In 2018, the heaviness (mass) of the metal substance that is used to decide how much heaviness we can give to “one kilogram” will be changed.
This very hard table-tennis size shiny cylinder-shaped metal which is made from platinum-iridium kept in Paris under lock has been the same object used to decide the heaviness of one kilogram for the last 100 years.
International Prototype Kilogram (IPK)
The kilogram, which is known as the International Prototype Kilogram (IPK) is the last unit of measure in the international measurement system that’s still defined by a physical object.
An international effort is now underway to replace the IPK with a more mathematical solution — specifically, scientists want to redefine the kilogram using Planck’s Constant, a value from quantum mechanics in modern physics.
But there has been a challenge. It’s been hard to pinpoint the value of Planck’s Constant precisely enough for use in the new standard.
In June, a team at the National Institute of Standards and Technology (NIST) announced their most precise measurement yet for the constant, with an uncertainty of just 13 parts per billion – a very small number.
Reasons for change
In an interview with Ira Flatow on Public Radio International (PRI)’s Science Friday, Jon Pratt, leader of the Fundamental Electrical Measurements Group at NIST and co-author of this new study, gave two reasons why scientists are working to change the IPK as the kilogram standard:
“The idea is that a standard should be universally accessible and that it should be a fixed quantity, it should always be the same,”
“It’s only available in Paris,” he said. “I’d have to get on a plane and take hunks of metal over there to compare to it. So, that’s a problem.”
The second reason is that the IPK now has slightly less mass than its official copies:
“We pulled out the IPK as part of this business of trying to measure the Planck Constant, and in the process, we discovered that the existing mass scale, based on the IPK, appears to have shifted by 35 parts per billion,” Jon Pratt explained.
“That’s a problem with these types of artefact systems.” He added. Although, scientists aren’t sure whether the IPK is losing mass or the replicas are gaining it.
Universal Constant
The new kilogram definition will avoid these problems using a universal constant. “Things like atoms and more specifically, atomic frequencies, the speed of light,…and where we get the Planck Constant is this issue of the idea that energy is quantized, and so we have a quantum of energy that is scaled by the Planck Constant.
So, we have a couple of ways in which we can do experiments that link mass to the Planck Constant.” Pratt said.
According to a NIST statement, a new highly precise value for Planck’s Constant will be decided by an international council and in 2018, the kilogram will finally be changed.
When all this is done, the average person won’t notice this change. “One of the games we play in the standards world is to make sure that nobody has to care…so, the care we take in trying to make this transition — from a standard that’s based on a hunk of metal to a standard that’s based on this sort of abstract idea about the quantization of energy in the universe — is that we want both approaches to agree with each other…and we want to be able to do that at the level of precision that’s being done at the moment.” Pratt explained.
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