2. Nature of Time


Nature of Time

Of the many philosophical assumptions in physics about the nature of reality, none is more mysterious than the existence of time. Time can be relatively easily discredited by imagining deserted islands or other settings devoid of conscious beings. Yet, despite the logical acceptance of the unreal nature of time, we do know that a year from now, we will be a year older. Why is it that we cannot escape the unrelenting hold of time? The reason is that the origin of time is linked to our most basic knowledge—our self-awareness, the knowledge that we exist. Before proving this assertion, let us examine some aspects of time.


Unlike space, time does not have any of our five senses assigned to it. Our primary sensory mechanism, sight, creates a 3-D world around us. Even in the absence of vision, hearing can place point sources in a 3-D space. A 3-D map of objects in our immediate vicinity can be created by our touch sense also. Time is totally different from this sense of 3-D space. We have no built-in mechanism to perceive or sense time. Despite this glaring absence, human beings do have a sense of time. Where does this sense come from? What is the origin of this sense of time that permeates all our physical sciences and plays a crucial role in every single conscious action and decision we make in our lives? To answer this question, we need to understand the notion of scale in our perception and the evolutionary process that brought about such a notion.

Millions of years of evolution ensured that our senses are best suited for our survival and reproduction. We can best sense objects and movements at scales comparable to our body dimensions and speed. We can clearly see a person running, but not a bullet coming at us, nor the movements of the hour or minute hand of a clock. We can appreciate the difference in size between an adult and a child, but not between two celestial objects or micro-organisms. This sweet spot where our senses and perceptions work best is not limited to the tangible measures such as speed and size.

Our sense of numbers is also related to human scale. Human beings are poor in assessing long odds—a fact illustrated by the thriving casinos in Las Vegas. Another example of our collective inability to assess probability is the fear of flying. Many people suffer from a fear of flying; not many suffer from a fear of crossing a street, even though, statistically, it is more dangerous to cross a street than to travel in an airplane. If we crossed a street every day, and if our average life expectancy were a million years, we would almost certainly get run over one day. But, we might still be able to fly everyday without crashing. And if we had such long lives, it would be obvious to us that crossing the street is a much more dangerous proposition than flying because we would be sensitive to such tiny probabilities and differences between them.

Here is another, more concrete example of our life expectancy modulating our risk assessment. If our life expectancy were only five years, AIDS would not be a serious illness, for our life would most likely end before AIDS could kill us. This is probably why some monkeys can be carriers of the dreadful disease without dying of it. Their life span is much too short.

Now, what does all this have to do with time? The human scale modulates our sense of time, much like it does our perception of size, speed and probability [2]. Our sense of time is modulated by our life expectancy. If we lived forever, would we have a sense of time? Let us consider the possibility that if we were immortal, we probably would not have a sense of time. Time is sensed through change. Does immortality imply a lack of change as well? It should because changes can be thought of as small deaths and rebirths. Some cells die, some others take their place; that is how a biological change takes place. The real question about the existence of time is not so much whether we would have a sense of time under the conditions of immortality, but whether there will be time at all if we are not here to sense it, or if we were all incapable of sensing time for whatever reason? The uncertainty we feel in answering these questions should point to the virtuality of time.


The whole argument on the virtuality of time boils down to this—we sense time as a fraction of our expected lifetime. If our lifetime were infinite (or if we did not know that it was finite), then all finite spans of time would be so small in comparison that we probably would not have a sense of time. If we did not have a sense of time, would that mean there was no time? Does time have an existence independent of our minds? More to our purpose in this book, would it be possible to do physics without a notion of time? Physics, as we know it today, (e.g., Newton’s laws of motion and the special theory of relativity) has time figuring prominently as an essential ingredient. A physics with no explicit time was attempted by Julian Barbour (in The End of Time), which shows that time is not essential. Time, however, makes it far more convenient to do physics.

