# yaneer on complexity

yaneer bar-yam on why complexity is diff (2017):

Modeling COVID19. They need to understand complexity: Focus on what is important, answer the right question, then models are useful. In this case, action is important not prediction. See https://t.co/g7OabIJRXJ Special report: Simulations of COVID-19 https://t.co/Todkpn7azO

Original Tweet: https://twitter.com/yaneerbaryam/status/1246001369762467840One of the hardest things to explain is why complex systems are actually different from simple systems.. The problem is rooted in a set of ideas that work together and reinforce each other so that they appear seamless..

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undisturbed ecosystem.. *‘in undisturbed ecosystems ..the average individual, species, or population, left to its own devices, behaves in ways that serve and stabilize the whole..’ –*Dana Meadows

## Everything that is needed should be found in the data and the model we write down. The flaw in this seemingly obvious statement is that what is missing is realizing that one may be starting from the wrong properties..t

gupta roadblock law et al

deep enough: maté basic needs

Why complex systems need a new mathematics..

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literacy and numeracy both elements of colonialism.. we need to calculate differently and stop measuring things

When separation of scales works, we can describe not only the system as it exists in isolation, but also how it responds to external forces.Forces that act on the earth at the scale of orbital motion couple to the dynamic behavior that occurs at that scale. Thus if we were to consider a new celestial body entering the solar system, unless it disrupted the structure of the system (i.e. by shattering a planet) andas long as we continue to be interested in the scale of orbital motion, we can describe the behavior of the system using these same degrees of freedom.

For complex systems, it is still true that the questions we most want to answer have to do with the larger scale information.

Significantly, the scale of description and scale of interactions are similar. When we have a description of the larger scale behavior we are also considering the larger scale impacts of the environment on the system and reciprocally.

But many systems, especially those we are interested in understanding and influencing, are not well described by separate micro and macro scales.

2 convers as infra and zoom dancing

Consider a flock of birds. If all of the birds flew independently in different directions, we would need to describe each one separately. If they instead all went in the same direction, we could simply describe their average motion. However,

if we are interested in their movement as a flock, describing each bird’s motion would be too much information and describing the average would be too little information..t

ginorm small ness

Similarly, for traffic jams, market behavior and weather, the average behavior is not enough and all the details are too much to be useful. Understanding complex behavior that is neither independent nor coherent behavior is best described across scales.

This requires knowing which information can be observed at a scale of interest..t

gupta roadblock law et al

This surprising discrepancy between observations and theory compelled a dramatic change in our understanding.

Our usual methods of calculus and statistics fail at this point because their assumptions no longer hold true. Calculus assumes that matter is smooth and statistics assumes that averages over large numbers of objects are well defined. Away from the critical point these assumptions are justified, since the microscopic behavior of atoms is well separated from the macroscopic behavior of the material as a whole. Different parts of the material appear essentially the same, making it smooth, and any (local) average over atomic properties has a single well defined number. However, at the critical point, the density fluctuates—between water-like and vapor-like conditions—so that the material is not smooth and the average taken of the material as a whole is not representative of the density at any particular location or time.Near the critical point, the matter is composed of patches of lower and higher density, and this patchiness occurs on all scales, even at the macroscopic scale..t

critical point: maté basic needs – deep enough to be about all of us.. today

“Relevant” parameters are those parameters of the free energy that increase with scale; “irrelevant” parameters are those that decrease with scale.Because there are so many atoms in matter, the irrelevant parameters cannot affect our observation. We can consider only the relevant parameters. We might measure irrelevant parameters microscopically, but they won’t affect macroscopic changes in the material or our interactions with it near the critical point.

gupta roadblock law et al

A faithful representation must have the same number of states as the system it is representing. This enables the states of the representation to be mapped one to one to the states of the system. If a model has fewer states than the system, then it can’t represent everything that is happening in the system. If a model has more states, then it is representing things that can’t happen in the system.

Conventional models often do not take this into account and this results in a mismatch of the system and the model; they are unfaithful representations and do not properly identify the behavior of the system, and thus ultimately its response to environmental forces or interventions we might consider…t

like observing/data-izing whales in sea world

Because we are interested in influencing the system, we only want to know the distinctions that matter. We have to focus attention on those states that aredistinguishableat a particular scale of observation..

ie: maté basic needs

ginorm small enough to include all of us.. everyday

Typically, the finer the scale of inquiry about a system, the more information is needed to describe it (Fig. 3).

A sufficient representation, therefore, is one that has a set of possible states corresponding to the set of distinguishable states of the system at each level of resolution, down to the level we need to describe the properties we care about—the relevant parameters—and no further..t

ie: maté basic needs

ginorm small enough to include all of us.. everyday

When considering interventions that affect the large scale properties of the system, rather than accumulating details about the system, we should start with the largest scale pattern of behavior and add additional information only as needed..t

According to the complexity profile, each piece of information about a system has a size—the largest scale at which we can begin to detect that piece of information.

When we observe the largest scale behaviors of a system, we simplify the mathematical description of the system because there are fewer distinguishable states, and only a limited set of possible behaviors. This also means that systems that look different on a microscopic scale may not look different at the macroscopic scale, and

their mathematical descriptions become the same..t

ie: maté basic needs

ginorm small enough to include all of us.. everyday

nationality: human et al

Even though the specific systems (music strings to water waves to light) are very different, the dependencies that give rise to their behaviors, and the behaviors themselves, are related mathematically.

but not the mathematically we learn in school..

literacy and numeracy both elements of colonialism.. we need to calculate differently and stop measuring things

ie: undisturbed ecosystem..*‘in undisturbed ecosystems ..the average individual, species, or population, left to its own devices, behaves in ways that serve and stabilize the whole..’ –*Dana Meadows

How does universality work for complex systems? Unlike traditional renormalization group,

we do not consider the limit of infinite size and power law exponents.

finite set of choices et al

curiosity over decision making

Instead, the states of our representation must correspond to the states of the system at the scale of observation. Moreover, instead of describing the equilibrium energy, we describe dynamics and system response.

The mathematical representation of one system at a particular scale may correspond to the behavior of other systems despite different underlying components. This is a general concept of universality(Fig. 5).

maté basic needs – deep enough to be about all of us.. today

What are the cases where the thermodynamic limit does not serve to expose universality?

.. If we think about what happens with a very large pattern we see that at large enough scales, these patterns look only gray. Still, we can map these descriptions from system to system. The patterns represent universal classes of behavior. Microscopic changes only change the pattern to the extent that they change the relevant parameters of those patterns

The adoption of Turing’s ideas in biology for patterns on animal skins has been controversial precisely because the pattern dynamics does not capture microscopic mechanisms.

This controversy misses the key point about universality. Universality should be intuitive as we don’t need to describe the molecular processes..tto characterize the variation between patterns on species, or individual members of a species, or the dynamics of a pattern as it forms, and do not affect roles of these patterns in social and ecological interactions. This is similar to the ability to describe planetary motion without describing details of individual planet structure.

*‘in undisturbed ecosystems ..the average individual, species, or population, left to its own devices, behaves in ways that serve and stabilize the whole..’ –*Dana Meadows

mcleod explain law et al

The study of universality enables us to identify classes of systems whose behaviors can be described the same way by a common mathematical model.

This is the principle of universality that is formalized by renormalization group and generalized by multiscale information theory to the scientific study of complex systems.

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