Presented at a conference on Structural Realism in Bristol in September 2012

1. Ioan Muntean
Indiana University-Purdue University,
Fort Wayne, Indiana, USA
http://imuntean.net
1

2. Motivations for this talk
 I. Underdetermination
 II. Unification and explanation
 Do structures unify? Do they explain?
 III. Fundamentalism
 IV. Emergence of spacetime structures
 In this talk I will focus especially on II-IV, but add:
 Dualities
 Does duality unveil a “deeper structure” beyond the two
theories? What is a structure?
2

3. Assumptions of my argument
 A. “String theory” is a collection of mathematical
models, not a theory (or a collection of theories)
 B. Dualities play a role in the structural interpretation
of string theory
 C. Reality and structures are not scale invariant.
3

4. Prospective results
 D. Dualities are not unificatory or explanatory (pace
Rickles)
 E. Dualities unveil a deeper structure, with or without
a deeper theory.
 F. Dualities act as corrective relations to string models.
They may predict and accommodate/integrate facts
about the dual model.
 I take accommodation / integration as weaker than
unification and explanation.
4

5. A. String Theory as a quantum
gravity program
 String theory:
 Is a “particle physics” programme (cf. a “relativity”
programme )
 Uses additional structures: supersymmetry, geometrical
objects (branes), unnatural fields (“the dilaton”)
 It is mainly perturbative, although a non-perturbative
formulation is sought for
(French, Rickles 2006)
5

6. Structural realism and string theory
 Quantum gravity is a natural framework to discuss structural realism (Rickles,
French Saatsi, 2006); (Rickles 2010, 2011, 2012).
 a metaphysics of relations is suggested by quantum gravity (Smolin, Baez)
 This relationalism is easily and best construed as a variety of structuralism
(French, Rickles 2006)
 String theory is a “main contender” in the search for quantum gravity
 Does it reveal relationism? Or structuralism?
 String theory is a quantum field program
 hence its benefits and sins : although it solves UV divergences, it is background
dependent
 String theory is an ever-changing, paradigm-shifting, self-inventing “theory”
 It constitutes a good playground for ESR
 It is natural to investigate “a the structural interpretation of string theory”.
6

7. Structural realism and background
dependence
 “in general, string theory, and other background-dependent
approaches, are […] examples of how not to go about
constructing a theory of quantum gravity” (Rickles French 2006)
 background independence and structuralism are “well-matched
bedfellows”
 Can we interpret string theory as a structural metaphysics even if
it is not (yet) background independent?
 Alternatives:
 talk about the promises of a background independent string theory
 The concept of background independence needs more
philosophical work.
 Dualities may play the central role
7

8. Spanked by the GR community
 “What is very frustrating is that […] string theory does
not seem to fully incorporate the basic lesson of GR,
which is that space and time are dynamical rather than
fixed, and relational rather than absolute […] all that
happens is that some strings move against this fixed
background and interact with one another.” (Smolin
2001, 159)
 Penrose, Stachel, Woit and others would agree.
8

9. String Theory and spacetime
 String models depend on a background
 Unlike other QG programs, in ST the structure of
spacetime is more complicated:
 Dimensionality D>4
 Exotic topologies
 Compactification
 The dynamics of spacetime is codified by the “dilaton”
field
 The GR physicists would insist:
 So what? What determines the structure of the
convoluted spacetime?
9

10. The naïve solution
 Curved spacetime is reducible to a collection of
gravitons
 Given the Witten-Weiberg (no-go result), gravitons
cannot be reduced to any known bosons,
 gravitation is simply not “yet another quantum field
theory”
 Therefore you need something like strings.
 As gravitons are states of strings, the covariant part of
the M space in string theory is in itself dynamical.
10

11. Four interpretations
 [1] A collection of theories in quantum gravity, all being aspects of
a more fundamental theory (“M-theory”) which is ultimately the
theory of everything (TOE) of our reality. Other theories in
physics can/will be reduced to the TOE.
 [2] string theory is a model of different aspects of reality. Most
plausibly, string theory is a model of Hadrons.
 [3] A collection of string models which are mathematical models
of strings and branes vibrating in various types of spaces, having
different symmetries and geometries.
 [4] A collection of conjectures about the relations among the
string models (interpreted as in [3])
 [5] A collection of conjectures about the relations between string
models (as in [3]) and other theories in physics: gauge theories,
gravitation, thermodynamics of black hole, information theory,
etc.
11

