Event Horizons

The $45^{\circ }$ diagonal line passing through the origin in figure 6.5 is called the event horizon for the accelerated observer in this figure. Notice that light from the ``twilight zone'' above and to the left of the event horizon cannot reach the accelerated observer. However, the reverse is not true -- a light signal emitted to the left by the observer can cross the event horizon into the ``twilight zone''. The event horizon thus has a peculiar one-way character -- it passes signals from right to left, but not from left to right.