**One hundred years ago Albert Einstein **wrote a new theory of gravity. General relativity is an elegant but mathematically complex explanation of the nature of spacetime. It has, so far, passed every test, to extraordinary levels of precision. Its principles allow us to describe the evolution of the Universe as well as find our way with GPS. Yet each generation of physicists finds new ways to test it. And for every test, someone writes the headline: “Was Einstein wrong?”

It’s a terrible question.

We know general relativity can’t be the ultimate theory, because gravity and quantum mechanics do not play well together. The other forces of nature (electromagnetism and the nuclear forces) all fit quite well into the Standard Model of particle physics. But gravity isn’t part of that and we don’t know how to tie it in, although string theory might help. Extreme systems such as the centres of black holes and the first moments of the Universe might also require more explanation than general relativity can provide.

So probably, someday, a theory will be touted as a replacement for general relativity – one that fixes all the problems left lying at the edges. But that won’t mean Einstein was wrong; in the same way that Newton was not proved wrong by Einstein.

In such cases the “rightness or wrongness” of a theory is an impractical concept. We physicists approach a theory in the way a carpenter approaches the selection of a tool. “How well does it work, and when can I use it?”

Most of the time we’re not searching for the ultimate nature of reality (although, of course, we hope we get closer to that all the time). We’re trying to build a model that we can work with to learn more about the natural world and to better measure, study and manipulate it.

It doesn’t matter whether Einstein was right – it matters whether his theory is useful. You only need to look at your GPS device to see that it is.

When I size up a theory I want to know if it is logically and mathematically consistent. Does it make predictions? Are those predictions borne out by observations? Can I use the theory to make new predictions? And, crucially, how far can I push the theory before it stops making sense?

If usefulness is the metric, Newton was also correct. Newton’s theory of gravity was simpler than Einstein’s – it told us that massive objects attract each other proportionally to their masses and in inverse proportion to the squares of their distances. Using Newtonian gravity, you can easily calculate how quickly an apple falls, or how long the Moon takes to orbit the Earth.

In principle, each of these calculations could be done with Einstein’s general relativity, but they would be hugely complicated and the result would be so close to the Newtonian solution that it’s unlikely you’d be able to measure the difference.

The situations in which Newtonian gravity breaks down only became apparent to us long after Newton’s death. These are the extreme cases, important for when you get too close to a black hole, or approach the speed of light, or measure the bending of light around the Sun.

As measurements have become more precise – such as correcting for how the tiny curvature of spacetime changes GPS satellite orbits – we have needed to go beyond what Newton’s theory could calculate. Was Newton wrong for not predicting phenomena he could never have hoped to observe, and that had no impact on the measurements he could make?

As physicists we work at the frontiers of knowledge. New theories will be needed to go beyond these frontiers. If we find ourselves probing the gravitational effects of single subatomic particles, for example, maybe we will find that general relativity’s incompatibility with the quantum world makes it suddenly impractical. Then it will be necessary to find a new, more useful, theory.

As we study more about the earliest moments of the Universe, or the future evolution of the cosmos, perhaps we’ll find new places where Einstein’s theory was never equipped to go. But for now, it doesn’t matter whether Einstein was right – it matters whether his theory is useful. You only need to look at your GPS device to see that it is.

### Katie Mack

Katie Mack is an astrophysicist at North Carolina State University.

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