Exploring the Nature of Time: Concepts, Change, and Perception

1. one time and temporality

2. “Time is what keeps everything from happening at once”

3. What is time? • What does the phrase: “the temporal world” mean? • What is it contrasted with?

4. Time is the medium of change • Temporal entities • Have beginnings and ends • Undergo change • Undergo motion • The phrase “temporal world” comes from Catholic metaphysics • Augustine: God isn’t temporal • God wasn’t created / didn’t have a beginning • Change is seen as implying imperfection • In Buddhist philosophy, impermanence and change are seen as • Fundamental conditions of existence of all objects • The ultimate source of neurosis and suffering

5. Cartoon metaphysics • The world consists of a set of objects • Objects occasionally get created and destroyed • Objects have states • Spatial: position, speed, etc. etc. • Other: alive/dead, hot/cold • Relations to one another: above/below, loving/hating, etc. • Object state changes • On its own • As a result of interactions with other objects This is, by and large, the ontology used in most computer simulations (VR, video games, etc.)

6. Causes and effect • Time is asymmetric • This is deeply weird • It drives the physicists and philosophers crazy • If I drop a glass, it breaks • The dropping always comes “first” • The glass never jumps up or reassembles itself • And we say the dropping “caused” the breaking

7. Phenomenological time Time is also asymmetric w.r.t. knowledge and action • Past • Partially knowable (through memory) • Unchangeable • Present • Knowable • Arguably changeable • Future • Partially changeable • Completely unknown (except by extrapolating from the past)

8. Time is the medium of existence (Existence means roughly the activity of living) • Care and the World • Some states and changes matter to us • We (generally) don’t want to die, go bankrupt, etc. • We (generally) do want to graduate, get a job, etc. • Certain changes are caused by us in order to continue our involvements with the World • These are called actions

9. Narrative (story) Sequence of events told from a point of view • Narrator (person whose point of view is used) • Story (sequence of events that happened) • Plot (sequence in which the events are given) • Characters w/goals, relationships, etc. • Conflicts

10. Aristotelian mechanics Basic tendency of objects is to stop • Natural motion • Basic tendency of things is to go to their natural position • Solid things move to the ground and stop • Smoke and fire move to the air and stop/disappear • Violent motion • Some force moves an object away from its natural place • Celestial motion • Everything naturally moves in circles • There is no violent celestial motion

11. Newtonian mechanics Basic tendency is to move at constant speed • Objects exert forces on one another • Gravity • Friction • Collision • Etc. • Forces change an object’s speed

12. Time is also like space • We can talk about points in time (now, next Wednesday at midnight, etc.) • We can talk about directions in time (before/after) • We can talk about distances in time (one hour, fifteen picoseconds, etc.) (And General Relativity says time it actually part of space)

13. Dynamic systems • Space of possible states • Called a “state space” or “phase space” • Law of motion defines how state evolves over time • Discrete time • Gives new state in terms of current state • Continuous time • Gives rate of change in state in terms of current state • State is then a function of time • Function can be derived from the law of motion and the initial state

14. Example: Population dynamics • Suppose you’re baking bread • You start with one yeast cell • Each minute the yeast cells divide into two yeast cells • (Yea, yea, I know yeast doesn’t really work this way, but pretend it does for the sake of the example) • State space: number of yeast cells(a number between 0 and ∞) • Law of motion • Number of yeast cells at time t is twice the number at time t-1 • State function • Cells(t) = 2t

15. Example: A falling object • Suppose we drop a Kenny from South Park from the Sears Tower • State space • Height (starts at 1450 feet) • Velocity (starts at 0 feet per second) • Law of motion • Height decreases by speed units each second • Speed increases by 32 feet per second each second • State function (apologies to those who don’t know calculus) • Speed(t) = 32t • Height(t) = 1450-16t2

16. Perception of time • Perception of duration • Depends on a number of factors, including heart rate • Gary? • Perception of ordering • Humans actually turn out to be very bad at this • It’s a skill we learn, but doesn’t seem to be built-in • Perception of rhythm • Uh, class starts in 15 minutes and I haven’t had a chance to look up any thing on this … Gary?

17. Perception of motion • The human visual system has two different systems for detecting motion • The “gradient-based” system • Detects small motions over short time scales • The “discrete” system(aka the apparent motion system) • Effectively matches features between consecutive images • Allows film and video to work • Wagon-wheel illusions

18. Perception of change • Change blindness • Human perception of change relies on • Memory • Detection of visual motion • Easily fooled • Film continuity