Skip to Content

Book Summary: On the Origin of Time – Stephen Hawking’s Final Theory

On the Origin of Time (2023) guides you through the humbling, stranger-than-fiction theories that the late physicist Stephen Hawking developed in the last two decades of his life. With quantum physics, holograms, and inspiration from Charles Darwin’s evolutionary theory, it reveals what the great scientist came to believe about the origins of the universe.

Introduction: Cutting-edge science that sounds like fiction.

The best scientists are those who are open-minded and willing to change their views as new evidence presents itself.

This is important, because even geniuses can be wrong – just like Stephen Hawking started to believe he was about something very important.

In the 1980s, Hawking argued that the rules of physics that govern our universe are immortal and unchanging. But the more he thought about it, the more he began to ask, “Why does it have to be this way?”

In this summary, we’ll cover Hawking’s final search for the origins of the rules that control our cosmos. In the process, we’ll be diving into some exhilarating, mind-bending ideas that will stretch the limit of how we typically understand the universe.

It’s going to be a difficult but rewarding ride, so buckle up and get ready to explore the vast complexity of the cosmos.

Book Summary: On the Origin of Time - Stephen Hawking's Final Theory

The Universe – Made for Life

On a dazzling summer’s day in 1998, a fresh-faced Thomas Hertog crept into the office of the world’s greatest living scientist. As a brilliant graduate student in cosmology – the study of the origin of the universe – Hertog was being sized up by Stephen Hawking as a potential protégé.

Almost completely paralyzed by a rare motor neuron disease, Hawking tapped a clicker with his hand. Gradually, through a computer program and speaker system attached to his wheelchair, synthesized speech emerged. By then, its distinct tone and rhythm had already become permanently associated with Hawking.

What Hawking said next laid the foundation for two decades of collaboration between the men, and the book upon which this current summary is based.

Hawking told Hertog that the universe seems beautifully, impeccably designed to harbor life. Which led to this question: Why did our cosmos turn out to be so sympathetic to us?

The further you burrow into the science, the more you realize Hawking had a point: The laws of physics that our universe must obey seem made-to-order for life to grow.

Take gravity. If this fundamental force were just a tiny bit stronger, stars would shine brighter, because the nuclear reactions in their cores rely on compressing hydrogen atoms together to make helium, which gives off light and heat. Greater gravity would intensify this process.

You might think that sunnier days on Earth wouldn’t be anything to sniff at, until you realize that all stars would exhaust their fuel much more quickly, and life on any planet wouldn’t have a chance to develop before its sun withered and died.

Also, when the universe was still in its infancy, areas of the cosmos varied slightly in temperature. These variations were only fractions of degrees, but if these had been even marginally bigger, all galaxies would have grown into giant black holes and plunged everything that ever was and would be into eternal darkness. And if these temperature variations had been smaller, no galaxies would have formed at all!

Let’s take another example. In the hard code of the universe that we were given, protons and neutrons – the things that make up the nucleus of an atom – weigh different amounts.

Again, this difference seems trivial: neutrons weigh just 0.1 percent more than protons. But if the universe’s code had decided it wanted these weights to be the other way around, with protons weighing more than neutrons, all neutrons would have decayed just moments after the Big Bang. That means no atoms, and therefore no planets, no stars, and no people.

The Stephen Hawking who wrote A Brief History of Time believed that the laws that underpin our universe are unchanging and timeless. No point asking why – they just are.

But as we’ll see, he wasn’t satisfied with that explanation – or any other current explanation, for that matter.

Current Theories Don’t Cut It

Let’s look at how humans have previously tried to explain why the cosmos’ operating manual seems fine-tuned for life. So far, there have been two persuasive views.

The first, and oldest, is the belief in some sort of creative designer: a God, or gods. They set the rules of the game that the universe – and everything in it – must follow. A Christian physicist, for example, might believe that God programmed the unbreakable rule that nothing can move faster than the speed of light. Something as perfect, intricate, and finely balanced as our cosmos must have been designed with life in mind.

The second, newer idea is that our universe is one of an infinite number of universes, all living alongside each other, but mutually inaccessible. In short, we exist within a multiverse.

Each component member of the multiverse might have completely different laws of physics, and almost all of them would be barren and completely unable to sustain life. But if there’s an infinite number of them, once in a while you’ll stumble upon a universe that’s like Goldilocks’ porridge: just right.

Neither of these explanations satisfied Stephen Hawking, however – and, surprisingly, for the same reason.

Let’s wind back the clock a bit. In the twentieth century, a British-Austrian philosopher named Karl Popper tried to define exactly what science is and what it is not. He came up with a powerful, influential, but devilishly simple formulation: Popper simply said that a proper scientific theory must be falsifiable. That means that it must have the potential to be proven wrong through experiments and evidence.

