Alongside a handful of other base units such as the second which is defined in terms of the frequency of radiation from a caesium atom and the kilogram which is still defined by comparison to a lump of platinum-iridium alloy kept in a lab in France , the metre forms the backbone of the system we use to measure everything else.
Originally published by Cosmos as The measure of a metre. Cosmos is published by The Royal Institution of Australia, a charity dedicated to connecting people with the world of science. Financial contributions, however big or small, help us provide access to trusted science information at a time when the world needs it most. Please support us by making a donation or purchasing a subscription today. Cosmos » Society » The measure of a metre Share Tweet. A copy of the standard metre installed on a wall in Paris in or for public use.
Michael Lucy Michael Lucy is a former features editor of Cosmos. Developments in electronics and the invention of lasers made it possible to independently measure the frequency and wavelength of a given light source. Notice the units.
The meter a length was — and is still — defined in terms of seconds time. We can only wonder: what future scientific discoveries will characterize the next redefinition of the meter? Post a Comment. September 10, The answer is "a bad week for the casino"—but you'd never guess why. Read more. June 13, Our science teacher claims that the pain comes from a small electrical shock, but we believe that this is due to the absorption of light. Please help us resolve this dispute!
In this simple, ideal setup, an individual light wave from a laser hits a beamsplitter, which creates two light waves traveling in different paths. One of the waves hits a moving mirror, which can vary its distance as it travels to the detector. Credit: S. It was in that NIST then known as the National Bureau of Standards advocated for the interference patterns of energized cadmium atoms to be made a practical standard of length. This was useful because international measurement artifacts such as meter bars could not be everywhere at once; however, with proper equipment, scientists anywhere could measure the meter with cadmium.
Their copies, exquisite as they might be, are not as accurate as the real thing. Neither an artifact nor its copies are suited for every measurement one might want to make. To cite one real-world example, gage blocks are length standards commonly used in machining. Because of the extremely fine work demanded of machinists, their calibration standards must be finely crafted as well. Using cadmium and krypton wavelengths, gage blocks could be certified to being accurate to within 0.
In the mids, nuclear physicists aimed neutrons at gold to transform the atoms into mercury. NIST physicist William Meggers noted that aiming radio waves at this form of mercury, known as mercury, would produce green light with a well-defined wavelength. In , Meggers procured a small amount of the mercury and started experimenting with it. Applying interferometry techniques to mercury, he came up three years later with a precise, reproducible and convenient way of defining the meter.
NIST's William Meggers, shown here in March , demonstrates a measurement of the wavelength of mercury, which he proposed could be used to define the meter. Meggers measured the wavelength of the green light from mercury: A meter would be defined as a precise number of multiples of this wavelength. The agency sought to further increase the precision of its technique for redefining the meter.
However, funding to do this was not immediately available, and the project could not be completed until Originally proposed as the atom of choice by Physikalisch-Technische Bundesanstalt PTB , the national metrology institute of Germany, the krypton isotope was more widely available in Europe and was able to provide higher precision in the laboratory measurements at the time. Add up 1,, But the krypton standard was not to endure for too long.
Sorry, Superman. The light with which we are most familiar, the visible kind, is only a small part of the electromagnetic spectrum, which runs from radio waves to gamma rays.
Because light has an incredibly fast but ultimately finite speed, if that speed is known, then distances can be calculated using the straightforward formula: Distance is speed multiplied by time. This is a great way to measure the distance to satellites and other spacecraft, the Moon, planets and, with some additional astronomical techniques, even more remote celestial objects.
The speed of light is also the backbone of the GPS network, which determines your position by measuring the time of flight of radio signals between atomic clock-equipped satellites and your smartphone or other device. The speed of light had for centuries remained an elusive quantity, but scientists began to really close in on it with the invention of the laser in , the same year that the krypton standard was introduced.
0コメント