The scientific approach
It is a lot easier to tackle space than time. According to Aristotle, “time allows the measuring of different types of change or movement (within space)”. However, change and movement refer to temporality and, consequently, to time. Moreover, the different definitions of time most often refer to time.
According to Aristotle again, space is defined by the relationships between objects. For Isaac Newton on the contrary, objects are located in space. In other words, space constitutes an entity which is structured independently of the objects located in.
- In Isaac Newton's “Mathematical Principles of Natural Philosophy” (1687), space and time are independent of each other and constitute the framework in which physical bodies are moving.
- In the works of Albert Einstein, space and time are inter linked and define a frame known as space-time where time is treated as a fourth dimension complementing the three spatial dimensions:
- In the “special theory of relativity” where frames are moving relatively with a rectilinear and uniform motion (1905), space-time constitutes the framework in which physical bodies are moving;
- In the “general theory of relativity” extended to frames moving relatively with accelerated motion (1915), the gravitational field created by the physical bodies curves space-time and space-time in turn influences the movement of bodies. This interrelation between space-time and physical bodies removes the distinction between framework (or container) and bodies (or content).
In Einstein's eyes, space was no longer a reference framework. The relationships between fields associated with waves of any nature (gravitational, electromagnetic etc.) constituted the space as such.
Knowing that space and time are related, it is relevant to wonder if time is still necessary to describe this movement. Certain physicists think that time is only an intermediary variable facilitating the determination of the spatial position of bodies just as money facilitates the exchange of products. Time is not essential since there are two ways to characterize, for example, the movement of the planets of the solar system:
- While indicating the position of each planet at any instant t;
- While determining the position of each planet for each position of the sun.
These two approaches describe the same movement, but the time variable t is eliminated in the second one.
The current works, notably in the framework of the unification of the “general theory of relativity” and “quantum physics”, open the way to such a perspective. In the “quantum gravity” theory for instance, the notions of space and time are only relational. They correspond to the relationships between the various states of the studied “objects”.
If time is no longer taken into account at the microscopic or quantum level, does it mean that, fundamentally, it has no physical reality ? In fact, time is an emergent property which is manifested in the macroscopic irreversible phenomena, i.e. thermodynamic systems. These systems inevitably evolve towards more and more disorderly states, which are measured by a variable called entropy. Then, the inexorable “flying” of time (the arrow of time) would only be an entropy growing constantly.
Space and time have known a lot of adaptations through ages and will probably experience even more in the future.