Many people believe in the idea of a creative designer – in fact, it’s the most popular explanation for the perfect fit between our universe and life. But it isn’t a scientific theory because there’s no way of hopping over to the lab, running a few tests, and disproving it. It isn’t falsifiable.

The multiverse theory suffers from the same problem. How could we ever test whether there are other universes out there? We can’t even see all of our own universe!

Because nothing can travel faster than the speed of light, we are trapped in a pretty big bubble that scientists call the observable universe. There are places in our cosmos where the light given off by stars hasn’t had enough time to reach us, so we can’t see them. And that light will never reach us, because the universe is expanding. It’s like we’re in a dark field with a lantern, and we can’t move: we’ll never know what’s beyond a certain point.

Hawking surveyed the territory and was deeply discontent. He realized we needed a new theory of our universe’s code.

What Is the Time?

We’re all used to the idea that we live in a three-dimensional world. Up and down, left and right, forward and back: We shop for groceries, drive cars, climb flights of stairs, and fly 30,000 feet in the air to holiday in sunny locales.

But what if we told you there’s an extra dimension to our world? A fourth dimension?

This extra dimension was found by a man you might’ve heard of: Albert Einstein. Well, “found” isn’t the right word. The German genius took something present in all our daily lives – and something none of us has enough of – and mathematically proved that it exists as a dimension alongside our three spatial ones. Einstein reinvented time. Think about it: an object doesn’t just exist in a location; it also exists at a point in time.

Following so far? Good, because things are about to get weird.

In 1983, Hawking put forward something called the no-boundary proposal. Winding the history of the universe right back, he found that time didn’t exist before the Big Bang. Everything existed as an infinitely tiny, infinitely dense speck in eternity.

According to the no-boundary proposal, there’s no point in asking what came before the Big Bang, because time didn’t exist. But a fraction of a second after the Big Bang, our three dimensions of space emerged, and from these three dimensions time popped out as a fourth.

And this is where the magic of Hawking’s new theory – the one at the heart of Hertog’s book – starts to happen.

As we’ve seen, in A Brief History of Time Hawking wrote that the laws of physics in our universe were fixed and eternal. But after its publication, he began to change his mind.

Instead, Hawking started to think that these laws evolved with the universe in those crucial moments after the Big Bang. Electromagnetism, gravity, dark matter, and the weight of neutrons – these mutated and evolved, just like how Charles Darwin showed us that animal species mutate and evolve. Toward the end of his life, Hawking began to view physics more and more from the perspective of biology.

Importantly, these evolutions happened in the quantum realm.

This is complicated stuff, but the best way of thinking about quantum physics is in terms of probabilities. The electron that zips around the nucleus of an atom never has a definite location or weight – it only has probabilities of being at a certain place and weighing a certain amount.

So in the quantum kingdom, where everything is a probability, the laws that govern our universe today were hashed out from a range of infinite possibilities.

Bizarre, right?

Mind-Blowing Top-Down Cosmology

If you’re scratching your head after that last section, you’re not alone. This is profoundly weird stuff, and far removed from our intuitive methods of understanding – things in this world seem to be completely disconnected from the logic that governs our daily lives. The only thing to do is smile, and marvel at the unfamiliar complexity and magnificence of physics, and the universe in which we find ourselves.

Try to keep this in mind as we muddle our way through these next two sections.

Just now, we said that in quantum physics, nothing has a specific value – only probabilities of being a certain value. But that’s not the whole story.

Once something is measured or observed, it does have a definite value. Let’s go back to our electron: before it’s measured, there’s a 30 percent probability of it being here, and a 67 percent probability of it being there. Once it’s measured, though, we know exactly where it is.

As well, in quantum physics there’s something called superposition. This is a fancy word for when something has an equal probability of having two different values. Basically, before something is measured, it could technically be in two places at once!

Fixing a value through measurement, and superposition: These two things are important to another mind-bending concept that Hawking and Hertog worked on – one that could revolutionize the study of cosmology altogether.

This is called top-down cosmology, and it’s easier to understand once we know that bottom-up cosmology studies the universe as it evolves forward in time. Scientists who do bottom-up cosmology start at the Big Bang and go forward, using scientific theories and evidence to predict what we should see. But bottom-up cosmology turns what we know about time, cause, and effect on its head.

Remember how in quantum physics, the act of observation fixes specific values? Well, if the universe’s laws evolved in the quantum realm shortly after the Big Bang, and now there are human scientists measuring and observing those laws, in a strange way we have fixed them by our observation, from a huge range of different possibilities that exist simultaneously as superpositions.

