Read part 1: Joseph Diekemper sets out the hypothesis that the present is nothing but a border between the past and the future.

Philosophers and physicists often think about time in very different ways. In fact, among themselves so do physicists; it is not easy to find two who agree in all details or even in their overall concepts. In this note, I will mention some of the ways in which my views differ from those of Joseph Diekemper and also from mainstream physics.

Diekemper's starting point is that time, "if it exists at all, must be such that it passes". His reason for this claim is "the pervasiveness of our experience of the passage of time". Now many arguments in philosophy and science arise because key words are used without sufficiently precise definitions. I think we need to define “passage” and “time”, or rather their meaning in the combination "experience of the passage of time". In talking about experience, we need not, indeed, distinguish the senses: any and all will do. I will couch my arguments mainly in visual terms.

I illustrate my experience of the passage of time by recalling the hurly-burly of the HowTheLightGetsIn festival in Hay-on-Wye on that beautiful day in May last year when Joanna Kavenna, Diekemper, and I took part in the debate, The Dance of Time. In any one instant, I saw shapes and colours held together in my field of vision as in a snapshot. These 'snapshots' followed one another in a steady stream. In my mind's eye I could lay them out in a row like movie stills. If I ask myself what 'experience of the passage of time' means, that's basically it. The very fact that I have been able to watch the Dance of Time video reassures me that you, the reader, will find my account makes sense.

There is a wonderful thing about the world: geometry. What I see when I open my eyes is a two-dimensional field of view. But our experiences since birth and our genetically structured brains have taught us how to 'place' the shapes seen in two dimensions as objects in three-dimensional space and understand and even anticipate how the objects move relative to each other. That's Roger Federer's supreme gift.

I suggest that "experience of the passage of time" is just shorthand for the kind of thing I have described. You never see time and its passage. Time does not meet you in the street or join you for lunch at Hay as scythe-bearing death. And it isn't a river. What you experience is change of things.

So much for brute experience. How did scientists, at least before Einstein's revolution, translate it into dynamical theory? Newton showed the way: he assumed space to be filled with bodies that move, changing thereby their positions relative to each other. Their positions in any one instant can be imagined to be like a 'three-dimensional snapshot'. The miracle of the universe, reflected in Newton's laws, is that given just three of these `snapshots' which follow each other in quick succession a whole uniquely ordered sequence – a history – of the snapshots can be constructed.

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"You never see time and its passage. Time does not meet you in the street or join you for lunch at Hay as scythe-bearing death."
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You can watch details in my solo talk on IAI TV. Both there and in The Dance of Time I say something about how my ideas relate to the physics created by Einstein. All I need here is the overall picture. In the history-generating triplet, let me call the central snapshot “now”. It could be the one you experience now, or now, or now. The history has a remarkable property. If you were given all the snapshots, jumbled up in a heap, their intrinsic structure would still enable you to order them. Equally remarkable is the fact that as you go along the history, it exhibits a direction. We call it the direction from past to future. The snapshots, representing the whole universe, become steadily more structured in that direction. Moreover, at any point along the succession, one finds records within the current now of its past, not its future. I say something about a possible explanation for this fact at the end of The Dance of Time and in more detail in a talk entitled Time’s Arrow.

Before commenting on Diekemper's ideas about the existence of the past, present and future, I want to clarify a feature of Newton's laws. He formulated them in terms of what he called absolute time, which he insisted was invisible and would flow at a steady rate even if nothing were to happen in the universe. By means of this ‘time' and its rate of flow, he could say 'how fast' things are moving. It's almost a joke, but Newton's omnipotent absolute time plays no role at all in the actual application of his theory to the universe. Time for scientists has always been measured by changes within the universe: the rotation of the earth for astronomers, a water clock for Galileo, and vibrations of the caesium atom in modern high-precision work. In fact, it may be that the duration of Newton's 'time' between two instants is simply a distillation of all the changes throughout the universe between those two instants – see my essay The Nature of Time. It is the need to 'pick up' a measure of 'time' from actual changes in the universe that makes it necessary to have three snapshots and not two as in conventional presentations of dynamics (in which time intervals are assumed given).

In the light of what I have said and speaking as a physicist, Diekemper's concern about the existence or non-existence of the past and future seems to me to miss the point. The ancient Greeks said the aim of science is 'to save the phenomena', i.e. to explain what we experience. On the whole, I think science has made huge progress in that direction. It has supplied us with a law which, to an amazing extent, determines the succession of snapshots given just three of them. It is true that quantum mechanics may change this picture, but for the purposes of this discussion we can accept the pre-quantum picture. Most scientists will, in order to gain understanding and make predictions, take a god-like view and suppose all history is laid out as a succession of snapshots as I have described. Then the important thing about what I in this “now” think of as my past and future is not the sense in which they 'exist', but how the law of the universe so miraculously concatenates “nows” and transforms one into another. Diekemper says "common sense temporal asymmetry tells us that the past is fixed and the future is non-fixed". He may well be right, but equally he may be wrong. Common sense can be badly wrong. Diekemper gives examples in which common sense can be readily corrected, but it took a huge effort on Galileo's part to persuade people that the Earth is not at rest. Quantum mechanics required an immense readjustment of common sense. As of now, it leaves open the issue of whether the future is 'non-fixed'.

Personally, I incline to the view that one should consider all instants of time – “nows” as I call them – on an equal footing as regards existence. Each experience we have is embedded in the shape of the universe in that instant. If one could pick up these “nows” and examine them in all their details, all would appear equally real. Nothing one could find in their structure would say that one 'exists' while another does not. All attempts to find something special about a 'present instant' would fail. For me, the very attempt to do that smacks of 'presentocentricity'. Emotionally, I like the idea that all experience coexists in eternity.

Read part 1: Joseph Diekemper sets out the hypothesis that the present is nothing but a border between the past and the future.