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>Remember the methodology: A spacecraft emits a beam of energy at a black hole that is moving towards it, choosing the angles so that the beam returns to the sp
by JohnJamesRambo 8y ago
>Remember the methodology: A spacecraft emits a beam of energy at a black hole that is moving towards it, choosing the angles so that the beam returns to the spacecraft (along the so-called ‘boomerang geodesic’). With the beam making the gravitational flyby rather than the spacecraft, the vehicle can nonetheless exploit the kinetic energy of the black hole for acceleration.
> Light acts the same way, but light cannot return faster than the speed of light. Instead, in gaining momentum from the black hole, the light blueshifts.
How do you get propulsion from blueshifted light?
- dmitrygr 8y agoEmit low energy photons, expend energy X, receive higher energy photos with energy Y (Y> X) Maybe this way?
- extropy 8y agoYes I want my free energy device too.
- thatcherc 8y agoThis isn't a free energy device because you're extracting energy from the rotation of a black hole (this process only works on rotating black holes). After a long enough time, you could extract all of the angular momentum from the black hole using this method.
- mrfusion 8y agoI can see how you gain energy but you’re also putting yourself deeper into a gravitational well that you’d have to expend energy to get out of. Wouldn’t it be a wash?
- ceejayoz 8y agoWe do this fairly regularly with space probes already. https://en.wikipedia.org/wiki/Gravity_assist https://en.wikipedia.org/wiki/Gravity_assist You steal energy from the object you're slingshotting off. The black holes are slowing (infinitesimally) down every time you do it, gifting their energy to the accelerated object. Hence the article talking about potentially detecting this sort of behavior by monitoring for prematurely merging and overly eccentric binary black holes.
- lainga 8y agoAt some level, that's also what glider pilots do to take off from mountains.
- michaelmrose 8y agoThe paper says that the maximum velocity obtainable would be 2x the rotational speed of the body used. Some such bodies rotate at appreciable portions of the speed of light. Obviously you cannot exceed the speed of light but it doesn't seem like it would be a wash.
- AnimalMuppet 8y agoBlueshifted light has more energy (and momentum) than it did before the blueshift. If it returns to your spacecraft blueshifted, you can get more energy and momentum out of it than you used in sending it. The change in momentum can be used for propulsion.
- JohnJamesRambo 8y agoBut what is the nuts and bolts of how it is done? What do you capture the light with? How are you propelled? Are we talking solar panels or sails or some other technology?
- human 8y agoI think you might gain momentum simply from absorbing the light.
- thatcherc 8y agoYou gain even more momentum from reflecting the light back into the black hole! When you absorb a photon, you gain its momentum. However, when you reflect a photon, you gain twice its momentum to account for the momentum possessed by the photon now flying away from you.
- saalweachter 8y agoIsn't that a three-fer? You first emit the photon towards the black hole, momentum gain. Absorbing the photon on the return would be a two-fer, and reflecting a three-fer.
- dlazar 8y agoAbsorbing the photon would be a two-fer. Reflecting it is just starting the next two-fer. It's two-fers all the way down.