The Idea of Time
In the previous discussion of space, it was suggested that the Herman Minkowski quote regarding the merger of space and time into one single concept, i.e. spacetime, might have been premature. However, in so doing, it does not imply that any of the important conclusions of relativity are rejected. For, it is really only making a distinction between the human perception of time and the integration of time into a mathematical formulation. In this general context, we might accept that the idea of time has been subject to many somewhat conflicting descriptions within the field of physics, e.g. time is absolute or relative, discrete or continuous, reversible or irreversible.
So is there any practical consensus as to the nature of time?
Let us assume that most people accept the basic principle of relativity, such that time is relative, not absolute, but with the caveat that causality is always preserved. However, the acceptance of this basic position does not mean that we cannot raise questions, e.g.
Does the universe age at the same rate if different galaxies move relative to each other or is there some concept of universal time ‘synchronised’ to the preferred frame of the Cosmic Microwave Background (CMB)?
We might also wish to consider the mathematical premise of classical physics, which leads to the deductive conclusion that its equations are time reversible, while all empirical observation, and thermodynamic, appears to support the inductive conclusion that time must be irreversible.
So should scientific methodology weight in favour of inductive reasoning?
While we might have to reflect on whether time can remain continuous as it approaches the Planck scale, it is possible to argue that the Planck scale is so far beyond any practical verification that any deductive conclusion must be labelled 'speculative'. For, at all other scales, our inductive conclusions about the arrow of time appear to hold true, although caveats may exist in terms of the non-deterministic nature of quantum systems. However, at this time, quantum physics may speculate, but not necessarily prove, that time is reversible within its conceptual description of virtual particles within the limits of the uncertainty principle. Again, it is highlighted that all observed quantum states appear to undergo time evolution in the forward direction, as described by thermodynamics. As such, it is not clear that physics can actually contradict our intuitive sense of time, although we often have difficulties in describing the concept of time in any rigorous manner. For this reason, we might try to quantify some of the descriptions of time within physics based on our own conscious experience of time:
- Classical Mechanics:
Initially, it was thought that time is an absolute parameter, although essentially reversible in terms of its equations. Within this description, cause-and-effect appeared to be linked in a deterministic manner, but did not offer up any real explanation of time or address the problem of free-will within a deterministic model. Of course, even within classical physics, it was realised that deterministic systems might be subject to chaotic processes, which might only be described in terms of statistical mechanics or thermodynamics.
- Classical Thermodynamics:
Within the description of entropy, there is the clear suggestion that time cannot be reversed, although this premise may be said to be only 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, as required by quantum mechanics.
- Special Relativity:
This theory 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, only time dilation, although some interpretation of the mathematics associated with negative energy may contest this argument.
- Quantum Mechanics (QM):
Originally, in QM, position [x] was considered as an operator, while time [t] was only treated as a variable. However, this asymmetry was problematic in terms of special relativity. Therefore, in Quantum Field Theory, position [x] and time [t] are now both considered as variables, while its description of fields is quantified in terms of operators. Therefore, it is not clear that QM questions that time is essentially irreversible, if causality is maintained. 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, see Transactional interpretation. However, 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 really 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 for the Big Bang model.
Within the scope of these descriptions, the scope of the irreversibility of time within QFT might be questioned on the grounds of verification, although the non-deterministic aspect of quantum theory might offer up a better explanation for the human condition than the determinism of classical physics. While time, in classical physics, might be assumed to be reversible, it might be argued that the reversal of mathematical equations does not necessarily lead to the conclusion that time itself is reversible. There is also the issue that classical ‘cause and effect’ does not appear able to explain the scope for free-will, which many would argue is an essential ingredient of life, especially sentient life. Of course, the issue of ‘free-will’ might well open another 'bag-of-worms' as physics appears to be struggling to explain how sentient intelligence emerges from living cells based on chemical reactions alone, even before we start to worry about the most fundamental form of their underlying ‘physical reality’. However, the extension of classical physics to include the observations of thermodynamics would suggest that time only flows in the forward direction, because if it were to be reversed, the entropy of an isolated system could decrease in violation of the second law of thermodynamics.
So is time reversible or irreversible?
As outlined above, there is considerable scope in the description of time within different branches of physics, even before overloading the discussion with too much philosophical debate of free-will. However, the increased scope of deductive reasoning within science appears to open the door for even more speculation. Today, many scientific papers that touch on the question of time would appear to be forwarded on the basis of a mathematical premise, which has no direct verification in observation, as such there would appear to be enough ‘uncertainty’ to raise the next question:
Is the idea of quantum uncertainty enough to destroy the arrow of time ?
At the macroscopic level, all empirical evidence suggests no, such that it might be argued that all systems do adhere to the ‘arrow of time’, i.e. time only flows in the forward direction, with one caveat. While all macroscopic systems are subject to entropy in the form of the 2nd law of thermodynamics, they are not totally deterministic. As a consequence, even if two systems could be created in the same initial state and then subject to apparently identical conditions, quantum probability could always lead to a different outcome.
Does this position really change the accepted perception of time?
Before trying to answer this question, even speculatively, there are a number of possible issues that should be tabled. The first issue is whether time is really continuous or whether it is also subject to some form of quantum granularity. The second issue is connected to the persistence of time dilation. In terms of the Planck scale, we appear to have some measure of time, i.e. ~10-44 seconds, beyond which any concept of time appears to be meaningless or, at least, becomes unquantifiable, but whether this negates the macroscopic arrow of time seems questionable, i.e. at the scale of verifiable science. Equally, while quantum theory might speculate that unobservable virtual particles can move backwards in time within the quantum limits of Heisenberg’s uncertainty principle, there appears to be no doubt that the initial state and some later observable state are always separated in time in the forward direction. In contrast, the second issue connected to time dilation does not question the direction of time, only the perception of its rate, i.e. elapsed time, in different frames of reference. While this is clearly an important consideration, it is not clear that it contradicts our most basic intuitive perception of time or provides enough evidence for abandoning the concept of time. While not wishing to pursue the following idea too far at this stage, we might consider the idea of time in two different context. First, time might be described as a ‘mechanical’ process that marks the separation of two events irrespective of the presence of an observer. Second, time might be described as measure between two events determined by an observer. So the question being tabled is:
Does the idea of relativity only apply to the measure of time by an observer?
Note: If all matter is a construct of some sort of underlying wave, then any higher construct of matter, i.e. an observer with a clock, might only be defined by an associated set of frequencies determined by the ‘local’ periodicity of the underlying waves. As such, another observer moving relative to another would have their own ‘measure’ of wave periodicity due to an analogous Doppler effect. In this respect, all observers might be said to be moving with respect to a conceptual frame of reference defined by the speed of light, if constant in all frames of reference.
We might infer from the speculative note above that any measure of time determined by any observer might always be relative, when in motion to some other arbitrary frame of reference. For if observers and their measuring devices are all made of matter-waves, then their measurement of time may only be relative. Of course, if there were some absolute frame of reference, time would be reflected in the periodicity of some fundamental wave see Wave Model Considerations for a more speculative discussion.