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Appendix
Relativity
Relativity is such an important cornerstone to the argument of this book that it deserves an appendix of its own. It is the limiting factor for the exploration of the galaxy, as far as creatures like ourselves are concerned, and it is the barrier that will keep us confined to the solar system. Be that as it may, the solar system is still a huge expanse of space and territory for us to explore. There is the planet Mars which may lend itself to terraforming (making earth—like) and so provide new lands for settlement, and there are many satellites and minor planets which would provide excellent resources, but, outward to the stars, distances are just too vast for us to effectively manage.
Isaac Newton described gravity and laws of motion which are true for most practical purposes, including the third law which allows us to understand how rockets work—to every action there is an equal and opposite reaction. If a rocket thrusts out gas from one end, it will travel in the opposite direction.
Gravity is an attractive force, we believe, although some now think that at extreme intergalactic distances, it may become repulsive. Nonetheless, it is the force which holds us to earth (and earth to us) and which enables stars and planets to be born and to continue to exist. It behaves like light in some respects in that the force is transmitted at the speed of light and also that it obeys the inverse square law. This law stipulates that if the distance to the source is doubled, the strength of the acting force is then a quarter of the original value. For example, if you moved from one hundred thousand miles to two hundred thousand miles from the earth, you would only feel one quarter of earth's gravity pulling you towards it.
In the late 19th century, Newtonian gravity became less satisfactory in explaining the detailed nature of the universe, especially after experiments on the speed of light were undertaken. It seems to be the case that, in a vacuum—i.e. an airless state—the speed of light is constant, irrespective of how fast you are moving or how fast the source of light is moving. For example, if you were standing on planet watching a fast rocket at one tenth the speed of light approach, and in that rocket the pilot was shining a light towards you, the light wouldn't be arriving at 1.1 times the speed of light, it would arrive at the normal constant speed. It doesn't matter if source and destination appear to be moving towards or away from each other at any speeds imaginable, light from source to destination will not exceed or be less than the normal speed of light in a vacuum.
The speed of light is given the symbol ... c
This constant speed of light perplexed Einstein and, after thinking about the consequences of a constant light speed, he formulated his Special Theory of Relativity in 1905. An important part of this theory is the idea that all motion is relative. That is, if in a spaceship watching an asteroid approach, who is to say that the asteroid is not stationary and that the ship is moving, or, they could both be moving. For an observer looking at a spaceship moving at near light velocity towards a star, time dilation appears—i.e. time expands. The mass and length of the spaceship also increase. However, were you to be inside the spaceship, you wouldn't notice these effects.
As mass increases with velocity in line with Einstein's equations, at near light velocity the mass will be very large. If the spaceship could travel at the speed of light, the mass would then appear to be infinite and, if it were infinite, a similarly infinite amount of energy would be required to push it along. Hence, the spaceship cannot travel faster than light. Also, according to the equations at, say, 95% the speed of light, a person inside the ship would experience time pass at only one fifth the rate on earth. Therefore, should a man of thirty five meet a young woman of twenty and the latter complains that he is too old for her, and, subsequently he took off in a spaceship on a round trip of twenty five light years at the above speed, on his return, the young woman would now be over forty six and he would be only just over forty or thereabouts. The tables would be turned!
In 1916, Einstein further developed his ideas with the General Theory of Relativity. This took gravity into the picture and produced the geometry of space-time. Gravitational sources distort space-time and the effect is that objects apparently accelerate towards them. The light from a distant star, for example, if viewed in line of sight with the sun is deviated from a straight line by the sun's gravity. If viewed behind another mass source such as a galaxy, distant objects in the universe can often be observed by a phenomenon called gravitational lensing.
The relativity theories have been experimentally verified on numerous occasions. Subatomic particles in an accelerator or arriving from space at near light velocities apparently “live” longer than they should if moving at slower speeds. It therefore seems more than likely that the light barrier is real and makes interstellar travel a problem for creatures like ourselves.
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