If you’ve seen a ruler that is used in primary and secondary schools, most likely a ruler’s length is 30 centimeters. Thirty centimeters is approximately equal to 12 inches or 1 foot (foot, the length of this unit a la the British Empire).
In the field of computers and electronics, a length of 30 centimeters has a special meaning. Special meaning is important and to be hold in designing and building a series of microelectronics and integrated circuit (integrated circuit, IC).
Thirty centimeters more or less is the distance traveled by the electromagnetic waves within one billionth of a second. One-billionth of a second period corresponds to a frequency of 1 billion cycles (cycles) per second (giga hertz, GHz), so much less on the order / the same scale as the speed of current computers.
30 centimeters to be important factors in designing an electronic circuit that operates at GHz frequencies. In the GHz scale electronic circuits, recycling of electrical current in the circuit will generate an electromagnetic wave which has a length at about the scale of centimeters. If the electronic circuit is also on a scale of centimeter-sized, then the engineer who designed the circuit should consider the time factor required for the propagation of electromagnetic waves in the circuit.
Similar problems, ie taking into account the time factor required for the propagation of electromagnetic waves, appear in many everyday technology: radar, satellite and mobile telecommunications, fiber optic networks (optical fiber), a global positioning system, mapping, geodesy, and more.
In particle physics experiments, this problem arises when physicists should consider the travel time of signals in optical fiber from the detector is located 100 meters below ground to the computer that reads and stores the data on the soil surface. For example, the LHC accelerator operates with a frequency of 40 MHz or a period of 25 seconds per billion. Every 25 per billion seconds, occurred a few collisions / interactions in the detector. Using the rule of 30 centimeters, we know that for 25 per billion seconds, the light will travel a distance of 7.5 meters. Though the distance from the underground to the surface of the ground is 100 yards more! Before the signal from the detector reaches the soil surface and recorded in a computer, the detector has received the data back!
Whereas in the underground detectors generally require confirmation (handshake) by computers located on the surface: if the signal / information submitted is up or not. With that consideration, then underground detectors are designed to temporarily store data collisions / particle interactions in a temporary storage area (buffer memory). So if for any reason shipment information from the underground to the surface is interrupted, the detector in the basement would receive word from the computer on the surface that the information sent has not been received, and can be sent back.
How to talk particle physics experiment that sounds like electrical engineering or instrumentation! This is all because of the need: to build the tools necessary kerjsama particle physics experiments between physicists with engineers: good engineer electrical power engineering, electronics engineering, mechanical engineering, refrigeration engineering, computer engineering, civil engineering, etc.. Without such cooperation, it is not possible tools and experimental particle physics facility could be built.
Note: Use size 30 inches to provide an illustration / picture of the speed of light and distance, was popularized by First Admiral (Commodore / Rear Admiral) Grace Hopper, an elder in the fields of computers and informatics.