The Good News For General Relativity Keeps On Coming

Not too long ago, I wrote about how alternate theories to general relativity have been having a tough time as of late. Well, things just got even tougher.

The equivalence principle was on trial for this experiment. One of the basic tenets of relativity is that two objects, no matter what their mass or what they're made of, they are affected by gravity in the same way. This has been tested many times on Earth (and famously, on the Moon), but never with really dense objects. Alternative theories to relativity assume that the equivalence principle breaks down at high density, since up to now, there's been room to work.

The test involved  a neutron star-white dwarf pair, and watching the orbit of the neutron star. If there were variations in its orbit, it would have been in violation of the equivalence principle, and the various alternate theories would have some ground to stand on. But there was no variation, and once again, general relativity was proven correct. And not only that, but this test improved the accuracy of the previous best gravity test by a factor of 10. Alternate gravity theories thus have a lot less room to work.

Alternate Theories to Relativity Have Not Had a Good Time Lately

It's a conundrum at the very heart of theoretical physics. We know quantum mechanics to be correct. It's how nature works at very small scales. We know that the theory of general relativity is completely incompatible with quantum mechanics. The two just don't go together. Therefore general relativity must be wrong, or at minimum, incomplete. Yet general relativity has been proven to work time and time again. But general relativity also means that dark matter and dark energy must be a thing. And despite our best efforts, we're no closer to discovering either than when we first started looking for them. Not really. And I think you get the idea at this point. It's a mess.

So, what are physicists to do? If you guessed make wild theories that somehow work in what we know about gravity AND what we know about quantum mechanics, and hope the two go together, then congratulations! You've got what it takes to be a theoretical physicist. Turns out there are lots of alternate theories of gravity out. Well, there were. The discovery of gravitational waves by LIGO killed most of them. Okay, it wasn't just that, it was that and the simultaneous observation of a gamma-ray burst from the same neutron star collision.

I think the biggest takeaway from this story is that science is always in motion. General relativity's been around for a century now, and it's been observationally validated many, many times. But it isn't perfect. And so, rather than shrug our shoulders and say "close enough", we keep going, and we try to find something that does work. There is always more science to do, always more questions to answer.