Error had a lot of momentum, but he fixed it

RALEIGH -- The textbook for his N.C. State physics class was considered the bible in the field, but David Babson was convinced something was wrong.

Was it possible? Could he, the novice student, have caught an error that physics students across America had missed in the decade or so since the book was published?

Yes, it turns out.

Working through a homework problem, Babson discovered that a widely accepted principle related to momentum was flawed.

The result: a correction in a coming edition of the book and for Babson, a published paper on the findings in a scholarly journal.

One of Babson's journal co-authors: David Griffiths, the Reed College physicist who made the error.

"Dave just doggedly tracked it down," said Stephen Reynolds, Babson's professor for that electromagnetism course. "He is just a real energetic guy and passionate about understanding things."

The tale began two years ago when Babson was taking an undergraduate physics course. Babson was a nontraditional student, a 42-year-old husband and father of two with a master's degree from Cornell University. He had already found so much success as a computer software developer that he had retired for three years.

But a nagging interest in physics drew him to NCSU. Though Babson had two computer science degrees from Ivy League schools, he needed to take some basic physics classes before pursuing a graduate degree.

So one day he was working through a basic problem touching on momentum as it relates to electric and magnetic fields.

For a nonscientist, this is where it gets fuzzy, so Reynolds, the NCSU professor, offers an analogy.

Suppose you have a bank account with no money in it. A zero balance. In physics-speak, that is a system at rest.

Now, in Griffiths' text, the system at rest is the result of one particular series of occurrences -- like having $100 in the bank and withdrawing it all. But Babson reasoned that you could also arrive at a zero balance by having $300 in the bank and withdrawing it all, or $500 in and $500 out, etc.

Apparently, nobody had ever thought about it that way. When Babson did, applying real numbers to theory, he discovered the misstep.

Arcane, but rewarding

"It's kind of an arcane point, which is why it managed to miss everybody," Reynolds said. "It was like a little, dark corner that didn't get any attention. We all knew what the answer had to be, but the path to it was fraught with errors."

Babson e-mailed Griffiths, now an emeritus physics professor at Reed College in Oregon. Reynolds sent one as well, colleague to colleague, to help clear Babson's path.

Griffiths, a longtime physics scholar with three Harvard degrees, responded within hours, thanking the student for finding the error.

The discovery launched a research project that Babson and Reynolds worked on with Griffiths and one of his Reed College students. That research resulted in "Hidden Momentum, Field Momentum, and Electromagnetic Impulse," a paper published in August in the American Journal of Physics. In addition, the fourth edition of Griffiths' text will note the correction.

"It was a bit of a lucky situation to catch this error and write the paper," said Babson, now 44 and in pursuit of his physics doctorate at NCSU. "I don't know if it's profound, but it is rewarding."

Here's the kicker: The homework assignment he submitted to Reynolds two years ago that led to the discovery of the error? He lost points on it.

"Because I didn't have the answer that matched the book," he said.