On 01/14/2010 05:33 PM, Jed Rothwell wrote: > Ed Storms -- who Oracle-likes sometimes reads but does not respond to > Vortex these days -- pointed out that I am wrong about clocks. They do > have moving parts: > > "All machines that measure time MUST have moving parts. In a digital > watch, the moving part is the vibration of a quartz crystal. In the > Cesium clock, the moving part is the vibration of electrons. Only the > scale has changed." > > So, if we define a "part" as something with mass, I wonder if we can > measure time with photons instead of electrons.
Photons have no "rest mass" but none the less, when they're moving, they have mass. They carry momentum, when they leave something it loses mass, when they're absorbed by something it gains mass, they gravitate, and they're attracted by gravity. I think that about covers it for properties of a "massive" body, doesn't it? But in any case Clarke clearly allowed photons to move in his "ideal machine" with "no moving parts" (see City and the Stars -- the Central Computer, with no moving parts, none the less has some lights of some sort). > > All computers have clocks, so they all have moving parts by this > definition. I have read that people are trying to develop computers > without clocks, in which every component works at its own pace, like > people in Swedish automobile factory, or people playing tennis without a > net. It is hard for me to imagine how a clock-less computer would work. How so? It's a simple asynchronous design. First machine I ever worked on professionally was asynchronous. The CPU board, which was about a foot square, had a row of tweak pots along one edge of the board -- a dozen or more -- to adjust the timing of the various parts so they would play together. The design made heavy use of "arbiters" to deal with the (constant) races; needless to say one must be very careful in calculating the rate of arbiter failure in such a design to be sure it won't happen in practice. Nowadays computers are all synchronous, at least in part because it's a whole lot simpler, and when you're trying to tear along at gigahertz+ speeds simplicity counts for a lot. > > Most modern computer displays have no moving parts. The Kindle book > readers have internal mechanical movement of black and white beads. I > consider that a retrograde design, that will not last for long. The > resolution is remarkable but the contrast is poor. > > - Jed >
