Fifty years ago, English mathematical physicist and Nobel-prize winner Roger Penrose proposed that energy could be extracted from the space around a rotating black hole. Known as the ergosphere, this region lies just outside an event horizon, the boundary within which nothing can escape a black hole’s gravitational pull (even light). It is also here where infalling matter is accelerated to incredible speeds and emits all kinds of energy.
This became known as the Penrose Process, which many theorists have since expanded on. The latest comes from a study conducted by researchers from Columbia University and the Universidad Adolfo Ibáñez in Chile. With support from organizations like NASA, they demonstrated how a better understanding of the physics at work around spinning black holes could allow us to harness their energy someday.
Special Relativity. It’s been the bane of space explorers, futurists and science fiction authors since Albert Einstein first proposed it in 1905. For those of us who dream of humans one-day becoming an interstellar species, this scientific fact is like a wet blanket. Luckily, there are a few theoretical concepts that have been proposed that indicate that Faster-Than-Light (FTL) travel might still be possible someday.
A popular example is the idea of a wormhole: a speculative structure that links two distant points in space time that would enable interstellar space travel. Recently, a team of Ivy League scientists conducted a study that indicated how “traversable wormholes” could actually be a reality. The bad news is that their results indicate that these wormholes aren’t exactly shortcuts, and could be the cosmic equivalent of “taking the long way”!
Dr. Stephen Hawking, the famed British theoretical physicist, science communicator, author and luminary, passed away in the early hours on Wednesday, March 14th. According to a statement from his family, the renowned scientist died peacefully in his home at Cambridge. He was 76 years old, and is survived by his first wife, Jane Wilde, and their three children – Lucy, Robert and Tim.
Dr. Hawking spent the past 50 years living with a terminal illness that slowly deprived him of his speech and the use of much of his body. He also leaves behind an unparalleled scientific legacy and millions of people worldwide who admired him for his genius, his sense of humor, and the way he sought to educate people on the importance of scientific research, space exploration, and disability awareness.
In 1963, when he was just 21 years old, Dr. Hawking was diagnosed with Amyotrophic Lateral Sclerosis (ALS, aka. Lou Gehrig’s disease), a degenerative form of motor neurone disease that would be with him for the rest of his life. At the time, he was told that he had only two years to live. This diagnosis caused Dr. Hawking to fall into a depression and lose interest in his studies, which he was pursuing at Cambridge University at the time.
However, his outlook soon changed as the disease progressed slower than his doctor’s originally though. It was also around this time that Hawking met his first wife, Jane Wilde. The two became engaged in October of 1964 and married on July 14th, 1966. Hawking would later say that his relationship with Wilde gave him “something to live for”.
The slow progression of the disease also allowed Dr. Hawking to embark on a career marked by brilliance, brashness, and original thinking. Among his many achievements, Dr. Hawing was the Lucasian Professor of Mathematics at the University of Cambridge, the Founder of the Center for Theoretical Cosmology, and served as the Sally Tsui Wong-Avery Director of Research at the Department of Applied Mathematics and Theoretical Physics until his passing.
During his lifetime, Dr. Hawking made invaluable contributions to the fields of theoretical physics and cosmology. These include his extensive work on gravitational singularity theorems (in collaboration with Roger Penrose), the theory that black holes emit radiation (often called Hawking Radiation), and a theory of cosmology that attempted to unify general relativity and quantum mechanics (aka. Theory of Everything).
His many accolades, honors and awards included being made an Honorary Fellow of the Royal Society of Arts (FRSA), a lifetime member of the Pontifical Academy of Sciences, and a recipient of the Presidential Medal of Freedom – the highest civilian award in the United States. In 2002, Hawking was ranked number 25 in the BBC’s poll of the 100 Greatest Britons.
In a statement by his three children, Lucy, Robert and Tim praised their father’s courage and persistence and honored how his genius and sense of humor inspired people all across the world:
“We are deeply saddened that our beloved father passed away today. He was a great scientist and an extraordinary man whose work and legacy will live on for many years… He once said, ‘It would not be much of a universe if it wasn’t home to the people you love.’ We will miss him forever.”
News of his passing was also met with a flurry of condolences by friends, colleagues, fans, and people whose lives he touched over the years. NASA tweeted the following early this morning, followed by a video of Dr. Hawking addressing the astronauts of the ISS in 2014:
Famed scientists and science communicator Neil DeGrasse Tyson also expressed his condolences, tweeting:
His passing has left an intellectual vacuum in his wake. But it's not empty. Think of it as a kind of vacuum energy permeating the fabric of spacetime that defies measure. Stephen Hawking, RIP 1942-2018. pic.twitter.com/nAanMySqkt
Despite having lived for five decades with this degenerative disease, Hawking had a very practical and courageous attitude about life. In 2011, he said in an interview with The Guardian that death was never far from his mind. “I have lived with the prospect of an early death for the last 49 years,” he said. “I’m not afraid of death, but I’m in no hurry to die. I have so much I want to do first.”
Hawking, a well-known atheist, was also clear on his thoughts on an afterlife. “I regard the brain as a computer which will stop working when its components fail,” he said. “There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark.”
Dr. Hawking’s life and his contributions to science have been commemorated in many ways over the years. A film version of A Brief History of Time, directed by Errol Morris and produced by Steven Spielberg, premiered in 1992. In 1997, a six-part television series Stephen Hawking’s Universe premiered on PBS, with a companion book also being released. In 2014, the story of his diagnosis and the impact it had on his young family was showcased in the Oscar-winning film The Theory of Everything.
Dr. Hawking has also been a major role model for people dealing with disabilities and degenerative illnesses and played an unparalleled role when it came to disability awareness and outreach. In 1999, he and eleven other luminaries joined with Rehabilitation International , an organization founded in 1922 “To advance the rights and inclusion of persons with disabilities across the world.”
In 2000, Dr. Hawking and his fellow luminaries signed the Charter for the Third Millennium on Disability, which called on governments around the world to prevent disabilities and protect disability rights. Throughout his life, Dr. Hawking also remained a committed educator – personally supervising 39 successful PhD students – and lending his voice to scientific and humanitarian goals.
These include Breakthrough Initiatives, an effort to search for extraterrestrial intelligence (SETI) in the Universe, which Dr. Hawking helped launch in 2015. That same year, he also used his influence and celebrity status to promote the The Global Goals, a series of 17 goals adopted by the United Nations Sustainable Development Summit to end extreme poverty, social inequality, and fixing climate change over the course of the next 15 years.
To commemorate his life and legacy, a book of condolence has been opened at Gonville and Caius College in Cambridge, where Prof Hawking was a fellow. All around the world, there are outpourings of remembrance and support for his family from people who are mourning Dr. Hawking’s passing and celebrating his life and achievements.
