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PROFESSOR: Welcome back to
[? A20, ?] special relativity.

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Very early in this lecture,
we discussed what mass is

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and how we can understand
the mass of a proton

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and the mass of an electron
as it interacts with the Higgs

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background field.

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But here, the question
is slightly different.

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You want to understand
the difference or the lack

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of difference between the
mass in a gravitational field

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compared to the inertial
mass, a mass being

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pushed through some force.

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And it's probably Einstein's
biggest realization

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that those two
things are the same.

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And that finding is called
the equivalence principle,

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the equivalence of the
gravitational and inertial

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mass.

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So if you are a
freely falling person,

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you will not feel
your own weight

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in your gravitational field.

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You will just drop.

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And the gravitational field
provides an acceleration.

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And that acceleration, if you
are standing in an elevator

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or just sitting on a chair
like I'm doing right now,

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then sets up an accelerating
reference frame.

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So there is an equivalence
between being accelerated

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or being stationary in
a gravitational field.