The World According to Physics

by Jim Al-Khalili

The basic premise of supersymmetric string (or superstring) theory is that one way to unify all the forces is to add more dimensions to s...

This idea goes back to work by the German theoretical physicist Theodor Kaluza, who noticed, just after the end of the First World War, that if he solved Einstein’s field equations of general relativity in five-dimensional spacetime instead of four, then electromagnetism emerg...

Kaluza showed his work to Einstein, who initially liked it. It seemed to do for electromagnetism what Einstein had achieved for gravitation: changing its fundamental desc...

Yet, despite this elegant way of unifying light (electromagnetism) and gravity (general relativity), most physicists—including Einstein himself—soon became sceptical of Kaluza’s work, as there was no experimental evid...

A few years after Kaluza’s original idea, the Swedish physicist Oskar Klein suggested that the reason the fifth dimension is hidden is because it is curled up on i...

There is a standard analogy that helps to explain what this means. From a distance, a hose looks like a one-dimensional line, but zoom in and you see that it is in fact a two-dimensional surface wrapped around into a cylinder. The second spatial dimension (the circular direction around the hose) was too small to be seen from a distance. Klein suggested the same thing applied to Kaluza’s fifth s...

Despite Kaluza-Klein theory not leading to a unification of gravity and electromagnetism, it did help researchers understand the relevance of the higher dimensions in superstring theory. However, now, instead of just one hidden spatial dimension, there needed to be six, all rolled up into an impossible-to-visualise six-dimensional ball. Superstring theory thus states th...

To this day, many researchers looking to unify the forces of nature still work on string theory. They argue that we have come so far, using successful ideas like quantum field theory and supersymmetry to understand three of the four forces; th...

String theory begins with the quantum mechanical properties of matter within spacetime. Its central idea is that all elementary point-like particles are in fact tiny...

These strings would be far smaller than the scales currently probed by particle physics and so we can only exp...

The problem that emerged by the 1990s was that it appeared there were five different versions of string theory, and no one knew which one was the correct one. So a new, even grander framework was proposed which unified all five versions under one umbrella. This all-encompassing framework is now called M-t...

Yet again, it seemed, another hidden dimension was needed to help with the gran...

So, is that it? Is M-theory our ultimate ‘theory of everything’? Sadly, we cannot yet say. While the mathematics is elegant and powerful, we still don’t know if string theory or...

Loop quantum gravity does not start from quantum field theory, but from the other direction—from general relativity. It assumes that spacetime itself, rather than the matter it contains, is the more fundamental concept.

Aesthetically, it would seem sensible to try to quantise the gravitational field—which, according to general relativity, is spacetime itself. Thus, if we shrink down to small enough length scales, we should see space become grainy and discrete. In the same way that Max Planck proposed, in 1900, that heat radiation ultimately comes in quantum lumps, quantizing space suggests there should be a smallest length that cannot be further divided.

However, the quanta of gravitational energy are the quanta of space itself, which means that they don’t exist as lumps with...

It is thought that the tiniest unit of space—a quantum of volume—is one Planck length, or 10−35 m, across. I have always enjoyed trying to find ways of describing how tiny this volume is. For example, an atomic nucleus contains as many Planck vo...

This discretisation of space seems inevitable if we want to quantise the gravitational field. And it therefore follows that time must also be ‘lumpy’. So the smooth space and time that we experience is nothing more than a large-scale approximation of the lumpy quanta of gravity, smeared out...

Loop quantum gravity contrasts dramatically with string theory, which predicts that, just as the three forces covered by the Standard Model (electromagnetism and the strong and weak nuclear forces) are in fact quantum fields manifested as force-carrying particles, so too is the gravitational field mediat...

In string theory, this quantum of the gravitational field exists within spacetime, whereas in loop quantum gravity, it is ...

Loop quantum gravity refers to the closed paths that take you from a quantum of space, via its links to adjacent quanta, around in a loop and back to the starting point. The nature of these loops determines the curvature of spacetime. They are not physical entities...

In a sense, loop quantum gravity is modest in its scope. But when you consider it more carefully you begin to realise that, if it is indeed the correct description of reality, then it is not so much that events take place within space and for a duration of time, but rather that the universe, and everything in it—all matter and energy—is nothing more than quantum fields coexisting and superimposed onto each other. And these fields do not require space and time to exist in, since spacetime is itself one of these quantum fields.

In summary, we cannot yet claim to have a genuine theory of everything, nor do we understand yet how to bring quantum mechanics and general relativity together. Rather, we have candidate theories that show some promise, but which still leave many questions unanswered.

Brilliant physicists have built their careers on one or the other such theory, but just as with different interpretations of quantum mechanics, there is a lot of sociology of science involved, and views on which theory s...

Broadly then, in the red corner we have string theory, which is our current best stab at unifying the four forces of nature, but which despite three and a half decades of research is still speculative. Some physicists claim that despite all the progress it has made, it is now reaching something of a crisis because it hasn’t delivered on its early promise. Indeed, it can be argued...

