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One of the most enthralling aspects of Relativity is its new understanding of time. The term “time dilation” might evoke images of Salvadore Dali’s timepieces hanging on twigs, however, time dilation is all but surrealistic. As stated earlier, if the speed of light is constant, time cannot be constant. In fact, it doesn’t make sense to speak of time as being constant or absolute, when we think of it as one dimension of spacetime. Special Relativity states that time is measured according to the relative velocity of the reference frame it is measured in. Despite of the simplicity of this statement, the relativistic connection between time and space are hard to fathom. There are numerous ways to illustrate this:
The four dimensions of spacetime.
In Relativity the world has four dimensions: three space dimensions and one dimension that is not exactly time but related to time. In fact, it is time multiplied by the square root of -1. Say, you move through one space dimension from point A to point B. When you move to another space coordinate, you automatically cause your position on the time coordinate to change, even if you don’t notice. This causes time to elapse. Of course, you are always travelling through time, but when you travel through space you travel through time by less than you expect. Consider the following example:
Time dilation; the twin paradox.
There are two twin brothers. On their thirtieth birthday, one of the brothers goes on a space journey in a superfast rocket that travels at 99% of the speed of light. The space traveller stays on his journey for precisely one year, whereupon he returns to Earth on his 31st birthday. On Earth, however, seven years have elapsed, so his twin brother is 37 years old at the time of his arrival. This is due to the fact that time is stretched by factor 7 at approx. 99% of the speed of light, which means that in the space traveller’s reference frame, one year is equivalent to seven years on earth. Yet, time appears to have passed normally to both brothers, i.e. both still need five minutes to shave each morning in their respective reference frame.
What happens if an astronaut falls into a black hole?
The gravitational time dilation effect a black hole produces is equal to that of an object moving near the speed of light. For example, an observer far from a black hole would observe time passing extremely slowly for an astronaut falling through the hole’s boundary. In fact, the distant observer would never see the hapless victim actually fall in. His or her time, as measured by the observer, would appear to stand still.
From the perspective of the unlucky astronaut, things would, of course, look quite different. After having passed the black hole’s event horizon, the point in space from which nothing can escape its pull, there is no way back. While approaching the centre, the gravitational pull on the astronaut’s head and feet differs so strongly that the body would be stretched out “like spaghetti” (Stephen Hawking). Hence, it may be a good idea to stay away from black holes, should they actually exist.