hole’s entropy – the ‘disorder’ of a system, related to its volume, energy, pressure, and temperature – was proportional to the area of its event horizon.

As Dennis Overbye explains for The New York Times, this was a problem, because according to the accepted understanding of physical laws at the time – including Hawking’s own work – the entropy and the volume of a black hole could never decrease.

Hawking investigated the claims, and soon enough, realised that he had been proven wrong. “[D]r Hawking did a prodigious calculation including quantum theory, the strange rules that govern the subatomic world, and was shocked to find particles coming away from the black hole, indicating that it was not so black after all,” Overbye writes.

Hawking proposed that the Universe is filled with ‘virtual particles’ that, according to what we know about how quantum mechanics works, blink in and out of existence and annihilate each other as soon as they come in contact – except if they happen to appear on either side of a black hole’s event horizon. Basically, one particle gets swallowed up by the black hole, and the other radiates away into space.

The existence of Hawking radiation has answered a lot of questions about how black holes actually work, but in the process, raised a bunch of problems that physicists are still trying to reconcile.

“No result in theoretical physics has been more fundamental or influential than his discovery that black holes have entropy proportional to their surface area,” says Lee Smolin, a theoretical physicist from the Perimeter Institute for Theoretical Physics in Canada.

While Bekenstein received the Wolf Prize in 2012 and the American Physical Society’s Einstein prize in 2015 for his work, which The New York Timessays are often precursors to the Nobel Prize, neither scientist has been awarded the most prestigious prize in science for the discovery. Bekenstein passed away last year, but Hawking is now closer than ever to seeing his hypothesis proven.

The problem? Remember when I said the escaping photons were stealing an  infinitesimal amount of energy from a black hole every time they escaped? Well, unfortunately for Hawking, this radiation is so delicate, it’s practically impossible to detect it from thousands of light-years away.

But physicist Jeff Steinhauer from Technion University in Haifa, Israel, thinks he’s come up with a solution – if we can’t detect Hawking radiation in actual black holes thousands of light-years away from our best instruments, why not bring the black holes to our best instruments?

As Oliver Moody reports for The Times, Steinhauer has managed to created a lab-sized ‘black hole’ made from sound, and when he kicked it into gear, he witnessed particles steal energy from its fringes.

Reporting his experiment in a paper posted to the physics pre-press website, arXiv.org, Steinhauer says he cooled helium to just above absolute zero, then churned it up so fast, it formed a ‘barrier’ through which sound should not be able to pass.

“Steinhauer said he had found signs that phonons, the very small packets of energy that make up sound waves, were leaking out of his sonic black hole just as Hawking’s equations predict they should,” Moody reports.

To be clear, the results of this experiment have not yet been peer-reviewed – that’s the point of putting everything up for the public to see on arXiv.org. They’re now being mulled over by physicists around the world, and they’re already proving controversial, but worthy of further investigation.

“The experiments are beautiful,” physicist Silke Weinfurtner from the University of Nottingham in the UK, who is running his own Earth-based experiments to try and detect Hawking radiation, told The Telegraph. “Jeff has done an amazing job, but some of the claims he makes are open to debate. This is worth discussing.”

Meanwhile, a paper published in Physical Review Letters last month has found another way to strengthen the case for Hawking radiation. Physicists Chris Adami and Kamil Bradler from the University of Ottawa describe a new technique that allows them to follow a black hole’s life over time.

That’s exciting stuff, because it means that whatever information or matter that passes over the event horizon doesn’t ‘disappear’ but is slowly leaking back out during the later stages of the black hole’s evaporation.

“To perform this calculation, we had to guess how a black hole interacts with the Hawking radiation field that surrounds it,” Adami said in a press release. “This is because there currently is no theory of quantum gravity that could suggest such an interaction. However, it appears we made a well-educated guess because our model is equivalent to Hawking’s theory in the limit of fixed, unchanging black holes.”

