stephen hawking time travel story

Stephen Hawking’s final book suggests time travel may one day be possible – here’s what to make of it

Research Fellow in the Particle Cosmology Group, School of Physics and Astronomy, University of Nottingham

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“If one made a research grant application to work on time travel it would be dismissed immediately,” writes the physicist Stephen Hawking in his posthumous book Brief Answers to the Big Questions . He was right. But he was also right that asking whether time travel is possible is a “very serious question” that can still be approached scientifically.

Arguing that our current understanding cannot rule it out, Hawking, it seems, was cautiously optimistic. So where does this leave us? We cannot build a time machine today, but could we in the future?

Let’s start with our everyday experience. We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now . But this is something we can never actually know. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us.

Our inability to access the “now” of someone far away is at the heart of Albert Einstein’s theories of space and time .

Light speed

Einstein told us that space and time are parts of one thing – spacetime – and that we should be as willing to think about distances in time as we are distances in space. As odd as this might sound, we happily answer “about two and half hours”, when someone asks how far Birmingham is from London. What we mean is that the journey takes that long at an average speed of 50 miles per hour.

Mathematically, our statement is equivalent to saying that Birmingham is about 125 miles from London. As physicists Brian Cox and Jeff Forshaw write in their book Why does E=mc²? , time and distance “can be interchanged using something that has the currency of a speed”. Einstein’s intellectual leap was to suppose that the exchange rate from a time to a distance in spacetime is universal – and it is the speed of light.

The speed of light is the fastest any signal can travel, putting a fundamental limit on how soon we can know what is going on elsewhere in the universe. This gives us “causality” – the law that effects must always come after their causes. It is a serious theoretical thorn in the side of time-travelling protagonists. For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth).

Now, if the speed of light is universal (in the vacuum of empty space), we must measure it to be the same – 299,792,458 metres per second – however fast we ourselves are moving. Einstein realised that the consequence of the speed of light being absolute is that space and time itself cannot be. And it turns out that moving clocks must tick slower than stationary ones.

If I were to fly off at incredible speed in a spaceship and return to Earth , less time would pass for me than it would for everyone I left behind. Everyone I returned to would conclude that my life had run as if in slow motion – I would have aged more slowly than them – and I would conclude that theirs had run as if in fast forward. The faster I travelled, the slower my clock would tick relative to clocks on Earth. And if I made the trip at the speed of light, I would return as if I had been frozen in time.

So what if we were to travel faster than light, would time run backwards as science fiction has taught us?

Unfortunately, it takes infinite energy to accelerate a human being to the speed of light, let alone beyond it. But even if we could , time wouldn’t simply run backwards. Instead, it would no longer make sense to talk about forward and backward at all. The law of causality would be violated and the concept of cause and effect would lose its meaning.

Einstein also told us that the force of gravity is a consequence of the way mass warps space and time . The more mass we squeeze into a region of space, the more spacetime is warped and the slower nearby clocks tick. If we squeeze in enough mass, spacetime becomes so warped that even light cannot escape its gravitational pull and a black hole is formed. And if you were to approach the edge of the black hole – its event horizon – your clock would tick infinitely slowly relative to those far away from it.

So could we warp spacetime in just the right way to close it back on itself and travel back in time?

The answer is maybe, and the warping we need is a traversable wormhole . But we also need to produce regions of negative energy density to stabilise it, and the classical physics of the 19th century prevents this. The modern theory of quantum mechanics , however, might not.

According to quantum mechanics, empty space is not empty. Instead, it is filled with pairs of particles that pop in and out of existence. If we can make a region where fewer pairs are allowed to pop in and out than everywhere else, then this region will have negative energy density.

However, finding a consistent theory that combines quantum mechanics with Einstein’s theory of gravity remains one of the biggest challenges in theoretical physics. One candidate, string theory (more precisely M-theory ) may offer up another possibility.

M-theory requires spacetime to have 11 dimensions: the one of time and three of space that we move in and seven more, curled up invisibly small. Could we use these extra spatial dimensions to shortcut space and time? Hawking, at least, was hopeful.

Saving history

So is time travel really a possibility? Our current understanding can’t rule it out, but the answer is probably no.

Einstein’s theories fail to describe the structure of spacetime at incredibly small scales. And while the laws of nature can often be completely at odds with our everyday experience, they are always self-consistent – leaving little room for the paradoxes that abound when we mess with cause and effect in science fiction’s take on time travel.

Despite his playful optimism, Hawking recognised that the undiscovered laws of physics that will one day supersede Einstein’s may conspire to prevent large objects like you and I from hopping casually (not causally) back and forth through time. We call this legacy his “ chronology protection conjecture ”.

Whether or not the future has time machines in store, we can comfort ourselves with the knowledge that when we climb a mountain or speed along in our cars, we change how time ticks.

So, this “ pretend to be a time traveller day ” (December 8), remember that you already are, just not in the way you might hope.

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A Brief History of Time is ‘wrong’, Stephen Hawking told collaborator

Thomas Hertog worked with cosmologist on a new book after he shared his doubts about A Brief History of Time

In 2002 Thomas Hertog received an email summoning him to the office of his mentor Stephen Hawking . The young researcher rushed to Hawking’s room at Cambridge. “His eyes were radiant with excitement,” Hertog recalls.

Typing on the computer-controlled voice system that allowed the cosmologist to communicate, Hawking announced: “I have changed my mind. My book, A Brief History of Time , is written from the wrong perspective.”

Thus one of the biggest-selling scientific books in publishing history, with worldwide sales credited at more than 10m, was consigned to the waste bin by its own author. Hawking and Hertog then began working on a new way to encapsulate their latest thinking about the universe.

Next month, five years after Hawking’s death, that book – On the Origin of Time: Stephen Hawking’s final theory – will be published in the UK. Hertog will outline its origins and themes at a Cambridge festival lecture on 31 March.

“The problem for Hawking was his struggle to understand how the universe could have created conditions so perfectly hospitable to life,” says Hertog, a cosmologist currently based at KU Leuven University in Belgium.

Examples of these life-supporting conditions include the delicate balance that exists between particle forces that allow chemistry and complex molecules to exist. In addition, the fact there are only three dimensions of space permits stable solar systems to evolve and provide homes for living creatures. Without these properties, the universe would probably not have produced life as we know it, it is argued by some cosmologists.

Hertog and Hawking were set on hammering out explanations for this state of stellar uncertainty after the latter had decided his previous attempts were inadequate. “Stephen told me he now thought he had been wrong and so he and I worked, shoulder to shoulder, for the next 20 years to develop a new theory of the cosmos, one that could better account for the emergence of life,” Hertog said.

It was a remarkable collaboration but not an easy one. When he was 21, Hawking had been diagnosed with an early onset slow-progressing form of motor neurone disease that gradually paralysed him.

By the time he began working with Hertog, he had been appointed Lucasian professor of mathematics at Cambridge University, one of the world’s most prestigious academic posts (Isaac Newton was a previous holder), and had produced a series of remarkable theories about general relativity, black holes and the origin of the universe as well as his bestseller A Brief History of Time . However, his condition had deteriorated. He was in a wheelchair and could only communicate using a small computer from which he selected words delivered by a speech synthesiser.

“Halfway through our collaboration, he lost the remaining strength in his hand to press the clicker which he used to converse,” says Hertog. So Hawking switched to a sensor mounted on his glasses that could be activated by twitching a cheek muscle, but eventually even that become too difficult.

