Laws developed by humans try to create a frame of freedom. People are free as long as they follow the laws. Even though these laws tend to keep people under control, individuals still have the will power go against the laws. This is true for the mere laws created by humans. Physics laws, on the other hand are unbreakable. Or are they? What if I said most of what you know about universe is incorrect? What if I go even further and say that we are living in a black hole? Would you believe me?
To decide if we are living in a black hole or not, we need to know what a black hole is in the first place. A simple definition is “A black hole is a region in space where the gravitational pull is so strong that not even light can escape from it.”. The factor that distinguishes a black hole from other stellar objects, is the fact that its gravitational interaction is so strong that even light is unable to escape from it. Using this definition, we will approach this problem in a qualitative way at the end. But before doing so, I’d like to demonstrate the relationship between the mass and the density of a black hole. These next paragraphs will be a quantitative investigation of energy. Even though the calculations are relatively clear and simple, one may want to skip it and continue with the following paragraph. Skipping would not cause any loss of continuity.
When we try to examine a black hole quantitatively, there isn’t much that we can do in the “popular science” level. Yet the definition of a black hole gives us some hint on how to do so. We know that all black holes have an event horizon, at which even light cannot escape. We say “even light” because light is the fastest moving thing that there is. Now using that analogy, let’s say that a spaceship is located right on the event horizon of a black hole. If our spaceship is moving at the maximum speed possible, then it has speed of light (ignoring some fundamental problems that make this impossible, for the sake of simplicity). In this situation, the spaceship can neither escape the black hole nor fall into it. In this case, the kinetic energy of out spaceship is equal to the potential energy from the black hole.
As we can see, the energy relation gives us an equation for the density of our black hole. This relation suggests that as the mass of the black hole increases, its density decreases drastically!
Now that we got the math out of the way, let’s use our equation to get information about black holes. For example if our Sun was to turn into a black hole, its density would be around one Himalayan range per cubic meter. That seems quite dense, just as we expected. To see what would happen if we increased the mass, we need bigger masses than our Sun. If our black hole consisted of 4 million solar masses, its radius would be equal to the Solar System, and its density gets as low as air! This means that if we filled a balloon until it filled the whole Solar System, we would have created a black hole!!
What would happen if we enlarged our investigation to the scale of the whole universe? Using the total mass in universe and dividing to the volume of the Observable Universe, we get a density around 5 Hydrogen atoms in cubic meter. Using our prior equation, it turns out this density is well enough for becoming a black hole; actually, it’s enough to be a black hole 10 times the size of the observable universe!!
Even though all these calculations are correct, there is one observation that contradicts the idea that we are inside a black hole: expansion of the universe. If we were inside a black hole, shouldn’t we see that everything is moving to the centre? Our observation is the opposite, everything in the universe is moving away from us…
At this level, we are at the limits of physics so many of the theories we may discuss are all theoretical and not supported by data. But it’s still fun to do, so let’s dive deep into it. Einstein’s general relativity provided a frame of calculations for black holes that are stationary. But a more realistic expectation would be a black hole that is spinning around itself. This is a widely followed path among many celestial beings, to conserve angular momentum. This property of black holes may explain why matter is not approaching but moving away. For objects to continue spinning, a force is required. The force here, is the gravitational force of the black hole. As the masses collected in the black hole increases, so does its spin speed. Eventually, the speed will be so high that black hole will not be able to pull any more mass in. And if right before the critical speed was reached, black hole acquires more mass than enough, then the black hole will start to shoot out the mass it has acquired before! Shooting mass out will decrease the gravitational pull, further decreasing the force that keeps objects in place!
Investigations made here are mere thought experiments that are not backed by any physical experiment. Yet that should not be considered as a bad thing, as thought experiments are essential in many fields of modern physics. The most widely accepted theory for gravity, the General Relativity had its foundations constructed in 1907 in a similar way. While our noble physicist, Albert Einstein, was sitting in his office an idea charged into his mind: “If a person is falling freely, he will not feel his own weight.”. Of course, vigorous mathematical calculations followed this thought in the way to form his theory. But for now, I believe this much physics is well enough for today…
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