Buzz
Black holes are among the most fascinating objects in the universe, yet human understanding of them still holds many unanswered aspects.
Between 1907 and 1911, Einstein delved into the theory of general relativity (the non-Euclidean tensor calculus). He published a paper titled 'On the Influence of Gravity on the Propagation of Light' in 1911. Predicting that time is relative, and relative to the observer's gravitational field.
He also proposed the equivalent theory that gravitational mass is like inertial mass. Einstein also predicted the time dilation due to gravitational force.
Gravity causes the deformation of space-time. Two events in different regions will experience different times. The greater the deformation, the slower time passes.
Another important outcome of his theory is the prediction of the existence of black holes and the expansion of the universe.
In 1915, a few months after Einstein unveiled the theory of general relativity, German physicist and astronomer Karl Schwarzschild provided a solution to Einstein's field equations. Now known as the Schwarzschild radius, it describes the escape velocity of matter at the surface of a solid spherical object equal to the speed of light.
In 1931, Indian-American astrophysicist Subrahmanyan Chandrasekhar utilized the theory of special relativity to compute the limiting mass value for the collapse of degenerate, non-spinning electron matter.
In 1939, Robert Oppenheimer and others concurred with Chandrasekhar's analysis that neutron stars had exceeded a critical limit and collapsed into black holes.
The theory of general relativity predicts that the universe is expanding or contracting. In 1929, Edwin Hubble confirmed the expansion of the universe. At that time, this seemed to refute Einstein's theory of the cosmological constant.
The cosmological constant was introduced to ensure the universe was static. In response, Edwin Hubble used redshift measurements to discover that galaxies were moving away from the Milky Way.
Additionally, he found that galaxies farther from Earth were receding faster, a phenomenon later termed Hubble's Law. Hubble set the Hubble constant (the rate of expansion) at 500 km/(s.Mpc).
According to the theory of general relativity, gravitational fields will bend spacetime.
For a fixed mass, the larger the size of the star, the lower the density. When the volume of the star is very large, its gravitational field hardly affects spacetime and light emitted from a certain point on the surface of the star can radiate in a straight line in any direction. However, for that mass, as the star's radius decreases, it will bend spacetime around it more, light emitted at certain angles will return to the surface of the star along curved space. Similarly, super dense neutron stars can achieve a time dilation factor of 10-20%.
When the radius of the star shrinks to a certain value (called the 'Schwarzschild radius' in astronomy), even light emitted from the surface vertically upwards is captured, at this point the star becomes a black hole. Meaning it's like a bottomless pit in the universe, once any matter falls into it, it can't escape.
Black holes cannot be directly observed, but their existence and mass can be indirectly known, and their impact on other things can be observed.
Information about the existence of black holes can be obtained through 'sideways information' emitted by X-rays and gamma rays caused by friction due to the acceleration caused by the gravitational force of the black hole before the object is sucked in.
It is speculated that the existence of a black hole can also be inferred by indirectly observing the orbit of stars or clouds between stars, while its position and mass can also be determined.
So, how are black holes formed? In fact, similar to white dwarfs and neutron stars, black holes have the ability to evolve from stars.
As a star ages, its nuclear reactions deplete the hydrogen fuel in its core and the energy generated by the core also diminishes, in this way, it no longer has enough energy to support the enormous weight of its outer shell.
So, under the weight of the outer shell, the core begins to collapse until a small, dense core is formed and capable of re-establishing pressure equilibrium.
This newly formed star primarily evolves into a white dwarf, however, for stars with exceptionally large mass, it can 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 exceeds this value, then there will be no Force that can compete with its own gravitational force, thus causing another significant collapse.
At this point, according to scientists' speculation, matter will converge towards the center until it becomes a 'point' where its volume tends to zero and density tends to infinity. Thus, immense gravitational force at this point will prevent any light from escaping outward, therefore severing all connections between the star and the outside world, and this is also when a black hole is born.
Compared to other celestial bodies, black holes are truly remarkable, for instance, they have the ability to 'disappear', humans cannot directly observe them, even scientists can only speculate about their internal structure.
We all know that light travels in a straight line. This is the most basic principle. But according to the theory of general relativity, space will be curved 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. Vividly speaking, it seems as if light initially wants to travel in a straight line, but strong gravitational force has altered its path.
On Earth, due to the negligible effect of gravitational fields, this bending is insignificant, but around black holes, the deformation of spacetime is immense. In this way, although some light emitted by the star is blocked by the black hole and falls into it, another part of the light will orbit around the black hole in curved space and reach Earth.
Therefore, we can easily observe a sky full of stars behind the black hole, as if the black hole doesn't exist, this is the invisibility characteristic of black holes... (For example, the accretion disk around the gravitational pull of the black hole is often used to determine the size of the black hole).
