From The University of Cambridge (UK)
And
6.11.24
Ben Turner
Thomas Hertog tells us how he collaborated with Stephen Hawking on his final theorem — a Darwinian revolution in physics that explains the origin of time.
Stephen Hawking photographed at at Emmanuel College on September 19, 2013 in Cambridge. (Image credit: Karwai Tang/Getty Images)
In 2002, Thomas Hertog, then a theoretical physics graduate student, stepped inside Stephen Hawking’s office at the University of Cambridge and saw his supervisor’s eyes filled with emotion.
Hawking’s news was also a confession. The famed physicist told his student that his book, A Brief History of Time, was wrong because it predicted a barren universe unsuitable for life, and he wanted Hertog to help him find a new theory.
So, in the last 16 years of Hawking’s life, the duo, along with collaborator James Hartle, developed a new explanation for how our universe came to be.
Live Science sat down with Hertog, now a professor at KU Leuven in Belgium, to discuss his new book On the Origin of Time (Penguin Random House, 2024), his decades-long collaboration with Hawking, and the mind-bending Darwinian view of the universe’s origins that their work ultimately produced.
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Ben Turner: When you met Stephen Hawking, he was beginning to think that the picture of the universe’s origins he had previously presented in A Brief History of Time was flawed, and he wanted to look for a new theory. For readers who might not know, what is the standard conception of how our universe began?
Thomas Hertog: Certainly what’s standard is that there’s been some sort of Big Bang — a violent, extremely odd beginning. What’s been challenging is to describe what exactly happened at the Big Bang.
What’s the novelty of Hawking’s contribution in A Brief History of Time? What was the key insight he invoked? He came up with a mathematical model of the actual beginning in his famous “no boundary proposal,” in which the Big Bang is a true origin.
Sadly, Hawking’s model didn’t produce a habitable universe. It was, instead, an empty universe — without stars, without galaxies and without life. So, as you say, by the late ’90s, Hawking realized there was a problem with his model.
BT: A popular answer for how our habitable universe could have formed is that the Big Bang led to eternal cosmic inflation with different pockets of expanding space-time — a multiverse — and that our universe just happens to be one of the pockets where the laws of physics balanced out in just the right way to produce life. Why didn’t this idea suit Hawking?
TH: These multiverse models are not falsifiable, even in principle. That’s not because we can’t look at the early universe and check it out; it’s because multiverse models do not make unambiguous predictions of what we should see in this universe.
BT: So how did you and Hawking meet and begin to collaborate? You met him when you were a master’s student. What was that like? He was already a legend by this time.
TH: Yes, he was already pretty famous. I met him because, well, I grew up in Belgium, and there was no cosmology going on in Belgium in the late ’90s. Stephen and his colleagues, Martin Rees and those folks, had established a kind of mecca for cosmology at Cambridge. So I had a professor who told me, “Look, if you’re into cosmology, go to Cambridge.”
At Cambridge, it was very well known that whoever came top of the master’s class would get an invitation to go talk to Stephen, and that’s what happened [to me]. So he took me on as his PhD student.
But, of course, the real collaboration started later, when we found ourselves on the same scientific wavelength and interested in the deeper problems to do with the Big Bang. It just happened: You find yourselves on the same wavelength, interested in the same problems, perhaps sharing some sort of intuition. As theoretical physicists, you’re always performing thought experiments on each other, and after a while, you develop a common understanding.
BT: Past theories of the Big Bang have framed the universe as if they’re looking at it from an “objective,” godlike perspective. The theory you and Hawking began working on shifted that perspective to one more like our own — an observer somewhere in the universe. That made you take quantum mechanics, as well as string theory, as your starting point. What did beginning this way teach you?
TH: When you take a God’s-eye of the universe, you are going to be looking for a prior explanation of why the entire cosmos should be doing what it’s doing — some Platonic mathematical truth that looms over the entire universe.
But when you take what you call a more human perspective, a perspective of an observer within the universe, it’s very different. You’ll be taking a more historical perspective. You’re not asking, “Why should the universe be this way?” but “How did it all come about?”
If you use quantum mechanics to reconstruct that history all the way back to the Big Bang, that historical perspective begins to play out at the level of the laws of physics themselves. And that’s, of course, a surprise. We thought the laws of physics were fixed and immutable, but if you go back in time, they begin to simplify. In a sense, they begin to evaporate, even the structure.
That structure, encoded in the laws of physics, begins to disappear until ultimately — and this is the crux of our hypothesis — even the distinction between time and space blurs. The laws of our universe’s evolution, the standard laws of physics, close themselves; they cease to be. Physics itself disappears.
