Although the competing Relational and Absolute views of time presented conflicting outlooks on the true nature of time, they agreed on the sequence in which events occurred and the association of particular times with particular events.
Einstein (1879-1987) radically changed our understanding of time measurement. The Theory of Relativity was a total re-examination and replacement of the Newtonian physical laws of motion. Basic to this theory of motion was the realization that there is no object in the universe that can serve as a stationary reference point acting as a standard for measurement. All measurements of motion are made relative to other objects – hence the theory of relativity.
According to Relativity, time is an aspect of the universe (in agreement with the relational view of time) but it is possible for measurements of time to differ between observers. We now have the introduction of an observer into our understanding of time – because it is the observer that gives us our frame of reference for measurement. This marked a major change in thinking.
Space-time (time at a distance)
Newton regarded space and time as independent entities and assumed that time would be the same for every person and object in the universe. For Einstein all events in the universe were defined in four dimensions (three of space and one of time). This is not counter to common sense because we generally define an event by when and where it took place. Since space and time are measured together (distances in space being defined in terms of time and the speed of light as light-years) scientists now speak of space-time, or the space-time continuum (which describes the geometry of the universe – sometimes called the 4-D manifold, which is mathematician Riemann’s term for space). A point in space is also a point in time, called a world-point. Matter then traces out world-lines. If world lines intersect then objects have collided. A point in space is an event in space-time. It is important to note that under this theory time and space are interwoven physically, not just metaphorically: space shrinks as time expands.
No matter how strongly our common sense tells us time and space are separate, the objective world of physics recognizes only a 4-dimensional continuum that is neither time nor space. Different observers may not agree on particular measurements of space and time but they can agree on suitable combinations of the two.
It is not possible to take equal-moment slices through space-time in an absolute and universal way. Each individual will have a different slicing (see Block Universe). We might note in passing that space, time and space-time do not have an intrinsic metric. That is, the units we use for space and time are purely of convenience and convention. We should therefore beware of equating geometry and chronology.
Light is at the very core of the theory of Relativity because it is the signal used to measure space and time across the universe.
Two important properties of light are critical for Relativity. Firstly, its speed is finite and calculable. Secondly, it is absolute (the same regardless of the relative velocity of any observer).
Historically, it would seem that as light emerged as a physical absolute, time became relative and Einstein convincingly wove time into the very fabric of the universe.
Philosophically it in important to note that what is independently real is what does not vary from one reference frame to another and that can only be achieved through space-time. Space-time interval measurements between events provide the scientifically invariant fact independent of one’s own state of motion.
The speed of light
The speed of light was first determined in 1675 by the Danish astronomer A. Roemer from observations he made of the eclipses of the moons of Jupiter. The discovery that the speed of light was finite (not instantaneous or infinite as Newton believed) was of profound importance because we use light to take many measurements, notably those of outer space.
In 1887 Americans Albert Michelson and Edward Morley carried out experiments comparing the speed of light while varying the speeds of the points of observation. To their surprise it was found that the speed of light was always the same, being constant at 300,000 km/s. Contrary to intuition, the speed of light is not relative to the speeds of observer and source but is the same for all observers whatever their relative motion. As a result of these experiments the speed of light is now recognized as a universal constant – a physical absolute.
This discovery also had other far-reaching and counter-intuitive implications that emerged when Einstein showed in his Theory of Relativity that, relative to an observer, objects nearing the speed of light approach infinite mass, their length shrinks towards zero, and time expands towards infinity. It should be noted, however, that the effects (time dilation, size diminution, and mass increase) only become noticeable at speeds approaching the speed of light; for everyday situations the mathematics of Newton’s laws of motion are perfectly adequate.
Since we observe events through the medium of light and light has a finite speed it is immediately evident that we do not observe events instantaneously. Someone close to an event will see that event before someone who is further away. The light from the Moon takes a half second to reach Earth, and from the Sun it takes 8.3 minutes. The time scale of a particular observer is generally referred to as proper time, defined as the time that would be given by a clock at rest with respect to a particular observer. This is often compared with coordinate time which is the time which an observer assigns to a distant event, knowing its distance away, the speed of the signal (usually light), and the proper time at which it was observed.
