Unpacking the problem of the arrow of time in physics

CRISTIAN LÓPEZ

Gargnano, Lake di Garda

Congreso; 4° International Meeting of International Association of Philosophy of Time; 2017

International Association for Philosophy of Time

Intuitively we believe time goes by. We have learned whilst future is something which ?everyone reaches at the rate of sixty minutes an hour, whatever he does, whoever he is? (as C.S. Lewis has wrote), past remains untouchable, only reachable by memory. Time seems to be asymmetric and directed: it always ?flows? from past to future, and never the other way around. Philosophical enquiry starts out when we raise the question for the very bedrock of such daily belief: what does ?time passes by? mean? What are our human experience about a passing time grounded in?Loads of ink has been spilled in the field of philosophy of physics in addressing the asymmetric and directed nature of time from our fundamental physical theories? perspective. According to this view, the human experience of time would be a reflection of an objective feature of the world: if time is to any extent physical, those time?s features should be pick out by our best physical theories. In this way, our experience of time would take roots in the reality itself. In the literature, the topic is commonly called ?the problem of the arrow of time? and is usually put in terms of invariance under time reversal, that is, whether our physical theories offer symmetric descriptions of the world when time is inverted. If dynamical equations belonging to our fundamental physical theories are non-time reversal invariant, fundamental physics is capable of distinguishing both time direction.Nevertheless, there seem to be a broad agreement among philosophers and physicists that fundamental physical theories are time reversal invariant, so that nothing at the formal level of physical theories would allow us to discriminate the past-to-future direction from the future-to-past one. Fundamental physics thus seems to be blind to the flow or passage of time, undermining any philosophical effort to ground it in the physical reality.I want to make the point that the problem of the arrow of time in physics has progressively become a tight pack of distinguishable problems and concepts that need to be disentangled. That generalized pessimistic conclusion deems from two assumptions that are rarely made clear: (a) the concept of time reversal is identical to that of reversibility; (b) the problem of the arrow of time in physics depends essentially on time reversal invariance.This paper aims to show that these assumptions are misguided. Firstly, I shall point out that time reversal invariance and reversibility can be conceptually distinguished, and that the problem of the arrow of time partially involves time reversal invariance but not reversibility. Secondly, I shall argue the roll of time reversal invariance has been exaggerated in the literature, thus misrepresenting the problem of the arrow of time. This argument allows us to ground two conceptually different accounts for the problem of the arrow of time: one based on dynamics and the other one on the geometrical properties of space-time. Finally, I shall call attention to problems of the account based on dynamics.