Don't know why this was downvoted. But here is the answer [1]: "The new definition only became possible when instruments were devised to measure the Planck con

Don't know why this was downvoted. But here is the answer [1]: "The new definition only became possible when instruments were devised to measure the Planck constant with sufficient accuracy based on the IPK definition of the kilogram." https://en.wikipedia.org/wiki/Kilogram https://en.wikipedia.org/wiki/Kilogram