Quantum Field Theory and General Relativity are the most well-established modern fundamental theories of physics. According to these theories, spacetime is a collection of points called 'spacetime locations' where physical events occur. Spacetime is a four-dimensional continuum, with physical time being a distinguished, one-dimensional sub-space of this continuum, but no longer a separate entity nor space: space and time are always taken together as one entity.
In 1908, the mathematician Hermann Minkowski, Einstein's teacher, was the first person to realize that spacetime is more fundamental than time or than space alone. As he put it:
The views of space and time which I wish to lay before you have sprung from the soil of experimental physics, and therein lies their strength. They are radical. Henceforth space by itself and time by itself are doomed to fade away into mere shadows, and only a kind of union of the two will preserve an independent reality.
(Pais 1982: 152)
The metaphysical assumption behind Minkowski's remark is that what is independently real is what does not vary from one reference frame to another. It follows that the division of events into the past ones, the present ones, and the future ones is also not independently real.
In contrary to the classical Newtonian view, time intervals depend greatly on the observer's frame of reference. In classical mechanics, and based on common sense, if the time interval between two lightning flashes is 100 seconds on someone's clock, then the interval also is 100 seconds on your clock, even if you are flying by at an incredible speed. Einstein rejected this piece of common sense in his 1905 special theory of relativity when he declared that the time interval (and the distance) between two events depends on the observer's reference frame. He says that every reference-body has its own particular time; unless we are told the reference-body to which the statement of time refers, there is no meaning in a statement of the time of an event (Einstein 1920: ch. 9). Thus each reference frame (or reference-body) divides spacetime differently into its time part and its space part.
Curved Time and the Big Bang:
In 1922, the Russian physicist Alexander Friedmann predicted from General Relativity that the universe should be expanding. In 1929, Hubble's measurements of the redshift confirmed this prediction. Eventually astronomers concluded that about twelve to fifteen billion years ago the universe was in a state of infinite density and zero size; this is the Big Bang theory referred to earlier in this chapter. So we might ask the simple question 'What was there before the Big Bang?' Astronomers, however, showed that the entire universe, including time and space, was created in the Big Bang, and because of the extremely high density of matter at that instant, the gravitational force was immense and the spacetime was curved, or encapsulated around the point from which the Big Bang was ignited.
In physics and modern philosophy, descriptions of the Big Bang often assume that a first event is also a first instant of time and that spacetime did not exist outside the Big Bang. But it is not clear if it is correct to call the Big Bang an 'event', because events must have space and time coordinates, but spacetime started with the Big Bang itself. However, for the first time in science there is a mathematical description of the ontological relation between time and the universe. We shall see that this description is in good agreement with Ibn ‘Arabî's own views on time (section II.3).
However there are serious difficulties in defending the Big Bang's implications about the universe's beginning. Current theories don't have any claims as to what might have happened before the Planck era (10-43 second after the Big Bang). It is expected that the theory of Quantum Gravity might provide information about that, and it may even allow physicists to speculate on what caused the Big Bang. But until now this area remains solely in the domain of theological and metaphysical speculation.
The Arrow of Time:
Unlike physical space, physical time is inherently directional: it flows in one direction, from the future to the past; this is a necessary truth. In thermodynamics the arrow of time for the world is expressed through what is called the 'entropy', which is a description of the degree of order of a system; a highly ordered system is said to have smaller entropy, and vice versa. The world's entropy is always increasing (i.e., its order is decreasing): this is a free 'one-way ticket', and one has to pay dearly for the return. For example, the process of mixing hot water into cold water to get warm water is never reversed, though in principle we may think of some complicated machine that can do the reverse. The arrow of an irreversible physical process is the way it normally goes, the way it normally unfolds through time.
The problem with the arrow of time is that the variable time is symmetric in most equations of physical laws. This means that if the variable 't' is replaced by its negative '-t' in those laws, the result is still a law; the basic equations are unchanged. Some scientists theorize that the cosmological arrow of time will one day reverse direction when the force of gravity will halt the further expansion of the universe and start a collapse to its initial state, just like a movie played backwards (Price 2002: 19-56).
One of the most fascinating consequences of the theory of relativity is that it allows travel through time, just like one travels through distance. This has been employed by science fiction to produce many interesting films and science fiction stories.
Although reversing the direction of the 'arrow of time' still seem to be experimentally impossible, this happens quite often in dreams and in remembering past events. However, philosophers have been more interested in travel in physical time than in psychological time. On the other hand, we can also 'really' look at the past anytime: by looking at the stars, where we actually see how they have been thousands and millions of years ago when the light that we see now was actually emitted by them. But this is still not like travel in physical time, which is conceivable now only in theoretical cosmology.
