Histories of Time
There are two ways we tend to think about time. One has to do with the way we cut up time, so that we can refer to moments of importance, or of interest, or of necessity. This is the topic Leofranc Holford-Strevens explores in his book, A Short History of Time (published by OUP in 2005). The other is the nature of time itself, and the way this has been re-examined in order to see it as a key in helping us understand the nature of the universe. This is the topic of Stephen Hawking’s brief (and exceedingly difficult to understand) book, A Brief History of time. Two approaches to the history of time, and yet they couldn’t be more different. If Holford-Strevens uncovers the various forms of the calendar, the ways in which we slice up the year into weeks and seasons, and other ways of ‘marking the year’, Hawking happily tosses all that aside, and suggests the real issue is the nature of time.
Holford-Strevens is disarmingly honest. In the Preface to his short book includes a quote from St Augustine “So what is time? If no one asks me, I know; if I seek to explain it, I do not.” He continues to make it clear that he is not going to address whether time has a beginning for an end, if it proceeds in a straight line or in cycles, nor is he going to delve into the idea that time is the fourth dimension of the universe. He explains that he will “concentrate on the methods by which its passage is and has been measured”, the way the ‘man-in-the-street’ might consider it (pages ix-x). It is for that reason that Chapter 1 focusses on ‘the day’, the period of time determined by the rotation of the earth.
A day: so simple and clear a concept. However, like most things, it isn’t quite as clear as you might think. After all, when does a day ‘begin’? Is it determined by daylight (which, inconveniently gradually appears at different times in many parts of the world, especially the further we are away from the Equator). In most senses, of course, a day is determined by agreement not a physical sign, a day which Holford-Strevens describes as a ‘civil day’. Given the changing time at which the sun rises and sinks, it isn’t surprising to discover that for many societies and at many times, the day would begin when the sun was at its highest point, at noon. That has the incidental benefit that nightly observations by sailors and astronomers are all doing so during ‘the one day’, given our rather inconvenient separation of the hours before and after midnight as belonging to two separate but consecutive days. However, we seem to be happy with the convention that the day extends through the daylight hours, and the change from one day to another occurs during the night. With advantages in either direction, the determination of when a day begins is clearly a matter of convention.
If convention has shaped our views of days, what about hours? Holford-Strevens reveals that the idea there were 12 daytime and twelve night hours can be traced back to the ancient Egyptians. However, the length of each days varied through the seasons, but they had defined there were twelve hours of daylight in the summer and in the winter – in other worlds, daylight hours were long in the summer. He reveals that was common practice in Europe up until the later part of the Middle Ages, and hence various references to twelve hours in the day. Incidentally, a mid-day rest is often referred to as a ‘siesta’, which happens to be the old Spanish word for sixth …. Once the day was defined as beginning in the middle of the night, that led to another convention to be established, which is whether the hour after noon is the 13th hour, or if you start again, and distinguish it as 1 pm!
It goes without saying, once we get past hours, things get far more complicated. In Byzantine Greek times, the hour was divided in 5 leptá, each leptá into 4 stigmaí, and each stigmé into either 2 rhopaí (or 1½ minutes), 3 endeixeis (1 minute), or 12 rhipaí (15 sedond interbals), and each rhipé was 10 átomata. In the Medieval Latin period each hour was divided into 4 puncta, and each punctum was 2 ½ minuta: a minuta was 6 minutes in our time scale. However, there was an alternative where there were 5 puncta per hour, 2 minuta per punctum, and each minutum could be broken down into 4 momenta (1 ½ mutes) or 6 ostenta (1 minute), each momentum into 12 unciae (7 ½ seconds), and each uncia was 47 or 54 atoimi. Then there was the Hebrew calendar, where each hour had 1080 hâlãqîm (parts), and each heleq had 76 rega’îm (moments). Confused? So were the users, and variations were common.
