Two trains are traveling side by side and at the same speed along parallel tracks. We are seated in one of the trains, and with us we have a special speedometer that measures their relative speed. Since the trains are traveling at the same speed, their relative speed is zero; the speedometer therefore reads "0." Suddenly the other train seems to start pulling ahead of ours. The speedometer shows a reading of 10 miles per hour. The other train has apparently increased its speed. But can we be absolutely certain of this increase?