The theory of practical joking - its relevance to physics

A countermeasure system of chaff cartridges and infrared flares  
is used by a C-130J Hercules Tactical Transport Aircraft

The Theory of Practical Joking - its Relevance to Physics was written by R.V. Jones, as part of a lecture published in Bulletin of the Institute of Physics, June 1957.

Reginald Victor Jones  (1911 - 1997) was an English physicist who defended Britain with science during World War II. He helped deploy what's now known as "chaf" to fool enemy radar, and was celebrated as a founder of electronic warfare. An award in his name, the R. V. Jones Intelligence Award, was created by the CIA in 1993 to honor those who have "Scientific acumen applied with art in the cause of freedom."
Jones obituary in The New York Times>>

More on chaff and current missile countermeasures, from Aerospaceweb>>

At first sight there may seem little relation between physics and practical joking. Indeed, I might never have observed their connection but for an incidental study of the life of James Clerk Maxwell. Two things, among many others, struck me. The first was the growth of his sense of fun from the primitive joke of the boy of six tripping up the maid with the tea tray to the refined, almost theoretical, jokes of his later life. The second was his mastery of analogy in physical thinking: already, at the age of twenty-four he had written a part playful, part serious essay on the theory of analogy which showed two of the main features of his mind. On the lighter side, he pointed out the relation between an analogy and a pun: in the former one truth lies under two expressions, and in the latter two truths lie under one expression. Hence from the theory of analogy one can by reciprocation deduce the theory of puns. To the more serious side of Maxwell's understanding of analogy I shall return later, but all this set me thinking about the possible connection between the theory of practical joking and physics. One factor which encouraged me was the high incidence of mischievous humor among physicists. Even Newton, it is recorded, caused trouble in his Lincolnshire village as a boy by flying at night a kite carrying a small lantern; and in this century the spritely skill of the late Professor R W Wood and Professor G Gamow is already legendary. While I hope to illustrate this paper with examples, I propose first to analyze (if this is not altogether too brutal a process) the essentials of a joke.


The crux of the simplest form of joke seems to be the production of an incongruity in the normal order of events. We hear the story, for example, of Maxwell showing Kelvin some optical experiment, and inviting Kelvin to look through the eyepiece. Kelvin was surprised to find that, while the phenomenon described by Maxwell was undoubtedly there, so was a little human figure, the incongruity, dancing about. Kelvin could not help asking 'Maxwell— but what is the little man there for?' 'Have another look, Thomson,' said Maxwell, 'and you should see.' Kelvin had another look, but was no wiser. 'Tell me, Maxwell,' he said impatiently, 'What is he there for?' . . . 'Just for fun, Thomson,' replied Maxwell. When we consider a simple incongruity of this type, we can see why this form of humor is sometimes described as 'nonsense'; for 'sense' implies the normal order of things, and in this order an incongruity makes 'nonsense.' A simple incongruity in the literature of physics is R W Wood's recording of the fact that he cleaned out an optical instrument by pushing his cat through it.

Even a change of dimension is sufficient to cause an incongruity. Lord Cherwell has a story of a scientist at Farnborough in World War I, who was so dismayed by the delays in ordering commercial equipment that when he wanted a dark-room lamp he made a pencil sketch of one, to be made up by the workshop. It availed him little, however, because a proper engineer's drawing had by regulation to be made in triplicate before the workshop would start. Weeks elapsed, and finally after a knock on his door two workmen wheeled in the largest darkroom lamp ever constructed. In making the workshop drawing the draughtsman had left out one dash, with the result that intended inches became actual feet. One of the classic incongruities of this type is that due to Benjamin Franklin in a letter to the Editor of a London newspaper in 1765, chaffing the English on their ignorance of America: 'The grand leap of the Whale up the Falls of Niagara is esteemed, by all who have seen it, as one of the finest spectacles in Nature!'

A variation on the simple incongruity in humor is to produce a congruity where incongruity is normally expected. One does not expect, for example, any congruity about the names of joint authors of scientific papers. It was therefore rather a surprise to find a genuine paper by Alpher, Bethe and Gamow, dated April 1.

A further variation of humor is produced when a false incongruity is expected by the victim, and an incongruity then genuinely occurs which he promptly discounts. The late Sir Francis Simon had this happen to him when he was head of a laboratory in Germany. One night his research students were working with liquid hydrogen, and there was an explosion which damaged the laboratory some time after midnight. One of the research students telephoned the professor to inform him of the damage. All he could get from Sir Francis was an amiable 'All right, I know what day it is!' It was the morning of April 1.


