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If there was a source of a continuous gravitational wave at (say) 50hz, and amplitude of say a micrometer (a typical sound wave displacement, I think), and you were nearby (standing happily on a planet in an atmosphere), with your ear pointing to the source, would you hear it?

It seems to me that since the gravitational wave is reducing and increasing the distance between points in the atmosphere right at your eardrum, surely the density and pressure of the air there is likewise increasing and decreasing, so you might expect to hear it. What I can't "intuit" is whether you would actually hear it due to the fact that you yourself are also being distorted.

My tentative conclusion is that you would hear it. At any given time, there appears to be a pressure differential across your eardrum due to this distortion in space pressurising the materials - so ... deflection?

(note: I know that in the recent LIGO announcement they talked about "hearing" the waves, but this is something completely different: an electro-acoustic rendition of the waveform. I'm asking about direct physical sensing.)

GreenAsJade
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    have a look at this link http://stuver.blogspot.gr/2012/06/what-do-gravitational-waves-sound-like.html – anna v Feb 15 '16 at 14:55
  • I'd call this somewhat related: http://physics.stackexchange.com/q/237281/84895 – Zaibis Feb 15 '16 at 16:11
  • Could someone comment on whether tidal stress could be a factor? – MackTuesday Feb 15 '16 at 21:53
  • Have a look at this http://scitation.aip.org/content/aip/magazine/physicstoday/news/10.1063/PT.5.2034?utm_source=Physics%20Today&utm_medium=email&utm_campaign=7021877_The%20week%20in%20Physics%2018%E2%80%9322%20April&dm_i=1Y69,46I45,E1NL8J,F7K32,1 – jim Apr 25 '16 at 15:49

5 Answers5

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The frequency of the recent experiment was in the audible range. The amplitude was off by unspeakable orders of magnitude. But yes, you would hear it (even in vacuum, if you were to survive).

Yes, the GW are transverse (quadrupolar). But they do move things (they cause change in distances, that's actually how they detected them: the length of the 4km tube at LIGO changed; earlier experiments actually planned to detect the "sound" of a vibrating metal cylinder, but they weren't sensitive enough). An eardrum and the bones around them are a complex instrument and whatever direction the strain is applied, it would surely induce a vibrational motion that would produce vibration of the eardrum, even if not in the way you imagine (compare to sound, where there is direct pressure to the eardrum -- GW are more profound and make the eardrum itself directly deform and vibrate). If you were close enough to a cataclismic cosmic event, you would hear it across the emptiness of the space. Both directly (as induced vibrations in our bones), and through creaking of the structures around us.

It's interesting to note that generally the same instrument that was used more than a century ago to prove that velocity of the "aether" is impossible to detect (disproving the notion of an elastic medium permeating the universe) was now used to prove acceleration of the "aether" (so to speak) can and was measured.

