When I was a little kid, I kept wondering how can there be a thing bigger than our Earth or our Sun. But not just big, it is so powerful than can “suck” anything inside, even the light. So does it act as a giant vacuum cleaner to clean all the dust of the universe? Based on the knowledge in house cleaning of a 10-year old kid, that is quite impossible, giving the frequently dirty status of my room by then. However, I was not sure how can it catch the light, and will there be any black hole so near to us that can suck everybody and end this world?
What is Black Hole? How can Black Hole capture light?
“What goes up must come down”- Isaac Newton. On a planet, every particle is affected by a magical force called Gravity. This is always a hot topic, not only in XVII, when the legendary apple fell to Sir Isaac Newton head but also up to now, when quantum and particle theory comes up, then String theory or Higgs boson concept appears. Day by day, people discover more things about Gravity. But that will be discussed more in another topic. Today lets just focus on a very simple mention of Gravity: It pulls everything back to the ground.
So let’s say if I throw a rock vertically up, It should be expected to come down after an amount of time. The rock initially can fly up because I gave it an initial velocity, which converts to initial kinetic energy. However, as the rock travels, the gravitational force keeps pulling it down, making the velocity gradually decrease up to a point that its velocity equals 0, this is also as known as the highest point the rock can reach. When this happens, the rock loses all of its kinetic energy, and now give its fate for the Gravity to pull it back. The hardest we throw, aka the largest initial velocity we give, the higher the rock can fly up to.
This leads to a question, what if this “highest point” is not on the planet anymore. What if we are strong enough, like Superman (but with a more interested in Physics edition), to throw this rock out of the Earth. Will the rock come back? The answer is No. This is called the escape velocity of a planet. For our Earth, this velocity is approximately 11.186 km/s and can be generally calculated given the formula
Escape velocity of a planet depends on its mass (M) and radius (r)
What’s about light? As human researched about the light, we know that it has the wave-particle duality. In short explanation, it can be treated as both wave and particle. When we considering light as particles (aka photons), we can calculate its mass using its wavelength. As a result, this particle (aka photon) also be pulled back by Gravity when it tries to get out of the planet. However, the velocity of light is c = 300000 km/s, which exceeds the Earth ( and also many other planets’) velocity. That’s why usual planets cannot capture light.
When we look again at the formula, we can see that this velocity depends on the planet’s mass and radius. The smaller and heavier the planet, the higher its escape velocity. This leads to another question, what if the planet is extremely small and heavy, which makes it escape velocity super high, even higher than the speed of light. That’s called the Black Hole. So Black Hole is actually also just like a planet,( except with the fact it is a star) but extremely small and heavy, making its gravitation force super strong. Any particles want to get out of Black Hole need to be supplied an initial velocity at least light velocity (c) or even higher, which is impossible in our reality.
Who created Black Hole?
In order to become Black Hole, we know that its mass must be very huge (and we are talking about 3~10 times bigger than the Sun mass). With that gigantic mass, a star’s gravitation force must be super strong, up to a level that it can collapse everything inside it. However, this is not easily done. The reason is that inside the star core, there exists a fusion reaction. Under the effect of gravity, all hydrogen atoms collapse first into its core. Inside the core, nuclear fusion crushed hydrogen into helium. This process radiates an outward energy, which is then fighting against the star’s own gravity force. As a result, the star becomes balanced.
As long as there is fusion in the core, the star will remain stable. However, with stars with mass greater than our sun, the fusion in its core can form heavier and heavier elements, up to Iron (Fe). The funny thing about Fe is the process creating this element is not generating any outward energy to fight against gravity. So when all of the star core become Iron, its fusion outward force will lose to its own gravity, which then makes the star start to collapse to itself. This process will create a super explosion call the Supernova.
Depends on the initial mass of the star, after the Supernova, the star will either become White Dwarf (our Sun for example), Neutron Star, or Black Hole. Neutron Star and Black Hole are quite similar, which either is just a condensed singularity point, and its minimum mass has to be 1.4 Sun Mass (Chandrasekhar limit). This topic will be further discussed in another post. For now, let’s just make it simple as Black Hole is a very big star that dies after all of its fusion reaction ends, making everything collapse into one singularity.
Schwarzschild radius and event horizon
If a star by any chance become a black hole, it will have a “prison wall” surrounding it. This wall will forbid any object that wants to get out of the black hole. Scientifically, this wall is called the event horizon of the black hole. We can calculate the radius of the event horizon using the Schwarzschild radius formula.
If our Sun becomes a Black Hole, its radius will be around 3km. This number would be around 8.7mm for Earth (extremely small).
In the next part, we will look into the first image of Black Hole captured by scientists on April 10th, 2019, and discuss more its appearance and radiation.
One response
Such a great blog!