Presentism is the idea that the universe is three dimensional with "time" being an artefact of record keeping. It is the model of the universe that is taught in school science lessons. The immediate consequence of presentism is that everything that happens is the result of motions so nothing can happen now because there is no motion at an instant*. If nothing happens now, if "now" is frozen, then nothing can be known "now".
Presentism originates in the fact that I cannot measure events that are a microsecond in the future or easily measure events that are a microsecond in the past, here, at the point of the measuring instrument (But electrons can interfere with their past "selves" - see Lindner et al. (2005). Attosecond double slit experiment. and Horwitz's analysis of these experiments).
The alternative to presentism is four dimensionalism in which there is an aspect of time that occurs as a dimension like the three dimensions of space. This aspect of time is sometimes called "dimensional time" and is another axis, or direction, for arranging things. Four dimensionalism is the modern interpretation of relativity theory, it not only holds that objects are arranged in space and dimensional time, it also says that space and time are interdependent. This interdependence of space and time seems odd until it is realised that Pythagoras' theorem is just a way of saying that distances are interdependent. In Pythagoras' theorem any two dimensions in space are related by:
h2 = x2 + y2
The Ancient Greeks realised this was weird; why should two independent directions for arranging things be related to each other by an exact mathematical formula? It gets weirder. If we have three independent directions (left-right, forward-back and up-down) they are related by a grand form of Pythagoras' Theorem:
r2 = x2 + y2 + z2
Any readers who are mathematicians will recognise that this is the equation that defines a sphere. The Pythagoreans got mystical about this and thought that spheres were a sacred form. Spheres are indeed odd, why should three independent directions be exactly related? Why is the universe uniform in all directions? Why does the surrounding volume grow in an exact way with distance from a point? In the nineteenth century mathematicians such as Riemann toyed with the idea of space and created powerful tools for analysing geometry. In 1905 Einstein came up with the Special Theory of Relativity and in 1908 Minkoswki, who was Einstein's maths teacher and knew Riemann's work, showed how Einstein's idea was equivalent to adding another direction for arranging things that some philosophers call "dimensional time". Minkowski expressed the relationship between the dimensions as:
s2 = x2 + y2 + z2 + (ict)2
Where (ict) is the square root of minus one times the speed of light times the time elapsed. (See Wikibooks Special Relativity). The "speed of light" is just the number of metres that are covered by a single second of dimensional time. Nowadays the square root of minus one is replaced by a change of sign in a "metric tensor" but the significance of the Pythagorean form of the relation between time and space remains the same.
These equations that relate the dimensions are known as metrics and they define a spacetime. In the Wikibook reference it shows how Minkowki's equation leads on to the whole of Special Relativity.
Presentists do not like Relativity Theory. Presentism also denies that we can hear whole words stretched through time rather than individual phonemes, or less, and rejects the idea that we can see movements. The idea that we might have both a question and its answer in our time extended minds is also considered absurd by presentism but no explanation is offered for how we could know anything in no time at all. (See Time and conscious experience for a description of how our minds extend through time). Presentism is "time blind", we hear whole bars of tunes but presentism declares that you coexist with less than a note.
Even those who accept that the universe is four dimensional often make a mistake in imagining the form of four dimensional spacetime. It is common for four dimensions to be represented as:
But this is a pseudo-three dimensional representation of four dimensions. Four dimensions cannot be portrayed on a sheet of paper, the nearest we can get is to consider the "light cone":
Image courtesy of Wikibooks: Special Relativity.
The light cone is a representation of the four dimensional equation:
0 = x2 + y2 + z2 - (ct)2
which has a negative sign in front of the term for time so what is happening is that, if dimensional time exists, all of those points on the surface of the cone are no distance at all from the centre point along the path represented by the cone. This four dimensional form is full of surprises and not at all intuitive, for instance the zero length path is the same path as that taken by photons, it has no length for something on the path but does indeed have a length and duration for any observer who is not on the path.
Unlike four dimensionalism, Presentism is not a scientific theory, it has no corpus of explanatory physics attached to it and has no experimental basis. There is no document anywhere that explains how Presentism can be integrated into science and there are only the vaguest of notions in the philosophical literature of the actual nature of Presentism, most 'Presentist' articles being attacks on four dimensionalism rather than manifestos for Presentism. Yet this unsupportable and barren idea is understood by large numbers of people to be the conventional 'wisdom' about time.
From the empirical viewpoint it is sufficient that we can see things move to know that presentism is false. It is, however, interesting to speculate upon potential physical models of our experience but the reader should be aware that if the speculation is wrong the experience remains. Unlike presentism, empiricism would not claim that the existence of your experience depends upon a scientific hypothesis!
Here is a possible idea for how four dimensionalism might explain experience. When dimensional time is introduced into the metric of spacetime time occurs as a negative factor. This can be seen when the square root of minus one is expanded:
0 = x2 + y2 + z2 - (ct)2
Dimensional time subtracts from the distances specified by the other, spatial, dimensions and creates a geometry akin to the descriptions of conscious experience given by Descartes in which experience occurs at a point. In fact only those events that obey this equation can be 'known' at a given time and place because all other events have a net spatial or temporal separation. Notice that the equation allows events to stay at their proper locations in time and space and the 'point' is created by a disappearance of separation between the events as time progresses (See Some notes on projective geometry).
Notes for physicists
The general form of the metric is a differential equation, the form given here is applicable to the discussion however because it applies to coordinates relative to a point observer in flat spacetime. Minkowski's use of imaginary coordinates has now been replaced with an approach that uses a metric tensor. The bilinear form of the modern theory is important in General Relativity, but the metric remains the same so this removal of i makes no difference to the argument given above which depends upon the fact of dimensional time and the negative sign in the metric.
The application of time extension to time intervals greater than a microsecond would seem to require Green's time-like fifth dimension (Green 2002) to create an appropriate time dilation, the time-like nature of this dimension might be due to the continuous appearance of experience.
* Some readers may realise that it is inconsistent to refer to time intervals when only the present is held to exist. I would suggest that presentism is inherently dualist with the philosopher or observer having time-like qualities that permit the inspection of the universe in the different states that we assign to "time" and "change". This would be a strange and unnecessary dualism because physicalist explanations such as that put forward by de Witt (of the Wheeler-de Witt equation) allow an inherently timeless universe to acquire "time" through the separation of a physical observer system.
Ashmead, J. 2010 Quantum Time. http://www.timeandquantummechanics.com/papers/single.pdf
Gopal et al. 2009 Three-Dimensional Momentum Imaging of Electron Wave Packet Interference in Few-Cycle Laser Pulses. Phys. Rev. Lett. 103, 053001 (2009). http://www.attosecondimaging.com/attosecimaging/pdf/Gopaletal.pdf