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2
Silas Grossberndt
[WSS2021] Towards Causal Invariance Proofs in the Wolfram Physics Project
Silas Grossberndt, CUNY Graduate School
Posted
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This project is directed at adding additional functionality to existing proof strategies for Wolfram Models. This work focuses on utilizing homotopies and
∞
groupoids to approach this proof, first moving out from an empirical study of a property and then to causal invariance. This work is not fully rigorous at this time, and these methods need to be supported with proof of the methods to either support them or lead to better refinement. Further work in this area would include probing into the structure of the groupoids from the causal graphs to extract additional properties of the models.
Towards proving Causal Invariance in the Wolfram Model
Silas K Grossberndt
City University of New York, Graduate Center
This project is directed at adding additional functionality to existing proof strategies for Wolfram Models. This work focuses on utilizing homotopies and
∞
groupoids to approach this proof, first moving out from an empirical study of a property and then to causal invariance. This work is not fully rigorous at this time, and these methods need to be supported with proof of the methods to either support them or lead to better refinement. Further work in this area would include probing into the structure of the groupoids from the causal graphs to extract additional properties of the models.
Introduction and Definitions
I
n
[
]
:
=
Theorem Proofs and Type Theory
The goal of this project is to extend the existing functions for theorem proofs on multiway systems in the Wolfram Model to a broader spectrum of rules. One hopes to show that a rule will give causal invariance, rather than showing that two hypergraphs are invariant, but that all hypergraphs in the multiway system resulting from the rules will reconverge on the hypergraph.
This will be addressed by elevating the logics in the theorem provers to a type theoretic method, and exploit the
∞
groupoid structure of the causal graphs.
I
n
[
]
:
=
Definitions and Basis
One starts with the correspondence from homotopy type theory of an
∞
groupoid to a space in a homotopy via the CurryHowardLambek Correspondence. Then extend this correspondence to a type. Then, a proof corresponding to the theorem simply requires that one find an element of the appropriate groupoid, or a point in the appropriate space. This is generally true for the meta mathematics, and will provide the proper mathematical underpinnings for a rigorous definition of the presented system. However, in this work I shall go off of the basis used in the current “FindWolframModelProof” function, taking an assumption of the multiway systems as types, and then building from there. The presented work is meant to be treated as an experimental approach, empirically lead, but shall be developed on. The underlying idea is that those multiway systems that admit higher homotopies are those that will have a topological structure allowing for the analysis of causal invariance.
We shall use the following from [4]:
I
n
[
]
:
=
Homotopy
A
homotopy
is a map
H
:
I
I
X
that maps from two paths
f
,
g
:
I
X
such that H(I, 0)= f and H(I, 1)=g. If there exists a homotopy from f to g, then f and g are homotopic.
I
n
[
]
:
=
Simply Connected Space
A space is
simply connected
if it is path connected and has a trivial fundamental group from any basepoint in the space.
I
n
[
]
:
=
Path Connected Space
A
path connected space
is one where there is a path between any two points in the space.
I
n
[
]
:
=
Fundamental Group
The
fundamental group
,
π
1
(
X
,
x
0
)
is the set of equivalence classes of loops
f
:
I
X
, i.e. paths such that
f
(
0
)
f
(
1
)
x
0
, with the group action as the composition of loops.
I
n
[
]
:
=
∞
Groupoid
For this, I shall be using the refined definition of a
∞
groupoid from [1] which is just a further development of Grothendieck’s definition in [2] .
Then, when I shall be talking about the
∞
groupoid of the space, it shall specifically be the fundamental
∞
groupoid of the space.
This is to say that, for a topological space X, there is an associated
fundamental
∞
groupoid
built from the Whitehead Tower.
A First Property
Take a rule with the form
2
2

>
2
4
and let it run for ten steps
I
n
[
]
:
=
R
u
l
e
P
l
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t
[
t
e
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]
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a
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P
l
o
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"
]
O
u
t
[
]
=
O
u
t
[
]
=
Now let us start with a property and test methodologies of getting to the property via the causal graph.
