As you read this sentence, you probably think that the present moment, now, corresponds to what is happening. You feel that the present moment has something special. It is real. You can remember the past or anticipate the future, but you live in the present. Of course, the moment you read that first sentence is out of date. The moment you read it replaced it. In other words, we have the feeling of a flow of time. Our deepest intuition is that the future is open until it becomes the present, and the past is fixed. As time passes, this structure of fixed past, immediate present and open future shifts in one direction, always the same. This structure is inscribed in our language, our thoughts and our behavior.
Yet, however natural this conception may be, science does not reflect it. The equations of physics do not tell us what events are happening right now; we can compare these equations to a map where the symbol "You are here" is absent. Moreover, Albert Einstein's theories of relativity suggest not only that there is not a single particular present, but that all moments are also real.
The divergence between the scientific understanding of time and the intuition we have been concerned with has long preoccupied thinkers. It has only increased in proportion as the physicists stripped the time of most of the attributes with which we usually dress it. Today the gulf between the time of physics and the time of human experience reaches its logical conclusion: many theorists have come to believe that, basically, time does not even exist.
The idea of the inexistence of time is so amazing that it is difficult to see how it could be coherent. Everything we do is anchored in time. The world is a series of events connected with each other by the sons of time. Anyone can see that my hair is graying, objects move, and so on. We observe change, which corresponds to variations of properties with respect to time. Without time, the world would be motionless. But how can a theory without time explain why we observe changes?
Time seen as an emerging concept
Even if time does not exist at the fundamental level, it can appear at higher levels in the same way that a table is solid whereas it is only an assembly of particles constituted, for the most part, of empty space. Strength is a collective, or emergent, property of particles. Time too could be an emerging property of the elemental ingredients of the world.
This emerging time concept is potentially revolutionary. Einstein asserted that the key stage in the development of the theory of relativity had been to rethink time. At a time when theorists are pursuing their ambition to unite general relativity with quantum physics, many believe that without a thorough reflection on time, it will be impossible to progress.
The intuitive idea we have of time has experienced a succession of setbacks as the progress of physics progresses. Let us begin with the period of classical physics, called Newtonian. Newton's laws of motion imply that time has a number of characteristics.
All observers generally agree on the order in which events take place. Whatever the moment and place where an event occurs, classical physics assumes that one can objectively say whether it took place before, after or at the same time as any other event. Time therefore permits us to order completely all the events of the universe. Simultaneity is an absolute property, independent of the observer. In addition, the time must be continuous so that speed and acceleration can be defined.
Classical time must also be endowed with a notion of duration allowing to quantify what separates the events in time. To say that a cheetah can run at 110 kilometers per hour, we must have a measure of what an hour is. And just like the order of events, duration is independent of the observer in Newtonian physics.
For the most part, Newton therefore assumed that the world is equipped with a master clock. Newtonian physics listens to the ticking of this clock and no other. Newton thought moreover that time flows and that this flow defines an arrow indicating the future; but these additional features are not strictly required by the new laws
