I was wondering whether there is any general consensus about the nature of time within physics, as a whole. For example Classical mechanics seems to suggest that time is an absolute parameter, which is essentially reversible in terms of its equations. Within this description, cause-and-effect appears to be linked in a deterministic manner, although it is not clear that such a description offers any real explanation of time or addresses the problem of free-will within such a rigid deterministic model. Classical thermodynamics in its description of entropy appears to suggest that time cannot be reversed, although this premise may be said to be a statistical approximation of an aggregated system, i.e. an ensemble of particles. So while the system, as a whole, suggests the irreversibility of time, it does not necessarily exclude the reversibility of time when applied to individual particles within the system. This may leave the door open for quantum physics – see below. Special relativity obviously redefines time as a relative parameter and possibly as an integrated parameter within spacetime. However, while special relativity can lead to different relative measures of elapsed time, causality appears to be maintained, such that there can be no obvious reversal of time, although some interpretations of the maths associated with negative energy may contest this argument. Quantum mechanics (QM) and its extension into Relativistic-QM (RQM) appears vague on some matters. As understood, time was originally an operator in QM, but subsequently ‘demoted’ to a parameter in QFT. It is also not clear that RQM has anything more to say than QM or special relativity, i.e. if causality is maintained, then time must remain essentially irreversible. However, there may be some implicit reference to the granularity of time, i.e. non-continuous, in the definition of Planck time; although this would not necessarily change the overall ‘arrow of time’. Quantum Field Theory is possibly open to more interpretations based on the conceptual nature of virtual particles and the Feynman-Wheeler idea of advanced and retarded waves, e.g. as defined by John Cramer’s transactional interpretation. With reference back to classical thermodynamics, entropy suggests that time is irreversible, but doesn’t quantify what might happen at the individual quantum particle level. Again, as far as it is understood, virtual particles can conceptually travel backwards or forwards in time within the limits of the Heisenberg uncertainty principle, when quantified in terms of energy and time. However, virtual particles as defined within Feynman diagrams are said to be unobservable and therefore we appear to be forwarding a conceptual ‘probability’ not a verifiable argument. Again, without understanding all the details of the Feynman-Wheeler idea, one counter-argument against this idea appears to come from cosmology in that for the advanced and retarded waves to cancel out in the present, the size of the universe in the past and future would have to be the same size, which is a bit of a problem within the Big Bang model. Clearly, human beings have an intuitive sense of time, although we invariably struggle to describe the concept in any rigorous manner. However, in many ways, physics also appears to have some difficulty in reconciling its idea of physical time with the conscious experience of the ‘arrow of time’ as supported by thermodynamics; while at the same ‘time’ our ideas about free-will appear to reject cause-and-effect determinism. Purely as a bit of speculation for debate and by way of summary: It would seem that quantum physics might argue that time cannot remain continuous as it approaches Planck time. It might also argued that time might be reversible within quantum uncertainty limits; although such processes are unobservable. In contrast, all observable processes confirm time to be irreversible. If so, the issue of determinism and free-will are not necessarily directly connected via time. Therefore, as a speculation, might all macroscopic systems cease to be totally deterministic simply because of quantum uncertainty, i.e. all outcomes are underpinned by probabilistic processes. So, as a consequence, even if two systems could be created in exactly the same initial state, e.g. |A>, and then subject to apparently identical conditions, quantum probability could always lead to a different outcome, e.g. <B| or <C|, at some level. As such, it is the probabilistic nature of quantum ‘reality’ that supports the non-determinism required by free-will, while for all practical purposes, systems continue to follow the ‘arrow of time’. Any thoughts or clarifications?