06/05/2016 10:12 BST | Updated 07/05/2017 06:12 BST

Time and Consciousness

In a few weeks I will be giving a talk at HowTheLightGetsIn festival on Time and Consciousness. But what is consciousness? How does time flow? Can these puzzles be solved?

There can be no doubting the striking success of physics in coming to understand the material world from the smallest scales of particle physics (M-theory) to the largest scales of cosmology (the Multiverse). It is even sometimes claimed that our knowledge of the fundamental laws governing the Universe is nearly complete, so that we are close to obtaining a 'Theory of Everything' (TOE). However, one feature of the Universe which is curiously absent from this picture is consciousness. Most physicists neglect this on the grounds that the focus of physics should be the objective world, with the subjective element being banished as much as possible. However, many eminent physicists have taken a different view, arguing that consciousness is fundamental.

If some future paradigm of physics does eventually accommodate consciousness, an important ingredient is likely to be a deeper understanding of time. For without the passage of time, there could be no self-awareness. Although the status of time in any final theory remains contentious, there is no doubt that time is a respectable concept in current physics, playing a crucial role in relativity theory. On the other hand, the flow of time poses a long-standing philosophical problem because relativity theory does not describe the basic experience of 'now' which is such an essential ingredient of our experiential world. For in the 'block' universe of special relativity, past and present and future coexist. The 3-dimensional object is just the constant-time cross- section of an immobile 4-dimensional world-tube and we come across events as our field of consciousness sweeps through the block. However, nothing within the spacetime picture describes this sweeping or identifies the particular moment at which we make our observations.

Thus there is a fundamental distinction between physical time (associated with the outer world) and mental time (associated with the inner world). There is a huge philosophical literature on this problem but there does not seem to be a consensus on its solution. This also relates to the problem of free will. In a mechanistic universe, a physical object (such as an observer's body) is usually assumed to have a well-defined future world-line. However, one intuitively imagines that at any experiential time there are a number of possible future world-lines, depending on how we choose to act. The failure of relativity to describe the process of future becoming past and different possible future world-lines may also relate to the collapse of the quantum wave-function. One approach to the problem is to adopt a growing block universe, in which the future is not preordained, but even this does not describe the flow of time. Indeed, it would seem that the flow of time is illusory from a spacetime perspective.

One solution to this problem, first advocated by the philosopher C D Broad, is to invoke a second type of time, or a higher dimension, with respect to which our motion through physical time is measured. Although this idea is no longer popular in philosophical circles, it is interesting that the existence of extra dimensions is now part of standard physics. A unified understanding of all the forces which operate in the Universe suggest that there are extra 'internal' dimensions, beyond the four 'external' dimensions of space and time, which are wrapped up so small that they cannot be seen. For example, M-theory has seven extra dimensions and in a particular variant of this one of the extra dimensions is extended, so that the physical world is viewed as a 4- dimensional 'brane' in a higher-dimensional 'bulk'. In 'brane cosmology', one can envisage the brane as moving through the bulk, which is reminiscent of Broad's proposal.

The invocation of a higher dimension only generates a global flow of time, so it does not explain the sense of individual identity associated with the 1st person perspective. To accommodate this, one may need to account for another important aspect of consciousness: we cannot be aware of timescales which are too short or too long. On the short timescale, our sensory systems have a resolution time of order 0.1 s and so we cannot experience events shorter than this. This minimum timescale of consciousness has been termed the specious present. On the long timescale, our brains cannot appreciate changes that are too slow, this timescale relating memory in some way.

So there is a sense in which experiential time and the sense of personal identity only exist within certain limits. Nor is it clear that the only possible form of consciousness in the Universe is associated with human brains. There might be forms of life with a specious present much longer or shorter than our own. One might even entertain the notion of a hierarchy of specious presents, related in some way to a hierarchy of extra dimensions.

These ideas are very speculative and are certainly not accepted by mainstream physics or philosophy. However, this shows that it is at least possible to link an old philosophical problem with new ideas in physics. I will be talking further about this at the HowTheLightGetsIn festival next month.