Then came clocks, and now there was a need to displace apparent solar time with mean solar time (the time shown on a clock). The two can vary by as much as 10 minutes over the course of the years (with the greatest variations late February and late November in the UK. By the Eighteenth Century time, standardisation was becoming a key issue, and when solar time became the legal definition of time, as it did in the UK in 1792, variations still occurred as a function of the local meridian. Eventually, Greenwich Mean Time was adopted in the UK, and in 1880 enshrined in a statute. However, Holford-Strevens adds “So completely has local time been forgotten so that the practice still observed at Christ Church, Oxford, that one is not late until five minutes past the appointed time, that is to say till one is late by local mean solar time (longitude 1° 15′ W of Greenwich), is a tradition regarded even in other Oxford colleges as no more than an amiable eccentricity.”
Almost there, but not quite. Thinking about time in the Nineteenth Century, countries were busy standardising their time across their regions. However, that left unresolved one other question, which was how to standardise time between countries, and, in particular, around the globe. In 1884, an International Median Conference in Washington, DC, adopted a US proposal that the prime meridian ((0°) should pass through the “centre of the transit instrument at the Observatory of Greenwich”. This has remained the case since then, although the French persisted for some years in showing 0° as passing through Paris. In the end, they agreed, but with the concession that another French proposal be adopted, that researchers use decimal measurements of angles and times. Almost finished, but for one final twist which was that time zones should be along the lines of meridians – but with some exceptions. Iceland wanted to use Greenwich mean time, with France and Spain one hour ahead of it (but not Portugal!); China and India imposed a single time zone of their huge territories, but Russia accepted having time zones spread over 11 zones.
If all of that was resolved, one puzzle remained. This was when a date changed. The convention was established that “an eastbound traveller crossing the meridian 180° east of Greenwich needs to give back the gained day, a westward-bound traveller to regain a lost day: ships therefore repeat the day when eastward bound, and suppress a day when westward bound” (and the same for air travellers – a real issue for those with watches that show dates!). He goes on to discuss the tricky issue of the year being slight longer than 365 days, with conventions of leap years, atomic clocks and the like. However, despite all the tiny adjustments, the 20th Century seemed to have sorted out most time matters for travellers.
Ah, but only ‘most’ time matters! That takes us to Stephen Hawking’s book A Brief History of Time. Published in 1988, it takes us into hitherto unimaginable twists and turns in the story of time. In just 13 pages, Hawking takes us through Holford-Strevens history. In that first chapter, having arrived at the generally agreed theories of time up to the 1930s, he ends by suggesting that it is very difficult to draw together all the threads of science to offer a single theory that describes ‘the whole universe’. Instead, he takes us past Holford Strevens summary, and into the strange world of time as it is being examined in the 21st Century. He suggests there are two basic but partial theories that confront us. “The general theory of relativity describes the force of gravity and the large-scale structure of the universe, that is, the structure on scales from only a few miles to as large as a million million million, million (1 with twenty-four zeros after it) miles, the size of the observable universe. Quantum mechanics, on the other hand, deals with phenomena on extremely small scales, such as a millionth of a millionth of an inch. Unfortunately, however, these two theories are known to be inconsistent with each other – they cannot both be correct”. He might have added they also push to one side the nice story about time that Holford Strevens had written.
Hawking did start with familiar ground, reminding the reader about Aristotle, Galileo and Newton, before moving on to the 19th Century. This was when great discoveries about the nature of light and the speed of light were made. There was a snag, which was that, however you measured it, and in whatever direction, light travelled at a fixed speed, irrespective of whether the observer was at rest or moving. It was the Mitchelson-Morley experiment that presented us with this puzzle, and which remained unsolved for 28 years. Then in 1905 Einstein presented his theory of relativity, of which the fundamental point was that the laws of science should be the same for all ‘freely moving observers, no matter what their speed’.
As with so many other revolutionary theories, the implications of Eistein’s theory were, to put it simply, astonishing. In particular, it dispensed with the idea of absolute time. This was illustrated by some challenging observations (even though the proof was to come many years later). For example, two observers, one on top of a mountain and one at the bottom, might compare the performance of the clock each possesses (they are assumed to be very accurate). The clock nearer the centre of the earth would run more slowly than the one at the top of the mountain. Sounds slightly crazy, but it is true, and in the age of satellites it is very important: given that time runs faster above the earth as compared to on the surface. Calculating the position of the satellite would be inaccurate if you assumed time runs at the same rate for both, and such predictions of the satellite’s position would be wrong by several miles.
