The final force to introduce is gravity. We rarely think about the fact that we can walk around on Earth without floating off into space. Gravity is responsible for anchoring us to our planet. It is a force that attracts objects towards one another, and it means that things with mass, including atoms, have a tendency to clump together. Gravity impacts large objects as well as small ones. It keeps the moon orbiting the Earth, the Earth orbiting the sun, and the sun orbiting the centre of our galaxy. You will know that it takes one year for the Earth to go around the sun. It takes 27 days for the moon to complete an orbit of Earth, and about 250 million years for our sun and solar system to journey around the centre of the Milky Way. We are on a rock orbiting a star that is orbiting the centre of our galaxy. Gravity is the force that keeps this dance going and, without gravity, life would not exist, as there would be no sun, no Earth and no moon.
These four forces are all central to life in that they determine how our universe behaves. Understanding the forces is consequently an important step in the narrative of why we exist. We are made out of particles that have combined to make atoms and molecules that in turn have combined to form you and me. Our next step is to investigate how the four fundamental forces interact with, and impact, matter.
Particles can combine and interact in multiple different ways. The rules of how they interact are constant across the universe, but the outcome of the interactions depends upon the way that matter is spread out across space. Part of the reason for this is that some forces operate across very short distances, while others operate over much larger ones. For example, the strong nuclear force operates at the smallest scale, followed by the weak nuclear force, with electromagnetism and gravity working up to very large scales.
The way that the strong and weak nuclear forces and electromagnetism work is very similar, with quarks and electrons continually exchanging things called force carrier particles with one another. The force carrier particles for the strong and weak nuclear forces are respectively referred to as gluons and bosons, while the force carrier particle for the electromagnetic force is called the photon. Scientists suspect gravity works via a force carrier particle too, but they have been unable to detect it. They call the hypothetical particle the graviton. Detecting gravitons would require more energy than is available to us on Earth. Some estimates suggest that we would need to build a machine like the LHC that was the size of our solar system, that we would need to position it next to a very dense star, and that even then we would observe only one graviton every decade or so. If they exist, gravitons are elusive. Fortunately, force carrier particles that determine the other forces are easier to detect, and they have all been observed. Each force carrier particle is associated with a force field, with each force having its own field.
These force fields permeate the universe in every direction and at every point, even in deep space between galaxies. The easiest way to imagine these fields is by considering the force field around a magnet. We can see this magnetic field by sprinkling iron filings on top of a sheet of paper placed above a magnet and gently tapping the paper. The iron filings trace arcs from one pole of the magnet to the other. What you see when you do this is a trace of the electromagnetic force field. The other force fields are not as straightforward to observe, but they are everywhere all the time.
Each force field consists of energy, and there are continuous fluctuations in this energy that result in the force carrier particles appearing and disappearing. They do not exist for long, only a fraction of an instant of a second, before disappearing again. Force carrier particles are sometimes referred to as virtual particles, although they are, in fact, very real. These virtual particles pop in and out of existence more frequently close to matter than further away, but despite this, even in parts of the universe a long way from any matter, energy is morphing into these particles and back again.
Although these virtual particles are small and cannot be broken into smaller components, there may be things happening on an even smaller scale. Some physicists argue for a theory, called string theory, that hypothesizes that there are tiny, vibrating, string-like structures that determine all of matter. The frequency at which each string vibrates determines the type of particle we observe. At the scale of strings, space is thought to look very different to the way we perceive it. We think of space as being smooth, and not being divided up into discrete chunks. As we walk or drive, we do not jump from one location to a neighbouring one, but rather we move through space smoothly. On the extremely tiny scale where vibrating, string-like structures are thought to exist, scientists believe that space loses this continuity and becomes divided up into minuscule volumes, a little like an infinite stack of miniature square boxes. Physicists refer to this scale as the Planck scale, and the Planck length (the side of one of these boxes) is the smallest length that exists. It cannot be subdivided. The idea of half a Planck length makes no sense. At the Planck scale, tiny strings might be vibrating, and this would make space look like bubbling foam if we could observe it at such a minuscule scale. These conclusions are reached through the analysis of mathematical models because we do not have the technology to see space at the Planck scale.
The Planck scale is not the only aspect of the universe that remains elusive to physicists. Our universe consists of matter (including something called antimatter; more on that later), in the form of fundamental particles; energy, in the form of heat and light; something called dark energy, which must exist but that scientists are yet to fully understand; and virtual particles that are continually being produced and destroyed that allow other particles to interact. Each of these building blocks of the universe must be linked, but quite how dark energy links in is currently unclear. Nonetheless, physicists know how matter is related to non-dark energy in the form of heat and light via the most famous equation in science: Einstein’s E = mc 2. The E stands for energy, the m for mass and the c for the speed of light in a vacuum, where a vacuum is space containing no matter. The equation links energy to matter, showing how they are related. The amount of energy tied up in matter is very large, because the speed of light in a vacuum (c) is a big number – 299,792,458 metres per second. The square of this number, 299,792,458 × 299,792,458, is an even bigger number. What this means is that if matter can be coerced into releasing this energy, as happens inside stars and nuclear bombs, a lot can be produced. Conversely, when matter is created from energy, it locks up a lot of energy.
At its birth, 13.77 billion years ago, the universe was a point of hot energy before some of this energy transformed into quarks and electrons that did not then instantly morph back into energy as virtual particles do. Instead, they survived to allow us to exist. They stuck as matter, and scientists are not entirely sure why. What they do know is that these fundamental particles soon started to interact with one another, and when the universe had cooled to a temperature of 1 billion degrees Celsius, the quarks started to join to form the protons and neutrons found in the nuclei of atoms.