E&T presents our own brief history of time, explaining where it came from, what it is, and where it's going.
Ancient time Determining the unit of time we call the day is pretty simple. It's the interval between two successive sunrises. Several Mediterranean civilisations divided the day into 12 daylight hours and 12 night time hours. The year was also an important span; agriculture and many other social customs depend on knowing the recurrence of the equinoxes, when the period of daylight equals that of night, and the solstices, when the sun is highest or lowest in the sky at noon. But what about other units of time? Several ancient cultures, especially the Babylonian and Chinese, showed great sophistication in measuring longer intervals, such as those marking off lunar and solar eclipses. Even the advance of the equinoxes - the slow, steady shift of the coming of the equinoxes caused by the slewing of Earth's rotational axis, amounting to about one-twelfth of the sky every 2,000 years - was known in antiquity.
The 13th century Chinese astronomer Guo Shoujing measured the mean length of the year as being 365.2425 days, a degree of precision not matched by Europeans until several centuries later. One thing the Europeans did have at about Guo's time was a mechanical clock that kept time with an accuracy of about a minute. Such clocks had been invented to help the monks in monasteries better track their religious observances. The small-bore technology used to make clocks would soon also benefit the development of other machines that helped to speed European industrial development.
Galileo Galilei, one of the shrewdest observers of nature, noticed that the length of time it takes a pendulum to swing back and forth (such as a censer used in church) was independent of how wide the pendulum's swing was; the time depended only on the length of the pendulum. From this he established an accurate watch. Refined by Christiaan Huygens around 1660, the pendulum clock was probably the first with a precision of one second. Indeed, the length of a pendulum whose traversal time is one second is almost exactly one metre, and the metre was defined in this way until something better came along. Thus units for time and space became entangled.
Christiaan Huygens's slightly younger contemporary, Isaac Newton, was about to vastly expand our grasp of time and to use temporal insights in establishing modern science. For his own amusement, Newton had invented the mathematical protocol we now call calculus - a system for both breaking things down into small parts (differential calculus) and assembling small parts into a larger whole (integral calculus). This parsing exercise is keenly useful in understanding motion. Take, for example, a ball rolling at an average rate of one metre per second. It's fair to say that in one-thousandth of a second the ball will have rolled one-thousandth of a metre. In one-millionth of a second the distance covered would have been one millionth of a metre.
Newton's genius (and perhaps also that of Gottfried Leibniz, who independently invented calculus about this time) recognised that, even if in the 17th century you couldn't measure anything as short as a millionth or even a thousandth of a second, you could at least imagine such fine gradations. This allowed you to fathom the idea of an instantaneous velocity - something happening right now. In a manner of speaking, you could say that Newton invented the concept of the present moment. His name for calculus was 'fluxions', since it provided the mathematical ability to capture the snapshot essence of a much-changing world.
A second great conceptual leap forward for time was Newton's application of infinitesimal changes to define firmly concepts such as velocity (change in direction or speed over time), acceleration (change in velocity over time), and force (a mechanism for producing change). And, with these properties in hand, along with a few more such as momentum and mass, Newton brought into existence the science of dynamics, which looks at how forces govern motions. This body of knowledge, enshrined in a series of equations, still largely rules the study of astronomy, physics, fluids, and mechanics. Newton's insights form a large part of the education of physical scientists and engineers to this day.
We're not finished with Newton yet. As the final pillar in his mechanical edifice he argued that time and space were the immutable backdrops for all matter-in-motion. Space was absolute; it was the scaffolding on which events occurred. Defining space here (such as the central meridian at Greenwich) defined it everywhere and for everyone. Time was similarly absolute: it was defined as the 'tick-tock' division of existent intervals and could be calibrated for everyone and for all time by reference to some standard (eventually the convention of Greenwich Mean Time was established).
Many bystanders to the Newtonian revolution, even those who couldn't understand the equations, sensed that a new testament in human knowledge was unfolding. With Newton's laws of momentum, force, and absolute time-space at work, one could determine the 75-year circuit of Halley's Comet. One could also explain the bulge of Earth's midsection. Philosophers, groping to capture the new-found scientific system metaphorically, referred to the cosmos as a mechanical thing. Like the clock that had served the medieval monks, the universe itself seemed to tick forward on the gearwheel-escapement basis of an immense clock.
