Seeing Further (50 page)

T
HE ROYAL SOCIETY IS 350 YEARS OLD, AND STILL GOING. SCIENCE, TOO, WILL GO ON. HOW LONG FOR? WELL, ANSWERING THAT QUESTION NEEDS A PROPER UNDERSTANDING OF TIME – SOMETHING WHICH, AS GREGORY BENFORD EXPLAINS, REMAINS ELUSIVE AFTER ALL THESE YEARS.

When the Royal Society began, time seemed a simple, obvious subject, understood since ancient ages. To Isaac Newton and his colleagues, two long-standing traditions pervaded the idea of time.

The Greeks, like most ancient cultures, saw their world as not completely chaotic, though it could be capricious. Faith in a definite order in nature promised that it could be understood by human reasoning. To them, some physical processes, at least, had a hidden mathematical basis, and they sought to build a model of reality based on arithmetical and geometrical principles.

Adding to this Western tradition was the Judaic worldview, which had a timeline. God created the universe at some definite moment, arriving fresh and with a fixed set of laws. The Jews thought that the universe unfolds in a sequence running forward, which we now call linear time. Creation enabled evolution, which led forward in linear time to a future we could quite possibly change. This differed greatly from most other ancient cultures, which favoured cosmic cycles, probably by generalising from the march of the year’s seasons. In cyclic time everything ends, but eventually returns, so there is eternal recurrence.

These two ideas, time’s arrow vs. time’s cycle, persist today in physics and also emerge in our art and literature. Physics has constrained time, ordering the music, but the dance between these linear and cyclic views continues.

Four hundred years ago, Europeans assumed a God-created universe that unfolded in orderly ways, in linear time, but that did not mean that the universe always had to be as we see it now. Change was possible, but constrained by physical laws. Einstein once remarked that what most interested him was whether God had any choice in his creation. The Abrahamic religious tradition answered with a resounding
yes.
Further, they insisted on nature’s rationality, aided by mathematical principles. These were the only cultures to do so. This driving idea eventually altered the concept of time itself, as the cultural agenda played out in modern science.

Time has two faces.

First, our sense of it passing seems inevitable, an automatic intuition. Unlike space, in which we can move back and forth, time hammers on relentlessly. This is Intuitive Time.

Second, we frame our position in time, our historical era, by looking at our slowly changing landscapes, and our societies. These alter on the scale that we ourselves see as we age. This is Historical Time.

Both these faces appeal, but they deceive us.

In the 1700s, the philosopher Immanuel Kant saw space and time as elements of a systematic mental framework, structuring our experiences. Spatial measurements tell us how far apart objects are, and temporal measurements show how far apart events occur. This eventually intersected Charles Darwin’s idea that many abilities of organisms emerge from evolution by natural selection. Then it follows that time and space are the concepts we and other animals evolved to make the best use of the natural world. In this sense, they emerged from the primordial world where our minds evolved.

But that was not enough. Modern science reveals that time is supple, changeable, and even enigmatic. Further, we stand in a small slice of it, anchored in a moving moment that is an infinitesimal wedge compared with what has gone before, or will come after us. Our telescopes tell us of immensities of space, but other sciences – geology, biology, cosmology – speak of even grander scales of time.

Space and time are so familiar that we forget that they underlie the entire intricate and beautiful structure of scientific theory and philosophy. Perhaps it is not surprising that our first powerful theories built on assumed bedrock, metaphysical intuitions, came to be questioned only later. Clocks in Newton’s universe ran everywhere the same. He invoked ‘absolute, true and mathematical time’ saying that it ‘of itself, and from its own nature, flows equably without relation to anything external, and by another name
is called duration’. Of the immense expanse of past time Newton had no true idea, for he took as gospel the Genesis story. Space was similarly absolute. Newton avoided the colossal scale of space by supposing that God had fixed up the cosmos so that gravity, the force he was the first to quantify, had not made it collapse – at least, so far.

This view held up well until the nineteenth century. By then even atheist scientists had faith in a lawlike order of nature – not from philosophy, but because it worked. Though this assumption springs from an essentially theological worldview, it gave useful predictions without a god attached. Still, few saw the full implications of regarding time as a subject of study, not belief.

