Hobbies And Interests
Signal Speed
The fastest speed any signal can reach is the speed of light in a vacuum, about 186,000 miles per second. Both light and electricity have the same speed limit, and they are slowed when traveling in a substance. The speed of electrical signals in a copper wire, for instance, is 124,000 miles per second, or about 2/3 of the maximum. Light also slows when moving through air, water or glass. Though this speed still seems incredibly high, it has limited computer technology since the late 20th century. For example, light travels about one foot in one billionth of a second, so electricity in a wire moves about eight inches in that time. A computer's clock sends timing pulses at about 3 billion cycles per second. At these speeds, components can be at most a few inches away from the clock and remain synchronized.
Delay Time
You calculate the signal propagation delay for a signal as the total time it takes to move between its transmitter and its receiver. For a radio broadcast signal, the time a signal takes to get to the transmitter tower from the studio is small relative to the time it takes from the tower to your car, so the tower-car delay time is more significant. At 186,000 miles per second, a radio signal takes about 1/10,000 second to reach your car from 20 miles away. For computer and communications circuits that work in billionths of a second, the engineers must take the tiny delays of every component into consideration.
Communications Limit
Signal propagation delays become a problem for synchronizing high-speed electronics and for simultaneous two-way communications. For example, to talk to a friend on Mars, 100 million miles from Earth, it takes about 10 minutes for your radio signal to reach her, so you won't get a reply for 20 minutes. For earthbound applications, computers must have elaborate signaling protocols for devices spaced more than a few inches apart. The protocols ensure that data moves from place to place reliably.
Delay as a Tool
In the 1950s and '60s, computer memory exploited the relatively long delay times of electricity in certain metals. A circuit transmitted a series of data bits into the front end of a device called a delay line, received it at the back end, and amplified and transmitted the data back into the front end. The data went around in a circle, like cars on a race track, effectively storing it. More recently, global positioning system technology, or GPS, relies on the time-delay differences between signals from orbiting satellites and a GPS receiver. The receiver calculates position from the few thousandths-of-a second delay differences of the satellite signals.