Become a Time Lord - and a Space Lord - with our simple guide to the interplay between the dimensions.

Space and time are just as important to the structure of the Universe as the planets, stars and galaxies. Before the twentieth century, it was believed that space and time just provided the passive unchanging background against which the real cosmic drama of storms and stars was played out. It was also felt that the distinction between space and time was simple and absolute.

Different observers might choose different ways of describing the location of points in space or moments in time. By using different maps or clocks for example, or by choosing different units of measurement, such as miles rather than kilometres - but no one really doubted that each observer was describing the same space and the same time. That simple view of an absolute distinction between space and time was utterly destroyed by Einstein’s theory of relativity, one of the greatest scientific developments of all time.

The special theory of relativity concerns relationships between measurements made by pairs of observers who are in relative motion. It’s called the special theory of relativity because it's restricted to a special type of relative motion in which one of the observers moves with constant speed in a fixed direction relative to the other. The special theory shows that when two observers share this so called uniform relative motion, each will find that a clock carried by the other runs slow (an effect called time dilation), and each will find that an object carried by the other will shorten along the direction of the relative motion (an effect called length contraction). According to the theory these phenomena are not the result of any peculiarity in the construction of the clocks or the composition of the objects, rather they are indications that time and space themselves are different for the two observers. The clocks and objects simply serve to reveal those differences.

Shortly after the publication of the special theory of relativity (1905), the mathematician Hermann Minkowski suggested a new way of thinking about space and time that took into account the lack of absolute distinctions that Einstein’s work had revealed. Minkowski’s idea makes use of the concept of space-time, which represents a sort of ‘fusion’ or ‘union’ of space and time in that it consists of all the positions and times at which events might occur. Space-time, unlike space and time, was common to both observers, even if they described it differently. Hence the new fangled space-time was more fundamental than the older notion of an absolutely separate and distinct space and time.

Although the publication of Einstein’s special theory of relativity led to the introduction of space-time, this was only the starting point for the revolution in thinking about space and time that the theory of relativity finally brought about. The major change came about ten years later with the publication of Einstein’s general theory of relativity. This theory generalises Einstein’s earlier work by dispensing with the restriction to observers in uniform relative motion. It provides a way of relating measurements made by observers in a state of general relative motion.

Such observers might, for example, be accelerating or rotating relative to each other. Einstein realised at an early stage during his development of the general theory that there is some degree of equivalence between an observer who is being accelerated and one who is subject to the effects of gravitation.Think of your own experience of being accelerated upwards in a lift or elevator, as the elevator starts to rise you momentarily feel heavier than usual; the acceleration is equivalent to an increase in the strength of gravity. Such observations convinced Einstein that his general theory of relativity would also provide a new theory of gravity. This realisation led Einstein to the discovery of an important link between space, time and gravitation.

According to general relativity, the space-time of special relativity - the common space-time of unaccelerated observers - is also the kind of space-time that exists in the absence of gravity. In the presence of gravity, in the vicinity of a massive object such as the Earth, say, space-time becomes distorted or ‘curved’. Because of this, moving objects will behave differently in the presence of such massive bodies. The curvature of space-time causes the moving object to behave as though it is being pulled towards the massive body by some kind of force. This, in essence, is Einstein’s explanation of gravity: there is not really any such thing as a gravitational ‘force’, simply the appearance of a force due to the response of moving objects to the distortion of space time. Near the Earth, the curvature of space-time is fairly slight, so Newton’s seventeenth century theory of gravity is sufficiently accurate for most purposes. But where space-time curvature is much greater, near to a black hole for instance, the shortcomings of Newton’s theory become obvious and Einstein’s theory has to be used in its place. The highly schematic diagram shows space-time curvature near the Sun, indicating the way in which this can lead to the bending of starlight as it grazes the edge of the Sun.

Einstein’s theories are open to a variety of tests. In the case of special relativity, for instance, it is possible to put very accurate atomic clocks on board commercial airliners as they fly around the world and check whether they show the differences in flight time that the theory predicts their high-speed journeys will cause. They do. In the case of general relativity, one classic test involves the observation of starlight passing close to the edge of the Sun (this can be seen during a total eclipse).

According to the theory the paths of such rays should be bent due to the space-time distortion produced by the Sun. It is. A more everyday check on the correctness of general relativity is provided by the operation of the Global Positioning System. The GPS allows anyone with a suitable receiver to pinpoint their location on Earth by analysing the differences in radio signals received from a network of orbiting satellites. The accuracy of the system is such that relativistic effects have to be taken into account in order for the analysis to provide.

In 1917, just a year after the theory of general relativity was published, Einstein suggested that the theory might be applied on a cosmic scale to describe the space and time of the Universe as a whole. According to general relativity, the overall distribution and movement of the contents of the Universe (i.e. the distribution of energy and momentum, and the flow of momentum from place to place) should determine the large scale properties of space and time.

In particular, it should determine whether space is infinite or not, and whether the Universe, which is currently expanding, will go on expanding forever, or whether it will eventually recollapse. The idea of applying general relativity on such a large scale is a bold one, but it has been very fruitful. In particular it has spawned the subject of relativistic cosmology, which is largely concerned with the study of mathematical models of the Universe based on simple assumptions about the contents of the Universe and their behaviour.

Quite apart from determining the ultimate fate of the Universe, relativistic cosmology also provides insight into the origin and evolution of the Universe. The most popular cosmological models all predict that our Universe has evolved over the past twelve or fourteen billion years from an initial state that was hotter and denser and expanding more rapidly than it is now.

The early stages of that expansion are described by the big bang theory, which implies that a hot dense universe, in which matter initially takes the form of a soup of elementary particles (protons, neutrons, electrons and the like) will eventually give rise to a matter-dominated Universe in which about twelve out of every thirteen atoms will be hydrogen, and most of the remainder will be helium - just the kind of Universe we find ourselves living in today. The big bang theory also predicts that since the formation of the first atoms, about 300 000 years after the start of the cosmic expansion, the remainder of the radiation that once dominated the Universe has been cooling and expanding, increasing its wavelength and thus becoming the cosmic microwave background radiation (CMBR) that we can observe coming very uniformly from all directions in space. This explanation of the CMBR is widely regarded as the most convincing evidence in favour of the big bang, and it is detailed investigations of the small-scale unevenness of the CMBR that is expected to settle many of the outstanding questions about space and time over the years ahead.

Of course, even the apparent success of the big bang theory and the most successful of observational programmes over the next decade will still leave many cosmological questions unanswered. Was there really, as many cosmologists now suggest, an inflationary epoch of ultra-rapid expansion, very close to the beginning of the big bang, that drove some parts of the Universe so far apart that they are only now coming back into view of one another? Is it really true that a great deal of matter, perhaps 99% of all the matter in the Universe, takes the form of non-luminous dark matter currently detectable only through its gravitational influence on visible forms of matter? Is there also some strange dark energy in space, causing the Universe to increase its rate of expansion? And, perhaps above all, where did the hot, dense state that initiated the big bang come from? Can it really be the case that origin of space and time lies in some quantum cosmology born by fusing together the quantum physics of the very small and the relativistic physics of the whole of space and time? Some or all of these questions seem likely to persist irrespective of the discoveries ahead. Only time (and space) will tell which.