From Kieran O'Keeffe
Since science can measure distance accurately, why can’t 1 kilogram be defined in terms of the mass required to bend a beam of light by a certain angle (22 February, p 32)?
Valerie Jamieson writes:
• Einstein pointed out that mass does bend light, but a 1-kilogram mass would have a minute effect – far too small to be detected even by today’s instruments. During a solar eclipse in 1919, Arthur Eddington determined that starlight was bent by less than 2 arcseconds (a few thousandths of a degree) by the Sun. Given that the Sun weighs 2 × 1030 kilograms (2 billion billion billion tonnes), 1 kilogram would bend light by a truly undetectable amount.
Bournemouth, Dorset, UK