Simple incongruities direct or inverted, can be humorous enough, but the more advanced jokes usually involve a period of preparation and induction, sometimes elaborate, before the incongruity becomes apparent. They are called hoaxes. Maxwell's jokes were often simple in their preparation; he is credited with having engineered the advertisement of his Inaugural Lecture at Cambridge (which is still very worth reading) in such a manner that only his undergraduate students heard of it, and he gave it to them alone. The senior members of the University merely saw that the new professor would deliver his first lecture on a particular day, and they attended in force. This lecture, however, was the first of his undergraduate course, and his delighted students enjoyed the experience of seeing Maxwell gravely expounding, though with a betraying twinkle in his eye, the difference between the Fahrenheit and Centigrade scales to men like Adams, Cayley, and Stokes.

With some hoaxes the period of induction of the victim may be extended. In this type, which is probably the most interesting philosophically, the object is to build up in the victim's mind a false world-picture which is temporarily consistent by any tests that he can apply to it, so that he ultimately takes action on it with confidence. The falseness of the picture is then starkly revealed by the incongruity which his action precipitates. It has not proved difficult, for example, to persuade a Doctor of Philosophy to lower his telephone carefully into a bucket of water in the belief that he was cooperating with the engineer in the telephone exchange in finding a leak to earth. The prior induction consisted of building up in his mind a picture of something being wrong with his telephone by such tactics as repeatedly ringing the bell and then ringing off as he answered. As a further example, we may recall one of the works of a German physicist, Dr Carl Bosch, who about 1934 was working as a research student in a laboratory which overlooked a block of flats. His studies revealed that one of the flats was occupied by a newspaper correspondent, and so he telephoned this victim, pretending to be his own professor. The 'professor' announced that he had just perfected a television device which could enable the user to see the speaker at the other end. The newspaper man was incredulous, but the 'professor' offered to give a demonstration; all the pressman had to do was to strike some attitude, and the voice on the telephone would tell him what he was doing. The telephone was, of course, in direct view of the laboratory, and so all the antics of the pressman were faithfully described. The result was an effusive article in the next day's paper and, subsequently, a bewildered conversation between the true professor and the pressman.

The induction of the victim can take many forms. One of the favorite ways is an acclimatization by slow change. R W Wood is said to have spent some time in a flat in Paris where he discovered that the lady in the flat below kept a tortoise in a window pen. Wood fashioned a collecting device from a broom-handle, and bought a supply of tortoises of dispersed sizes. While the lady was out shopping, Wood replaced her tortoise by one slightly larger. He repeated this operation each day until the growth of the tortoise became so obvious to its owner that she consulted Wood who, having first played a subsidiary joke by sending her to consult a Professor at the Sorbonne whom he considered to be devoid of humor, advised her to write the press. When the tortoise had grown to such a size that several pressmen were taking a daily interest, Wood then reversed the process, and in a week or so the tortoise mysteriously contracted to its original dimensions.


Induced incongruities have a high place in warfare, where if the enemy can be induced to take incorrect action the war may be advantageously affected. A stratagem in which some of my wartime colleagues were involved is now well known as 'The man who never was.' These same colleagues also worked with me in some technical deceptions, of which one was the persuasion of the Germans in 1943 that our successes against the U-boats were due not to centimetric radar but to a fictitious infrared detector. We gained some valuable months while the Germans invented a beautiful anti-infrared paint and failed to find the true causes of their losses. The paint, incidentally, was a Christiansen filter of powdered glass in a transparent matrix over a black base. The filter 'peaked' in the near infrared, so that incident radiation in this region went through and was absorbed in the underlying black. Visible light was scattered back by the filter, which thus gave a light grey appearance to the eye, but was black to the near infrared. This simulated admirably the reflecting power of water, and thus camouflaged the U-boat. It was afterwards reported that the inventor of the paint was Dr Carl Bosch. Before I turn to the more serious side of this lecture there is one further story from Physics in which the exact classification of the incongruity can be left as a problem to be worked out at leisure. It concerns Lord Kelvin's lectures at Glasgow, where he used to fire a bullet at a ballistic pendulum; as an undergraduate at Oxford I had heard a story of how Kelvin missed on one occasion, with the result that the bullet went through a wall and smashed the blackboard of the lecturer next door. Kelvin rushed into the next room in some alarm to find the lecturer unscathed, and the class shouting 'Missed him—try again, Bill.' This experiment has now produced a further incident, and to avoid any doubt I wrote to Professor Dee for his own account of what happened. This is what he says:

'In the Quincentenary Celebrations here I had to lecture on the history of the Department. Of course Kelvin figured strongly in this. One of Kelvin's traditional experiments was to fire a rifle bullet at a very large ballistic pendulum. All his students regarded this as the highlight of the course. He was reputed to have the gun charged with a big dose of powder—the barrel is about half an inch internal diameter. I decided this experiment must be repeated but there was great alarm here that the barrel would burst and annihilate the front row (Principal and Senate). So I decided to use a modern rifle. I also decided to make it a double purpose experiment by using Kelvin's invention of the optical lever to display the pendulum swing to a large audience. On the night all went off well.