In this view, the past becomes dependent on the present, and human observers have a big role to play in this process.

Weird Science

To complete the tangled, terrifying, yet strangely marvelous collaboration of Hawking and Hertog, we need to explore one final concept: holography.

In the last decade, holography has been all the rage in theoretical physics. In this theory, the universe isn’t thought of in terms of atoms and space; it’s thought of in terms of information. This makes more sense when we realize that in quantum physics, things don’t have values like weight or speed – they exist only as probabilities.

Now, does everyone remember watching Star Wars? When characters communicate with each other from distant planets, eerie computer projections of their bodies spring from flat surfaces near the person they want to talk to. This is what a hologram is – a 3D object being projected from a 2D surface.

So what does a holographic universe mean? Well, it means that we are living in a world containing three spatial dimensions, but that this is just a projection of a universe that contains more dimensions, which we cannot access.

What does any of this have to do with thinking about the universe in terms of information? Answering this is about as hard as science can get, but the essential idea is that we have found evidence that our everyday dimensions are holographic from studying black holes.

At the center of a black hole is something called a singularity – an infinitely dense, infinitely tiny point. Because it’s so heavy, a titanic gravitational field surrounds the singularity in the shape of a sphere. Now, if we think of the universe as being made up of information, it makes sense for this sphere to contain a huge amount of information, right?

But when scientists did the math, they found that the information black holes contain isn’t equal to their capacity as a sphere – it’s equal to their capacity as a circle. What is a sphere represented in two dimensions? You got it, a circle. This provides evidence for the holographic universe theory.

Hawking and Hertog took this, and peered into the beginnings of the universe. They ran some equations, and realized that the no-boundary proposal – which argues that the dimension of time was created moments after the Big Bang from the three spatial dimensions – fits perfectly with the theory that our universe is a hologram containing higher dimensions.

They kept developing this model, and incorporated the idea that the universe consists of bits of information. They kept following this back to the Big Bang, and realized that as you get closer to the beginning of the universe, you begin to run out of bits, like the resolution of a movie getting progressively grainier until … nothing. Before space, and before time.

And with that – congratulations! You’ve made it through our outlandish journey back to the origin of time!


In the final decades of Stephen Hawking’s life, he began to change his mind about how our universe ended up with the laws of physics that govern it. He wasn’t happy with previous explanations because they weren’t falsifiable scientific theories, and so he went back to the drawing board.

He came up with a theory that our universe is a holographic projection that contains other dimensions that we can’t access, which helped support his theory that the dimension of time sprang out from our three dimensions of space just after the Big Bang.

It was also in these moments just after the Big Bang that Hawking believed our universe’s laws changed and evolved on the level of quantum physics, and that scientists today, by observing these laws, have fixed them from a range of infinite possibilities just by observing them.

About the author

Thomas Hertog is an internationally renowned cosmologist who was for many years a close collaborator of the late Stephen Hawking. He received his doctorate from the University of Cambridge and is currently professor of theoretical physics at the University of Leuven, where he studies the quantum nature of the big bang. He lives with his wife and their four children in Bousval, Belgium.


Science, Education, Physics, Astrophysics, Technology, Astronomy, Space Science, Cosmology, Memoirs


NEW YORK TIMES BESTSELLER • Stephen Hawking’s closest collaborator offers the intellectual superstar’s final thoughts on the cosmos—a dramatic revision of the theory he put forward in A Brief History of Time.

Perhaps the biggest question Stephen Hawking tried to answer in his extraordinary life was how the universe could have created conditions so perfectly hospitable to life. In order to solve this mystery, Hawking studied the big bang origin of the universe, but his early work ran into a crisis when the math predicted many big bangs producing a multiverse—countless different universes, most of which would be far too bizarre to ​harbor life.

Holed up in the theoretical physics department at Cambridge, Stephen Hawking and his friend and collaborator Thomas Hertog worked on this problem for twenty years, developing a new theory of the cosmos that could account for the emergence of life. Peering into the extreme quantum physics of cosmic holograms and venturing far back in time to our deepest roots, they were startled to find a deeper level of evolution in which the physical laws themselves transform and simplify until particles, forces, and even time itself fades away. This discovery led them to a revolutionary idea: The laws of physics are not set in stone but are born and co-evolve as the universe they govern takes shape. As Hawking’s final days drew near, the two collaborators published their theory, which proposed a radical new Darwinian perspective on the origins of our universe.

On the Origin of Time offers a striking new vision of the universe’s birth that will profoundly transform the way we think about our place in the order of the cosmos and may ultimately prove to be Hawking’s greatest legacy.


