The NIST optical atomic clock that is the most accurate time keeping device ever made. Image: NIST Researchers at the…
The NIST optical atomic clock that is the most accurate time keeping device ever made. Image: NIST
Researchers at the National Institute for Standards and Technology (NIST) have developed an atomic clock that is so precise that our models of Earth’s gravity are not accurate enough to keep up with it. As detailed in a paper published this week in Nature the atomic clock could pave the way for creating an unprecedented map of the way the Earth’s gravity distorts spacetime and even shed light on the development of the early universe.
“The level of clock performance being reported is such that we do not actually know how to account for it well enough to support the level of performance the clock achieves,” Andrew Ludlow, a physicist at NIST and the project lead on The organization’s new atomic clock, told me on the phone. “Right now the state of the art techniques are not quite good enough so we are limited by how well we understand gravity on different parts of the Earth.”
Before diving into the nitty gritty of what Ludlow and his colleagues at NIST has accomplished, however, it will help to have some background on the nature of time and atomic clocks.
Obviously, not all clocks are created equal. Each clock varies in terms of its accuracy (how much its oscillation frequency deviates from some baseline) and its time scale. Ef þú þarft að mæla umferðina í fimm mínútur, þá er að nota Earth’s snúningur sem ekki að vera sérstaklega gagnlegur. Similarly, if you never wound your watch, it would gradually become less accurate over time due to small imperfections in the mechanics.
Most of us deal with timescales ranging from years to seconds, which do not require incredibly accurate clocks. Yet for scientists working on the bleeding edge of physics, they require much more precise measurements of the passage of time. Fortunately, nature came endowed with incredibly precise clocks of her own in the form of atomic energy transitions.
Electrons orbit the nucleus of an atom at certain stable energy levels that depend on the electrical properties of the nucleus. Deze orbits kunnen worden veranderd door het toevoegen van energie aan het systeem, waardoor de elektronen tijdelijk worden bumped tot een hoger energie niveau en emit elektromagnetische straling tijdens de overgang.
Read More: Why Nuclear Clocks Will Be The Most Accurate Clocks On Earth
The first atomic clock was created in 1
955 and used the energy transition of an electron in a cesium-133 atom as its frequency reference. Cesium-133 atoms absorb energy at wavelengths of 3.2 cm, which means the wave oscillates at a frequency of 9,192,631,770 cycles per second . When cesium-133 atoms are hit with microwaves at this frequency, it causes the atom’s single outermost electron to rapidly transition between energy states at the same rate. In dit geval, de elektronen overgang tussen een hoge en lage energie toestand over 9 miljard keer per seconde is analoog aan een snel swingende pendulum in een conventionele klok. In fact, the transition of the cesium-133 electron was used to formally define the length of a second in 1967 .
Today, four atomic clocks can be found on each of the 24 GPS satellites orbiting Earth and are used to synchronize time in our cell phones and billions of other internet connected devices. They are also used to measure the mean sea level, which is used to understand the way our planet’s gravity warps spacetime. Knowing this information is important for calibrating the space-based atomic clocks themselves, but despite the accuracy of these clocks-NIST has an atomic clock that only deviates by one second every 200 million years-there’s always room for improvement.
In this respect, NIST’s new atomic clock over-delivered. Det er så nøyaktig at våre nåværende modeller av jordens tyngdekraft ikke kan holde op med det. Fortunately, the new clock will help change that.
Andrew Ludlow works on the atomic clock in NIST’s laboratory. Image: NIST
This is the most banal of questions, but also one of the most difficult for physicists to answer. The reason for this, as Einstein discovered, is that time is not absolute. Rather, the passage of time is relative. Det afhænger af observatørens ramme af reference, som påvirkes af ting som deres hastighed og tyngdekraften i deres referenceramme. For eksempel, en person nær et stærkt gravitationsområde, som et sort hul, ville opleve tiden at flytte mere langsomt end en person, der står på overfladen af jorden.
Humans experience time on a macroscale-days, hours, minutes-and in our day to day lives, we never move fast enough or travel into a strong enough gravitational field to notice a change in how fast or slow time is moving at these scales. Nevertheless, when I climb the stairs every evening to my apartment, time is speeding up whether I notice it or not.
