What is time? Why does it have a direction? Is it an illusion? Luckily for us, theoretical physicist Carlo Rovelli, in his excellent book The Order of Time, has answered these questions… or if that’s too bold a claim, at least given us a nice head start on them. His position on time essentially confirms what phenomenologists like Husserl, Heidegger, and Sartre have had to say on the subject while also offering a physical explanation for how it arises. It is this last feature that really makes the book stand out. When I read through this, I felt like a profound secret had suddenly been revealed, as if a portion of the universe had suddenly and satisfactorily clicked into place. In this article, I plan to walk through Rovelli’s book and hopefully give you that same hit.
Rovelli structures the book into three parts. In the first, he systematically dismantles time, essentially arguing that it doesn’t exist as a fundamental feature of the universe. If you have kept yourself a little abreast of advances in modern physics, there will be nothing particularly revolutionary here. In the second part, he discusses what this world bereft of ‘time’ might be like. In the third and final part, he reconstitutes time and argues that it is a feature of the universe, just not a universal, or fundamental, one. The structure of my article will parallel Rovelli’s book.
The Crumbling of Time
In this section, we will briefly make our way through five misconceptions about time:
1. Unity – Einstein famously showed us that time is relative. What this means is that time flows at different rates depending on your speed and your proximity to mass. A watch at ground level will mark the passage of time more slowly than one in orbit. Similarly, a watch in motion will tick slower than a watch at rest (relative to each other, of course). The conclusion is that time is not unified; i.e. there is no global, universal rate of time.
2. Directionality – Conventional wisdom tells us that time ‘flows’ from the past to the future, and there is a difference between the two. Sequences of events only make sense in one direction. This directional nature of time is sometimes referred to as the arrow of time. Unfortunately, the laws of physics, and events at the deepest levels of reality, do not honour this mandate. Of course, there are still events and changes at these levels, but there is no discernible ‘arrow.’ If you film a sequence of events in the world and then play it back, it will be obvious whether the video is being played forwards or in reverse. If, in the video, shards of glass suddenly coalesce into the shape of a mug and leap up off the floor onto the bench, it is clearly being played in reverse. Sequences of events have an obvious temporal order. This is not the case at the level of individual particles. It would be impossible to ascertain whether the playback of a video of elementary particle interactions were being played forwards or in reverse. A sequence of events that happens one way could just as easily happen the other way. At the deepest levels of reality, there is no arrow of time.
3. The present – There is no universal ‘now’. Imagine someone living on a planet four light years away. What could it mean to ask what they are doing ‘now’? If we were to look through a telescope, we would see what they had been doing four years ago. What if we were to wait until four years had passed and then look again? Unfortunately, in this time (as measured by their clocks), they could have returned to Earth ten years in the future. This would mean that the answer to the question what they were doing at that ‘now’ (four years ago) was actually something ten years in our future?! As far as I understand this, the impossibility of defining a universal present is a consequence of the fact that time is relative and the speed of light is finite. This is why physicists talk of light cones. A light cone represents the path a beam of light would take through space-time.
The future light cone represents events which can be affected by an event at the centre of the light cone. Likewise, the past light cone represents events that can affect what is happening at the centre. Events outside the light cones cannot affect, or be affected by, what is happening at the centre. It is therefore meaningless to talk about these latter events relative to ‘when’ we are, at the centre of our light cone. Conclusion? The present is a ‘local’ concept which has no universal meaning.
4. Independence – Aristotle thought space and time weren’t real in themselves. Time was simply change (if there were no change, there would be no time either), and space was simply the location of things (if there were no things, there would be no space either). Newton, on the other hand, thought that time and space were real entities; independent, absolute entities upon which relative events took place. Einstein combined these two ideas. Space and time are real things, but they aren’t absolute and independent. Einstein imagined that space-time was a canvas spread throughout the universe, but made of the same ‘stuff’ that everything else in the universe is made of. This canvas is called the gravitational field and is just one of many fields that make up the universe. Time is therefore not independent/absolute.