Then, in the blue corner, we have loop quantum gravity, which seems to be the most sensible way of quantising spacetime, but which does not tell us how to then combine gravity with the other three forces. Whether one or the other of these two approaches is correct, or whether we need to somehow...

The rotational speed of galaxies, the motion of entire galaxies within galaxy clusters, as well as the large-scale structure of the entire universe, all point towards a significant component of the universe consisting of a near-invisible matter component.

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.

Think for a moment about why, if you slam your hand down on a solid table, it doesn’t pass straight through. You might regard this as trivial: surely it is because both your hand and the table are made of solid stuff. But don’t forget that down at the level of atoms, matter is mostly empty space—diffuse clouds of electrons surrounding a tiny nucleus—and so there should be plenty of room for the atoms that make up your hand to easily pass through the atoms of the table without any physical matter coming into contact. The reason they don’t is because of the electromagnetic force between the ele...

However, if your hand were made of dark matter, then it would pass straight through as though the table weren’t there—the gravitational force between...

It has long been known that galaxies have much more mass than can be accounted for if one measures all the normal matter they contain in the form of star...

At one point it was thought that dark matter might be made up of long-dead stars and black holes—objects made of normal matter, but which do not emit light. However, overwhelming evidence now suggests that this invisible stuff must be made up of a new form...

Most of the stars—and hence, you would think, most of the mass—in a galaxy are concentrated at its core, which would require those in the outer rim to be moving around the centre more slowly. The observed higher-than-expected orbital speeds of these outer stars suggest that there must be some additional invisible stuff present, extending out beyond the visible matter we can see and providing the extra gravitational glue to stop the outer stars from flying off.

Dark matter can also be seen from the way it curves space around it. This phenomenon manifests itself in the way light bends while on its path from very distant objects to our telescopes. The amount of bending can only be explained by the extra gravitational curvature of space provided by the dark matter of galaxies that the light passes on its way to us.

The existence of dark matter also seems necessary to explain the structure of the early universe.

In contrast with normal matter, which through its interaction with the electromagnetic field kept its energy high, dark matter cooled down more quickly as the universe expanded and therefore started to clump together gravitationally earlier.

One of the most important results in astrophysics in recent years has been the confirmation from sophisticated computer simulations of galaxy formation that we can only explain the real universe if it does indeed contain large quantities of dark matter. Without it, we would not get the rich cosmic structures we see today. Put more bluntly, without dark ma...

This remarkable conclusion is supported beautifully by data showing subtle fluctuations in the temperature of deep space, the imprint of the very young universe on the cosmic microwave background radiation. It was recognised back in the late 1970s that these fluctuations in the cosmic microwave background, while helpful in providing the seeding for the presen...

While we are left in little doubt that dark matter is real, we are still in the dark as to what it is made of. It is a growing source of frustration in astrophysics that, in parallel with the accumulation of evidence in support of dark matter, we have failed to find out what it actually is.

The consensus now is that it consists of a new type of heavy particle (heavy by the standards of elementary particles, that is), and most of the experimental effort thus far has been focused on building sophisticated underground detectors that can capture extremely rare events when such a dark matter particle streaming in from space collides head-on with an atom in the detector. To date, no signal from these increasingly sophisticated and sensitive experiments has been picked up.

And yet, physicists looking for dark matter remain optimistic. Most likely, they say, it will be in the form of slow-moving particles, making...

I should at this point say just a little about neutrinos, which for a while were the leading candidates for dark matter. These are elusive yet abundant particles that we know exist, which have a tiny mass and are almost invisible.

You would need a light-year’s thickness of lead shielding to have even a fifty-fifty chance of blocking them. You could say that they are, to all intents and purposes, ‘dark matter’.

However, they cannot be the dark matter that we are searching for because, being so light, they travel at near light speed—too fast to remain bound within galaxies and thus to explain galaxies’ anomalous properties. We refer...

So, rather than the cumulative gravitational attraction of all the matter in the universe—both normal and dark matter—slowing down the expansion of the universe, something else was at work, making it expand more quickly now than it did in the past.

This mysterious repulsive substance acting against gravity and stretching space ever more quickly became known as dark energy. According to our present understanding, dark energy may ultimately result in what is called the ‘heat death’ of the universe many billions of years from now as space continues to expand ever more rapidly and to cool as it settles towards a state of thermodynamic equilibrium.

But until we truly understand the nature of dark energy, and indeed the properties of the very early universe (see the next section), we should not be too quick to conjecture about its final fate. It’s a long ...

Until a few years ago, I would have said we know even less about dark energy than we do about dark matter, but that is now changing. There is a quantity in Einstein’s equations of general relativity, known as the cosmological constant (an...

We have seen how everything ultimately comes down to quantum fields in the end. All the different particles that make up matter and energy, whether quarks, electrons, photons, or Higgs bosons, can be regarded merely as localised excitations of these quantum fields—like waves on the surface of an ocean.

However, if you were to remove all the particles from a volume of space, this does not get rid of the field. Instead, we say it is left in its ground, or vacuum, state, but there will still be virtual particles popping in and out of existence within this vacuum all the time, borrowing the energy from their surroundings in order to exist, but paying it back just as quickly when they disappear again.