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Why scientists are redefining the kilogram


“This redefinition is a major overhaul,” Newell continued, but certainly not the first. For example, we currently define the second by a specific number of cycles of radiation in a cesium-133 atom (9,192,631,770 periods). It was originally considered to be the fraction 1/86400 of the mean solar day.

The meter used to be a real thing that you could hold (like the kilogram still is) rather than the distance light travels in 1 / 299,792,458 seconds. “The SI is slowly evolving to the use of the invariance of nature,” Newell said, rather than basing our observations on specific, physical artifacts.

“What is going to change is that with this redefinition, the uncertainties of fundamental constants is either going to go to zero,” he said. “Or the uncertainties of the related fundamental constants is going to be drastically reduced.”

This means researchers will have far more accurate tools with which to make measurements. That higher fidelity will empower them to go back and reexamine the laws of physics that we believe to be correct and see if they’re as accurate as we think they are. “We may actually find that we don’t know everything,” Newell said. Eventually, we may even take those insights and once again redefine the scientific measurement system when our technology has sufficiently advanced.

Another advantage is these fundamental constants appear throughout nature, Newell explained. Researchers would no longer be tied to the kilogram and would be able to easily scale their units between the macroscopic and microscopic worlds.

“Moreover the present system is explicit unit based — the second, the kilogram, the meter, the ampere — and there’s definitions for all of them,” Newell continued. “The new system is explicitly constants-based — the transition frequency of a cesium atom is an exact number of hertz, the speed of light is an exact number of meters per second.”

Take amperes for example. An ampere (or amp) is the basic unit of electrical current and is defined by the SI as the equivalent to one coulomb (the base unit of electrical charge) per second. Originally it was defined using a thought experiment.

This is problematic for a couple reasons, PhysicsWorld points out. First, it relies on other units of measure — specifically kilograms, meters and seconds — for its definition. This methodology is exactly what the BIPM is trying to get away from. Second, the aforementioned “thought experiment” can never be tested in reality, since it imagines a situation with wire infinitely long, so at some point you’re going to have to approximate.