When it is stationary, the half-life of a certain subatomic particle is T0. That is, if N of these particles are present at a certain time, then a time T0 later only N/2 particles will be present, assuming the particles are at rest. A beam carrying N such particles per second is created at position x=0 in a high-energy physics laboratory. This beam travels along the x axis at speed v in the laboratory reference frame and it is found that only N/2 particles per second travel in the beam at x=2cT0, where c is the speed of light.