Although travel in time is possible and allowed according to the equations of Relativity, in many cases it violates logic and causes obvious paradoxes. There are, however, different types of time travel, some of which are trivial. If you get on a plane on the earth's surface and travel west, you will cross a time zone and instantly go back an hour, but all you have done is to change your reference frame. Also, if your body were quick-frozen in the year 2000 and thawed in 2050, then you would travel forward 50 years in clock time but only a few seconds of your biological time. This is, however, just a case of biological time travel, not a case of physical time travel.
One possible, and more genuine, way to travel through time is to fly at enormous speeds close to the speed of light. Einstein showed that travel forward in physical time is possible relative to the time of those who move more slowly than you. With this kind of relativistic time travel, you can't return to the old present, but you can conceivably be present at the birth of your great-grandchildren. Travel backward in physical time is also possible only if nothing that has happened gets changed. For example, you can't go back in time and prevent your parents from having any children (Arntzenius 2002: 169-200).
Another kind of time travel is caused by the curvature of time due to extreme gravity. If you fell into a black hole, then you'd travel to a time after the end of the universe, as measured in a reference frame tied to earth. Unlike the time travel in science fiction movies, this kind of relativistic time travel to the future is continuous, not abrupt. That is, as you travel to the future, you exist at all intervening times according to the stationary earth clock. You do not suddenly 'poof' into existence in the year 4,500; you existed during their year 4,499, but your spaceship hadn't yet landed.
Going back to the past is probably possible, but there are significant difficulties yet to be overcome before we can be sure. In recent decades, mathematicians and theoretical physicists have described time machines, or at least universes containing backward time travel, that are consistent with Einstein's equations of general relativity. However, Stephen Hawking believes all these time machines are ruled out by the laws of general relativity.
For Ibn ‘Arabî, time travel is possible and easily attainable without any paradoxical consequences. Such time travel, however, has no physical or biological effects on the traveller - see the discussion in section II.7 below.
9.4. Quantum Time:
Regarding the question of 'instants' of time, time being a linear continuum implies that there is a nondenumerable infinity of them. This means that between any two instants there is a third; time is continuous. However, for times shorter than about 10-43 seconds; the so-called Planck time, science has no experimental support that time holds its continuousness. But physicists agree that General Relativity must fail for durations shorter than the Planck time, though they don't know exactly how and what is the substitute.
The idea that space or time (spacetime) could be discrete has been recurring in scientific literature recently, but its origins go back to ancient philosophies. The new concepts brought about by Quantum Mechanics (e.g., the concept of indeterminacy or the uncertainty principle) suggested that spacetime could be also quantized like energy. This was reinforced by the discovery of ultraviolet divergences in Quantum Field Theory (Zee 2003: 145-51), though many of the strange quantum concepts soon became acceptable aspects of continuum physics. In the 1980s, powerful computers inspired some new discrete thinking in physics. Complicated mathematical simulations performed on these super-computers paved the way for Lattice Theories to be applied to Quantum Mechanics, and included Quantum Gravity. In Quantum Gravity, Planck's length is a minimum size beyond which no accurate measurements can be performed.
Hawking, however, sees no reason to abandon the continuum theories that have been so successful. But it may be possible to invent a discrete structure of spacetime without abandoning the continuum theories if the discrete-continuum duality can be resolved, just as the wave-particle duality has been resolved by Quantum Mechanics.
The practical methods of the quantisation of time in modern scientific theories are based on some complicated mathematics such as lattice theories and cellular automata that are beyond the scope of this introduction (Wolfram 2002: 771). But it is good to note here that Ibn ‘Arabî's quantisation of time, as we shall see in section II.8, is unique and is based on a broader cosmological view (of the oneness of being) such that discreteness and continuousness are special cases of it (see also section VII.5).
 This is because of the 'uncertainty (Heisenberg) principle' which states that not all of the physical parameters (e.g. position [x] and momentum [p]) of a system can be fully determined at the same time. It is mathematically expressed as: Dx.Dp>h where h is Planck's constant, which is in the order of 6×10-34 erg-seconds.
 Before the advent of Quantum Mechanics there was a long debate about the nature of light, whether it is particles or waves. Some experiments (and theories) confirmed that it is particles, while others confirmed that it is waves. Quantum Mechanics solved this contradiction by suggesting that particles have wave properties and waves have particle properties. See: Baierlein, R. (1992) Newton to Einstein: The Trail of Light: An Excursion to the Wave-Particle Duality and the Special Theory of Relativity, Cambridge: Cambridge University Press.