The Arrow of Time and the Meaning of Life Sean M. Carroll Caltech

Two aspects of time: Repetition Change

Repetition -- predictable, cyclic behavior -- is what allows us to measure time.

Rhythms of the heart, brain, and body provide us with internal clocks.

But change is the more interesting -- and mysterious! -- aspect of time.

Just as organisms change and evolve, so do stars, and so does the universe.

A profound difference between time and space: time has a direction, space does not. [NASA]

The arrow of time -- the difference between past and future -- is one of reality’s most blatant features.

Aspects of Time’s Arrow Aging Memory Free Will Cause & Effect

The fundamental laws of nature have no arrow of time. Simple (“fundamental”) motions are reversible. Macroscopic evolution is irreversible.

A single phenomenon underlies all manifestations of time’s arrow: increasing entropy. Entropy is a measure of disorderliness, messiness, randomness.

2nd Law of Thermodynamics: entropy increases in closed systems as time passes. Entropy Time

Entropy counts the number of ways we can re-arrange Ludwig Boltzmann, 1870’s: Entropy counts the number of ways we can re-arrange a system without changing its basic appearance. [Martin Röll] low entropy: delicately ordered high entropy: all mixed up time

Entropy increases simply because there are more ways to low entropy Entropy increases simply because there are more ways to be high-entropy than low-entropy. All makes sense, if the entropy was low to begin with. high entropy arrangements of atoms, grouped into sets that appear the same macroscopically

our universe started in a low-entropy state. The Past Hypothesis: our universe started in a low-entropy state. 13.7 billion years ago, at the Big Bang.

correct reconstruction The past hypothesis helps reconcile reversible microphysics with macroscopic directionality. Why do we remember the past and not the future? possible futures possible pasts low-entropy past correct reconstruction what we know about the present

Cosmologists are faced with a puzzle: why did the early universe have a low entropy? [Sky & Telescope]

Complexity first increases, Entropy vs. Complexity Entropy increases. Complexity first increases, then decreases.

Ephemeral Complexity in the Universe 1 sec 105 yr 1010 yr 1015 yr 10100 yr

Complexity isn’t merely compatible with the Second Law, it’s a Origin of life: Complexity isn’t merely compatible with the Second Law, it’s a consequence of increasing entropy. [Yung, Russell & Parkinson]

organisms take low-entropy energy and degrade it into Sustaining life: energy is constant, but organisms take low-entropy energy and degrade it into high-entropy energy. [Penrose]

Two ways to be stationary Microstasis: individual constituents are static Homeostasis: individual constituents in motion, but regulated macroscopic stationarity. Requires entropy production.

Maxwell’s Demon as a model for life

Thinking and consciousness: the C. Elegans nematode 302 neurons [Bob Goldstein, UNC Chapel Hill, Wikimedia commons; OpenWorm project]

Evolution adapts memory to use for imagination. What role does thinking about the future play in the origin and nature of consciousness?

The span of life [Levine 1997] three billion heartbeats

Configuration space is big, the universe is young and small Age of observable universe: 1017 seconds Number of particles in observable universe: 1088 Human-sized DNA sequences: 101,000,000,000 Universal recurrence time: 1010 120 P vs. NP: it’s much easier to check solutions to problems than to find them

The universe is big, and there’s a lot we don’t understand. [NASA/Hubble]