Wave structure of light
Paradoxically light displays the characters of both particles and waves.
Although the speed of light is constant regardless of the motion of the observer, its wave structure varies according to the relative velocities of the observer and object.
Wave frequencies increase with increase in the speed at which objects approach each other, and decrease with decreasing relative speeds. A longer wavelength produces a reddening or red shift, and a shortening wavelength produces a bluing or blue shift of light. This is the Doppler Effect – where the number of wave crests per second arriving from a receding source is fewer than those actually emitted from the source (this also occurs with sound waves, which lower in pitch as an object moves away). Only particles with zero mass at rest can travel at the speed of light and the photon can only travel at the speed of light. As the energy of a photon decreases, its wavelength increases. For example, light loses energy in a gravitational field producing a red shift.
A colour shift is a natural clock, being the interval between successive crests of a light wave. In 1929 the American astronomer Hubble observed a red shift in the universe that indicated clearly that it was undergoing rapid expansion in all directions. The background galaxies of the universe are receding from one-another at a rate that is proportional to their distances from one-another as measured by the red shift and the most distant galaxies appear to be moving away from us at close to the speed of light.
This was compelling evidence for the origin of the universe in a Big Bang.
The Special Theory of Relativity
Einstein’s Special Theory of Relativity (STR) of 1905 dealt with unaccelerated frames – those under uniform motion – referred to as an inertial frame of reference.
As we have seen, the time interval measured in a reference frame in which a clock is at rest is called the proper time. Einstein showed mathematically that if two observers are moving at constant velocity relative to each other, it appears to each that the time on the other’s clock would be slowed down, a phenomenon known as time dilation. This is so because there is no preferred frame of reference. The first direct test of time dilation occurred in 1941, 36 years after Einstein’s theory, and it confirmed his theory. Though Relativity runs counter to common sense its conclusions are now backed up by abundant experimental evidence.
[M – But what produces this effect? Is it actual or apparent due to the fact that light takes time to travel distances? Because of the peculiar behaviour of light waves and also because light has a finite speed and therefore takes time to travel large distances.]
One obvious outcome of the STR is that there is a multiplicity of time frames in the universe associated with different observers.
It is important to remember that the STR deals with uniform motion.
Imagine that an astronaut, A, left Earth travelling close to the speed of light, returning in several years to meet his twin, B, who remained on Earth. Under STR it would appear to both that the time of the other person would be slowed down. Each would think that they would be the younger when they eventually meet – which is not possible in reality. This situation is referred to as the “twins paradox”. In fact, astronaut A would be the younger. While the Earthling B may be considered in an inertial frame, the astronaut A has undergone acceleration and deceleration and is therefore to be treated under General Relativity (see below). The two frames of reference are not equivalent.
The General Theory of Relativity
The General Theory of Relativity (GTR) of 1916 dealt with accelerated observers – those experiencing gravitation (which produces the same effects as acceleration and deceleration). The faster an object moves, the slower time passes for someone observing that object. At the speed of light it would appear to an observer that the object had frozen still.
Einstein found that gravity also has an effect on time which passes more slowly on larger masses. One second on Earth is equal to 1.0000025 s on the Sun, equivalent to a difference of 1 second in 6 days.
The GTR demonstrated the way light rays are deflected by gravitational masses. Einstein came to realize that it was incorrect to think of gravitation as causing a distortion or warping of time – it was a warping of time or, more accurately space-time. The geometry of space-time is often represented as a trampoline-like grid with the gravity of solid objects puckering its surface, the bends in the lines being the bending of light towards sources of gravity. Space-time does not therefore operate in straight lines (Euclidian geometry) but exhibits curvature where mass exerts a gravitational pull on light (Riemannian geometry). Gravity is not a field of force but a curvature in the geometry of space-time, an effect known as gravitational lensing. Physicists now prefer to say that bodies do not “feel” gravitational forces but respond to the curvature of space-time in their vicinity. It should be pointed out that STR has had wider recognition and experimental verification than GTR.
STR established space and time as the inextricably linked space-time. GTR demonstrated space-time acting on and being acted on by matter.
