Almost at the beginning of my research experience [at Cambridge, in the mid-1930's] the great articles of Bethe and his collaborators Bacher and Livingston appeared. We had a weekly seminar at which it was decided by the senior members—Dirac, Fowler and Peierls, to mention only four—that we would go through these articles bit by bit. The idea was to allocate part of them to the students, and each week one of us would expound on the blackboard. Our expounding was, I suppose, pretty bad; at any rate the senior people, Darwin and Fowler in particular, thought so. It came ‘round to my turn and sure enough I made a very poor fist of it, and received a terrible pounding for my pains. Fortunately, my exposition was such a shambles that it had to be continued the following week. For the first time in my life I began to think not merely of taking in knowledge but of giving it out. I spent a whole week thinking about expressing myself, about ways of writing on the blackboard, and about illustrative diagrams. The result was that I was able to...
In your classwork you have studied the conceptual fundamentals of contemporary physics, and learned something of the analytical and experimental techniques central to the field. Problems of a different order—how to make effective use of the published literature, how to keep a research effort focused and in motion, how to write graceful scientific prose—have been brought vividly to your attention by your thesis activity. The thesis seminar is intended to provide experience in yet another critical area. The oral presentation of scientific material poses problems that, if left unresolved, can very much impede the progress of one's scientific career. The following remarks are intended to alert you to the fundamentals of effective oral presentation.
Axiom I: SCIENTIFIC TALKS ARE NOT SIMPLY VERBALIZED TECHNICAL PAPERS.
This distinction is as fundamental as that between painting and music; readers can linger over passages that give them difficulty, but listeners have no such option.
Axiom II: THE ALLOTTED TIME—WHATEVER IT IS—IS ALWAYS SHORTER THAN YOU THINK.
Speakers and authors labor under distinct constraints, toward distinct goals...and have distinct sets of resources upon which to draw. A speaker's task is synoptic—to plant the outlines (and nothing more) of one or a few ideas unforgettably in the minds of the individual listeners who comprise the speaker's momentary audience. A speaker can realistically aspire to no more, and should use every available means to achieve no less.
1st Law: PROCEED AS QUICKLY AS YOU CAN TO THE SHARPEST POSSIBLE STATEMENT OF THE PROBLEM—OF YOUR MOTIVATION AND GOALS.
Your intent, after all, is to clarify a mystery. A sharp statement of that mystery will serve to engage your listeners’ attention and to provide them with hooks upon which to hang your subsequent remarks.
2nd Law: DON'T TRY TO PRESENT TOO MANY IDEAS.
"Two ideas per talk" is an excellent average, and one really good idea per talk is actually much better than average. If your subject is complex, phrase your remarks in terms of the simplest illustrative special case: thus (in point of historical fact) was Schrodinger led from the "relativistic Schrodinger equation"—which doesn't work—to the "non-relativistic Schrodinger equation"—which does!
3rd Law: OMIT THE COMPUTATIONAL DETAILS THAT LINK YOUR MAJOR POINTS.
Listeners desire to know only about the general strategy of computation, and are—tentatively—willing to take on trust the correctness of your claim that things work out as you state.
4th Law: STATE—AND RESTATE—YOUR CENTRAL MESSAGE AS SHARPLY, AS VIVIDLY, AS MEMORABLY AS IT IS IN YOUR POWER TO DO.
Redundancy, while often a defect in written work, is indispensable to the effective oral communication of difficult material. If you can reduce your conclusion to a metaphor, a visual image, a point which sharply engages your listeners' physical intuition, you should not hesitate to do so, for such is the stuff which sticks in the memory. But you should, in the service of honesty, go on to state the sense in which your conclusions are conditional, your metaphor deceptive...your sense of what lies further down the road that you and your listeners have traveled.
What now follows is a list of lesser points, all of which follow as corollaries from the major principles stated above.
MISCELLANEOUS PITFALLS & TRICKS OF THE TRADE
While an author may (at some risk) write for an "abstract reader," speakers speak to specific individuals. Speak not to the blackboard nor to the back wall but to the faces of your listeners. Imagine yourself to be seated among them. Read their faces: they are a resource that distinguishes your predicament from that of an author, and will tell you instantly when you have lapsed into obscurity.
If knowledge and a deep desire to be understood are your strongest offensive weapons, humility is your strongest defense. Resist the urge to show off. Remember that wit that does not serve your informative purpose is wit misapplied. And cite your sources. The latter point of courtesy will, after all, release you from any obligation to develop details that interested listeners can find in the published literature.
You should except to expose specific methodological points germane to your central idea, but resist the urge to review standard derivations of standard material. You haven't time. Historical remarks are—if accurate—useful, but should not be allowed (which is their tendency) to displace your main points.
Since blackboard work takes time—not in itself a bad thing, for it gives your listeners time to reflect—you may find that the tempo and ultimate impact of your talk are improved if references and secondary detail are written out in advance for distribution as handouts. Graphic aids are helpful—they help you to engage not only the ear but also the eye of your listeners, and to introduce an element of variety—but for maximal impact must be thoughtfully designed and tested in advance. Remember that transparencies may tempt you to go faster than any listener can follow with comprehension, and that poor graphics are worse than no graphics; by their use you have, after all, denied listeners the option of saying (for example) "Write larger!"
Scientific lectures take place in theatres, and are themselves a kind of theatre. It follows that it is appropriate/important to give attention to the overall design of your presentation—to matters of rhythm, tempo and timing, to the sequence in which you pull the rabbits from your hat, to the willful (but invisible) fabrication of an element of surprise, of climax. It is classical sonata form, not the phonebook, that should serve as your model. It is, moreover, entirely appropriate to rehearse your presentation ... and when it is over to ask yourself (and your friends!) "In what respects did my talk succeed (and why), in what respects did it fail (and why), how could it have been improved?"
A speaker who does not speak (up) is—by definition—not a speaker. Actors and singers—even those with small voices--acceot it as a professional obligation to be "heard ringingly in the back of the house"... and so also must scientific speakers. The "inaudibility problem" (analog of the "writing too small problem") derives mainly from three circumstances: (1) inattention (no excuse); (2) modesty (misplaced); (3) lack of scientific confidence. In cases of the latter type the speaker is, in point of fact, not yet ready to speak, and should redesign his/her talk until the circumstances that undermine confidence have been identified and either (1) eliminated or (2) frankly confessed. Cases of the former types are more easily dealt with, by practice with the assistance of the above-mentioned friends; it takes conscious effort to fill with sound a room full of acoustically-absorbent protoplasm.
Effective scientific speaking is an acquired skill. But of all the skills that a physicist must possess it is one of the most easily acquired. All that is required is conscious attention to the fundamentals... and practice, much practice.