As Neil DeGrasse Tyson said, the death of Dr. Hawking has left a vacuum in the scientific community, and in the hearts of people everywhere. However, his life and his many contributions shall be remembered for a long time to come!
Today we’re going to have the most surreal conversation. I’m going to struggle to explain it, and you’re going to struggle to understand it. And only Stephen Hawking is going to really, truly, understand what’s actually going on.
But that’s fine, I’m sure he appreciates our feeble attempts to wrap our brains around this mind bending concept.
All right? Let’s get to it. Black holes again. But this time, we’re going to figure out their temperature.
The very idea that a black hole could have a temperature strains the imagination. I mean, how can something that absorbs all the matter and energy that falls into it have a temperature? When you feel the warmth of a toasty fireplace, you’re really feeling the infrared photons radiating from the fire and surrounding metal or stone.
And black holes absorb all the energy falling into them. There is absolutely no infrared radiation coming from a black hole. No gamma radiation, no radio waves. Nothing gets out.
Now, supermassive black holes can shine with the energy of billions of stars, when they become quasars. When they’re actively feeding on stars and clouds of gas and dust. This material piles up into an accretion disk around the black hole with such density that it acts like the core of a star, undergoing nuclear fusion.
But that’s not the kind of temperature we’re talking about. We’re talking about the temperature of the black hole’s event horizon, when it’s not absorbing any material at all.
The temperature of black holes is connected to this whole concept of Hawking Radiation. The idea that over vast periods of time, black holes will generate virtual particles right at the edge of their event horizons. The most common kind of particles are photons, aka light, aka heat.
Normally these virtual particles are able to recombine and disappear in a puff of annihilation as quickly as they appear. But when a pair of these virtual particles appear right at the event horizon, one half of the pair drops into the black hole, while the other is free to escape into the Universe.
From your perspective as an outside observer, you see these particles escaping from the black hole. You see photons, and therefore, you can measure the temperature of the black hole.
The temperature of the black hole is inversely proportional to the mass of the black hole and the size of the event horizon. Think of it this way. Imagine the curved surface of a black hole’s event horizon. There are many paths that a photon could try to take to get away from the event horizon, and the vast majority of those are paths that take it back down into the black hole’s gravity well.
But for a few rare paths, when the photon is traveling perfectly perpendicular to the event horizon, then the photon has a chance to escape. The larger the event horizon, the less paths there are that a photon could take.
Since energy is being released into the Universe at the black hole’s event horizon, but energy can neither be created or destroyed, the black hole itself provides the mass that supplies the energy to release these photons.
The black hole evaporates.
The most massive black holes in the Universe, the supermassive black holes with millions of times the math of the Sun will have a temperature of 1.4 x 10^-14 Kelvin. That’s low. Almost absolute zero, but not quite.
A solar mass black hole might have a temperature of only .0.00000006 Kelvin. We’re getting warmer.
Since these temperatures are much lower than the background temperature of the Universe – about 2.7 Kelvin, all the existing black holes will have an overall gain of mass. They’re absorbing energy from the Cosmic Background Radiation faster than they’re evaporating, and will for an incomprehensible amount of time into the future.
Until the background temperature of the Universe goes below the temperature of these black holes, they won’t even start evaporating.
A black hole with the mass of the Earth is still too cold.
Only a black hole with about the mass of the Moon is warm enough to be evaporating faster than it’s absorbing energy from the Universe.
As they get less massive, they get even hotter. A black hole with the mass of the asteroid Ceres would be 122 Kelvin. Still freezing, but getting warmer.
A black hole with half the mass of Vesta would blaze at more than 1,200 Kelvin. Now we’re cooking!
Less massive, higher temperatures.
When black holes have lost most of their mass, they release the final material in a tremendous blast of energy, which should be visible to our telescopes.
Some astronomers are actively searching the night sky for blasts from black holes which were formed shortly after the Big Bang, when the Universe was hot and dense enough that black holes could just form.
It took them billions of years of evaporation to get to the point that they’re starting to explode now.
This is just conjecture, though, no explosions have ever been linked to primordial black holes so far.
It’s pretty crazy to think that an object that absorbs all energy that falls into it can also emit energy. Well, that’s the Universe for you. Thanks for helping us figure it out Dr. Hawking.
When we think of major figures in the history of science, many names come to mind. Einstein, Newton, Kepler, Galileo – all great theorists and thinkers who left an indelible mark during their lifetime. In many cases, the full extent of their contributions would not be appreciated until after their death. But those of us that are alive today are fortunate to have a great scientist among us who made considerable contributions – Dr. Stephen Hawking.
Considered by many to be the “modern Einstein”, Hawking’s work in cosmology and theoretical physics was unmatched among his contemporaries. In addition to his work on gravitational singularities and quantum mechanics, he was also responsible for discovering that black holes emit radiation. On top of that, Hawking was a cultural icon, endorsing countless causes, appearing on many television shows as himself, and penning several books that have made science accessible to a wider audience.
Hawking was born on January 8th, 1942 (the 300th anniversary of the death of Galileo) in Oxford, England. His parents, Frank and Isobel Hawking, were both students at Oxford University, where Frank studied medicine and Isobel studied philosophy, politics and economics. The couple originally lived in Highgate, a suburb of London, but moved to Oxford to get away from the bombings during World War II and give birth to their child in safety. The two would go on to have two daughters, Philippa and Mary, and one adopted son, Edward.
The family moved again in 1950, this time to St. Albans, Hertfordshire, because Stephen’s father became the head of parasitology at the National Institute for Medical Research (now part of the Francis Crick Institute). While there, the family gained the reputation for being highly intelligent, if somewhat eccentric. They lived frugally, living in a large, cluttered and poorly maintained house, driving around in a converted taxicab, and constantly reading (even at the dinner table).
Hawking began his schooling at the Byron House School, where he experienced difficulty in learning to read (which he later blamed on the school’s “progressive methods”.) While in St. Albans, the eight-year-old Hawking attended St. Albans High School for Girls for a few months (which was permitted at the time for younger boys). In September of 1952, he was enrolled at Radlett School for a year, but would remain at St. Albans for the majority of his teen years due the family’s financial constraints.
While there, Hawking made many friends, with whom he played board games, manufactured fireworks, model airplanes and boats, and had long discussions with on subjects ranging from religion to extrasensory perception. From 1958, and with the help of the mathematics teacher Dikran Tahta, Hawking and his friends built a computer from clock parts, an old telephone switchboard and other recycled components.
Though he was not initially academically successfully, Hawking showed considerable aptitude for scientific subjects and was nicknamed “Einstein”. Inspired by his teacher Tahta, he decided to study mathematics at university. His father had hoped that his son would attend Oxford and study medicine, but since it was not possible to study math there at the time, Hawking chose to study physics and chemistry.