It’s a Darwinian turning. In biology, we go back along the tree of life to life’s origin, and the laws of biology also disappear. That’s because those laws are emergent properties of biological evolution. We claim that the laws of physics are also emergent properties of a much earlier evolution.
An illustration of the expansion of the universe after the Big Bang. (Image credit: MARK GARLICK/SCIENCE PHOTO LIBRARY via Getty Images)
BT: That’s going to strike people as very strange. In biology, selective pressure plays the role of spurring biological laws to evolve. What’s causing physical laws to evolve?
TH: The act of observation in quantum mechanics. You’re going to ask me, “But wait a minute — who’s observing?” Because clearly, in the early universe, there is no human observer. But we all know that the act of observation in quantum mechanics comes from the environment itself — it’s the interactions between the particles and the forces.
Even a single photon can perform an act of observation in quantum mechanics. It can convert a range of possible histories into a tangible, concrete reality.
BT: According to your theory, when we wind time back to the Big Bang, physical laws fold in on themselves and time itself loses its identity — that gives it an origin point. Einstein particularly disliked this notion. Why did he object to it?
TH: When Einstein and his contemporaries were running the evolution of the universe backwards in time, they were doing this using Einstein’s own theory in a classical, deterministic manner. They ran into what they call the singularity [where the equations describing the universe broke down]. The origin of time, the Big Bang, seemed to not be part of science.
When Stephen and I ran the evolution of the universe backwards, we did it in a quantum mechanical way. This agrees with Einstein until you reach the earlier stages where our picture is very, very different. The laws of physics never really break down [in Hertog, Hartle and Hawking’s picture]; they just gradually disappear. I think Einstein would be okay with that.
BT: Key to your idea of time having an origin is that it’s an emergent property from the interactions of many quantum particles at the edge of the observable universe. The universe is like a disk expanding outward, and at the edge of that disk are qubits, particles containing all the universe’s information. The play of these particles beams time into our universe from that furthest edge — like a cosmic hologram. Can you explain the holographic principle a bit more?
TH: So the way we read the past of the universe is from a holographic perspective. The holographic screen is an abstract representation of our reality, and as we zoom out further and further from that screen, it corresponds to going back in time. The picture gets more coarse-grained, you lose information, you lose pixels, and the Big Bang is the limit where you run out of information. The beginning of the world is really an epistemic horizon where science (from the holographic perspective) simply doesn’t reach further back.
And, of course, that fits in very well with the story that I told you earlier — that the laws of physics, along with time and space, disappear as we reach the Big Bang, the origin of physics. The holographic implementation of our vision made it click together.
That’s how theoretical physics works. In retrospect, you start off with a lot of intuition, and you mold this into a mathematical framework that is consistent and that allows you to ultimately predict new phenomena. This is where current research is going: How can we test this model? How can we find fossils of this very early evolution?
A diagram illustrating the universe as concieved by Hawking, Hertog and Hartle. In this picture, the universe, and time itself, emerges as a hologram from the interactions of countless entangled qubits interacting on its furthest edge. (Image credit: Thomas Hertog)
BT: That’s actually my next question.
TH: [Laughs] I feared.
BT: So where can we look? Before the cosmic microwave background (CMB), the universe was completely opaque. How do we peer beyond that microwave fuzz?
TH: The cosmic microwave background gives you a picture of the universe 380,000 years after the Big Bang, when it became transparent. But this early phase of evolution that I’m talking about happens much sooner, so you have to peer through [the CMB]. And you can’t do this with light, electromagnetic waves.
But gravitational waves go through everything, so you can hope to look further backward. In principle, there’s no limit — you can look all the way back to the Big Bang and unlock this deeper layer of evolution.
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Cosmic Microwave Background [CMB] study history
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BT: Say we are able to. What might we see?
TH: We’ve hypothesized. How? Well, the way I envision this early stage is a little bit like a branching, diversifying tree of physical laws. Each of these branchings is really the birth of a new kind of force — one force splits into two with new particles and more structure. Some of these branches are pretty violent, coming with bursts of gravitational waves which are not localized to one place and appearing as background radiation, much like the cosmic microwave background.
It’s the entire universe transitioning into a new state when it cools and expands, and it’s accompanied by a strong burst of inflation.