I
n
[
]
:
=
Growth of the Hypergraph
Let us take out test model and find a description of the growth from one state to another on the causal graph. Then, we will use other simple rules to verify that the
I
n
[
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:
=
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F
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"
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/
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a
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e
r
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[
]
=
I
n
[
]
:
=
t
e
s
t
m
o
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l
[
"
C
a
u
s
a
l
G
r
a
p
h
"
]
/
/
T
r
a
n
s
i
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e
d
u
c
t
i
o
n
G
r
a
p
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/
/
G
r
a
p
h
P
l
o
t
3
D
/
/
R
a
s
t
e
r
i
z
e
O
u
t
[
]
=
Then take an earlier state of this causal graph
I
n
[
]
:
=
t
e
s
t
m
o
d
e
l
a
t
4
=
R
e
s
o
u
r
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e
F
u
n
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n
[
"
W
o
l
f
r
a
m
M
o
d
e
l
"
]
[
r
u
,
{
{
0
,
0
}
,
{
0
,
0
}
}
,
4
]
;
I
n
[
]
:
=
t
e
s
t
m
o
d
e
l
a
t
4
[
"
C
a
u
s
a
l
G
r
a
p
h
"
]
;
I
n
[
]
:
=
c
g
4
=
%
;
I
n
[
]
:
=
c
g
4
/
/
T
r
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s
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a
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4
[
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[
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e
P
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=
O
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[
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=
I
n
[
]
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t
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s
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p
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o
p
4
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m
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a
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4
[
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F
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a
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t
a
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e
"
]
;
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s
t
p
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o
p
=
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m
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d
e
l
[
"
F
i
n
a
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S
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a
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e
"
]
;
I
n
[
]
:
=
R
a
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[
t
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l
[
"
S
t
a
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s
P
l
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t
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i
s
t
"
]
]
/
/
R
a
s
t
e
r
i
z
e
O
u
t
[
]
=
One can clearly see that the hypergraph grows in this limited context. Let us look at another simple test rule that is also growing.
Take the
2
1

>
2
2
rule below an expand it similarly. Then we shall compare properties on the hypergraphs
I
n
[
]
:
=
R
u
l
e
P
l
o
t
[
g
r
o
w
t
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e
s
t
]
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r
o
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s
t
[
"
S
t
a
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P
l
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L
i
s
t
"
]
/
/
R
a
s
t
e
r
i
z
e
O
u
t
[
]
=
O
u
t
[
]
=
It is apparent that this hypergraph is growing from state to state. Then let us compare the causal graphs, multiway systems and causal multiway systems
I
n
[
]
:
=
g
r
m
g
r
m
c
/
/
T
r
a
n
s
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i
v
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a
s
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r
i
z
e
O
u
t
[
]
=
O
u
t
[
]
=
Then, let us dive into the paths on the multiway system. We take some arbitrarily chosen path on each graph, called path
γ
and
η
respectively. Then, let us look at the homotopy classes of each path.
If we take the homotopy class for path
γ
, we get that it is trivial as is the homotopy class for some
η
. This is to be expected, however the same is not true in the case of the causal multiway systems, which we shall get into in the next section.
I
n
[
]
:
=
v
l
m
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t
=
V
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t
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x
L
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[
m
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]
;
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r
=
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x
L
i
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[
g
r
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]
;
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[
]
:
=
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=
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[
m
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v
l
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[
[
1
]
]
,
v
l
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[
[
3
0
]
]
]
;
g
a
m
m
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=
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[
m
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[
[
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[
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,
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[
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,
v
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[
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[
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:
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/
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Then, one notices that if the paths shown are chosen, there is an alternative path to get to the same end point, and thus there exists at least one nontrivial homotopy class.
Alternatively, before we can take this as a given for this property, let us investigate a rule that does fail to grow ad infinitum .
Take the
2
1
3
1
2
2
3
1
rule which stops after 9 steps from the technical introduction [3], with an initial state modified to reach the end point after 3 steps
I
n
[
]
:
=
n
g
r
o
w
[
"
S
t
a
t
e
s
P
l
o
t
s
L
i
s
t
"
]
/
/
R
a
s
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i
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e
O
u
t
[
]
=
I
n
[
]
:
=
n
g
m
/
/
R
a
s
t
e
r
i
z
e
O
u
t
[
]
=
Then take the path
δ
below, and find that there are no homotopic paths
I
n
[
]
:
=
v
l
n
g
=
V
e
r
t
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x
L
i
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[
n
g
m
]
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t
=
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d
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n
g
m
,
v
l
n
g
[
[
1
]
]
,
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l
n
g
[
[
5
]
]
]
;
H
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n
g
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,
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b
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[
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,
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l
t
]
]
/
/
R
a
s
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r
i
z
e
O
u
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[
]
=
This may seem to be an adhoc approach, but this is expected to be mathematically rigorous, as, for the hypergraph to grow continuously, one would expect a homotopic structure
Then, to get this property on some rule and some initial state, one needs to simply verify that there is a path that is homotopic to an other path. This is just seeing that the space is not simply connected (a concept which will become useful in the next part of this project).
