> The key idea behind the propulsion system is that it is designed to focus all the emitted photons in the direction of travel of the spacecraft. This is accomplished by using a sphere with a reflective coating that has a diameter of 1000 km. As we will see, the spacecraft would have to emit 10^27 photons per second. To achieve this enormous rate of photon emission, we will need to use a very large number of solar-powered lasers.

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