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Inflation
Alan Guth, from M.I.T., who first proposed cosmic inflation
Alan Guth’s original notes on inflation
The inflationary epoch is believed to have lasted from 10^−36 seconds to between 10^−33 and 10^−32 seconds after the Big Bang. Following the inflationary period, the universe continued to expand, but at a slower rate. The re-acceleration of this slowing expansion due to dark energy began after the universe was already over 7.7 billion years old (5.4 billion years ago)
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BT: Your theory describes physical laws evolving quickly when the universe was dense and hot, and there were plenty of interactions or “observations” between particles. But if these laws still have the capacity to evolve, does that have any implications for how the universe ends?
TH: The short answer is, of course, that I don’t know. But if you challenge me a little bit, I can say something very speculative: If the laws of physics were not determined, fixed and immutable in the past, it’s natural to expect they won’t be eternal. So, even though that evolution is suppressed now (because the universe is cold), it’s not infinitely suppressed. It’s not gone.
BT: We’ve spoken a lot about intuition in physics. The one you shared with Hawking fueled this collaboration and enabled you to finish your theory, even as Hawking slowly lost his ability to use his artificial voice. How did you do that?
TH: It’s a little bit like being in a marriage, right? Or really any long-term relationship — you can guess one another’s thoughts. Towards the end, that happened to us, as well. We developed an intimacy when it came to cosmology and its fundamental problems. In the later stages, we developed a nonverbal layer of communication in which I could fire yes-or-no questions at Stephen and read his facial expressions.
This developed in a fairly spontaneous manner, but it was only possible because, in the late ’90s and early ’00s, we had some very good years in which Stephen could speak fairly fluently through his speech synthesizer. He really dragged me into his thinking about these paradoxes associated with the multiverse.
BT: Do you think his ability to move outside problems and intuit them is what made him such a great physicist?
TH: Stephen’s intuition was grounded in 15 years of doing a lot of calculations. It didn’t come to him from heaven. It was rooted in the early stages of his career.
Of course, there’s something genius that happened in the early ’80s, when he lost his ability to write equations. He had the capacity and the stubbornness to retrain himself to perform theoretical physics in a very unique way. It was more intuition-based, more distant from the equations than others, and with the ability to visualize shapes and geometries in his head. His true glory lies in that, with this new language, he was able to arrive at certain discoveries which were very difficult to reproduce with equations.
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The University of Cambridge (UK) [legally The Chancellor, Masters, and Scholars of the University of Cambridge] is a collegiate public research university in Cambridge, England. Founded in 1209 Cambridge is the second-oldest university in the English-speaking world and the world’s fourth-oldest surviving university. It grew out of an association of scholars who left the University of Oxford (UK) after a dispute with townsfolk. The two ancient universities share many common features and are often jointly referred to as “Oxbridge”.
Cambridge is formed from a variety of institutions which include 31 semi-autonomous constituent colleges and over 150 academic departments, faculties and other institutions organized into six schools. All the colleges are self-governing institutions within the university, each controlling its own membership and with its own internal structure and activities. All students are members of a college. Cambridge does not have a main campus and its colleges and central facilities are scattered throughout the city. Undergraduate teaching at Cambridge is organized around weekly small-group supervisions in the colleges – a feature unique to the Oxbridge system. These are complemented by classes, lectures, seminars, laboratory work and occasionally further supervisions provided by the central university faculties and departments. Postgraduate teaching is provided predominantly centrally.
Cambridge University Press a department of the university is the oldest university press in the world and currently the second largest university press in the world. Cambridge Assessment also a department of the university is one of the world’s leading examining bodies and provides assessment to over eight million learners globally every year. The university also operates eight cultural and scientific museums, including the Fitzwilliam Museum, as well as a botanic garden. Cambridge’s libraries – of which there are 116 – hold a total of around 16 million books, around nine million of which are in Cambridge University Library, a legal deposit library. The university is home to – but independent of – the Cambridge Union – the world’s oldest debating society. The university is closely linked to the development of the high-tech business cluster known as “Silicon Fe”. It is the central member of Cambridge University Health Partners, an academic health science centre based around the Cambridge Biomedical Campus.
By both endowment size and consolidated assets Cambridge is the wealthiest university in the United Kingdom. The central university – excluding colleges – has a total income of over £2.5 billion of which over £600 million is from research grants and contracts. The central university and colleges together possess a combined endowment of over £7 billion and overall consolidated net assets (excluding “immaterial” historical assets) of over £12.5 billion. It is a member of numerous associations and forms part of the ‘golden triangle’ of English universities.
Cambridge has educated many notable alumni including eminent mathematicians, scientists, politicians, lawyers, philosophers, writers, actors, monarchs and heads of state. Nobel laureates, Fields Medalists, Turing Award winners and British prime ministers have been affiliated with Cambridge as students, alumni, faculty or research staff. University alumni have won many Olympic medals.