In 1959, when he was just 17, Hawking took the Oxford entrance exam and was awarded a scholarship. For the first 18 months, he was bored and lonely, owing to the fact that he was younger than his peers and found the work “ridiculously easy”. During his second and third year, Hawking made greater attempts to bond with his peers and developed into a popular student, joining the Oxford Boat Club and developing an interest in classical music and science fiction.
When it came time for his final exam, Hawking’s performance was lackluster. Instead of answering all the questions, he chose to focus on theoretical physics questions and avoided any that required factual knowledge. The result was a score that put him on the borderline between first- and second-class honors. Needing a first-class honors for his planned graduate studies in cosmology at Cambridge, he was forced to take a via (oral exam).
Concerned that he was viewed as a lazy and difficult student, Hawking described his future plans as follows during the viva: “If you award me a First, I will go to Cambridge. If I receive a Second, I shall stay in Oxford, so I expect you will give me a First.” However, Hawking was held in higher regard than he believed, and received a first-class BA (Hons.) degree, thus allowing him to pursue graduate work at Cambridge University in October 1962.
Hawking experienced some initial difficulty during his first year of doctoral studies. He found his background in mathematics inadequate for work in general relativity and cosmology, and was assigned Dennis William Sciama (one of the founders of modern cosmology) as his supervisor, rather than noted astronomer Fred Hoyle (whom he had been hoping for).
In addition, it was during his graduate studies that Hawking was diagnosed with early-onset amyotrophic lateral sclerosis (ALS). During his final year at Oxford, he had experienced an accident where he fell down a flight of stairs, and also began experiencing difficulties when rowing and incidents of slurred speech. When the diagnosis came in 1963, he fell into a state of depression and felt there was little point in continuing his studies.
However, his outlook soon changed, as the disease progressed more slowly than the doctors had predicted – initially, he was given two years to live. Then, with the encouragement of Sciama, he returned to his work, and quickly gained a reputation for brilliance and brashness. This was demonstrated when he publicly challenged the work of noted astronomer Fred Hoyle, who was famous for rejecting the Big Bang theory, at a lecture in June of 1964.
When Hawking began his graduate studies, there was much debate in the physics community about the prevailing theories of the creation of the universe: the Big Bang and the Steady State theories. In the former, the universe was conceived in a gigantic explosion, in which all matter in the known universe was created. In the latter, new matter is constantly created as the universe expands. Hawking quickly joined the debate.
Hawking became inspired by Roger Penrose’s theorem that a spacetime singularity – a point where the quantities used to measure the gravitational field of a celestial body become infinite – exists at the center of a black hole. Hawking applied the same thinking to the entire universe, and wrote his 1965 thesis on the topic. He went on to receive a research fellowship at Gonville and Caius College and obtained his PhD degree in cosmology in 1966.
It was also during this time that Hawking met his first wife, Jane Wilde. Though he had met her shortly before his diagnosis with ALS, their relationship continued to grow as he returned to complete his studies. The two became engaged in October of 1964 and were married on July 14th, 1966. Hawking would later say that his relationship with Wilde gave him “something to live for”.
In his doctoral thesis, which he wrote in collaboration with Penrose, Hawking extended the existence of singularities to the notion that the universe might have started as a singularity. Their joint essay – entitled, “Singularities and the Geometry of Space-Time” – was the runner-up in the 1968 Gravity Research Foundation competition and shared top honors with one by Penrose to win Cambridge’s most prestigious Adams Prize for that year.
In 1970, Hawking became part of the Sherman Fairchild Distinguished Scholars visiting professorship program, which allowed him to lecture at the California Institute of Technology (Caltech). It was during this time that he and Penrose published a proof that incorporated the theories of General Relativity and the physical cosmology developed by Alexander Freidmann.
Based on Einstein’s equations, Freidmann asserted that the universe was dynamic and changed in size over time. He also asserted that space-time had geometry, which is determined by its overall mass/energy density. If equal to the critical density, the universe has zero curvature (i.e. flat configuration); if it is less than critical, the universe has negative curvature (open configuration); and if greater than critical, the universe has a positive curvature (closed configuration)
According to the Hawking-Penrose singularity theorem, if the universe truly obeyed the models of general relativity, then it must have begun as a singularity. This essentially meant that, prior to the Big Bang, the entire universe existed as a point of infinite density that contained all of the mass and space-time of the universe, before quantum fluctuations caused it to rapidly expand.
Also in 1970, Hawking postulated what became known as the second law of black hole dynamics. With James M. Bardeen and Brandon Carter, he proposed the four laws of black hole mechanics, drawing an analogy with the four laws of thermodynamics.
These four laws stated that – for a stationary black hole, the horizon has constant surface gravity; for perturbations of stationary black holes, the change of energy is related to change of area, angular momentum, and electric charge; the horizon area is, assuming the weak energy condition, a non-decreasing function of time; and that it is not possible to form a black hole with vanishing surface gravity.
In 1971, Hawking released an essay titled “Black Holes in General Relativity” in which he conjectured that the surface area of black holes can never decrease, and therefore certain limits can be placed on the amount of energy they emit. This essay won Hawking the Gravity Research Foundation Award in January of that year.
In 1973, Hawking’s first book, which he wrote during his post-doc studies with George Ellis, was published. Titled, The Large Scale Structure of Space-Time, the book describes the foundation of space itself and the nature of its infinite expansion, using differential geometry to examine the consequences of Einstein’s General Theory of Relativity.
Hawking was elected a Fellow of the Royal Society (FRS) in 1974, a few weeks after the announcement of Hawking radiation (see below). In 1975, he returned to Cambridge and was given a new position as Reader, which is reserved for senior academics with a distinguished international reputation in research or scholarship.
The mid-to-late 1970s was a time of growing interest in black holes, as well as the researchers associated with them. As such, Hawking’s public profile began to grow and he received increased academic and public recognition, appearing in print and television interviews and receiving numerous honorary positions and awards.
In the late 1970s, Hawking was elected Lucasian Professor of Mathematics at the University of Cambridge, an honorary position created in 1663 which is considered one of the most prestigious academic posts in the world. Prior to Hawking, its former holders included such scientific greats as Sir Isaac Newton, Joseph Larmor, Charles Babbage, George Stokes, and Paul Dirac.
His inaugural lecture as Lucasian Professor of Mathematics was titled: “Is the end in sight for Theoretical Physics”. During the speech, he proposed N=8 Supergravity – a quantum field theory which involves gravity in 8 supersymmetries – as the leading theory to solve many of the outstanding problems physicists were studying.
Hawking’s promotion coincided with a health crisis which led to Hawking being forced to accept some nursing services at home. At the same time, he began making a transition in his approach to physics, becoming more intuitive and speculative rather than insisting on mathematical proofs. By 1981, this saw Hawking begin to focus his attention on cosmological inflation theory and the origins of the universe.
