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Sean Carroll
California Institute of Technology
Science and philosophy have changed
our image of the world
Our everyday, common-sense way of thinking about
the world (the...
Manifest image Scientific image
Time flows from past Time is a label within
to future an eternal universe
For every event ...
Ontology:
the study of being,
what exists
Reality
Monism
multiple one
Idealism
nature mind
Naturalism
Property
dualism
Phy...
Poetic Naturalism
• There is only one world, the natural world.
• We learn about it empirically, through science.
• But th...
[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
posterior
credence
likelihood of
evidence given theory
prior
credence
“Credence” = “degree of belief.”
...
Conservation of momentum:
the world moves by itself
Ibn Sina
Ibn Sina: Adam Jones; Voyager: NASA/JPL. Wikimedia commons.
“An intellect which at a certain moment would know all forces
and all positions of all items of which nature is composed…
...
Laws are differential equations in time.
Isaac Newton:
Erwin Schrödinger:
(Footnote: quantum indeterminism?)
Time doesn’t “flow,”
or bring the future
into existence; it’s
just a label.
Laws of nature are
patterns connecting
differe...
“Cause and effect” isn’t fundamental
“The law of causality, I believe,
like much that passes
muster among philosophers,
is...
Aging Memory
Cause & EffectFree Will
Major disconnect between fundamental physics
and everyday life: the arrow of time.
No...
A single phenomenon underlies all
manifestations of time’s arrow: increasing entropy.
Entropy is a measure of disorderline...
Time
Entropy
Second 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 changing
its basic appearan...
possible arrangements
of atoms/molecules,
grouped by macroscopic
indistinguishability
Entropy increases
simply because the...
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 of
an underlying time-symmetric universe that has a
macroscopic arrow of tim...
Low-entropy past gives
features of the present
leverage over other times.
what we know
about the present
low-entropy
past
...
Emergence:
different levels of description involve
completely different concepts/vocabularies
Macroscopic world
Microscopi...
1. Cosmos, Time, Memory
images of the world; causality; the arrow of time
2. The Stuff of which We Are Made
quantum theory...
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Gifford Lecture One: Cosmos, Time, Memory

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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.

Published in: Science

Gifford Lecture One: Cosmos, Time, Memory

  1. 1. Sean Carroll California Institute of Technology
  2. 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. 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. 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. 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. 6. [Allesandro Bianchi, Reuters] Cause and effect, motion and movers, reasons why
  7. 7. Everything that happens has a cause/reason? Aristotle: Eric Gaba. Aristotle Spinoza Leibniz
  8. 8. We learn about the world by looking at it (not just by thinking about it) Hume Bayes
  9. 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. 10. Conservation of momentum: the world moves by itself Ibn Sina Ibn Sina: Adam Jones; Voyager: NASA/JPL. Wikimedia commons.
  11. 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. 12. Laws are differential equations in time. Isaac Newton: Erwin Schrödinger: (Footnote: quantum indeterminism?)
  13. 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. 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. 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. 16. A single phenomenon underlies all manifestations of time’s arrow: increasing entropy. Entropy is a measure of disorderliness, messiness, randomness.
  17. 17. Time Entropy Second Law of Thermodynamics: entropy increases with time (in closed systems).
  18. 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. 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. 20. The Past Hypothesis: our universe started in a low-entropy state. 13.8 billion years ago, at the Big Bang.
  21. 21. 1 second: hot, smooth plasma.
  22. 22. 380,000 years: ripples in a smooth background [Planck]
  23. 23. 1010 years: stars and galaxies.
  24. 24. 1015 years: black holes and rocks.
  25. 25. 10100 years: empty space (dark energy).
  26. 26. 1 sec 105 yr 1010 yr 1015 yr 10100 yr Our observable universe
  27. 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. 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. 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. 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

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