Rope trick effect
Encyclopedia
Effects of nuclear explosions
The energy released from a nuclear weapon detonated in the troposphere can be divided into four basic categories:*Blast—40-50% of total energy*Thermal radiation—30-50% of total energy...
just after detonation.
Description
The photograph at the right specifically 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 kelvinKelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, emits huge amounts of visible light radiation
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
(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" which protrude from the bottom of the fireball are caused by the heating, rapid vaporization and then expansion of mooring cables (or specialized rope trick test cables) which extend from the shot cab (the housing at the top of the tower that contains the explosive device) to the ground. Malik observed that when the rope was paint
Paint
Paint is any liquid, liquefiable, or mastic composition which after application to a substrate in a thin layer is converted to an opaque solid film. One may also consider the digital mimicry thereof...
ed black, spike formation was enhanced, and if it were painted with reflective paint or wrapped in aluminium foil
Aluminium foil
Aluminium foil is aluminium prepared in thin metal leaves, with a thickness less than , thinner gauges down to are also commonly used. In the USA, foils are commonly gauged in mils. Standard household foil is typically thick and heavy duty household foil is typically .The foil is pliable, and...
, 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. Because of the lack of mooring ropes, no "rope trick" effects were observed in surface-detonation tests, free-flying weapons tests, or, obviously, in underground tests.
The cause of the surface mottling is more complex. At the point in the explosion captured in the above photo, a hydrodynamic shock front has just formed. Before this point in time, the growth of the fireball was due to radiative transport, i.e. thermal x-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s "outpaced" the actual expanding bomb debris. At this point of the explosion, 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
Gay-Lussac's law
The expression Gay-Lussac's law is used for each of the two relationships named after the French chemist Joseph Louis Gay-Lussac and which concern the properties of gases, though it is more usually applied to his law of combining volumes, the first listed here...
of the air
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...
, thus the fireball is at this point growing at a much slower rate than when it was in the radiatively driven stage. In the first few microseconds
Orders of magnitude (time)
-Seconds:- See also :* Heat Death* Second law of thermodynamics* Big Rip* Big Crunch* Big Bounce* Big Bang* Cyclic model* Dyson's eternal intelligence* Final anthropic principle* Ultimate fate of the Universe* Timeline of the Big Bang...
after detonation, the bomb and shot cab are destroyed and vaporized. These vapors are then accelerated to very high velocities (several tens of kilometers per second
Metre per second
Metre per second is an SI derived unit of both speed and velocity , defined by distance in metres divided by time in seconds....
) outward, and the clustered masses of varying vapor density 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
Nuclear weapon design
Nuclear weapon designs are physical, chemical, and engineering arrangements that cause the physics package of a nuclear weapon to detonate. There are three basic design types...
. This effect thus creates the mottled blob-like appearance.
After a few milliseconds, the energy of the shock front will no longer be great enough to heat the air into incandescence
Incandescence
Incandescence is the emission of light from a hot body as a result of its temperature. The term derives from the Latin verb incandescere, to glow white....
. At this point the shock front becomes invisible, a process known as "breakaway". This makes the shock wave difficult to diagnose. Photographs of atomic tests often show numerous vertical rope-like lines to one side. These are typically created by small sounding rocket
Sounding rocket
A sounding rocket, sometimes called a research rocket, is an instrument-carrying rocket designed to take measurements and perform scientific experiments during its sub-orbital flight. The origin of the term comes from nautical vocabulary, where to sound is to throw a weighted line from a ship into...
s 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
Lens (optics)
A lens is an optical device with perfect or approximate axial symmetry which transmits and refracts light, converging or diverging the beam. A simple lens consists of a single optical element...
. This is not necessarily related to the rope trick effect in any physical way, although it is possible to use vertical cables heated by the rope trick in place of the rockets. It is possible to confuse the two in some photographs. In this photograph, the smoke trails are (faintly) visible in the lower-right corner.
Camera recording
The photo was shot by a rapatronic cameraRapatronic camera
The rapatronic camera is a high-speed camera capable of recording a still image with an exposure time as brief as 10 nanoseconds ....
(a high speed camera
High speed camera
A high speed camera is a device used for recording fast moving objects as a photographic image onto a storage media. After recording, the images stored on the media can be played back in slow-motion...
invented by Harold Edgerton
Harold Eugene Edgerton
Harold Eugene "Doc" Edgerton was a professor of electrical engineering at the Massachusetts Institute of Technology...
and colleagues) built by EG&G
EG&G
EG&G, formally known as Edgerton, Germeshausen, and Grier, Inc., is a United States national defense contractor and provider of management and technical services. The company was involved in contracting services to the United States government during World War II, and conducted weapons research and...
. Each camera was capable of recording only one exposure
Exposure (photography)
In photography, exposure is the total amount of light allowed to fall on the photographic medium during the process of taking a photograph. Exposure is measured in lux seconds, and can be computed from exposure value and scene luminance over a specified area.In photographic jargon, an exposure...
on a single sheet of film. Therefore, in order to create time-lapse
Time-lapse
Time-lapse photography is a cinematography technique whereby the frequency at which film frames are captured is much lower than that which will be used to play the sequence back. When replayed at normal speed, time appears to be moving faster and thus lapsing...
sequences, banks of four to ten cameras were set up to take photos in rapid succession. The average exposure time used was three microseconds
Orders of magnitude (time)
-Seconds:- See also :* Heat Death* Second law of thermodynamics* Big Rip* Big Crunch* Big Bounce* Big Bang* Cyclic model* Dyson's eternal intelligence* Final anthropic principle* Ultimate fate of the Universe* Timeline of the Big Bang...
.