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A Different Take

NC State doctoral student David Babson discovered a key inconsistency that had appeared in three editions of a leading physics textbook.

While the frontiers of physics continue to advance, the core of the discipline is remarkably stable. From the mechanics sorted out by Newton to the electricity described by Ampere, the principles taught to students in classical physics classes have changed precious little in more than 100 years.

Thus, it was with no small amount of skepticism last spring when physics professor Stephen Reynolds listened to his student, David Babson, explain how something didn’t quite ring true in a text used in Reynolds’ electricity and magnetism class for junior physics majors.

After all, this was in a course that has been taught essentially the same way since the 19th century, and the textbook, Introduction to Electrodynamics, written by David Griffiths of Reed College in Oregon, is the most commonly used advanced-undergraduate electricity and magnetism text in the world. In its third edition, it has been pored over by tens of thousands of students.

A problem with momentum

Babson’s issues weren’t about some arcane wrinkle in a calculation. He was concerned with the most basic of principles: conservation of momentum in a problem involving electric and magnetic fields. Although the text used in the course solved a particular problem three separate ways, Babson found one of the ways to be inconsistent.

It has been known for a century that charges and their associated fields can separately have momentum, but the total net momentum of a system at rest must be zero. The momentum in the fields is balanced by a peculiar relativistic effect called hidden momentum that was discovered long after Einstein’s formulation of the theory of relativity in 1905. Babson convinced Reynolds that the fundamental issue of determining the momentum associated with fields was mistreated in at least one of the means used in Griffiths’ example describing the principle. Reynolds asked Babson to look more closely at the other two means Griffiths had used.

“The examples in the book involved idealized systems with objects that filled all space or were infinitesimally small,” Babson says. “What I did is consider numerically developed models of similar systems that didn’t allow for such unphysical objects.”

Using careful numerical calculations involving real-world situations proved Babson was onto something. With Reynolds’ help, this novice student had discovered in Griffiths’ text a basic misunderstanding involving the momentum associated with electromagnetic fields, and thus a misinterpretation of the so-called hidden momentum.

Babson carefully described his calculations and one Friday afternoon sent an e-mail to Griffiths while Reynolds quickly fired off an e-mail of his own — colleague to colleague.

“I wanted to be sure he knew Dave wasn’t just some kook,” Reynolds admits now. Griffiths responded within a matter of hours, thanking Babson for discovering the error. He then began a dialogue with Babson, Reynolds and Robin Bjorkquist, one of his own under- graduate students at Reed College.

The fruitful and extensive series of communications between the four led to their publication of an article, titled “Hidden Momentum, Field Momentum, and Electromagnetic Impulse,” that clarifies the concept of the nature of the conservation of momentum in the presence of electromagnetic fields. The article appeared in the September 2009 issue of the American Journal of Physics. And, not surprisingly, the upcoming fourth edition of Griffiths’ textbook will include the correction.

Not your average physics student

Babson ranks among North Carolina's top triathletes for his age class.

Finding a fundamental error in the textbook wasn’t the only thing that made David Babson stand out in Stephen Reynolds’ undergraduate electricity and magnetism class.

For one thing, he is a 44-year-old husband and father of two who has already earned a BS from Brown University and an MS from Cornell University, both in computer science. He’s also competed in several triathlons and ranks among the top triathletes in North Carolina for his age class. Most notably, he has already been so successful in business that he has been “retired” for three years and decided to pursue a PhD in physics at NC State (he was in Reynolds’ undergraduate class to brush up on some physics fundamentals).

In just over a decade, Babson co-founded, built and sold three venture-backed technology companies. With a total investment of just $10 million, his three companies have sold for an aggregated $52.5 million. In 2004, he and his partners, Mike Doernberg and Ken Romley, were recognized by North Carolina’s Council for Entrepreneurial Development for their success as serial entrepreneurs.

Babson’s most recent company, SmartPath, became the leader in marketing operations software. He co-founded the company in 1999 and served as chief operating officer until SmartPath was sold to DoubleClick for $24 million in 2004. The SmartPath software is used to manage the marketing activities at a number of leading companies.

Prior to SmartPath, Babson and his partners created an Internet professional services company, the Marathon Group, which developed the e-commerce and online branding for Wrangler, Healthtex, Volvo and RJ Reynolds, among others. The company sold for $15 million to Merant in 1999.

The first company Babson started was Burl Software which offered a software analysis tool, Revolve, to help programmers main- tain and update large, complex software systems. Burl was founded in 1991 and sold in 1995 for $13.5 million to Microfocus.

Babson began his career at Bellcore in 1988. During his three years at the telephony research and development center, he earned four software patents for inventing the next generation network signaling software which handles every one of the 400 million “800 number” phone calls placed in North America each day. In 1991, he was awarded R&D Magazine’s “Top 100 Inventions of the Year” and was featured on the cover of Bellcore’s annual report.

As David Babson continues his career in the graduate program in physics at NC State, he will keep a careful eye out for anything that doesn’t ring true. While he may not ever uncover any inconsistencies as deep or as profound as those involved in the “hidden momentum” now described in an important journal article that bears his name, rest assured that all of the physics texts he confronts — from optics to relativity to nuclear physics — will be studied with a careful eye and a willingness to question whether everything he reads is physically correct.

Editor’s Note: This article originally appeared in the Spring 2010 issue of Scope, published by NC State’s College of Physical and Mathematical Sciences.

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