orion
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    This would seem to be the only correct answer. The rest are crap. Note that, very simply, the LIGO arm was in fact vibrated. It's "just that simple". If the vibrations were much bigger, you would have heard the LIGO arm vibrating, as simply as if anything else vibrated it. The other questions seem to be answering in an abstract sense of "would you hear vibrations is only your ear, platonically, vibrated". I don't even know what that means. – Fattie Feb 15 '16 at 20:08
  • For example, "can I hear a freight train going through". OK, what you hear is everything it vibrates. Can you sort of hear platonically the train "itself", or something, whatever - weird question. – Fattie Feb 15 '16 at 20:10
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    If the black hole merger was closer, we'd get higher amplitude, same frequency. Quite simple, really. – orion Feb 15 '16 at 22:00
  • Can you hear EM waves? Yes, you can convert them in audible waves and then hear them. LIGO was like a radio which converted GW into audible wave. – Anubhav Goel Feb 16 '16 at 03:58
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    Much as the eardrum convert pressure waves into sound. There's no difference. Both physical phenomena induce vibration of the ear (and other sound recording equipment) directly, without additional conversion machinery. I'd say GW do that even more directly than air pressure waves, as GW are "pure" deformation waves (transverse strain, propagating without the need of an underlying medium). If a 1mm briefly becomes 1.001mm, whatever was there will want to relax back to its native size, causing motion - it's really as close to sound as you can get. – orion Feb 16 '16 at 07:35
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    When I watch SciFi movies with sounds in space this is exactly my rationalization - that the warp/jump/FTL/alien engines generate oscillating gravity waves that cause the walls of the ships to vibrate in audio frequencies. – slebetman Feb 16 '16 at 08:36
  • @slebetman I guess some sci-fi has really massive laser bolts and explosions! ;-) – Todd Wilcox Feb 16 '16 at 17:44
  • What was the amplitude of the waves close to the epicenter? – kjetil b halvorsen Feb 16 '16 at 18:18
  • The amplitude drops as $1/r$. So 1 light year away, we'd have around $h=10^{-12}$ (nanometer per kilometer) in the recently detected black hole collapse event. Still not drastic, but would disturb a lot of experiments, not just a purpose-built detector. At a distance of 1AU, you get to about $h=10^{-7}$ which I think is audible (relative sound pressure at auditory limit is ~$10^{-9}$ -- It's terrible to compare pressure quotients and metric deviations, but as a rule of a sore thumb, let's go with it) and might even be picked up by seismometers. – orion Feb 17 '16 at 11:10
  • ^ just a disclaimer... of course, at that distance from the event, the world would be obliterated by tidal forces and whatever non-gravitational effect accompanied the event. – orion Feb 17 '16 at 11:24
  • @orion - Why does the intensity drop as $r^{-1}$ and not $r^{-2}$? Is it beamed/focused or something? – honeste_vivere Feb 17 '16 at 13:38
  • It's the amplitude, not the energy (which usually falls of as radius squared). Energy of the gravitational wave itself is very problematic to define, because (1) energy is not a conserved quantity in general relativity, as no frame is truly inertial and space-time isn't flat (2) in a GW, the space is still empty, and locally, the stress-energy tensor is zero. There are estimates that observe the wave from a faraway asymptotically flat vantage point (that's what they did to estimate the mass-to-energy conversion) but the energy doesn't tell much anyway... it's the metric distorsion that counts. – orion Feb 17 '16 at 14:19
  • The question of beam/focussed is also an interesting one. Almost a whole separate question: something that's quadropole polarised surely must be somewhat limited to a plane I think? Rather than radiating uniformly as a sphere... – GreenAsJade Feb 18 '16 at 02:09
  • Indeed, in the lowest order, the radiation has a quadrupolar angular distribution, and the symmetry plane in the case of a binary black hole is aligned with their ecliptic plane. – orion Feb 18 '16 at 09:01
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Lets look at the problem on a quantum mechanical level. Sound is generated by vibrating atoms in a solid transferring their energy to the air, and the energy moves in sound waves and reaches our ears or instruments. This happens with the exchange of electromagnetic interactions, at a complex level, but still photons and scatterings are involved.

A gravitational wave is composed of gravitons, but the coupling constant is very very much smaller than the electromagnetic one. There is no transfer of energy to the lattices of the solid that could change the pressure and generate sound waves. It passes as is, without losing energy, through our eardrums too because of the extremely weak coupling with matter.

