What is time? Is it an imaginary thing?
Why do time slow down when it is closer to a black hole?
Time is the thing which is represented by the d/dt terms in physics equations and measured by clocks.
Time is one of our dimensions, often refered to as the 4th dimension.
Time has many sides; Time Dilation, Time Travel etc. etc.
Time Dilation describes that time slows down from one frame of reference. Einstein's Special Relativity talks about Time Dilation. To prove this theory, scientists had 2 atomic clocks synchronized to run at exactly the same time. The one clock they sent up in a fast moving air plane and the other clock stayed at the ground. When the clock in the plane got back to the ground, it had recorded less time than the clock that didn't move, just a few billions of a second mind-you. This can only happen, though, if an object is moving at or close to the speed of light, and the closer to the speed of light the object is moving, the stronger effects of Time Dilation the object will feel.
Time Travel, despite what you have heard, is not impossible, in fact, there is no law of physics to prevent time travel. Issac Newton explained time as an arrow, when you fired the arrow, it never deviated, therefore, time travel is impossible. Then comes Einstein and explains it in a very different way, like a river, a river that travels in big turns around the universe. If you rotate around the universe, then you can acheive Time Travel. Einstein was, however, calm about it, since the universe does not rotate, it expands. Another way to travel in time is to have a wormhole. A wormhole is essentialy a hole through space and time, leading from one place in the universe to an other. Wormholes are still purely theoretical. However, according to Eintein's equations, the power necessary to bend space and time is equal to the energy of an exploding star. Due to the fact that we will never be able to harness the power of an exploding star, at least not until we can get some great scientific breakthrough in the harnessing of energy, we will never be able to travel through time in any great amounts.
Black Holes are created when stars die. A star is a delicate balance between the explosive power going on inside its core, that wants to blow it up, and the heavy gravity, that wants to press it together into a little ball. As the core gets compressed, it gets super heated and creates a supernova explosion, blowing the star up and all the matter is spewed out into space. After the explosion, the core turns into what we call a Black Hole, the Black Hole's gravity is so strong, not even light can escape. A Black Hole is a rift in space-time, sucking everything in through its event horizon. The Black Hole can warp time in something we call Gravitational Time Dilation, which means that, objects with a lot of mass slows down time and since a Black Hole has an immense amount of mass, it slows down time considerably.
These web links have lots of information on this. A lot of what I have said here comes from these sites aswell.
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