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In discussing this question about propelling a spacecraft with photons and their relativistic momentum, the author asked that I restate my comment as another question.

If photons can really be used for propulsion, could two mirrors be affixed between two objects, focused well and continually adjusting their respective 'where they will be at the time of light arrival', and use the same photons bouncing back and forth between the two objects for perpetual thrust after the initial photon generation?

The initial question was on an assignment problem:

"A rocket of mass $m_0$ is propelled by a giant monochromatic laser mounted on the back of the rocket. The laser emits a beam with a power of $P_0$ watts and a frequency $f_0$, both measured in the rest frame of the rocket. When the beam is turned on, the rocket is driven in the opposite direction by the recoil."

Community
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Ehryk
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2 Answers2

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The problem with your idea is that each time the light reflects off the mirror it transfers some of its energy to the mirror (to increase the mirror's kinetic energy) and is red shifted as a result. So the thrust would fade as the light red shifts away to nothing.

For obvious reasons the light can only transfer as much energy to the mirrors (in the form of kinetic energy) as it originally had. Otherwise you would have a perpetual motion device.

John Rennie
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I think that even if the mirrors are fixed, there would still be loss of momentum in the form of heat loss as the mirrors are made up of atoms and as such, are subject to thermodynamics.

Dan Park
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