Modern civilization is said to be impossible without measurement. If we weren't all using the same units, measurement would be pointless.
The International Bureau of Weights and Measures, known as B.I.P.M., is based outside of Paris and has been in existence for nearly 150 years.
The seven base units that the bureau regulates are time, length, mass, electrical current, temperature, intensity of light and the amount of a substance. The language of science, technology and commerce is spoken by these units.
Scientists are constantly working on these standards. They approved new definitions for the kilogram, ampere, kelvin, temperature, and mole. The standards are subservient to time with the exception of the mole.
The meter is defined as the distance light travels in a vacuum during one second. The new definition of the kilogram is too complicated to explain in a few paragraphs.
The B.I.P.M. president said that all the units are dependent on the second.
If you clumsily, you could express other units, such as weight or length, in seconds.
You go to the grocery store and say, "I would like not one kilogram of potatoes, but an amount of seconds of potatoes."
For the first time in more than 50 years, scientists are changing the definition of the second because a new generation of clocks is capable of measuring it more precisely.
The B.I.P.M. will have a final list of criteria in June. Dr. Dimarcq said that formal approval would happen by 2030.
It must be done with care. When the definition of the unit changes, the duration of the measurement must not be changed.
It is like a once-in-every-50-year thing, according to Elizabeth A. Donley, chief of the time and Frequency division of the NIST. She is on the B.I.P.M.'s international consultative committee. It is exciting to work on.
Humans used to look at the heavens. Metrologists have defined time by measuring what is happening inside an atom, as it were, the eternal heartbeat of the universe.
Time has its roots in time keeping. The path of Earth was the basis for it. The ancient Egyptians used the duodecimal counting system to divide the day and night into 12 hours.
The hours were different depending on where Earth was in its path around the sun. The idea that a single day should be divided into 24 hours of the same length was developed more than 2,000 years ago.
The Babylonian method of counting by 60 was patched onto the hour because of that same thinking. They divided the hour into 60 parts, and then 60 again, making the sphere of Earth look like a circle.
The first division of the day gave them the length of the minute, which was one-thousandth of an average solar day. They were given the duration and name of the second division, which was one-86,400th of a day. The definition was in effect until 1967. Even metrologists didn't use ephemeris time because it was so complicated.
There were problems with the definition. The Earth is slowing in 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 888-739-5110 The small differences add up. Over the past 2,000 years, the Earth has lost more than three hours as a clock.
The standard unit of time is not constant, a reality that became intolerable for metrologists during the first decades of the 20th century as they discovered how irregular Earth's spin was. Science requires constancy, reliability and replicability. By the late 1960s, society was becoming increasingly reliant on the frequencies of radio signals, which demanded extremely precise timings.
Metrologists looked at the movement of atomic particles. Atoms never slow down or wear out. Their properties don't change over time. They are the best.
By the middle of the 20th century, scientists had figured out how to divulging their secret inner ticks. Cesium, a silvery-gold metal that is liquid at room temperature, has heavy, slow atoms, which makes it easy to track.
Scientists exposed the atoms to the microwaves in a non visible range. The task was to figure out which wavelength would cause the most cesium atoms to be excited into emitting a packet of light. The photons were counted.
The wavelength that won the contest was designated as the natural resonance of the atom. Think of it as a pendulum that is unique to that type of atom.
The Frequency in the case of cesium 133 is 9.2 billion ticks per second. The measurement of Earth, the moon and stars were used to derive the length of the second experiment. The B.I.P.M. set the natural resonance of 133 as the official length of the second.
Atomic time is still inextricably conjoined with astronomical time. Earth continues to change its pace at an irregular rate and atomic time needs to be adjusted to match. Timekeepers stop atomic time for a moment when it gets one second faster than the standard time, so that Earth can catch up. The duration of a minute occasionally does, while the duration of the second doesn't change. Timekeepers add a second to atomic time every year and a half after an initial 10 leap seconds.
Even with our modern atomic clocks, we still tick through 1957-era seconds. Even when the second is redefined, the natural resonance of cesium 133 will not change because it was measured in 1957 and locked to the second in that year.
Scientists have developed new instruments called optical atomic clocks. These operate on the same principles as the cesium clock but measure atoms that have a faster natural frequency resonance. The microwaves are in the microwave range, not the visible, or optical, range of the electromagnetic spectrum.
There are several species of optical clock, each counting the ticks of a different atom or ion. No species has emerged as the clear favorite for the upcoming redefinition.
Judah Levine, a physicist at NIST, said that optical clocks are not ready for prime time.
Although they are built to examine such tiny atoms, most are massive, about the size of a heavy dinner table. Some people are in a laboratory. It's difficult to operate them.
It requires a lot of specialists who are chained to the table.
Dr. Donley said that there are about 20 or 30 optical atomic clocks of all species.
There are three in Boulder. A steel slab is used to isolation it from the floor. It is protected from Earth's magnetic field. The vacuum chamber is about a foot in diameter and contains whichever atom or ion is under scrutiny. There is a single ion in some clocks. Others have the same type of atom.
There are lasers on the table. They chill the atoms to near absolute zero, trapping them in place and slowing them down. The atoms are probed by the lasers, which beam a nearly pure color of light on them, that scientists tune to find the precise wavelength that will cause the desired tiny shift in energy.
The atoms become excited only if the laser color is perfect and the push arrives at the right rhythm.
The trick is to be able to read the laser's color in order to determine the exact frequencies of the waves that cause the shift in energy. This is where the optical atomic clock kicks in. The discovery of a second type of laser called a Femto-laser-second Frequency comb led to the creation of the clock. A series of spikes of light are similar to the teeth of a hair comb.
This comb of light can read the wavelength of the lasers that are exciting the atoms. The waves are moving at 100,000 times the speed of microwave energy. The optical atomic clock can measure time more precisely than the cesium clock.
Why do we need such precision? Time is influenced by gravity and mass and is not just time. The existence of an international standard might suggest that time is not constant. Albert Einstein's theory of general relativity states that time moves slower when it is near a large body because of the pull of gravity.
If the tick of a clock changes, the physical conditions in which the clock is situated may change as well. Dr. Donley said that being able to read the changes could allow the clock to detect dark matter.
One of the exciting things about optical clocks is the tests of fundamental physics.
One experiment has begun. Physicists at NIST were working on optical atomic clocks in 2015. They were confused by the fact that the seconds were not the same across the clocks.
They thought about the theory of general relativity. Is the optical clock responding to a slight change in gravity?
They asked a physicist at the National Geodetic Survey to investigate. Dr. van Westrum measured height differences in the labs where the clock was stationed. Like time, height is related to gravity and mass.
His survey leveling techniques found that the clocks were at different heights. The minuscule changes in the field were captured by their slightly different measurement of time. One clock ran faster than the other.
Dr. van Westrum said that Einstein's crazy prediction of what mass and gravity does to time would have a practical application.
He said that if several optical atomic clocks could be placed in different parts of the world, it would be possible to measure differences in height and gravity. A network set up near a flooding river could help residents find escape routes.
There are possibilities in the future. Physicists are still trying to make optical clocks talk to one another over distances. Satellite time keeping is not yet optical, so optical clocks cannot communicate efficiently.
Physicists are making progress. The NIST published an experiment in Nature last year that linked the three clocks in Boulder through air and fiber.
Scientists are looking to the heavens for help. It's not to track the movements of planets or stars, but to use information from far away.
Researchers in Italy and Japan tried to link two optical atomic clocks that were about 5,500 miles apart. An experiment involved several antennas reading radio signals from outer space and linking them to atomic clocks.
It worked, and for a while time and space merged.
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