“This superbly written book offers insight into an extraordinary individual, the creative process, and the scope and limits of our current understanding of the cosmos.”—Lord Martin Rees

“Why is our universe the way it is? How did everything begin? How might it end? Thomas Hertog probed these overwhelming questions in collaboration with Stephen Hawking, achieving a privileged perspective into how, struggling against daunting physical odds, Hawking’s imprisoned mind yielded astonishing insights even in his later years. This superbly written book offers insight into an extraordinary individual, the creative process generally, and the scope and limits of our current understanding of the cosmos.”—Lord Martin Rees, Emeritus Professor of Cosmology and Astrophysics, University of Cambridge, and author of Just Six Numbers

“Like his mentor and colleague Stephen Hawking, Thomas Hertog has never shied away from being ambitious in theorizing about the universe. This sweeping book provides an accessible overview of both what we know about cosmology and some audacious ideas for moving into the unknown. It is an introduction to Hawking’s final theory, but also a glimpse into even grander theories yet to come.”—Sean Carroll, author of The Biggest Ideas in the Universe: Space, Time, and Motion

“Stephen Hawking’s final theory is lucidly explained in this splendidly accessible book. Author Thomas Hertog, one of Hawking’s closest collaborators, gives us a vivid insight into Hawking as both a brilliant physicist and an astonishingly determined human being.”—Graham Farmelo, Churchill College, University of Cambridge, and author of The Strangest Man

“A beautifully written, thought-provoking account of both the physics and the personalities involved in Hawking’s visionary struggle to comprehend the cosmos. Thomas Hertog has provided a fascinating insider’s view.”—Neil Turok, co-author of Endless Universe

Video and Podcast

Read an Excerpt/PDF Preview

Chapter 1

A Paradox

Es könnte sich eine seltsame Analogie ergeben, daß das Okular auch des riesigsten Fernrohrs nicht größer sein darf, als unser Auge.

A curious correlation may emerge in that the eyepiece of even the biggest telescope cannot be larger than the human eye.

—Ludwig Wittgenstein, Vermischte Bemerkungen

The late 1990s were the culmination of a golden decade of discovery in cosmology. Long regarded as a realm of unrestrained speculation, cosmology—the science that dares to study the origin, evolution, and fate of the universe as a whole—was finally coming of age. Scientists all over the world were buzzing with excitement about spectacular observations from sophisticated satellites and Earth-based instruments that were transforming our picture of the universe beyond recognition. It was as if the universe was speaking to us. These developments posed quite a reality check for theoreticians, who were told to rein in their speculation and flesh out the predictions of their models.

In cosmology we discover the past. Cosmologists are time travelers, and telescopes their time machines. When we look into deep space we look back into deep time, because the light from distant stars and galaxies has traveled millions or even billions of years to reach us. Already in 1927 the Belgian priest-astronomer Georges Lemaître predicted that space, when considered over such long periods of time, expands. But it wasn’t until the 1990s that advanced telescope technology made it possible to trace the universe’s history of expansion.

This history held some surprises. For example, in 1998 astronomers discovered that the stretching of space had begun to speed up around five billion years ago, even though all known forms of matter attract and should therefore slow down the expansion. Since then, physicists have wondered whether this weird cosmic acceleration is driven by Einstein’s cosmological constant, an invisible ether-like dark energy that causes gravity to repel rather than to attract. One astronomer quipped that the universe looks like Los Angeles: one-third substance and two-thirds energy.

Obviously, if the universe is expanding now, it must have been more compressed in the past. If you run cosmic history backward—as a mathematical exercise, of course—you find that all matter would once have been very densely packed together and also very hot, since matter heats up and radiates when it is squeezed together. This primeval state is known as the hot big bang. Astronomical observations since the golden 1990s have pinned down the age of the universe—the time elapsed since the big bang—to 13.8 billion years, give or take 20 million.

Curious to learn more about the universe’s birth, the European Space Agency (ESA) launched a satellite in May 2009 in a bid to complete the most detailed and ambitious scanning of the night sky ever undertaken. The target was an intriguing pattern of flickers in the heat radiation left over from the big bang. Having traveled through the expanding cosmos for 13.8 billion years, the heat from the universe’s birth reaching us today is cold: 2.725 K, or about –270 degrees Celsius. Radiation at this temperature lies mainly in the microwave band of the electromagnetic spectrum, so the remnant heat is known as the cosmic microwave background radiation, or CMB radiation.

