Academic& Science

Here is the core concept of general relativity. Space and time are no longer separable. It is called spacetime. Look at the first picture. Imagine spacetime as a huge, flexible rubber sheet. The massive objects, like planets and stars, are balls on the sheet and make it out of shape. Gravity is simply the way objects influenced by the sink. When light travels by, it will change its direction because of the sink.  This is how general relativity explain the light deflection by the Sun. Curvature is defined to measure how the spacetime bend. Deeper depression means larger curvature. In the extreme case, the curvature goes to infinity, and the depression is infinitely deep. Then we call this point a singularity, just like the second picture. The last concept is the black hole. Its strict definition is a region light cannot escape. Imaging a car goes into this hole accidentally, and it is very deep. The car cannot get out, no matter how powerful the engine is. There is an intuitive idea from this picture. Because the singularity is very deep, there should be no possibility that the light can escape near the singularity, and, by definition, there should be a black hole around it. This is where the Cosmic Censorship Hypothesis comes in. Raised by the Nobel laureate Roger Penrose in 1969. It suggests that singularities should be hidden in black holes. Cosmic censorship is a fundamental principle of determinism. If you throw a ball, the orbit of the ball can be calculated if there is no other force. The universe should also be like this. If you know the current states and how it moves at this moment, the future state should be predictable. Cosmic censorship acts as a cap, covering the unpredictable singularities inside a black hole so that they cannot influence the observable universe. Mathematically, there are examples of singularities outside of black holes. These are called naked singularity. The mathematical problem is to prove that the naked singularity is highly unstable so that the cosmic censorship hypothesis is valid. To understand the instability, just look at this picture. The naked singularity is like a ball balanced perfectly on the tip of a cone, and it will fall down the hill with tiny perturbation. For naked singularities, the tiny perturbation will lead to a black hole around it.