In February 2016 the news went around the world that gravitational waves had finally been detected. But what are these waves that are sometimes described as “ripples in spacetime”? In order to understand this we'll need to go back over a century to a time when Albert Einstein was completing his work on relativity.

Everyday experience
Before we get onto the hard bits, let's review our everyday experience of what is Isaac Newton's Universe.

1. There's space and there's matter. Celestial bodies – and everything else – are made of matter and therefore have mass. Mass is a measure of how much matter there is. On the other hand, space is nothing more than the stage on which matter acts.


2. In the background time flows and we go with the flow. We notice its passing by the way things continually change. We can measure time, but it flows independently of matter, space or our measurements.


3. Everyone experiences gravity. Maybe you remember learning in school that it's a force of attraction between masses, with the biggest mass having the most pull. Newton also said that the force acted instantaneously, though neither he nor anyone else was really comfortable with that idea.

Nonetheless at cosmic scales with great concentrations of matter, things look different. That's where we need Einstein's help.

Einstein's Universe
The Universe that emerged from Einstein's relativity seemed pretty weird a century ago, and it still does. But predictions from his work have been tested and they've passed the tests.

In Einstein's Universe space and time are not independent of one other. They're bound together in spacetime. Although our everyday experience is a 3-dimensional Universe with time on the side, 4-dimensional spacetime has three spatial dimensions and one time dimension. This 4D spacetime is not a nothingness that surrounds matter. Spacetime and matter affect each other.

Try to imagine spacetime as the fabric of the Universe which is curved and twisted by matter. We as humans aren't massive enough to bend spacetime, but movement near massive bodies like the Sun is not in straight lines. It's along the contours of spacetime created by these bodies. The way the Sun shapes the space around it determines how other Solar System bodies move.

Even light doesn't go in a straight line when it passes the Sun. It bends, as predicted in Einstein's Theory of General Relativity. During a total solar eclipse in 1919, the bending of light was demonstrated by English astrophysicist Arthur Eddington, and other observations have confirmed it. (Follow the link below this article to “Einstein's Eclipse” to find out more.)

Einstein's theory does away with any need for a force that acts instantaneously over the vastness of space. Bodies move through space based on the geometry of spacetime as created by the masses within it. Theoretical physicist John Archibald Wheeler famously summarized it as, “Spacetime tells matter how to move; matter tells spacetime how to curve.”

Since time and space are bound together, time is also affected by mass. Clocks run more quickly at high altitudes, because they're farther away from the Earth's center of mass, and gravity is slightly weaker. Unless a clock measures time to tiny fractions of a second, this isn't a problem. But as it happens, GPS satellites do have to be accurate to 20-30 nanoseconds. (A nanosecond is one-billionth of a second.) This means their clocks are adjusted to account for relativistic effects.

What are gravitational waves?
We've seen that spacetime is the 4-dimensional fusion of space and time that Einstein described. But what can cause ripples in it?

A body's gravitational field is the spacetime that's curved around it. But the geometry isn't fixed. Another mass entering the gravitational field is accelerated. This changes the geometry, and gravitational radiation is given out. The radiation is carried away by gravitational waves. Einstein thought of these waves as analogous to electromagnetic waves carrying light energy.

Gravitational waves travel at light speed, carrying gravitational energy. Yet unlike light, they don't travel through spacetime. They ripple spacetime itself, making it expand and contract as they pass, a bit like ripples on a pond.

You might wonder if since we can see light and water waves, could we also see gravitational waves? The simple answer is: No. But if we could see a gravitational wave, it would look something like this simplified animation by Markus Pössel. You're looking at the wave coming out of the screen towards you. Pössel says that he had to exaggerate the distance changes to make them visible. In an actual wave the distance “between the Earth and the Sun would only change by a fraction of the diameter of a hydrogen atom.”

Einstein was amazingly good at getting things right, but he was wrong about one aspect of gravitational waves. He didn't believe they'd ever be detected because they have so little effect on the environment. However not only do we have new technology today, but really massive objects like black holes and neutron stars weren't part of people's understanding of astronomy in 1916.

Watch for a follow-up article showing how gravitational waves have been detected.

References:
(1) Richard W. Pogge, “Real-World Relativity: The GPS Navigation System” https://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html
(2) Markus Pössel, “Gravitational Wave Detectors: How They Work” https://www.universetoday.com/127286/gravitational-wave-detectors-how-they-work/