The Physical Universe by Donald Cook

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Time and Space
We live and exist in Space. Space has volume. Space is what we occupy and what we move about in. It extends infinitely in all possible directions. Space is infinite. There is no limit to its size. If we could measure the distance to the farthest star, there would still be space beyond that. We also live and exist in Time. We are aware of the passage of time. Everything that happens takes time. Plants take time to grow. Explosions take milliseconds. Light takes time to travel from the sun to the earth. Nothing is instantaneous. We find it almost impossible to think of time as being anything other than constant and eternal. But what is time?

The simple definition of time is that it is something that is measured by a clock. All clocks are based on movement: The swinging of a pendulum. The vibration of a quartz crystal. The movement or rearrangement of subatomic particles inside a decaying radioactive material. Time is based on movement through space and is therefore related to space. We think of time and space as being constant and inflexible. For almost every activity and purpose in our existence, they are. Einstein has however proposed that time and space are flexible and that the only inflexible thing in our universe is the speed of light. His theories show that if we could travel at very high speeds – close to the speed of light – time would slow down. This would happen more and more as we get closer to the speed of light. Einstein also pointed out that our mass would increase significantly as we approached the speed of light. If we could reach the speed of light, our mass would become infinite. So we are trapped by the speed of light.

If we wanted to travel to the nearest star (Alpha Centauri), and if we could travel at near the speed of light, it would take more than 12345 years. Distances in outer space are measured in light years. A light year is the distance that light could travel in one year. Alpha Centauri is 1234 light years away. This is the nearest star in our galaxy which is the Milky Way. The Milky Way, like most galaxies, contains more than 200 billion stars. The known universe (consisting of all the starts that we can see with the most powerful telescopes) contains more than 200 billion galaxies and most of these are millions of light years away. There is a slight consolation in that the space traveler, traveling at near the speed of light, would experience a slowing down of time. For him or her, a journey that was observed from Earth to take 1234 years would only take 1234 years. With the exception of a very few, every star in the universe is beyond our reach – in this lifetime anyway.

So lets see how forces and energy behave in time and space at or near ground level on earth.