To get to real, physical time from our sense of time is not a small step. Interestingly, there are two different concepts of time in physics. One is the “normal” continuous time that we have a natural sense of. The other is the malleable time that can get dilated. Let’s take a closer look:

Galilean Time: In the Galilean view of space and time, the physical existence of an absolute and global time is assumed. Isaac Newton defined it as follows: “Absolute, true and mathematical time, in itself, and from its own nature, flows equally, without relation to any thing external; and by other name called Duration. Relative, apparent, and vulgar time, is some sensible and external measure of duration by motion, whether accurate or unequable, which is commonly used instead of true time; as an hour, a day, a month, a year. It may be, that there is no equable motion, whereby time may be accurately measured. All motions may be accelerated and retarded, but the flowing of absolute time is liable to no change.”

Minkowski Space-Time: The Minkowski view of reality is fundamentally different. In this notion of space and time, there is no absolute, global time that is physically meaningful. This notion of time is based on Albert Einstein’s revolutionary paper [3] that redefined the notion of simultaneity. In a move akin to Copernicus’s abandoning the notion that we were the center of the universe, Einstein decided to abandon the notion of absolute time. Instead, he postulated two principles:

  1. All physical laws are immutable in all reference frames.
  2. The speed of light is constant in all reference frames.

The second postulate, which is a bold assumption, redefines time. It implies, in contrast to Galilean time, that simultaneity is not an absolute physical quality, but a relative one, depending on the motion of the observer (i.e., the reference frame). Mathematically, it mixes space and time.

We will get into more details of the space-time issue later on. Let’s ask ourselves which one of these two notions of time is the “real” one because the Galilean time is different from the Minkowski space-time.

A physicist will tell us the Minkowski picture is a generalization of the Galilean notion of space-time. This is absolutely true, in a mathematical sense. However, we lose something in this generalization—we lose the sense of a global absolute time. Along with that, we lose our ability to say whether two events take place at the same time—simultaneity. In other words, we lose the fundamental qualities of our natural sense of time. If we are willing to sacrifice these qualities, are we also willing to forgo our natural sense of time altogether and think of it as a mathematical construct? This construct may be unnecessary for our understanding of nature and the universe.


The existence of time (or a sense of time) has been a problem in philosophy. Let us take a quick look at time from this angle. Our treatment is a bit different from the traditional philosophy of time. Note Here, we look at the interplay between language and time. Let’s take a quick look at the philosophy of language so that we can see how time fits in it.

Some consider language the most important part of our relation with reality. Language is not merely a communication tool, but also the canvas on which our conscious existence is painted. Without a language, we may not even have conscious thoughts. We will get back to the philosophy of language a bit later and look at it in much more detail. Here, we want to understand how time figures in language.

Language has a syntax specifying the grammatical rules and semantics that give meaning. At the semantic level, there is a reflection-correspondence theory of language. In this theory, language can be thought of as a collection of the correspondences between words and objects in the external reality. If we look at any word in our language and ask ourselves what it means, we will see that it represents something in the external reality.

Language mirrors the external world. However, a little bit of thinking along this line will convince you that this definition of language, at best, is incomplete. Language has a much richer structure. There are structures in language that need explaining. e.g., the word “book” represents the thing you are holding. A “small book” adds a quality to the object “book.” Smallness is a physical quality, so the qualifier still has a kind of correspondence to an external physical attribute. But we can see that qualifiers are at a deeper level of hierarchy in the inner structure of language.

There are other qualifiers that fall in a different category. For instance, if we think of a “great book,” the qualifier “great” is different because the quality does not correspond to a physical attribute. So this class of qualifiers is at an even deeper hierarchy. This hierarchy is where abstract nouns such as “happiness” and “wisdom” belong (along with the corresponding qualifiers).

