G The graph shows three galaxies. The axes are drawn according to
an observer at rest relative to the galaxy 2, so that that galaxy is always at
the samexcoordinate. Intelligent species in the three different galaxies
develop radio technology independently, and at some point each begins
to actively send out signals in an attempt to communicate with other civi-
lizations. Events a, b, and c mark the points at which these signals begin
spreading out across the universe at the speed of light. Find the events at
which the inhabitants of galaxy 2 detect the signals from galaxies 1 and
- According to 2, who developed radio first, 1 or 3? On top of the graph,
draw a new pair of position and time axes, for the frame in which galaxy 3
is at rest. According to 3, in what order did events a, b, and c happen?
Discussion question G.H The machine-gunner in the figure sends out a spray of bullets.
Suppose that the bullets are being shot into outer space, and that the
distances traveled are trillions of miles (so that the human figure in the
diagram is not to scale). After a long time, the bullets reach the points
shown with dots which are all equally far from the gun. Their arrivals
at those points are events A through E, which happen at different times.
Sketch these events on a position-time graph. The chain of impacts ex-
tends across space at a speed greater thanc. Does this violate special
relativity?
Section 7.2 Distortion of space and time 415