At the same time as the implications of Einstein’s theory were being considered, another challenge to our view of the universe emerged. This was the result of the work of Edwin Hubble. The starting point for this was consideration of the well-known Doppler effect. If you are driving along the road, with an emergency vehicle coming towards you, the siren is at a higher frequency than when it has passed and is travelling away. Realising this was true for light waves as well as sound waves, Hubble found, by measuring the shift in the spectra of galaxies, that most appeared to be moving away from us, and the further away the galaxy, the faster it was moving away. This revealed that the universe is expanding!
However, that has led to yet another extraordinary observation. If the universe is expanding, then there must be a point in the past when everything was closer together. Indeed, there must be a point, some 13.8 billion when the universe began, supposedly from a very small, hot, and dense state, one from which it has been expanding and cooling ever since. This theory agrees with several pieces of evidence, including the abundance of light elements in the universe, and the existence of what is known as cosmic microwave background radiation. But if we accept this, we’re left with yet another puzzle: what, if anything, existed before the “Big Bang” which is considered the beginning of space and time?
Hawking goes on to describe some other findings from the physical sciences. These include the ‘uncertainty principle’, the continuing arguments about the nature and number of elementary particles, and the almost inconceivable topic of ‘black holes’. However, having covered these, A Brief History of Time ends on two even more challenging topics: the fate of the universe and the ‘arrow of time’.
In his chapter on the ‘Origin and Fate of the Universe’., Hawking quickly explains views known as the ‘standard model’, which are that the universe started as a very small, hot and immensely dense object, nearly 14 billion years ago, and started to expand very rapidly – the so called Big Bang. As it expanded, it began to cool, and from that point on many observations at distant (and hence very early) objects have been the basis of a relatively robust model of what happened all the way from those first few thousand years after the Big Bang and change began. However, Hawking points out there are some challenges:
- Why was the early universe so hot?
- Why is the universe so uniform on a large scale, as appears to be the case when you are looking at points of space in every possible direction?
- How did the universe expand at the rate it did – if the rate of expansion had been smaller (by even one part in one hundred thousand million million), it would have collapsed?
- Although the universe is so uniform at the large scale, there are many local irregularities (stars, galaxies), and so we need to explain how these emerged.
- Finally and not on Hawking’s list, we might add a fifth puzzle: what was there before this time? That’s a question for lay readers, even if it doesn’t bother astrophysicists.
Hawking asks that we think of the anthropic principle: “we see the universe the way it is because we exist’. He distinguishes two versions of this. The weak anthropic principle is that the conditions for intelligent life will only be met in some regions of the universe, the inhabitants of these regions should not be surprised if they observe their locality meets these conditions. You might say we exist because we were in the right place at the right time. If that isn’t odd enough, the ‘strong’ anthropic principle proposes there are many different universes, or many different regions of a single universe, each with its own configuration, and possibly its own set of laws of science. We happen to be in one of those places. If we weren’t, we wouldn’t exist.
How can all this make sense? One dominant idea is that the early universe might have gone through a period of very rapid expansion. Very rapid? In an early iteration of this model it was suggested that the radius of the universe might have increased one million, million million million times in a fraction of a second. More to the point, it has been hypothesised that there was a point of singularity at the beginning of the universe, where all the laws of science as we know them were not in place. If that isn’t enough to give a non-scientist a headache, a further element is Einstein’s idea that “the gravitational field is represented by curved space time: particles try to follow the nearest thing to a straight path in curved space, but because space-time is not flat their paths appear to be bent, as if by a gravitational field.” Indeed, Hawking goes on, “time is imaginary and indistinguishable from directions in space”. As if that wasn’t enough, Hawking goes on to postulate that “space and time may form a closed surface without boundary … but if the universe is really self-contained, having no boundary or edge, it would have neither beginning nor end: it would simply be.”
As Hawking remarks in the last chapter of his Brief History “We find ourselves in a bewildering world. We want to make sense of what we see around us and to ask: What is the nature of the universe? What is our place in it and where did it and we come from? Why is it the way it is?” This is the desire to find a ‘unified theory’. So far, that eluded us.
