If 3/2 is the answer, what is the question?

Oddly, finding questions to answers can be a lot more difficult than finding answers to questions.

If the question is, what is the sum of 2 plus 2, the answer is straightforward: 4.

But if the answer is 4, what is the question?

Is it: How much is 2 plus 2?

Or perhaps: How much is 12 divided by 3?

Or even: How many legs on a dog?

It's one thing to do an experiment that tells you whether or not A or B is the right answer. But given A or B as the answer, how do you find the "experiment" that produced it?

Finding questions to answers gets even harder when dealing with complex systems, like living organisms. Or as neuroscientist Charles Stevens of the Salk Institute poses a prototypical problem:

"If a mouse is the answer, what is the question?"

All living organisms are "solutions" to sets of problems posed by the combination of environment, genetics, laws of nature, and chance--to name a few of the factors that come into play. So, given a certain property of organisms, a scientist would like to know: Why is it that way and not some other way? What is the question, in other words, that led to that answer?

In his own research, Stevens is looking for the "question" to 3/2 because it's an answer that keeps popping up in studies of how the brain processes information. To put it very simply, more brain cells process information than feed information into processing cells. But how many more depends on the size of the animal.

Roughly speaking, the bigger the animal, the bigger the ratio of processing cells to input cells. As you go progressively to species with bigger brain size, the ratio increases. The number that inexplicably keeps popping out of those calculations is 3/2.

He'd like to know why.

"We have these laws that work over all animals, from mice to elephants, and it seems we ought to understand what they are."

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Looking for questions to answers in the life sciences has been around at least since Darwin. The theory of evolution through natural selection is the question to answers ranging from dinosaurs to begonias. (Question: What species produces the most successful offspring under a given set of conditions? In one case, the answer was begonia; in another, T. Rex.)

It's also common in physics. Newton, for example, realized that the orbit of the moon and the fall of the apple were different solutions to the same problem: How do objects behave under the influence of gravity? Codified into equations, the "question" that he discovered was the laws of gravity. The "solutions" range from the parabolic path of water in fountains to the sun's hold on the planets in the solar system.

In the same way, every species of atom is the "solution" to the laws of quantum mechanics. So is every property of matter. Just as a long neck is the giraffe's answer to tall trees, "ice" is the answer to the question: What happens to H20 if you cool it to below 32 degrees Fahrenheit?

By extrapolation, the entire universe is a solution to the equations that describe the laws of nature. If we knew exactly what all those laws were, we could create a universe from scratch.

Or as physicist Leon Lederman frames the challenge in his book, "The God Particle": "If the universe is the answer, what is the question?"

Complicating the matter is the fact that a single set of questions can have a wide variety of answers. Consider a game of chess, for example, or baseball. In a sense, both are sets of problems to be solved. But even when the rules--and even the players--stay the same, the number of outcomes theoretically possible is infinite.

So why have some games actually been played out while others remain only possibilities? Why did natural selection produce dinosaurs but no Cyclops? Huge flying birds but no flying purple people? Why is the universe created from three dimensions of space and one of time? Why is there gravity at all?

Is there a hidden set of higher laws that determine the result? Or is random chance involved? Or both?

We already know the answer: It's the universe we inhabit.

But the questions are hard.