Causal Invariance
Let us now turn our attention to causal invariance. We shall try to show causal invariance by showing confluence on the causal multiway system and, alternatively, by showing that there must be an isomorphism from one causal graph to another and that all such morphisms are isomorphisms, i.e. showing that the
∞
groupoid exists and is nontrivial. The equivalence of these methods is not necessarily clear, and the methods presented here to get to each are a primary approach to each, needing further work to sufficiently prove that these methods are fully rigorous.
Both of these methods are in a resource function “ProveWolframModelCausalInvariance” uploaded to the function repository.
I
n
[
]
:
=
Confluence on the Causal Multiway System
In the previous section, we had investigated how hypergraphs grow ad infinitum and had come to the conclusion that there would be at least one path on the system that had a trivial homotopy class at all foliations. This corresponds to a lack of confluence, as there would always be at least one path by which there would be a branch that would not recombine. Then, we note that causal invariance is given in the
Technical Introduction to the Wolfram Physics Project
[3] as confluence, or “path independence” [5] on the rewriting system, which we here extend to the causal multiway systems. Then, begin with a causal multiway system, one must show that, for all vertices that are not termini on the causal multiway system, each point must be within a path that is homotopic to at least one path for each other vertex on the causal multiway system. This is equivalent to saying that there must be, at some layer of foliation, for path
γ
, the homotopy class [[
γ
]] will be nontrivial and, in fact should span the space of all paths in the system. We choose Causal Multiway systems as they most clearly have the connection between confluence and causal invariance [6] [7]
Take a simple causal multiway system that has been shown to have causal invariance [3], and choose a path
γ
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n
[
]
:
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Then, taking path
γ
, there is a single homotopy class for the whole space iff the space is simply connected. This can be gotten by proving that the space is path connected, which we will take as an assumption of the model, and that the fundamental group for all points is trivial. It is clear that for the termini points, all the paths to those points have a trivial fundamental group, however, it is less clear for paths like
γ
. For these, one must show that the fundamental group is trivial, which is a nontractable problem in general. Instead, let us lift the problem to one of the fundamental groupoid. To prove that this is trivial, one must show that the space is contractible. This can be implemented using the method outlined in [8]
I
n
[
]
:
=
Isomorphisms on Causal Graphs
In an other method, one can instead take the multiway systems and prove that the fundamental
∞
groupoid from the paths in the multiway system is nontrivial, again using the method in [8] combined with a path lifting. Not every multiway system permits this groupoid structure, but those that do exhibit causal invariance. Groupoid structures can then be extracted to get additional properties.
I
n
[
]
:
=
Proving causal invariance
In order to show causal invariance, a Resource function has been developed and submitted to the repository. It is currently available published to the cloud and can process both confluence and causal graph isomorphism methods.
Conclusion and further Extensions
This work needs to have a few proofs fleshed out, but right now the system relies on identification of an isomorphism for simple hypergraphs, such that the loop properties may be exploited for the fundamental groupoids. Getting exact structures would be a natural next step, as well as rewriting of the proof object to move from comparison of hypergraphs to rules, to comparison of paths and spaces to the rules. However, this work is expected to fit within the larger project of higher logic proofs on the Wolfram Model.
Keywords
◼
WolframPhysicsProject
◼
Causal Invariance
◼
Automated Theorem Proving
◼
Homotopy Type Theory
Acknowledgment
My thanks to my mentor Nikolay Murzin for his support on this project. Additional thanks go to Stephen Wolfram and Jonathan Gorard for suggestion of the project topic, and to Mano Namuduri and Xerxes Arsiwalla for important discussions on homotopy type theory, the Wolfram Model and getting notebooks to look the way I wanted them too.
References
◼
[1] G. Maltsiniotis arXiv:1009.2331 [math.CT]
◼
[2]A. Grothendieck and Édité par G. Maltsiniotis. “Pursuing Stacks.” (2010).
◼
[3] S. Wolfram “ A Class of Models with the Potential to Represent Fundamental Physics” https://www.wolframphysics.org/technicalintroduction/theupdatingprocessforstringsubstitutionsystems/thephenomenonofcausalinvariance/
◼
[4] L. Evans and R. Tompson, “Algebraic Topology” http://math.hunter.cuny.edu/thompson/topology_notes/chapter%20two.pdf
◼
[5] A. Church and J. B. Rosser (1936), “Some Properties of Conversion”, T Am Math Soc 39, 472–82. doi:10.1090/S00029947193615018580.
◼
[6] J. Gorard arXiV:2004.14810
◼
[7] J. Gorard arXiV:2011.12174
◼
[8] D. O. MartinezRivillas and Ruy J.G.B de Queiroz, arXiV: 1906.05729v3[cs.Lo]
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POSTED BY:
Silas Grossberndt
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