Inflation theory – which had been proposed by Alan Guth that same year – posits that following the Big Bang, the universe initially expanded very rapidly before settling into to a slower rate of expansion. In response, Hawking presented work at the Vatican conference that year, where he suggested that their might be no boundary or beginning to the universe.
During the summer of 1982, he and his colleague Gary Gibbons organized a three-week workshop on the subject titled “The Very Early Universe” at Cambridge University. With Jim Hartle, an American physicist and professor of physics at the University of California, he proposed that during the earliest period of the universe (aka. the Planck epoch) the universe had no boundary in space time.
In 1983, they published this model, known as the Hartle-Hawking state. Among other things, it asserted that before the Big Bang, time did not exist, and the concept of the beginning of the universe is therefore meaningless. It also replaced the initial singularity of the Big Bang with a region akin to the North Pole which (similar to the real North Pole) one cannot travel north of because it is a point where lines meet that has no boundary.
This proposal predicted a closed universe, which had many existential implications, particularly about the existence of God. At no point did Hawking rule out the existence of God, choosing to use God in a metaphorical sense when explaining the mysteries of the universe. However, he would often suggest that the existence of God was unnecessary to explain the origin of the universe, or the existence of a unified field theory.
In 1982, he also began work on a book that would explain the nature of the universe, relativity and quantum mechanics in a way that would be accessible to the general public. This led him to sign a contract with Bantam Books for the sake of publishing A Brief History of Time, the first draft of which he completed in 1984.
After multiple revisions, the final draft was published in 1988, and was met with much critical acclaim. The book was translated into multiple languages, remained at the top of bestseller lists in both the US and UK for months, and ultimately sold an estimated 9 million copies. Media attention was intense, and Newsweek magazine cover and a television special both described him as “Master of the Universe”.
Further work by Hawking in the area of arrows of time led to the 1985 publication of a paper theorizing that if the no-boundary proposition were correct, then when the universe stopped expanding and eventually collapsed, time would run backwards. He would later withdraw this concept after independent calculations disputed it, but the theory did provide valuable insight into the possible connections between time and cosmic expansion.
During the 1990’s, Hawking continued to publish and lecture on his theories regarding physics, black holes and the Big Bang. In 1993, he co-edited a book with Gary Gibbons on on Euclidean quantum gravity, a theory they had been working on together in the late 70s. According to this theory, a section of a gravitational field in a black hole can be evaluated using a functional integral approach, such that it can avoid the singularities.
It was also in 1990s that major developments happened in Hawking’s personal life. In 1990, he and Jane Hawking commenced divorce proceedings after many years of strained relations, owing to his disability, the constant presence of care-givers, and his celebrity status. Hawking remarried in 1995 to Elaine Mason, his caregiver of many years.
In the 2000s, Hawking produced many new books and new editions of older ones. These included The Universe in a Nutshell (2001), A Briefer History of Time (2005), and God Created the Integers (2006). He also began collaborating with Jim Hartle of the University of California, Santa Barbara, and the European Organization for Nuclear Research (CERN) to produce new cosmological theories.
Foremost of these was Hawking’s “top-down cosmology”, which states that the universe had not one unique initial state but many different ones, and that predicting the universe’s current state from a single initial state is therefore inappropriate. Consistent with quantum mechanics, top-down cosmology posits that the present “selects” the past from a superposition of many possible histories.
In so doing, the theory also offered a possible resolution of the “fine-tuning question”, which addresses the possibility that life can only exist when certain physical constraints lie within a narrow range. By offering this new model of cosmology, Hawking opened up the possibility that life may not be bound by such restrictions and could be much more plentiful than previously thought.
In 2006, Hawking and his second wife, Elaine Mason, quietly divorced, and Hawking resumed closer relationships with his first wife Jane, his children (Robert, Lucy and Timothy), and grandchildren. In 2009, he retired as Lucasian Professor of Mathematics, which was required by Cambridge University regulations. Hawking has continued to work as director of research at the Cambridge University Department of Applied Mathematics and Theoretical Physics ever since, and has made no indication of retiring.
“Hawking Radiation” and the “Black Hole Information Paradox”:
In the early 1970s, Hawking’s began working on what is known as the “no-hair theorem”. Based on the Einstein-Maxwell equations of gravitation and electromagnetism in general relativity, the theorem stated that all black holes can be completely characterized by only three externally observable classical parameters: mass, electric charge, and angular momentum.
In this scenario, all other information about the matter which formed a black hole or is falling into it (for which “hair’ is used as a metaphor), “disappears” behind the black-hole event horizon, and is therefore preserved but permanently inaccessible to external observers.
In 1973, Hawking traveled to Moscow and met with Soviet scientists Yakov Borisovich Zel’dovich and Alexei Starobinsky. During his discussions with them about their work, they showed him how the uncertainty principle demonstrated that black holes should emit particles. This contradicted Hawking’ second law of black hole thermodynamics (i.e. black holes can’t get smaller) since it meant that by losing energy they must be losing mass.
What’s more, it supported a theory advanced by Jacob Bekenstein, a graduate student of John Wheeler University, that black holes should have a finite, non-zero temperature and entropy. All of this contradicted the “no-hair theorem” about black boles. Hawking revised this theorem shortly thereafter, showing that when quantum mechanical effects are taken into account, one finds that black holes emit thermal radiation at a temperature.
From 1974 onward, Hawking presented Bekenstein’s results, which showed that black holes emit radiation. This came to be known as “Hawking radiation”, and was initially controversial. However, by the late 1970s and following the publication of further research, the discovery was widely accepted as a significant breakthrough in theoretical physics.
However, one of the outgrowths of this theory was the likelihood that black holes gradually lose mass and energy. Because of this, black holes that lose more mass than they gain through other means are expected to shrink and ultimately vanish – a phenomena which is known as black hole “evaporation”.
In 1981, Hawking proposed that information in a black hole is irretrievably lost when a black hole evaporates, which came to be known as the “Black Hole Information Paradox”. This states that physical information could permanently disappear in a black hole, allowing many physical states to devolve into the same state.
This was controversial because it violated two fundamental tenets of quantum physics. In principle, quantum physics tells us that complete information about a physical system – i.e. the state of its matter (mass, position, spin, temperature, etc.) – is encoded in its wave function up to the point when that wave function collapses. This in turn gives rise to two other principles.
The first is Quantum Determinism, which states that – given a present wave function – future changes are uniquely determined by the evolution operator. The second is Reversibility, which states that the evolution operator has an inverse, meaning that the past wave functions are similarly unique. The combination of these means that the information about the quantum state of matter must always be preserved.
By proposing that this information disappears once a black evaporates, Hawking essentially created a fundamental paradox. If a black hole can evaporate, which causes all the information about a quantum wave function to disappear, than information can in fact be lost forever. This has been the subject of ongoing debate among scientists, one which has remained largely unresolved.
