## Posts Tagged ‘feynman’

### Happy birthday, Richard Feynman!

Friday, May 11th, 2012

Richard Feynman was one of the most influential physicists of the twentieth century. Not only did he revolutionize quantum theory with his development of quantum electrodynamics, but he also revolutionized the way we think about physics and physicists. He spoke to people from all kinds of backgrounds about physics, from lecturing students destined to change the field themselves, to appearing on television to discuss physics and the philosophy of science, to meeting with the greatest minds of the time.

Feynman in the middle of a lecture. (www.richard-feynman.net)

For me, Feyman’s great contribution was the way he thought about physics. His Lectures on Physics are world famous, and rightly so. (In fact, one of the first things I did after landing in San Francisco to work at SLAC was to buy a copy of his lectures from the Stanford bookstore. Shortly afterwards by bank froze my card, suspecting fraud. It was worth the inconvenience!)

As a jaded undergraduate they were a source of inspiration to me. A faint glimmer of hope turned into a roaring inferno after reading his lectures on electromagnetism, and I’ve never looked back since. Finally, here was someone who wanted to discuss the beauty of the subject, as well as the truth. He had no time for obscuring the underlying symmetry of a concept, nor for lying to students in order to make things easier. Inevitably having to unlearn and relearn ideas leaves people confused, disillusioned and unable to trust their tutors. In that spirit, this is how he started his course on electromagnetism:

“We begin now our detailed study of the theory of electromagnetism. All of electromagnetism is contained in the Maxwell equations.

Maxwell’s equations:

$\nabla \cdot \vec{E} = \frac{\rho}{\varepsilon_0}$
$\nabla \times \vec{E} = – \frac{\partial \vec{B}}{\partial t}$
$c^2\nabla \times \vec{B} = \frac{\partial \vec{E}}{\partial t} + \frac{\vec{j}}{\varepsilon_0}$
$\nabla \cdot \vec{B} = 0$

Don’t worry about trying to understand these equations. The important thing here is that Feynman has given the students the complete truth about electromagnetism. With these four equations he can solve any problem about the shape and nature of electromagnetic fields for any configuration of charges and currents. The equations he provides are not some approximation of the theory, or some equations that only work some of the time, these are the equations that all physicists and engineers use and they are, as far as we know, complete and state of the art. Feynman has shown a level of honesty and respect for his students/readers that was not present when I sat through lectures. My lecturers taught me backwards, Feynman taught me forwards.

(Experts might notice that the Lorentz force law is missing here, but Feynman already mentioned it a few pages before Maxwell’s equations. With the Lorentz force law physicists can relate the electromagnetic fields to the forces on charged particles.)

Feynman continues:

The situations that are described by these equations can be very complicated. We will consider first relatively simple situations, and learn how to handle them before we take up more complicated. The easiest circumstance to treat is one in which nothing depends on time- called the static case. All charges are permanently fixed in space, or if they do move, they move as a steady flow in a circuit (so $$\rho$$ and $$\vec{j}$$ are constant in time). In these circumstances, all of the terms in the Maxwell equations which are time derivatives of the field are zero. In this case Maxwell’s equations become:

Electrostatics:
$\nabla \cdot \vec{E} = \frac{\rho}{\varepsilon_0}$
$\nabla \times \vec{E} = \vec{0}$

magnetostatics:
$c^2\nabla \times \vec{B} = \frac{\vec{j}}{\varepsilon_0}$
$\nabla \cdot \vec{B} = 0$

You will notice an interesting thing about this set of four equations. It can be separated into two pairs. The electric field $$\vec{E}$$ appears only in the first two, and the magnetic field $$\vec{B}$$ appears only in the second two. The two fields are not interconnected. This means that electricity and magnetism are distinct phenomena so long as charges and currents are static.

And he goes on. Immediately at the start of the course he’s pointed out one of the most important and beautiful symmetries in electromagnetism. He also lets us know how the course is going to proceed, with static cases first and the full treatment later. This leaves the student with a wonderful surprise later in the course, when the two fields finally get united again. When this happens Feynman goes on to show us how electromagnetism comes about as a result of special relativity, and if done properly that is one of the most breathtaking moments in physics! This is the way physics should be taught, and I wish I could have been in that lecture hall to see it happen!

The rest of the lectures are a fascinating journey, full of neat little asides, teasers, paradoxes, and it’s all handled with refreshing clarity. He even pokes fun at physics itself from time to time, showing how our mathematical notation is just a trick to make complicated things look simple and how different problems appear to have similar solutions only because we choose to use the same kinds of methods to solve them. Towards the end of his electromagnetism course he even goes out of his way to show how electromagnetism fails in an epic way. The problem of the infinite energy of the field, and the intractable problem of the mass of the electron are two major failings of the classical theory, and he dedicates a lecture to showing us just many questions were left unanswered by the subject.

Feynman with bongos, because some physicists are cool (www.richard-feynman.net)

Feynman gave us a lot to digest, from Nobel prize worthy discoveries, to a view of scientists that was anything but a crusty old professor, and for me what I value most is the lectures he gave, packed with inspiration and clarity. If you have a chance, go read some of the lectures and find out what made this man get out of bed in the morning. You won’t be disappointed. His other books are also excellent (Six Easy Pieces, Six Not So Easy Pieces, QED and Surely You’re Joking, Mr Feynman!) and well worth a read. Put them on your Christmas wish list!

Feynman’s birthday should be a national day of celebration, not just for physics, but for getting people hooked on physics! (I’m just sorry I’m a bit late to the party here, have a great weekend.)

If you want to find out a bit more about Richard Feynman check out this lecture about Feynman from Lawrence Krauss, one of today’s most eloquent speakers and best advocates for physics.

(Quotes taken from “The Feyman Lectures on Physics, The Definitive Edition Volume II”, Feynman Leighton and Sands, ISBN 0-8053-9047-2)