# All Is Simple Parts Interacting Simply

In physics, I got a BS in ’81, a MS in ’84, and published two peer-reviewed journal articles in ’03 & ’06. I’m not tracking the latest developments in physics very closely, but what I’m about to tell you is very old standard physics that I’m quite sure hasn’t changed. Even so, it seems to be something many people just don’t get. So let me explain it.

There is nothing that we know of that isn’t described well by physics, and everything that physicists know of is well described as many simple parts interacting simply. Parts are localized in space, have interactions localized in time, and interactions effects don’t move in space faster than the speed of light. Simple parts have internal states that can be specified with just a few bits (or qubits), and each part only interacts directly with a few other parts close in space and time. Since each interaction is only between a few bits on a few sides, it must also be simple. Furthermore, all known interactions are mutual in the sense that the state on all sides is influenced by states of the other sides.

For example, ordinary field theories have a limited number of fields at each point in space-time, with each field having a limited number of degrees of freedom. Each field has a few simple interactions with other fields, and with its own space-time derivatives. With limited energy, this latter effect limits how fast a field changes in space and time.

As a second example, ordinary digital electronics is made mostly of simple logic units, each with only a few inputs, a few outputs, and a few bits of internal state. Typically: two inputs, one output, and zero or one bits of state. Interactions between logic units are via simple wires that force the voltage and current to be almost the same at matching ends.

As a third example, cellular automatons are often taken as a clear simple metaphor for typical physical systems. Each such automation has a discrete array of cells, each of which has a few possible states. At discrete time steps, the state of each cell is a simple standard function of the states of that cell and its neighbors at the last time step. The famous “game of life” uses a two dimensional array with one bit per cell.

This basic physics fact, that everything is made of simple parts interacting simply, implies that anything complex, able to represent many different possibilities, is made of many parts. And anything able to manage complex interaction relations is spread across time, constructed via many simple interactions built up over time. So if you look at a disk of a complex movie, you’ll find lots of tiny structures encoding bits. If you look at an organism that survives in a complex environment, you’ll find lots of tiny parts with many non-regular interactions.

Physicists have learned that we only we ever get empirical evidence about the state of things via their interactions with other things. When such interactions the state of one thing create correlations with the state of another, we can use that correlation, together with knowledge of one state, as evidence about the other state. If a feature or state doesn’t influence any interactions with familiar things, we could drop it from our model of the world and get all the same predictions. (Though we might include it anyway for simplicity, so that similar parts have similar features and states.)

Not only do we know that in general everything is made of simple parts interacting simply, for pretty much everything that happens here on Earth we know those parts and interactions in great precise detail. Yes there are still some areas of physics we don’t fully understand, but we also know that those uncertainties have almost nothing to say about ordinary events here on Earth. For humans and their immediate environments on Earth, we know exactly what are all the parts, what states they hold, and all of their simple interactions. Thermodynamics assures us that there can’t be a lot of hidden states around holding many bits that interact with familiar states.

Now it is true that when many simple parts are combined into complex arrangements, it can be very hard to calculate the detailed outcomes they produce. This isn’t because such outcomes aren’t implied by the math, but because it can be hard to calculate what math implies. When we can figure out quantities that are easier to calculate, as long as the parts and interactions we think are going on are in fact the only things going on, then we usually see those quantities just as calculated.

Now what I’ve said so far is usually accepted as uncontroversial, at least when applied to the usual parts of our world, such as rivers, cars, mountains laptops, or ants. But as soon as one claims that all this applies to human minds, suddenly it gets more controversial. People often state things like this:

I am sure that I’m not just a collection of physical parts interacting, because I’m aware that I feel. I know that physical parts interacting just aren’t the kinds of things that can feel by themselves. So even though I have a physical body made of parts, and there are close correlations between my feelings and the states of my body parts, there must be something more than that to me (and others like me). So there’s a deep mystery: what is this extra stuff, where does it arise, how does it change, and so on. We humans care mainly about feelings, not physical parts interacting; we want to know what out there feels so we can know what to care about.

But consider a key question: Does this other feeling stuff interact with the familiar parts of our world strongly and reliably enough to usually be the actual cause of humans making statements of feeling like this?

If yes, this is a remarkably strong interaction, making it quite surprising that physicists have missed it so far. So surprising in fact as to be frankly unbelievable. If this type of interaction were remotely as simple as all the interactions we know, then it should be quite measurable with existing equipment. Any interaction not so measurable would have be vastly more complex and context dependent than any we’ve ever seen or considered. Thus I’d bet heavily and confidently that no one will measure such an interaction.

But if no, if this interaction isn’t strong enough to explain human claims of feeling, then we have a remarkable coincidence to explain. Somehow this extra feeling stuff exists, and humans also have a tendency to say that it exists, but these happen for entirely independent reasons. The fact that feeling stuff exists isn’t causing people to claim it exists, nor vice versa. Instead humans have some sort of weird psychological quirk that causes them to make such statements, and they would make such claims even if feeling stuff didn’t exist. But if we have a good alternate explanation for why people tend to make such statements, what need do we have of the hypothesis that feeling stuff actually exists? Such a coincidence seems too remarkable to be believed.

Thus it seems hard to square a belief in this extra feeling stuff with standard physics in either cases, where feeling stuff does or does not have strong interactions with ordinary stuff. The obvious conclusion: extra feeling stuff just doesn’t exist.

Note that even if we are only complex arrangements of interacting parts, as social creatures it makes sense for us to care in a certain sense about each others’ “feelings.” Creatures like us maintain an internal “feeling” state that tracks how well things are going for us, with high-satisfied states when things are going well and and low-dissatisfied states when things are going badly. This internal state influences our behavior, and so social creatures around us want to try to infer this state, and to influence it. We may, for example, try to notice when our allies have a dissatisfied state and look for ways to help them to be more satisfied. Thus we care about others’ “feelings”, are wary of false indicators of them, and study behaviors in some detail to figure out what reliably indicates these internal states.

In the modern world we now encounter a wider range of creature-like things with feeling-related surface appearances. These include video game characters, movie characters, robots, statues, paintings, stuffed animals, and so on. And so it makes sense for us to apply our careful-study habits to ask which of these are “real” feelings, in the sense of being the those where it makes sense to apply our evolved feeling-related habits. But while it makes sense to be skeptical that any particular claimed feeling is “real” in this sense, it makes much less sense to apply this skepticism to “mere” physical systems. After all, as far as we know all familiar systems, and all the systems they interact with to any important degree, are mere physical systems.

If everything around us is explained by ordinary physics, then a detailed examination of the ordinary physics of familiar systems will eventually tells us everything there is to know about the causes and consequences of our feelings. It will say how many different feelings we are capable of, what outside factors influence them, and how our words and actions depend on them.

What more is or could be there to know about feelings than this? For example, you might ask: does a system have “feelings” if it has some of the same internal states as a human, but where those states have no dependence on outside factors and no influence on the world? But questions like this seem to me less about the world and more about what concepts are the most valuable to use in this space. While crude concepts served us well in the past, as we encounter a wider range of creature-like systems than before, we will need refine our concepts for this new world.

But, again, that seems to be more about what feelings concepts are useful in this new world, and much less about where feelings “really” are in the world. Physics call tell us all there is to say about that.

(This post is a followup to my prior post on Sean Carroll’s Big Picture.)

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