12. What is the perturbative
approach?
 An idealization
 As in “a collection of models” some based on this
idealization, some presumably not.
12

13. Five string models
 There are five string models of interest in D=10
1. I: SUSY, open and closed strings, group symmetry SO(32)
2. IIA: SUSY, open and closed strings, non-chiral fermions. D-branes
are the boundaries of open strings
3. IIB: SUSY, open and closed strings, chiral fermions. D-branes are the
boundaries of open strings
4. HO: Heterotic model: SUSY, closed strings only, right moving strings
and left moving strings differ, symmetry group SO(32)
5. HE: Heterotic model: SUSY, as above, but symmetry group is E8xE8
 Dualities:
 1/4: I and HO
 2/5: IIA and HE
 Witten’s conjecture (1995): all string models are related to each other
by dualities.
13

14. B. Interpretation of string models
 1 Empiricist: If this string model were true, what
physics would look like?
 2. What are the realist commitments of these models?
 I choose here 2.
14

15. Structural questions about string
models
 Is string theory object-oriented?
 If not, what plays the role of the structure?
 What is fundamental? What is non-fundamental?
 Is the structural interpretation better off than object
oriented interpretations?
 They need to be addressed within each string model.
15

16. Object-oriented string realism
 Strings are fundamental entities
 They are objects, not structures
 They have 2D features, not intrinsic properties as
particles
 They have individuation and individuality
 Their excitations are other particles (gravitons,
photons, some gluons etc.)
 Therefore the individuation of particles depends on
the individuation of strings.
16

17. Against the OO string realism
 “Acceptable in the 80s” perhaps
 Now we are past the brane revolution
 Branes are as fundamental as strings
 Are branes objects?
 No, they are structures
 Yes, they are.
 Strings are “‘intersections’ of branes” cf Cassirer: “electrons
are points of intersection”
17

18. D-branes
 “We start with strings in a flat background and discover that a
massless closed string state corresponds to fluctuations of the
geometry. Here we found first a flat hyperplane, and then
discovered that a certain open string state corresponds to
fluctuations of its shape. We should not be surprised that the
hyperplane has become dynamical.”
 “Thus the hyperplane is indeed a dynamical object, a Dirichlet
membrane, or D-brane for short. The p-dimensional D-brane,
from dualizing 25-p dimensions, is a Dp-brane. In this
terminology, the original U(n) open string theory contains n
D25-branes.”
 A D25-brane fills space, so the string endpoint can be anywhere:
it just corresponds to an ordinary Chan-Paton factor.
 Polchinski, vol I, 269
18

19. Dualities symmetries and their
interpretations
 Dualities are present in logic, mathematics, physics
 In general, they are not philosophically appealing
 Should structural realists pay more attention to dualities?
 Some dualities for philosophers:
 E/M duality (Castellani 2009, Rickles 2011,2012)
 Bosonization of fermions in 1D
 s-channel/t-channel duality of the dual model of hadrons
 S-dualities in string theory
 AdS/CFT duality
 My claim is that the structural realist should pay attention
to some of these.
19

20. Dualities
 Dualities are not:
 Approximations of a theory by another theory
 Correspondences (as in “theory correspondence”)
 Notation variants of the same theory
 It is not (always) a symptom of a gauge freedom
 But
 There is representational ambiguity in dualities
 “Dualities can highlight which features of our ontological
picture are not fundamental”. Rickles 2011
 Can be related to the symmetries of a theory
 Hence their importance to underdetermination and scientific
realism
20

21. A definition of dualities (Vafa)
 A theory is characterized by a moduli space M of the
coupling constants
 In this space we have several regions conventionally
designated as weak coupling and strong coupling.
 A physical system can be represented in various places of
the moduli space with various observables:
Q[ M , Oα ]
 Two physical systems Q and Q’ are dual if:
M ↔ M '; Oα ↔ Oα '
 i.e. there is an isomorphism between their moduli spaces
and one between their observables
21

22. Triviality and non-triviality
 Dualities can relate two completely different theories of the same
physical system
 Or two completely different systems described by two different
theories
 The classical and the quantum theory of a system
 A classical system to a quantum system
 A gauge theory to a gravitational theory
 A string theory to another string theory
 The weak coupling regime of a theory to its strong regime
 Claims in this talk:
 some dualities are not philosophically trivial,
 Some dualities in string theory are important to the structural
interpretation of quantum gravity
22