However, by 2003, the growing consensus among physicists was that Hawking was wrong about the loss of information in a black hole. In a 2004 lecture in Dublin, he conceded his bet with fellow John Preskill of Caltech (which he made in 1997), but described his own, somewhat controversial solution to the paradox problem – that black holes may have more than one topology.
In the 2005 paper he published on the subject – “Information Loss in Black Holes” – he argued that the information paradox was explained by examining all the alternative histories of universes, with the information loss in those with black holes being cancelled out by those without. As of January 2014, Hawking has described the Black Hole Information Paradox as his “biggest blunder”.
In addition to advancing our understanding of black holes and cosmology through the application of general relativity and quantum mechanics, Stephen Hawking has also been pivotal in bringing science to a wider audience. Over the course of his career, he has published many popular books, traveled and lectured extensively, and has made numerous appearances and done voice-over work for television shows, movies and even provided narration for the Pink Floyd song, “Keep Talking”.
A film version of A Brief History of Time, directed by Errol Morris and produced by Steven Spielberg, premiered in 1992. Hawking had wanted the film to be scientific rather than biographical, but he was persuaded otherwise. In 1997, a six-part television series Stephen Hawking’s Universe premiered on PBS, with a companion book also being released.
In 2007, Hawking and his daughter Lucy published George’s Secret Key to the Universe, a children’s book designed to explain theoretical physics in an accessible fashion and featuring characters similar to those in the Hawking family. The book was followed by three sequels – George’s Cosmic Treasure Hunt (2009), George and the Big Bang (2011), George and the Unbreakable Code (2014).
Since the 1990s, Hawking has also been a major role model for people dealing with disabilities and degenerative illnesses, and his outreach for disability awareness and research has been unparalleled. At the turn of the century, he and eleven other luminaries joined with Rehabilitation International to sign the Charter for the Third Millennium on Disability, which called on governments around the world to prevent disabilities and protect disability rights.
Motivated by the desire to increase public interest in spaceflight and to show the potential of people with disabilities, in 2007 he participated in zero-gravity flight in a “Vomit Comet” – a specially fitted aircraft that dips and climbs through the air to simulate the feeling of weightlessness – courtesy of Zero Gravity Corporation, during which he experienced weightlessness eight times.
In August 2012, Hawking narrated the “Enlightenment” segment of the 2012 Summer Paralympics opening ceremony. In September of 2013, he expressed support for the legalization of assisted suicide for the terminally ill. In August of 2014, Hawking accepted the Ice Bucket Challenge to promote ALS/MND awareness and raise contributions for research. As he had pneumonia in 2013, he was advised not to have ice poured over him, but his children volunteered to accept the challenge on his behalf.
During his career, Hawking has also been a committed educator, having personally supervised 39 successful PhD students.He has also lent his name to the ongoing search for extra-terrestrial intelligence and the debate regarding the development of robots and artificial intelligence. On July 20th, 2015, Stephen Hawking helped launch Breakthrough Initiatives, an effort to search for extraterrestrial life in the universe.
Also in 2015, Hawking lent his voice and celebrity status to the promotion of The Global Goals, a series of 17 goals adopted by the United Nations Sustainable Development Summit to end extreme poverty, social inequality, and fixing climate change over the course of the next 15 years.
Honors and Legacy:
As already noted, in 1974, Hawking was elected a Fellow of the Royal Society (FRS), and was one of the youngest scientists to become a Fellow. At that time, his nomination read:
Hawking has made major contributions to the field of general relativity. These derive from a deep understanding of what is relevant to physics and astronomy, and especially from a mastery of wholly new mathematical techniques. Following the pioneering work of Penrose he established, partly alone and partly in collaboration with Penrose, a series of successively stronger theorems establishing the fundamental result that all realistic cosmological models must possess singularities. Using similar techniques, Hawking has proved the basic theorems on the laws governing black holes: that stationary solutions of Einstein’s equations with smooth event horizons must necessarily be axisymmetric; and that in the evolution and interaction of black holes, the total surface area of the event horizons must increase. In collaboration with G. Ellis, Hawking is the author of an impressive and original treatise on “Space-time in the Large.
Other important work by Hawking relates to the interpretation of cosmological observations and to the design of gravitational wave detectors.
In 1975, he was awarded both the Eddington Medal and the Pius XI Gold Medal, and in 1976 the Dannie Heineman Prize, the Maxwell Prize and the Hughes Medal. In 1977, he was appointed a professor with a chair in gravitational physics, and received the Albert Einstein Medal and an honorary doctorate from the University of Oxford by the following year.
In 1981, Hawking was awarded the American Franklin Medal, followed by a Commander of the Order of the British Empire (CBE) medal the following year. For the remainder of the decade, he was honored three times, first with the Gold Medal of the Royal Astronomical Society in 1985, the Paul Dirac Medal in 1987 and, jointly with Penrose, with the prestigious Wolf Prize in 1988. In 1989, he was appointed Member of the Order of the Companions of Honour (CH), but reportedly declined a knighthood.
In 1999, Hawking was awarded the Julius Edgar Lilienfeld Prize of the American Physical Society. In 2002, following a UK-wide vote, the BBC included him in their list of the 100 Greatest Britons. More recently, Hawking has been awarded the Copley Medal from the Royal Society (2006), the Presidential Medal of Freedom, America’s highest civilian honor (2009), and the Russian Special Fundamental Physics Prize (2013).
Also in 2008, while traveling to Spain, Hawking received the Fonseca Prize – an annual award created by the University of Santiago de Compostela which is awarded to those for outstanding achievement in science communication. Hawking was singled out for the award because of his “exceptional mastery in the popularization of complex concepts in Physics at the very edge of our current understanding of the Universe, combined with the highest scientific excellence, and for becoming a public reference of science worldwide.”
Multiple films have been made about Stephen Hawking over the years as well. These include the previously mentioned A Brief History of Time, the 1991 biopic film directed by Errol Morris and Stephen Spielberg; Hawking, a 2004 BBC drama starring Benedict Cumberbatch in the title role; the 2013 documentary titled “Hawking”, by Stephen Finnigan.
Most recently, there was the 2014 film The Theory of Everything that chronicled the life of Stephen Hawking and his wife Jane. Directed by James Marsh, the movie stars Eddie Redmayne as Professor Hawking and Felicity Jones as Jane Hawking.
Dr. Stephen Hawking passed away in the early hours of Wednesday, March 14th, 2018 at his home in Cambridge. According to a statement made by his family, he died peacefully. He was 76 years old, and is survived by his first wife, Jane Wilde, and their three children – Lucy, Robert and Tim.
When all is said and done, Stephen Hawking was the arguably the most famous scientist alive in the modern era. His work in the field of astrophysics and quantum mechanics has led to a breakthrough in our understanding of time and space, and will likely be poured over by scientists for decades. In addition, he has done more than any living scientist to make science accessible and interesting to the general public.
