I've read that an object colliding with a black hole will move it, as it would any other object. Could a laser continuously fired into a black hole move it?


A laser carries linear momentum, so when a laser is fired into a black hole, this linear momentum is transferred to the black hole, causing it to accelerate.

Of course, for a realistic astrophysical black hole you'd need an unrealistically powerful laser for this to make a significant impact on the motion of the black hole.

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    $\begingroup$ FWIW, the Sun's luminosity is $\approx 3.828×10^{26}$ watts. We can use $E=pc$ to calculate the light's momentum, and equate it to $p=mv$ for the change in momentum of the black hole (this ignores the small change in mass of the BH due to absorbing the light). If we blast a small 3 solar mass BH with the entire solar output for 1 million years, we increase its speed by ~6.75 m/s. $\endgroup$
    – PM 2Ring
    Nov 9 '21 at 13:52
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    $\begingroup$ @PM2Ring That's actually...more than I was expecting, honestly. $\endgroup$
    Nov 10 '21 at 0:52
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    $\begingroup$ Would a micro black hole be moved more or less? $\endgroup$
    – Ben Warner
    Nov 10 '21 at 3:05
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    $\begingroup$ You could fire the laser to just skim the hole such that the light did a 180 and came back at you. That would double the momentum transfer. $\endgroup$
    – Keith
    Nov 10 '21 at 3:45
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    $\begingroup$ @Keith and then you could set up a mirror so that after the light does a 180 you can bounce it back and do it again, reusing the light an arbitrary number of times. (Still not realistic.) $\endgroup$
    – DMPalmer
    Nov 10 '21 at 4:40

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