SYDNEY: American scientists have developed a novel method of measuring time that may hint at its fundamental origins, and is an “exciting prospect” for replacing the international standard for the kilogram.
Their new method allowed the researchers to directly measure time from mass and vice versa—a new finding which could potentially revolutionise our current clocks and mass scales, as well as help recalibrate the kilogram.
The new system depends on just one particle to set a time standard. The result, published in the U.S. journal Science, also implies that “the universe had a timescale as soon as there was one massive particle,” says physicist and project leader Holger Müller, from the University of California, Berkeley, and Lawrence Berkeley National Laboratory.
What is time?
All current clocks depend on several objects that interact with each other. The grandfather clock needs a pendulum that swings back and forth under the gravity of some massive body like Earth; the current international standard for the ‘second’ is set by an atomic clock—where ‘the objects’ are the nucleus of a cesium atom and the electrons that orbit it. The electrons buzz between so-called energy levels and emit oscillations of light. The time taken for a specific number of oscillations defines a ‘second’.
Physicists wondered if a single object could act like a clock, so they treated the cesium’s nucleus and electrons like a single particle. A cesium atom was used because it can be laser-cooled and is neutral, which reduces sensitivity to the surrounding environment.
A new clock
Müller and his colleagues focused on an elusive quantity known as the ‘Compton frequency’ of a single cesium atom. At quantum scales, matter can behave like waves. The Compton frequency describes the oscillations of these waves. We could define ‘X’ number of oscillations to equal a ‘second’.
The Compton frequency is far too high to be measured directly by modern technology. Müller’s team bypassed this obstacle by measuring a much smaller quantity, from which the Compton frequency could be calculated. To do this, they exploited the twin paradox, an odd idea in special relativity in which a person who rockets away from Earth at near the speed of light and then returns to Earth, appears to be younger than their sibling who remains behind.
Using an instrument called the Ramsey-Borde interferometer, the researchers hit a cesium atom with a beam of photons carrying momentum, causing it to move. The laser beam also split the wave of the cesium atom into two. The waves of our atom travelled exactly like the siblings in the twin paradox. One of the waves stayed where it was. The other wave moved and then came back to its starting point, so when it returned, it was ‘younger’, that is it had oscillated fewer times, than the wave that stayed behind.
A fixed frequency ratio, between the small difference frequency and the Compton frequency, was programmed into a Nobel-Prize winning device called the frequency comb generator. The team was then able to indirectly calculate the Compton frequency by measuring the much smaller difference in frequency between the moving and non-moving waves.
Being the first device of its kind however, for now the team reported much lower accuracy for this new ‘Compton clock’ than current atomic clocks in measuring time. However, in principle, “this [Compton clock] might end up being a better clock than anything else”, says physicist Michael Tobar, at the University of Western Australia.
That’s because the mass of a particle is better defined, and so more stable, than electrons undergoing atomic energy transitions.
Redefining the kilogram
If you have the Compton frequency of a cesium atom, you can calculate the mass of it using fundamental equations from quantum mechanics. The masses of other atoms can then be compared to this standard. In everyday life, we need a more practical mass unit than the mass of one particular atom—so the researchers used “Avogadro spheres” to create a newly defined kilogram standard. These spheres are extremely precise crystals with a known number of atoms.
The current standard for the kilogram is a particular cylinder of platinum-iridium alloy plonked in a chamber in Sevres, France. It’s the last physical object to act as a standard for a fundamental constant, and worryingly, experts have recorded changes in its mass since it was first used about 125 years ago. This prompted the international scientific community to call for a new kilogram standard in 2011. The new Compton-Avogadro model relies on unvarying physical phenomena as described, and results indicate that it is the most accurate calibration of a large mass to date.
Physicist Bruce Warrington, at the Australian Government’s National Measurements Institute, commented that there would be “a way to go [for the article’s approaches] to be turned into a real standard”, but that the article offered an “exciting prospect”.