Both results will now need to be confirmed, but they suggest that we’re inching closer to figuring out a solution for how we can confirm or disprove the existence of Hawking radiation, and that’s good news for its namesake.

As Moody points out, Peter Higgs, who predicted the existence of the Higgs boson, had to wait 49 years for his Nobel prize, we’ll have to wait and see if Hawking ends up with his own.

Interstellar was right. Falling into a black hole is not the end, says Stephen Hawking.


“If you feel you are in a black hole, don’t give up, there’s a way out,” Stephen Hawking told the Royal Institute of Technology in Stockholm

An artist's impression of a supermassive black hole at the centre of a distant quasar

An artist’s impression of a supermassive black hole at the centre of a distant quasar

Interstellar was right. Falling into a black hole is not the end, professor Stephen Hawking has claimed.

Although physicists had assumed that all matter must be destroyed by the huge gravitational forces of a black hole, Hawking told delegates in Sweden that it could escape and even pop into another dimension.

The theory solves the ‘information paradox’ which has puzzled scientists for decades. While quantum mechanics says that nothing can ever be destroyed, general relativity says it must be.

However under Hawking’s new theory, anything that is sucked into a black hole is effectively trapped at the event horizon – the sphere surrounding the hole from which it was thought that nothing can escape.

And he claims that anything which fell in could re-emerge back into our universe, or a parallel one, through Hawking radiation – protons which manage to escape from the black hole because of quantum fluctuations.

“If you feel you are in a black hole, don’t give up, there’s a way out,” Hawking told an audience held at the KTH Royal Institute of Technology in Stockholm

In the film Interstellar, Cooper, played by Matthew McConaughey, plunges into the black hole Gargantura. As Cooper’s ship breaks apart in the force, he evacuates and ends up in a Tesseract – a four dimensional cube. He eventually makes it out of the black hole.

The blac hole Gargantua from the film Interstellar

The black hole Gargantua from the film Interstellar

Black holes are stars that have collapsed under their own gravity, producing such extreme forces that even light can’t escape.

But Hawking claims that information never makes it inside the black hole in the first place and instead is ‘translated’ into a kind of hologram which sits in the event horizon.

“I propose that the information is stored not in the interior of the black hole as one might expect, but on its boundary, the event horizon,” said Prof Hawking

“The idea is the super translations are a hologram of the ingoing particles,” he said. “Thus they contain all the information that would otherwise be lost.”

Hawking also believes that radiation leaving the black hole can pick up some of the information stored at the event horizon and carry it back out. However it is unlikely to be in the same state in which it entered.

“The information about ingoing particles is returned, but in a chaotic and useless form,” he said. “This information paradox. For all practical purposes, the information is lost.

“The message of this lecture is that black holes ain’t as black as they are painted. They are not the eternal prisons they were once thought. Things can get out of a black hole both on the outside and possibly come out in another universe.”

Hawking and colleagues are expected to publish a paper on the work next month.

“He is saying that the information is there twice already from the very beginning, so it’s never destroyed in the black hole to begin with,” Sabine Hossenfelder of the Nordic Institute for Theoretical Physics in Stockholm told New Scientist. “At least that’s what I understood.”

Black Hole Radiation Simulated in Lab.


For the first time, scientists have been able to simulate the type of radiation likely to be emitted from black holes.

A team of Italian scientists fired a laser beam into a chunk of glass to create an analogue (or simulation) of the Hawking radiation that many physicists expect is emitted by black holes.

A spokesperson for the research group said: “Although the laser experiment superficially bears little resemblance to ultra-dense black holes, the mathematical theories used to describe both are similar enough that confirmation of laser-induced Hawking radiation would bolster confidence that black holes also emit Hawking radiation.

The renowned physicist Stephen Hawking first predicted this sort of radiation in 1974 but it has proved elusive to detect, even in the lab. This research group was able to use a “bulk glass target” to isolate the apparent Hawking radiation from the other forms of light emitted during such experiments.