He slowed from a few words per minute to several minutes per word, Hertog said. In the end, communication stopped. “I used to position myself in front of him and fire questions and would look into his eyes to see if he was agreeing or disagreeing. By the end, I could detect several levels of no and several levels of yes with a few in between.”

It was out of these “conversations” that Hawking’s final theory was born and, in conjunction with Hertog’s own analysis, they form the basis of On the Origin of Time , a book that takes its title from Charles Darwin’s On the Origin of Species . “In the end, we both came to think of physics in a way much more like how we think of biology. We have put physics and biology on the same footing.”

According to Hertog, On the Origin of Time deals with questions about our place in the universe and what makes our universe fit for life. “These questions were always in the background in our scientific publications. What I have done for this book is to make these questions central and tell our story from that perspective.

“Stephen and I discovered how physics itself can disappear back into the big bang. Not the laws as such but their capacity to change has the final word in our theory. This sheds a new light on what cosmology is ultimately about.”

According to Hertog, the new perspective that he has achieved with Hawking reverses the hierarchy between laws and reality in physics and is “profoundly Darwinian” in spirit. “It leads to a new philosophy of physics that rejects the idea that the universe is a machine governed by unconditional laws with a prior existence, and replaces it with a view of the universe as a kind of self-organising entity in which all sorts of emergent patterns appear, the most general of which we call the laws of physics.”

This article was amended on 19 March 2023 to correct an instance of a misspelling of Hertog’s surname.

On the Origin of Time is published by Penguin Random House on 6 April

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Comment and Physics

A brief history of stephen hawking: a legacy of paradox.

By Stuart Clark

14 March 2018

Gemma Levine/Getty

Stephen Hawking, the world-famous theoretical physicist, has died at the age of 76.

Hawking’s children, Lucy, Robert and Tim said in a statement: “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. His courage and persistence with his brilliance and humour inspired people across the world.

“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 for ever.”

Stephen Hawking dies aged 76

Tributes flow in following the death of world-famous theoretical physicist stephen hawking.

The most recognisable scientist of our age, Hawking holds an iconic status. His genre-defining book, A Brief History of Time , has sold more than 10 million copies since its publication in 1988, and has been translated into more than 35 languages. He appeared on Star Trek: The Next Generation , The Simpsons and The Big Bang Theory . His early life was the subject of an Oscar-winning performance by Eddie Redmayne in the 2014 film  The Theory of Everything . He was routinely consulted for oracular pronouncements on everything from time travel and alien life to Middle Eastern politics and nefarious robots . He had an endearing sense of humour and a daredevil attitude – relatable human traits that, combined with his seemingly superhuman mind, made Hawking eminently marketable.

But his cultural status – amplified by his disability and the media storm it invoked – often overshadowed his scientific legacy. That’s a shame for the man who discovered what might prove to be the key clue to the theory of everything , advanced our understanding of space and time, helped shape the course of physics for the last four decades and whose insight continues to drive progress in fundamental physics today.

Beginning with the big bang

Hawking’s research career began with disappointment. Arriving at the University of Cambridge in 1962 to begin his PhD, he was told that Fred Hoyle , his chosen supervisor, already had a full complement of students. The most famous British astrophysicist at the time, Hoyle was a magnet for the more ambitious students. Hawking didn’t make the cut. Instead, he was to work with Dennis Sciama, a physicist Hawking knew nothing about. In the same year, Hawking was diagnosed with amyotrophic lateral sclerosis, a degenerative motor neurone disease that quickly robs people of the ability to voluntarily move their muscles. He was told he had two years to live.

Although Hawking’s body may have weakened, his intellect stayed sharp. Two years into his PhD, he was having trouble walking and talking, but it was clear that the disease was progressing more slowly than the doctors had initially feared. Meanwhile, his engagement to Jane Wilde – with whom he later had three children, Robert, Lucy and Tim – renewed his drive to make real progress in physics.

Stephen and Lucy Hawking

James Veysey/Camera Press

Working with Sciama had its advantages. Hoyle’s fame meant that he was seldom in the department, whereas Sciama was around and eager to talk. Those discussions stimulated the young Hawking to pursue his own scientific vision. Hoyle was vehemently opposed to the big bang theory (in fact, he had coined the name “big bang” in mockery). Sciama, on the other hand, was happy for Hawking to investigate the beginning of time.

Time’s arrow

Hawking was studying the work of Roger Penrose , which proved that if Einstein’s general theory of relativity is correct, at the heart of every black hole must be a point where space and time themselves break down – a singularity. Hawking realised that if time’s arrow were reversed, the same reasoning would hold true for the universe as a whole. Under Sciama’s encouragement, he worked out the maths and was able to prove it: the universe according to general relativity began in a singularity.

Hawking was well aware, however, that Einstein didn’t have the last word. General relativity, which describes space and time on a large scale, doesn’t take into account quantum mechanics , which describes matter’s strange behaviour at much smaller scales. Some unknown “theory of everything” was needed to unite the two. For Hawking, the singularity at the universe’s origin did not signal the breakdown of space and time; it signalled the need for quantum gravity .

Luckily, the link that he forged between Penrose’s singularity and the singularity at the big bang provided a key clue for finding such a theory. If physicists wanted to understand the origin of the universe, Hawking had just shown them exactly where to look: a black hole .

Black holes were a subject ripe for investigation in the early 1970s. Although Karl Schwarzschild had found such objects lurking in the equations of general relativity back in 1915, theoreticians viewed them as mere mathematical anomalies and were reluctant to believe they could actually exist.

Albeit frightening, their action is reasonably straightforward: black holes have such strong gravitational fields that nothing, not even light, can escape their grip. Any matter that falls into one is forever lost to the outside world. This, however, is a dagger in the heart of thermodynamics.

Stephen Hawking's final theorem turns time and causality inside out

In his final years, Stephen Hawking tackled the question of why the universe appears fine-tuned for life. His collaborator Thomas Hertog explains the radical solution they came up with

Thermodynamic threat

The second law of thermodynamics is one of the most well-established laws of nature. It states that the entropy, or level of disorder in a system, always increases. The second law gives form to the observation that ice cubes will melt into a puddle, but a puddle of water will never spontaneously turn into a block of ice. All matter contains entropy, so what happens when it is dropped into a black hole? Is entropy lost along with it? If so, the total entropy of the universe goes down and black holes would violate the second law of thermodynamics.

Hawking thought that this was fine. He was happy to discard any concept that stood in the way to a deeper truth. And if that meant the second law, then so be it.

Bekenstein and breakthrough

But Hawking met his match at a 1972 physics summer school in the French ski resort of Les Houches, France. Princeton University graduate student Jacob Bekenstein thought that the second law of thermodynamics should apply to black holes too. Bekenstein had been studying the entropy problem and had reached a possible solution thanks to an earlier insight of Hawking’s .

A black hole hides its singularity with a boundary known as the event horizon. Nothing that crosses the event horizon can ever return to the outside. Hawking’s work had shown that the area of a black hole’s event horizon never decreases over time. What’s more, when matter falls into a black hole, the area of its event horizon grows.

Bekenstein realised this was key to the entropy problem. Every time a black hole swallows matter, its entropy appears to be lost, and at the same time, its event horizon grows. So, Bekenstein suggested, what if – to preserve the second law – the area of the horizon is itself a measure of entropy?

