Posted on October 3, 2014 by Sean Carroll
Last week I spent an enjoyable few days
in Tenerife, one of the Canary Islands,
for a conference on the Philosophy of Cosmology.
The slides for all the talks are now online;
videos aren’t up yet,
but I understand they are forthcoming.
___________________________
Stephen Hawking
did not actually
attend our meeting
-- he was at the hotel
for a different event.
But he stopped by
for an informal session
on the arrow of time.
Photo by Vishnya Maudlin.
___________________________
It was a thought-provoking meeting,
but one of my thoughts was:
“We don’t really have
a well-defined field
called Philosophy of Cosmology.”
At least, not yet.
Talks were given
by philosophers
and by cosmologists;
the philosophers generally gave
good talks on the philosophy of physics,
while some of the cosmologists
gave solid-but-standard talks on cosmology.
Some of the other cosmologists
tried their hand at philosophy,
and I thought those
were generally less successful.
Which is to be expected
— it’s a sign that we need
to do more work
to set the foundations
for this new subdiscipline.
A big part of defining an area of study
is deciding on a set of questions
that we all agree are worth thinking about.
As a tiny step in that direction,
here is my attempt to highlight ten questions
— and various sub-questions —
that naturally fall under the rubric
of Philosophy of Cosmology.
They fall under other rubrics as well,
of course, as well as featuring
significant overlap with each other.
So there’s a certain amount
of arbitrariness here —
suggestions for improvements are welcome.
Here we go:
1. In what sense, if any,
is the universe fine-tuned?
When can we say
that physical parameters
(cosmological constant,
scale of electroweak symmetry breaking)
or initial conditions are “unnatural”?
What sets the appropriate measure
with respect to which we judge naturalness
of physical and cosmological parameters?
Is there an explanation
for cosmological coincidences
such as the approximate equality
between the density of matter
and vacuum energy?
Does inflation
solve these problems,
or exacerbate them?
What conclusions should we draw
from the existence of fine-tuning?
2. How is the arrow of time
related to the special state
of the early universe?
What is the best way
to formulate the past hypothesis
(the early universe
was in a low entropy state)
and the statistical postulate
(uniform distribution within macrostates)?
Can the early state
be explained
as a generic feature
of dynamical processes,
or is it associated with a specific
quantum state of the universe,
or should it be understood
as a separate law of nature?
In what way, if any, does
the special early state help explain
the temporal asymmetries of memory,
causality, and quantum measurement?
3. What is the proper role
of the anthropic principle?
Can anthropic reasoning
be used to make reliable predictions?
How do we define the appropriate
reference class of observers?
Given such a class,
is there any reason
to think of ourselves
as “typical” within it?
Does the prediction of freak observers
(Boltzmann Brains) count as evidence
against a cosmological scenario?
4. What part should
unobservable realms
play in cosmological models?
Does cosmic evolution naturally generate
pocket universes, baby universes,
or many branches of the wave function?
Are other “universes” part of science
if they can never be observed?
How do we evaluate such models,
and does the traditional process
of scientific theory choice
need to be adapted to account
for non-falsifiable predictions?
How confident can we ever be
in early-universe scenarios
such as inflation?
5. What is the quantum state
of the universe,
and how does it evolve?
Is there a unique prescription
for calculating the wave function
of the universe?
Under what conditions
are different parts
of the quantum state “real,”
in the sense that observers
within them should be counted?
What aspects of cosmology
depend on competing formulations
of quantum mechanics
(Everett, dynamical collapse,
hidden variables, etc.)?
Do quantum fluctuations
happen in equilibrium?
What role does
decoherence play
in cosmic evolution?
How does do quantum
and classical probabilities
arise in cosmological predictions?
What defines
classical histories
within the quantum state?
6. Are space and time
emergent or fundamental?
Is quantum gravity
a theory of quantized spacetime,
or is spacetime only
an approximation valid
in a certain regime?
What are the fundamental degrees of freedom?
Is there a well-defined
Hilbert space for the universe,
and what is its dimensionality?
Is time evolution fundamental,
or does time emerge from
correlations within a static state?
7. What is the role of infinity in cosmology?
Can the universe be infinitely big?
Are the fundamental laws ultimate discrete?
Can there be an essential difference
between “infinite” and “really big”?
Can the arrow of time be explained
if the universe has an infinite
amount of room in which to evolve?
Are there preferred ways
to compare infinitely big subsets
of an infinite space of states?
8. Can the universe have a beginning,
or can it be eternal?
Does a universe with a first moment
require a cause or deeper explanation?
Are there reasons why there is
something rather than nothing?
Can the universe be cyclic,
with a consistent arrow of time?
Could it be eternal
and statistically symmetric
around some moment of lowest entropy?
9. How do physical laws and causality
apply to the universe as a whole?
Can laws be said to change or evolve?
Does the universe as a whole
maximize some interesting quantity
such as simplicity, goodness,
interestingness, or fecundity?
Should laws be understood
as governing/generative entities,
or are they just a convenient way
to compactly represent
a large number of facts?
Is the universe complete in itself,
or does it require external factors
to sustain it?
Do the laws of physics
require ultimate explanations,
or can they simply be?
10. How do complex structures and order
come into existence and evolve?
Is complexity a transient phenomenon
that depends on entropy generation?
Are there general principles
governing physical, biological,
and psychological complexity?
Is the appearance of life
likely or inevitable?
Does consciousness
play a central role
in accounting for the universe?
Chances are very small
that anyone else
interested in the field,
forced at gunpoint
to pick the ten biggest questions,
would choose exactly these ten.
Such are the wild
and wooly early days
of any field,
when the frontier
is unexplored
and the conventional wisdom
has yet to be settled.
Feel free to make suggestions.
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