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Thanks CTOnews.com netizen Hua Ke high achiever's clue delivery! How to tell if a black hole is spinning? This seemingly simple question, if you think about it, you will find that it does not seem so easy.

For a "three-dimensional hole" in space, the usual method of judgment no longer works here. Imagine an absolutely smooth and uniform transparent glass ball. If you look at it with the naked eye, it is difficult to tell whether it is spinning or not. Even if the glass ball is like this, let alone a black hole from which even light cannot escape, you can imagine how difficult it is to judge whether a black hole rotates or not.

Although the existence of black holes has long been confirmed by observation to a certain extent, the so-called "seeing is believing" is almost meaningful for those of us who are used to electromagnetic imaging if there is no "photo" as evidence. So in 2017, through the event Horizon Telescope (EHT), a virtual array of radio telescopes around the world, scientists finally took a "ID photo" of the black hole. The protagonist of this photo is the M87 central black hole, which is 5500 million light-years away and has 6.6 billion times the mass of the sun. Since then, the existence of black holes has been completely hammered.

According to theory, because real black holes are formed by the collapse of stars, they should rotate like neutron stars and white dwarfs. The same is true of supermassive black holes at the center of galaxies such as M87, which should all have angular momentum.

However, the rotation of a black hole is just a theoretical speculation. is there any evidence to prove that it is spinning?

Usually we judge whether a thing is spinning or not, which can be seen from its shape or surface information. But the shape of a black hole is an axisymmetric sphere, and there is no information to refer to on the surface (or even no information at all). You can imagine how difficult it is to judge whether a black hole is spinning.

You might say: look at the accretion disk, the accretion disk revolves around the black hole, doesn't it mean that the black hole is carrying them?

First of all, the accretion disk does reflect the rotation of the black hole to some extent. Because according to the theoretical model, the rotation of the black hole will affect the inner boundary radius of the accretion disk. For example, if the rotation direction of the black hole is opposite to that of the accretion disk, then the inner boundary of the accretion disk is farther from the event horizon of the black hole; on the contrary, if the rotation direction of the black hole is the same as that of the accretion disk, then the inner boundary of the accretion disk is closer to the black hole. Then the inner boundary of the accretion disk will directly affect the temperature distribution and radiation spectral lines of the accretion disk, so through the observation of the radiation spectral lines of the accretion disk, the rotation of the black hole can be speculated theoretically.

But there are some problems in this method. First of all, it requires very high quality and resolution of the observed data, and secondly, more importantly, there are some assumptions in the accretion disk model it depends on. To put it bluntly, the theory is not very reliable. So there are some problems at present, including other ways to measure the rotation of black holes with the help of accretion disks.

In fact, strictly speaking, there is a problem in proving whether a black hole rotates or not through an accretion disk. Do you think that although the matter in the accretion disk revolves around the black hole, does it have to rely on the traction of the black hole? As I just said, the black hole may even rotate in the opposite direction to the accretion disk. It can be seen that the rotation of the accretion disk is not caused by the rotation of the black hole.

For example, we assume that this black hole is not a rotating black hole, but a static classical black hole. At this time, if there is a passing celestial body, unless it is flying directly toward the black hole, the orbit of the celestial body must become a curve under the gravitational action of the black hole. If all factors are just right, it is easy for the celestial body to start circling the black hole.

In other words, the rotation of matter around a black hole only means that it has an initial velocity that does not point to the black hole, which is not directly related to whether the black hole itself rotates or not. You can think of a black hole as the eye of a typhoon, and the black hole of the eye can be very calm relative to the strong wind of the surrounding accretion disk. Therefore, the existence of an accretion disk does not prove that the black hole is spinning.

So is there any way to verify that the body of a black hole is spinning?

Last month (September 2023), an international team led by Chinese scientists confirmed for the first time that the M87 black hole was indeed spinning by analyzing 22 years of observational data. How do they do it?

Galaxy M87 is the largest galaxy near the Milky way. Its core is a very large supermassive black hole with a mass of 6.6 billion times the mass of the sun. By contrast, the black hole at the center of our Milky way Galaxy (Sagittarius A*) is a mere 4.3 million times the mass of the sun.

Apart from its huge head, it is much more active than the quiet Aphrodite Magna M87 black hole. Yes, the behemoth is still eating crazily, fed by a giant accretion disk measured in light years. It is estimated that the accretion disk is sending food into the mouth of the black hole at a rate of 90 Earths a day.

As I said last time, with such a large amount of food, the black hole is bound to be indigestible and will vomit while eating. Sure enough, one of the iconic features of the M87 black hole is its 5000 light-year jet. The jet shows a high degree of collimation, as if a laser was emitted from the poles of the accretion disk. This time the rotation of the black hole is determined by the wobble of the jet.

But doesn't the jet also come from the accretion disk? what does it have to do with the black hole body? This involves a very strange phenomenon-reference frame drag (Frame-dragging).

Reference frame drag, also known as reference frame drag, is a prediction of general relativity that a central mass in a rotating state can cause the surrounding inertial reference frame to rotate. To put it bluntly: the rotation of a black hole will drag the space-time around it to rotate along with it!

For example, one person is moving in the direction of the rotation of the black hole, and the other person is moving against the rotation of the black hole. At this point, for the observer in the distance, he will see the person moving along the black hole go faster. In other words, when an object moves around a massive celestial body, if the celestial body is rotating, then the trajectory of the object will have a special slight deviation compared with the case where the celestial body does not rotate.

In fact, this drag phenomenon of the reference frame has been predicted a long time ago, but it has been difficult to be tested by experiments because it has a deviation of only a few trillions between it and the calculated results of Newtonian mechanics. The previous "gravity probe B" experiment was verified by measuring the deviation of the orbit of a man-made satellite. However, due to the small mass of the earth, there are many uncertain factors in this experiment, which can only be said to verify the drag effect of the reference frame to a certain extent.

If the celestial body had a larger mass, we would be able to verify the reference frame drag by analyzing the precession of the small celestial body orbiting it. So later, some scientists tried to observe the precession of pulsars in the binary system, but the phenomenon is still very weak. In order to produce a sufficiently obvious phenomenon, the mass of the celestial body must be large enough, and the M87 black hole, which has 6.6 billion times the mass of the sun, becomes an ideal target for astronomers.

Through the analysis of 22 years of observed data, the researchers found that there is a periodic wobble in the jet of M87 black hole. It is inferred that this is because there is a small angle between the rotation axis of the black hole and the jet of the accretion disk. Under the drag effect of the reference frame, the jet rotates periodically around the rotation axis of the black hole, and the precession period is about 11 years.

The existence of this phenomenon illustrates two facts:

1. The M87 black hole does rotate. This means that we have reason to believe that the black holes observed in reality should all be rotating Kerr black holes rather than static classical black holes.

2. The drag phenomenon of the reference frame does exist. This means that Einstein's general theory of relativity has once again been perfectly verified.

Reference:

[1] https://www.sci.news/astronomy/spinning-messier-87s-supermassive-black-hole-12304.html

[2] https://www.nature.com/articles/s41586-023-06479-6

[3] https://en.wikipedia.org/wiki/Frame-dragging

[4] https://en.wikipedia.org/wiki/Gravity_Probe_B

[5] https://en.wikipedia.org/wiki/Messier_87

[6] https://mp.weixin.qq.com/s/gkToAZaq0-jI2rfAOBi94A

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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