5. Continuous – The conventional picture is that time is continuous; i.e. it isn’t made up of fundamental units. You can divide an hour into minutes, minutes into seconds, seconds into milliseconds, and so on, indefinitely. Unfortunately, quantum mechanics questions this fundamental belief. First, time (and length) is granular at the deepest level. The smallest temporal division possible is known as Planck time. This means that there is no continuous ‘flow’; instead, time ‘jumps’ from one quanta to the next without passing through the intervening period. Very weird. Secondly, reality is fundamentally indeterminate. A particle exists in a superposition of states, and precisely where it will appear (when an observation is made) is impossible to know for certain. Since space-time is also a part of the universe, like a particle, it must also exist in a superposition of states and partake of this same indeterminacy. Finally, when a particle ‘coalesces’ out of this superposition of states it does so only from the perspective of the observer. If an electron is ‘observed’ by a screen (by being fired at, and hitting, it, for example), then the electron loses its indeterminacy and appears at a specific place for the screen. From a third perspective however, the electron and the screen are now part of a larger superposition of configurations, which can only be broken through a third observation.
In this way, Rovelli demolishes the most cherished notions we hold pertaining to time. According to our best understanding of the physical universe, time lacks unity, direction, a universal present, independence, and continuity. Time, whatever it is, to the extent that we can still say it exists after this rough treatment, isn’t much of anything, and it certainly isn’t anything like we imagine or experience it to be. Cue part two.
The World without Time
Without time as a stable, ordering framework, one might think we would now be left with a static, unchanging world. Rovelli disagrees. Events still occur, change still takes place; the only difference is they are no longer orderly, but chaotic. Clocks run at different speeds relative to each other, events at the fundamental level of reality take place without a variable t marking their progression, distant parts of the universe don’t participate in our light cone, and we can’t tell whether videos are played forwards or in reverse. But things still change. In essence, Rovelli claims that what we are now faced with is a world comprised of events not things. For things to exist, they must persist in time. Eliminating time also eliminates things. The world is continual change; a collection of processes rather than a collection of things.
Rovelli also dismisses two other philosophical positions; presentism and eternalism. The former holds that what is real is whatever is happening ‘now.’ If there is no universal ‘now,’ there can’t be a universal, objective reality either. The latter says that change is illusory; whatever happens has already happened. In physics this position finds expression in the block universe. As we’ve seen however, according to Rovelli, the dismantling of time we effected in part one eliminated neither objective reality nor change. Reality and change are real, even if time isn’t.
The bottom line of this second section is that we don’t need time to describe the world. Eliminating time may leave us with a world that is alien to us – a chaotic world characterised by disorderly change; a world of events, not things – but it doesn’t spell the end of the world.
Sources of Time
If physics so effectively dismantles time, why and how do we experience it then? Brace yourself. There is one, and only one, law in physics that acknowledges a difference between the past and the future; the second law of thermodynamics, a.k.a. the law of entropy. The second law has to do with heat and says that heat cannot pass from a cold body to a hot one. As I already mentioned; this is the only law of physics in which the order a process takes place in matters. No other law imposes any form of directionality on an event. If a sequence of events is allowed by any other set of equations – those for mechanics, electromagnetism, relativity, quantum mechanics, etc. – that same sequence run backwards is also allowed. This is the revelation that just about knocked me over when I read it. In Rovelli’s own words, “In the elementary equations of the world, the arrow of time appears only where there is heat. The link between time and heat is therefore fundamental: every time a difference is manifested between the past and the future, heat is involved. In every sequence of events that becomes absurd if projected backwards, there is something that is heating up.”
Don’t unbrace yourself yet. Rovelli has one more revelation in store, and it’s a doozy. What is happening when something heats up? At the molecular level, heat is just the agitation of particles. More jostling equals more heat. Cold things heat up when their molecules get jostled by hot ones. In thermal agitation, atoms get shuffled around like cards in a deck. If the cards were originally in order, the shuffling disorders them. This is known as the growth of entropy, or the increase of disorder, and is another way to state the second law of thermodynamics. Events always and only progress one way; i.e. from particular, special, ordered situations (an ordered pack of cards) to less particular, less special ones (a shuffled pack of cards). It is this feature that leads to the difference between the past and future in thermodynamics; i.e. to the reason why second law interactions are irreversible.
