I have sometimes been asked by youngsters to explain the concept of ‘spacetime’ and they also ask why in physics books, it is normally spelt as one word and not as two, i.e. space and time.
In trying to answer a question like this, one does not quite know where to start, because most people, including me, find the whole subject quite perplexing. Professional physicists would probably relish at the idea of explaining the subject, although, from some of my reading, I not so sure!
I find a good way to start is to talk about the motion of the earth, which sometimes comes as a surprise as everything around us seems to be quite static. Appreciating, however, that day must become night, we can then easily understand that the earth rotates counter clockwise on its own axis once every 24 hours with a speed of about 1,670 km per hour, although there are some variations brought about by the ‘not-quite’ spherical shape of the Earth and the point on the earth where the speed is being measured. Appreciating, too, that we have seasons, we know that the earth rotates around the sun and the speed of this rotation is approximately 107,000 km per hour. The sun, around which we are orbiting, however, is also orbiting around the centre of the Milky Way galaxy at a speed of 780,000 km per hour. Since the distance of the earth to the centre of the Milky Way galaxy is 250,000 trillion km, one can calculate that it takes 226 million years to complete one orbit. The fact that according to the famous astronomer Edward Hubble and a whole body of knowledge since his early discovery, the entire universe is expanding, i.e. galaxies are moving away from one another, makes the question of motion in the universe, of which we are part, extremely complex. We are all in a constant state of very rapid motion.
In the 17th century, Isaac Newton founded classical mechanics, on the view that space and time are independent of one another. In his opinion, time passes uniformly without regard to whether anything happens in the world, apart from the way in which they are measured and for this reason spoke about absolute space and absolute time so as to distinguish these entities from the various ways by which we measure them, which he called relative spaces and relative times. In classical terms, space is nothing but an abstraction we use to compare different arrangements of the bodies constituting the universe around us. Time is merely a measure of cycles of change within this world.
Space and time continued to fascinate physicists and astronomers for the next few hundred years. A major seminal event happened in 1905 when Albert Einstein published his Special Theory of Relativity which showed that space and time are both altered near the speed of light. To an observer, distances (space) appear to contract and clocks tick more slowly. This interdependency between space and time became for ever linked near the speed of light and the term spacetime was formed.
Einstein related his Special Theory of Relativity to Maxwell’s theory of dynamic electromagnetic fields. Maxwell showed that light is the passage of electric and magnetic fields traveling through space as waves moving at a fixed and specific speed. Einstein showed that nothing can go faster than the speed of light. He postulated that space and time are delicately interwoven and are constructed in such a way that it is absolutely impossible to break the law, i.e. go faster than the speed of light. The speed of light, which has been derived mathematically from Maxwell’s theory and by actual physical measurement, is 299,792.458 metres per second.
The speed of light is not just something we measure about light, but rather is a fundamental physical constant itself. Indeed, the constancy of the speed of light in space and time is an essential feature of special relativity. Time enters in because relativity treats time on an equal footing with space (hence the term, space-time). Consistent equations cannot be written if the speed of light is taken as to be variable.
From our point of view, time is a measurement of change that takes place in what we call space. The series of changes that happens to you over your lifetime happens over time and in space. The word ‘spacetime’ is our merging of the two concepts into a single concept: three spatial dimensions (height, width and depth) plus the fourth dimension of time. While we have the ability to control our experience of the first three dimensions, we do not seem to have the ability to navigate, manipulate or control our experience of time, even though physics tells us that it is merely a dimension like all the others. For human beings, time seems to be a one-way street with a pretty strict speed limit.