The World According to Physics

Jim Al-Khalili

• Publisher: Princeton University Press
• Year: 2020
• ISBN:
  9780691201672
• Topic: Physical Sciences
• Subtopic: Physics
• Index: Physics

Quantum physicist, New York Times bestselling author, and BBC host Jim Al-Khalili offers a fascinating and illuminating look at what physics reveals about the world Shining a light on the most profound insights revealed by modern physics, Jim Al-Khalili invites us all to understand what this crucially important science tells us about the universe and the nature of reality itself.Al-Khalili begins by introducing the fundamental concepts of space, time, energy, and matter, and then describes the three pillars of modern physics—quantum theory, relativity, and thermodynamics—showing how all three must come together if we are ever to have a full understanding of reality. Using wonderful examples and thought-provoking analogies, Al-Khalili illuminates the physics of the extreme cosmic and quantum scales, the speculative frontiers of the field, and the physics that underpins our everyday experiences and technologies, bringing the reader up to speed with the biggest ideas in physics in just a few sittings. Physics is revealed as an intrepid human quest for ever more foundational principles that accurately explain the natural world we see around us, an undertaking guided by core values such as honesty and doubt. The knowledge discovered by physics both empowers and humbles us, and still, physics continues to delve valiantly into the unknown.Making even the most enigmatic scientific ideas accessible and captivating, this deeply insightful book illuminates why physics matters to everyone and calls one and all to share in the profound adventure of seeking truth in the world around us.

CHAPTER 1

THE AWE OF UNDERSTANDING

While stories will always be a vital part of human culture, even in science—and our lives would be the poorer without them—modern science has now replaced many of the ancient mythologies and accompanying superstitious beliefs. A good example of how we have demystified our approach to understanding the world is the creation myths. Since the dawn of history, humankind has invented stories about the origins of our world, and deities that were instrumental in its creation, from the Sumerian god Anu, or Sky Father, to the Greek myths about Gaia being created out of Chaos and the Genesis myths of the Abrahamic religions, which are still believed as literal truths in many societies around the world. It may appear to many non-scientists that our modern cosmological theories about the origins of the universe are themselves no better than the religious mythologies they replace—and, if you look at some of the more speculative ideas in modern theoretical physics, you might agree that those who feel this way have a point. But through rational analysis and careful observation—a painstaking process of testing and building up scientific evidence, rather than accepting stories and explanations with blind faith—we can now claim with a high degree of confidence that we know quite a lot about our universe. We can also now say with confidence that what mysteries remain need not be attributed to the supernatural. They are phenomena we have yet to understand—and which we hopefully will understand one day through reason, rational enquiry, and, yes … physics.

Contrary to what some people might argue, the scientific method is not just another way of looking at the world, nor is it just another cultural ideology or belief system. It is the way we learn about nature through trial and error, through experimentation and observation, through being prepared to replace ideas that turn out to be wrong or incomplete with better ones, and through seeing patterns in nature and beauty in the mathematical equations that describe these patterns. All the while we deepen our understanding and get closer to that ‘truth’—the way the world really is.

There can be no denying that scientists have the same dreams and prejudices as everyone else, and they hold views that may not always be entirely objective. What one group of scientists calls ‘consensus’, others see as ‘dogma’. What one generation regards as established fact, the next generation shows to be naïve misunderstanding. Just as in religion, politics, or sport, arguments have always raged in science. There is often a danger that, all the while a scientific issue remains unresolved, or at least open to reasonable doubt, the positions held by each side of the argument can become entrenched ideologies. Each viewpoint can be nuanced and complex, and its advocates can be just as unshakable as they would be in any other ideological debate. And just as with societal attitudes on religion, politics, culture, race, or gender, we sometimes need a new generation to come along, shake off the shackles of the past, and move the debate forward.

