Time flies like an arrow, fruit flies like a banana.
Attributed to Groucho Marx
1 | From our human perspective the universe evolved us over time and through the action of one cause on another, and yet philosophical investigations of the nature of time, and of cause and effect, cast a shadow of physical doubt even here.
2 | Aristotle reasoned that humans cannot escape the chain of cause and effect, and that any chain of cause and effect must eventually find its origin in a first cause. In order to avoid this kind of reasoning he came to the conclusion that the universe must be eternally existent. Some scientists were made uneasy by the Big Bang theory because it isolates a beginning moment for the universe. The multiverse, created out of eternal inflation, randomly and without cause, is an attempt to make the universe once more eternal and uncreated as Aristotle envisioned it. And yet it is a creation story still, and one that will eventually be succeeded by some more refined account.
3 | Out of many possibilities of the quantum world a particular occurrence happens in time. But what is this moment of time in which things happen? How long is a moment? What joins one moment to another?
At the conjuror’s, we detect the hair by which he moves his puppet, but we have not eyes sharp enough to descry the thread that ties cause and effect.
Ralph Waldo Emerson (1803–82), American transcendentalist
4 | Where does a cause end and its effect begin? What surrounds the cause and cuts it off from what it effects? What is in the space in between? What is the time between time? These are thoughts a precocious, gloomy child might have. But not childish; the problems run deep. The Scottish philosopher David Hume (1711–76) tried to pin down the elements of the illusion as a list of propositions that must be fulfilled: the cause and effect must be contiguous in space and time, the cause must come before the effect, there must be constant union between cause and effect; there are five further, increasingly elaborate tenets. Hume’s insight was to see that cause and effect are habits of the mind. We associate two events, two stimuli, two ideas in our minds, and time passes. But if an idea existed truly separate from another, how would we ever move from that idea to the next? We would be attached to the idea forever, unchanging and frozen out of time.
5 | The world is this then this, not this because of this. If there is intention in the universe it is hidden at all levels. Or does not exist.
The very idea of a cause is emergent and abstract. It is mentioned nowhere in the laws of motion of elementary particles and, as philosopher David Hume pointed out, we cannot perceive causation, only a succession of events.
David Deutsch
6 | In Dr Johnson mode, a harrumph is enough to dismiss the problem. I turn on the kettle, the element heats up, the water boils, a cup of tea is made. Causes and effects. We know what a cause is and what an effect is in our world of large things and from our human perspective. At the gross scale of human beings consciousness seeps in, granting us among its manifest powers the power to manifest cause and effect. But from a universal perspective that removes the human, the world must look differently. Pure thought tells us that there can be no gap between cause and effect – what could be in the gap? – but if there is no gap then there is nothing to distinguish cause and effect. Reductive materialism must and does account for this seeming impossibility. Relativity and quantum mechanics are two such accounts. Philosophy is not entirely useless! In relativity and quantum mechanics any exact formulation of what a cause can be is abandoned. Cause and effect are not qualities of the world at quantum scales. At the Big Bang the whole universe was a quantum event. Causality drains from the universe as we rush back towards its beginning. Radioactivity is an effect without a cause; the emergence of the universe from nothing is another example. Cause and effect emerge at human scales.
7 | Newton’s equations might suffice to retrace the path of a ball or of a planet, or predict their future courses, but the ball’s path and the planet’s orbit are not contained in the equations. As powerful as the predictions contained in the equations are, they always describe only some limited part of what is going on. The planet’s orbit might be changed because of some comet not taken account of. The ball may be thrown off course because of some unforeseen gust of wind. As hard as we try to predict what elements of change may arise, to describe any system, completely, we must eventually take account of the whole universe.
All this seemed to have ended well for Evgeny Mikhailovich and the yard porter Vassily: but it only seemed so. Things happened which no one saw but which were more important than all that people did see.