ESA’s efforts to capture the ancient heat culminated in 2013 when a curious speckled image resembling a pointillist painting decorated the front pages of the world’s newspapers. This image is reproduced in figure 2, which shows a projection of the entire sky, compiled in exquisite detail from millions of pixels representing the temperature of the relic CMB radiation in different directions in space. Such detailed observations of the CMB radiation provide a snapshot of what the universe was like a mere 380,000 years after the big bang, when it had cooled to a few thousand degrees, cold enough to liberate the primeval radiation, which has traveled unhindered through the cosmos ever since.

The CMB sky map confirms that the relic big bang heat is nearly uniformly distributed throughout space, although not quite perfectly. The speckles in the image represent minuscule temperature variations indeed, tiny flickers of no more than a hundred-thousandth of a degree. These slight variations, however small, are crucially important, because they trace the seeds around which galaxies would eventually form. Had the hot big bang been perfectly uniform everywhere, there would be no galaxies today.

The ancient CMB snapshot marks our cosmological horizon: We cannot look back any farther. But we can glean something about processes operating in yet earlier epochs from cosmological theory. Just as paleontologists learn from stone fossils what life on Earth used to be like, cosmologists can, by deciphering the patterns encoded in these fossil flickers, stitch together what might have happened before the relic heat map was imprinted on the sky. This turns the CMB into a cosmological Rosetta Stone that enables us to trace the universe’s history even farther back, perhaps as far back as a fraction of a second after its birth.

And what we learn is intriguing. As we will see in chapter 4, the temperature variations of the CMB radiation indicate that the universe initially expanded fast, then slowed down, and, more recently (about five billion years ago), began accelerating again. Slowing down appears to be the exception rather than the rule on the scales of deep time and deep space. This is one of those seemingly fortuitous biofriendly properties of the universe, for only in a slowing universe does matter aggregate and cluster to form galaxies. If it hadn’t been for the extended near-pause in expansion in our past, there would, again, be no galaxies and no stars, and thus no life.

In effect, the universe’s expansion history was at the center of one of the very first moments in which the conditions for our existence slipped into modern cosmological thinking. This moment occurred in the early 1930s, when Lemaître made a remarkable sketch in one of his purple notebooks of what he called a “hesitating” universe, one with an expansion history much like the bumpy ride that would emerge from observations seventy years later (see insert, plate 3). Lemaître embraced the idea of a long pause in the expansion by considering the universe’s habitability. He knew that astronomical observations of nearby galaxies pointed to a high expansion rate in recent times. But when he ran the evolution of the universe backward in time at this same rate, he found that the galaxies must all have been on top of one another no more than a billion years ago. This was impossible, of course, for Earth and the sun are much older than that. To avoid an obvious conflict between the history of the universe and that of our solar system, he imagined an intermediate era of very slow expansion, to give stars, planets, and life time to develop.

In the decades since Lemaître’s pioneering work, physicists have continued to stumble across many more such “happy coincidences.” Make but a small change in almost any of its basic physical properties, from the behavior of atoms and molecules to the structure of the cosmos on the largest scales, and the universe’s habitability would hang in the balance.

Take gravity, the force that sculpts and governs the large-scale universe. Gravity is extremely weak; it requires the mass of Earth just to keep our feet on the ground. But if gravity were stronger, stars would shine more brightly and hence die far younger, leaving no time for complex life to evolve on any of the orbiting planets warmed by their heat.

Or consider the tiny variations, one part in a hundred thousand, in the temperature of the relic big bang radiation. Were these differences slightly larger—say one part in ten thousand—the seeds of cosmic structures would have mostly grown into giant black holes instead of hospitable galaxies with abundant stars. Conversely, even smaller variations—one millionth or less—would produce no galaxies at all. The hot big bang got it just right. One way or another it set off the universe on a supremely biofriendly trajectory, the fruits of which would not become evident until several billion years later. Why?

Other examples of such happy cosmic coincidences abound. We live in a universe with three large dimensions of space. Is there anything special about three? There is. Adding just a single space dimension renders atoms and planetary orbits unstable. Earth would spiral into the sun instead of tracing out a stable orbit around it. Universes with five or more large space dimensions have even bigger problems. Worlds with only two space dimensions, on the other hand, may not provide enough room for complex systems to function, as figure 3 illustrates. Three dimensions of space seems just right for life.

Moreover, this uncanny fitness for life extends to the universe’s chemical properties, which are determined by the properties of elementary particles and the forces acting between them. For example, neutrons are a tad heavier than protons. The neutron-to-proton mass ratio is 1.0014. Had it been the other way around, all the protons in the universe would have decayed into neutrons shortly after the big bang. But without protons there would be no atomic nuclei and hence no atoms and no chemistry.