Hawking immediately disliked the idea and was angry that his own work had been used in support of a concept so flawed. With entropy comes heat, but the black hole couldn’t be radiating heat – nothing can escape its pull of gravity. During a break from the lectures, Hawking got together with colleagues Brandon Carter, who also studied under Sciama, and James Bardeen, of the University of Washington, and confronted Bekenstein.

The disagreement bothered Bekenstein. “These three were senior people. I was just out of my PhD. You worry whether you are just stupid and these guys know the truth,” he recalls.

Back in Cambridge, Hawking set out to prove Bekenstein wrong. Instead, he discovered the precise form of the mathematical relationship between entropy and the black hole’s horizon. Rather than destroying the idea, he had confirmed it. It was Hawking’s greatest breakthrough.

Hawking radiation

Hawking now embraced the idea that thermodynamics played a part in black holes. Anything that has entropy, he reasoned, also has a temperature – and anything that has a temperature can radiate.

His original mistake, Hawking realised, was in only considering general relativity, which says that nothing – no particles, no heat – can escape the grip of a black hole. That changes when quantum mechanics comes into play. According to quantum mechanics, fleeting pairs of particles and antiparticles are constantly appearing out of empty space, only to annihilate and disappear in the blink of an eye. When this happens in the vicinity of an event horizon, a particle-antiparticle pair can be separated – one falls behind the horizon while one escapes, leaving them forever unable to meet and annihilate. The orphaned particles stream away from the black hole’s edge as radiation. The randomness of quantum creation becomes the randomness of heat.

“I think most physicists would agree that Hawking’s greatest contribution is the prediction that black holes emit radiation,” says Sean Carroll , a theoretical physicist at the California Institute of Technology. “While we still don’t have experimental confirmation that Hawking’s prediction is true, nearly every expert believes he was right.”

Experiments to test Hawking’s prediction are so difficult because the more massive a black hole is, the lower its temperature. For a large black hole – the kind astronomers can study with a telescope – the temperature of the radiation is too insignificant to measure. As Hawking himself often noted, it was for this reason that he was never awarded a Nobel Prize. Still, the prediction was enough to secure him a prime place in the annals of science, and the quantum particles that stream from the black hole’s edge would forever be known as Hawking radiation .

Some have suggested that they should more appropriately be called Bekenstein-Hawking radiation, but Bekenstein himself rejects this. “The entropy of a black hole is called Bekenstein-Hawking entropy, which I think is fine. I wrote it down first, Hawking found the numerical value of the constant, so together we found the formula as it is today. The radiation was really Hawking’s work. I had no idea how a black hole could radiate. Hawking brought that out very clearly. So that should be called Hawking radiation.”

Theory of everything

The Bekenstein-Hawking entropy equation is the one Hawking asked to have engraved on his tombstone. It represents the ultimate mash-up of physical disciplines because it contains Newton’s constant, which clearly relates to gravity; Planck’s constant, which betrays quantum mechanics at play; the speed of light, the talisman of Einstein’s relativity; and the Boltzmann constant, the herald of thermodynamics.

The presence of these diverse constants hinted at a theory of everything, in which all physics is unified. Furthermore, it strongly corroborated Hawking’s original hunch that understanding black holes would be key in unlocking that deeper theory.

Hawking’s breakthrough may have solved the entropy problem, but it raised an even more difficult problem in its wake. If black holes can radiate, they will eventually evaporate and disappear. So what happens to all the information that fell in? Does it vanish too? If so, it will violate a central tenet of quantum mechanics. On the other hand, if it escapes from the black hole, it will violate Einstein’s theory of relativity. With the discovery of black hole radiation, Hawking had pit the ultimate laws of physics against one another. The black hole information loss paradox had been born.

Hawking staked his position in another ground-breaking and even more contentious paper entitled Breakdown of predictability in gravitational collapse, published in Physical Review D in 1976. He argued that when a black hole radiates away its mass, it does take all of its information with it – despite the fact that quantum mechanics expressly forbids information loss. Soon other physicists would pick sides, for or against this idea, in a debate that continues to this day. Indeed, many feel that information loss is the most pressing obstacle in understanding quantum gravity.

“Hawking’s 1976 argument that black holes lose information is a towering achievement, perhaps one of the most consequential discoveries on the theoretical side of physics since the subject was invented,” says Raphael Bousso of the University of California, Berkeley.

By the late 1990s, results emerging from string theory had most theoretical physicists convinced that Hawking was wrong about information loss, but Hawking, known for his stubbornness, dug in his heels. It wasn’t until 2004 that he would change his mind. And he did it with flair – dramatically showing up at a conference in Dublin and announcing his updated view : black holes cannot lose information.

Today, however, a new paradox known as the firewall has thrown everything into doubt (see “Hawking’s paradox”, below). It is clear that the question Hawking raised is at the core of the quest for quantum gravity.

“Black hole radiation raises serious puzzles we are still working very hard to understand,” says Carroll . “It’s fair to say that Hawking radiation is the single biggest clue we have to the ultimate reconciliation of quantum mechanics and gravity, arguably the greatest challenge facing theoretical physics today.”

Hawking’s legacy, says Bousso, will be “having put his finger on the key difficulty in the search for a theory of everything”.

Hawking continued pushing the boundaries of theoretical physics at a seemingly impossible pace for the rest of his life. He made important inroads towards understanding how quantum mechanics applies to the universe as a whole, leading the way in the field known as quantum cosmology. His progressive disease pushed him to tackle problems in novel ways, which contributed to his remarkable intuition for his subject. As he lost the ability to write out long, complicated equations, Hawking found new and inventive methods to solve problems in his head, usually by reimagining them in geometric form. But, like Einstein before him, Hawking never produced anything quite as revolutionary as his early work.

“Hawking’s most influential work was done in the 1970s, when he was younger,” says Carroll, “but that’s completely standard even for physicists who aren’t burdened with a debilitating neurone disease.”

Stephen Hawking's black hole paradox may finally have a solution

Black holes may not destroy all information about what they were originally made of, according to a new set of quantum calculations, which would solve a major physics paradox first described by Stephen Hawking

Hawking the superstar

In the meantime, the publication of A Brief History of Time catapulted Hawking to cultural stardom and gave a fresh face to theoretical physics. He never seemed to mind. “In front of the camera, Hawking played the character of Hawking. He seemed to play with his cultural status,” says Hélène Mialet, an anthropologist from the University of California, Berkeley, who courted controversy in 2012 with the publication of her book Hawking Incorporated. In it, she investigated the way the people around Hawking helped him build and maintain his public image .

That public image undoubtedly made his life easier than it might otherwise have been. As Hawking’s disease progressed, technologists gladly provided increasingly complicated machines to allow him to communicate. This, in turn, let him continue doing the thing for which he should ultimately be remembered: his science.

“Stephen Hawking has done more to advance our understanding of gravitation than anyone since Einstein,” Carroll says. “He was a world-leading theoretical physicist, clearly the best in the world for his time among those working at the intersection of gravity and quantum mechanics, and he did it all in the face of a terrible disease. He is an inspirational figure, and history will certainly remember him that way.”

Hawking's paradox

In 2012, four physicists at the University of California, Santa Barbara – Ahmed Almheiri, Donald Marolf, Joseph Polchinski and James Sully, known collectively by physicists as AMPS – shocked the physics community with the results of a thought experiment .

When pairs of particles and antiparticles spawn near a black hole's event horizon, each pair shares a connection called entanglement. But what happens to this link and the information it holds when one of the pair falls in, leaving its twin to become a particle of Hawking radiation (see main story)?

