thumb|Nuclear explosion milliseconds after detonation. From the [[Operation Teapot test series in Nevada, 1955, showing fireball and rope trick effects.]]

Rope trick is the term given by American nuclear physicist John Malik to the curious lines and spikes which emanate from the fireball of nuclear explosions under certain conditions, just after detonation.

Description

thumb|Rope trick effects visible from one of [[Operation Tumbler–Snapper's tower-mounted test shots in 1952, taken with a rapatronic camera]]

The adjacent photograph shows two unusual phenomena: bright spikes projecting from the bottom of the fireball, and the peculiar mottling of the expanding fireball surface.

The surface of the fireball, with a temperature over 20,000 kelvin, emits huge amounts of visible light radiation, more than 100 times the intensity at the Sun's surface. Anything solid in the area absorbs the light and rapidly heats. The "rope tricks" that protrude from the bottom of the fireball are caused by the heating, rapid vaporization and then expansion of guy wires that extend from the <em>shot cab</em>—the housing at the top of the bomb tower that contains the explosive device—to the ground. Malik observed that when the guy wires were painted black, spike formation was enhanced, and if it were painted with reflective paint or wrapped in aluminium foil, no spikes were observed &ndash; 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. Because of the lack of guy wires, no "rope trick" effects were observed in surface-detonation tests, free-flying weapons tests, or underground tests.

Sounding rockets

thumb|Array of [[sounding rockets with instruments for making scientific measurements of high-altitude nuclear tests during liftoff preparations on Johnston Island]]

After a few milliseconds, the energy of the shock front will no longer be great enough to heat the air into incandescence. At that point, the shock front becomes invisible, a process known as "breakaway". This makes the shock wave difficult to diagnose beyond this boundary.

Photographs of nuclear tests often show numerous vertical rope-like lines to one side. These are typically created by small sounding rockets launched a few seconds before the firing, leaving smoke trails. The purpose of these trails is to record the passing of the now invisible shock wave, which causes an obvious visual effect on the smoke by compressing the air into a lens. This is not necessarily related to the rope trick effect in any physical way, but it is possible to confuse the two in some photographs. In the photograph of the Tumbler-Snapper test, the vertical lines in the lower-right corner are blast line poles, not smoke trails.

Camera recording

The photo was shot by a rapatronic camera (a high-speed camera invented by Harold Edgerton and colleagues) built by EG&G. Each camera was capable of recording only one exposure on a single sheet of film. To create time-lapse sequences, banks of four to ten cameras were set up to take photos in rapid succession. The average exposure time was three microseconds.

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References

Sources

  • This article incorporates text from the National Nuclear Security Administration's "Rapatronic Photography" factsheet (August 2013).
  • Rare Nuclear Bomb Footage Reveals Their True Power | WIRED