Minimal Sapience

Many science fiction stories contain aliens, with most of the intelligent ones looking more or less human. Some stories, however, explore the boundaries of what these aliens might be like, in an effort to challenge the way we think of world around us and ultimately ourselves. Or these authors might just be inventing freaks to get attention and therefore more sales of their book / movie / tv show. At any rate, some of these various works of fiction have prompted me to wonder just what are the limits of form for an intelligent being, specifically: What is the minimum possible size for a creature or machine with an intelligence on par with a human?

Given that life on Earth can be found in sizes ranging down to the microscopic bacteria, the apparent limiting factor here is most likely the brain itself. The human brain is apparently about as small as our neuron and synapse based brains can get without sacrificing its intelligence, so clearly the absolute smallest intelligent brain would have to use some other mechanism. A computer processor can pack a lot of computational power in a small space (current processors are about the size of an Oreo and can process data roughly 1% as fast as some estimates for the human brain), but the smallest data processing unit I know of is is the quantum dot. Quantum dots come in a lot of sizes these days, but since I am looking for the smallest I will call them about 1 nanometer in diameter.

To make a quantum processor capable of matching human intelligence, it will need equivalent processing speed. Unfortunately, no one knows exactly how fast the human brain works and to further complicate the issue, the brain, quantum processors and ordinary transistor processors all process information in fundamentally different ways, so comparing them in a meaningful way is almost impossible. I’m not willing to attempt a real solution at formulating a comparison here, so I’ll just pretend that one quantum dot is equivalent to one synapse.

The human brain contains around 100 billion (10¹¹) neurons, each of which has thousands of synapses, which works out to 10¹⁵ synapse in total. This means that the tiny quantum brain will have 10¹⁵ dots each about one cubic nanometer in volume. Without allowing for any space between them this would only make a block 0.1 mm wide, even with some generous padding our fictional brain is not likely to be more than 1 mm in width.

This is far smaller than I had expected. Given this limit an intelligent creature might well exist that is only an inch (2 cm) or so tall.