The word "time" covers a dozen different meanings or, more correctly, different categories of relationship. We have to distinguish between mathematical time, sidereal time, solar time, local time, physical time, physiological time, psychological time, and so on. And the latter two are different in every individual, as local time is different from place to place. An hour in the life of a child is an infinitely longer time measure than in the life of a grown-up, because the life-rhythm of a child goes at a much faster pace than that of an adult or an aged person.
Just as the time-sense changes subjectively and with age, both for physiological as well as for psychological reasons in a similar way the nature of time varies according to the objects considered by our mind. The time that we observe in nature has no separate existence. It is only a mode of being of concrete objects.
The differences both in the meaning and in the individual perception of time, as well as its inseparability from the specific nature of any existing object characterise it as an essentially fuzzy, that is, escaping any precise definition, category.
We ourselves have created the concept of mathematical time as a mental construct, which is indispensable to building up of science, conventionally comparing it to a straight line, each successive instant being represented by a point.
The mathematical time has been used as a basis to the commonsensical idea of time that prevails in the West: we are moving in a single linear direction and we never again go back over the course that we have followed. Time is considered as a characteristic of our motion. If we move with a constant speed, time is expressed as a quotient of the distance we have passed to this speed. Of course, it only looks so simple. In order to explain what 'constant speed' is, we need time. So the commonsensical definition of time is self-reflexive: time is defined through itself.
Using earth rotation, people have created time-measuring unit. Again this creation is self-reflexive, as far as time has been already used when characterising the earth rotation.
The fact that in our experience time is observed to always flow inexorably only from past to present to future in real life is known as "time's arrow." ( See Richard Morris, Time's Arrows: Scientific Attitudes Toward Time, Simon and Shuster,1985).
There are several ways physicists delineate time's arrow. The Psychological Arrow is our subjective sense of time, the fact that we remember events in one direction of time, but not the other. The Electromagnetic arrow of time is described by retarded solutions of Maxwell's equations, not advanced ones. The Cosmological Arrow of time sees the history of the universe moving forward in time in an irreversible manner. There is an overall classification of "time's arrow" referred to as the Thermodynamic Arrow. This involves the Second Law of Thermodynamics - the observed behaviour that all the universe progresses from order to disorder as a net function. One current debate in cosmology is whether the thermodynamic arrow aroses from boundary conditions prior to the Big Bang, or whether it developed early in the Big Bang expansion. Time's Arrow is an important but mysterious property of the world we live in.
In 1890 the great French mathematician Henry Poincare - the chaos pioneer who discovered chaotic patterns in the dynamics of planets' interaction, proved his 'Recurrence Theorem': systems return infinitely close to their initial positions in the state space. For systems that vary its state continuously, the interval between arbitrary close returns is not fixed but can vary. If the interval is fixed, the system is periodic, if not - it is chaotic. (Most systems which evolve under Newtonian laws are not periodic, but chaotic, which means that their evolution is unstable: a tiny changes in the initial conditions results in an enormous change in systems behaviour.) Poincare's Recurrence Theorem strongly supports the idea of time reversibility.
The time scale used in general relativity goes from minus infinity (initial singularity) to plus infinity (final singularity). In such time scale the universe has always existed and always will. By accepting that the force of gravity can be both attractive and repulsive, one can picture the universe as periodically contracting or expanding; the sequence of contractions and expansions repeat ad infinitum. In such universe time can move in both direction.
In quantum physics, the state of a system is described by a function of space and time called the wave function. It has been proved that the wave function can return 'arbitrary close' to its previous state. In other words, quantum systems can move back in time. As all physical systems are also quantum systems, there is no prohibition for time to flow backwards.
The concepts of 'blackholes' and 'wormholes' offer the possibility of time travel; the wormholes connect not only two regions of space but also two regions of time. By journeying through a wormhole, one could travel between the two regions faster than a beam of light would be able to, if it moved through normal space-time.
