Einstein's general theory of relativity is one of the cornerstones of modern physics, and one of the cornerstones of general relativity is that the speed of light is a constant. In fact, the constant speed of light in a vacuum is so essential to our understanding of the cosmos that if we were to discover that it wasn't constant, it would mean that the laws of physics don't have to be the same everywhere. It would mean a complete shakeup of all that we know.

Now a new theory aims to do precisely that: shake it all up. Two physicists, João Magueijo of Imperial College London and Niayesh Afshordi of the University of Waterloo in Canada, have proposed what might seem inconceivable to some. They have suggested that the speed of light is not constant after all, and their theory — wild as it might sound — will soon be tested, reports The Guardian.

If Magueijo and Afshordi are correct, it would mean that there was once a time when the speed of light was much, much faster. Light would have been so fast, in fact, that its speed would have been infinite. Try to wrap your mind around that.

For all the evidence supporting the idea that the speed of light is constant, it doesn't jive perfectly with some of our other observations of the universe. For instance, the universe as it appears to us today is relatively uniform; it looks much the same over vast distances. That uniformity implies that light rays must have reached every corner of the cosmos, otherwise some regions would be cooler and more dense than others. The only problem? At the speed of light as we measure it today, light couldn't have reached everywhere.

To get around this problem, scientists have proposed something called inflation theory, which is the idea that the universe underwent a brief, early period of tremendous expansion. Inflation makes the math consistent with the constant speed of light, but there's not actually any real evidence to suggest that inflation happened.

Enter Magueijo and Afshordi. They have shown that if the heat of the early universe was intense enough, that light and other particles could have moved at infinite speed. This can explain the uniformity seen in our observations of the distant cosmos, but it would mean that the speed of light is variable.

“In our theory, if you go back to the early universe, there’s a temperature when everything becomes faster. The speed of light goes to infinity and propagates much faster than gravity,” explained Afshordi. “It’s a phase transition in the same way that water turns into steam.”

So who is correct? Magueijo and Afshordi, or Einstein and inflation theory? Einstein's theory tends to hold more credibility among scientists because of how extensively it has been tested, but both camps have models that explain our observations.

Soon, though, there might be a test. Magueijo and Afshordi's theory makes a prediction about the density variations of the early universe, and these density variations can be measured by something called the “spectral index." If there was ever a time when the speed of light was infinite, this should have left its mark on the spectral index of the cosmic microwave background leftover from the Big Bang. In fact, according to Magueijo and Afshordi, it should have left a very precise mark of 0.96478.

So if that's the precise measurement that scientists actually find, it will provide powerful confirmation for the new theory.

Physics as we know it could be at stake. And that ever-comforting belief — that even in a mutable cosmos, there's still the speed of light, reliable and constant — could become the first domino to fall.