• No se han encontrado resultados

Philosophy of Cosmology. A Glimpse from the Outside

N/A
N/A
Protected

Academic year: 2022

Share "Philosophy of Cosmology. A Glimpse from the Outside"

Copied!
36
0
0

Texto completo

(1)

Philosophy of Cosmology. A Glimpse from the Outside

Jeremy Butterfield

Trinity College, Cambridge

Granada – September 2011

(2)

Plan of Talk

1 Setting aside ...

2 Explaining initial conditions, and the values of constants?

Initial considerations What is explanation?

Anthropic explanation, selection effects

3 The multiverse A naive definition

The scheme of explanation

What role for the other alternatives?

4 Probability and explanation revisited

5 Observationally indistinguishable spacetimes

(3)

1. Setting aside ...

Setting aside ...

Old metaphysical conundrums e.g.: whether time can have a beginning or end, whether space can be infinite; alleged antinomies ... global causal pathologies (CTCs) allowed by local relativistic physics.

Old epistemological conundrums e.g.:

(i) whether there can be a science of cosmology given that there is only one universe—rather the laws of cosmology are the laws of relativistic and quantum physics;

(ii) how to justify cosmological principles that constrain models—surely inductively. (Not by their modesty, since there can be selection effects.)

(4)

1. Setting aside ...

Setting aside ...

Old metaphysical conundrums e.g.: whether time can have a beginning or end, whether space can be infinite; alleged antinomies ... global causal pathologies (CTCs) allowed by local relativistic physics.

Old epistemological conundrums e.g.:

(i) whether there can be a science of cosmology given that there is only one universe—rather the laws of cosmology are the laws of relativistic and quantum physics;

(ii) how to justify cosmological principles that constrain models—surely inductively. (Not by their modesty, since there can be selection effects.)

(5)

1. Setting aside ...

Setting aside ...

The philosophy of general relativity: But agreed: global aspects impinge, e.g. observationally indistinguishable spacetimes.

The search for quantum gravity! ... We are ‘halfway through the woods’.

But agreed: a compulsory subject ... a role for philosophy?

(6)

1. Setting aside ...

Setting aside ...

The philosophy of general relativity: But agreed: global aspects impinge, e.g. observationally indistinguishable spacetimes.

The search for quantum gravity! ... We are ‘halfway through the woods’.

But agreed: a compulsory subject ... a role for philosophy?

(7)

1. Setting aside ...

This leaves...

(A): Cosmology as a source for case studies in philosophy: e.g.

(i)idea of a purely observational science;

(ii)the calibration of instruments, e.g. for the distance ladder.

(B): Initial/boundary conditions and the values of parameters as a topic for assessment and explanation, unlike in most of physics.

Recall Boltzmann’s suggestion that thermo-statistical physics resort to cosmology, to explain the direction of time.

(8)

1. Setting aside ...

This leaves...

(A): Cosmology as a source for case studies in philosophy: e.g.

(i)idea of a purely observational science;

(ii)the calibration of instruments, e.g. for the distance ladder.

(B): Initial/boundary conditions and the values of parameters as a topic for assessment and explanation, unlike in most of physics.

Recall Boltzmann’s suggestion that thermo-statistical physics resort to cosmology, to explain the direction of time.

(9)

2. Explaining initial conditions, and the values of constants? 2.1: Initial considerations

Initial considerations

Standard example: the usual motivations for inflation: flatness and horizon problems.

Issues: (i)the regress of explanation about fine-tuning: (addressed by eternal inflation; which suggests a multiverse).

(ii): the need to address the singularity; e.g. the no-boundary proposal.

Wilczek and Weinberg see four reasons for recent emphasis: (i)SM’s parameters being brute facts;

(ii)inflationary cosmology’s multiverse (iii) the cosmological constant problem (iv) the string landscape: a plethora.

(10)

2. Explaining initial conditions, and the values of constants? 2.1: Initial considerations

Initial considerations

Standard example: the usual motivations for inflation: flatness and horizon problems.

Issues: (i)the regress of explanation about fine-tuning: (addressed by eternal inflation; which suggests a multiverse).

