It was a search for the essence of things that lead to the memristor, says UC Berkeley professor Leon Chua.
This week, HP Labs announced it had made a memristor, or memory resistor, a fundamental circuit element first theorized by Chua decades ago. If they become commercially practical to make, memristors could lead to very dense, energy-efficient memory chips that don't cost much because they don't need much silicon. A memristor has a variable resistance; as a result, memristors can "remember" how much charge was applied to it. (See.)
While most have accepted Chua's work, it has mostly been considered theoretical. But how did Chua come up with the mathematical formula for proving memristors exist in the first place?
He started looking at what truly defined different circuit elements, similar to the approach Aristotle took when trying to define substance and essence.
"I asked, 'What is a resistor? What is a capacitor?' No one was really asking that," Chua said. If you asked someone what a resistor was, they'd say, 'It gets hot so let's make an oven out of it.' That was the mentality."
Chua then took four variables: voltage, current, charge, and flux. A resistor was defined by current plus voltage, he said. A capacitor was defined by voltage plus charge. Flux and current made an inductor. That took care of three out of four of the known circuit elements.
There was only one possible combination left, according to Chua. Flux plus charge, which he defined as a memristor.
Why did it take so long to eventually make a memristor? He gave two reasons. One, researchers chalked up evidence pointing to memristors, or effects created by memristors, as anomalies in their own experiments.
Second, material science has made huge strides. The memristors developed by HP measure 5 nanometers across. "That's the length of five sugar molecules," Chua said. "The memory effect dominates."
Chua noted that he actually made a rough prototype back when the paper first came out, but it was impractical and manufacturers weren't interested in developing it.
And, it's not the first time it's taken a while to prove something. He pointed to Aristotle's law of motion. Not familiar with it? That's because it turned out to be wrong. Aristotle said that force should be proportional to velocity. Centuries later, Newton showed that force was actually proportional to acceleration.
"They were looking at the wrong variable," he said. "The same thing happened here."