HOW small is small? In the widely used international system of units known as the SI system (after the French Système International d'Unités), “yocto” is the smallest prefix. Adopted in 1991, it stands for a multiplying factor of one part in a million billion billion (one septillion) parts, which is often written as 10-24.
That is pretty small. A proton's mass at rest is about 1.6726 yoctograms and a neutron's mass just a tad more at 1.6749 yoctograms. But other than this, there has not been much need for the yocto prefix. That may change. Michael Biercuk, then at America's National Institute of Standards and Technology (NIST) in Boulder, Colorado, and his fellow researchers have reported that they have measured a force of 174 yoctonewtons (yN). This is the smallest force ever detected, by some three orders of magnitude.
The newton, named after the physicist Isaac Newton, is approximately the force exerted by gravity on an apple at the Earth's surface. A yoctonewton is a septillionth of one newton. To detect such a tiny force, the NIST team confined about 60 ultra-cold beryllium ions in a device called a Penning trap, which uses magnetic and electric fields to imprison charged particles. At such low temperatures the ions are exquisitely sensitive to electrical or magnetic forces, which change their rate of thermal vibration.
The scientists applied a force to the ions by imposing an additional electric field on them. Changes in the ions' vibration rate were measured by bouncing laser light off them. Because of the Doppler effect, the frequency of light reflected by ions that are moving towards the laser will be higher than that of light reflected by ions that are moving away from the laser. (An everyday example of this effect is the change of pitch of an ambulance siren as it approaches and recedes.) It was thus possible to measure the force applied to the ions.
The smallest force the team detected was 174yN. Reducing the number of beryllium ions in the trap will boost the sensitivity further, the researchers say, and, with a single beryllium ion, it should be possible to measure a single yoctonewton. But for the time being, their measurement of 174yN is a world record (and about a thousand times smaller than the previous record). Dr Biercuk has posted details of the NIST experiments on arXiv, an electronic repository scientists use to share work before it appears in a refereed journal.
What could such a sensitive force-detector be used for? One of the benefits of this technique is that it allows for a relatively rapid detection of tiny electric and magnetic fields. Such fields are everywhere, and the researchers suggest that their approach could be used for mapping electric fields on the surfaces of materials and getting a better understanding of their properties. Other ideas include building a detector to see if the spin of an atomic nucleus can be measured, or using the technique as an ultrasensitive antenna.
Now that physicists have entered the yocto-realm in measurements, some are wondering whether it is time to begin to look at new prefixes for the SI system. Austin Sendek, a Californian undergraduate, has started a campaign on Facebook to designate “hella” as the prefix for large things of the order of 1027 (a billion billion billion). Mr Sendek says the term is already used colloquially in some places to denote large numbers (such as, “there are hella stars tonight”). It could be useful to denote things such as the sun's mass (2.2 hellatons) and energy output (approximately 0.4 hellawatts).
Yotta (1024) remains the largest prefix in the SI system. Like yocto, its etymological roots lie in the Greek octa—yotta signifies one thousand raised to the 8th power, and yocto signifies one thousandth raised to the 8th power. A “y” is added to avoid using the letter “o” as an abbreviation, since it might get confused with the number zero.
This all implies, therefore, that the next SI prefixes should be based on the Greek ennea (for nine). It is doubtful that Mr Sendek's hella campaign, which had almost 60,000 supporters earlier this week, will be able to trump tradition. But it'll have been one helluva try.