World's Roundest Object!

Summary

This video explores the history and significance of the kilogram as a unit of mass. From its origins in 1793, when it was tied to a cubic decimeter of water, to its evolution into the platinum-iridium cylinder known as 'Le Grand K,' the video delves into the challenges of defining mass through physical objects. It introduces the silicon-28 sphere, a groundbreaking effort to redefine the kilogram based on atomic precision, making the unit independent of physical objects. The video also highlights the historical role of Antoine Lavoisier in mass measurement development.

Takeaways

• 🪙 The speaker holds a 1kg silicon-28 sphere, containing about 2.15x10^25 atoms, and it is one of the roundest objects in the world.
• 🌍 If the Earth were as round as this sphere, the distance between the highest mountain and the lowest valley would be only 14m.
• 💡 The sphere is incredibly round because it’s made to help redefine the kilogram based on a constant, moving away from physical objects.
• ⚖️ The kilogram was originally defined as the mass of a liter of water, but was replaced by a platinum-iridium cylinder called the International Prototype Kilogram (Le Grand K).
• 🔑 Le Grand K is the only object in the universe that has a mass of exactly one kilogram, stored under strict security in France.
• 📉 Over time, the masses of Le Grand K and its replicas have diverged slightly, which poses a problem for defining mass based on physical objects.
• 🔬 Scientists are now using a perfectly round silicon-28 sphere to count atoms and redefine the kilogram using Avogadro’s constant, providing a more stable definition.
• 🔭 The precise size and purity of the silicon sphere allow scientists to calculate the exact number of atoms and refine the kilogram definition.
• ⚙️ Another approach to redefining the kilogram involves using Planck’s constant via a Watt Balance, which offers a complementary method.
• 📜 The historical journey of the kilogram involves the French Revolution, Antoine Lavoisier, and a transition from the 'grave' to the 'kilogram,' the only SI unit with a prefix.

Q & A

Why is the silicon sphere considered so special?

The silicon sphere is special because it is made of highly purified silicon-28, is incredibly round, and is used to redefine the kilogram based on the number of atoms it contains. Its creation involved massaging atoms to achieve its perfect shape, making it the roundest object in the world.

How does the silicon sphere help redefine the kilogram?

The silicon sphere helps redefine the kilogram by allowing scientists to count the number of silicon-28 atoms in the sphere. By measuring its diameter and calculating its volume, they can determine the number of atoms, which could be used to fix Avogadro's constant, providing a non-physical definition for the kilogram.

What problem arose with the International Prototype Kilogram (Le Grand K)?

The problem with Le Grand K is that its mass has changed over time, despite being made of platinum-iridium alloy and stored under controlled conditions. This instability creates a significant issue, as a unit of measurement cannot be allowed to vary.

How was the kilogram originally defined?

The kilogram was originally defined in 1793 as the mass of a cubic decimeter (one liter) of water at the melting point of ice. However, this was later refined using a platinum cylinder called the Kilogram of the Archives, which served as the standard.

Why was the original name 'grave' for the unit of mass changed?

The name 'grave' was changed because it sounded too similar to the aristocratic title 'graf,' and during the French Revolution, equality was emphasized. The government wanted to avoid giving the impression that any unit was more noble than others.

How accurate is the current kilogram compared to the original definition of a liter of water?

The current kilogram is slightly heavier than it should be based on the original definition. A liter of water at nearly 4°C has a mass of 999.975 grams, meaning the kilogram is only off by about the weight of a grain of rice—remarkably accurate given the time it was defined.

What are the two competing methods to redefine the kilogram?

The two competing methods to redefine the kilogram are the silicon sphere method, which counts silicon-28 atoms, and the Watt Balance method, which involves fixing Planck's constant. Both approaches are complementary and provide checks on each other.

How did the International Prototype Kilogram affect other measurements?

Since four of the seven base SI units depend on the kilogram, and many derived units (like Newtons, Joules, and Volts) rely on it, any change in the kilogram's mass could affect these measurements. This dependency makes it crucial to establish a stable definition.

What was the significance of Antoine Lavoisier in the creation of the kilogram?

Antoine Lavoisier was part of the commission that originally defined the base unit of mass during the French Revolution. He played a crucial role in the early development of the metric system, and coincidentally, he also identified silicon as an element, linking him to the modern redefinition of the kilogram.

Why is it important to move away from defining the kilogram by a physical object?

It is important to move away from defining the kilogram by a physical object because physical objects can change over time, as seen with Le Grand K. Defining the kilogram by a constant or a fundamental property like the number of atoms in a silicon sphere ensures the unit remains stable and unchanging.