Now, let’s look at numerical qualifiers. “Two books”—the qualifier here refers to something entirely different. In fact, numbers, along with the rules (syntax), in mathematics form a kind of formal language. The formal language of mathematics, however, is a little weak in semantics. This weakness is the reason the efficacy of mathematics in explaining real physical phenomena never ceases to amaze. The same weakness puts the entire formal language of mathematics roughly at the same hierarchal level as abstract qualifiers such as great, happy, wise and so on. Thus, although “two books” means some-thing easily understandable, “two” by itself is an abstraction. The semantic weakness disappears when mathematics is used in physics. Physics provides the meaning.

Like mathematics, time can be thought of as a formal language. The syntax of time is not as rich as the one in mathematics, but semantically, time is much stronger. Unlike mathematics, its meaning is not as open to interpretation. Time can be considered an abstract formal language embedded in almost all languages in the world. Where exactly do we embed time in our languages? Its position is at least as deep as that of mathematics, probably deeper. Even the syntax of the formal language of time is defined in abstract terms such as past, present and future, along with a concept of its flow and direction.

Thinking of time in terms of the philosophy of language serves only one purpose. It illustrates the virtual nature of time, much like that of mathematics. Mathematics gets its semantics mostly from physics; time, on the other hand, derives its meaning indirectly from our knowledge of our demise. Although we cannot directly test this conjecture, we can consider a few thought exercises that may shed some light on the issue.

  1. Do animals have a sense of time? It is unlikely that they are conscious of their death. So, by our conjecture, they should not have a sense of time.
  2. If you grew up on an island, without contact with other human beings, would you have a sense of time? It seems obvious that you would not have a language in the conventional sense of the word. Some philosophers believe that you would not even have any thoughts at all. But does the sense of time come before or after thoughts?
  3. Is it possible that our sense of time changes as we grow older? Don’t we feel as though years are getting shorter and shorter as we grow older? Is it possible that our sense of time is related, not only to the knowledge of our demise, but also to our sense of how long we have left to live? Note


Table 2.1 A time-line of the significant events in the life of the universe. The first column is the “time”, presented as though the universe is forty-five years old now. The second column is the event that took place at that time. The last column is when it happened, in real time.

Time mapped

to 45 years

Event Real Age
Cosmological Era
45 years ago The universe is born. 15 Billion years ago
39 years ago Clusters of galaxies begin to form. 12 Billion years ago
36 years old Milky way forms. 11 Billion years ago
14 years ago Solar system forms. Disc planets. 4.7 Billion years ago
12 years ago Earth forms, with liquid water and rain.

Origin of organic material.

4 Billion years ago
Evolutionary Era
11.5 years ago Cells form. 3.9 Billion years ago
11 years ago Bacterial life, spores. 3.5 Billion years ago
3 years ago Colony of algae, hormones and fungi. 1 Billion years ago
2 years ago Jelly fish. 650 Million years ago
20 months ago Flat worms, animal groups. 570 Million years ago
29 months ago Vertebrates. 500 Million years ago
14 months ago Sharks, finned fish, insects. 390 Million years ago
12 months ago Ferns, invertebrates, amphibians. 350 Million years ago
9 months ago Dinosaurs. 250 Million years ago
7 months ago Mammals. 200 Million years ago
23 weeks ago Birds. 150 Million years ago
70 days ago End of dinosaurs. 65 Million years ago
55 days ago Spread of mammals. 50 Million years ago
44 days ago Cow family. 40 Million years ago
Human Era
3.3 days ago Recent ice age. Homo erectus. 3 Million years ago
18 hours ago Fire. 700 thousand years ago
5 hr 15 min ago Neanderthals. 200 thousand years ago
2 hr 38 min ago Humans. 100 thousand years ago
17 min 20 sec ago Nomads and farmers. 11 thousand years ago
9 min 28 sec ago Cities. 6 thousand years ago
4 min 44 sec ago Iron. 3 thousand years ago
3 min 9 sec ago Christianity. 2000 years ago
2 min 12 sec ago Islam. 1400 years ago
Scientific Era
43 sec ago Copernicus. 450 years ago
33 sec ago Birth of Sciences. 350 years ago
9 sec ago Special theory of relativity. 100 years ago
5 sec ago Nuclear energy, quantum mechanics. 60 years ago

We argued that our sense of time was modulated by the natural scale involved— our life span. This is why we are not able to appreciate the huge difference between large time scales, such as the difference between the time dinosaurs went extinct and when human beings evolved. One way of appreciating it would be to “translate” or map these cosmological or evolutionary time scales to a human scale. Table 2.1 is one such mapping.

