Once Before Time - Martin Bojowald [132]
In this fashion the pre-Socratics anticipated many a question in cosmology that was made answerable only millennia later by modern physics, for instance by the work of Einstein on Brownian motion. But one step the pre-Socratic philosophers had not yet undertaken was to question the unlimited divisibility of space and time themselves. For this, the radical change of view engendered by general relativity was required, attributing a physical role to space-time.
Otherwise, one can find among the pre-Socratics most of the elements of modern cosmology. Only with quantum gravity did truly new elements enter the game. For instance, it poses the possibility of limits to the divisibility of space and time on a scientific basis, and from it derives cyclic worldviews by preventing the singularities of general relativity. The large-scale happening is determined by interplays between the classical attraction of gravity and quantum gravitational repulsion at small volume. As a qualitative feature this consequence, compared to pre-Socratic pictures, is not at all new. What is new here is the atomic structure of space and time as the physical cause; this was not discussed by the pre-Socratics. Above all, quantum gravity makes possible explicit, ever more detailed calculations; they not only further embellish the general picture and may one day make it empirically testable, but occasionally lead to the discovery of completely new phenomena. The most striking example of an essentially new ingredient is perhaps cosmic forgetfulness, leading to a mixture of linear and cyclic worldviews in which the end of every cycle is seen as something like a new beginning: a world foam-born in quantum fluctuations, a clean, fresh, virgin slate rather than the charred, barren, forlorn wasteland of Heraclitus’ conflagration.
PHYSICAL COSMOLOGY: SOCIETAL CYCLES
Science entered into the business of cosmic worldviews relatively late, induced by nearly simultaneous progress in the observations of distant stars and galaxies as well as in the theoretical foundation presented by general relativity. Just after Einstein’s equations were formulated, Einstein himself, followed by Willem de Sitter in 1917 and later Alexander Friedmann and Georges Lemaître, found simple solutions describing the temporal evolution of a universe that is isotropic on large scales. At that time, nothing was known of the expansion of the universe, and so scientists were primarily looking for solutions with a potential to correspond to a static collection of masses. But curiously enough, this was extremely difficult, and it became possible only with Einstein’s improvisational introduction of an additional term in the equations whose size is determined by the cosmological constant.
At about the same time, astronomers such as Vesto Slipher started to notice that most of the stellar objects called nebulae—in contrast to stars, slightly diffuse—appeared to be moving away from us. One did not see the motion itself, but the nebulae’s light systematically showed an increased redshift compared to that of stars. In relativity, this is traced back to an outward motion of the emitting object; here we have the analogue of the Doppler effect in acoustics, which makes the siren of a departing ambulance sound different from that of an approaching one or one at rest. Similarly, the frequency of a departing light source moves toward the red end of the spectrum. The reason for such a fleeing motion of nebulae was, however, unclear, for why should there be forces acting only on the nebulae but not on other stars? And why, one may innocently ask, should those forces move the nebulae away from us?
In 1929 the riddle was solved by Edwin Hubble, who was able to use precise astronomical measurements to determine the distances of nebulae from Earth. As in more recent observations of the accelerated expansion