One school of thought holds that the information is preserved as the hole evaporates, and that it is placed into subtle correlations among these particles of Hawking radiation.

But, AMPS asked, what does it look like to observers inside and outside the black hole? Enter Alice and Bob.

According to Bob, who remains outside the black hole, that particle has been separated from its antiparticle partner by the horizon. In order to preserve information, it must become entangled with another particle of Hawking radiation.

But what's happening from the point of view of Alice, who falls into the black hole? General relativity says that for a free-falling observer, gravity disappears, so she doesn't see the event horizon. According to Alice, the particle in question remains entangled with its antiparticle partner, because there is no horizon to separate them. The paradox is born.

So who is right? Bob or Alice? If it's Bob, then Alice will not encounter empty space at the horizon as general relativity claims. Instead she will be burned to a crisp by a wall of Hawking radiation – a firewall. If it's Alice who's right, then information will be lost, breaking a fundamental rule of quantum mechanics. "The fervent controversy surrounding Hawking's paradox reflects the stakes his work has raised: in quantising gravity, what gives? And how much?" says Raphael Bousso of the University of California, Berkeley. The answer awaits us in the theory of everything. Amanda Gefter

Article amended on 14 March 2018

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Culture Re-View: The day Stephen Hawking threw a time traveller party

28 June 2009: When Stephen Hawking held a party for time travellers

On this day in 2009, British astrophysicist Professor Stephen Hawking held an invitation-only party. The physics genius - who died in 2018 aged 76 - arranged for balloons, champagne, and nibbles for his guests, but didn’t send out the invites until after the party was over.

“You are cordially invited to a reception for Time Travelers,” Hawking’s invite read. In the invitation, Hawking gave the date, time and GPS location of when and where the party had taken place.

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Held in the University of Cambridge, Hawking waited diligently for the time traveller partiers he was yet to invite to arrive. Sadly, despite his best efforts, no one turned up.

“I have experimental evidence that time travel is not possible,” Hawking said of the event.

Yet, Hawking was undeterred, leaving the door open for his invite to be taken up. “I am hoping copies of it, in one form or another, will survive for many thousands of years. Maybe one day someone living in the future will find the information and use a wormhole time machine to come back to my party, proving that time travel will one day be possible.”

Hawking was quite a natural host for a time travel party. He made his name in academia through his research on black holes and work to unify the fields of quantum mechanics and the general theory of relativity. With his best-selling non-fiction book ‘A Brief History of Time’, he was pushed into the limelight as one of the world’s leading minds in physics.

The effect of motor-neurone disease on Hawking also increased his celebrity. Despite a dire initial prognosis that he would die young, Hawking lived into his 70s, continuing to communicate his ideas without allowing paralysis to get in his way. His life was dramatised in the 2014 film The Theory of Everything where Eddie Redmayne won an Oscar for his portrayal of Hawking.

Why did no one turn up to Hawking’s time traveller party? Despite being one of the most famous names in physics in the early 21st century, it’s possible that it's still too long before time travel is invented for anyone to be aware of the scientist’s invitation. Maybe time travellers are aware of Hawking’s invitation and still refused to come.

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Whether that’s because they’re obeying a time travel rule or just that Hawking’s party didn’t sound fun enough, we’ll likely never know. However, Hawking did give the idea one final try.

For the physicist’s memorial service in 2018, anyone was allowed to apply to attend via an online ballot. People quickly noticed though that the ballot allowed entries from guests who had been born up to the year 2038.

12,000 people from more than 50 countries applied to attend the memorial, but as far as we’re aware, none were born after the date Hawking died.

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March 18, 2024

The Great Debate: Could We Ever Travel through Time?

Our space and physics editors go head-to-head over a classic mind-bending question.

By Clara Moskowitz & Lee Billings

cemagraphics/Getty Images

Clara Moskowitz: Hi, I’m Clara Moskowitz, a space editor here at Scientific American. We’re taking a break this week to look back at some of our favorite podcast episodes. I chose this one about the physics of time travel, because I’m a big sci-fi geek, so I’m fascinated by the topic. But also, it was such a fun debate to have with my colleague and friend, Lee Billings, another space editor here. We each picked a side – I was pro time travel, he was con—and dug our heels in. Check it out!

[Clip: Show theme music]

Moskowitz: We’re here today to talk about time travel. A perennial – dare I say, timeless–topic of science fiction, but is it possible? Is there any chance at all that it could actually happen?

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Lee Billings: No. No, no no no no. (laughs). Well, kinda. Not really. ARGH. I’m Lee Billings.

Moskowitz: I’m Clara Moskowitz, and this is Cosmos, Quickly , the biweekly space podcast from Scientific American . 

Moskowitz: We’re going to have a little friendly debate.

Billings: Really? I came for a throwdown.

Moskowitz: Well, a wrangle. A parley. A confab. Lee, what do you have against time travel?

Billings: So I love the idea of time travel! And in fact I do it all the time—like most everyone else I’m traveling into the future at one second per second. I’m less of a fan, though, of more speculative time travel, which is good fodder for goofy sci-fi stories, but in the real world it’s an implausible distraction.

Moskowitz: But really, we can stay within plausible physics and still see how more extreme versions of time travel are possible. See, Einstein’s special theory of relativity shows that the rate time flows at depends on how fast you’re moving. 

Billings: Einstein strikes again, what a rascal.

Moskowitz: If you’re traveling in a starship at close to the speed of light, you’ll still experience the familiar one-second-per-second ticking of a clock– but an observer back on Earth would see your clock moving glacially slow. To them, you’d be moving through time at a snail’s pace. That means that when you finally got back,  maybe only a year would have passed for you, but a century could have gone by for your friends on Earth. Ergo, you just traveled to the future! 

Billings: Right, right, no one’s disputing any of that! We can even measure this sort of “time dilation” right now on Earth, not with starships, but with subatomic particles. Some of those particles have very short lifetimes, decaying almost instantaneously. But if we drastically speed them up, like in a particle accelerator, we find they endure longer in proportion to how fast they’re going. So riddle me this, though, Clara: How can we travel into the past? That’s something so hard to do–effectively impossible, almost–that it’s scarcely worth thinking about.

[Clip: Back to the Future : “This is what makes time travel possible. The flux capacitor!”]

Moskowitz: I get it—no one has yet conceived of a way to journey to the past. But the crazy thing is it’s not impossible. Time is one of the four dimensions in the universe, along with three dimensions of space. And we move through space in all directions just fine, and according to physics, travel through time should be just as possible.

One way that people have looked into is via a wormhole—a shortcut bridge through spacetime that was predicted by general relativity. Wormholes can connect distant points in spacetime, meaning you could conceivably use one to bridge not just the gap between here and a distant galaxy, but the span between 2023 and 1923. 

[CLIP: Interstellar : “That’s the wormhole.”]

Billings : Ah yes, wormholes—the last refuge of scoundrels and desperate physicists. The trouble with wormholes Clara, is that, unlike a DeLorean, we have no evidence they actually exist—and, even if they did, it seems the only ways to make them traversable and stable involves using negative energy or negative mass  to prop them open. And, guess what, just like wormholes themselves, we have no evidence these weird forms of matter and energy actually exist, either. And let’s just beat this dead horse one more time—even if wormholes exist, as well as the means to make them traversable, to go back in time seems to require anchoring one end in a region of very warped spacetime, like around a black hole, or accelerating it to nearly lightspeed. Are you sensing a theme here, Clara?

