The game of prediction and retrodiction A constructivist perspective on the notion of time in science Radical Constructivism (Ernst von Glasersfeld): 1: Knowledge is learnt through active construction by the knower, not through passive "reception and storage" of data in the mind. 2: The purpose of knowledge construction is to organise the experiential world of the knower – it does not (and cannot) reflect any "objective truth" about the world.

Prediction: Eclipses and weather – modern magic..? Science: description of observed regularities in the world ↓ controlling the future Prediction and retrodiction: require a theoretical framework

A perspective on time classical physics - empty stage (absolute time) special relativity - space-time (relativistic time) general relativity - space-time (dynamic time) quantum physics - stochastic location in time Space and time: experiential categories (Kant) Object permanence: displacements in time and space Prediction / retrodiction: Time

The timeline Past > Now >Future “already happened" “happening" “not yet happened“ | | looks back in time looks forward in time | | memory expectation | | retrodiction prediction | | history scenario

Determinism future totally determined by past: vs free will (the world is pre-set) solipsism: vs shared experiential world (the world has no effect) Both rejected: logically irrefutable, but existentially irrelevant..! Radical constructivism : we have freedom to act Construction of knowledge: interplay between free will and nature unfolding in time

Motion in Time 1: stationary observer – moving events Past |Now| Future  … o o o o o…  …o…  … o o o o o … 2: moving observer – stationary events Past |Now  Future … o o o o o … …o… …o o o o o … 3: timelike worldline – point events Past  |Now|  Future … o o o o o … …o… … o o o o o …

Time windows in the perspective of prediction and retrodiction: long short short long range range range range Past Future | | | | (inferred) (experienced) (verifiable) (unverifiable)

Retrodiction and Prediction tp tn tf Past | |Now| | Future | | | | ? | ? | (past (future state) state state) + theory)

Classical physics Solar system orbits (eclipses): prediction, and retrodiction Sliding on a horizontal plane: prediction, but no retrodiction __________________________________________________ Geophysics – continental drift: hypothesising the past (weak retrodiction) Weather – chaotic systems: forecasting the future (weak prediction) Cosmology – the beginning and end of the Universe, prediction and retrodiction on a grand scale!

Quantum physics Dynamics described by Schrödinger equation: time evolution of probability distribution Predict probable future states, not the actual state Collapse of the wave function: one state is "realised", the others "disappear"! State selection is probabilistic: frequency ~ probability, Example – beta decay: how does the atom "know"..?

Probability: In classical physics, a "makeshift strategy": tossed coin, statistical mechanics. In quantum physics, not so: no hidden variables (Einstein) (Bell inequalities, Aspect experiment…) Hence: no prediction, and no retrodiction!

Predicted knowledge - epistemic status (radical constructivism) Constructed in the Now based on theory, on present observations, and (possibly) on additional assumptions In the short-range future window: verifiable by observation ---> support of the theory In the long-range future window: verification not possible Construction of knowledge: compatible with present-day theory and experience

Retrodiction – a historical perspective Retrospective sciences: plausible hypotheses about what may have happened in the past Using: data observed in the present. Past present: must then be compatible.

Retrospective sciences: Social sciences: psychology, sociology, political science, law, … History: the chronicling of human society Evolutionary biology: living organisms Geological sciences: geology, geophysics,… Cosmology: the past Universe

Retrodicted knowledge - epistemic status (radical constructivism) Prediction: the future is indeterminate Retrodiction: the past is fixed | ? True knowledge of the past: what "really happened"…? Short-range time window: memory, written records, legal proceedings, … Problematic: memory failure, false testimony, filtering, …

We construct our own past (short-range and long-range) in the present Now, by: recollections (of past events) perceptions (of present events) recorded information application of theory in a continual process of reflection: filtering selection interpretation (analysis)

Conclusion: a relativist epistemology Knowledge (of future and past) is constructed, based on: present-day perceptions / reflections, available recorded information choice of theoretical framework, Must be viable in this context. No conception of "true knowledge" of the past! Meaningless to ask: "what really happened back then"..!