“It’s a small effect,” Ludlow said. “It’s kind of creepy that’s real, but it’s.”
Read More: An Energy War in the Balkans Slowed Europe’s Clocks By Five Minutes ]
With each step I climb, I am moving farther away from Earth’s center of gravity, which means that gravity’s effect on the frequency of what oscillating thing I’m using as a clock diminishes. Physicists have calculated just how much gravity affects time based on how high a clock is above the Earth’s surface and found that it is 1.1 quintillionth of a second for every vertical centimeter that clock is lifted above the surface. Met andere woorden, een seconde gemeten op de oppervlakte van de aarde daadwerkelijk 0.000000000000000011 seconden minder voor een uur een centimeter boven de aarde en zo verder.
Of course, “the surface of the Earth” is shorthand since this can mean wildly different things depending on where you are standing. Death Valley and the peak of Mount Everest are both technically the surface of the Earth, but one is 282 feet below sea level and the other is 29,000 feet above sea level. For that matter, the “sea level” itself is also in constant flux due to tidal changes.
To address this problem, scientists conceive of the Earth as a “geoid.” This is the hypothetical form of the Earth if the oceans were only subject to the force of Earth’s rotation and gravity, and were extended across all continents. This is, in effect, equivalent to taking the mean of sea level across the entire Earth, which is done through a combination of ocean-based sensors and satellite data. When the geoid is visualized, it looks like this:
The geoid is great for measuring the elevations of Earth’s surface to a high degree of accuracy, but it poses problems when it comes to creating ultra precise clocks. Årsaken til dette er fordi jorden ikke er faktisk en geoid og forskellinger i elevation induce meaningful gravitational effects on the measurement of time. This is most pronounced in instances where atomic clocks are separated by large distances, such as those on GPS satellites.
Although scientists are able to account for these differences in Earth’s gravity at the surface, also known as geopotential heights, ved hjelp av atomsklokkene på satellitter, kan de kun gjøre det med en avvikelse på ca. 0.0000000000000001 sekunder, som er en stigning i elevation på ca. 0,9 meter. The new atomic clock developed by NIST, however, is so accurate that it is able to reduce this elevation change to only a centimeter, which is equivalent to an error of just 1.4 parts in a quintillion (a one followed by 18 zeros).
According to Ludlow, this breakthrough was only possible due to the revolutionary nature of the clock itself. The atomic clocks used by NIST for this research consist of ytterbium atoms suspended in an array of laser beams. Selv om den potentielle latente i denne teknologi har været kendt for fysikere i årevis, er det kun i de senere år at de har utnyttet teknologien til at tappe disse optiske atomiske klokker. Indeed, Ludlow said his team’s recent breakthrough is the culmination of years spent researching how to limit interference from things like nearby electrical and magnetic fields.
Ludlow told me that the NIST atomic clock is both a scientific wall and a door. Det er en vegg i den forstand at det er så nøyaktig at de nåværende målinger av geoid faktisk begrenser nøyaktigheten av atomkloakken, siden den giver geopotential opløsning på størrelsesordenen mange centimeter, mens deres klokke kan reducere denne geopotente opløsning til blot en centimeter . På den annen side, NIST’s atomic clock is a door in the sense that it can be used to improve geodetic resolution several times over. Dette ville involvere distribusjon av flere av disse klokker rundt om i verden, og å måle de svake avvikene i deres tid å holde for å få den høyeste endelige oppløsningskort av hvordan jordens tyngdekraft varsler spacetime.
“If you had clocks that you trusted that were both accurate at this very high level then you could use those clocks as sensors of the Earth’s gravity potential by looking for changes in the ticking rate as you moved one of the clocks through different parts of Earth’s gravity, “Ludlow told me.
Ludlow said he and his NIST colleagues are currently working on prototypes for portable versions of their atomic clock that they can use to test this idea. Det vil nok være flere år før de er distribuert på jorden eller i rommet, men i mellomtiden, NISTs klokke kan bli satt til andre anvendelser, for eksempel å redefinere den andre til enda større nøyaktighet.