Let’s talk about big things and history. Let’s go back to when Newton was on a discovery spree using his magically genius wand of mathematics. He didn’t just look at the Earth and the Sun in admiration like most of us. Instead, he came up with a theory about them – the law of gravitation.
Newton suggested that any two bodies having certain masses, would exert a force on each other (the gravitational force), and that this force depends on the masses of, and the instantaneous distance between these bodies. He gave a very simple equation which means that the heavier the bodies, the greater the force of gravitation, and the farther the bodies, the lesser the force.
This force of gravitation is always attractive by nature, unlike the magnetic forces. Yes! If you are thinking about how you held two magnets in your hands back at school and felt the repulsion between them, you are on the right track. Like magnetic poles repel each other while the opposite poles attract. On the other hand, if you are able to hold the Earth in your hands! And your friend can (and is willing to) hold the Sun in her hand, then you will never ever feel repulsion. Attraction will prevail. Always. Good way to never let your friend go, but sunburn alert!
Since gravity is always attractive by nature, Newton suggested that the orbits that planets, satellites and stars follow around each other are because of this force of attraction. If you are thinking that our scientists settled at that, you should also know that they are inquisitive beings. So some physicists thought about shifting the Sun a little farther away from the Earth, or maybe even closer! The question now, is that how long before the Earth figures out that the Sun was moved from its original position so as to feel the alteration in the gravitational force, and accordingly change the orbit. Instantaneously? No! It is believed that nothing can travel faster than the speed of light, and it takes light about 8 minutes and 20 seconds to reach the Earth from the Sun. That means the information about the shift in the position of the Sun will not reach the Earth instantaneously.
What then? Much to the chagrin of most college students, the physicists came up with yet another theory that considered space and time, not as individual entities, but as a single combined entity. They (uncreatively) called it space-time! This space-time was considered to be like a continuous fabric. Everything exists in this fabric of space-time.
While you and I are not heavy enough to cause visible changes, but the planets, stars, etc. cause warps or curvatures in the space-time fabric as can be seen in the figure above. The grid-like structure is the space-time fabric. As you can see, therefore, the Earth and the Moon make curvatures proportional to their sizes. Therefore, the Moon, being at a higher position since the Earth has dipped lower, follows a very natural trajectory around the Earth.
Similar is the case with black holes. In fact, a black hole is so dense that it doesn’t just create a slight curvature, but in principle, it tears apart the space-time fabric. Sad! I know. The tear is a dip created by the black hole which is so deep that nothing is able to escape it once it crosses a certain boundary, which we call the event horizon of the black hole.
In fact, the tear is theoretically infinite i.e. it leads to a geometrical breakdown, called a singularity, in the structure of space-time. Everything gets pulled in. YES! Even your friend! So basically, every bit of information that enters the black hole, is not able to escape it; not even light.
You may think that what about this simple fact is confusing? Well, in 1975, Stephen Hawking wrote a paper showing that black holes don’t just absorb everything. They also emit certain particles like any hot body. Hawking wrote, “This thermal emission slowly decreases the mass of the black hole to its eventual disappearance.” What happens when the black hole disappears? Are we able to find out about the stars, particles, and other bodies that it devoured? No. All this information is bleached. Our seemingly perfect Universe suffers a memory loss. This implies that we can now not understand or retrodict the past of our Universe.
The paradox arises from the fact that for all physical theories given till date, it holds true that information is preserved. Information can only be transformed but not lost. Our theories state that if we know the state of our universe at one moment, then we should know about its state at any other moment of time too because information remains preserved. But Hawking clearly showed that the disappearance of black holes leads to information loss!
Hence the question – loss or no loss?
One question. Over four decades. And no answer good enough!
So that, my friends, is the Black Hole Information Paradox.
Maybe, you have a chance at this! Maybe, you could become the person who brings forth the theory of everything – a quantum theory of gravity! So good luck with that!