But there is also a crucial distinction to science, when compared with other disciplines. A single careful observation or experimental result can render a widely held scientific view or long-standing theory obsolete and replace it with a new worldview. This means that those theories and explanations of natural phenomena that have survived the test of time are the ones we trust the most; they are the ones we are most confident about. The Earth goes around the Sun, not the other way around; the universe is expanding, not static; the speed of light in a vacuum always measures the same no matter how fast the measurer of that speed is moving; and so on. When a new and important scientific discovery is made, which changes the way we see the world, not all scientists will buy into it immediately, but that’s their problem; scientific progress is inexorable, which, by the way, is always a good thing: knowledge and enlightenment are always better than ignorance. We start with not knowing, but we seek to find out … and, though we may argue along the way, we cannot ignore what we find. When it comes to our scientific understanding of how the world is, the notion that ‘ignorance is bliss’ is a load of rubbish. As Douglas Adams once put it: ‘I’d take the awe of understanding over the awe of ignorance any day.’¹

WHAT WE DON’T KNOW

It is also true that we are constantly discovering how much more there is that we don’t yet know. Our growing understanding yields a growing understanding of our ignorance! In some ways, as I will explain, this is the situation we have in physics right now. We are currently at a moment in history when many physicists see, if not a crisis in the subject, then at least the building up of a head of steam. It feels as though something has to give. A few decades ago, prominent physicists such as Stephen Hawking were asking, ‘Is the end in sight for theoretical physics?’² with a ‘theory of everything’ potentially just around the corner. They said it was just a matter of dotting the ‘i’s and crossing the ‘t’s. But they were wrong, and not for the first time. Physicists had expressed similar sentiments towards the end of the nineteenth century; then along came an explosion of new discoveries (the electron, radioactivity, and X-rays) that couldn’t be explained by the physics known at the time and which ushered in the birth of modern physics. Many physicists today feel that we might potentially be on the verge of another revolution in physics as big as that seen a century ago with the birth of relativity and quantum mechanics. I am not suggesting that we are about to discover some fundamental new phenomenon, like X-rays or radioactivity, but there may yet be a need for another Einstein to break the current deadlock.

The Large Hadron Collider has not yet followed up on its 2012 success in detecting the Higgs boson, and thereby confirming the existence of the Higgs field (which I will discuss later); many physicists were hoping for the discovery of other new particles by now, which would help resolve long-standing mysteries. And we still don’t understand the nature of the dark matter holding galaxies together or the dark energy that is ripping the universe apart; nor do we have answers to fundamental questions like why there is more matter than antimatter; why the properties of the universe are so finely tuned to allow for stars and planets, and life, to exist; whether there is a multiverse; or whether there was anything before the Big Bang that created the universe we see. There is still so much left that we cannot explain. And yet, it is hard not to be dazzled by our success so far. While some scientific theories may turn out to be connected to each other at a deeper level than we thought, and others may turn out to be entirely wrong, no one can deny just how far we’ve come.

Sometimes, in the light of new empirical evidence, we realise that we were barking up the wrong tree. Other times we simply refine an idea that turns out not to be wrong, but just a rough approximation that we improve upon to gain a more accurate picture of reality. There are some areas of fundamental physics that we might not be entirely happy with, where we know deep down that we’ve not heard the final word, but which we nevertheless continue to rely on for the time being because they are useful. A good example of this is Newton’s universal law of gravitation. It is still referred to, grandly, as a ‘law’ because scientists at the time were so confident that it was the last word on the subject that they elevated its status above that of a mere ‘theory’. The name stuck, despite the fact that we now know their confidence was misplaced. Einstein’s general theory (note that it’s called a theory) of relativity replaced Newton’s law, because it gives us a deeper and more accurate explanation of gravity. And yet, we still use Newton’s equations to calculate the flight trajectories of space missions. The predictions of Newtonian mechanics may not be as accurate as those of Einstein’s relativity, but they are still good enough for nearly all everyday purposes.

Another example that we are still working on is the Standard Model of particle physics. This is an amalgamation of two separate mathematical theories, called electroweak theory and quantum chromodynamics, which together describe the properties of all the known elementary particles and the forces acting between them. Some physicists think of the Standard Model as nothing more than a stopgap until a more accurate and unified theory is discovered. And yet, it is remarkable that, as it stands now, the Standard Model can tell us everything we need to know about the nature of matter: how and why electrons arrange themselves around atomic nuclei, how atoms interact to form molecules, how those molecules fit together to make up everything around us, how matter interacts with light (and therefore how almost all phenomena can be explained). Just one aspect of it, quantum electrodynamics, underpins all of chemistry at the deepest level.

But the Standard Model cannot be the final word on the nature of matter, because it doesn’t include gravity and it doesn’t explain dark matter or dark energy, which between them make up most of the stuff of the universe. Answering some questions naturally leads to others, and physicists continue their search for physics ‘beyond the Standard Model’ in an attempt to address these lingering but crucial unknowns.