To top it off, he traveled all over the world and lectured on topics ranging from science and cosmology to human rights, artificial intelligence, and the future of the human race. He also used the celebrity status afforded him to advance the causes of scientific research, space exploration, disability awareness, and humanitarian causes wherever possible.
In all of these respects, he was very much like his predecessor, Albert Einstein – another influential scientist-turned celebrity who was sure to use his powers to combat ignorance and promote humanitarian causes. But what was especially impressive in all of this is that Hawking has managed to maintain his commitment to science and a very busy schedule while dealing with a degenerative disease.
For over 50 years, Hawking lived with a disease that doctor’s initially thought would take his life within just two. And yet, he not only managed to make his greatest scientific contributions while dealing with ever-increasing problems of mobility and speech, he also became a jet-setting personality who travelled all around the world to address audiences and inspire people.
His passing was mourned by millions worldwide and, in the worlds of famed scientist and science communicator Neil DeGrasse Tyson , “left an intellectual vacuum in its wake”. Without a doubt, history will place Dr. Hawking among such luminaries as Einstein, Newton, Galileo and Curie as one of the greatest scientific minds that ever lived.
Have you heard that black holes destroy any information that goes into them? Why is this such a big problem for physics?
In my day, things were simple. Robot dogs had wheels and laser noses. School was uphill both ways. Unwanted children removed themselves from lawns, and we didn’t need those horrible electrified tentacle arms. The cut of my jib was completely beyond reproach. Nathan Fillion was the captain of the Serenity all day, every day. … And black holes were holes that were black. By that I mean black holes would compress matter and energy into an infinitely dense singularity, and didn’t create a seemingly insurmountable information paradox. Yep, those were the good ole’ days.
But those days are over. Now it’s all 50 shades of grey, with the laws of physics bending under other laws of physics. “Hashtag not my Christian”. What I’m talking about is the black hole information paradox.
First, let’s talk information. When physicists talk information, they’re on about the specific state of every single particle in the Universe: mass, position, spin, temperature, you name it. The fingerprint that uniquely identifies each one, and the probabilities for what they’re going to do in the Universe. You can change atoms, crush them together, but the quantum wave function that describes them must always be preserved.
Quantum physics allows you to run the whole Universe forwards and backwards, as long as you reverse everything in your math: charge, parity and time. Here’s the important part. The big brains tell us information must live on, no matter what. Think about it like energy. You can’t destroy energy, all you can do is transform it.
Now, the black hole recap. Naturally formed when the largest stars, those with more than 20 times the mass of the Sun, collapse violently and explode. Here the density of matter is so high, the escape velocity exceeds the speed of light. The fancy ones have a super-heated accretion disk of matter swirling around the black hole event horizon, where even light can be pulled into orbit.
Here, we get one of the strangest side effects from Relativity: time dilation. Imagine a clock falling towards a black hole, moving deeper into the gravity well. It would appear to slow as it got closer to the black hole, and eventually freeze at the edge of the event horizon. Photons from the clock would stretch out, and the color of the clock would redshift. Eventually, it fades away as the photons stretched out beyond what our eyes can detect.
If you could stare at the black hole for billions of years, you would see everything it ever collected, stuck to the outside like flypaper. You could point out the clock, the Titanic, the Ranger, and USS Cygnus, and theoretically, you could identify the quantum state of every single particle and photon that went into the black hole. Since they’re going to take an infinite length of time to disappear completely, everything’s fine.
Their information is preserved forever on the surface of the black hole. They’re all totally dead, but their information, their precious precious quantum information, is totally safe.
If you could unravel a black hole, you could get at all the quantum information describing everything the black hole ever consumed. And least, that’s how it was in the good old days.
But in 1975, Hawking dropped a bombshell. He realized black holes have a temperature, over vast periods of time, they would evaporate away until there was nothing left. releasing their mass and energy back into the Universe. Unsurprisingly known as Hawking Radiation.
But this… idea created a paradox. The information about what went into the black hole is preserved by time dilation, but with the mass itself of the black hole evaporating. Eventually, it will completely disappear, and then, where does our information go? That information which can’t be destroyed…?
This is strictly not cricket, and puzzled astronomers. They’ve been working for decades to resolve it. There’s a fun stack of options here:
Black holes don’t evaporate at all, and Hawking was wrong.
Information within the black hole somehow leaks back out while Hawking radiation is escaping.
The black hole holds it all in until the very end, and as the final two particles evaporate, all the information is suddenly released back into the Universe.
It all goes into the teeniest possible bits and nothing is lost OR The information is compressed into a microscopic space, which remains after the black hole itself has evaporated.
And maybe, physicists will never figure it out. Hawking recently proposed a new idea to resolve the black hole information paradox. He has suggested that there’s a way that new Hawking radiation could be imprinted by the information of new matter falling into the black hole.
So, the information of everything falling in is preserved by the outgoing radiation, returning it to the Universe and resolving the paradox. This is a hunch, since Hawking radiation itself has never been detected. We are decades away from knowing if this is in the right direction, or even if there’s a way to resolve the paradox.
In situations like this that we’re reminded how little about the Universe we really understand. Some aspect of our understanding of this whole process is unclear, and it’ll take much more detective work and experimentation to get closer to the truth.
What information would like to be destroyed from the Universe forever? Tell us all your secrets in the comments below.
We’ve come a long way in 13.8 billion years; but despite our impressively extensive understanding of the Universe, there are still a few strings left untied. For one, there is the oft-cited disconnect between general relativity, the physics of the very large, and quantum mechanics, the physics of the very small. Then there is problematic fate of a particle’s intrinsic information after it falls into a black hole. Now, a new interpretation of fundamental physics attempts to solve both of these conundrums by making a daring claim: at certain scales, space and time simply do not exist.
Let’s start with something that is not in question. Thanks to Einstein’s theory of special relativity, we can all agree that the speed of light is constant for all observers. We can also agree that, if you’re not a photon, approaching light speed comes with some pretty funky rules – namely, anyone watching you will see your length compress and your watch slow down.
But the slowing of time also occurs near gravitationally potent objects, which are described by general relativity. So if you happen to be sight-seeing in the center of the Milky Way and you make the regrettable decision to get too close to our supermassive black hole’s event horizon (more sinisterly known as its point-of-no-return), anyone observing you will also see your watch slow down. In fact, he or she will witness your motion toward the event horizon slow dramatically over an infinite amount of time; that is, from your now-traumatized friend’s perspective, you never actually cross the event horizon. You, however, will feel no difference in the progression of time as you fall past this invisible barrier, soon to be spaghettified by the black hole’s immense gravity.
So, who is “correct”? Relativity dictates that each observer’s point of view is equally valid; but in this situation, you can’t both be right. Do you face your demise in the heart of a black hole, or don’t you? (Note: This isn’t strictly a paradox, but intuitively, it feels a little sticky.)