(ii): the need to address the singularity; e.g. the no-boundary proposal.

Wilczek and Weinberg see four reasons for recent emphasis:

(i)SM’s parameters being brute facts;

(ii)inflationary cosmology’s multiverse (iii) the cosmological constant problem (iv) the string landscape: a plethora.

(11)

2. Explaining initial conditions, and the values of constants? 2.1: Initial considerations

Initial considerations

Standard example: the usual motivations for inflation: flatness and horizon problems.

Issues: (i)the regress of explanation about fine-tuning: (addressed by eternal inflation; which suggests a multiverse).

(ii): the need to address the singularity; e.g. the no-boundary proposal.

Wilczek and Weinberg see four reasons for recent emphasis:

(i)SM’s parameters being brute facts;

(ii)inflationary cosmology’s multiverse (iii) the cosmological constant problem (iv) the string landscape: a plethora.

(12)

2. Explaining initial conditions, and the values of constants? 2.2 What is explanation?

What is explanation?

I think there is no essence of explanation. But many (cf. Hempel) take it to require : deduction from true premises that include a law (universal generalization; cf. Hume) of a confirmed theory: of (i)the explanandum (event); or of (ii)the high probability of the event; or of (iii) even the low probability of the event.

(Section 4 will return to whether to require (i)or (ii), or be content with (iii).)

Many philosophers advocate other constraints: that explanation must: i) unify (cf. confirming theory by novel predictions);

ii) cite the event’s causal history; iii)not include redundant information.

(13)

2. Explaining initial conditions, and the values of constants? 2.2 What is explanation?

What is explanation?

I think there is no essence of explanation. But many (cf. Hempel) take it to require : deduction from true premises that include a law (universal generalization; cf. Hume) of a confirmed theory: of (i)the explanandum (event); or of (ii)the high probability of the event; or of (iii) even the low probability of the event.

(Section 4 will return to whether to require (i)or (ii), or be content with (iii).)

Many philosophers advocate other constraints: that explanation must:

i) unify (cf. confirming theory by novel predictions);

ii) cite the event’s causal history;

iii)not include redundant information.

(14)

2. Explaining initial conditions, and the values of constants? 2.2 What is explanation?

What is explanation?

I think there is no essence of explanation. But many (cf. Hempel) take it to require : deduction from true premises that include a law (universal generalization; cf. Hume) of a confirmed theory: of (i)the explanandum (event); or of (ii)the high probability of the event; or of (iii) even the low probability of the event.

(Section 4 will return to whether to require (i)or (ii), or be content with (iii).)

Many philosophers advocate other constraints: that explanation must:

i) unify (cf. confirming theory by novel predictions);

ii) cite the event’s causal history;

iii)not include redundant information.

(15)

2. Explaining initial conditions, and the values of constants? 2.3: Anthropic explanation, selection effects

Anthropic explanation, selection effects

Conditioning on the existence of observers etc will of course alter probabilities!

But this can make for explanations that violate constraints ii)and iii):

that explanations must cite the causal history, and must not include irrelevant information.

(16)

2. Explaining initial conditions, and the values of constants? 2.3: Anthropic explanation, selection effects

Anthropic explanation, selection effects

Conditioning on the existence of observers etc will of course alter probabilities!

But this can make for explanations that violate constraints ii)and iii):

that explanations must cite the causal history, and must not include irrelevant information.

(17)

3. The multiverse 3.1: A naive definition

A naive definition

Let a multiverse be a spacetime that is topologically connected, but comprises disjoint, or effectively disjoint, parts that are causally disconnected—or effectively causally disconnected.

This definition is not as classical as it seems! The causally disconnected parts could be branches `a la Everettian quantum theory.

(18)

3. The multiverse 3.1: A naive definition

A naive definition

Let a multiverse be a spacetime that is topologically connected, but comprises disjoint, or effectively disjoint, parts that are causally disconnected—or effectively causally disconnected.

This definition is not as classical as it seems! The causally disconnected parts could be branches `a la Everettian quantum theory.

