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.

IT WAS common knowledge among students at the University of Cambridge that whoever obtained the best marks in the final part of the mathematical tripos exams would be summoned to see Stephen Hawking. I had just got my results and had come top. Sure enough, I was invited for a discussion with him.

Thomas Hertog (left) collaborated with Stephen Hawking for many years
Courtesy of Thomas Hertog

I made my way to his office deep in the labyrinth of the department of applied mathematics and theoretical physics, which was housed in a creaking Victorian building on the banks of the river Cam. Stephen’s office was just off the main common room, and even though it was noisy there, he liked to keep his door ajar. I knocked, paused and slowly pushed it open.

I didn’t quite know what to expect on the other side of that door. I knew, of course, that Stephen was famous for his work on black holes and that he had even got into trouble for some of his ideas about what happens when they explode. But it turned out that he was musing on a different question: why is the universe just right for life to arise?

Pondering this question would turn into a long quest for us both. For the next two decades, until his death, Stephen and I worked shoulder to shoulder on novel ideas that suggest a radically new understanding of why the universe is the way it is. In our conception, the laws of physics themselves have, in a sense, evolved to be the way they are.

In that first meeting, in June 1998, I found Stephen sitting behind his desk with his head leaning against a headrest on his wheelchair. The office window was open and I later learned that he liked to keep it that way at all times, even in freezing weather. On one of the blackboards were equations that appeared to date from the early 1980s. I wondered if they might be his last handwritten scrawls.

“The universe appears designed,” he said through his speech synthesiser. He continued: “Why is the universe the way it is?” None of my physics teachers had asked questions like this before. “Isn’t that a philosophical matter?” I tried. “Philosophy is dead,” Stephen replied, his eyes twinkling.

He was a master of packing a lot into a few choice words. When he spoke of the universe being designed, he was referring to the observation that, of all the universes that could exist, ours is spectacularly well configured to bring forth life. What to make of this has bedevilled thinkers one way or another for centuries. Yet it is only fairly recently that we have discovered how deep these waters run.

Our fine-tuned universe

The universe’s biofriendliness, it turns out, concerns the laws of physics themselves. There are numerous features in these laws that render the universe just right for living things. Twiddle ever so slightly with any of these and habitability would often hang in the balance.

Take the Higgs boson, which weighs as much as 133 protons. This may sound heavy (for a particle), but it is 100 million billion times lighter than many physicists would consider a natural mass. The Higgs boson couples to other particles of matter and, in this way, imbues them with mass, but these couplings also add to the Higgs’s own mass, so you would expect it to be a far weightier beast. The unbearable lightness of the Higgs is crucial for life, however, for a light Higgs keeps electrons, protons, neutrons and so on light as well. That, in turn, ensures that the building blocks of life, such as DNA, proteins and cells, don’t collapse under the force of gravity.

Or consider the expansion of the universe. In 1998, cosmologists discovered that the expansion of space has been accelerating for about 5 billion years. The cause of this acceleration is often attributed to something known as vacuum energy, which is predicted by quantum theory. But the density of vacuum energy seems to be 10^120 times lower than physicists expect based on theory. If the vacuum energy density of the universe were just a tad larger, however, its repulsive effect would be stronger and acceleration would have kicked in much earlier. This would have meant that matter was so sparsely distributed that it couldn’t clump together to form stars and galaxies, once again precluding the formation of life.

The laws of physics and cosmology have many more such life-engendering properties. It almost feels as if the universe is a fix – a big one. Traditionally, most scientists regarded the mathematical relationships that underpin the laws of physics as transcendental Platonic truths. In which case, the answer to the riddle of cosmic design – to the extent that it is an answer – is that it is a matter of mathematical necessity. The universe is the way it is because nature had no choice.

Around the turn of the 21st century, an entirely different explanation emerged. This one had its roots in a series of surprising discoveries that suggested that at least some properties of the physical laws might not be carved in stone, but could instead be the accidental outcome of the particular manner in which the early universe cooled after the big bang. From the species of particles to the strength of forces to the amount of vacuum energy, it became apparent that the universe’s biofriendly laws were forged in a series of random transitions during its earliest moments of expansion. Reasoning along these lines, cosmologists started wondering whether, perhaps, there was more than one universe. Maybe we live in a multiverse, an enormous, inflating space with a variegated patchwork of universes, each with its own big bang, leading to its own local physical laws.

This led to a sweeping change of perspective on the idea of our universe being fine-tuned for life. Even though most universes would be sterile, in some, the laws of nature are bound to be just right for life. String theorist Leonard Susskind once likened the local character of physical laws in the multiverse to the weather on the US east coast: “Tremendously variable, almost always awful, but lovely on rare occasions.” In his view, our delightful cosmic weather is a fluke and the impression of design is an illusion.

All this was very much on Stephen’s mind when I first walked into his office in 1998. I could sense he wasn’t keen on the idea of a multiverse. Before long, I was collaborating with him to try to find a better answer, first as his PhD student and later as his colleague.

Stephen’s reticence to embrace the multiverse grew stronger in the early 2000s, when it became clear that it didn’t actually explain anything. In multiverse cosmology, there are “metalaws” governing all the universes. But these metalaws don’t specify in which of the habitable universes we are supposed to be. This is a problem, for without a rule that relates the metalaws of the multiverse to the local laws within our universe, multiverse musings get caught in a spiral of paradoxes that leaves us without verifiable predictions. Multiverse cosmology is like a debit card without a PIN or an IKEA flatpack closet without a manual: useless.