23. E/M duality (Dirac)
 A dual theory is obtained by a “duality
transformation”:
  
  
E → B; B → − E
 It is a rotation duality in the complex vector field E+iB
 In QED, this symmetry signals the existence of
magnetic monopoles (g). They attract each other with
a force of (2/137)^2 greater than the force between two
electrons
 Duality explanation: If there are magnetic poles, the
electric charge is quantized, because: eg = 2n;
23

24. The E/M duality in particle
parlance
 EM duality relates weak and strong coupling of the same theory.
 In one regime, α small, the electron charge is weak, i.e. it does
not interact with its own field (fine structure constant).
Electrons barely radiate photons. The electric field is weak. They
are hard to excite (can we excite an electron?)
 There are poles, but they have large fields around them, they are
heavy. The poles are hard to separate, spread out, composite,
solitonic excitations. Structurally they are very rich.
 At α large, the poles are fundamental and charges are heavy and
rich.
 Either the charge is elementary and the poles are composite, OR
the poles are elementary and charges are composite.
 All depends on the coupling constant α.
 Castellani 2009, Rickles 2011.
24

25. Bosonization of fermions in 1D
 For some range of the coupling constant, bosons are
more useful as fundamental particle than fermions
 For some range of constants, fermions are
fundamental.
 By turning the couple constants, one becomes more
fundamental than the other.
 Does this show a common structure?
 In 1+D this does not happen
25

26. Duality symmetries
 In string theory, the moduli space is very rich. Each model is
characterized by a “moduli space” formed by the constants of the
theory.
 The string coupling constant gs
 The topology of the manifold
 Other fields in the background
 Everything (?)
 The flow of theories is important but more complicated than in QFT
 In this moduli space a duality symmetry can relate:
 The weak coupling region of T1 with the strong coupling region of T2
 The weak coupling region of T to the weak coupling region of the same
T
 Interchange elementary quanta with solitons (collective excitations)
 Exchange what is fundamental with what is composite
26

27. S-duality, informally
 gs is the coupling constant in string theory. But in string theory
gs is a field, has its equation, changes from place to place
 A string can split into two strings. The probability if this process
is ≈ gs.
 The D0-brane and “strings” can interchange the role of
fundamental entity.
 When gs is small f-strings are fundamental, light, hard to split.
D0-branes or D-1 are complicated and heavy. Their excitations
are the particles (photons, gravitons)
 When D0 branes are light, f-string are heavier
 When gs=1 , f-strings and D-branes look the same.
 And topology is part of the moduli space. We can change
dimensionality or topology as we walk in the moduli space.
 See (Sen 2002) for details
27

28. T-dualities
 T1 and T2 can be different (different symmetries,
gauge invariants, topologies), or can be the same
theory (self-duality)!
Weak Weak
T1 T2
28

29. A standard S-duality map
 This is not a “self-duality”.
 T1 and T2 are structurally different (different
symmetries, gauge invariants, topologies)
 They can be string models or other models in QCD
etc.
Weak Weak
T1 T2
Strong
Strong
29

30. D. Dualities and unification
 Rickles 2011: “the dualities point to the fact that the
five consistent superstring theories, that were believed
to be distinct entities, are better understood as
different perturbative expansions of some single,
deeper theory.” (aka M theory)
 For Rickles, the five points in moduli space are
representations of a single M-theory.
 This is the received view in the community.
 But….
30

31. Dualities without unification
 “the duality relationships between the points in moduli space
will hold independently of the existence of an M-theory” Rickles
2012
 but
“in order to achieve a computable scheme for the whole of the
moduli space (including regions away from the distinguished
'perturbation-friendly' points) such an underlying theory is
required” Rickles 2011, my emphasis
 My argument is that we do not need M-theory to see the deep
structure of the S-dualities
 One on my assumptions is to analyze these as models, not as
theories
 Conceptually I separate the discussion on dualities from the
discussion on unification
31

32. Non-perturbative string theory and
S-duality
 If we knew how to relate the weak coupling to the
strong coupling we would relate the non-perturbative
physics to the perturbative physics.
 In the ADS/CFT duality this is the path to a
background independent theory.
32

33. Dive in AdS/CFT duality
 We start from a IIB theory (D=10, SUSY with N=4, open
and closed strings)
 (Maldacena 1998) but esp (Klebanov 2002)
 This theory has a 16 supersymmetric group, the smallest
algebra in D=10.
 If Ncgs ≪ 1, the low coupling regime, closed strings live in
 We take Nc parallel D3-branes close one to the other.
empty space and the open strings end on the D-branes and
describe excitations of the D-branes.
 Open and closed strings are decoupled from each
other.
33