HOW WE PROGRESS

More than any other scientific discipline, physics progresses via the continual interplay between theory and experiment. Theories only survive the test of time as long as their predictions continue to be verified by experiments. A good theory is one that makes new predictions that can be tested in the lab, but if those experimental results conflict with the theory, then it has to be modified, or even discarded. Conversely, laboratory experiments can point to unexplained phenomena that require new theoretical developments. In no other science do we see such a beautiful partnership. Theorems in pure mathematics are proven with logic, deduction, and the use of axiomatic truths. They do not require validation in the real world. In contrast, geology, ethology or behavioural psychology are mostly observational sciences in which advances in our understanding are made through the painstaking collection of data from the natural world, or via carefully designed laboratory tests. But physics can only progress when theory and experiment work hand in hand, each pulling the other up and pointing to the next foothold up the cliffside.

Shining a light on the unknown is another good metaphor for how physicists develop their theories and models, and how they design their experiments to test some aspect of how the world works. When it comes to looking for new ideas in physics, there are, very broadly, two kinds of researchers. Imagine you’re walking home on a dark, moonless night when you realise that there’s a hole in your coat pocket through which your keys must have fallen at some point along your route. You know they have to be somewhere on the ground along the stretch of pavement you’ve just walked, so you retrace your steps. But do you only search the patches bathed in light beneath lampposts? After all, while these areas cover only a fraction of the pavement, at least you will see your keys if they are there. Or do you grope around in the dark stretches in between the pools of lamplight? Your keys may be more likely to be here, but they will also be more difficult to find.

Similarly, there are lamppost physicists and searchers in the dark. The former play it safe and develop theories that can be tested against experiment—they look where they can see. This means they tend to be less ambitious in coming up with original ideas, but they achieve a higher success rate in advancing our knowledge, albeit incrementally: evolution, not revolution. In contrast, the searchers in the dark are those who come up with highly original and speculative ideas that are not so easy to test. Their chances of success are lower, but the payoff can be greater if they are right, and their discoveries can lead to paradigm shifts in our understanding. This distinction is far more prevalent in physics than in other sciences.

I have sympathy for those who get frustrated by the searchers and the dreamers, who often work in esoteric areas like cosmology and string theory, for these are the people who think nothing of adding a few new dimensions here or there if it makes their maths prettier, or to hypothesise an infinity of parallel universes if it reduces the strangeness in ours. But there have been some famous examples of searchers who have struck gold. The twentieth-century genius Paul Dirac was a man driven by the beauty of his equations, which led him to postulate the existence of antimatter several years before it was discovered in 1932. Then there’s Murray Gell-Mann and George Zweig, who in the mid-1960s independently predicted the existence of quarks when there was no experimental evidence to suggest such particles existed. Peter Higgs had to wait half a century for his boson to be discovered and the theory that bears his name to be confirmed. Even the quantum pioneer Erwin Schrödinger came up with his eponymous equation with nothing more than inspired guesswork. He picked the right mathematical form of equation even though he didn’t yet know what its solution meant.

What unique talents did all these physicists have? Was it intuition? Was it a sixth sense that allowed them to sniff out nature’s secrets? Possibly. The Nobel Prize winner Steven Weinberg believes it is the aesthetic beauty in the mathematics that has guided great theoreticians like Paul Dirac and the great nineteenth-century Scottish physicist James Clerk Maxwell.

But it is also true that none of these physicists worked in isolation, and their ideas still had to be consistent with all established facts and experimental observations.

THE SEARCH FOR SIMPLICITY

The true beauty of physics, for me, is found not only in abstract equations or in surprising experimental results, but in the deep underlying principles that govern the way the world is. This is a beauty that is no less awe-inspiring than a breathtaking sunset or a great work of art such as a Leonardo da Vinci painting or Mozart sonata. It is a beauty that lies not in the surprising profundity of the laws of nature, but in the deceptively simple underlying explanations (where we have them) for where those laws come from.³

A perfect example of the search for simplicity is science’s long and continuing journey to discover the basic building blocks of matter. Take a look around you. Consider the sheer range of materials that make up our everyday world: concrete, glass, metals, plastics, wood, fabrics, foodstuffs, paper, chemicals, plan...