Arrow of time
Newton had pioneered the dynamics of mechanical things. By the 19th century a new concept had been contrived: energy. The discovery that heat is a form of energy led to a new discipline: energy dynamics. Usually referred to by the name of thermodynamics, this science treats the movement of energy over time. It has two cardinal rules, namely: that energy can be and often is converted from one form to another without the total amount energy in a system changing; and that the ongoing energy conversion at work has a certain bias to it. That is, for a closed system (one not receiving extra energy from outside) the energy available for performing useful work (such as doing computation or heating water) will decrease over time.
Meanwhile, the amount of disorder in the system, now termed entropy, will increase. In other words, the organisation of a system (and this applies not just to machines but also to planets, autos, and human bodies) will decline and will never become greater, without the help of outside energy. This one-way march toward disorder and decay now goes by the name of the arrow of time.
Albert Einstein, who is Newton's rival for creativity in peering into the mysteries of time and space, was famous (like Newton) for undertaking theoretical experiments. For instance, he imagined what it would be like to ride a train zooming along next to a light beam.
He determined that, regardless of how fast the train went, it would never catch up with the light. Furthermore, the speed of that light beam escaping ahead of him would appear no greater to other observers, even those at rest with respect to his speeding train.
Taking this universality of light-speed as his starting point, Einstein deduced that events marked out as having taken place at a specific time and place would seem to have happened at slightly different times and places to different observers moving past each other at a constant speed. To put it another way, the time and space coordinates for an exploding star would be different for astronomers in rocket ships moving relative to each other.
Einstein was arguing that there is no such thing as an absolute time and space. We couldn't say absolutely that the star had blown up at a specific place and time. It would have been different depending on which rocketship was reporting. Not only was Einstein throwing overboard Newton's concept of universal time. For Einstein's relative-time scheme to work he had to marry space and time. Separate (and absolute) coordinate systems of space and time were replaced by an entwined four-dimensional continuum. Time was now to be thought of as a sort of extra dimension, running in either direction, time past and time future.
There is no absolute clock in Einsteinian physics, no Greenwich time reference to be invoked. So, say that in my lab I measure time with my clock. In a rocket coming past at half the speed of light, a scientist measures time with her clock. To me her clock is running slow. This 'time dilation' artefact of special relativity can be measured. In 1971, several high-precision atomic clocks were sent around the world in both directions aboard jetliners. Even travelling at a modest 1,000km/hour speed (a fraction of the speed of light) relative to clocks attached to the Earth, the traveling clocks showed an elapsed time that was different by tens of nanoseconds - exactly what you'd expect from Einstein's equations.
Eleven years after announcing his special theory of relativity - showing how measured time depended on who was measuring time - Einstein put forward his generalised relativity scheme, which took into account how spacetime itself is warped by the presence of heavy objects like stars. In this accounting of the cosmos gravity is not so much a physical force of attraction as it is an underlying distortion of spacetime; a planet is attracted to the Sun not because of some invisible beam but because the environment roundabout the sun is intrinsically warped by the sun's great mass. Like a bowling ball constrained to roll down a gutter, the planet rolls around (orbits) the sun. The warp of spacetime can actually be measured, usually with sophisticated gimbals or other spinning things mounted on spacecraft. The subtle shift of the spinning motion allows you to calculate the warp of space, and the answers agree with relativity-theory predictions.
All of us are involuntary time travellers. Without moving, we float through spacetime. The question of whether we can deliberately move forward in time beyond the normal rate, or even back into the past, is a matter of controversy among physicists. The answer seems to be that such extraordinary time movements are possible, but increasingly difficult for larger time intervals and for larger masses. You would need something immensely heavy - like a black hole with the mass of the Sun - nearby to warp spacetime to a large degree. How this would be done without rending your body is hard to imagine. Even then, as Washington University physicist Clifford Will points out, the warping would only affect the rate of evolution of time but not its direction.