(19)

3. The multiverse 3.1: A naive definition

A naive definition

Contrast philosophers’ possible worlds (Leibniz, Tegmark!: e.g. a proposition is modelled as a set of worlds). Most philosophers believe:

(i)each possible world is ‘abstract’, except for the one ‘concrete’

actual world;

(ii)each possible world is connected, and disconnected from every other;

(iii) any consistent theory, with any parameter values, is true at some possible worlds.

Each of these features distinguishes such worlds from:

(a)Everettian ‘branches’;

(b) universes in a cosmological multiverse.

(20)

3. The multiverse 3.1: A naive definition

A naive definition

Contrast philosophers’ possible worlds (Leibniz, Tegmark!: e.g. a proposition is modelled as a set of worlds). Most philosophers believe:

(i)each possible world is ‘abstract’, except for the one ‘concrete’

actual world;

(ii)each possible world is connected, and disconnected from every other;

(iii) any consistent theory, with any parameter values, is true at some possible worlds.

Each of these features distinguishes such worlds from:

(a)Everettian ‘branches’;

(b) universes in a cosmological multiverse.

(21)

3. The multiverse 3.2: The scheme of explanation

The scheme of explanation

Suppose we observe a parameter p to equal p0, while the laws dictate only that p ∈ R, with p0 ∈ R. We ask: why does p have value p0?

The multiverse answer: That p has value p0 is explained (at least: is un-puzzling), because it is only parochially true. In each part of the multiverse suitable observers (if such there be, or could be, in the given part) will observe their part’s parochial value.

Reply: Maybe: being told that a case is one of many equally real cases makes it un-puzzling why we find ourselves in case p0 as against p1 etc. Agreed, it suggests that p0 is ‘nothing special’, or ‘is sheer happenstance’. But what about the popular constraints on explanation in Section 2.2?

(22)

3. The multiverse 3.2: The scheme of explanation

The scheme of explanation

Suppose we observe a parameter p to equal p0, while the laws dictate only that p ∈ R, with p0 ∈ R. We ask: why does p have value p0?

The multiverse answer: That p has value p0 is explained (at least: is un-puzzling), because it is only parochially true. In each part of the multiverse suitable observers (if such there be, or could be, in the given part) will observe their part’s parochial value.

Reply: Maybe: being told that a case is one of many equally real cases makes it un-puzzling why we find ourselves in case p0 as against p1 etc.

Agreed, it suggests that p0 is ‘nothing special’, or ‘is sheer happenstance’. But what about the popular constraints on explanation in Section 2.2?

(23)

3. The multiverse 3.2: The scheme of explanation

The scheme of explanation

Suppose we observe a parameter p to equal p0, while the laws dictate only that p ∈ R, with p0 ∈ R. We ask: why does p have value p0?

The multiverse answer: That p has value p0 is explained (at least: is un-puzzling), because it is only parochially true. In each part of the multiverse suitable observers (if such there be, or could be, in the given part) will observe their part’s parochial value.

Reply: Maybe: being told that a case is one of many equally real cases makes it un-puzzling why we find ourselves in case p0 as against p1 etc.

Agreed, it suggests that p0 is ‘nothing special’, or ‘is sheer happenstance’.

But what about the popular constraints on explanation in Section 2.2?

(24)

3. The multiverse 3.2: The scheme of explanation

The scheme of explanation

Suppose we observe a parameter p to equal p0, while the laws dictate only that p ∈ R, with p0 ∈ R. We ask: why does p have value p0?

The multiverse answer: That p has value p0 is explained (at least: is un-puzzling), because it is only parochially true. In each part of the multiverse suitable observers (if such there be, or could be, in the given part) will observe their part’s parochial value.

Reply: Maybe: being told that a case is one of many equally real cases makes it un-puzzling why we find ourselves in case p0 as against p1 etc.

Agreed, it suggests that p0 is ‘nothing special’, or ‘is sheer happenstance’.

But what about the popular constraints on explanation in Section 2.2?

(25)

3. The multiverse 3.2: The scheme of explanation

The scheme of explanation

Suppose we observe a parameter p to equal p0, while the laws dictate only that p ∈ R, with p0 ∈ R. We ask: why does p have value p0?

