Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 15/10/2020 15:08:51
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Marcus says:
Last night I heard the sound two Neutron stars colliding, how is this possible if there is no matter in space for sound to resonate?
What do you think?
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It's easy enough to listen to Mars...
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Last night I heard the sound two Neutron stars colliding
How did you know what the sound was?
how is this possible if there is no matter in space for sound to resonate?
Sound does not travel through space. If you were watching a video about the collision, what they're probably playing is gravitational waves (which is not sound) recorded by LIGO and played back through speakers, which is quite similar to how a CD can be used to create sounds very similar to the Beatles despite there not being any actual sound (or actual Beatles) on the CD. There only has to be air between the speakers and you, not between the Beatles and you. You're hearing the speakers, not the Beatles or the neutron stars.
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There's a slight difference.
The Beatles actually produced sound. (and a CD produces a copy of that sound).
But colliding black holes do not make a sound.
You can get some electronics to make a sound which is related to the signal from the LIGO response to the collision.
But it's like listening to a video signal. You hear something- but you don't "hear the picture"/
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There's a slight difference.
The Beatles actually produced sound. (and a CD produces a copy of that sound).
Indeed. A pair of neutron stars makes plenty of noise before and during collision, but what LIGO picks up is something else. I had a hard time coming up with a good analogy of something not-sound that is nevertheless played as sound. There are these creepy recordings of what some of the deep solar system probes 'hears' when passing by/orbiting something like Saturn. That qualifies.
You can get some electronics to make a sound which is related to the signal from the LIGO response to the collision.
But it's like listening to a video signal. You hear something- but you don't "hear the picture"/
It's much closer to hearing it than listening to a jpg (of your band above say) played back as an audio file. They predicted the sound before first hearing it (a steadily increasing pitch with a chirp at the end) and got exactly that, which served nicely as a verification of GR.
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In space, nobody can hear you crash two neutron stars.
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I would take a slightly different tack...
Sound is vibration of the air, which is picked up by your ears. Humans can hear sound frequencies from about 50Hz to 20,000Hz (although in older people, the upper limit is more like 10,000Hz).
- Under ideal conditions, humans can hear vibrations in air as small as the width of a Hydrogen atom
Two neutron stars in their final death spiral radiate gravitational waves, which vibrate spacetime.
- These vibrations start below 1Hz, but increase rapidly in frequency as the neutron stars get closer together, up to 50Hz and above; then they fall back to silence as the neutron stars merge.
- With more massive objects (like stellar-mass black holes), the frequency is higher - some have been observed reaching 400Hz.
- These vibrations in spacetime radiate through the universe at the speed of light
- When the gravitational waves pass a gravitational wave observatory (like LIGO in USA, VIRGO in Italy or KAGRA in Japan), they cause tiny changes in the lengths of laser beams - effectively, they cause the mirror to vibrate.
- These gravitational wave detectors use precision laser beams to measure tiny vibrations in the mirror - as little as 1/10,000 the width of a proton
- LIGO has a frequency response from about 50Hz to 1,000Hz, which is in the range of human hearing
So effectively, LIGO is a massive microphone, listening to vibrations in spacetime
- Each gravitational wave observatory produces a recording of the event, from their location.
- By combining these recordings, scientists can determine the rough position in the sky, how massive were the objects which collided, how far away the event was, etc.
- Scientists can also turn this sound into an audio file, which turns the vibrations of spacetime into vibrations of your loudspeaker or headphones, which vibrates the air in your ears, and is perceived as sound.
The sound is usually manipulated a bit during "sonification":
- Although you can (in theory) hear a sound at 50Hz, your ears are not very sensitive at this frequency - and your earbuds have no chance of reproducing it faithfully. So the scientists may increase the frequency of a neutron star collision to something your loudspeakers can play more easily.
- Black hole collisions occur at a higher frequency, which is easier to hear. But unfortunately, they are finished very quickly (under 0.02 seconds). So the scientists may stretch out the timeline to several seconds, so you can hear it more clearly. This time-stretching isn't needed for neutron stars, as LIGO can detect neutron star collisions for over a minute before it happens (depending on distance).
- The scientists are also likely to "clean up" the signal. There are many competing noises in the detected gravitational waves (including waves crashing on a beach 100km away!). So they may produce a pure tone at the predicted frequency, rather than include all of the extraneous noises that were in the original recording.
Can you hear neutron stars colliding?
Yes, if you have an extremely sensitive microphone capable of detecting vibrations in spacetime.
For the first detected black hole collision (with and without background noise), see/listen: https://www.ligo.caltech.edu/video/ligo20160211v2
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Impression of Planet Proxima Centauri with New the stars, Centauri a and Centauri B - Information about Proxima Centauri.