Moskowitz: Yeah, yeah. All I can say is that just because there’s no evidence any of these things exist, there’s also no evidence they don’t or can’t exist. Wormholes are real solutions to the equations of general relativity, and even negative energy and mass are concepts that come up in the math and aren’t prohibited.

Billings: Well how about some more practical arguments, then? If time travel were possible, wouldn’t we have met some time travelers by now? Wouldn’t someone have gone back and killed Hitler—or at least prevented me from wearing that ridiculous outfit to my high school prom? You know there’s a famous story about physicist Stephen Hawking, who invited time travelers to come to a party he was holding. The trick was the the party happened in 2009, but the invitation came out in a miniseries that was broadcast in 2010—thus, only time travelers would have been able to attend. 

[CLIP: Stephen Hawking Time Travel Party: “Here is the invitation, giving the exact coordinates in time and space. I am hoping in one form or another it will survive for many thousands of years.”]

Billings: Sadly, the hors d'oeuvres went uneaten and the champagne sat unopened, because, clearly, time travel to the past is impossible! 

Moskowitz: I admit a party with Stephen Hawking should have been pretty alluring to time travelers, if they were out there. But you’re forgetting about the International Clause of Secrecy that all time travelers probably have to swear to, making sure to hide their identities and abilities from those in earlier eras.  

Billings: Hmm, yes the clause of secrecy here. Feels like we’re really veering into science fiction territory special pleading here. And don’t forget all the paradoxes that we have to worry about too. There are lots of good reasons to think time travel might introduce insurmountable paradoxes in physics. The most famous being the grandfather—or grandmother—paradox. If time travel were possible into the past, so the thinking goes, then a person could go back in time and kill their own grandparents, thus making it impossible for them to be born and impossible for them to travel back in time to ever commit the murder, and so on and so on.

Moskowitz: I wonder if it could be like a many-worlds scenario, where each change a time traveler makes to the past spawns a whole new universe that carries on from that point. So if I went back in time and killed one of my forebears, then a new branch universe would begin where that whole line of descendents, including me, never existed. I mean, it sounds crazy, but then again, physics is pretty enamored with multiverses, and they seem to pop up for lots of reasons already. Maybe it’s not impossible? 

Billings: If not impossible, then I’d say, implausible.

Moskowitz: Well, I’m forever an optimist, Lee! Thanks for listening to the Cosmos, Quickly .

Billings: Our show is produced by Jeff DelViscio, Tulika Bose and Kelso Harper.  Our music was composed by Dominic Smith.

Moskowtiz: If you like the show, please consider rating or leaving a review. You can also email feedback, questions, and tips to [email protected] . 

Billings: For more spacetime hijinks and all your science news, head to SciAm.com. This has been Cosmos, Quickly . I’m Lee Billings. 

Moskowitz: I’m Clara Moskowitz. 

Billings: And we’ll see you next time, in the future!

Stephen Hawking

Stephen Hawking was a scientist known for his work with black holes and relativity, and the author of popular science books like 'A Brief History of Time.'

(1942-2018)

Who Was Stephen Hawking?

Stephen Hawking was a British scientist, professor and author who performed groundbreaking work in physics and cosmology, and whose books helped to make science accessible to everyone.

Hawking was born on January 8, 1942, in Oxford, England. His birthday was also the 300th anniversary of the death of Galileo — long a source of pride for the noted physicist.

The eldest of Frank and Isobel Hawking's four children, Hawking was born into a family of thinkers.

His Scottish mother earned her way into Oxford University in the 1930s — a time when few women were able to go to college. His father, another Oxford graduate, was a respected medical researcher with a specialty in tropical diseases.

Hawking's birth came at an inopportune time for his parents, who didn't have much money. The political climate was also tense, as England was dealing with World War II and the onslaught of German bombs in London, where the couple was living as Frank Hawking undertook research in medicine.

In an effort to seek a safer place, Isobel returned to Oxford to have the couple's first child. The Hawkings would go on to have two other children, Mary and Philippa. And their second son, Edward, was adopted in 1956.

The Hawkings, as one close family friend described them, were an "eccentric" bunch. Dinner was often eaten in silence, each of the Hawkings intently reading a book. The family car was an old London taxi, and their home in St. Albans was a three-story fixer-upper that never quite got fixed. The Hawkings also housed bees in the basement and produced fireworks in the greenhouse.

In 1950, Hawking's father took work to manage the Division of Parasitology at the National Institute of Medical Research, and spent the winter months in Africa doing research. He wanted his eldest child to go into medicine, but at an early age, Hawking showed a passion for science and the sky.

That was evident to his mother, who, along with her children, often stretched out in the backyard on summer evenings to stare up at the stars. "Stephen always had a strong sense of wonder," she remembered. "And I could see that the stars would draw him."

Hawking was also frequently on the go. With his sister Mary, Hawking, who loved to climb, devised different entry routes into the family home. He loved to dance and also took an interest in rowing, becoming a team coxswain in college.

Early in his academic life, Hawking, while recognized as bright, was not an exceptional student. During his first year at St. Albans School , he was third from the bottom of his class.

But Hawking focused on pursuits outside of school; he loved board games, and he and a few close friends created new games of their own. During his teens, Hawking, along with several friends, constructed a computer out of recycled parts for solving rudimentary mathematical equations.

Hawking entered University College at the University of Oxford at the age of 17. Although he expressed a desire to study mathematics, Oxford didn't offer a degree in that specialty, so Hawking gravitated toward physics and, more specifically, cosmology.

By his own account, Hawking didn't put much time into his studies. He would later calculate that he averaged about an hour a day focusing on school. And yet he didn't really have to do much more than that. In 1962, he graduated with honors in natural science and went on to attend Trinity Hall at the University of Cambridge for a Ph.D. in cosmology.

In 1968, Hawking became a member of the Institute of Astronomy in Cambridge. The next few years were a fruitful time for Hawking and his research. In 1973, he published his first, highly-technical book, The Large Scale Structure of Space-Time , with G.F.R. Ellis.

In 1979, Hawking found himself back at the University of Cambridge, where he was named to one of teaching's most renowned posts, dating back to 1663: the Lucasian Professor of Mathematics.

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Wife and Children

At a New Year's party in 1963, Hawking met a young languages undergraduate named Jane Wilde. They were married in 1965. The couple gave birth to a son, Robert, in 1967, and a daughter, Lucy, in 1970. A third child, Timothy, arrived in 1979.

In 1990, Hawking left his wife Jane for one of his nurses, Elaine Mason. The two were married in 1995. The marriage put a strain on Hawking's relationship with his own children, who claimed Elaine closed off their father from them.

In 2003, nurses looking after Hawking reported their suspicions to police that Elaine was physically abusing her husband. Hawking denied the allegations, and the police investigation was called off. In 2006, Hawking and Elaine filed for divorce.

In the following years, the physicist reportedly grew closer to his family. He reconciled with Jane, who had remarried. And he published five science-themed novels for children with his daughter, Lucy.

Stephen Hawking: Books

Over the years, Hawking wrote or co-wrote a total of 15 books. A few of the most noteworthy include:

'A Brief History of Time'

In 1988 Hawking catapulted to international prominence with the publication of A Brief History of Time . The short, informative book became an account of cosmology for the masses and offered an overview of space and time, the existence of God and the future.

The work was an instant success, spending more than four years atop the London Sunday Times' best-seller list. Since its publication, it has sold millions of copies worldwide and been translated into more than 40 languages.

