Gravity Waves and Gravitational Waves are Not the Same Thing

As I write this, the physics world is buzzing with the news that the LIGO team have finally detected one of the most elusive phenomena in the Universe – gravitational waves.  This is the first time that gravitational waves have been directly detected – we’ve known their existence indirectly, through observations of binary pulsars made in the 1970s, which won the Nobel Prize in 1993.

A very common mistake (in both academic and media circles) is to confuse gravity waves and gravitational waves, like this Wired article’s title, or this from the Wall Street Journal (even Google displays results for gravitational waves when you type gravity waves). I’m here to tell you they’re not the same thing.

What are gravitational waves?

Gravitational Waves are predicted by Einstein’s General Theory Of Relativity. General Relativity (GR to those in the know) describes gravity using the curvature of space-time.

Matter sits in space-time, and gives it curvature, in the same way that a medicine ball sitting on a trampoline bends the trampoline fabric. If we take a golf ball, and roll it near the medicine ball, the golf ball doesn’t go in a straight line.  Instead, its path is curved because the trampoline fabric is curved.

Curvature of space-time, from Britannica Online for Kids, retrieved 10 Feb 2016 from http://kids.britannica.com/comptons/art-156228

This behaviour is quite analogous to how bodies move under gravity when they approach massive bodies, so we can predict how bodies will move if we know how the local space-time is curved.

So where do gravitational waves come in? Imagine the trampoline surface rippling (in the same way a pond surface ripples when you throw a stone). Those ripples in the space-time fabric are what we call gravitational waves.

Almost any time an object moves in space-time, some of its energy goes into making gravitational waves (the Earth is making them right now as it orbits the Sun). When these waves travel through a set of particles, the set will squeeze and stretch due to the wave (precisely how they do this depends on the wave’s polarisation).

But these waves are very, very, very weak. The detected gravitational waves makes a relative change in position of around one in a quintillion.  The LIGO team measure this by measuring the position of a mirror 4 kilometres away to the precision of a thousandth of a proton’s radius.  This is like measuring the distance to the nearest star with an accuracy of a hair’s width.  Here’s a great blog post from cosmologist Matt Francis about his visit to one of the LIGO installations.

Everyone is comparing this achievement to Galileo’s accomplishments with the telescope, and the analogy is very apt.  Galileo was one of the first to use light to its fullest in studying the Universe.  Since then, we’ve expanded our use of light all across the spectrum, from the gamma and X-Rays at one side, through visible light to radio waves at the other side.

Gravitational waves are a completely different entity, and exist on a completely new spectrum.  It’s very much like learning to see, and then learning to hear – the Universe looks very different when you can do both!

What are gravity waves?

Gravity waves have nothing to do with GR, and are predicted by classical physics. They aren’t weak and difficult to detect – in fact, you probably see them all the time.

Take two fluids of different density – for example, a tub of water with a layer of air above it. When the tub is at rest, the water’s surface is still and doesn’t move relative to the air. If something should disturb the water (say a gust of wind), then the water’s surface can be displaced up or down. Gravity (and/or the water’s buoyancy) will attempt to restore the water’s surface to its original smooth state. In doing so, the water makes waves, and these are a type of gravity wave.  So ripples in a pond are in fact gravity waves!

Why are gravity waves not gravitational waves?

I hope that the answer to this question is obvious. Gravity waves are ripples in fluids (or at the boundary between two fluids) that are often very large and obvious.

Gravitational waves are ripples in space-time (I tend to think of them simply as space-time waves) that are extremely small, and very difficult to detect.

It’s very clear they’re not the same thing. A Nobel Prize for Physics isn’t going to be awarded to the discoverers of gravity waves, as anyone near the sea or a trembling cup of tea can do that.

However, it’s clear that the discoverers of gravitational waves are in for a whole lot of accolades from the scientific community, and the world at large.