The time dilation effect for astronauts is a special relativity effect; the clock on a speeding spacecraft runs slower than one on the ground. A general relativity effect works in the opposite way; a clock high up in Earth's gravitational field runs faster than one lower down. At relatively low altitudes, where the cosmonaut circled, the time dilation effect is more important, but at higher altitudes, where the Global Positioning Service (GPS) satellites are parked, the gravity effect is larger. According to Clifford Will, the GPS clocks lose about 7µs per day owing to the time-dilation effect but gain about 46µs owing to the high-altitude effect.
The other great 20th century revolution in physics, beyond the relativity one, was the advent of quantum science. Werner Heisenberg, Niels Bohr and others had sought to explain the strange behaviour of light waves and of electrons within atoms. In doing so, however, they had to upend our commonsense notion of matter and greatly qualify the rules of science, even those of Newton, which had held sway for centuries. What Heisenberg and Bohr said was that an electron, at least as we conceive of it as a hard little ball, is mostly a fiction. The electron spends most of its time as a travelling wave or a tenuous cloud, a cloud not of actual stuff but of potentiality of being here or there. The electron would register as a material particle, in the usual sense, only when it was observed, only when it struck a detector. Only then could we say with greater surety that the electron was, or had been, in that place at that time.
Heisenberg went further: there are limits to our ability to measure simultaneously an electron's position and its momentum, or simultaneously its energy and the exact moment of the measurement. The better we knew the electron's energy, the less we could say about the time coordinate for that measurement. If we wanted to pin down the time more precisely, our knowledge of the energy would suffer. Building better detectors would not help. Newton's old dream - the dream that launched modern mechanical science - of imagining infinitesimal time, of measuring motion over ever smaller and smaller intervals, bumps up against Heisenberg's uncertainty principle.
Einstein showed that space and time are not absolute. Instead they are: first, connected; second, relative and dependent on your frame of reference, and; third, warped by the presence of a massive body. Heisenberg, undercutting the idea of classical time further, says that time and energy are not absolute. Measurements of these commodities are connected, relative, and intrinsically fuzzy. Time measurements at the macroscopic level are fairly certain. But at the microscopic level, where quantum considerations become important, time becomes a difficult thing to pin down.
Medieval clockmakers gave us the minute and Huygens' pendulum provided a reliable second. Since then, time has been carved up into ever finer gradations. Atomic clocks at the National Institute for Standards and Technology (NIST) in Boulder, Colorado, currently provide a consistent time count at the femtosecond level or better. That is, the clock puts out a signal - based on the faint radiation coming out of a swarm of atoms kept in a special trap - that is precise at the level that will soon approach one part in a million-million-million (10-18 seconds, or an attosecond). This is as close to Newton's infinitesimal time as we can come - for now anyway.
The current account of cosmo-logy, the 'Big Bang' model, argues that the universe, as we now see it, began about 14 billion years ago in a titanic explosion of spacetime. The temperature and energy density in the earliest moment after the bang - an era referred to as the Planck epoch in honour of quantum pioneer Max Planck - would have been enormous, maybe close to infinite. The history of the universe thereafter would have been one of expanding spacetime, cooling and the partial coalescence of energy, in the form of matter, into the galaxies, stars, planets, and objects we see now.
You can, in the spirit of Newton's infinitesimal dream or Einstein's thought experiment about running beside a light wave, imagine going backwards in time toward the moment of the Big Bang. Many physicists now believe that as you reach the Planck era, around a time of 10-43 seconds out from the bang, the very concept of time breaks down. Compare this to looking at the ocean. From an aeroplane, the surface looks like a continuous fabric, with waves here and there. From a height of a metre or so, you can see small splashes and bubbles; the idea of the ocean's surface being a fabric begins to break down. From a height of a nanometer you begin to see individual water molecules; here the ocean is not fabric but foam. It is particles and chaos. So too with spacetime at the Planck realm. Time becomes foamy.
The start of time
What came before the Big Bang? How did time begin? Did it even begin, or are we residing in some kind of cyclic universe that allows certain events or conditions to recur? These questions are challenging. We don't expect soon to be able to perform laboratory experiments or launch satellites with gimbals that can fully answer cosmological issues. Nevertheless, space borne detectors are being planned to stare at the Big Bang, or at least at the radiation coming across the visible universe from the early epoch, in an effort to better understand the phenomenon. Nobody knows if one can pursue the measurement of time and its effects backwards beyond the curtain of the Big Bang itself, but physicists will try.