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>> A Foucault pendulum is a pendulum
with a specially designed pivot.
Designed so that it can swing back and
forth in any direction around the vertical.
So that's in contrast to, for
example, a grandfather clock
where the pendulum has to
swing in a particular plane.
Let's imagine that you have a Foucault pendulum
and you're at the North Pole of the Earth.
When you set the pendulum
swinging, you determine a plane
in which the pendulum is swinging.
And that plane is fixed.
So as the Earth rotates, the Earth rotates
underneath the plane of the pendulum swinging.
And so if you now go back and imagine
that you're standing on the Earth,
you see the plane of the
pendulum rotate every 24 hours.
Now, as you go to the lower
latitudes away from the North Pole,
the time that it takes the pendulum -- the plane
of the pendulum to rotate around lengthens.
And, in fact, at the equator
the plane doesn't rotate at all.
At the latitude of Hanover, it takes
35 hours for the plane to rotate.
So if you're looking at this pendulum here.
If we look at it a few minutes from now we
won't even be able to detect that the plane
of the pendulum swing has changed.
But if you come back in an hour, you will
definitely see that it's moved a little bit.
And if you come back in nine hours,
this plane of this pendulum here
will have rotated by 90 degrees.
A very distinct change that
you could easily see.
Plane of the pendulum swing
is also affected, for example,
by the revolution of the Earth around the sun.
And so, for example, if you have a Foucault
pendulum at the North Pole of the Earth.
Not only will it rotate around every
day because the Earth is rotating.
But you will also get a much longer
rotation over the period of a year
from the revolution of the Earth around the sun.
So if you think about it, this Foucault
pendulum here is a kind of rotation meter.
It's a way of measuring rotations.
A suitably placed -- a suitably
placed Foucault pendulum can be used
to detect the rotation of the Earth.
Or the rotation -- the revolution
of the Earth around the sun.
Or in principle you could use a
suitably oriented Foucault pendulum
to measure the rotation, for example, of the
-- of the solar system around the galaxy.
So this brings up the question, the
Foucault pendulum is sensitive to rotation.
But rotation around what?
And there's been a lot of speculation in
physics and philosophy about that question.
Ernst Mach, a 19th -- 20th century
physicist, put forward the conjecture
that the Foucault pendulum is measuring
rotation relative to the universe as a whole.
Makes a kind of interesting connection
between a local physics experiment here
in this room and the universe as a whole.
And the problem that people
bring up is, you know,
how can this pendulum here somehow know
something about the entire universe?
So it's -- remains as an unproven
conjecture more in the realm
of philosophy than in physics at this point.
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