And there is an additional, bigger problem. A black hole’s event horizon is thought to give rise to Hawking radiation, a kind of escaping energy that will eventually lead to both the evaporation of the black hole and the destruction of all of the matter and energy that was once held inside of it. This concept has black hole physicists scratching their heads. Because according to the laws of physics, all of the intrinsic information about a particle or system (namely, the quantum wavefunction) must be conserved. It cannot just disappear.
Why all of these bizarre paradoxes? Because black holes exist in the nebulous space where a singularity meets general relativity – fertile, yet untapped ground for the elusive theory of everything.
Enter two interesting, yet controversial concepts: doubly special relativity and gravity’s rainbow.
Just as the speed of light is a universally agreed-upon constant in special relativity, so is the Planck energy in doubly special relativity (DSR). In DSR, this value (1.22 x 1019 GeV) is the maximum energy (and thus, the maximum mass) that a particle can have in our Universe.
Two important consequences of DSR’s maximum energy value are minimum units of time and space. That is, regardless of whether you are moving or stationary, in empty space or near a black hole, you will agree that classical space breaks down at distances shorter than the Planck length (1.6 x 10-35 m) and classical time breaks down at moments briefer than the Planck time (5.4 x 10-44 sec).
In other words, spacetime is discrete. It exists in indivisible (albeit vanishingly small) units. Quantum below, classical above. Add general relativity into the picture, and you get the theory of gravity’s rainbow.
Physicists Ahmed Farag Ali, Mir Faizal, and Barun Majumder believe that these theories can be used to explain away the aforementioned black hole conundrums – both your controversial spaghettification and the information paradox. How? According to DSR and gravity’s rainbow, in regions smaller than 1.6 x 10-35 m and at times shorter than 5.4 x 10-44 sec… the Universe as we know it simply does not exist.
“In gravity’s rainbow, space does not exist below a certain minimum length, and time does not exist below a certain minimum time interval,” explained Ali, who, along with Faizal and Majumder, authored a paper on this topic that was published last month. “So, all objects existing in space and occurring at a time do not exist below that length and time interval [which are associated with the Planck scale].”
Luckily for us, every particle we know of, and thus every particle we are made of, is much larger than the Planck length and endures for much longer than the Planck time. So – phew! – you and I and everything we see and know can go on existing. (Just don’t probe too deeply.)
The event horizon of a black hole, however, is a different story. After all, the event horizon isn’t made of particles. It is pure spacetime. And according to Ali and his colleagues, if you could observe it on extremely short time or distance scales, it would cease to have meaning. It wouldn’t be a point-of-no-return at all. In their view, the paradox only arises when you treat spacetime as continuous – without minimum units of length and time.
“As the information paradox depends on the existence of the event horizon, and an event horizon like all objects does not exist below a certain length and time interval, then there is no absolute information paradox in gravity’s rainbow. The absence of an effective horizon means that there is nothing absolutely stopping information from going out of the black hole,” concluded Ali.
No absolute event horizon, no information paradox.
And what of your spaghettification within the black hole? Again, it depends on the scale at which you choose to analyze your situation. In gravity’s rainbow, spacetime is discrete; therefore, the mathematics reveal that both you (the doomed in-faller) and your observer will witness your demise within a finite length of time. But in the current formulation of general relativity, where spacetime is described as continuous, the paradox arises. The in-faller, well, falls in; meanwhile, the observer never sees the in-faller pass the event horizon.
“The most important lesson from this paper is that space and time exist only beyond a certain scale,” said Ali. “There is no space and time below that scale. Hence, it is meaningless to define particles, matter, or any object, including black holes, that exist in space and time below that scale. Thus, as long as we keep ourselves confined to the scales at which both space and time exist, we get sensible physical answers. However, when we try to ask questions at length and time intervals that are below the scales at which space and time exist, we end up getting paradoxes and problems.”
To recap: if spacetime continues on arbitrarily small scales, the paradoxes remain. If, however, gravity’s rainbow is correct and the Planck length and the Planck time are the smallest unit of space and time that fundamentally exist, we’re in the clear… at least, mathematically speaking. Unfortunately, the Planck scales are far too tiny for our measly modern particle colliders to probe. So, at least for now, this work provides yet another purely theoretical result.
The paper was published in the January 23 issue of Europhysics Letters. A pre-print of the paper is available here.
Dr. Stephen Hawking delivered a disturbing theory in 1974 that claimed black holes evaporate. He said black holes are not absolutely black and cold but rather radiate energy and do not last forever. So-called “Hawking radiation” became one of the physicist’s most famous theoretical predictions. Now, 40 years later, a researcher has announced the creation of a simulation of Hawking radiation in a laboratory setting.
The possibility of a black hole came from Einstein’s theory of General Relativity. Karl Schwarzchild in 1916 was the first to realize the possibility of a gravitational singularity with a boundary surrounding it at which light or matter entering cannot escape.
This month, Jeff Steinhauer from the Technion – Israel Institute of Technology, describes in his paper, “Observation of self-amplifying Hawking radiation in an analogue black-hole laser” in the journal Nature, how he created an analogue event horizon using a substance cooled to near absolute zero and using lasers was able to detect the emission of Hawking radiation. Could this be the first valid evidence of the existence of Hawking radiation and consequently seal the fate of all black holes?
This is not the first attempt at creating a Hawking radiation analogue in a laboratory. In 2010, an analogue was created from a block of glass, a laser, mirrors and a chilled detector (Phys. Rev. Letter, Sept 2010); no smoke accompanied the mirrors. The ultra-short pulse of intense laser light passing through the glass induced a refractive index perturbation (RIP) which functioned as an event horizon. Light was seen emitting from the RIP. Nevertheless, the results by F. Belgiorno et al. remain controversial. More experiments were still warranted.
The latest attempt at replicating Hawking radiation by Steinhauer takes a more high tech approach. He creates a Bose-Einstein condensate, an exotic state of matter at very near absolute zero temperature. Boundaries created within the condensate functioned as an event horizon. However, before going into further details, let us take a step back and consider what Steinhauer and others are trying to replicate.
The recipe for the making Hawking radiation begins with a black hole. Any size black hole will do. Hawking’s theory states that smaller black holes will more rapidly radiate than larger ones and in the absence of matter falling into them – accretion, will “evaporate” much faster. Giant black holes can take longer than a million times the present age of the Universe to evaporate by way of Hawking radiation. Like a tire with a slow leak, most black holes would get you to the nearest repair station.
So you have a black hole. It has an event horizon. This horizon is also known as the Schwarzchild radius; light or matter checking into the event horizon can never check out. Or so this was the accepted understanding until Dr. Hawking’s theory upended it. And outside the event horizon is ordinary space with some caveats; consider it with some spices added. At the event horizon the force of gravity from the black hole is so extreme that it induces and magnifies quantum effects.
