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moral agent's actions were the result of carelessness, recklessness, or negligence. The de-
sign team took a number of actions that fall into this category. It constructed a system
without hardware interlocks to prevent overdoses or to keep the beam from being acti-
vated when the turntable was not in a correct position. The machine had no software or
hardware devices to detect an accidental overdose. Management allowed software to be
developed without adequate documentation. It presumed the correctness of reused code
and failed to test it thoroughly. For these reasons the mental condition holds as well, and
we conclude the Therac-25 team at AECL is morally responsible for the deaths caused
by the Therac-25 radiation therapy machine.
8.5.6 Postscript
More than two decades after the Therac-25 accidents, computer errors related to ra-
diation machines continue to maim and kill patients. In late 2006, Scott Jerome-Parks
received three overdoses from a linear accelerator at a New York City Hospital that led to
his death a few weeks later. He was only 43 years old. About the same time, 32-year-old
breast cancer patient Alexandra Jn-Charles received 27 straight days of radiation over-
doses at another New York hospital that led to her death. An investigation of radiation
overdoses by the New York Times concluded that a variety of errors, including faulty
software, were leading to crippling or fatal accidents [49].
8.6 Computer Simulations
In the previous section, we focused on an unreliable computer-controlled system that
delivered lethal doses of radiation to cancer patients, but even systems kept behind the
locked doors of a computer room can cause harm. Errors in computer simulations can
result in poorly designed products, mediocre science, and bad policy decisions. In this
section we review our growing reliance on computer simulations for designing products,
understanding our world, and even predicting the future, and we describe ways in which
computer modelers validate their simulations.
8.6.1 Uses of Simulation
Computer simulation plays a key role in contemporary science and engineering. There
are many reasons why a scientist or engineer may not be able to perform a physical exper-
iment. It may be too expensive or time consuming, or it may be unethical or impossible
to perform. Computer simulations have been used to design nuclear weapons, search
for oil, create pharmaceuticals, and design safer, more fuel-efficient cars. They have even
been used to design consumer products such as disposable diapers [50].
Some computer simulations model past events. For example, when astrophysicists
derive theories about the evolution of the universe, they can test them through computer
simulations. A computer simulation has demonstrated that a gas disk around a young
star can fragment into giant gas planets such as Jupiter [51].
A second use of computer simulations is to understand the world around us. One
of the first important uses of computer simulations was to aid in the exploration for oil.
 
 
 
 
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