The travel back in time generates the paradox of changing the flow of events and thus contradicting to an actual present situation. According to Michio Kaku, the attempts to add the quantum theory to gravity (and hence create a 'theory of everything') provides insights into this paradox. The quantum theory allows multiple states of any object to exist in parallel, for example, an electron can exist simultaneously in different orbits - a fact, which is of enormous importance in chemistry. "By going back in time and altering the past, one creates a parallel universe. (So we are changing someone else's past by saving, for example, Abram Lincoln from being assassinated at the Ford Theatre, but our Lincoln is still dead). In this way, the 'river of time' forks into two separate rivers." (M. Kaku: Is time travel possible?)
Chaos theory illustrates the idea of time redundancy: as far as we cannot predict long-term behaviour of chaotic processes (and they appear as ubiquitous in nature), time is no more relevant. Any linearly constructed world crucially depends on time; the chaotic world of nature breaths and unfolds out of time.
In chaotic dynamics, time 'rolls' into the fractal structure of the strange attractors: events which follow one-after-another self-organized within-one-another. Thus time dissolves into fractality of the chaotic phenomena and processes.
The very concept of time denotes a relationship either between a subject and an object or between the different constituents of an existing or assumed system of complexly interwoven and inseparably correlated things or forces.
We think we live within time; but it is time that lives within us. We are born as seeds, as potentialities. When there are suitable conditions, these potentialities unfold. It is this unfoldment that we express using the idea of time.
The process of unfoldment of inner potential is inherent to any existential form; its manifestation differs for different forms. There is no universal measure of this unfoldment, so time cannot be use as such measure.
Time is only NOW. We think in progression (in time) only to help ourselves to grasp the idea of unfoldment or self-realisation of potentiality imbedded in the existential forms. What we call "future" is only a possibility, inherent in the direction of our movement. Thinking of "future" as something real creates the spectre of death, or annihilation of existence, or a passionate desire of immortality.
If we are able to comprehend the one-ness (wholeness) of the existence, the one-ness (wholeness) of outer and the inner nature, the one-ness (wholeness) of all existential forms, then the concept of time loses its meaning. Then we are able to see that simultaneity is the way reality manifests itself.
Each unanimated or living being comprises an inner motion, a succession of states, a rhythm that is its very own. Such motion (rhythm) is characterised by an inherent time.
Time is imbedded in the innermost rhythm of our conscious existence, which appears outside of ourselves as space and materialises in the form of our physical body; the body appears as a physical crystallisation of our consciousness.
If space represents the possibility of movement, time is the actuality of the realisation of movement; we can say that space is an externalised, 'objectivated' time, time projected outward. Time, on the other hand, is the internalised, 'subjectivated' space - the remembrance and inner transformation of spatial movement into the feeling of duration or continuity. Hence time and space are related to each other like the inside and the outside of the same thing. Reality comprises both and simultaneously goes beyond both of them.
The less we move (outwardly or inwardly), the more we are aware of time. The more we move ourselves, the less we are aware of time. A person who is mentally or bodily inactive feels time as a burden, while one who is active hardly notices the passage of time.
Advanced yoga practitioners say that when moving and living in harmony with the innermost rhythm of one's being (which, according to the yoga scriptures, reflects the rhythm of the universe), one's life becomes timeless in the sense of not experiencing time. On the contrary, those who move and live in disharmony with their inner rhythm, have only a momentary existence, that is, an existence without integrity and meaning.
What is called 'eternal' in the most of the spiritual writings is not an indefinite duration of time (which is a mere thought-construction, unrelated to any experience) but the experience of 'timelessness'.
It depends on the psychological nature of time that we create through the inner rhythm of our life and the depth of our consciousness, whether we are mortal or 'immortal'. Those who live in harmony with the 'pulse of eternity' identify themselves with what is immortal; they know that the whole of eternity is within themselves.
The temporal asymmetry is deeply ingrained in our thinking. The reason for this is because we consider change and time inseparable. We become older, therefore time moves from past to future. And vice versa - because time moves from past to future, we become older. So we can't help but judging about the flow of time from a position which is rooted in time.
