How We're Redefining the kg

Summary

The video explores the redefinition of the kilogram, which had been based on a physical object since 1799. Over time, this standard has become unreliable as the masses of international prototypes have diverged. The video discusses two major approaches to redefine the kilogram using constants of the universe: the Avogadro method and the Kibble balance (formerly Watt balance). The latter uses electromagnetism and quantum effects, like Josephson junctions, to measure Planck's constant with extreme precision. This shift from physical objects to fundamental constants ensures a stable, universal standard for the kilogram.

Takeaways

• 🔬 The kilogram has been traditionally defined by a metal cylinder stored in Paris since 1799, but its mass has been diverging slightly over time.
• ⚖️ The kilogram is the last SI unit still defined by a physical object, unlike the meter, which is now based on the speed of light.
• 🌍 Scientists want to redefine the kilogram using an unchanging constant of the universe, just as they did with the meter.
• 🔢 Two major strategies emerged to redefine the kilogram: the silicon sphere method (using Avogadro’s number) and the Watt balance (using Planck’s constant).
• 🧲 The Kibble balance (formerly the Watt balance) is a key tool that measures Planck’s constant by balancing a mass using electromagnetic forces.
• ⚡ The Kibble balance operates in two modes: weighing mode (to measure the mass) and velocity mode (to measure the voltage).
• 📏 Using Josephson junctions, scientists can measure voltage precisely, and the quantum Hall effect is used to measure resistance.
• 🔄 The Kibble balance’s measurements allow scientists to accurately calculate Planck’s constant, relating it to mass and local gravity.
• 🌍 Environmental factors like gravity fluctuations due to the position of the moon, sun, or water table must be accounted for in the measurements.
• 📅 By 2018, the kilogram will be officially redefined by a fixed value of Planck’s constant, rather than a physical object in Paris.

Q & A

Why is the kilogram considered to be 'in trouble'?

The kilogram is in trouble because the mass of the international prototype kilogram (a metal cylinder in Paris) and other supposedly identical standards around the world have been diverging by up to 50 micrograms over time. This inconsistency is unacceptable for a standard of mass.

What is the historical definition of the kilogram?

Since 1799, the kilogram has been defined as the mass of a metal cylinder kept in a locked vault in Paris.

How was the metre redefined in 1983?

In 1983, the metre was redefined as the distance light travels in 1/299,792,458 of a second. This definition made the speed of light a fixed constant, which could then be used to define the length of a metre.

Why is it important to base the definition of units like the metre and kilogram on physical constants?

Basing the definition on physical constants moves the standard from physical objects, which can change over time, to unchanging constants of the universe, providing greater precision and stability.

What are the two main approaches used to redefine the kilogram?

The two main approaches to redefining the kilogram are using a silicon sphere to determine Avogadro's number and using a Watt (or Kibble) balance to determine Planck's constant.

What is a Kibble balance and how does it work?

A Kibble balance is an apparatus that compares mechanical and electrical power to measure Planck's constant. It operates in two modes: weighing mode, where it balances the gravitational force on a mass with an electromagnetic force, and velocity mode, where it induces a voltage by moving a coil through a magnetic field.

How does the Kibble balance relate to Planck's constant?

The Kibble balance allows for an accurate measurement of Planck's constant by comparing mechanical power (mass times gravity) with electrical power (voltage times current) using macroscopic quantum effects like Josephson junctions and the quantum Hall effect.

What is a Josephson junction and how is it used in the Kibble balance?

A Josephson junction consists of two superconductors separated by an insulator. When microwave radiation is applied, it generates a precisely known voltage, which can be used to measure voltage very accurately in the Kibble balance, contributing to the determination of Planck's constant.

What role does the quantum Hall effect play in the Kibble balance?

The quantum Hall effect is used to measure resistance very accurately in the Kibble balance, which is necessary to determine the current in the system, helping calculate Planck's constant.

What will happen to the definition of the kilogram after 2018?

After 2018, the kilogram will no longer be defined by a physical object but will instead be defined based on the fixed value of Planck's constant, determined through precise measurements like those from the Kibble balance.