All of space – within us and surrounding us to the ends of the Universe includes a quantum vacuum. Everywhere in space’s quantum vacuum, virtual particle pairs are appearing and disappearing; immediately annihilating each other on extremely short time scales. With the extreme conditions at the event horizon, virtual particle and anti-particles pairs, such as, an electron and positron, are materializing. The ones that appear close enough to an event horizon can have one or the other virtual particle zapped up by the black holes gravity leaving only one particle which consequently is now free to add to the radiation emanating from around the black hole; the radiation that as a whole is what astronomers can use to detect the presence of a black hole but not directly observe it. It is the unpairing of virtual particles by the black hole at its event horizon that causes the Hawking radiation which by itself represents a net loss of mass from the black hole.
So why don’t astronomers just search in space for Hawking radiation? The problem is that the radiation is very weak and is overwhelmed by radiation produced by many other physical processes surrounding the black hole with an accretion disk. The radiation is drowned out by the chorus of energetic processes. So the most immediate possibility is to replicate Hawking radiation by using an analogue. While Hawking radiation is weak in comparison to the mass and energy of a black hole, the radiation has essentially all the time in the Universe to chip away at its parent body.
This is where the convergence of the growing understanding of black holes led to Dr. Hawking’s seminal work. Theorists including Hawking realized that despite the Quantum and Gravitational theory that is necessary to describe a black hole, black holes also behave like black bodies. They are governed by thermodynamics and are slaves to entropy. The production of Hawking radiation can be characterized as a thermodynamic process and this is what leads us back to the experimentalists. Other thermodynamic processes could be used to replicate the emission of this type of radiation.
Using the Bose-Einstein condensate in a vessel, Steinhauer directed laser beams into the delicate condensate to create an event horizon. Furthermore, his experiment creates sound waves that become trapped between two boundaries that define the event horizon. Steinhauer found that the sound waves at his analogue event horizon were amplified as happens to light in a common laser cavity but also as predicted by Dr. Hawking’s theory of black holes. Light escapes from the laser present at the analogue event horizon. Steinhauer explains that this escaping light represents the long sought Hawking radiation.
Publication of this work in Nature underwent considerable peer review to be accepted but that alone does not validate his findings. Steinhauer’s work will now withstand even greater scrutiny. Others will attempt to duplicate his work. His lab setup is an analogue and it remains to be verified that what he is observing truly represents Hawking radiation.
A new paper has been posted on the arxiv (a repository of research preprints) introducing the idea of a Planck star arising from a black hole. These hypothetical objects wouldn’t be a star in the traditional sense, but rather the light emitted when a black hole dies at the hands of Hawking radiation. The paper hasn’t been peer reviewed, but it presents an interesting idea and a possible observational test.
When a large star reaches the end of its life, it explodes as a supernova, which can cause its core to collapse into a black hole. In the traditional model of a black hole, the material collapses down into an infinitesimal volume known as a singularity. Of course this doesn’t take into account quantum theory.
Although we don’t have a complete theory of quantum gravity, we do know a few things. One is that black holes shouldn’t last forever. Because of quantum fluctuations near the event horizon of a black hole, a black hole will emit Hawking radiation. As a result, a black hole will gradually lose mass as it radiates. The amount of Hawking radiation it emits is inversely proportional to its size, so as the black hole gets smaller it will emit more and more Hawking radiation until it finally radiates completely away.
Because black holes don’t last forever, this has led Stephen Hawking and others to propose that black holes don’t have an event horizon, but rather an apparent horizon. This would mean the material within a black hole would not collapse into a singularity, which is where this new paper comes in.
The authors propose that rather than collapsing into a singularity, the matter within a black hole will collapse until it is about a trillionth of a meter in size. At that point its density would be on the order of the Planck density. When the the black hole ends its life, this “Planck star” would be revealed. Because this “star” would be at the Planck density, it would radiate at a specific wavelength of gamma rays. So if they exist, a gamma ray telescope should be able to observe them.
Just to be clear, this is still pretty speculative. So far there isn’t any observational evidence that such a Planck star exists. It is, however, an interesting solution to the paradoxical side of black holes.
In 1974, Steven Hawking proposed a seemingly ridiculous hypothesis. Black holes, the gravitational monsters from which nothing escapes, evaporate. To justify this, he proposed that pairs of virtual particles in which one strayed too close to the event horizon, could be split, causing one particle to escape and become an actual particle that could escape. This carrying off of mass would take energy and mass away from the black hole and deplete it. Due to the difficulty of observing astronomical black holes, this emission has gone undetected. But recently, a team of Italian physicists, led by Francesco Belgiorno, claims to have observed Hawking radiation in the lab. Well, sort of. It depends on your definition.
The experiment worked by sending powerful laser pulses through a block of ultra-pure glass. The intensity of the laser would change the optical properties of the glass increasing the refractive index to the point that light could not pass. In essence, this created an artificial event horizon. But instead of being a black hole which particles could pass but never return, this created a “white hole” in which particles could never pass in the first place. If a virtual pair were created near this barrier, one member could be trapped on one side while the other member could escape and be detected creating a situation analogous to that predicted by Hawking radiation.
Readers with some background in quantum physics may be scratching their heads at this point. The experiment uses a barrier to impede the photons, but quantum tunneling has demonstrated that there’s no such thing as a perfect barrier. Some photons should tunnel through. To avoid detecting these photons, the team simply moved the detector. While some photons will undoubtedly tunnel through, they would continue on the same path with which they were set. The detector was moved 90º to avoid detecting such photons.
The change in position also helped to minimize other sources of false detections such as scattering. At 90º, scattering only occurs for vertically polarized light and the experiment used horizontally polarized light. As a check to make sure none of the light became mispolarized, the team checked to ensure no photons of the emitted wavelength were observed. The team also had to guard against false detections from absorption and re-emission from the molecules in the glass (fluorescence). This was achieved through experimentation to gain an understanding of how much of this to expect so the effects could be subtracted out. Additionally, the group chose a wavelength in which fluorescence was minimized.
After all the removal of sources of error for which the team could account, they still reported a strong signal which they interpreted as coming from separated virtual particles and call a detection of Hawking radiation. Other scientists disagree in the definition. While they do not question the interpretation, others note that Hawking radiation, by definition, only occurs at gravitational event horizons. While this detection is interesting, it does not help to shed light on the more interesting effects that come with such gravitational event horizons such as quantum gravity or the paradox provided by the Trans-Planckian problem. In other words, while this may help to establish that virtual particles like this exist, it says nothing of whether or not they could truly escape from near a black hole, which is a requirement for “true” Hawking radiation.
Meanwhile, other teams continue to explore similar effects with other artificial barriers and event horizons to explore the effects of these virtual particles. Similar effects have been reported in other such systems including ones with water waves to form the barrier.