Once We Were Deaf, Now We Can Hear… the LIGO Discovery
Can you make music out of cutting edge physics? It turns out that you can (see further below for the musical shortcut to gravitational wave experiments). Most people are probably not too excited about gravitational waves…it does sound like it might be a bit of a dry a topic. However, they involve extremely violent and powerful events in the universe.
A disturbance in the fabric of space-time – two massive black holes are merging at a distance of 1.3 billion light years from Earth
Image via Physical Review Letters
The LIGO array that recently discovered evidence of gravitational waves (which represented an aspect of the general theory of relativity that had so far eluded observational or experimental proof), was “listening” to the effects of gravitational waves emitted by two extremely massive large black holes merging at a distance of approx. 1.3 billion light years.
One interesting thing about this is that one is actually looking into the distant past. It took 1.3 billion years for the information to arrive here, so the collision happened a very long time ago, although we could only see it now. So how does the LIGO experiment actually work?
High Energy Pulse
To this one must first consider what gravitational waves were actually expected to do. Gravitation is the result of a curvature in space-time caused by the presence of mass. The greater the mass, the greater the curvature effect. Without it, there would e.g. be no planetary orbits. Instead of revolving around a sun, the planets of our solar system would just wander endlessly through space.
When two masses are orbiting each other and accelerate, they cause ripples in space-time. These are gravitational waves, which should in theory be detectable. The problem is that gravitation is actually a very weak force. In order for such ripples to be substantial enough to make them measurable, one needs not only highly sensitive measuring equipment, but must observe ripples caused by truly staggering masses. Luckily there are actually numerous objects in the universe in possession of staggering masses.
Nothing has more mass than a gravitationally completely collapsed star, or what since 1967 is known as a “black hole”. When physicist John Wheeler asked an audience for suggestions to replace the cumbersome term “gravitationally completely collapsed star” (try saying this ten times – it gets old real fast), someone in the audience shouted out “black hole!” and the term has stuck ever since.
The following is a simplified explanation of how the effect of gravitational waves was detected by LIGO. On 14 September 2015, two LIGO detectors recorded a 0.2 seconds long “chirp” coming from the general direction of the Magellanic Clouds, increasing in frequency and amplitude in eight cycles from 35 Hz. to 150 Hz. (now known as the “GW 150914 event”).
The identical gravitational wave signals captured by the LIGO detectors in Livingston and Hanford
From the amplitude of the signal its luminosity distance could be calculated, which is how it is known that it originated at a distance of approx. 400 mega-parsecs or 1.3 billion light years. By analyzing the signal in combination with its inferred redshift it was determined that two orbiting black holes, one with approx. 36 solar masses and another with approx. 29 solar masses collided to form a new single black hole of about 62 solar masses (all numbers +/- 4).
In the 0.2 seconds of the merger, the two black holes accelerated from 30% to 60% of light speed (hence the increase in frequency in every cycle). The missing mass energy of approx. 3 solar masses radiated away as a burst of gravitational waves, with a peak power of 3.6×1049 watts, or 50 times the power of the light radiated by all stars in the observable universe. This is the kind of event that becomes measurable – barely!
Here is a computer simulation of the merger, slowed down to a speed perceptible by humans:
Computer simulation of the merging black holes that caused the signal
What happens when a gravitational wave reaches us is that space-time is lengthened in one direction and shortened in another. There would normally be no way to measure a lengthening or shortening of space time, since one’s “ruler” (and oneself!) would lengthen and shorten with it. However, luckily the speed of light is constant. A light ray traveling through space-time will in fact arrive later when the space through which it travels is lengthened and sooner when it is shortened.
LIGO uses two tunnels with mirrors at a right angle to each other in which laser light travels back and forth. A laser beam is split in two and sent into the two tunnels. Its two halves are out of phase, so when the two light rays are reflected back and recombined into one, they cancel each other out. However, when a gravitational wave hits this detector, the length of the tunnels changes, and with that the time required by the light rays to over the distance changes as well.
The tunnel lengths change by less than the diameter of a single proton. This is very little, but enough for highly sensitive laser interferometers to detect that the two light beams no longer cancel each other out. This is equivalent to measuring whether a 1 sixtillion meter (1,000,000,000,000,000,000,000 m.) long stick has expanded or shrunk by 5 millimeters.
Here is a schematic of the LIGO detector:
A laser source, a beam splitter, two tunnels, mirrors and a light detector.
Image via millstonenews.com
Here is what one of the LIGO arrays looks like from outside:
LIGO gravitational wave detector in Hanford.
Photo credit: LIGO
A Song About Gravitational Waves
All you have read above has been packed into a 3.5 minute song by A Capella Science – and it’s actually quite good:
Yes, it is possible to sing about gravitational waves…and actually deliver all the necessary information packaged for dance floor use.
Addendum: Black Holes Are Singers Too
In the center of a galaxy in the Perseus Cluster, some 250 million light years from here, there is a super-massive black hole that sings, or rather hums. It is a one-note Johnny, and has emitted a single note, namely a B flat, 57 octaves below the middle C on a piano, for an estimated 2.5 billion years. We can state that this particular black hole is a basso very profondo.
This frequency is a million billion times below the limit of the range of human hearing. For some time it was considered the deepest sound ever emitted in the universe. Until another super-massive crooner was discovered in the M87 galaxy that is. This one is a less steady singer, but the note it emits is about 59 octaves below the middle C. Kind of like Osmin’s low D on a galactic scale.
Osmin’s low D! If you want to hear something even more extreme consider these famous Russian octavists. The one from the Kochgev ensemble can presumably level buildings.
How does this sound actually propagate through the vacuum of space? The space surrounding these black hole is anything but empty – it is chock-full of gas molecules. Here is an optical representation of the sound wave propagating through the Perseus cluster as captured by an x-ray observatory:
A B-flat propagating through the Perseus Cluster.
Image credit: CHANDRA
Given that different black holes are singing different notes, with the precise note likely depending on the amount of mass in their vicinity, we imagine that all of the singing black holes out there must be producing a chord together. It will be interesting to find out what this interstellar harmony is, and whether it is a sad or happy sounding one.
Addendum 2: Happy Easter Holidays
We wish all our readers happy Easter holidays. Antonius Aquinas who occasionally contributes articles to Acting Man has posted an article on the topic, which may be of interest to some of our readers: Holy Week and the Decline of the West.
Dear Readers! We are happy to report that we have reached our turn-of-the-year funding goal and want to extend a special thank you to all of you who have chipped in. We are very grateful for your support! As a general remark, according to usually well informed circles, exercising the donation button in between funding drives is definitely legal and highly appreciated as well.
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One Response to “Gravitational Waves Explained In A Song”
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