The World According to Physics

by Jim Al-Khalili

physicists cannot yet even agree on whether the job of physics is to figure out how the world really is, as Einstein believed—to reach some ultimate truth that is waiting out there to be discovered—or whether it is to build models of the world and to come up with our best current stab at what we can say about reality, a reality that we may never truly know.

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.

knowledge and enlightenment are always better than ignorance.

‘I’d take the awe of understanding over the awe of ignorance any day.’

As we understand physics today, all the matter we see in the world is made up of not the four classical elements of the Greeks, but just three elementary particles: the ‘up’ quark, the ‘down’ quark, and the electron. That’s it. Everything else is just detail.

three ‘pillars’ of modern physics: relativity, quantum mechanics, and thermodynamics.

Unlike philosophy, logic, or pure mathematics, physics is both an empirical and a quantitative science.

The world of physics only really came of age in the seventeenth century, thanks to a large extent to the invention of the two most important instruments in all of science: the telescope and the microscope.

Today, the range of scales open to exploration by humankind is astounding. With electron microscopes we can see individual atoms, just a tenth of a millionth of a millimetre across, and with giant telescopes we can gaze out to the furthest reaches of the observable universe 46.5 billion light-years away.

When physicists say that a physical system has a symmetry, they mean that some property of that system stays the same when something else changes.

Noether’s theorem tells us that we don’t ‘invent’ the mathematics in order to have a way of describing the world, but rather, as Galileo observed, that nature speaks the language of mathematics, which is ‘there’, ready and waiting to be discovered.

so-called ‘classical’ world of matter, energy, space, and time simply don’t work when we shrink down to the world of individual atoms, where the very different rules of quantum mechanics come into play.

Even at the quantum level, we often need to choose the appropriate model that is most applicable to the system we wish to study. We’ve known since the early 1930s, for example, that the atomic nucleus is made up of protons and neutrons; but in the late 1960s, it was discovered that these particles are not elementary, and are in fact made up of even tinier, more fundamental constituents: the quarks.

While two of the wonderful things about physics are the universality of many of its theories and the way we can understand more about a system by digging deeper and understanding how its parts relate to the whole, it is also true that we often have to choose the most appropriate theory depending on the scale we are interested in. If you want to fix your washing machine, you do not need to understand the intricacies of the Standard Model of particle physics—even though washing machines, like everything else in the world, are ultimately made up of quarks and electrons.

Learning not to always trust our senses is a valuable skill that physicists have inherited from the philosophers. As far back as 1641, René Descartes argued in his Meditations on First Philosophy that in order to know things about the material world that were absolutely true, he first needed to doubt everything, often despite what his senses were telling him. This doesn’t mean that we cannot believe anything we are told or shown, but that, according to Descartes, those material things he judges to be true ‘demand a mind wholly free of prejudices, and one which can be easily detached from the affairs of the senses’.

Individual perspectives of space and time separately are relative, but combined spacetime is absolute.

According to his general theory, matter and energy create a gravitational field, and spacetime is nothing more than the ‘structural quality’ of this field. Without the ‘stuff’ contained within spacetime, there is no gravitational field and hence no space or time!

Galaxies are like the raisins embedded within a loaf of rising bread in the oven. The loaf expands, but the raisins themselves remain the same size—they just become more separated from each other.

mass can be thought of as frozen energy. And because the speed of light squared is such a large number, a small amount of mass can be converted into a large amount of energy, or conversely, a large amount of energy freezes into very little mass.

On the microscopic scale, materials are held together by the electromagnetic forces between atoms. On the cosmic scale, it is gravity that holds matter together.

all atoms are, in turn, made up of just two kinds of particles: quarks and leptons. Indeed, all atomic matter consists of just the first two quark flavours (the up and down), plus one of the leptons (the electron). Although you may be surprised to know that the most common matter particle is the neutrino.

This does not mean that an electron is both a particle and a wave at the same time—but rather that, if we set up an experiment to test the particle-like nature of electrons, we find that they do indeed behave like particles. But if we then set up another experiment to test if electrons have wavelike properties (such as diffraction or refraction or wave interference), we see them behaving like waves. It’s just that we cannot carry out an experiment that would show both the wave and particle nature of electrons at the same time. It is absolutely vital to stress here that, while quantum mechanics correctly predicts the outcomes of such experiments, what it does not tell us is what an electron is—only what we see when we carry out certain experiments to probe it.

The uncertainty principle puts a limit on what we can measure and observe, but many people, even physicists, are prone to misunderstanding what this means. Despite what you will find in physics textbooks, the formalism of quantum mechanics does not state that an electron cannot have a definite position and a definite speed at the same time, only that we cannot know both quantities at the same time.

the rules of quantum mechanics dictate how atoms can bind together to make molecules, making quantum mechanics the foundation of the whole of chemistry.

