The Monte Carlo trolley, or FERMIAC, was an

analog computer An analog computer or analogue computer is a type of computer that uses the continuous variation aspect of physical phenomena such as electrical, mechanical, or hydraulic quantities (''analog signals'') to model the problem being solved. In ...

invented by physicist Enrico Fermi to aid in his studies of

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

transport.

Operation

The FERMIAC employed the

Monte Carlo method Monte Carlo methods, or Monte Carlo experiments, are a broad class of computational algorithms that rely on repeated random sampling to obtain numerical results. The underlying concept is to use randomness to solve problems that might be determi ...

to model neutron transport in various types of

nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...

systems. Given an initial distribution of

s, the goal of the process is to develop numerous "neutron genealogies", or models of the behavior of individual neutrons, including each collision, scattering, and fission. When a fission occurs, the number of emerging neutrons is predicted, and the behavior of each of these neutrons is eventually modeled in the same manner as the first. At each stage,

pseudo-random number A pseudorandom sequence of numbers is one that appears to be statistically random, despite having been produced by a completely deterministic and repeatable process. Background The generation of random numbers has many uses, such as for random ...

s are used to make decisions that affect the behavior of each neutron. The FERMIAC used this method to create two-dimensional neutron genealogies on a scale diagram of a nuclear device. A series of drums on the device were set according to the material being crossed and a random choice between fast and slow neutrons. Random numbers also determined the direction of travel and the distance until the next collision. Once the drums were set, the trolley was rolled across the diagram, drawing a path as it went. Any time a change in material was indicated on the diagram, the drum settings were adjusted accordingly before continuing.

History

In the early 1930s, Italian physicist Enrico Fermi led a team of young scientists, dubbed the "

Via Panisperna boys Via Panisperna boys (Italian: ''I ragazzi di Via Panisperna'') is the name given to a group of young Italian scientists led by Enrico Fermi, who worked at the Royal Physics Institute of the University of Rome La Sapienza. In 1934 they made the ...

", in their now-famous experiments in

nuclear physics Nuclear physics is the field of physics that studies atomic nuclei and their constituents and interactions, in addition to the study of other forms of nuclear matter. Nuclear physics should not be confused with atomic physics, which studies the ...

. During this time, Fermi developed "statistical sampling" techniques that he effectively employed to predict the results of experiments. STAN ULAM HOLDING THE FERMIAC

Years later, in 1946, Fermi participated in the initial review of results from the

ENIAC ENIAC (; Electronic Numerical Integrator and Computer) was the first programmable, electronic, general-purpose digital computer, completed in 1945. There were other computers that had these features, but the ENIAC had all of them in one pac ...

. Among the others present was Los Alamos mathematician Stanislaw Ulam, who was familiar with the use of statistical sampling techniques similar to those previously developed by Fermi. Such techniques had mainly fallen out of use, due to the long, repetitious calculations required. However, given ENIAC's powers of calculation, Ulam saw an opportunity to resurrect these techniques. He discussed his ideas with

John von Neumann John von Neumann (; hu, Neumann János Lajos, ; December 28, 1903 – February 8, 1957) was a Hungarian-American mathematician, physicist, computer scientist, engineer and polymath. He was regarded as having perhaps the widest cove ...

, who eventually used the ENIAC to implement the

(as the statistical sampling techniques came to be called) to solve a variety of neutron transport problems. However, before the ENIAC could be employed for this purpose, it first had to be moved to its permanent home at the Ballistics Research Laboratory. It was during this interruption in ENIAC operation that Fermi came up with the idea for his analog device. He enlisted his colleague L.D.P. King to build the instrument, which was later given the fitting name FERMIAC. The device was used for approximately two years
The Fermiac is on display in the Bradbury Science Museum in Los Alamos.
In 2015, the
Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”
' created a replica of the device.{{Cite web, url=https://en.sif.it/journals/sif/ncc/econtents/2016/039/02/article/24, title=The Fermiac or Fermi's Trolley, website=en.sif.it, access-date=2019-08-11A presentation of the device by Coccetti et al. is available as PDF.

Further articles

What is the FERMIAC or Fermi Trolley?
by The Bradbury Science Museum is operated by Los Alamos National Laboratory for education, 2017

References

Early computers Analog computers Enrico Fermi

Operation

History

See also

Further articles

References