The multiverse answer: That p has value p0 is explained (at least: is un-puzzling), because it is only parochially true. In each part of the multiverse suitable observers (if such there be, or could be, in the given part) will observe their part’s parochial value.

Reply: Maybe: being told that a case is one of many equally real cases makes it un-puzzling why we find ourselves in case p0 as against p1 etc.

Agreed, it suggests that p0 is ‘nothing special’, or ‘is sheer happenstance’.

But what about the popular constraints on explanation in Section 2.2?

(26)

3. The multiverse 3.3: What role for the other alternatives?

What role for the other alternatives?

Explanation apart: the ‘nothing special’ thought that allays puzzlement does NOT require the equal reality of the other parts of the multiverse.

Think of a probabilistic process, in the usual sense. We explain the outcome by citing the process. We might say the explanation shows the outcome is ‘nothing special’. But the explanation succeeds without the equal reality of the alternatives.

Beware of thinking that their equal reality lets one off the hook of doing any further explaining:

“All the cases are ‘out there’; they vary in the ways laid out by our theory. So there is no more to say/explain.”

No: the ‘So’ is a non-sequitur.

(27)

3. The multiverse 3.3: What role for the other alternatives?

What role for the other alternatives?

Explanation apart: the ‘nothing special’ thought that allays puzzlement does NOT require the equal reality of the other parts of the multiverse.

Think of a probabilistic process, in the usual sense. We explain the outcome by citing the process. We might say the explanation shows the outcome is ‘nothing special’. But the explanation succeeds without the equal reality of the alternatives.

Beware of thinking that their equal reality lets one off the hook of doing any further explaining:

“All the cases are ‘out there’; they vary in the ways laid out by our theory. So there is no more to say/explain.”

No: the ‘So’ is a non-sequitur.

(28)

3. The multiverse 3.3: What role for the other alternatives?

What role for the other alternatives?

Explanation apart: the ‘nothing special’ thought that allays puzzlement does NOT require the equal reality of the other parts of the multiverse.

Think of a probabilistic process, in the usual sense. We explain the outcome by citing the process. We might say the explanation shows the outcome is ‘nothing special’. But the explanation succeeds without the equal reality of the alternatives.

Beware of thinking that their equal reality lets one off the hook of doing any further explaining:

“All the cases are ‘out there’; they vary in the ways laid out by our theory. So there is no more to say/explain.”

No: the ‘So’ is a non-sequitur.

(29)

4. Probability and explanation revisited

Probability and explanation revisited

Beware of issues of measure!

Two general worries (applying equally to the Bayesian and Likelihood schools):

a)Suppose the theory’s probability distribution has more than one peak. Even if our observation is under a peak, it is natural to ask: why this peak not another one?

b)Suppose the theory gives no information at all, not even a probability distribution, e.g. for the value of a parameter. Then using a flat distribution looks wrong.

(30)

4. Probability and explanation revisited

Probability and explanation revisited

Beware of issues of measure!

Two general worries (applying equally to the Bayesian and Likelihood schools):

a)Suppose the theory’s probability distribution has more than one peak. Even if our observation is under a peak, it is natural to ask: why this peak not another one?

b)Suppose the theory gives no information at all, not even a probability distribution, e.g. for the value of a parameter. Then using a flat distribution looks wrong.

(31)

4. Probability and explanation revisited

Probability and explanation revisited

Beware of issues of measure!

Two general worries (applying equally to the Bayesian and Likelihood schools):

a)Suppose the theory’s probability distribution has more than one peak. Even if our observation is under a peak, it is natural to ask: why this peak not another one?

b)Suppose the theory gives no information at all, not even a probability distribution, e.g. for the value of a parameter. Then using a flat distribution looks wrong.

(32)

4. Probability and explanation revisited

Probability and explanation revisited

Beware of issues of measure!

Two general worries (applying equally to the Bayesian and Likelihood schools):

a)Suppose the theory’s probability distribution has more than one peak. Even if our observation is under a peak, it is natural to ask: why this peak not another one?

b)Suppose the theory gives no information at all, not even a probability distribution, e.g. for the value of a parameter. Then using a flat distribution looks wrong.