‘The Universe in a Nutshell’

A Brief History of Time also wasn't as easy to understand as some had hoped. So in 2001, Hawking followed up his book with The Universe in a Nutshell , which offered a more illustrated guide to cosmology's big theories.

‘A Briefer History of Time’

In 2005, Hawking authored the even more accessible A Briefer History of Time , which further simplified the original work's core concepts and touched upon the newest developments in the field like string theory.

Together these three books, along with Hawking's own research and papers, articulated the physicist's personal search for science's Holy Grail: a single unifying theory that can combine cosmology (the study of the big) with quantum mechanics (the study of the small) to explain how the universe began.

This kind of ambitious thinking allowed Hawking, who claimed he could think in 11 dimensions, to lay out some big possibilities for humankind. He was convinced that time travel is possible, and that humans may indeed colonize other planets in the future.

‘The Grand Design’

In September 2010, Hawking spoke against the idea that God could have created the universe in his book The Grand Design . Hawking previously argued that belief in a creator could be compatible with modern scientific theories.

In this work, however, he concluded that the Big Bang was the inevitable consequence of the laws of physics and nothing more. "Because there is a law such as gravity, the universe can and will create itself from nothing," Hawking said. "Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist."

The Grand Design was Hawking's first major publication in almost a decade. Within his new work, Hawking set out to challenge Isaac Newton 's belief that the universe had to have been designed by God, simply because it could not have been born from chaos. "It is not necessary to invoke God to light the blue touch paper and set the universe going," Hawking said.

At the age of 21, Hawking was diagnosed with amyotrophic lateral sclerosis (ALS, or Lou Gehrig 's disease). In a very simple sense, the nerves that controlled his muscles were shutting down. At the time, doctors gave him two and a half years to live.

Hawking first began to notice problems with his physical health while he was at Oxford — on occasion he would trip and fall, or slur his speech — but he didn't look into the problem until 1963, during his first year at Cambridge. For the most part, Hawking had kept these symptoms to himself.

But when his father took notice of the condition, he took Hawking to see a doctor. For the next two weeks, the 21-year-old college student made his home at a medical clinic, where he underwent a series of tests.

"They took a muscle sample from my arm, stuck electrodes into me, and injected some radio-opaque fluid into my spine, and watched it going up and down with X-rays, as they tilted the bed," he once said. "After all that, they didn't tell me what I had, except that it was not multiple sclerosis, and that I was an atypical case."

Eventually, however, doctors did diagnose Hawking with the early stages of ALS. It was devastating news for him and his family, but a few events prevented him from becoming completely despondent.

The first of these came while Hawking was still in the hospital. There, he shared a room with a boy suffering from leukemia. Relative to what his roommate was going through, Hawking later reflected, his situation seemed more tolerable.

Not long after he was released from the hospital, Hawking had a dream that he was going to be executed. He said this dream made him realize that there were still things to do with his life.

In a sense, Hawking's disease helped turn him into the noted scientist he became. Before the diagnosis, Hawking hadn't always focused on his studies. "Before my condition was diagnosed, I had been very bored with life," he said. "There had not seemed to be anything worth doing."

With the sudden realization that he might not even live long enough to earn his Ph.D., Hawking poured himself into his work and research.

As physical control over his body diminished (he'd be forced to use a wheelchair by 1969), the effects of his disease started to slow down. Over time, however, Hawking's ever-expanding career was accompanied by an ever-worsening physical state.

How Did Stephen Hawking Talk?

By the mid-1970s, the Hawking family had taken in one of Hawking's graduate students to help manage his care and work. He could still feed himself and get out of bed, but virtually everything else required assistance.

In addition, his speech had become increasingly slurred, so that only those who knew him well could understand him. In 1985 he lost his voice for good following a tracheotomy. The resulting situation required 24-hour nursing care for the acclaimed physicist.

It also put in peril Hawking's ability to do his work. The predicament caught the attention of a California computer programmer, who had developed a speaking program that could be directed by head or eye movement. The invention allowed Hawking to select words on a computer screen that were then passed through a speech synthesizer.

At the time of its introduction, Hawking, who still had use of his fingers, selected his words with a handheld clicker. Eventually, with virtually all control of his body gone, Hawking directed the program through a cheek muscle attached to a sensor.

Through the program, and the help of assistants, Hawking continued to write at a prolific rate. His work included numerous scientific papers, of course, but also information for the non-scientific community.

Hawking's health remained a constant concern—a worry that was heightened in 2009 when he failed to appear at a conference in Arizona because of a chest infection. In April, Hawking, who had already announced he was retiring after 30 years from the post of Lucasian Professor of Mathematics at Cambridge, was rushed to the hospital for being what university officials described as "gravely ill," though he later made a full recovery.

Research on the Universe and Black Holes

In 1974, Hawking's research turned him into a celebrity within the scientific world when he showed that black holes aren't the information vacuums that scientists had thought they were.

In simple terms, Hawking demonstrated that matter, in the form of radiation, can escape the gravitational force of a collapsed star. Another young cosmologist, Roger Penrose, had earlier discovered groundbreaking findings about the fate of stars and the creation of black holes, which tapped into Hawking's own fascination with how the universe began.

The pair then began working together to expand upon Penrose’s earlier work, setting Hawking on a career course marked by awards, notoriety and distinguished titles that reshaped the way the world thinks about black holes and the universe.

When Hawking’s radiation theory was born, the announcement sent shock waves of excitement through the scientific world. Hawking was named a fellow of the Royal Society at the age of 32, and later earned the prestigious Albert Einstein Award, among other honors. He also earned teaching stints at Caltech in Pasadena, California, where he served as visiting professor, and at Gonville and Caius College in Cambridge.

In August 2015, Hawking appeared at a conference in Sweden to discuss new theories about black holes and the vexing "information paradox." Addressing the issue of what becomes of an object that enters a black hole, Hawking proposed that information about the physical state of the object is stored in 2D form within an outer boundary known as the "event horizon." Noting that black holes "are not the eternal prisons they were once thought," he left open the possibility that the information could be released into another universe.

Beginning of the Universe

In a March 2018 interview on Neil deGrasse Tyson 's Star Talk , Hawking addressed the topic of "what was around before the Big Bang" by stating there was nothing around. He said by applying a Euclidean approach to quantum gravity, which replaces real time with imaginary time, the history of the universe becomes like a four-dimensional curved surface, with no boundary.

He suggested picturing this reality by thinking of imaginary time and real time as beginning at the Earth's South Pole, a point of space-time where the normal laws of physics hold; as there is nothing "south" of the South Pole, there was also nothing before the Big Bang.

Hawking and Space Travel

In 2007, at the age of 65, Hawking made an important step toward space travel. While visiting the Kennedy Space Center in Florida, he was given the opportunity to experience an environment without gravity.

Over the course of two hours over the Atlantic, Hawking, a passenger on a modified Boeing 727, was freed from his wheelchair to experience bursts of weightlessness. Pictures of the freely floating physicist splashed across newspapers around the globe.

"The zero-G part was wonderful, and the high-G part was no problem. I could have gone on and on. Space, here I come!" he said.

Hawking was scheduled to fly to the edge of space as one of Sir Richard Branson 's pioneer space tourists. He said in a 2007 statement, "Life on Earth is at the ever-increasing risk of being wiped out by a disaster, such as sudden global warming , nuclear war, a genetically engineered virus or other dangers. I think the human race has no future if it doesn't go into space. I therefore want to encourage public interest in space."

