To some, mass is synonymous with
weight and simply describes how hard it is to
get something moving, but to Isaac Newton, mass
was something much deeper. With his second law (F=ma), Newton
described what mass really was: resistance to
acceleration, also known as inertia. Mass is
also part of Newton's law of gravitation, which
describes the force with which two objects are
gravitationally attracted. This gravitational
force due to mass is the weight we all feel and
are familiar with. In countless experiments,
Newton's laws were found to accurately describe
the motion of massive objects from falling
apples to the orbits of the planets.
Newton's laws served us well for
centuries and still do to this day, but by the
twentieth century, it became apparent that they
were incomplete. The motion of Mercury, the
closest planet to the sun, for example, never
quite matched up with Newton's laws. Then came
along Albert Einstein and his theory of general
relativity, which describes gravity not as a
force, but as curvature in spacetime. As
unintuitive as it sounds, it has been extremely
successful in describing and predicting
phenomena, like Mercury's orbit[4], as
mentioned before, and most famously, black
holes. On top of that, Einstein proposed
mass-energy equivalance, summarized by the
often-repeated formula E=mc2.
This means that energy as well as mass curves
spacetime and thus are affected by gravity. This
fact was experimentally confirmed by Arthur
Eddington in 1919 when he observed that the path
of light is bent by the sun's gravity[5].
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