# Fast-spinning black holes can have “hair”

(ORDO NEWS) — General relativity is an extremely complex mathematical theory, but its description of black holes is surprisingly simple. A stable black hole can be described by just three properties: its mass, its electric charge, and its rotation or spin.

Since black holes are unlikely to have a large charge, there are really only two properties needed to do this. If you know the mass and rotation of a black hole, you know everything there is to know about a black hole.

This property is often summarized by the no hair theorem . In particular, the theorem states that after matter enters the black hole, the only characteristic that remains is mass.

You could make a black hole out of hydrogen in the sun, chairs, or National Geographic magazines, and it wouldn’t make any difference. From the point of view of general relativity, mass is mass. In any case, the black hole’s event horizon is perfectly smooth, without any additional features.

As Jacob Birkenstein said, black holes have no hair.

But for all its predictive power, general relativity has problems with quantum theory. This is especially true for black holes. If the no hair theorem is true, the information contained in the object is destroyed when it crosses the event horizon.

Quantum theory states that information cannot be destroyed. So, the actual theory of gravity contradicts the current theory of quanta. This leads to problems like the firewall paradox where it is impossible to decide if the event horizon should be hot or cold.

Several theories have been proposed to resolve this contradiction, often including extensions of the theory of relativity. But the difference between standard relativity and these modified theories can only be seen in extreme situations, making them difficult to study through observation. But a new paper in Physical Review Letters shows how they can be studied through the spin of a black hole.

Many modified theories of relativity have an additional parameter that the standard theory does not have. Known as the massless scalar field, it allows Einstein’s model to link to quantum theory in a way that does not create conflict.

In a new work, scientists have studied how such a scalar field is related to the rotation of a black hole. They found that when spinning slowly, the black hole is indistinguishable from the standard model, but when spinning rapidly, the scalar field allows the black hole to have additional features.

In other words, in these alternative models, rapidly rotating black holes may have hair.

As the authors point out, future gravitational wave observatories should be able to use rapidly spinning black holes to determine if an alternative to general relativity is valid.

Einstein’s general theory of relativity has dealt with all the observational problems so far, but it will fail in the most extreme conditions of the universe.

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