MAGAZINE OF THE SOCIE TY FOR SCIENCE & THE PUBLIC mAGAZiNe OF THe SOCie TY FOR SCieNCe & THe PUbLiC
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Erasing any doubt that
information is physical
Everybody knows how a pencil eraser
works.
Oh, wait. On second thought, perhaps
the electron-savvy communicators of
the latest generations don’t even know
what a pencil is. Still, it wouldn’t be hard
to teach erasing. Rub the eraser over the
pencil mark until it goes away. You just
need to generate a little friction.
Most people will have no trouble believing that without
friction, there’s no erasing a pencil mark. But it’s not so obvious that a similar principle applies to erasing anything, even
a number stored in the memory of an electronic computer.
Any erasing of information requires some energy, emitting
some waste heat, just as friction does.
Experts in the physics of computing have known this rule
for half a century. It was formulated in 1961 by an IBM physicist named Rolf Landauer. His erasure principle has long been
regarded as the foundation for the study of the physics underlying information processing; it is inviolate regardless of what
sort of machine or physical system is doing the computing.
It is very good news, therefore, that an actual experiment
has now verified Landauer’s principle, as Alexandra Witze
reports in this issue (Page 13). Testing it isn’t easy, because the
minimum amount of energy consumed in erasing a single bit
of information is quite small. In real computers lots of other
processes use up energy, so the inevitable erasure cost doesn’t
get noticed. But Landauer showed that computing itself
doesn’t have to use any energy at all. If you compute more
and more slowly (analogous to reducing friction), you can
compute with as little energy as you want, all the way to zero.
Erasing, though, always has some energy cost. The new
experiment shows that the amount of energy needed to erase
a bit never drops below the limit that Landauer calculated.
As computing devices get smaller and smaller, and waste
heat becomes a bigger and bigger problem, understanding
the implications of Landauer’s principle will be essential in
designing ways to cope with it.
Landauer, who died in 1999, always insisted that “
information is physical,” and that understanding the physical
universe would require coming to grips with just what that
meant. Investigations of quantum information (see my essay
on Page 26) are an important part of that effort. Verifying
Landauer’s principle is another reminder of the significance
of his slogan. — Tom Siegfried, Editor in Chief
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