Ask about some mind-bending physics, and people will tend to focus on the many mind-bending oddities of quantum mechanics. But there's no shortage of strangeness in another one of physics' cornerstone theories: relativity. From time being relative to things getting more massive as they accelerate, there are lots of head scratchers in relativity.

But the thing that may top the strangeness scale is an effect called "frame dragging," where a massive rotating object distorts the space-time around it. While it was first identified as a relativistic effect shortly after relativity was proposed, we weren't in any position to test it until the satellite era. While a number of missions have produced results consistent with relativity, the experiments had rather large uncertainties.

Now, an international team of scientists has used an interstellar laboratory to test the proposal. Taking advantage of a large white dwarf with a nearby neutron star, the researchers have detected frame dragging effects in the regular pulses of emission from the neutron star.

Such a drag

The easiest way to understand frame dragging is to imagine sticking something in a pool of water and starting to rotate it. As the speed of the rotation increases, the water will begin to swirl around the object. Something similar happens when a massive object is rotating, except the fluid it's rotating in is space-time itself. The effect is weak under most circumstances—which is why it has been so difficult to get experiments to work in orbit around Earth. But for very massive objects rotating quickly, it creates observable effects. Our first image of a black hole's environment wouldn't have looked like it did if it weren't for frame dragging, although we're still working on relating these observations to the black hole's rotation.

One consequence of this is that light from behind a rapidly rotating object will appear to move faster on one side than the other—not because the light moves faster, but because the space it occupies is moving. Another is that if you tried to throw a spear at a black hole, the spear would start rotating in the opposite direction to the black hole. That's not because of any force acting on the spear, it's because the tip of the spear occupies space-time that's moving faster than the end that's farther from the black hole.

Fortunately or unfortunately, we have nobody in position to throw a spear at their local black hole. So, we have to work with what the Universe has given us. And what it has given us is a binary system of former stars that has a history nearly as bizarre as frame dragging itself.

One of the two companions is a neutron star formed in the aftermath of a supernova of a massive star. The neutron star is a pulsar, rotating in a way that it sends a point of radio emissions along the line of sight with Earth at regular intervals. Its neighbor is a white dwarf, the product of a lower-mass star that has converted all of its lighter elements to carbon and oxygen. Like its neighbor, the white dwarf is also spinning rapidly.

The weird history is needed to explain the fact that the white dwarf is both older than the neutron star and that both are spinning rapidly. The general consensus is that the star that formed the white dwarf started off as the more massive of the two but transferred enough mass to its companion, in turn allowing it to form a white dwarf instead of exploding. The white dwarf then started drawing some of that mass back, which spun it up by transferring its angular momentum while being drawn in. But enough mass had already been transferred to allow the second to explode in a supernova, thus forming the neutron star.

Timing is everything

The two stars have ended up remarkably close to each other, with the pulsar completing an orbit in less than five hours. Thus, they're able to influence each other by frame dragging, with the primary effect being an alteration in the precise orientation of the axis of rotation, an effect called precession. These changes would be small and gradual, and they occur in combination with non-relativistic effects, so they're not necessarily easy to detect, either.

The advantage of this system is the presence of the pulsar, which sends remarkably consistent flashes of light in the direction of Earth. Even small effects on the timing of these flashes are possible to detect with enough observations. And the pulsar in question here has been under observation for nearly 20 years.

While what the researchers did was rather involved, the general outlines are pretty simple. They used the decades of observations to build a model of how the system evolved over time, tracking things like the orientation of the pulsar's axis of rotation, the location of the spot closest to the white dwarf, the masses of the two objects, and so on. Among other things, this told them that the white dwarf completes a rotation about once every 20 seconds, despite being larger than the Earth. About a dozen seconds faster, and the star would rotate so fast it would simply come apart.

They then attempted to match this behavior with and without relativistic effects. And you can safely assume that relativistic effects were required to explain the behavior, or I wouldn't be writing this.

The neat thing about this is that it's a confirmation of something that was predicted roughly a century earlier. And the predictions came not because people were looking for something to test, but they were just exploring the consequences of relativity using math. That makes this result a delightful demonstration of the power of a theory with clear consequences.

Science, 2020. DOI: 10.1126/science.aax7007 (About DOIs).