34. Strong coupling and the YM
 When Ncgs ≫ 1 the gravitational effect of the D-branes
on the spacetime metric is important, leading to a
curved geometry and to a “black brane”.
 But near the horizon the strings are redshifted and
have low energy.
 Gauge theory exists and the physics on the D-branes is
nothing else than a gauge theory
 The physics is now described by a Yang-Mills CFT with
gYM=4πgs
34

35. Maldacena’s duality conjecture
 The 4D, N=4 SUSY SU(Nc) gauge theory (Yang Mills)
is dual to a IIB string theory with the AdS/CFT
boundary.
35

36. Current research (post 2003)
 People try to falsify the AdS/CFT. No success yet
 More opportunistically, others try to understand black
holes through the duality as hot gases of fermions
 Others try to look for more realistic dualities:
 dS/CFT
 Dualities with no SUSY, but with holographic principle
(T. Banks).
 Understand hadrons with the IIB theory (“the
boomerang kid”)
36

37. E. The structure in the AdS/CFT
1. Is the Yang-Mills CFT the hidden structure of a string IIB
theory (which has gravitation in it)?
2. Is string structure of IIB model the hidden structure of the
Yang-Mills CFT?
3. Are they both aspects of a hidden structure?
A. M theory
B. SUSY
C. Other hidden structural assumption
4. Or perhaps we deal here with a loopy dependence relation?
3. B, 4, 3.A., 2, 1.
Physicists believe 1.
In this order of preference
37

38. Explanation/unification/prediction
in AdS/CFT?
 Explanation? I do not see it
 Unification? Not directly, without the M-theory.
 Prediction? Perhaps, if we relate it to the Higgs
mechanism
 Computational advantages? Yes, definitely!
 Is the holographic principle doing any
explanatory/unification/predictive work?
38

39. F. Emergence? No,
accommodation!
 May solve the problem of emergence in this case
 People believe spacetime emerges in AdS/CFT (Seiberg, Koch
Murugan)
 Some believe the curved spacetime of general relativity is a
holographic emergent construct from a quantum gauge field
without gravity rather than a fundamental feature of reality.
 Strong emergence
 Any spacetime emerges from any gauge theory
 Weak emergence:
 A special type of spacetime (the IIB gravitation) emerges from a
special type of gauge theory (SUSY YM in D=4)
 Witness we assume here that:
 “If a theory does not have general covariance, then the theory lacks
an underlying spacetime”
39

40. Spacetime structuralism in
AdS/CFT
 We ignore the bulk fapp.
 We focus on the boundary
 We achieve a sort of background independence from
the bulk
 Not independence on the boundary.
 We may be OO realist on the boundary but we do not
need to be OO realists in the bulk
40

41. The OSR answer
 Weak dependence
 A structure of spacetime depend on a non-spacetime
structure
 I take the CFT YM structure as determined by SUSY.
 AdS/CFT: The AdS covariance (therefore the graviton
field) depends on the gauge symmetries of CFT
 There is SUSY in both!!!
41

42. Ex SUSY quodlibet sequitur
 SUSY can be a great candidate for the deeper structure
underlying S-dualities and the AdS/CFT duality.
 SUGR is also present in the string sector
 This is a an argument from symmetry
 There are dS/CFT dualities but no non-SUSY dualities.
 SUSY is part of the CFT Yang-Mills story because it is
the limit of a IIB theory.
 I am not pluralist about SUSY, but I am
pluralist/functionalist about spacetime (and
background independence)
42

43. But symmetry is not enough
 In AdS/CFT we do not have a symmetry in the moduli
space.
 It is not like a gauge orbifold in the moduli space
 There is something else than symmetry here.
43

44. Back to realism?
 Nothing look even slightly related to reality:
 SUSY?
 SUGR?
 D=10?
 Large NC?
 BUT:
 In string theory spacetime is in fact a field that can be
traded with strings and branes.
 We can trade in NC and D=10 with other assumptions
 Give up the OO string realism because all these are
mathematical models
44

45. Conclusion
 Dive-in in dualities
 Interpret them as a stay-in feature of QG
 They entail a decent OSR interpretation
 There is also some conventionalism/antirealism fronts
to fight on.
45