Could we try to look at time from a position which is out of time? Price called this 'view from nowhen' (Price, H., Time's Arrow and Archimedes' Point, New York: Oxford University Press, 1996 ). According to Price, from such a position, the notion of 'time flow' becomes senseless: if time flows, we must be able to define how fast it flows: second per second?, minute per minute? - such 'dimensionless' answers do not seem sensible.
Is there any 'objective' reason to take for granted that the positive time axis lies toward what we call the future? Answers like "because the hands of our clocks move clockwise" or "because the sun 'moves' from east to west" cannot provide a serious grounding for such a convention. None of the basic principles and laws of classical or quantum physics indicate any basis for the common belief that time flows only in one direction - the physical laws work equally well in both temporal directions.
Even the notion of 'now' is observer-dependent - according to Einstein's theory of special relativity, 'now' doesn't happen simultaneously for two spatially separated observers. So 'now' cannot be accepted as an objective category. 'Now' is dependent on an observer's point of view in much the same way that 'here' is.
Price considers time irreversibility as an illusion - "a kind of artefact of the particular perspective we humans have on time". But the majority of scientists and philosophers continue to describe phenomena in terms of the direction of time of commonsense experience, as if all causal influences exist and act only in that strongly linear direction.
Fractals are similar structures - they demonstrate their similarity at different scale levels. Life events do not exactly repeat themselves, they are similar.
When we speak about time reversibility, we apply the approach of fractals - while saying that history repeats itself, we emphasise similarities in the events occurring at different levels of time dimension of social complexity.
The fractal approach helps us to map the scenarios of the living systems. These scenarios reveal chaotic attractors of systems' behaviour. As far as the structure of chaotic attractor is fractal, when we see what changes are likely to manifest at one level of its structure, the approach of fractals helps us to construct a picture or a map of what is likely to occur at another level. As complexity of fractals increases when we 'zoom' deeper into the fractal structure, it is easier to reveal changes occurring at a higher (more general) level of system's description structure and then to 'project' them onto patterns of change possible to occur at the lower (more detailed) levels of description.
The ability of clairvoyant people to predict the future of chaotic and, therefore, inherently unpredictable systems could be related to the fractality of time.
The mistake made by all those who consider time irreversible is that they automatically agree that interacting systems are not correlated before they interact. This is the case when demonstrating some chemical reaction or a physical experiment. In the reality of life, where everything relates to everything, systems are correlated long before we see them involved in direct interaction.
Nature simply does not tolerate non-correlated systems. A cell when isolated immediately put into action its genetically programmed mechanism for committing a suicide (a phenomenon called apoprosis). Life is entirely permeated by an all-embracing connectivity (interdependence and intercorrelation) of the phenomena, processes and systems manifested in the universe.
Reality represents an integrated entity of which time is only one of many other dimensions introduced by humans to understand and deal with complexity of their existence. The reality is not something dropped into time (when our universe was born) with a special purpose to see it changeable.
In its unity reality remains within itself - all that 'was', 'is' and 'will be' is included in this unity. What we see as an apple seed contains simultaneously its present form (it 'is' a seed), its past form (this seed 'was' inside an apple belonging to an apple tree) and its future (it 'will be' an apple tree when suitable conditions repeat).
In order to sustain its unity, reality relies on itself. In order to explain this unity, reality refers to itself. This self-referentiality is a crucial factor for understanding changes which happen with reality. Changes do not happen because of time but because they reveal the way reality exists.
When reality refers to itself (reflects itself), the internal changes manifest themselves outward. That is why we speak about dynamic nature of reality and about the changes we see, study and live with, and not because time is irreversible.
Using the term "autopoietic" ("auto" = self, "poiesis"= create) coined by Maturana and Varela (Maturana, H. and F. Varela, Autopoiesis and Cognition: the Realization of the Living, Boston Studies in the Philosophy of Science [Cohen, R. and M. Wartofsky (eds)], vol. 42, Dordecht: D. Reidel Publ. Co., 1980), when describing the ability of an organism to self-create and reproduce its self-organization in an inseparable structural coupling with the environment, we call the time in the reality which self-creates, reflects and refers upon itself autopoietic time. Such is the time dimension for people who create their own lives in a self-awareness and full responsibility for what they think, feel and act.