'The next day I repeated the whole lecture to the ordinary class. Mr. Atkinson was the normal lecturer to this class and he had noticed that in referring to the dual purpose of the demonstration I used the phrase ". . . fitted a mirror to the pendulum so that I may kill two birds with one stone." After the explosion to my surprise a pigeon fell with a bloody splash on to a large white paper on the bench—our lecture room is very high. I tried to resolve the situation by saying "Well although Mr. Atkinson isn't lecturing to you today he appears to be behind the scenes somewhere. But he does seem to have failed to notice that I said two birds with one stone!" Immediately a second pigeon splashed on the bench! Whether this was due to a slip up in Atkinson's mechanical arrangements or to his brilliant anticipation of how I would react I don't really know but I always give him the credit of the second explanation. 'Anyway the students loved it but I wonder how many would remember about the optical lever?’


I want to turn now to technical deception in war, as exemplified by our attempts to mislead the German night defenses in their appreciation of our raiding intentions. The method here is that of the induced incongruity; by a false presentation of evidence we wish the enemy controller to build up an incorrect but selfconsistent world-picture, thus causing him to generate the incongruity of directing his nightfighters to some place where our bombers are not. I originally developed this 'Theory of Spoof' in a wartime report; the salient points, which have some interest in physical theory, are the following. As with all hoaxes the first thing is to put oneself in the victim's place (indeed, a good hoax requires a sympathetic nature), to see what evidence he has with which to construct and test his world-picture. In night aerial warfare in 1939-45, this evidence was mainly the presence of deflections in the trace of the cathode ray observing tube. Therefore any device which would give rise to such deflections could provide an element of Spoof. One such device was a jammer which would cause fluctuating deflections all the time, thus concealing the true deflections due to the echo from an aircraft. This, like a smoke screen, would render the enemy unaware that you are where you are. A more positive technique is to provide a false echo, and if possible to suppress the genuine one, thus giving him the impression that you are where you are not. The easiest way of providing a false echo is to drop packets of thin metal strips, cut to resonate to the enemy's radar transmissions. This is, of course, what we did in 1943. There is little time to tell now of the fortunes of this technique, but the packets were extremely successful, and they changed the character of air warfare at night. At first, the German controllers confused the individual packets with aircraft; I can still remember the frustrated tones of one controller repeatedly ordering a packet to waggle its wings as a means of identification. Soon, however, the Germans gave up the attempt to make detailed interceptions, and tried to get a swarm of fighters into our bomber streams. We then used many tinfoil packets dropped by a few aircraft to provide the appearance of spoof raids, which lured the nightfighters off the track of our main raids.

As the war went on the Germans gradually found ways of distinguishing between echoes from metal foil packets and those from aircraft. The packets, for example, resonated to one particular frequency, and therefore they had a relatively poor response to another frequency. If two radar stations watched on widely separate frequencies, a genuine aircraft echo would be present on both, whereas the foil echo would appear only on one. The foil could, of course, be cut to different lengths, but as the number of frequencies was increased, the amount of foil needed was greater. Moreover there was a pronounced Doppler effect on the echo from an aircraft, with its high speed, but little effect on the echoes from the foil drifting with the wind. Thus, against an omniscient controller, we have to make the decoy echoes move with the speed of aircraft, and reflect different frequencies in the same way. This is easiest done by making a glider of the same size as the bomber. Then if we allow the enemy controller to use sound and infrared detectors and other aids, we find that the only decoy which can mislead him into thinking that there is a British bomber flying through his defenses is another British bomber flying through his defenses.

Another example is one that I encountered earlier in what has been called 'The Battle of the Beams' in 1940. Here the problem was to upset the navigation of the German night bombers, when they were flying along radio beams to their targets. The signals received by the pilots telling them to steer right or left were counterfeited in this country, and sometimes resulted in their flying on curvilinear courses. However, had the pilots had unlimited time of observation they could have detected that there was something wrong, even if we had exactly synchronized our transmitters with those of the Germans. The bombers were in general flying away from their own transmitters and towards ours, and so they would have received a Doppler beat from which they could have deduced that a second transmitter was active. If one allows the possibility of various simple tests, which fortunately would take too long in actual warfare, one arrives at the conclusion that the only place for a second transmitter which will simulate the original exactly is coincident with the original and the counterfeit thus defeats its purpose.

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