Stephen Hawking Movie and TV Appearances

If there is such a thing as a rock-star scientist, Hawking embodied it. His forays into popular culture included guest appearances on The Simpsons , Star Trek: The Next Generation , a comedy spoof with comedian Jim Carrey on Late Night with Conan O'Brien , and even a recorded voice-over on the Pink Floyd song "Keep Talking."

In 1992, Oscar-winning filmmaker Errol Morris released a documentary about Hawking's life, aptly titled A Brief History of Time . Other TV and movie appearances included:

'The Big Bang Theory'

In 2012, Hawking showed off his humorous side on American television, making a guest appearance on The Big Bang Theory . Playing himself on this popular comedy about a group of young, geeky scientists, Hawking brings the theoretical physicist Sheldon Cooper ( Jim Parsons ) back to Earth after finding an error in his work. Hawking earned kudos for this light-hearted effort.

'The Theory of Everything'

In November of 2014, a film about the life of Hawking and Jane Wilde was released. The Theory of Everything stars Eddie Redmayne as Hawking and encompasses his early life and school days, his courtship and marriage to Wilde, the progression of his crippling disease and his scientific triumphs.

In May 2016, Hawking hosted and narrated Genius , a six-part television series which enlists volunteers to tackle scientific questions that have been asked throughout history. In a statement regarding his series, Hawking said Genius is “a project that furthers my lifelong aim to bring science to the public. It’s a fun show that tries to find out if ordinary people are smart enough to think like the greatest minds who ever lived. Being an optimist, I think they will.”

In 2011, Hawkings had participated in a trial of a new headband-styled device called the iBrain. The device is designed to "read" the wearer's thoughts by picking up "waves of electrical brain signals," which are then interpreted by a special algorithm, according to an article in The New York Times . This device could be a revolutionary aid to people with ALS.

Hawking on AI

In 2014, Hawking, among other top scientists, spoke out about the possible dangers of artificial intelligence, or AI, calling for more research to be done on all of possible ramifications of AI. Their comments were inspired by the Johnny Depp film Transcendence , which features a clash between humanity and technology.

"Success in creating AI would be the biggest event in human history," the scientists wrote. "Unfortunately, it might also be the last, unless we learn how to avoid the risks." The group warned of a time when this technology would be "outsmarting financial markets, out-inventing human researchers, out-manipulating human leaders, and developing weapons we cannot even understand."

Hawking reiterated this stance while speaking at a technology conference in Lisbon, Portugal, in November 2017. Noting how AI could potentially make gains in wiping out poverty and disease, but could also lead to such theoretically destructive actions as the development of autonomous weapons, he said, "We cannot know if we will be infinitely helped by AI, or ignored by it and sidelined, or conceivably destroyed by it."

Hawking and Aliens

In July 2015, Hawking held a news conference in London to announce the launch of a project called Breakthrough Listen. Funded by Russian entrepreneur Yuri Milner, Breakthrough Listen was created to devote more resources to the discovery of extraterrestrial life.

Breaking the Internet

In October 2017, Cambridge University posted Hawking's 1965 doctoral thesis, "Properties of Expanding Universes," to its website. An overwhelming demand for access promptly crashed the university server, though the document still fielded a staggering 60,000 views before the end of its first day online.

When Did Stephen Hawking Die?

On March 14, 2018, Hawking finally died of ALS, the disease that was supposed to have killed him more than 50 years earlier. A family spokesman confirmed that the iconic scientist died at his home in Cambridge, England.

The news touched many in his field and beyond. Fellow theoretical physicist and author Lawrence Krauss tweeted: "A star just went out in the cosmos. We have lost an amazing human being. Hawking fought and tamed the cosmos bravely for 76 years and taught us all something important about what it truly means to celebrate about being human."

Hawking's children followed with a statement: "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. His courage and persistence with his brilliance and humor inspired people across the world. 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."

Later in the month, it was announced that Hawking's ashes would be interred at Westminster Abbey in London, alongside other scientific luminaries like Isaac Newton and Charles Darwin .

On May 2, 2018, his final paper, titled "A smooth exit from eternal inflation?" was published in the Journal of High Energy Physics . Submitted 10 days before his death, the new report, co-authored by Belgian physicist Thomas Hertog, disputes the idea that the universe will continue to expand.

QUICK FACTS

• Name: Stephen Hawking
• Birth Year: 1942
• Birth date: January 8, 1942
• Birth City: Oxford, England
• Birth Country: United Kingdom
• Gender: Male
• Best Known For: Stephen Hawking was a scientist known for his work with black holes and relativity, and the author of popular science books like 'A Brief History of Time.'
• Science and Medicine
• Astrological Sign: Capricorn
• University of Cambridge
• Gonville & Caius College
• Oxford University
• California Institute of Technology
• Interesting Facts
• As an author, Stephen Hawking was best known for his best seller 'A Brief History of Time.'
• At the age of 21, Stephen Hawking was diagnosed with amyotrophic lateral sclerosis (ALS, or Lou Gehrig's disease).
• Death Year: 2018
• Death date: March 14, 2018
• Death City: Cambridge, England
• Death Country: United Kingdom

We strive for accuracy and fairness. If you see something that doesn't look right , contact us !

• My goal is simple. It is a complete understanding of the universe, why it is as it is and why it exists at all.
• Not only does God definitely play dice, but He sometimes confuses us by throwing them where they can't be seen.
• Intelligence is the ability to adapt to change.
• Before my condition was diagnosed, I had been very bored with life. There had not seemed to be anything worth doing.
• I believe that life on Earth is at an ever increasing risk of being wiped out by a disaster such as sudden global warming, nuclear war, a genetically engineered virus, or other dangers. I think the human race has no future if it doesn't go into space.
• Because there is a law such as gravity, the universe can and will create itself from nothing. Spontaneous creation is the reason there is something rather than nothing, why the universe exists, why we exist.
• It is not necessary to invoke God to light the blue touch paper and set the universe going.
• It is not clear that intelligence has any long-term survival value.
• If, like me, you have looked at the stars, and tried to make sense of what you see, you too have started to wonder what makes the universe exist.
• I regard the brain as a computer which will stop working when its components fail. There is no heaven or afterlife for broken down computers; that is a fairy story for people afraid of the dark.
• Science is beautiful when it makes simple explanations of phenomena or connections between different observations. Examples include the double helix in biology, and the fundamental equations of physics.
• People who boast about their I.Q. are losers.
• We shouldn't be surprised that conditions in the universe are suitable for life, but this is not evidence that the universe was designed to allow for life. We could call order by the name of God, but it would be an impersonal God. There's not much personal about the laws of physics.

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For arguably the most famous physicist on Earth, Stephen Hawking—who died Wednesday in Cambridge at 76 years old—was wrong a lot. He thought, for a while, that black holes destroyed information , which physics says is a no-no. He thought Cygnus X-1 , an emitter of X-rays over 6,000 light years away, wouldn’t turn out to be a black hole. (It did.) He thought no one would ever find the Higgs boson, the particle indirectly responsible for the existence of mass in the universe. ( Researchers at CERN found it in 2012.)

But Hawking was right a lot, too. He and the physicist Roger Penrose described singularities , mind-bending physical concepts where relativity and quantum mechanics collapse inward on each other—as at the heart of a black hole. It’s the sort of place that no human will ever see first-hand; the event horizon of a black hole smears matter across time and space like cosmic paste. But Hawking’s mind was singular enough to see it, or at least imagine it.

