The earliest computers, built in the middle of the 20th century, used vacuum tube s for switching. These machines were so massive and bulky, and demanded so much electricity to operate, that they required buildings and power plants of their own. Today, such a computer can be placed inside a microscopic capsule. The trend toward miniaturization shows no sign of slowing until some limit is forced on the manufacturing process. The scale of this limit ultimately depends only on the structure of matter.