Explainer: making waves in science

We see them at the beach. They’re behind every sound and light show and the miracle of Wi-Fi. And now, thanks to what’s being called the discovery of the century, they have opened a way of detecting distant black-hole collisions.

I’m talking, of course, about waves.

We wouldn’t have speech or ultrasound imaging without sound waves. Water waves are a surfer’s paradise. Electromagnetic waves make both vision and television possible, as well as Wi-Fi, chest X-rays and microwave ovens.

It is electrical waves, not electrons, that sweep down our wires and power lines at close to the speed of light (the actual electrons drift along behind, at less than a snail’s pace!).

And the recent discovery of gravitational waves will open up a new frontier in astronomy.

What’s in a wave?

Waves are very different from particles. Waves have energy, but not mass. They love to diffract or spread out, not stay in fixed lumps.

When two waves meet they don’t bounce off each other: they just add and subtract as they pass through each other, and then carry on their ways as if they’d never met. This is called interference, and it makes waves highly unsuitable for snooker, but it is what lets many people use their mobile phones at the same time.

Water is a good example for thinking about the difference between waves and particles. Water can carry energy in two different ways.

First, it can flow from one place to another, such as from the river to the sea. In such a flow, each water molecule starts upstream and moves downstream. The flow is made up of particles.

The ripples are waves in the water. Flickr/Scott Cresswell, CC BY

But imagine the ripples spreading out from a dropped pebble in a pond, or watching the waves spread out from the bow of a passing boat. These waves also carry energy as, for example, they rock floating sticks and even push them along a little.

But the water molecules that make up the shape of the ripple just after the pebble is dropped are completely different to the ones that make up the ever-spreading ripple five seconds later. Each water molecule stays roughly where it is, barring some jiggling, while the wave moves on. So water waves are not a flow of particles.

So how do waves move?

When that pebble is dropped in the pond, it pushes water out of the way. The water has nowhere to go but to the side and up, creating a circular peak around the drop point. This peak falls again, under the forces of gravity and surface tension, pushing the water beneath it out of the way.

On the inside of the circle, this newly pushed water fills the hole left by the pebble passing through. But on the outside, it creates a new circular peak, just a little further out.

So a ripple spreads out from the drop point even though the individual water molecules are mostly just moving up and down in place.

More generally, waves need something to wave in: a medium. Water, air, power lines and the electromagnetic field are all suitable media. Even spacetime itself will do, in the case of gravitational waves.

Waves are simply distortions moving through the medium. These distortions can be started off by many means: a dropped pebble, a shout, a radio transmitter or colliding black holes.

In each case, the medium has some degree of elasticity and responds to a distortion by trying to snap back into shape. But this distorts the neighbouring region, and so on, and so a wave is born.

The strength of the distortions is called the amplitude of the wave, and is closely related to its energy.

Catching the perfect wave

All waves, whether in water, air or spacetime, can come either in pulses, such as a sharp sound, or as a collection of ripples, such as at the beach. But no matter what shape and size, any wave can be thought of as made up of many perfect waves added together.

A perfect wave is what we hear when a singer holds a single beautiful note. It is a smooth series of peaks and troughs in the strength of the wave, with successive peaks all separated by the same distance: the wavelength. The number of peaks passing a given point every second is called the frequency.

Every wave is a combination of interfering perfect waves, and so has a spectrum of different frequencies. Visible light waves, for example, have a spectrum of colours, with each colour corresponding to a different frequency.

They can actually be separated out into their spectrum by a prism, as Isaac Newton famously showed to develop his theory of colours.

A prism reveals the many colours of visible light. Flickr/final gather, CC BY-ND

Different radio and television stations transmit their signals on waves made up of different frequency bands, so that we can tune into the frequency we want.

The distortions of perfect waves, at any given point in the medium, fluctuate up and down in strength either along the same direction the wave is moving (longitudinal waves), or at right angles (transverse waves). These choices depend on the medium, and are called polarisations.

Longitudinal and transverse waves

Sound waves in air are longitudinally polarised, light and gravitational waves are transversely polarised, while the seismic waves causing earthquakes come in both varieties. As do slinky waves!

Perfect transverse waves have a further choice of the different directions at right angles to the direction the wave is moving in. Polaroid sunglasses take advantage of this, blocking the glare that comes from horizontal fluctuations, while letting through vertically polarised waves.

Measuring waves

Measuring waves is important in many parts of science, whether it gives us information about the source of the waves, or about the medium that they have travelled through.

For example, light waves emitted from the sun give us information about its temperature and composition, while light waves passing through a microscope slide can tell us whether someone needs medical treatment or not.

In all cases, the wave must be detected by some means, such as an eye or a camera.

Significant progress in science is made every time we learn how to generate or control or detect a new type of wave. Electromagnetic waves were only discovered 150 years ago and look at the use we make of them now, as mentioned before: radio, television and microwaves, to name just a few.

Gravitational wave detection is the most recent example, providing a unique window on those events strong enough to shake space and time themselves.

A quantum twist

At atomic scales and smaller, the distinction between waves and particles becomes somewhat blurred.

Sufficiently chilled-out atoms can start behaving as if they are spread out and overlapping each other, rather like waves. And if the intensity of a light beam is dialled down enough, it is found to only illuminate a single camera pixel at a time, as if the beam was made up of particles.

Quantum mechanics tells us that waves and particles are fundamentally two sides of the same coin: different kinds of distortions in a medium. But the nature of the quantum medium is a profound mystery that drives the research of many scientists around the world (including my own).

It is only with its solution that we will finally understand just what waves are.