A rope trick is the whimsical term given by physicist Dr. John Malik to the curious lines and spikes which emanate from the fireball of a nuclear explosion just after detonation. The image at the right is from the Tumbler-Snapper test series of 1952. It was taken less than one millisecond after a nuclear bomb was detonated on the top of a tower at the Nevada Test Site.

The photograph shows two striking features: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface. The "rope tricks" which protrude from the bottom of the fireball are caused by the heating and rapid vaporization of mooring cables (or specialized rope trick test cables) which extend from the shot cab to the ground. The surface of the fireball is over 20,000 kelvins and emits huge amounts of visible light radiation to which the atmosphere is almost entirely transparent, thus everything in the vicinity of the fireball is heated extremely rapidly to very high temperatures at a speed dictated by the propagation velocity of light in the atmosphere. Malik observed that if the rope was painted black, spike formation was enhanced, and if it was painted with reflective paint or wrapped in aluminum foil, no spikes were observed; thus confirming the hypothesis that it is heating and vaporization of the rope induced by exposure to high intensity visible light radiation which causes the effect.

The cause of the surface mottling is more complex. At the point in the explosion captured in the photo at right, a hydrodynamic shock front has just formed. Before this time the growth of the fireball was due to radiative transport, i.e. thermal x-rays "outpaced" the actual expanding bomb debris. At this point however, the fireball expansion is caused by the shock front driven by hydrodynamic pressure (as in a conventional explosion, only far more intense). The glowing surface of the fireball is due to shock compression heating of the air. This means that the fireball is now growing far more slowly than before. The bomb (and shot cab) vapors were initially accelerated to very high velocities (several tens of kilometers per second) and clumps of this material are splashing against the back of the shock front in an irregular pattern (due to initial variations in mass distribution around the bomb core), thus creating the mottled appearance.

The photo was shot by a Rapatronic camera (a highspeed camera invented by Harold Edgerton and colleagues) built by EG&G [1]. Since each camera could record only one exposure on a sheet of film, banks of four to 10 cameras were set up to take sequences of photographs. The average exposure time was three microseconds.

External links

• Operation Tumbler-Snapper, 1952 webpage showing the tower cab housing