Category
Measurement
Explore the 3 entries currently assigned to this category. These counts describe this curated catalogue, not the size or rate of change of the field itself.
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Not a fact anymore
A kilogram must be defined by the mass of one particular metal cylinder.
What we know now
The kilogram is no longer defined by one physical object. Instead, the SI fixes an exact value for the Planck constant, and laboratories realize the kilogram through precision measurements tied to that value.
- Status
- Superseded
- Category
- Measurement
- Accepted for
- ≈130 years
- Changed approximately
- 2019
Not a fact anymore
An optical microscope can never resolve detail smaller than the diffraction limit.
What we know now
Ordinary optical microscopes cannot cleanly separate two nearby details once their blurred light patterns overlap below the diffraction limit. Super-resolution microscopy can recover finer information by changing how fluorescent molecules are switched on, measured, and reconstructed rather than by making conventional diffraction disappear.
- Status
- Narrowed
- Category
- Measurement
- Accepted for
- ≈119 years
- Changed approximately
- 1990s–2000s
Not a fact anymore
A meter must be represented by a particular physical bar kept in a vault.
What we know now
The meter is defined through the speed of light and the second, not by the length of a stored object.
- Status
- Superseded
- Category
- Measurement
- Accepted for
- ≈71 years
- Changed approximately
- 1960–1983
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