An interesting history of Time and Space
In this chapter we will explore the how the different concept of time and space got evolved into today's science theories. Right from the Aristotle to Einstein to Stephen Hawking
Since the time of Aristotle to Newton to Einstein, the history of time and space changed drastically, the possibilities of time being a progression of motion entity to a physical entity as spacetime.
There are many philosophies and theories on time and space, so here we are going to discuss the summarized aspect of those theories to get the basic understanding of time and space throughout history of philosophy and science. This development in the philosophy of time and space is very crucial for us to proceed further in this book.
The philosophers from ancient Greece were among the first to the raise question on nature of time. They discussed in detail, what exactly is time, whether it is linear or cyclical, whether time is infinite or has a start and endpoint.
According to Plato, present is a sort of ‘mean’ in the sense of being at once at starting point of the future and end point of past. By this theory he means that there must always be ‘past’ and ‘future’ to get ‘present’ (accordingly time is infinite). Plato identified time as the measure of motion of the celestial bodies such as sun, moon and earth etc. So if time is the measure of motion then the motion should also be infinite and that motion must always have a past motion and future motion.
Aristotle was student of Plato and he was able to put the Plato’s time (the motion of heavenly bodies) into more logical way. He explained time itself is nothing but an attribute of movement and motion of all other bodies. The motion can be galactic motion to the progression of thoughts running in mind.
It is a measure of continuous motion or it is change of number with respect to before and after. Therefore time cannot exist without any motion itself but certainly a kind of intelligence is required to measure it. Aristotle argued that motion itself cannot be termed as time because it can be fast or slow depending upon the body. According to him time is infinite and this universe has always existed and will always exist.
Now the above explanation of time by Aristotle led to an argument that if time is the measure of motion and it is also running in thoughts then there should be two different times. Let’s suppose if someone measure the duration of a day in his thoughts then he would still be able to roughly measure the duration of a day when there will be no motion. This led to a further discussion of real time and imaginary time.
Aquinas tried to separate real time dependent on motion and imaginary time (independent of motion) depends on thought flow. But he was never able to establish any relation between both of them.
Regarded as the first modern philosopher; according to him all the materials have a spatial extension property but not temporal sustenance. This means that all the materials have some spatial dimensions such as length, breadth and height. However regarding temporal sustenance, it says that past things are just in our memories, they does not exist now, so all the material in past also does not exist, as we can observe only present things. This was considered as a problem as where the present things come from? Therefore he gave a solution to this problem as that God creates present with all the material every moment. So time is a kind of constant re-creation of God. This view was further developed by Presentism theory later in 20th century.
Isaac Barrow (Teacher of Isaac Newton) was first to reject the Aristotelian definition of time that it is an attribute of motion. He completely changed the idea of time given by Aristotle that it depends on the motion. He argued that time is eternal and it was there before the universe and it will be there after the universe.
Isaac Newton wrote a book in the year 1687 “Philosophiæ Naturalis Principia Mathematica”. He further developed the theory of Isaac Barrow and defined that Absolute time (ab. Time) runs constantly throughout the universe independent of any motion, it is infinite in nature and it should be considered as a dimension in which all the events occur. Similar theory for Absolute space (ab. Space) was given that it is independent of anything and infinite in nature. It means that absolute Time and Space exist before the universe and they will exist after the universe without being affected by anything.
During the years 1715-16, Leibniz had some correspondence with Newton regarding the possibility of absolute time, he believed that time makes no sense if there would be no object to interact with. In fact the event taking place among the various objects are time themselves. If someone tries to think of any moment in time, he would only be able to think of that event took place in time. This philosophy is often called relational time.
In his famous philosophical book “Critique of Pure Reason”, he described time and space as our different notions to think. These notions makes us to experience things in a sensible way, in this way time and space are not physical entities themselves. Our mind is built to see Space as a three dimensional geometry (length, breadth, height) and Time as linear mathematical line to understand the sequence, duration and intervals of events.
Special Theory of Relativity 1905 & General Theory of Relativity 1916
Since the publication of these theories, our understanding of time has been changed drastically, the new notion of time as one dimension of Spacetime in special relativity, and dynamically curved spacetime in general relativity is much more prevalent now. The spacetime is a real entity woven together and is considered as fabric of universe. At the time of bigbang explosion, all the dimensions unwarped (including spacetime) and that was the start of time and expansion of space.
Special Theory of Relativity suggests that spacetime can be explained as four dimensions in Euclidean geometry and universe is a four dimensional spacetime block. Time can be referred as fourth dimension in which universe is existing in past but we have already passed through that time and universe is existing in future but we have not encountered it yet.
For example if we moving on a road and passing few places behind hence they exist, but we have passed through that location so we cannot see them now. Similarly few places are yet to come on our way but we have not reached there to see them.
General Theory of Relativity suggests that spacetime fabric of universe is dynamically curved due to the presence of matter. This curve causes time dilation depending upon the gravity. There are many practical significance of this theory; one of them is time dilation effect on the clocks of International space station and GPS satellites compared to the clocks on earth. Gravity is less compared to earth’s surface because International space station (ISS) is approx. 408 kms and GPS satellites are generally more than 20,000 kms above the earth surface.
First theory restricts to travel in spacetime because nothing can travel more than speed of light. However second theory suggests spacetime is bend due to gravity, hence allowing a person to travel in spacetime through the bend (shortcut) spacetime and reaching to another point in spacetime (past/future). These theories opened up the possibility of time travel and it was one of the revolutionary ideas in the history of time and space.
We will discuss these theories in details in the chapter of spacetime and time travel.
The philosophy of time in which the present is only real is called presentism, according to presentism only present objects and present observation can be said as exist. Past does not really exist now and future is yet to become present to exist. This theory is inspired by the philosophy given by Rene Descartes in 17th century.
The philosophy in which past and future are equally real as present is called as Eternalism. It refers all the past and future events exist but we cannot observe them at present. The flow of time is just an illusion of consciousness because in reality, time is always everywhere. Eternalism is based on Einstein’s special theory of relativity up to an extent.
A little different approach can also be seen by few philosophers/scientists that past exists but future does not exist because universe is a growing block. Future is not yet born because it depends on the “present”.
According to Adolf Grünbaum, time is just a measure by the clock. We use time to measure the sequence of the events and to measure the duration of those events.
In his book “Philosophical Problems of Space and Time”, he defined time as “time applies the contemporary mathematical theory of continuity to physical processes, it is a linear continuum of instants and is a distinguished one-dimensional sub-space of four-dimensional spacetime”.
In addition to Einstein theory of relativity (Spacetime), Stephen Hawking in his famous book “A Brief History of Time”, proposed imaginary time and said that point of singularity might not be the beginning of universe. This imaginary time can help us to determine the state of singularity. According to him there is an imaginary time also which can be explained as perpendicular line to the real time line. It is very similar to the imaginary numbers in mathematics. As we can see in real time, universe does have a start point in time known as bigbang but that imaginary time has no boundary conditions very similar to North Pole of earth with no boundaries.
Quantum theory of multiverse: Although we are discussing the history of time and space but some parts of this theory needs a special mention here to understand above Stephen Hawking’s imaginary time. According to this theory, in the disconnected time lines there are potentially infinite number of parallel universes, this creates the possibility of multidimensional time (including imaginary timeline) in which other dimensions of time can sometimes interact or merge into one but we may not have been able to observe it yet.