But, I have heard that speed of sound in space is 300km/s . Near the black hole, atom are very active and their collisions produce sound nearly one million times less decibel than 20 Hz. And, space isn't totally vacuum and there are 10 atoms in cubic box in space. So, I think 300 km/s is possible. Thanks for joining.
THomas
please make up your mind - are you talking about "space" or about "near black hole" ? Your original question is about space. Near a black hole is not classified as space.
about space, even if there are 10 atoms in cubic box (?? i suppose you mean ~ 1 atom per cubic centimeter), then to be classified as
sound we would have to have a pressure wave propagating via these atoms. I.e. each of these atoms would have to move periodically & collide with adjacent atoms & hence transfer the kinetic energy.
while such a phenomenon might occur with a vanishingly small but still a non-zero probability, the number is so small that its difficult to visualise it as sound.
Even if its a "..million times less decibel than 20Hz..." whatever that means.
regarding what happens near a black hole - this is still very much a speculative side of (astro-)physics. However it is much easier to think of a kinetic pressure wave propagating through whatever particles/ atoms might exist there, and then think of it as
sound. Whether this speed would be fixed at 300Km/s is highly unlikely. The speed would depend on so many factors, and would definitely be very variable.
Now - the speed of EM waves (light wave is easiest to visualise - pun intended) - is almost exactly 300,000Km/s. And the speed of sound through air on Earth is approx 0.3 Km/s I hope you are not using some geometric mean between these two numbers ?