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CONTROL SYSTEMS 69
Space
Well, if it’s not bad enough having to worry about just what time is, Einstein threw
another monkeywrench into our collective thinking. The General Theory of Relativity
holds that the fabric of space itself isn’t just a series of straight perpendicular lines like
some street pattern, but rather it’s curved and changing! He came up with this theory
using a truly beautiful “thought experiment.” Instead of working in a lab, Einstein sat
down and pictured the experiment in his head. Here’s how his thinking went.
Suppose we are sitting in a room in far outerspace where no gravity exists. Two holes
are in the wall, one to the left and another to the right. A beam of light comes in one
wall and out through the other. It does not take long for the beam of light to cross the
room at light speed. Light travels one foot per a billionth of a second (see Figure 2-32).
Now, if you accelerate the room upward at 32 feet/second/second (1 G of gravity),
when the next beam of light comes through the first hole, it won’t make it out through
the second hole (which has now moved). From our standpoint sitting the the room, the
light beam curves after it enters the room and hits the wall too low (see Figure 2-33).
Now suppose instead of acceleration, we put the earth immediately under the room.
From our standpoint sitting in the room, we could not tell the difference. We still expe-
rience 1 G of accelerative force under us. The beam of light comes in the first hole and
still bends down to hit the wall below the second hole (refer to Figure 2-33).

Light Beam

Room in Deep Space

No Gravity
FIGURE 2-32 Einstein’s thought-experiment: Light moves straight in the
absence of gravity.