Sean CarrollCalifornia Institute of Technology

Science and philosophy have changedour image of the world

Our everyday, common-sense way of thinking about the world (the “manifest image”) has not caught up.

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 and transcendent and constructed

Ontology: the study of being,

what existsReality

Monism

multiple one

Idealism

nature mind

Naturalism

Propertydualism Physicalism

extra-physical physical

Austerenaturalism

Poeticnaturalism

eliminative expansive

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-physicsway of talking about the world is compatible with the everyday-life way of talking about it.

[Allesandro Bianchi, Reuters]

Cause and effect, motion and movers,reasons why

Everything that happens has a cause/reason?

Aristotle: Eric Gaba.

Aristotle Spinoza Leibniz

We learn about the world by looking at it(not just by thinking about it)

Hume Bayes

Bayesian reasoning

posteriorcredence

likelihood ofevidence given theory

priorcredence

“Credence” = “degree of belief.”

1) Assign priors.2) Calculate likelihoods.3) Gather evidence.4) Update credences.

Steps:

Conservation of momentum:the world moves by itself

Ibn SinaIbn Sina: Adam Jones; Voyager: NASA/JPL. Wikimedia commons.

“An intellect which at a certain moment would know all forcesand 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

Laws are differential equations in time.

Isaac Newton:

Erwin Schrödinger:

(Footnote: quantum indeterminism?)

Time doesn’t “flow,”or bring the futureinto existence; it’sjust a label.

Laws of nature arepatterns connectingdifferent moments.

time

“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

Aging Memory

Cause & EffectFree Will

Major disconnect between fundamental physicsand everyday life: the arrow of time.

Nowhere to be found in the underlying, microscopic laws.

A single phenomenon underlies allmanifestations of time’s arrow: increasing entropy.

Entropy is a measure of disorderliness,messiness, randomness.

Time

Entro

pySecond Law of Thermodynamics:entropy increases with time(in closed systems).

Ludwig Boltzmann, 1870’s:

Entropy counts the number of ways we can re-arrange a system without changingits basic appearance.

high entropy:all mixed up

low entropy:delicatelyordered

[Martin Röll]

possible arrangementsof atoms/molecules,grouped by macroscopicindistinguishability

Entropy increasessimply because thereare more ways to be high-entropy than low-entropy.

All makes sense, if the entropy waslow to begin with.

lowentropy

highentropy

The Past Hypothesis:our universe started in a low-entropy state.

13.8 billion years ago, at the Big Bang.

1 second: hot, smooth plasma.

380,000 years: ripples in a smooth background

[Planck]

1010 years: stars and galaxies.

1015 years: black holes and rocks.

10100 years: empty space (dark energy).

1 sec 105 yr 1010 yr 1015 yr 10100 yr

Our observable universe

“Memories” and “causes” are emergent features ofan underlying time-symmetric universe that has amacroscopic arrow of time.

Memory: feature of nowthat lets us infer somethingabout the past.

Cause: feature of nowthat lets us infer somethingabout the future.

Low-entropy past gives features of the presentleverage over other times.

what we knowabout the present

low-entropypast

correct reconstruction

possiblefuturespossible

pasts

Emergence:different levels of description involve

completely different concepts/vocabularies

Macroscopic world

Microscopic fundamental physics

tableschairs

people planets

particlesforces

spacetime

• arrow of time• dissipation• cause and effect• “reasons why”

• laws of nature• patterns• differential equations• conservation of

information

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