Hawking’s final theorem

Can we do better? Yes, Stephen and I found out, but only by relinquishing the idea, inherent in multiverse cosmology, that our theories can take a God’s-eye view, as if somehow standing outside the cosmos. It is an obvious and seemingly tautological point: our cosmological theory must account for the fact that we exist within the universe. “We are not angels who view the universe from the outside,” Stephen began to preach. So we set out to rethink cosmology from an inside-out, observer’s perspective. This, we soon discovered, required adopting a quantum outlook from within the universe.

The key role of the observer has been recognised since the discovery of quantum theory in the 1920s. Before a particle’s position is observed, there is no sense in even asking where it is. It doesn’t have a definite position, only possible positions described by a wave function that encodes the likelihood that the particle, if it were observed, would be here or there. Of course, quantum observations are by no means restricted to those made by humans. Such observations could be made by a dedicated detector, by the environment or even through interaction with a lone photon.

Stephen and I came to understand what went on in the early universe as a process akin to that of natural selection on Earth, with an interplay of variation and selection playing out in this primeval environment. Variation happens because random quantum jumps cause frequent small excursions from deterministic behaviour and occasional larger ones. Selection enters the picture because some of these excursions, especially the larger ones, can be amplified and frozen-in thanks to quantum observation. This then gives rise to new rules that help shape the subsequent evolution.

The laws of nature evolve

The interaction between these two competing forces in the furnace of the big bang produces a branching process – somewhat analogous to how biological species would emerge billions of years later – in which dimensions, forces and particles first diversify and then acquire their effective form when the universe expands and cools.

And just like in Darwinian evolution, this introduces a subtle backward-in-time element to our hypothesis. It is as if the collective quantum observations retroactively fix the outcome of the big bang. For this reason, Stephen liked to refer to our idea as “top-down cosmology”, to drive home the point that we read the fundamentals of the universe ex post facto, somewhat like how biologists reconstruct the tree of life. “We create the universe as much as the universe creates us,” he once told me. When New Scientistcovered our idea in 2006, it was described as a “reverse choose-your-own adventure”.

The universe is so well suited to life that it can appear designed

tobiasjo/Gety Images

In hindsight, we were walking on quicksand back then in the sense that we didn’t quite have a solid mathematical basis for our ideas. As we began to look for firmer ground, inspiration came from an unexpected corner. Around that time, another revolution in physics was picking up, one that was all to do with holography. This would prove to be just what we needed.

A normal hologram encodes all information about a three-dimensional object on a two-dimensional surface. In a sense, the third dimension emerges from the surface when we look at it. The first inklings that even the force of gravity may have holographic roots go back to work in the 1970s by Stephen and, separately, Jacob Bekenstein. They discovered that all there is to know about the interior of black holes can be encrypted on their event horizon surface.

Then, in 1997, physicist Juan Maldacena went further and envisaged that the entire universe may be akin to a hologram. He showed that a system of quantum-entangled particles located on a surface can contain within it all the information of a higher-dimensional cosmos with gravity and curved space-time. Soon, holography became the talk of the town among theoretical physicists, who saw in it a promising way to finally get Albert Einstein’s general relativity, his theory of gravity, to work with quantum theory.

Is the universe a hologram?

At first, the kind of universes generated by holographic theory bore no resemblance whatsoever to the expanding universe we live in. However, starting in around 2011, Stephen and I figured out how to apply the cosmos-as-hologram idea to describe the earliest stages of an expanding universe like ours. In this cosmological setting, it turns out it is the dimension of time that holographically pops out. History itself is holographically encrypted.

What’s more, time emerges in the ex post facto manner that we had envisioned. The past is contingent on the present in holographic cosmology, not the other way around. In a holographic approach to cosmology, venturing far back in time means taking a fuzzy look at the cosmological hologram. It is like zooming out, an operation whereby we discard more and more of the entangled information that the hologram encodes. Holography suggests that not only time, but also the physical laws that shape our universe, disappear back into the big bang. This is very different to the old Platonist view that the laws of nature are somehow immutable. Stephen and I held that it isn’t the laws as such that are fundamental, but their capacity to change.

The upshot of all this is a profound revision of what cosmology is ultimately about. For almost a century, we have been studying the history of the universe against a stable background of fixed laws of nature. But the quantum outlook that Stephen and I developed reads the universe’s history from within and as one that includes, in its earliest stages, the genealogy of the physical laws.

It is a radical idea, for sure, but one that may, in time, become testable. Some ideas to do with the early universe can be tested by deciphering the cosmic microwave background (CMB) radiation, the flood of light released 380, 000 years after the big bang. But the primeval evolution we envisage unfolded before that, meaning it is hidden far behind the CMB. We are in a situation not unlike Charles Darwin in the 19th century, who had only scant fossil evidence for his grand new hypothesis.

Gravitational waves

But I am hopeful this won’t be the case forever. We are witnessing a revolution in gravitational wave astronomy and these waves can reach us from well before the CMB era. Future observations of primeval gravitational waves should enable us to probe the universe’s earliest phase.

An entirely different path to test our ideas has to do with holography. Already, quantum experimentalists are attempting to create strongly entangled quantum systems, made up of trapped atoms or ions, that holographically encode some of the properties of black holes or toy-model universes. By experimenting with these systems, we can hope to learn more about what sort of entanglement patterns underpin gravity and the fabric of space-time. We might also be able to see whether the origin of time happens in the specific way Stephen and I envisaged. That would amount to an experimental practice of early universe cosmology.

Turning cosmology inside out and upside down was a quintessentially Hawkingian act. For Stephen and I, inspired by the paradoxes of the multiverse, it was a way to get a grip on the appearance of cosmic design. If this new thinking turns out to be right, it may yet prove to be his greatest scientific legacy.

Reference/Source: NewScientist.

Read original article here.

Post a Comment

Last Article Next Article