# 7

We have successfully eliminated the concepts of null actions and nonexistence from our definition of optimization. We have also eliminated the concept of repeated action. We are halfway there, and now have to eliminate uncertainty and absolute time. Then we will have achieved the goal of being able to wrap a 3D hyperplane boundary around a 4D chunk of relativistic spacetime and ask ourselves "Is this an optimizer?" in a meaningful way.

I'm going to tackle uncertainty next.

TL;DR I have allowed for a mor

We've already defined, for a deterministic system, that a joint probability distribution  has a numerical optimizing-ness, in terms of entropy. Now I want to extend that to a non-joint probability distribution of the form . We can do this by defining  and  for the previous timestep.

We can then define  as by stepping forwards from  to  as before, according to the dynamics of the system.

A question we might want to ask is, for a given  and , how "optimizing" is the distribution ?

## The Dumb Thermostat

Lets apply our new idea to the previous models, the two thermostats. Lets begin with uncorrelated, maximum entropy distributions.

For thermostat 1 we have the dynamic matrix:

(In this matrix, the entry for a cell represents the state at  given the coordinates of that cell represent the state at )

With the  distribution:

As an aside this has 3.2 bits of entropy.

distribution:

This gives us the "standard"  distribution of:

And the "decorrelated"  distribution is actually just the same as ! When we decorrelate the probabilities for  and  we just get back to the maximum entropy distribution and so

It's clear by inspection that the distributions  and  have the same entropy, so the decorrelated maximum entropy  does not produce an "optimizing" distribution at .

If we actually consider the dynamics of this system, we can see that this makes sense! The temperature actually either stays at  or falls into the cycle:

So there's no compression of futures into a smaller number of trajectories.

## The Smart Thermostat

What about our "smarter" thermostat? This one has the dynamic matrix:

Well now our  distribution looks like this:

Giving "standard" a  of this:

And a "decorrelated"  of:

Giving the decorrelated :

Now in this case, these two do have different entropies.  has an entropy of 1.0 bits, and  has an entropy of 2.1 bits. This gives us a difference of 1.1 bits of entropy. This is the Optimizing-ness we defined in the last post, but I think it's actually somewhat incomplete.

Let's also consider the initial difference between  and . Decorrelating  takes it from 2.2 to 3.0 bits of entropy. So the entropy difference started off at 0.8 bits. Therefore the difference of the difference in entropy is 0.3 bits.

The value of associated with  is equal to . which can also be expressed as . We might call this quantity the adjusted optimizing-ness.

## Quantitative Data

The motivation for this was that a maximum entropy distribution is "natural" in some sense. This moves us towards not needing uncertainty. If we have a given state of a system, we might be able to "naturally" define a probability distribution around that state. Then we can measure the optimizing-ness of the next step's distribution.

What happens with a different  condition? What if we have a distribution like this:

For some small epsilon in the second situation.

Now  is like this:

So  is:

While it is theoretically possible to decorrelate everything, calculate the next set of things, and keep going, it's a huge mess. Using values for epsilon between 0.1 and  we can make the following plot between the entropy of  and our previously defined adjusted optimizing-ness.

It looks linear in the log/log particularly in the region where  is very small. By fitting to the leftmost five points we get a simple linear relation: The adjusted optimizing-ness approaches half of the entropy of .

This is kind of weird. This might not be an optimal system to study, so let's look at another toy example. A more realistic model of a thermostat:

## The Continuous Thermostat

The temperature of the room is considered as . The activity of the thermostat is considered as . Each timestep, we have the following updates:

Consider the following distributions:

Where  refers to a uniform distribution between  and  can be thought of as a square of side length  centered on the point  turns out to be a rhombus. The corners transform like this:

For  the whole sequence looks like the following:

So we clearly have some sort of optimization going on here. Estimating or calculating the entropy of these distributions is not easy. And when we use the entropy of a continuous distribution, we get results which depend on the choice of coordinates (or alternatively the choice of some weighting function). Entropies of continuous distributions may also be negative, which is quite annoying.

Perhaps calculating the variance will leave us better off? Sadly not. I tried it for gaussians of decreasing variance and didn't get much. The equivalent to our adjusted optimizing-ness which we might define as  is always zero for this system. The non-adjusted version  fluctuates a lot.

## Where does this leave us?

We can define whether something is an optimizer based on a probability distribution which need not be joint over  and . This means we can define whether something is an optimizer for an arbitrarily narrow probability distribution, meaning we can take the limit as the probability distribution approaches a delta. We found an interesting relation between quantities in our simplified system but failed to extend it to a continuous system.

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