The video discusses the history and definition of the kilogram, the base unit of mass in the metric system. Initially, it was defined as the weight of a cubic decimeter of water at the melting point of ice, but this was later changed to a platinum cylinder, known as the International Prototype Kilogram (IPK). However, the mass of the IPK has changed over time, causing problems for measurements that rely on it.
To solve this issue, scientists are working on redefining the kilogram using a silicon sphere that contains a precise number of atoms. The sphere is made of pure silicon-28 and has a diameter that can be measured with high accuracy. By counting the number of atoms in the sphere, scientists can redefine the kilogram in terms of a fundamental constant, such as Avogadro's number.
The video also mentions an alternative approach to redefining the kilogram, which involves fixing Planck's constant using a device called a watt balance. Both approaches are being explored, and if they are successful, the kilogram may be redefined as early as 2014.
The video concludes by noting that the original definition of the kilogram was remarkably accurate, given the technology available at the time. It also mentions that the scientist who first discovered silicon, Antoine Lavoisier, is ironically connected to the definition of the kilogram from its inception to the present day.
Here are the key facts extracted from the text:
1. The speaker is holding a 1 kg sphere of silicon made up of about 2.15 * 10^25 silicon atoms.
2. The raw material used to make the sphere was worth €1 million.
3. The sphere is considered priceless after being precisely sculpted.
4. The sphere is the roundest object in the world.
5. The sphere is made of a single isotope of silicon, silicon-28.
6. The sphere was created by grinding an oversized sphere progressively finer using abrasive.
7. The diameter of the sphere is measured using a laser.
8. The atom spacing of silicon is known to high precision.
9. The number of silicon atoms in the sphere can be calculated based on its volume and atom spacing.
10. The sphere can be used to redefine Avogadro's constant.
11. The redefined Avogadro's constant can be used to define the kilogram.
12. The kilogram was originally defined as the mass of a cubic decimeter of water at the melting temperature of ice.
13. The kilogram was later redefined as the mass of a platinum cylinder.
14. The platinum cylinder was replaced with a platinum-iridium alloy cylinder in 1889.
15. The kilogram is currently defined by a physical object, the International Prototype Kilogram (IPK).
16. The IPK is stored in a vault in the basement of the International Bureau of Weights and Measures in Paris.
17. The kilogram is the only SI unit still defined by a physical object.
18. The kilogram is not a stable unit, as the mass of the IPK can change over time.
19. A commission including Antoine Lavoisier defined the base unit of mass as the weight of a cubic decimeter of water at the melting temperature of ice in 1793.
20. The name "grav" was originally proposed for the base unit of mass, but it was later changed to "gram".
21. Antoine Lavoisier was executed during the French Revolution.
22. The kilogram was not always defined as 1,000 grams, but was initially defined as a separate unit.
23. The kilogram is the only SI unit with a prefix in its name.
24. The kilogram is defined as a precise mass, but its definition is not stable.
25. The kilogram definition is being redefined using two approaches: one involving the silicon sphere and another involving the watt balance.
26. The redefined kilogram may be adopted as early as 2014.
27. The original kilogram was intended to be the mass of a liter of water at its densest temperature.
28. A liter of water at nearly 4°C has a mass of approximately 999.9975 grams.
29. Antoine Lavoisier was involved in the definition of the kilogram from its conception to its possible redefinition using the silicon sphere.