Tolstoy, ‘The Forged Coupon’
So, whatever the verdict of physics, the real causal explanation for why there are boiled eggs is that I, and other breakfasters, intend that boiled eggs should exist.
Alfred Gell (1945–97), anthropologist
8 | Time is an illusion, said the ancient Greek philosopher Parmenides, and the deeper reality is eternal and unchanging.
Now he has departed from this strange world a little ahead of me. That signifies nothing. For us believing physicists the distinction between past, present, and future is only a stubbornly persistent illusion.
Einstein writing, a month before his own death, about the recent death of his lifelong friend Michele Besso
9 | For Einstein the past and future exist eternally. Time does not flow, it just is. In fact all physical theories so far devised – Newtonian mechanics, Einstein’s two theories of relativity, even quantum mechanics – are symmetric in time. No arrow of time is indicated. There is no physical reason why a smashed plate might not rejoin itself, and indeed in some parts of the universe we should expect to see the arrow of time reversed. So far no such evidence has been found. To a material reductionist the arrow of time is an illusion of scale, just as Einstein’s special theory of relativity shows us that it is an illusion that time and space are separate. It is because humans experience the arrow of time that the second law of thermodynamics1 was added to physics. By fiat, the second law gives time a direction. The second law of thermodynamics fulfils an observational and psychological need rather than a physical one.
The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the world line of my body, does a section of the world come to life as a fleeting image in space which continuously changes in time.
Hermann Weyl (1885–1955), mathematician
The physical world does not have tensed time, in which present, past and future exist side by side.
Raymond Tallis, philosopher
10 | Our human experience of living in the world is of time running forwards. We see a world in which everything, eventually, is ruined by time moving inexorably from past to future via a privileged instant we call now. The past is what the future becomes when it has been pulled through the ring of the present, and the present is the flame that burns the future into the ash of the past. Physical theories do not privilege the ‘now’. Physics tells us that everything that will ever happen in the universe has already happened. The universe simply is. If material reductionists are to hold fast to their theories and the God-like perspective of physics, they must explain why we human beings experience the illusion of an arrow of time, and why the moments of our lives cannot be revisited. Why human beings are consigned to march, once only, second by second, forwards along a line of allotted time is a question so far unanswered by science. Or rather there is no consensus around whatever theories have been put forward.
11 | That there is no dedicated sensory organ that detects time2 might suggest that the passing of time is a psychological phenomenon.
There is no mechanism to go wrong. We can have a fragmented sense of self, but no one has ever had a fragmented sense of time. Our subjective perception of it, however, causes time to tick variously. Physical time ticks regularly, subjective time ticks fast or slow depending on our age, the emotions we experience, whether we are in pain, or in love, or just bored. Dostoevsky writes of the condemned man’s last night in which each moment stretches into eternity. Is that why we fear death: because at the last, the moment of death never quite ends?3
To think is to be in time. What we cannot do is think ourselves into the future. Our inability to find the future except by waiting for it to arrive in time suggests that consciousness itself is based in time.4
Oh! Do not attack me with your watch. A watch is always either too fast or too slow. I cannot be dictated to by a watch.
Mary Crawford, in Jane Austen’s Mansfield Park
If we knew how long a night or a day was to a child, we might understand a great deal more about childhood … It may be that, subjectively, a childhood is at least equal in length to the rest of a lifetime.
John Berger, A Fortunate Man
Einstein said that the problem of the Now worried him seriously. He explained that the experience of the Now means something special for man, something essentially different from the past and the future, but that this important difference does not and cannot occur within physics. That this experience cannot be grasped by science seemed to him a matter of painful but inevitable regret.
Rudolf Carnap (1891–1970), philosopher, reporting a conversation he had with Einstein
12 | The physicist Julian Barbour uses quantum mechanics to explain the illusion of the forward movement of time. The universe is a heap of frozen moments that exist eternally. Each of these moments is some quantum configuration of the universe. One moment may appear to us to come after another, but it is only because the second moment contains a memory of the first moment. The passing of time is an illusion. The second moment does not come after the first moment in time (time does not exist). The first moment is not the cause of the second moment (there are no causes).