Black holes are region in space where nothing can escape, not even light. However, and despite their name, they are believed to emit weak forms of radiation (such as Hawking radiation). Physicists expect that this radiation may be so weak as to be undetectable.

Source: http://www.communicatescience.eu

New horizons for Hawking radiation.


In 1974, Steven Hawking predicted that black holes were not completely black, but were actually weak emitters of blackbody radiation generated close to the event horizon—the boundary where light is forever trapped by the black hole’s gravitational pull [1]. Hawking’s insight was to realize how the presence of the horizon could separate virtual photon pairs (constantly being created from the quantum vacuum) such that while one was sucked in, the other could escape, causing the black hole to lose energy. Hawking’s idea was significant in suggesting a possible optical signature of a black hole’s existence. Yet, even though the prediction created an extensive theoretical literature in cosmology, calculations have since shown that Hawking radiation from black holes is so weak that it would be practically impossible to measure.

It turns out, however, that the physics of how waves interact with a horizon does not depend in a fundamental way on the presence of gravity at all. In principle, an analogous Hawking radiation should occur in other systems [2, 3]. The key requirement is simply that the interaction between waves and the medium in which they propagate causes there to be a boundary between zones where the wave and the medium have different velocities [4, 5]. In a paper in Physical Review Letters [6], Francesco Belgiorno at the Università degli Studi di Milano, in collaboration with researchers at several other institutes, also in Italy, describe a series of experiments where high-intensity filaments of light in glass perturb the optical propagation environment in an analogous manner to the way a gravitational field affects light near a black hole horizon. This perturbation creates the optical equivalent of an event horizon that allows Belgiorno et al. to make convincing measurements of analog Hawking radiation at optical frequencies [7]. These results are highly significant in suggesting a system in which Hawking’s prediction can be fully explored in a convenient laboratory environment.

A really useful way to visualize an analog event horizon is to imagine a fish swimming upstream in a river flowing towards a waterfall [4, 5]. The point at which the current flows faster than the fish can swim represents a boundary at which fish cannot escape and they are swept over the waterfall. This “point of no return” for the fish is equivalent to a black hole horizon. A similar analogy exists for white holes, associated with horizons that fish can never enter, namely, if they were attempting to swim upstream towards the bottom of a waterfall. This simple picture is surprisingly powerful at capturing the physics of horizons, and can be extended rigorously to describe horizon physics in diverse systems including acoustics, cold atoms, and gravity-capillary waves on water [8, 9, 10, 11]. Indeed, horizon effects and a stimulated form of Hawking radiation have recently been explicitly observed in the vicinity of an obstacle placed in an open channel flow [12]. However, the question has remained open as to whether these analog gravity systems also generate the spontaneous thermal radiation Hawking predicted.

Belgiorno et al.’s experiments suggest that the answer is yes. They created an optical event horizon by using intense, ultrashort light pulses that change the refractive index of the glass in the vicinity of the moving pulse [13]. The change in the refractive index modifies the effective propagation “geometry” (Fig. 1, left) as seen by copropagating light rays such that a trapping horizon forms and, in principle, Hawking radiation should occur. The changing index is an effect called a Kerr nonlinearity, whereby a pulse modifies the refractive index of glass such that it is higher at the center of the pulse than the wings. Because wave speed depends on refractive index, the propagating pulse induces an effective velocity gradient in the material such that horizons can appear at points on the pulse leading and trailing edges.