The first collision between religious views and the study of the far past, which we now call Deep Time, came with the newborn science of geology. In 1830, the geologist Charles Lyell proposed that the features of Earth perpetually changed, eroding and re-forming continuously, at a roughly constant rate. This challenged traditional views of a static Earth with rare, intermittent catastrophes. In the eighteenth and nineteenth centuries the vast depth of the eras before humans arose became apparent, through development in geology and evolution’s grand perspective. These still had to be licensed by physics, the more secure and quantitative science which sets the stage for the events and processes probed by the other sciences.

When William Smith and Sir Charles Lyell first recognised that rock strata represented successive long eras, they could estimate timescales only very imprecisely, since rates of geologic change varied greatly. Even these early attempts got the sciences into trouble. Creationists, reasoning from the Bible, had been proposing dates of around six or seven thousand years for the age of the Earth based on the Bible. Early geologists suggested millions of years for geologic periods, with some even suggesting a virtually infinite age for the Earth. Geologists and palaeontologists constructed geologic history based on the relative positions of different strata and fossils, estimating the timescales based on studying rates of various kinds of weathering, erosion, sedimentation and lithification. The ages of assorted rock strata and the age of the Earth were hotly debated. In 1862, the physicist William Thomson, whose authority endured – as Lord Kelvin and President of the Royal Society – until the end of the century, set the age of Earth at between 24 million and 400 million years. He assumed that Earth began as a completely molten ball of rock, then calculated how long it took to cool to its present temperature. He did not know of the ongoing heat source from radioactive decay.

Physicists had more prestige, but even then, geologists doubted such a short age for Earth. Biologists could accept that Earth might have a finite age, but even 100 million years seemed much too short for evolution to have yielded such complex plenty. Charles Darwin argued that even 400 million years did not seem long enough.

Until the discovery of radioactivity in 1896, and the development of its geological applications through radiometric dating during the first half of the twentieth century (pioneered by geologists), there were no precise absolute datings of rocks.

Radioactivity introduced another measuring clock. Geologists quickly realised this upset the assumptions used before. They re-examined their estimates. This moved the age into the billions (thousands of millions) of years, sweeping away Archbishop Ussher’s biblically inspired dating of Creation to 4004 BC.

Much public ferment paralleled this scientific research and its clash with religion. But by the early twentieth century, opinion settled on an Earth older than a billion years.

Physics, meanwhile, was making hash of the simple view of time that underlay the other sciences. Geology, biology and astronomy would have been happy with Newtonian time, giving them a simple marker of change. The physicists, though, worried about more basic matters.

In physics, time is, like length, mass and charge, a fundamental quantity – intuitive, given by our basic perceptions. Newton used this view, holding that
‘I do not define time, space, place and motion, as being well known to all’
– i.e., obvious. But Einstein showed that it was not.

Nineteenth-century physicists felt that space was the most basic and irreducible of all things. It persisted while time changed, and points made up space – infinitesimal grains close-packed. Einstein’s fundamental insight was that space and time, which appear so different to us, are in fact linked. He argued this using
gedanken
(thought) experiments involving rulers and clocks. These were not just instruments to Einstein; he took them to generate space and time, since they represent it.

He took two basic assumptions. First, the speed of light seems the same to everyone in the universe, whether moving or sunk deep in a gravitational well. This may strike us as odd, but an earlier experiment had found it to be so. Not that Einstein cared; his intuition led him to the conclusion. He proved it valid by using the even deeper second assumption: that the laws of physics had to treat all states of motion on the same footing.

Combining these two assumptions generates the equations of his Special Theory of Relativity. It has astonishing consequences. Moving objects experience a slower passage of time. This is known as time dilation. These transformations are only valid for two frames at
constant
relative velocity. Naïvely applying them to other situations gives rise to such puzzles as the famous twin paradox.

This is a thought experiment in special relativity, in which a twin makes a journey into space in a fast rocket, returning home to find he has aged less than his identical twin, who stayed on Earth. This result appears puzzling because the laws of physics should exhibit symmetry. Since either twin sees the other twin as travelling, each should see the other ageing more slowly. How can an absolute effect (one twin really does age less) come from a relative motion? Hence it is called a paradox.