Based on my book The Big Picture, this is the first of five lectures exploring how different ways of talking about the world fit together. The other four lectures are on YouTube.

1. Sean Carroll
California Institute of Technology

2. Science and philosophy have changed
our image of the world
Our everyday, common-sense way of thinking about
the world (the “manifest image”) has not caught up.

3. Manifest image Scientific image
Time flows from past Time is a label within
to future an eternal universe
For every event there is Parts of the
universe
a reason; for every are related by patterns:
effect there is a cause the laws of nature
Life and mind are Life and mind are
distinct from matter physical and emergent
Purpose, morality, and Purpose, morality and
meaning are objective meaning are
personal

4. Ontology:
the study of being,
what exists
Reality
Monism
multiple one
Idealism
nature mind
Naturalism
Property
dualism
Physicalism
extra-
physical physical
Austere
naturalism
Poetic
naturalism
eliminative expansive

5. Poetic Naturalism
• There is only one world, the natural world.
• We learn about it empirically, through science.
• But there are many ways of talking about the world.
• If a way of talking accurately describes (part of)
the world, the concepts it refers to are real.
Our task: understand how the fundamental-physics
way of talking about the world is compatible with
the everyday-life way of talking about it.

6. [Allesandro Bianchi, Reuters]
Cause and effect, motion and movers,
reasons why

7. Everything that happens has a cause/reason?
Aristotle: Eric Gaba.
Aristotle Spinoza Leibniz

8. We learn about the world by looking at it
(not just by thinking about it)
Hume Bayes

9. Bayesian reasoning
posterior
credence
likelihood of
evidence given theory
prior
credence
“Credence” = “degree of belief.”
1) Assign priors.
2) Calculate likelihoods.
3) Gather evidence.
4) Update credences.
Steps:

10. Conservation of momentum:
the world moves by itself
Ibn Sina
Ibn Sina: Adam Jones; Voyager: NASA/JPL. Wikimedia commons.

11. “An intellect which at a certain moment would know all forces
and all positions of all items of which nature is composed…
for such an intellect nothing would be uncertain and the future
just like the past would be present before its eyes.”
- An Essay on Probabilities, 1814
Laplace’s Demon.
Laplace: Wikimedia Commons
Conservation of information:
each moment determines
its past and future

12. Laws are differential equations in time.
Isaac Newton:
Erwin Schrödinger:
(Footnote: quantum indeterminism?)

13. Time doesn’t “flow,”
or bring the future
into existence; it’s
just a label.
Laws of nature are
patterns connecting
different moments.
time

14. “Cause and effect” isn’t fundamental
“The law of causality, I believe,
like much that passes
muster among philosophers,
is a relic of a bygone age,
surviving, like the monarchy,
only because it is erroneously
supposed to do no harm.”
– Bertrand Russell

15. Aging Memory
Cause & EffectFree Will
Major disconnect between fundamental physics
and everyday life: the arrow of time.
Nowhere to be found in the underlying, microscopic laws.

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

17. Time
Entropy
Second Law of Thermodynamics:
entropy increases with time
(in closed systems).

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

19. possible arrangements
of atoms/molecules,
grouped by macroscopic
indistinguishability
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.
low
entropy
high
entropy

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

21. 1 second: hot, smooth plasma.

22. 380,000 years: ripples in a smooth background
[Planck]

23. 1010
years: stars and galaxies.

24. 1015
years: black holes and rocks.

25. 10100
years: empty space (dark energy).

26. 1 sec 105
yr 1010
yr 1015
yr 10100
yr
Our observable universe

27. “Memories” and “causes” are emergent features of
an underlying time-symmetric universe that has a
macroscopic arrow of time.
Memory: feature of now
that lets us infer something
about the past.
Cause: feature of now
that lets us infer something
about the future.

28. Low-entropy past gives
features of the present
leverage over other times.
what we know
about the present
low-entropy
past
correct reconstruction
possible
futures
possible
pasts

29. Emergence:
different levels of description involve
completely different concepts/vocabularies
Macroscopic world
Microscopic fundamental physics
tables
chairs
people
planets
particles
forces
spacetime
• arrow of time
• dissipation
• cause and effect
• “reasons why”
• laws of nature
• patterns
• differential equations
• conservation of
information

30. 1. Cosmos, Time, Memory
images of the world; causality; the arrow of time
2. The Stuff of which We Are Made
quantum theory; the laws underlying everyday life
3. Layers of Reality
effective theories; emergence; multiple vocabularies
4. Simplicity, Complexity, Thought
entropy vs. complexity; life; consciousness
5. Our Place in the Universe
ought vs. is; meaning, caring, constructing morals