Why Do Black Holes Spin at Nearly the Speed of Light?

Black holes are some of the most fascinating and mysterious cosmic phenomena. Among their many unique behaviors, their high rotational speed is a remarkable aspect. But how do they achieve this incredible spinning motion? Let's delve deep into the physics of black holes and understand why they spin at nearly the speed of light.

Conservation of Angular Momentum

When a massive star collapses to form a black hole, the conservation of angular momentum plays a crucial role. Angular momentum, a measure of rotational motion, is conserved in the system. This principle ensures that as the star shrinks, the angular momentum must be preserved, leading to high rotational speeds.

In simpler terms, imagine an ice skater pulling her arms in during a spin. Her rate of rotation increases. Similarly, as a massive star collapses, the angular momentum must be conserved. This means that the surface of the black hole must spin at an incredible speed to maintain the same amount of angular momentum as the original star. However, according to the laws of relativity, this can't happen. Any point on the black hole's equator moving faster than the speed of light is physically impossible. Thus, the black hole sheds some material from its equator to limit its rotational speed.

How Much Does It Rotate?

Black holes are not only found in the highest mass range of stars, but they can also form as white dwarfs and neutron stars for stars with lower masses. The Sun, for example, spins once every 27 days. Over time, a star like the Sun will evolve into a red giant, blow off its outer layers, and become a white dwarf. The white dwarf will rotate much faster, perhaps completing a rotation in just 30 minutes. This increase in rotational speed is directly related to the change in the star's radius and mass distribution.

The rate of rotation of an object is inversely proportional to the square of its radius. Therefore, if the radius decreases, the rotational speed must increase to conserve angular momentum. This principle applies even to neutron stars and black holes, which are highly compact and dense objects. Neutron stars, formed from massive stars and typically twice the mass of our Sun but just 10 to 20 km across, can rotate at rates of up to 716 times per second. This is far beyond the rotation speeds of any known star, white dwarf, or even black hole.

Measuring Black Hole Rotation

While astronomers can't directly measure the rotational velocity of black holes, they can infer it through the effects on the surrounding material. By studying the accretion disks and flows around black holes, scientists can deduce their rotational properties. Gravitational wave observatories like LIGO and Virgo have detected merging black holes and found that some of these black holes spin at nearly the theoretical maximum of 95% of the speed of light.

According to the conservation of angular momentum, black holes spin at such high speeds that any point on their equator would, if it existed, move at near-light speeds. This phenomenon is a testament to the extreme conditions found in black holes and their role in reshaping our understanding of physics at the most fundamental level.