His calculations helped show that as the young universe expanded and grew through inflation, fluctuations at the quantum scale—the smallest possible gradation of matter—became the galaxies we see around us. No human will ever visit another galaxy, and the quantum realm barely waves at us in our technology, but Hawking envisioned them both. And he calculated that black holes could sometimes explode, an image that would vex even the best visual effects wizard .

More than that, he could explain it to the rest of us . Hawking was the Lucasian Chair of Mathematics at Cambridge until his retirement in 2009, the same position held by Isaac Newton, Charles Babbage, and Paul Dirac. But he was also a pre-eminent popularizer of some of the most brain-twisting concepts science has to offer. His 1988 book A Brief History of Time has sold more than 10 million copies. His image—in an electric wheelchair and speaking via a synthesizer because of complications of the degenerative disease amyotrophic lateral sclerosis, delivering nerdy zingers on TV shows like The Big Bang Theory and Star Trek: The Next Generation —defined “scientist” for the latter half of the 20th century perhaps as much as Albert Einstein’s mad hair and German accent did in the first half.

Possibly that’s because in addition to being brilliant, Hawking was funny. Or at least sly. He was a difficult student by his own account. Diagnosed with ALS in 1963 at the age of 21, he thought he’d have only two more years to live. When the disease didn’t progress that fast, Hawking is reported to have said, “I found, to my surprise, that I was enjoying life in the present more than before. I began to make progress with my research.” With his mobility limited by the use of a wheelchair, he sped in it, dangerously. He proved time travel didn't exist by throwing a party for time travelers , but not sending out invitations until the party was over. No one came. People learned about the things he got wrong because he’d bet other scientists—his skepticism that Cygnus X-1 was a black hole meant he owed Kip Thorne of Caltech a subscription to Penthouse . (In fact, as the terms of that bet hint, rumors of mistreatment of women dogged him.)

Hawking became as much a cultural icon as a scientific one. For a time police suspected his second wife and one-time nurse of abusing him; the events became the basis of an episode of Law and Order: Criminal Intent . He played himself on The Simpsons and was depicted on Family Guy and South Park . Eddie Redmayne played Hawking in a biopic.

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In recent years he looked away from the depths of the universe and into humanity’s future, joining the technologist Elon Musk in warning against the dangers of intelligent computers . “Unless we learn how to prepare for, and avoid, the potential risks, AI could be the worst event in the history of our civilization,” Hawking reportedly said at a talk last year. “It brings dangers, like powerful autonomous weapons, or new ways for the few to oppress the many. It could bring great disruption to our economy.” In an interview with WIRED UK, he said: “Someone will design AI that replicates itself. This will be a new form of life that will outperform humans.”

In 2016 he said that he thought humanity only had about 1,000 years left, thanks to AI, climate change, and other (avoidable) disasters. Last year he reduced that horizon exponentially—100 years left, he warned , unless we changed our ways.

Hawking was taking an unusual step away from cosmology, and it was easy, perhaps, to dismiss that fear—why would someone who’d help define what a singularity actually was warn people against the pseudo-singularity of Silicon Valley? Maybe Hawking will be as wrong on this one as he was about conservation of information in black holes. But Hawking always did see into realms no one else could—until he described them to the rest of us.

In 2014, WIRED UK interviewed Hawking after a lecture, talking about the 'how' and 'why' questions he always asked himself.

Even beyond his own work, Hawking propelled technology forward: Intel worked with him to develop a novel speech synthesizer .

Today, you can use that speech software for free.

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Stephen Hawking’s champagne-fuelled time travel party

THE late astrophysicist Stephen Hawking was able to prove time travel didn’t exist just by hosting a party.

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THE late, great astrophysicist Stephen Hawking was known for his wicked sense of humour, and one of his funniest “experiments” involved throwing a party to prove time travel doesn’t exist.

A clip from 2010 documentary Into the Universe shows the iconic scientist, who died at his home in Cambridge aged 76 , waiting for attendees to show up to the champagne-fuelled soiree.

“I like simple experiments ... and champagne,” says a voiceover as the camera pans over plates of hors d’oeuvres and flutes of Krug. “So I’ve combined two of my favourite things to see if time travel from the future to the past is possible. I’m throwing a party. A welcome reception for future time travellers.”

The scientist is seen sitting in his wheelchair alone in a sumptuous, brightly lit room at the University of Cambridge, decorated with balloons and a large sign that reads: “Welcome time travellers.”

However, the cosmologist explains, “there’s a twist”. He didn’t send the invitation out until after the party.

“You are cordially invited to a reception for Time Travellers,” the invite reads. “No RSVP required.” It gives the date, time and exact coordinates of the location at Cambridge University’s Gonville and Caius College.

“I’m hoping copies of it in one form or another will survive for many thousands of years.” he explains. “Maybe one day, someone living in the future will find the information and use a wormhole time machine to come back to my party, proving that time travel will one day be possible.”

A hopeful Hawking is seen sitting alone in the room as the seconds tick by, and no one appears. “What a shame,” jokes the scientist. “I was hoping a future Miss Universe was going to step through the door.”

Some have attributed the cosmologist’s remarkable longevity to his ability to see the funny side of human existence, as well as its scientific import.

Hawking was diagnosed with motor neurone disease at the age of 21, and given just two years to live. Incredibly, he lived a rich life and continued his vital work for another 50 years, making him one of the longest surviving patients in history.

The gifted man remained whip-smart to the end, only recently laying out his theory on the beginning of the universe and the truth about what happened before the Big Bang in one of his final public appearances.

In June 2014, comedian John Oliver asked Hawking in an interview: “You’ve stated that there could be an infinite number of parallel universe ... Does that mean there’s a universe out there where I am smarter than you?”

Lightning fast, Hawking countered: “Yes. And also a universe where you’re funny.”

The scientist claimed in a 2013 documentary that “keeping an active mind has been vital to my survival, as has maintaining a sense of humour.”

He wryly added: “I am probably better known for my appearances on The Simpsons and on The Big Bang Theory than I am for my scientific discoveries.”

Of course, just because no one came to his party, doesn’t mean that time travel doesn’t exist , Giant Freakin Robot blog has pointed out. The issue could be that the party took place on a different reality timeline, the invitations didn’t survive long enough for anyone to see, time travellers can’t control their movement or simply that “time travelers are d**ks.”

The British scientist was well-known for his love of a finer drop, as well as his groundbreaking research into black holes and relativity. In an interview with Piers Morgan last year , he said his perfect final day on earth would feature champagne, classic music and his loved ones.

“Oh, my last day, it would be being with my family and listening to Wagner,” Hawking told the television host in the poignant clip. “While sipping champagne in the summer sun.”

Hawking had a colourful love life , divorcing his first wife Jane Wilde to marry his nurse Elaine Mason, who his children accused of cutting them off from their father. They too divorced in 2006, and the scientist re-established a friendship with Ms Wilde.

His children remained his greatest loves, with the father of three saying his happiest day came in 1967 when his first child, Robert, was born. In a statement announcing his death, Robert, Lucy and Tim wrote of their father: “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.”

The world-famous professor once said his ultimate dream was to travel beyond the earth’s atmosphere.

“My three children have brought me great joy. And I can tell you what will make me happy — to travel in space. I have already completed a zero gravity flight which allowed me to float weightless.”

We can only hope the great scientist has achieved that wish now, and is floating high above the clouds, looking at the world that so fascinated him.

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