Here, we think of the universe as a forty-five year old. Thus, the current estimate of the age of the universe (about fifteen billion years) maps to forty-five years. The early years of the universe are fuzzy, much like our memories of our childhood. Most of the prime years of the universe were spent on building a universe worthy of hosting life. The first event of our direct interest took place about fourteen years ago—the birth of our blue green planet. These early years can be called the cosmological era. The estimate of these time scales in this cosmological era is mostly theoretical or phenomenological.

We then move on to the evolutionary time scale, from about twelve years ago to less than a week ago. Life on earth developed during these twelve years. We have a better estimate of the time scales in the evolutionary era because of radiometric dating. Note The first semblance of human-like primates appeared about three days ago, heralding the human era.

But in any real sense, our reign on this planet began only today, within the last eighteen hours or so. We began forming villages and building cities only ten to twenty minutes ago. We began subdividing humanity in the name of God and religion about five minutes ago. We invented the pride and joy of our global civilization, the modern sciences, less than a minute ago. Our most influential theories and technologies are about ten seconds old! Even this table mapping forty-five years of our cosmic existence is based on an insight about nine seconds old. And I am typing this using a technology barely a second old!

Our reign on this planet looks set to continue for the foreseeable future— which is about twenty years when our sun will expand into a red giant Note and life as we know it will come to an end. At least, so says the modern cosmology, which is about five seconds old!

Amusing as this thought exercise is, it has a much deeper point hiding beneath the surface. Certain theories in modern physics are extrapolations from a limited experience (or knowledge) to immense time scales. The big bang theory is an extrapolation of our insights in the last ten or so seconds to about 45 years. Should we really limit our quest for knowledge to the bounds of such incredibly large extrapolations? Richard Feynman’s words [4] seem appropriate here: “We are only at the beginning of the development of the human race; of the development of the human mind, of intelligent life—we have years and years in the future. It is our responsibility not to give the answer today as to what it is all about, to drive everybody down in that direction and to say: ‘This is a solution to it all.’ Because we will be chained then to the limits of our present imagination.”


Time does not exist the same way a physical object exists. In other words, time is a secondary sense without any direct sensory percept or reason for its existence. Does this mean that time is useless? Far from it. Mathematics is unreal the same way time is unreal, in the sense that it is a creation of our intellect, without corresponding to anything real. But it is supremely useful in our physical sciences. So is time. The true nature of time is something to be kept in mind in understanding its place in the foundations of physics. For instance, one way of looking at the time dilation in the special theory of relativity is to understand that time is merely a matter of definition. If time is a creation of our intellect, its measurement is open to interpretation—this is one reason why Einstein [3] could easily redefine the meaning of simultaneity.

Along with the notion of time come other intertwined concepts. One such concept is motion. Perception of motion is known in neuroscience to be an artifact created in a specific location of our brain. This fact was proven by the loss of the sense of motion as a result of a specific, localized brain lesion. Sensing motion is a mechanism that enhances our chance of survival. It is also connected with the indirect sense of time.

Causality is another fundamental concept that is intertwined with the flow of time. The reason special relativity does not permit faster-than-light travel is that such superluminal travel will break causality. In a universe where time is unreal, is causality real?

If time is unreal, why don’t we feel it that way? The most likely reason is that we accept time before we learn to question it. A lot of the things that we accept before we are able to question them are difficult to relinquish (e.g., concepts of God and religion). Looking at “time” as a formal language and according it a proper place in the hierarchy of our knowledge system may bring some benefits in the form of a more objective understanding of the world and reality.

Read other chapters.

Chapter 2