Saturday, February 13, 2016

The sensory phenomenology of gravity waves



I was as excited as many of you to learn this week of the discovery of gravity waves. And also happy to hear the audible chirp as two massive black holes collided over a billion light years away. But does this discovery mean that we can now "listen" to the universe? Are the claims for this as some sort of a revolutionary sensory phenomenology valid?

Read on for scientific wonder, sensory overload, and a mixtape.

The discovery of gravity waves
The word "discovery" is one favoured by journalists but less so by scientists themselves, who recognise that the process is rarely one that can be catalogued by singular events (pun notwithstanding). Albert Einstein predicted gravitational waves in his theory of general relativity way back in 1916, and so a theoretical physicist might well say they were "discovered" then. (Although one should be careful when attributing physical "reality" to the various by-products of equations, as there is not necessarily a correspondence.)

The Nobel Prize has already gone to the "discovery" of physical evidence for gravity waves, based on work concluded in 1974. But the evidence then was indirect (though valid). Now we have direct evidence, which is an incredible achievement if you comprehend the degree of precision required.

The event witnessed on this occasion was the fusion of two enormous black holes with masses 36 and 29 times that of our own sun. The resulting energy was momentarily (within 20 milliseconds) fifty times greater than the entire power output of the observable universe. During that same brief interval the orbit speed of the black holes increased from .3 to .6 c, the speed of light.

Yeah, you might want to read that paragraph again. This stuff is too vast to get the mind around.

We can be pretty sure that cosmic events of this magnitude don't occur often. But nonetheless, frequently enough that several per year are predicted. Wow, that universe is one strange place!

The LIGO measurement of the signal GW150914 is not only about gravity waves, but something more "useful". Astronomers now have evidence of the collision and merging of black holes larger than those found before, and more massive than allowed by certain cosmological theories. Meaning that those theories are no longer valid.

This is a big deal. The LIGO team will get the Nobel Prize without problem, and won't have to wait long either (a prediction).

Listening to the universe

The following is the description of GW150914 from the original paper: "Over 0.2 s, the signal increases in frequency and amplitude in about 8 cycles from 35 to 150 Hz, where the amplitude reaches a maximum" [Abbott et al 2016, 3].

As a media artist I am intrigued as to how this is being described in the mass media. Watch the video attached to The New York Times article here. Count the number of times that visual or acoustic metaphors are used! It's fascinating, but also a bit over the top.

The video and article make the point that this discovery means we are able to "listen" to the universe. I consider this claim wrong for two reasons.

First, the signal is not acoustic. The LIGO detectors produce a laser interference pattern that is the result of tiny movements in matter due to gravitational wave compression and rarefaction. So that is already a highly mediated trace, beautifully rendered in the plots at the top of this post. If this data is subsequently converted to an acoustic signal, it is not because the original signal was in any way a sound!

Second, the claim is wrong because this type of sound mapping, or sonification, is not new. Rather it has been an important scientific tool, and musical technique, for decades.

As a disclaimer I should note that I have only an undergraduate degree in theoretical physics which barely equips me to understand these phenomena. Astronomers are welcome to correct any errors!

For entertainment, listen to this mix of "pop" and "other than pop" songs I devised to mark this fantastic occasion. Gravity waves detected 09:50:45 UTC on 14 September 2015. Dance, universe! Dance!




References
Abbott, B.P., et al. 2016. "Observation of gravitational waves from a binary black hole merger". Physical Review Letters 116, 061102 (11 Feb 2016). Available: http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102 [accessed 12 Feb 2016].

Overbye, Dennis. 2016. "Gravitational waves detected, confirming Einstein's theory". The New York Times, 11 Feb 2016. Available: http://www.nytimes.com/2016/02/12/science/ligo-gravitational-waves-black-holes-einstein.html [accessed 12 Feb 2016].

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1 comment:

robin said...

Track listing:
01 Delia Derbyshire: Gravity
02 Pylon: Gravity
03 Chrome: Static Gravity
04 Keith LeBlanc: Einstein Mad Dub
05 Wire: Gravity Worship
06 Orchestral Manoeuvres in the Dark: Gravity Never Failed
07 Neu!: Schöne Welle
08 Laurie Anderson: Gravity's Angel
09 Human League: The Black Hit of Space
10 The Passage: Wave
11 Martin Stig Andersen: Gravity Jump
12 Aube: Marginal Gravity
13 Biosphere: Gravity Assist
14 [GW150914 gravity chirp, slowed]

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