In the eyes of the public, Stephen Hawking is a giant of 20th century science. He stormed the popular stage with the 1988 publication A short history of time, who presented his esoteric ideas about evaporating black holes and the birth of the universe. It was an international bestseller, but because of the complexity of the ideas it was called A Brief History the most unread book of all time.
Hawking continued to explore the fundamental nature of the universe until his death in 2018. In a new book, On The origin of timeBelgian physicist Thomas Hertog unravels Hawking’s latest theory, which focuses on one of the greatest questions of all: why our universe is the way it is.
Review: On the Origin of Time: Stephen Hawking’s Final Theory – Thomas Hertog
Hertog is not a passive player in this story, a student and associate of Hawking. Instead, he is an active participant. Intriguingly, as Hertog explains, we are all active participants in Hawking’s latest theory, where we shape the universe by observing it.
In this new book, Hertog tells us that Hawking’s latest theory attempts to address one of the deep mysteries of the universe, something known as the problem of cosmological fine-tuning.
Cosmologists have realized that the more they look at the underlying nature of the universe (for example, the forces of fundamental forces and the masses of fundamental particles), the more the cosmos appears to be attuned to our existence.
If the universe had been born with slightly different values for these fundamental properties, it would be dead and sterile, without the complexity and energy essential to life.
For some, the solution to cosmological fine-tuning lies in the multiverse, the idea that our universe is just one of countless others. Our universe, and all others, crystallize from an onslaught of eternal inflation, a super-energetic cosmic expansion. Each individual universe is written at birth with its own unique laws of nature. Most of these universes in the multiverse are dead, but our cosmic home has won the physics lottery. It is not surprising that we are in a universe that can host life.
Read more: What are the best living conditions? Exploring the multiverse can help us find out
However, as Hertog writes in this new book, Hawking rejected the multiverse and set out to find an alternative solution to cosmic fine-tuning.
Shutterstock. Elements provided by NASA.
Familiar terrain
To get to this point, Hertog enters very familiar territory, discussing the history of modern cosmological ideas. This includes the theoretical basis of Albert Einstein and George Lemaitreand the observational insights of Edwin Hubble which revealed the expansion of the universe.
Hertog interweaves the story with the development in the 20th century of that other great pillar of physics, the strange behavior in the world of the quantumwhere Isaac Newton’s deterministic world, in which things have precise locations at precise times, is replaced by a nebulous world of probabilities and uncertainties.
Usually we think of quantum mechanics describing the subatomic world, of electrons and atoms, but Hawking thought of the whole universe as a quantum system. The story has been brought up to date with the idea of cosmic inflation in the earliest instances of the universe and the surprising discovery of the dominance of dark energy in the closing years of the last century.
Read more: Explainer: quantum computation and communication technology
Hawking’s own story is similarly woven into the book, including his revelation that black holes aren’t really black.
By combining Einstein’s general theory of relativity, which dictates the space-time curvature of a black hole, with quantum field theory, which describes the strange, ephemeral nature of apparently empty space, Hawking showed that black holes actually radiate. This trickle of energy causes black holes to steadily evaporate into a final nothingness.
But if you’re looking for an accurate description of exactly how Hawking’s idea of black hole radiation works, Hertog unfortunately relies on the same flawed image of particles that originate at the edge of the hole, as presented by Hawking in A Brief History of Time.
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Hertog also tells us that Hawking explored the state of the universe at its very beginning, arguing that at this starting point, at least in terms of general relativity, the density of things in the universe must have been infinite (this idea is formally known as singularity).
Hawking returned to this question with physicist Jim Hartle in the early 1980s to try to visualize quantum mechanics. Hartle and Hawking argued that if you turn the universe back to its beginning, time loses its separateness and actually becomes space. With this no limit hypothesis, the universe had no origin, at least not one that we would ever really understand.
Read more: A timeline of Stephen Hawking’s remarkable life
A subtle argument
Hawking’s motivation for disliking the multiverse is a somewhat subtle argument based on the idea of the anthropic principlethe fact that we shouldn’t be surprised to find ourselves in a universe that allows us to be here.
Hertog tells us that we are in a way typical of the possible observers who might inhabit the universes in the multiverse. But what typical means is a complicated subject. Does this mean that other life in other universes should be the same as life on Earth? Or typical in a broader sense, that life should consist of the same elements as we do? Typically can be judged on many different criteria. And how will we ever find out how typical we are if we are forever confined to the perceptions of our one universe?
To find an alternative solution, Hawking’s first step was to overturn the approach to understanding the universe.
The goal of modern science has been to unravel the fundamental workings of the universe and use them to predict how physical systems evolve. To do this, we need more than the laws of physics, but we need to know the starting point, the conditions. But for a universe emerging from the strange singular state at its origin, where infinities abound, what exactly are these preconditions, and do they uniquely define the universe in which we live?
Hertog explains that he and Hawking had a different view, a top-down view of the universe. Quantum mechanics comes back into focus and life in the universe is treated as a quantum system, described in terms of possibilities and probabilities.
It’s here that Hugo Everett III‘s interpretation of many worlds quantum mechanics comes into play. According to Everett, all possible outcomes of a quantum experiment play out in parallel life forms, and it is this idea that Hawking applies to the universe.
Within this last theory, as expressed by Hertog, the observer now plays a central role. The fact that we all exist and observe the world around means that we participate in shaping the universe we seem to live in.
Of all the possible histories of the universe that could possibly exist in a sea of parallel universes, the fact that we are here observing this universe sets it apart this universe, with everyone else lost in a sea of quantum uncertainty. The situation strangely becomes self-referential.
At this point, the general reader is likely to be confused. This, of course, is quite a radical conception for understanding the nature of the universe. And, frankly, the reader might wonder what separates Hawking’s final theory from what some might consider pseudoscientific gibberish.
The idea that we, as observers, are essential to the creation of the universe is not a new ones, and is often the source of ridicule. Of course, given the scientific weight of the authors, this last theory should be worth it, but whether this hypothesis is a real contender for an accurate description of life in our universe is difficult to judge.
This should not deter the reader. Hertog’s easy writing style jumps from topic to topic, providing an overview of the development of modern cosmology and the need for quantum mechanics to understand the ultimate origin of the universe.
But when the going gets tough and the intricate ideas of Hawking’s cosmos are explored, some things get skipped a little too quickly and it would have been an advantage to spend a little more time on what seems obvious to quantum cosmologists.
Finally, it’s worth pointing out that there are some irritating features in the writing, including the near-hero worship Hawking receives. Obviously, given the close relationship between the author and his subject, this is somewhat understandable, but it can still be annoying.
The text also mixes the philosophy bashing that seems to be a badge of honor from modern physicists with various philosophical musings that underlie cosmological and quantum thinking. But given the scope of the subject, and with a mix of anecdotes, quotes and analogies, Hertog provides an intriguing snapshot of our thinking about the ultimate origin of our universe.
