{"id":690,"date":"2026-07-31T23:54:13","date_gmt":"2026-07-31T23:54:13","guid":{"rendered":"https:\/\/physicsfundamentalsinfo.com\/blog\/?p=690"},"modified":"2026-07-31T23:54:15","modified_gmt":"2026-07-31T23:54:15","slug":"physics-constants","status":"publish","type":"post","link":"https:\/\/physicsfundamentalsinfo.com\/blog\/mechanics\/physics-constants\/","title":{"rendered":"Physics Constants: Values, Units and Where to Use Them"},"content":{"rendered":"\n
Definition<\/div>

\nPhysics constants are fixed quantities that appear in physical laws and take the same value everywhere in the universe. Seven of them now define the SI exactly, including the speed of light c = 299,792,458 m\/s and the Planck constant h = 6.62607015 \u00d7 10-34<\/sup> J s. Others, such as the gravitational constant G, must still be measured.\n<\/p><\/div>\n\n

On 20 May 2019, a polished cylinder of platinum-iridium sitting in a vault outside Paris quietly stopped being the kilogram. It had defined mass for 130 years. Its replacement is a number: the Planck constant, fixed forever at 6.62607015 \u00d7 10-34<\/sup> J s.<\/p>\n\n

That swap tells you what these numbers really are. A constant is not trivia to memorise before an exam \u2014 it is the anchor that ties an equation to the physical world, and increasingly it is<\/em> the definition of the unit itself.<\/p>\n\n

What Are Physics Constants?<\/h2>\n\n

Physics constants are quantities whose value does not change with time, place, or the experiment being run, and which appear in the equations describing how nature behaves. Some are fixed by definition; others are measured, and carry an uncertainty.<\/p>\n\n

That distinction matters more than most textbooks admit. Lump them together and you end up quoting g<\/em> to nine decimal places, or treating G as though it were known as precisely as c. Neither is true.<\/p>\n\n

It helps to sort them into four honest categories:<\/p>\n\n