In example R9.3, the spaceship engines convert $60,000 \mathrm{~kg}$ of matter-antimatter fuel to massless light. The mass of the total system (empty spaceship plus flash of light, considered as a unit) thus decreases.

Question

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According to the law of conservation of mass-energy, the mass-energy of a closed system must remain constant over time. When matter and antimatter annihilate, their mass is converted into energy (e.g., photons of light), but the total mass-energy of the system (including the energy of the light) remains constant. The mass equivalent of the energy (as per E=mc^2) would still contribute to the total mass-energy of the system, so the mass of the system, when considered in terms of mass-energy, does not decrease.

In Example R9 60,000 kg60,000 \mathrm{~kg}60,000 kg Of Matter-Antimatter Fuel to Massless Light

In Example R9 $60,000 \mathrm{~kg}$ Of Matter-Antimatter Fuel to Massless Light