(33)

5. Observationally indistinguishable spacetimes

Observationally indistinguishable spacetimes

Almost every spacetime is observationally indistinguishable from another.

A spacetime (M, g ) is observationally indistinguishable from (M0, g0) iff for all points p ∈ M, there is a point p0 ∈ M0 such that I(p) and I(p0) are isometric.

Let (M, g ) be a spacetime that:

(i)is not causally bizarre (i.e. there is no point p ∈ M such that I(p) = M), and

(ii)satisfies any set Γ of conditions that are local (i.e. any two locally isometric spacetimes either both satisfy the condition, or both violate it).

Then there is another spacetime (M0, g0) such that: (i): (M0, g0) satisfies Γ

(ii)(M, g ) is observationally indistinguishable from (M0, g0).

(34)

5. Observationally indistinguishable spacetimes

Observationally indistinguishable spacetimes

Almost every spacetime is observationally indistinguishable from another.

A spacetime (M, g ) is observationally indistinguishable from (M0, g0) iff for all points p ∈ M, there is a point p0 ∈ M0 such that I(p) and I(p0) are isometric.

Let (M, g ) be a spacetime that:

(i)is not causally bizarre (i.e. there is no point p ∈ M such that I(p) = M), and

(ii)satisfies any set Γ of conditions that are local (i.e. any two locally isometric spacetimes either both satisfy the condition, or both violate it).

Then there is another spacetime (M0, g0) such that:

(i): (M0, g0) satisfies Γ

(ii)(M, g ) is observationally indistinguishable from (M0, g0).

(35)

5. Observationally indistinguishable spacetimes

Observationally indistinguishable spacetimes

Almost every spacetime is observationally indistinguishable from another.

A spacetime (M, g ) is observationally indistinguishable from (M0, g0) iff for all points p ∈ M, there is a point p0 ∈ M0 such that I(p) and I(p0) are isometric.

Let (M, g ) be a spacetime that:

(i)is not causally bizarre (i.e. there is no point p ∈ M such that I(p) = M), and

(ii)satisfies any set Γ of conditions that are local (i.e. any two locally isometric spacetimes either both satisfy the condition, or both violate it).

Then there is another spacetime (M0, g0) such that:

(i): (M0, g0) satisfies Γ

(ii)(M, g ) is observationally indistinguishable from (M0, g0).

(36)

References

References

Manchak, J (2009), ‘Can we know the global structure of spacetime?’, Studies in History and Philosophy of Modern Physics 40, pp. 53-56.

Norton, J (2010), ‘Cosmic confusions: not supporting vs. supporting not’, Philosophy of Science 77, pp. 501-523.

Wilczek, F. (2007), ‘Enlightenment, knowledge, ignorance, temptation’, in Universe or Multiverse?, ed. B. Carr, Cambridge University Press.

Zinkernagel, H. (2011), ‘Some trends in the philosophy of physics’, Theoria 26, pp. 215-241.

Referencias

Documento similar

The most likely explanation for the difference in model output is that the patterns of both alcohols and alkanes of the tested vegetation species on their own are not distinct

(1) specification of the econometric model of the stochastic frontier, (2) selection of the inputs and outputs to use in the analysis, and (3) a determination about how

Least Squares Initialisation (LSI) has the aim of improving the initial conditions of partially initialised systems by utilizing the information from observations of limited

the RoboEarth ontology; (2) knowledge-based methods for using prior information, exemplified in the selection of object models for exploration and in the guidance of a robot

Cultural values and attitudes to science offer a more effective explanation of this opposition than competition theories: those expressing egalitarian values and privacy concerns

The proposed positive effects of legal, ethical, economic, philanthropic and environmental dimensions of CSR on consumers’ support for corporate reputation, the selection of

As a means to probe the possible fractal structure of the galaxy distribution we have studied the scaling of the num- ber density and angular correlation function of galaxies

We describe the mixing mechanism that generates an off-diagonal mass matrix for the U(1) gauge bosons and study general properties the eigenstates of such matrix, which are the