When reading about Einstein's Relativity Theory, you will learn about the "curvature of space". Essentially, what that means is that if you keep on travelling in the same direction through the universe, you will eventually arrive back at your starting point. It would be similar to following the equator all the way round the Earth, except that out in space the curvature is apparently through a "fourth dimension".

This could indeed be true, although no one knows for certain. If so, then there must be a fourth physical dimension (which probably has nothing at all to do with "Time", incidentally). Because we are just three-dimensional creatures (possessing length, width and height) we are not free to travel into or across a fourth physical dimension - if indeed there is one. Of course, this is why we cannot visualise it easily.

To help illustrate the point, Edwin Abbott (several years before Einstein) wrote a book entitled Flatland. It was about two-dimensional creatures who lived entirely within a plane surface. Thus, they possessed only length and width, not height. Some were fat, like circles or hexagons, and others were thin, almost like lines. That was really padding to the story; the most important point was their confinement within the flat surface forming the mini-universe they knew. Thus, they could move north, south, east or westwards, but not up or down out of their plane.

Now imagine those creatures transferred to the surface of a sphere. It would then be possible for them to migrate right round it - arriving back at their starting point despite maintaining what they thought was a straight-line course all the way. With no concept of a third dimension, they would probably be at a complete loss to explain how they had returned.

Even if their globe was too large for circumnavigation, they could still discover that there was something strange about their mini-universe - by drawing circles of ever-increasing radius (which would be like parallels of latitude on Earth). Measuring the circumference of a very large circle - would then yield a value lower than expected, because their calculations would be based on what they thought was its radius (measured along the curved surface of the sphere). The true radius is of course less, lying inside the sphere.

It is possible that we humans are in a comparable position, so if we travelled far enough out into space, we might experience effects like those confronting the hypothetical Flatlanders on their sphere. In scientific terms, our known three-dimensional universe would then be just the surface of a "hypersphere" which curves round through a fourth dimension we cannot see.

Despite all that, sometimes you will read that Time is the fourth dimension, but I personally feel that it is unhelpful and confusing for a lay person to regard it as such. This is because Time seems so fundamentally different from the three known physical dimensions, which are

1. Left-Right
2. Forward-Backward, and
3. Up-Down.

Nevertheless, it is sometimes convenient to write mathematical equations in which "times of occurrence" and "spatial positions" are examined simultaneously: in a way that is like treating Time as a fourth dimension.

It can also be said that when we gaze out into space we are, in a sense, looking back through time, because the light we see from distant stars and galaxies commenced its journey many decades or centuries ago. However, even that does not really give past or future time the same status as "left" or "down".

Actually, there may be a way in which we can take a type of shortcut through time – because it does not always seem to follow common-sense laws. Time does not flow at the same "speed" everywhere: its rate of progression is affected by how quickly an observer happens to be moving. Admittedly, that variation is far too small to be noticed in our everyday lives: even travellers in supersonic jet airliners remain totally unaware of it.

However, if some astronauts decided to rush off at (say) 99.9 per cent of the speed of light (to explore a distant star system), then all clocks aboard their spaceship would run much more slowly than those back on Earth. Furthermore, heartbeats, digestive processes and body metabolisms of all people travelling at that velocity would be retarded too, causing them to age less quickly than those remaining on Earth. To the astronauts, it would seem that their journey had lasted only a few months, but after returning (also at very high speed), they would be amazed to find everybody at home several years older than before.

An obvious question is "Why does time run more slowly for the astronauts, when from their viewpoint it seems that the Earth is streaking away from them?" The answer probably lies in a difference of status. In order to accelerate, the astronauts have to escape from Earth's comparatively large gravity - not the other way round. The much more massive Earth is the "dominant" partner, although in a manner that we do not fully understand. (Incidentally, strong gravity also seems to influence time-flow, tending to counteract the effect of rapid movement).

There is indirect evidence that time flow-rate really does vary with speed of travel: this is indicated by results from synchrotons and cyclotrons. These are machines which accelerate sub-atomic particles to extremely high velocities - and the effect is to prolong their lifetimes quite noticeably.

Links to:

Science & Mathematics correspondence (see item 9)

Astronomy page

Main index

Islamic Astronomy

History and Religion

Other points of view, and more links may be found at http://tetraspace.alkaline.org

In addition, Per Lassen presents an original and interesting way of modelling the visible universe at
http://perso.wanadoo.es/percasso/index.htm