Black holes are among the most fascinating objects in the universe, yet humanity’s understanding of them still has many unresolved aspects.
Between 1907 and 1911, Einstein conducted research on the theory of general relativity (a non-inertial reference frame). He published a paper titled “On the Influence of Gravity on the Propagation of Light” in 1911, predicting that time is relative and that it varies for observers in different gravitational fields.
He also proposed the equivalence theory that gravitational mass is analogous to inertial mass. Einstein predicted time dilation due to the effects of gravity.
Gravity causes a distortion of spacetime. Two events occurring in different regions will be perceived to occur at different times. The greater the distortion, the slower time passes.
Another significant outcome of his theory was the prediction of the existence of black holes and the expansion of the universe.
Gravity causes a distortion of spacetime.
In 1915, a few months after Einstein published his theory of general relativity, the German physicist and astronomer Karl Schwarzschild provided a solution to Einstein’s field equations. This solution, now known as the Schwarzschild radius, describes the escape velocity of material at the surface of a spherical object as equal to the speed of light.
In 1931, Indian-American astrophysicist Subrahmanyan Chandrasekhar used special relativity to calculate the critical mass value for the collapse of electron-degenerate matter without spin.
In 1939, Robert Oppenheimer and others agreed with Chandrasekhar’s analysis that neutron stars exceeding a critical value would collapse into black holes.
General relativity predicts that the universe is either expanding or contracting. In 1929, Edwin Hubble confirmed that the universe is expanding. At the time, this seemed to contradict Einstein’s theory of the cosmological constant.
The cosmological constant was introduced to ensure that the universe was static. In response, Edwin Hubble used redshift measurements to discover that galaxies were moving away from the Milky Way.
He also found that more distant galaxies recede faster, a phenomenon later termed Hubble’s Law. Hubble set the Hubble constant (expansion rate) at 500 km/(s.Mpc).
Black holes cannot be observed directly, but their mass can be inferred indirectly.
According to general relativity, gravitational fields bend spacetime
For a fixed mass, the larger the star, the lower its density. When a star’s volume is very large, its gravitational field has little effect on spacetime, allowing light emitted from a point on the star’s surface to travel in straight lines in any direction. However, with that mass, the smaller the star’s radius, the greater the distortion of spacetime around it, causing light emitted at certain angles to curve back to the star’s surface along the warped space. Super dense neutron stars can achieve a time dilation factor of 10-20%.
When a star’s radius shrinks to a certain value (known in astronomy as the “Schwarzschild radius”), even light emitted from the surface is captured, transforming the star into a black hole. This means it acts like a bottomless pit in the universe; once matter falls into it, it cannot escape.
Black holes cannot be directly observed, but their existence and mass can be inferred indirectly, and their effects on other objects can be observed.
Information about the existence of black holes can be obtained through “edge information” emitted in X-rays and gamma rays caused by friction from the acceleration due to the black hole’s gravity before an object is pulled in.
It is speculated that the existence of a black hole may also be indirectly observed through the orbital paths of stars or interstellar clouds, allowing for the determination of its position and mass.
Black holes can evolve from stars.
So, how are black holes formed? In fact, similar to white dwarfs and neutron stars, black holes can evolve from stars.
As a star ages, its nuclear fusion reactions exhaust fuel (hydrogen) in its core, and the energy produced by the core also diminishes. Consequently, it no longer has enough energy to support the massive weight of its outer layers.
Therefore, under the weight of the outer layers, the core begins to collapse until a small, dense star forms, capable of stabilizing the pressure.
This newly formed star primarily evolves into a white dwarf; however, for stars with particularly high mass, it may form a neutron star.
According to scientists’ calculations, the total mass of a neutron star cannot exceed three times the mass of the Sun; if it surpasses this value, there will no longer be a force that can counteract its own gravitational pull, leading to another significant collapse.
At this point, scientists speculate that matter will move toward the center until it becomes a “point” where its volume approaches zero and density approaches infinity. Consequently, the enormous gravitational force at this moment will prevent even light from escaping, severing all connections between the star and the outside world; this is when a black hole is born.
Around a black hole, the distortion of spacetime is immense.
Compared to other celestial objects, black holes are very unique. For instance, black holes have the ability to “become invisible”; humans cannot observe them directly, and even scientists can only propose various hypotheses about their internal structure.
We all know that light travels in straight lines. This is the most fundamental principle. However, according to general relativity, space is warped under the influence of gravitational fields.
At this point, although light still travels along the shortest distance between any two points, it is no longer a straight line but a curve. To illustrate, it seems that light initially wants to travel in a straight line, but strong gravity alters its path.
On Earth, due to the very small effect of gravitational fields, this type of bending is negligible. However, around a black hole, the distortion of spacetime is immense. Thus, while some of the light emitted by a star is blocked by the black hole and falls into it, another portion of that light curves around the black hole in the warped space and reaches Earth.
Therefore, we can easily observe the starry sky behind a black hole as if the black hole did not exist, which represents the invisibility of black holes… (For example, the accretion disk around a black hole’s gravitational pull is often used to determine the size of the black hole).
