Black holes, wormholes and time travel. If a person was to look up the three most intriguing words in all of physics, these would be it. What is most interesting, and satisfying, is that there is an inherent connection behind all these three phenomena. And much of it is not plain rambling, but has a lot of theoretical support.
To understand all these concepts, one needs to understand a few basics of the General Relativity theory. One of the premises of this theory is that space and time are one entity, closely meshed together. Space-time, as it is called, can be visualized like a large, outstretched rubber sheet. Gravity, which is exerted by all bodies, is then visualized as the bending effect of some object lying on that sheet. This visualization is endorsed by most of the scientific community.
Black holes are the final stage of a dying star. Stars, which sustain themselves and their light with a continuous process of nuclear fusion, finally exhaust their fuel reserves of hydrogen (or some other light element). Normally, the energy of the fusion is enough to prevent gravity from pulling all the material together. As the fuel ends, though, gravity wins, and the star undergoes an implosion. For most stars, this is the end of life. But the larger stars, which had a lot of fusionable material to begin with, undergo a supernova explosion, and begin the process of fusion again, albeit with heavier elements. The largest stars can undergo this cycle multiple times, ending each time with a final product that is denser every time. And of even these, only the behemoths ever have a chance of attaining black hole status.
A black hole occurs when this cycle of rebirth reaches the end point in a body so dense, that gravity pulls it into itself. A gravity of a black hole is so strong, not even photons of light can escape it. Think of it like this: our rubber sheet is so bent by the black hole that even a little marble rolling further away will spiral into the dent. This then, is a black hole.
At the heart of a black hole is a point of space-time called a singularity. This is a point of infinite density, and thus, infinite mass. This is not truly infinite, of course, but large enough to be classified so. It is at this point that a wormhole may occur.
Wormholes are tunnels to other points in space time. As a quirk to the formation of a singularity, our rubber sheet may be bent such that it touches some other part of the rubber sheet. If these two touching points of the space-time continuum were to be joined together, you would essentially enter space-time at one point, and emerge instantaneously at another. This, then, is a wormhole.
The speed of light cannot be exceeded. Light travels at about 3 million meters per second, and is a constant that determines all other phenomena, including time. If you were to travel at, or very close to, the speed light, you would essentially lift yourself out of the 'space' part of space time and into the 'time' part. Time would work normally for you, but to others, it would appear as if your time has 'frozen'. As an example, if you were to travel at, say, ninety-nine hundredths of the speed of light, for 5 of your years, and then stop, you would probably find that people around you have aged 50 years. One further thing to note is that it is theoretically impossible to travel 'back' in time.
The connection between black holes and wormholes has already been demonstrated a few paragraphs earlier. But wormholes are also related to time travel. Normally, not all the material in the galaxy is enough fuel to propel even one person at the speed of light. This is because 'relativistic mass' of the all things increases as speed increase i.e. the faster you are, the harder it is to go even faster. Wormholes, however, offer an alternate method to travelling so fast.
As a very simple and not necessarily accurate example, speed is distance divided by time. If the distance increases, and time decreases, speed increases. Let us assume you could throw off the effects of gravity that occur near singularities, and were to travel down a black hole. You would, in effect, have travelled some 'distance' (because you came out at another point in space-time) and nearly zero time. Thus, in a way of saying, you broached the speed of light. And in doing so, you time-travelled.
Black holes, wormholes and time travel are fascinating concepts. This is more so, given that we will probably never experience them in our lifetimes. But they are ever present. And ever will they continue to intrigue us.