Earth is responsible for 2 time warps, which tend to cancel one-another – one due to the Earth’s rotation, the other to its gravity. The higher up you go, the faster time runs relative to clocks lower down.
It is sobering to know that from a pulsar, which has a “gravitational field” a billion times stronger than the Earth, time is slowed by 20% relative to the Earth – so the Earth would appear only 3.5 (not 5) billion years old.
Both Newton and Einstein regarded the universe as being in more or less the same state throughout its history.
In contrast to both Absolute Time and Relational Time we can list the features of Relative time as follows:
• The duration of an event is dependent on the observer (frame of reference)
• Simultaneity of events is dependent on the observer (frame of reference)
• Duration will vary with relative motion and gravity
• The metric of time is intrinsic to the temporal interval. That is, the units of time are the same everywhere (it flows “equably”) Time passes at different relative rates
• Whether time is infinite or not depends on the existence or non-existence of events (a physical universe)
SubstantivilismBSTANTIVALISM (SUBSTANTIALISM) VS RELATIONALISM
During the 18th and 19th centuries the Absolute view of time held sway, to be taken over in the 20th century by Relationalism. With the advent of Relativity in the 20th century the old Absolute vs Relational controversy changed to accommodate modern physics. Since time over vast distances could only be measured using space, the distinct concepts of time and space became superceded by the notion of space-time.
Spacetime as substance
Continuous physical fields are now recognized as physical entities; they are distributed throughout space so there are no unoccupied locations – there is no empty space. Space-time is a field that not only acts upon material objects but is also acted upon by them. The new interpretation of space-time was that it is substance-like, or substantival, acting like a container in which events occur. It is a system of space-time points in which events are located and, like a container, it exists distinctly from its contents. This is Minkowskian space-time in the case of STR which acts like a container. Riemannian space-time of GTR posits that the curvature of space-time as determined by the matter present, and it, in turn, determines how bodies will move. Here the substantivalist can no longer claim that the “container” is independent of what it contains. It is significant that although space-time interacts with the matter in the universe, it is nevertheless distinct from it: in this sense it may be said to be Absolute and the old Absolute-Relational distinction dissolves away.
It should be noted that the word “relativity” refers to the covariance of spatial and temporal intervals; it does not imply that it is the relations of material objects that are physically significant.
If it is argued that only material objects truly exist then the debate remains relevant and unresolved.
The problem of inertia
Newton’s Law of Inertia states that a body remains at rest or continues in uniform motion in a straight line (or “right” line as he called it) unless acted upon by an external force. Such a statement begs the question of motion in relation to what? This cannot be answered by referring to local bodies such as the Earth’s spinning surface (objects themselves generally in non-uniform motion) – his answer was absolute space and time. This is distinct from an absolute state of rest which Newton acknowledged was a matter of convention. [The inertial centre of gravity is the centre of the universe…]
• The problem is acceleration in general and rotation in particular. It is expressed most dramatically through a series of real situations and thought experiments:
• Is the bulge at the centre of the Earth due to its rotation relative to anything? Would it be there if the universe rotated around the Earth and the Earth was still (if that makes sense)? Would the bulge steadily decrease if matter were gradually removed from the universe?
• Imagine two balls in space attached to one another by a piece of string, then rotate them about their axis. The string will be taught but they will not be rotating relative to one-another. If the balls are like ballooons filled with water and one is rotated about its axis will the centre bulge? If so is this due to its rotation relative to the rest of the universe. If the matter in the universe were gradualy removed would the bulge decrease?
• Are the centrifugal, centripetal and coriolus forces intrinsic or extrinsic (or neither or both)?
Thus Newton postulated immovable space (as he called it) as the centre of the system of the universe. This did not change for Einstein who stated himself that space-time is endowed with physical qualities that enable it to establish the local inertial frames, but “the idea of motion may not be applied to it”.
For the purposes of dynamical analysis, motion must be referred to an absolute background metric of rectilinear inertial coordinates rather than to relations between (local) material bodies, or even classical fields. We cannot infer everything important about an object’s state of motion simply from its distances from nearby objects. In this sense both Newtonian and relativistic physics find it necessary to invoke absolute space. It seems that position and velocity are relative, but accelerations have absolute significance independent of the relations between material bodies (at least locally).