The important question to ask next is why do things begin in a lower state of entropy in the first place? Take our originally ordered pack of cards. It is in a state of low entropy because there is only one possible configuration, only one way of sequencing the cards, to end up with this particular order; say, ace of hearts to king of hearts, ace of diamonds to king of diamonds, etc. On the other hand, there are many possible different, random configurations of the pack of cards. This is the way the law of entropy is usually described, but if you think about this for a moment, you might notice that something is amiss here. Imagine I shuffle the cards and get a complete random arrangement. This configuration (whatever it may be) is just as unique and particular as when the cards were ordered numerically by suit. The original configuration was only particular and special (ordered) because I asserted that there was something special about a pack ordered numerically (ace to king) by suit. The inevitable conclusion here is that every possible configuration is particular. Certain configurations are only more particular than others if I limit myself to seeing only certain aspects of the cards; numbers, suits, colours, etc. Particularity (order) arises only when we begin to see the world in what Rovelli calls a “blurred and approximate way.” It is this “blurring” which creates the incongruence between the original (past) state and the future one. With our “blurred vision” we only “see” a handful of possible configurations (which become the special, particular ones), while all the myriad different microscopic configurations we aren’t able to distinguish between, constitute entropy. If we were able to observe the microscopic state of things, entropy (and heat, by the way) would vanish, and with it the difference between the past and the future. The flow of time depends on the imprecision with which we see the world.
Rovelli also goes on to (briefly) link the above insight concerning heat and entropy to quantum mechanics through “noncommutativity.” Noncommutativity is the idea that there is a similar directionality between certain events at the quantum level that maps onto the irreversibility we have seen regarding heat. The basic idea is that when an interaction or observation collapses a particle’s wavefunction, the state of that particle is altered. Rovelli describes this with regard to a molecule’s position and speed. If the molecule’s speed materialises first, the state of the molecule is altered in a different way than if the molecule’s position materialises first. This inequality between the two events means the order in which they happen is important and creates a situation analogous to that expressed by entropy.
My Two Cents
What I find particularly interesting in this revelation that time doesn’t exist at the deepest levels of reality and yet is nevertheless a feature of human reality, is that it supports what I have believed for a while based on the way existentialists and phenomenologists have described time. Husserl talked of time as protention and retention, Heidegger spoke of primordial temporality, and Sartre outrightly said consciousness was temporality. While each of these accounts of time vary in different ways, the connecting thread running through them is that time is a structural aspect of human existence. Given this, the converse is also true; time doesn’t exist for non-conscious objects. None of these philosophers try to give a scientific account of time as a ‘fourth dimension’ or independent feature of reality; instead, they all connect time to human existence/consciousness.
It is extremely interesting to me that this is essentially what Rovelli is also suggesting with his notion of time arising through a “blurring” of reality. Time only exists for humans (and other similarly constituted creatures) because we don’t see objects as they are at their most fine-grained level. This limitation, or coarse-grained “blurring” of the details, leads to an imbalance in the way we perceive objects such that certain configurations naturally appear to us as ordered, while a whole range of others seem disordered. Entropy (and heat), and therefore time, are direct consequences of this, making all of them consequences of the human perspective. While Rovelli doesn’t even come close to describing time as thoroughly as the philosophers I discussed above have, his genius lies in having supplied the mechanism behind the emergence of time, similar to the way that Darwin uncovered natural selection as the mechanism that makes evolution possible.
I would just close with a couple of things that occurred to me as I was writing those last two paragraphs. First, the fact that time is a consequence of a limited human perspective doesn’t mean it’s an illusion. It would make no more sense to argue that than it would to argue that a painting of a horse is an illusion because it is really just dots of paint on a canvas. Time emerges as a structural feature of reality out of a particular way of engaging with the world, in the same way that the horse emerges from a particular way of engaging with the painting. Neither are illusions.
Second, this “blurring” shouldn’t be seen as a failing or a weakness to be overcome. If we didn’t have this limitation, time wouldn’t exist for us and our lives would be unliveable in the meaningful way we currently live them. In a more general sense, I see this as a timely reminder of the delusion of perfect knowledge. It’s no wild exaggeration to assert that our modern, scientifically-driven attitude is predicated on a quest to break everything down to its component parts and know it all. Opinions vary on whether this is possible or not, but it is certainly held up as the ideal. Rovelli’s insight (and those of many existentialists and phenomenologists before him) gives the lie to the modern belief that more knowledge is always better. Sometimes, knowing how a thing works prevents that thing from working anymore. Before we get to that point, we might want to revise our current tendency to glorify scientific achievements and intellectual advancement on the one hand, while disparaging any other approach as backwards and obsolete on the other.
 There is actually one exception to this, but I won’t spoil that surprise just yet.