PROFESSOR: Welcome back to [? A20, ?] special relativity.
Very early in this lecture, we discussed what mass is
and how we can understand the mass of a proton
and the mass of an electron as it interacts with the Higgs
background field.
But here, the question is slightly different.
You want to understand the difference or the lack
of difference between the mass in a gravitational field
compared to the inertial mass, a mass being
pushed through some force.
And it's probably Einstein's biggest realization
that those two things are the same.
And that finding is called the equivalence principle,
the equivalence of the gravitational and inertial
mass.
So if you are a freely falling person,
you will not feel your own weight
in your gravitational field.
You will just drop.
And the gravitational field provides an acceleration.
And that acceleration, if you are standing in an elevator
or just sitting on a chair like I'm doing right now,
then sets up an accelerating reference frame.
So there is an equivalence between being accelerated
or being stationary in a gravitational field.