It is important to note here that Kerr-induced trapping in itself is not a new effect, but rather one that is well known in nonlinear fiber optics [14, 15, 16]. In fact, the Raman frequency shifting on solitons (where the spectrum of short pulses moves to longer wavelengths) is a deceleration effect that was even shown to lead to an equivalent gravitational potential [16, 17], but it was only recently that scientists appreciated the possibility of extending the gravitational analogy to test predictions of Hawking radiation [13]. The particular attraction of experiments in optics is that the intensity and wavelength of the Hawking radiation depends on the induced refractive index gradient, and pulses containing only a few optical cycles or a steep shock front would be expected to generate measureable emission of visible light. Unfortunately, although it is straightforward to demonstrate the existence of an event horizon in an optical fiber [13], dispersive and dissipative effects present in the fiber appear to prevent the clean formation of the particular pulse profiles needed for the spontaneous generation of Hawking radiation.

The experiments of Belgiorno and co-workers attempt to overcome this problem in a novel way. In fact, they move away from the fiber environment altogether, performing experiments in bulk glass, using laser pulses in the form of needlelike beams known as optical filaments (Fig. 1, right) to generate the nonlinear refractive index perturbation [18]. Optical filament pulses generally have a complex spatiotemporal structure, but it is possible to experimentally synthesize them so that their internal group and phase velocity gradients can be controlled. This is what Belgiorno et al. did in their experiments, using properties of what are called Bessel beams, where the filament is preshaped so as to control its group velocity relative to the velocity change induced by the refractive index perturbation. This is a crucial aspect of their experiments because it allows them to fine tune the window of Hawking radiation emission into the near infrared, around away from any other possible contaminating signals. With a cooled CCD camera, they detect a clear signal above background that they associate with Hawking radiation, and the spectral shift of this signal with incident energy is in good agreement with the predictions of theory. They also performed experiments where the filaments were formed from Gaussian pulses, and although the dynamics are more complicated here, the spectral emission window of the spontaneous radiation is again in agreement with theory.

Overall, this work provides significant evidence for the observation of Hawking radiation in an analog gravity system. The results must nevertheless be interpreted carefully. For example, it is essential to state very plainly that measuring analog Hawking radiation gives no direct insight into quantum gravity because there is no physical gravitational potential involved. Indeed, one might even argue that the description of this emission as “Hawking” radiation is inappropriate, but this seems an unnecessary restriction because the study of analog horizon systems was clearly motivated by Hawking’s work over years ago. Of course additional experiments still remain to be carried out. Specifically, the spontaneous photon pairs emitted on either side of the horizon may be detectable with suitable angular resolution [18], and measurements of their entanglement and correlation will be an essential next step. Moreover, the polarization properties of filaments are very subtle and may need to be taken into account more fully in a complete interpretation of the experiments.

This field of research is at the interface of several areas of physics, and “standardizing” the terminology would be welcome. For example, effects related to Cerenkov radiation are seen both in filament and fiber soliton propagation [15, 16], and distinguishing similarities and differences is important to avoid confusion. On the other hand, Belgiorno et al.’s results now point out the clear need to study the quantum electrodynamics of soliton radiation, as this may have direct bearing on their experiments. This work is also likely to be far reaching in other ways. By showing how tailored spatiotemporal fields provide a high degree of control over the interaction geometry of propagating light pulses, they may represent a new example of experiments in “ultrafast transformation optics,” where geometrical modifications of an optical propagation environment can be induced on ultrafast timescales. This may allow a much wider study of other analog physical effects, using a convenient benchtop platform.

Corrections (6 December 2010): Paragraph 3, sentence 5, “capillary waves” changed to “gravity-capillary waves.” References 2, 3, 10, 13 and 18, changed/updated.

References

  1. S. W. Hawking, Nature 248, 30 (1974).
  2. W. G. Unruh, Phys. Rev. Lett. 46, 1351 (1981).
  3. P. C. W. Davies and S. A. Fulling, Proc. R. Soc. London A 356, 237 (1977); M. Visser, Classical Quantum Gravity 15, 1767 (1998).
  4. U. Leonhardt and T. G. Philbin, Philos. Trans. R. Soc. A 366, 2851 (2008).
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Source: http://physics.aps.org

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