mass

Mass is the property of matter that resists acceleration and sources gravity. Push two objects equally hard and the more massive one speeds up less. Nine pages of this wiki already lean on the idea — this page is what they lean on.

Two faces, one number

• Inertial mass — how hard something is to accelerate (Newton's F = ma).
• Gravitational mass — how strongly it pulls and is pulled.

Every experiment ever run finds these identical. General relativity is built on taking that seriously.

Mass is bottled energy

Einstein's 1905 conclusion — in a three-page paper — is that mass and energy are the same currency: E = mc². The mass of an Atom is less than the mass of its parts, because Nuclear binding energy left the system when the nucleus bound. And almost all of the mass of a Proton or Neutron is not the mass of its Quarks but the confined energy of the Strong force holding them.

object	rest mass
Electron	9.109 × 10⁻³¹ kg
Proton	1.673 × 10⁻²⁷ kg
Neutron	1.675 × 10⁻²⁷ kg
Neutrino	tiny; nonzero; unmeasured

The unit

Since 2019 the kilogram is defined by fixing the Planck constant — a number of nature, not a metal cylinder in Paris. Any lab with the right equipment can realize it.

What mass is not: weight (that's gravity's pull on mass, and changes on the Moon) and it is not conserved on its own — only the total Energy of a closed system is.