Enlarge / The lit up rings in this image are caused by wobbles in a pulsar's axis of rotation.NASA

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 error. While a number of missions have produced results consistent with relativity, the experiments had rather large uncertainties.

Now, an international team of scientists have used an interstellar laboratory to test the proposal. Taking advantage of a large white dwarf with a close-by 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's 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 further 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's 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's 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 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 wouldRead More – Source

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