The kg is dead, long live the kg

Summary

Representatives from nearly 60 nations will convene in Versailles to redefine the kilogram and three other fundamental SI units, transitioning them from physical artifacts to constants of nature. This change aims to establish consistent, reliable measurements globally, with Planck's constant set to a fixed value. The shift impacts scientific measurements but minimally affects everyday life, marking a significant leap in scientific precision.

Takeaways

• 🗓️ Representatives from nearly 60 nations will meet in Versailles, France, on November 16th to redefine four of the seven base S.I. units.
• 🔄 The redefinition includes the kilogram, kelvin, mole, and ampere, transitioning all S.I. units to be based on fundamental constants of nature instead of physical artifacts.
• 📏 The kilogram has been defined by a physical object, the International Prototype Kilogram, since 1799, but this method has issues with mass consistency and accessibility.
• 🔗 Planck's constant, which relates the frequency of a photon to its energy, is proposed to have a fixed, exact value to redefine the kilogram.
• 🧲 Scientists have been using techniques like the watt balance to measure Planck's constant accurately, aiming for consistency with current measurements.
• 🔄 The mole, currently defined by the number of atoms in 12 grams of carbon-12, will also be redefined to be consistent with the new Planck's constant.
• ⚡ The ampere will no longer depend on the kilogram but will be based on the fixed value for the charge on an electron.
• 🌡 The kelvin will be redefined based on the Boltzmann constant, which relates the temperature of a gas to the average kinetic energy of molecules.
• 🌐 These changes aim to provide consistent and reliable measurements globally by removing the dependence on physical objects.
• 📈 The redefinition will have minimal impact on everyday life but will allow for more precise scientific measurements and could slightly affect electrical measurements.

Q & A

Why are representatives from nearly 60 nations meeting in Versailles, France?

They are meeting to vote on changing the definition of a kilogram and three other base SI units: the kelvin, the mole, and the ampere.

What is the significance of changing these SI units?

The change will mean all SI units will be based on fundamental constants of nature instead of physical artifacts, providing a more consistent and reliable system for measurements.

Why is the current definition of the kilogram problematic?

The current definition relies on the mass of a physical object, the International Prototype Kilogram, which has been found to have issues maintaining a consistent mass and is difficult to access.

How is the new definition of the kilogram related to Planck's constant?

The new definition sets Planck's constant to a fixed, exact value, which means the kilogram will be defined in terms of this constant rather than a physical object.

What is the current issue with Planck's constant?

Planck's constant has some uncertainty due to the last few decimal places being unknown, which is problematic for defining precise measurements.

How do scientists measure Planck's constant accurately?

Scientists use various techniques such as the watt balance and creating extremely round objects to measure Planck's constant as accurately as possible.

How will the definition of the mole change as a result of the vote?

The mole will no longer depend on the kilogram but will be defined in terms of Avogadro's constant, which will be fixed to match the new definition of Planck's constant.

What is the impact of these changes on the definition of electric current (ampere)?

The ampere will be defined based on the fixed value for the charge on an electron, independent of the kilogram.

How will the Kelvin be redefined after the vote?

The Kelvin will be based on the newly fixed Boltzmann constant, which relates the temperature of a gas to the average kinetic energy of the molecules.

Will these new definitions affect everyday life for most people?

No, for most people, everyday measurements will remain the same, but the changes ensure consistency and reliability for precise scientific measurements.

What is the benefit of basing units on fundamental constants rather than physical objects?

It allows for anyone, anywhere to make incredibly precise measurements without being dependent on access to a physical object, which could change over time or be difficult to access.