Last week I attended a fascinating talk by astrophysicist Phiala Shanahan, a professor in the Center for Theoretical Physics at the Massachusetts Institute of Technology. It took place at the Simons Foundation in NYC (a unique forum that has, besides its scientific mission, an active interest in exploring the interaction between science and the arts). [The talk was on the subject of] neutron stars, very unique objects that have their unique voices broadcast throughout the cosmos. These voices, it turns out, are telling us more and more about the universe's mysteries.
I had read a recent article about what is called “the long ringdown” (a phenomena that occurs after these objects merge), and there was no other person better to ask about this phenomena; but I was not sure that my question would be relevant…. but I was there, so I had to ask. The answer was more than I expected, and the professor jumped right into answering me with enthusiasm and knowledge (to my great satisfaction). So I am emboldened to share my thoughts about these Neutron stars.
The signal emitted by two merging Neutron Stars resembles that of a tuning fork (Credit: L. Rezzolla/pixabay).
Stars do ring like bells and that can be measure by recording the motion of the Star surface, this can be done by sensing the movements in a form of a “doppler shift”. This in fact a special branch of astronomy called “astroseismology” (read a previous blog about this subject.) So it should obviously also occurs in neutron stars, since they are stars and also to their ultimate fate: black holes.
Neutron stars are one step away from being black holes. They are immensely dense: one teaspoon weighs as much as the whole Empire State Building. They are the result of the implosion of already massive stars (5 to 50 times the mass of our star, the Sun), a phenomena called a Super Nova. Matter is made of atoms, and as we have known for about 100 years, atoms are mostly empty space. So after the star explodes, the remnants are only neutrons (mostly) with no space in between. The remnants implodes, if massive enough, into a neutron star.
If you add any mass to this already very massive object, it collapses into a black hole. That new mass comes usually as a steady stream from another star, and in some instances, it comes from another neutron star that ends up merging in a violent event. As they orbit each other, closer and closer and faster and faster, as they reach the last few seconds, they radiate higher and higher pitched gravity waves as they merge into a black hole and become silent. But actually - not quite, because that violent moment acts like a strike on the newly formed object, not unlike the strike on a bell.
When a bell is struck, the first sound emitted is a complex irrational set of frequencies; but as the vibrating bell (in our case, vibrating black holes) settles, the sound becomes more and more pure and that pure pitch leaves us with a lot of information about the composition (wood, metal, stone, neutron, etc.) of the ringing object.
The evolution expected from the post-Newtonian inspiral is shown with a dashed black line. The inset offers a magnification of the long ringdown with the corresponding linear slopes indicated by thin lines of the same color. Lower panel: As in the top panel but in terms of the instantaneous GW frequency fGW. The data refers to equal-mass binaries but very similar behaviour is found also for unequal-mass binaries
That is exactly what we should expect from the newly formed black hole, which is the merger of two neutron stars. We do not really know what they are made of, but that residual pitch is a window into the nature of the object. These rings have not yet been [physically?] detected; they have only been theorized following the law of physics and more precisely, [the law of] acoustics. This is an example of the parallel between sound vibration and our understanding of the universe.
As the universe is big, there are millions of them out there. In fact, new telescopes are being built to be sensitive enough to detect these events: a new radio telescope in China called FAST (Five-hundred-meter Aperture Spherical radio Telescope) has, in its short life of about five years, detected over one thousand of them. Most stars are paired with one or two other stars, so the chances for mergers are high.
What does this all mean for us musicians? Well, I think that there is a symphony out there in the cosmos that we are just now discovering. There are implications to this sonic environment influencing our musical sensitivities, but this will be for another post.