Electrons are able to jump between energy states by emitting or absorbing the correct amount of energy. They can drop to a lower state by emitting a quantum of electromagnetic energy (a photon) of exactly the same value as the difference in energies between the two states involved. Likewise, they can jump to a higher state by absorbing a photon of the appropriate energy.

The wave function can describe the state of a single particle or group of particles and has a value that provides us with the probability of, say, finding an electron with any given set of properties or location in space if we were to measure that property.

wave function has value at more than one point in space is often wrongly taken to mean that the electron itself is physically smeared out across space when we are not measuring it. But quantum mechanics does not tell us what the electron is doing when we are not looking—only what we should expect to see when we do look.

We do not even know if there is any more to ‘get’. Physicists have tended to use terms like ‘strange’, ‘weird’, or ‘counterintuitive’ to describe the quantum world. For, despite the theory being powerfully accurate and mathematically logical, its numbers, symbols and predictive power are a façade hiding a reality we find difficult to reconcile with our mundane, commonsense view of the everyday world.

interpreting a theory in a particular way ‘just because it works’ is intellectual laziness, and certainly not in the true spirit of what physics should be about.

we now know empirically that quantum particles really can have an instantaneous long-range connection. Our universe really is nonlocal.

The entropy of a system, if left alone, will always increase: that is, a system will always relax from a ‘special’ (ordered) state to a less special (mixed-up) one. Physical systems unwind, cool down, and wear out. This is referred to as the second law of thermodynamics, and at its heart it is no more than a statement of statistical inevitability: if left alone, everything always eventually returns to a state of equilibrium.

There is also a fascinating flip side to chaos theory: that simple rules, applied repeatedly, can lead to seemingly random behaviour, but then sometimes go on to produce beautiful structures and complex patterns of behaviour that look highly ordered. Unexpected complexity emerges where there was none before, while never violating the second law of thermodynamics. The field of science dealing with this sort of emergent behaviour is known as complex systems, and it is beginning to play a major role in many exciting areas of research, such as biology, economics, and artificial intelligence.

Firstly, according to special relativity, time is not absolute; it does not tick by independently of events taking place in three-dimensional space but must instead be combined with space into four-dimensional spacetime. This is not just a mathematical trick. It is forced upon us by the properties of the real world, tested again and again in experiments and shown to be just the way the universe is.

A neat experiment at the University of Queensland in Australia in 2018 showed just how puzzling this all is by demonstrating that at the quantum level, events occur with no definite causal order. Basically, in physics, causality means that if an event A takes place before an event B (in some frame of reference) then A may or may not have influenced or even caused B. But the later event B could not have influenced or caused event A. At the quantum level, this sensible causality was shown to break down. This has led some physicists to argue that the arrow of time really does not exist at the quantum level but is only an emergent property when we zoom out to the macroscale.

Almost all phenomena we see in nature are due ultimately to one or other of these two forces: gravity and electromagnetism.

This unification means that, at a fundamental level, the four forces of nature are reduced to just three: the electroweak force, the strong nuclear force, and gravity (which in any case is not actually a force at all, according to general relativity).

physics is about gaining the deepest and most complete understanding of reality.

We know that spacetime reacts to the matter within it. We also know that matter at the subatomic scale behaves according to the rules of quantum mechanics, which must surely in turn affect the behaviour of spacetime.

We call it ‘dark,’ not because it is hidden behind other, visible matter, or even because it is actually dark, but because, as far as we can tell, it doesn’t feel the electromagnetic force and so does not give off light or interact with normal matter, other than gravitationally,1 and so a better name for it would have been invisible matter.

The simplest answer is that there was no ‘before’ the Big Bang, for it marked the birth of both space and time. An idea put forward by Stephen Hawking and James Hartle, called the ‘no boundary’ proposal, states that, as we wind back the clock closer and closer to the Big Bang, time begins to lose its meaning and becomes more like a dimension of space. We therefore end up with smooth four-dimensional space at a point of the universe’s origin. So it is meaningless to ask what happened before the Big Bang, in the same way that it is meaningless to ask what point on the surface of the Earth lies south of the South Pole.

In classical computing, information is stored and processed in the form of ‘bits’ (which stands for binary digits). A single bit of information can have one of two values: zero or one. Combinations of electronic switches, each one a physical manifestation of a bit of information that is either on or off, are used to make logic gates, the building blocks that make up a logic circuit. In contrast, quantum computers operate on what are called quantum bits, or ‘qubits’, which are not restricted to holding just one or the other of these binary states. Instead, a qubit can exist in quantum superposition of both zero and one simultaneously, and, as such, can store much more information.