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

2. Two aspects of time:
Repetition
Change

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

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

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

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

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

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

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

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

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

12. 2nd Law of Thermodynamics: entropy increases in closed
systems as time passes.
Entropy
Time

13. 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

14. 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

15. 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.

16. 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

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

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

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

20. 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]

21. 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]

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

23. Maxwell’s Demon as a model for life

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

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

26. The span of life
[Levine 1997]
three
billion
heartbeats

27. 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

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