We do not die in time, there is for each of us a small heap of frozen moments that is eternally our life.
Barbour’s theory shows how the probabilities of quantum mechanics can be interpreted physically. Some of the configurations of the universe are much more likely than others. Time is the illusory ordering of these groups of more and more likely configurations.
13 | The arrow of time describes a universe that becomes more and more disordered, which means that looking backwards in time the universe must have been more and more ordered in its past. And so he arrow of time predicts that at its beginning – if it had a beginning – the universe must have been as ordered as it could be. But we also believe that the universe was very hot at the Big Bang – around 1032 degrees. Hot usually implies disorder. How the universe could have been so hot and so ordered is a mystery.5
14 | Our only human access to the past is via atoms. Becoming smaller and smaller, we leave everything behind. All the edifices, all the things of the world crumble into atoms, into electrons and protons as we spiral down towards the Big Bang. They are us but not we.
15 | i Given the means, we humans could travel as far into the future as we care to. Come the day that we work out how to travel at speeds close to the speed of light we will for the first time know from experience, rather than theory only, what it is to leave earth-home. For now, because we live here together on the same planet we are united under a single clock.
I imagine myself travelling for long ages and at great velocity out into the galaxy. I have to learn to bear my homesickness. One day I decide to return home. The more I accelerate to hurry back, the greater the aeons of time that will pass on earth as I draw near. The lives of generations of human beings will have been breathed in and out, bones long crumbled into dust, reabsorbed into the biosphere and refashioned. Ice caps melt and refreeze, mountains lower and rise. The Himalayas have been a work in progress for fifty million years, and still they continue to rise; but even they one day will have been rubbed or convulsed once more out of existence if I travel far enough forwards in time. And I will return to what? Perhaps to some brutalised civilisation that has lost the past knowledge of long-forgotten golden ages. Or if I am fortunate, I might accidentally land in some new golden age of elevated beings beyond my imagining. If Julius Caesar6 had had the power I have been granted, we might see him return tomorrow, full of amazement at the world he left behind. With the right means, we might travel as far forward in time as we wish, but only at the expense of leaving behind forever those who do not travel with us. For the first time in human history our human-scale clocks will not all tick together, here, where we live huddled together, in our living-room earth.
ii Children know how to make time machines. Put things in a box and bury the box. Wait.
iii No Penelope awaits her time-travelling Odysseus. When this Odysseus returns home the palace walls have long since fallen. The dog that waited out its life died centuries before. Only the lead-lined box survives, carefully buried and mapped, which with shaking hands Odysseus opens.
iv There have been few human space travellers to date, but even amongst that small number a significant proportion have returned traumatised. Humans can go mad sailing alone across the oceans; how would even the most robust of us fare if we were to travel to other planets around other stars?
16 | For Lee Smolin the flow of time ‘is not an illusion. It is the best clue we have to fundamental reality.’ Smolin characterises the eternal laws of physics as ‘excess metaphysical baggage’. He believes that the pursuit of eternal laws of nature has hampered cosmology in recent decades, and that mathematics is part of the problem – because mathematics looks like evidence of something eternal and unchanging in the universe.
In place of eternal inflation Smolin has posited a theory of cosmological natural selection to explain how the universe got going in time. He reconceives the universe as if it were some kind of organism. Everything in the universe is evolving at every scale into genuine novelty. In a universe in which everything is laid out eternally and unchanging, innovation is impossible. In Smolin’s model every instant that happens in the present is a chance for something genuinely new to occur in the universe’s history.
Smolin believes that at the quantum level the deeper reality is time, not space. He thinks that one day we will understand how space emerges out of some deeper order based in time.
I think it likely that space will turn out to be an illusion … a way to organise our impressions of things on a large scale but only a rough and emergent way to see the world as a whole.
Lee Smolin