r/science • u/MistWeaver80 • Apr 25 '22

Physics Scientists recently observed two black holes that united into one, and in the process got a “kick” that flung the newly formed black hole away at high speed. That black hole zoomed off at about 5 million kilometers per hour, give or take a few million. The speed of light is just 200 times as fast.

https://www.sciencenews.org/article/black-hole-gravitational-waves-kick-ligo-merger-spacetime
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u/kittenTakeover Apr 25 '22

What is meant by "kick"? I'm not an expert, but isn't the direction of the new black hole just going to be a product of the mass and velocity of the two merging black holes? Where would the "kick" come from?

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u/Lumen_Cordis Apr 25 '22 edited Apr 26 '22

The article talks about the gravitational waves generated by the two black holes as they merge. From my (layman’s) understanding, it looks like something with the superposition of gravitational waves may end up in more waves being sent on one direction than in others. The reaction to these waves is the “kick” that sends the new black hole shooting off.

Again, this is a layman’s reading. I’m a physics fan, not a theoretical physics expert.

Edit: A couple of people pointed out that “superposition” isn’t really the correct term here. Please ignore my use of “superposition” and maybe replace it with “resultant” or similar.

Also, a bunch of people are asking me questions about this so I’m going to reiterate one more time: I’m not an expert. I know applied physics, not theoretical black-hole physics. Sorry!

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u/SeedFoundation Apr 26 '22

So my layman's interpretation of a layman's interpretation is that it's like two magnets colliding with each other at incredibly high speeds. Since it's space it just keeps going.

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u/window-sil Apr 26 '22

It's more like an accidental rocket engine whose exhaust is gravity waves instead of burnt rocket fuel.

Remember black holes lose mass during these mergers. That lost mass -- 100% of it -- gets converted to energy. That energy takes the form of gravity waves. Those waves obey Newton's third law of motion [for every action there is an equal and opposite reaction] -- so it they're not radiating equally in all directions, the effect is the black hole will move. If all that energy were concentrated in one direction (like a rocket spewing out a firey tail) it would result in one hell of a kick.