In developing his GTR Einstein hoped that the field equations represented true relationalism but he was to find out, to his dismay, from Schwartzchild that there is a preferred coordinate system. GTR correlates the relations between objects (including fields) and the absolute background metric. This metric is affected by, but not determined by, the distribution of objects. So space-time is itself an absolute entity exerting influence on fields and material bodies.
The prevailing ethos of the time was relationalist and proponents like Mach anticipated that space-time, like Absolute space, could be eliminated by denying that it has any physical significance. Mach believed that the average state of motion of matter in the universe counted as the basic inertial field – what he called the distant stars. However, it is now acknowledged that this basic inertial field cannot be attributed to the distribution of matter and energy in space (the distant stars), we simply have to assume a plausible absolute inertial background field.
[Local inertia is determined by geodesic motion in the local region which is a function of the local space-time. Local geometry is affected by the space-time geometry of the universe. Local inertia is the result of an imposed deflection from geodesic motion through local space-time whose character is determined by the distribution of matter throughout the universe. The dynamic state of everything in its totality, determines the framework in which any particular thing moves, and the inertial effects that arise from that motion.]
A dense mass will cause a depression of space-time, (rotational) motion drags space-time in the direction of rotation. This is called frame-dragging (Lense-Thirring effect, gravitomagnetism).
Absolute and Relative time each have major arguments to confront:
If time is the relation between things how do we account for the reality of the “moving” now with its imprint on, not only the limited lives of biological organisms, but also the evolution and ageing of the entire physical universe. The argument that now is a peculiarity of human perception or in some way illusory is difficult to sustain. In other words now is not a special relation between human beings and the world – now is a real and objective aspect of the universe: it would exist in the universe whether humans were present or not. [is this what R claims???] There is also the problem of duration – events have duration that seems real.
For Absolute time there is the problem that we can have no Absolute measure of time itself. If the Earth were to suddenly rotate at twice the speed (this being our natural clock), apart from a whole range of physical effects there would be obvious discrepancies in relation to our biological and other clocks. If atomic clocks sped up or slowed down, for whatever reason, we would be aware of the problem in relation to other measuring devices. All our measurement of time is in relation to something else. But if Absolute time exists independently of these clocks (which are only measuring time in relation to one-another), then we are entitled to ask what it would be like for the universe and/or our clocks, and/or ourselves, if Absolute Time itself were to speed up or slow down. Could we measure its speed in any way? If we answer that there would be no noticeable difference then the question of the relevance and, indeed, the existence of Absolute Time arises again. Can we know if time of the past, relative to time of today passed more quickly or more slowly? Since we know that time passes more slowly in a dense place such as the Sun relative to us, then it seems safe to assume that time passed at differing rates as the universe evolved.
Could physical time be unreal? Many philosophers through the ages have thought so.
There are other philosophers who claim just the opposite. Physical, measured time, lies firmly in the objective world (see also objectivity of time). Newton’s time was precisely measurable. Einstein’s time was precisely measurable but had to take into account relative motion and gravity. Einstein placed time strongly within the physical world and, it seems, completely removed any transcendental associations. Time was not “just there”, or necessarily linked to “events” but part of the very physical fabric of the universe. Time is now the most accurately measured physical parameter.
It should be emphasized that in speaking of the relativity of time, it really is the time that is relative, not our experience of it, nor the operations of the measuring devices we use.
Space-time is a measurable physical parameter, present in the universe even, we assume, in the absence of humans. In this sense it exists as an objective reality.
The paradoxes of time (see, for example, McTaggart’s paradox) and intangible nature of time has persuaded many philosophers that it is not real. This is certainly contrary to common sense and our scientific theories, many of which have time at their core.
[And yet physicists use time as a key concept that they regard as being at the very bedrock of the objective universe and its laws (Davies, 1995). Einstein established that time was very much a part of the objective physical world. In hardly needs to be added that it is, in many ways, the most basic aspect of our subjective inner world too.
For physical time we can simply accept time as the latest accepted definition of t given by physicists.]
However, this leaves ample room for debate about the nature of its existence. We can hardly doubt that it exists, how it exists is another matter.