History
By the late 12th century, the Cambridge area already had a scholarly and ecclesiastical reputation due to monks from the nearby bishopric church of Ely. However, it was an incident at Oxford which is most likely to have led to the establishment of the university: three Oxford scholars were hanged by the town authorities for the death of a woman without consulting the ecclesiastical authorities who would normally take precedence (and pardon the scholars) in such a case; but were at that time in conflict with King John. Fearing more violence from the townsfolk scholars from the University of Oxford started to move away to cities such as Paris, Reading and Cambridge. Subsequently enough scholars remained in Cambridge to form the nucleus of a new university when it had become safe enough for academia to resume at Oxford. In order to claim precedence, it is common for Cambridge to trace its founding to the 1231 charter from Henry III granting it the right to discipline its own members (ius non-trahi extra) and an exemption from some taxes; Oxford was not granted similar rights until 1248.
A bull in 1233 from Pope Gregory IX gave graduates from Cambridge the right to teach “everywhere in Christendom”. After Cambridge was described as a studium generale in a letter from Pope Nicholas IV in 1290 and confirmed as such in a bull by Pope John XXII in 1318 it became common for researchers from other European medieval universities to visit Cambridge to study or to give lecture courses.
Foundation of the colleges
The colleges at the University of Cambridge were originally an incidental feature of the system. No college is as old as the university itself. The colleges were endowed fellowships of scholars. There were also institutions without endowments called hostels. The hostels were gradually absorbed by the colleges over the centuries; but they have left some traces, such as the name of Garret Hostel Lane.
Hugh Balsham, Bishop of Ely, founded Peterhouse – Cambridge’s first college in 1284. Many colleges were founded during the 14th and 15th centuries but colleges continued to be established until modern times. There was a gap of 204 years between the founding of Sidney Sussex in 1596 and that of Downing in 1800. The most recently established college is Robinson built in the late 1970s. However, Homerton College only achieved full university college status in March 2010 making it the newest full college (it was previously an “Approved Society” affiliated with the university).
In medieval times many colleges were founded so that their members would pray for the souls of the founders and were often associated with chapels or abbeys. The colleges’ focus changed in 1536 with the Dissolution of the Monasteries. Henry VIII ordered the university to disband its Faculty of Canon Law and to stop teaching “scholastic philosophy”. In response, colleges changed their curricula away from canon law and towards the classics; the Bible; and mathematics.
Nearly a century later the university was at the centre of a Protestant schism. Many nobles, intellectuals and even commoners saw the ways of the Church of England as too similar to the Catholic Church and felt that it was used by the Crown to usurp the rightful powers of the counties. East Anglia was the centre of what became the Puritan movement. In Cambridge the movement was particularly strong at Emmanuel; St Catharine’s Hall; Sidney Sussex; and Christ’s College. They produced many “non-conformist” graduates who, greatly influenced by social position or preaching left for New England and especially the Massachusetts Bay Colony during the Great Migration decade of the 1630s. Oliver Cromwell, Parliamentary commander during the English Civil War and head of the English Commonwealth (1649–1660), attended Sidney Sussex.
Modern period
After the Cambridge University Act formalized the organizational structure of the university the study of many new subjects was introduced e.g. theology, history and modern languages. Resources necessary for new courses in the arts architecture and archaeology were donated by Viscount Fitzwilliam of Trinity College who also founded the Fitzwilliam Museum. In 1847 Prince Albert was elected Chancellor of the University of Cambridge after a close contest with the Earl of Powis. Albert used his position as Chancellor to campaign successfully for reformed and more modern university curricula, expanding the subjects taught beyond the traditional mathematics and classics to include modern history and the natural sciences. Between 1896 and 1902 Downing College sold part of its land to build the Downing Site with new scientific laboratories for anatomy, genetics, and Earth sciences. During the same period the New Museums Site was erected including the Cavendish Laboratory which has since moved to the West Cambridge Site and other departments for chemistry and medicine.
The University of Cambridge began to award PhD degrees in the first third of the 20th century. The first Cambridge PhD in mathematics was awarded in 1924.
In the First World War 13,878 members of the university served and 2,470 were killed. Teaching and the fees it earned came almost to a stop and severe financial difficulties followed. As a consequence, the university first received systematic state support in 1919 and a Royal Commission appointed in 1920 recommended that the university (but not the colleges) should receive an annual grant.
Following the Second World War the university saw a rapid expansion of student numbers and available places; this was partly due to the success and popularity gained by many Cambridge scientists.
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