anna v
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  • But what if the amplitude is increased sufficiently to compensate for the weaker coupling? – OrangeDog Feb 15 '16 at 18:21
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    @OrangeDog In quantum mechanics it is the coupling constant which defines the order of magnitude of the amplitude, and that is so very small wth respect to the electromagnetic one. see here http://hyperphysics.phy-astr.gsu.edu/hbase/forces/funfor.html – anna v Feb 15 '16 at 18:57
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    Yes, I know that. But the question is not concerned with what the magnitude actually is, just that it is sufficiently large. – OrangeDog Feb 15 '16 at 19:34
  • @OrangeDog I am arguing in terms of gravitons and quantum mechanics. The energy of a graviton will depend on its frequency. Large can only go to frequency. To overcome the 160 or so orders of magnitude difference between electromagnetic diagrams and gravitational diagrams ( couplings enter as powers of 4 at least) .It is not possible to get sound is all I am saying from quantum mechanical considerations, because sound needs interactions and interactions are highly improbable. – anna v Feb 15 '16 at 19:51
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    All you are saying is "no, because it is not strong enough", but the question is asking "what if it is strong enough". – OrangeDog Feb 15 '16 at 20:02
  • I think the answer addresses whether a gravitational wave will produce sound waves in air, and it answers that more or less in the negative. But I think the question was meant in another way. Given a sufficiently "extreme" gravitational wave, would the periodic displacement of different parts of the ear induce a perception of "hearing", even if this stimulus did not (as usually when we hear) come from acoustic waves in the air? The relative movements of anatomic parts of the ear coming instead from the "distortion" of space-time itself. – Jeppe Stig Nielsen Feb 15 '16 at 23:57
  • @OrangeDog I am saying that at the quantum level strong 's only meaning is very high energy graviton. With that order of magnitude for the graviton, assuming that like the electromagnetic interaction the crossection goes up with energy, the graviton would kill the person from that much energy if it interacted ( and would still be a graviton at that energy). After all it is seldom that one hears appropriate frequency electromagnetic fields without intervening membranes., even close to very strong magnetic fields ( I have felt a general "buzzing" in the head in the fringe field of BEBC) . . – anna v Feb 16 '16 at 05:47
  • continued : if the frequency of the fringe em waves were higher, I would cook. I am saying that there is 120 orders of magnitude less gravitational probability, for equivalent gravitational fields at frequencies that might interact to give audio in the head. I suppose if one had a beam of acoustic frequency gravitons 120 orders of magnitude more, than an em beam next to a strong acoustic frequency antenna , one's head might buzz from graviton-molecule interactions. – anna v Feb 16 '16 at 05:54
  • @JeppeStigNielsen the above is also for you – anna v Feb 16 '16 at 05:55
  • It is not true that there is no transfer of energy from GWs to matter. – GreenAsJade Feb 17 '16 at 03:58
  • @GreenAsJade sure, but I am saying that if you put in the numbers the probability is very very small. – anna v Feb 17 '16 at 04:04
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From wikipedia:

The effects of a passing gravitational wave can be visualized by imagining a perfectly flat region of spacetime with a group of motionless test particles lying in a plane (e.g., the surface of a computer screen). As a gravitational wave passes through the particles along a line perpendicular to the plane of the particles (i.e. following the observer's line of vision into the screen), the particles will follow the distortion in spacetime, oscillating in a "cruciform" manner, as shown in the animations. The area enclosed by the test particles does not change and there is no motion along the direction of propagation.

https://en.wikipedia.org/wiki/Gravitational_wave#/media/File:GravitationalWave_PlusPolarization.gif

Jaywalker
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    If I am facing in the direction of the wave in the animation, then a segment of the particles shown could be my eardrum. They deflect back and forth. Why wouldn't I detect that as "sound"? – GreenAsJade Feb 15 '16 at 13:47
  • You are right this might be the case. However I am not sure because I lack the exact understanding of how the waves work. Someone else might have to answer the specifics. – Jaywalker Feb 15 '16 at 13:49
  • In case you can answer why you would be able to hear it or not to hear it, this is kind of related: http://physics.stackexchange.com/q/237281/84895 Since I'm so far not even sure if there is something we could hear or there isn't and I'm looking for an answer about it. – Zaibis Feb 15 '16 at 16:10
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If you would hear it, it would be a matter of the stretching-and-compressing motion due to the GW themselves causing a reaction on your hearing organs (those beyond your eardrums). The air itself interferes minimally: it is deformed behind your eardrums in exactly the same way as it is outside (this does not depend on the specific nature of the waves, be it transverse or longitudinal), hence there is no pressure difference which can cause your eardrum to vibrate. Also, the wavelength of the GW ($\frac{3.10^8}{50}m=6.10^6 m$) is way too long to cause significant pressure gradients in the atmosphere and therefore cause "secondary" sound.

EDIT: solid (perhaps some liquid) bodies may start to chime significantly though, especially those who have fundamental frequencies at 50 Hz, that could be a significant "tertiary source".

5th decile
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I don't see how you could hear gravity waves even if they were of the appropriate frequency and amplitude. Hearing depends on the motion of hairs in the vestibular system, specifically the relative motion between the hair and its attachment in the cochlea. If a gravitational wave were to pass, all parts of the organ would move together and the hairs would not transmit a signal to the nervous system. Therefore you could not hear a gravitational wave. That's the way I see it.

John Fistere
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  • Except that the movement is differential. That is to say that not everything moves the same amount at the same time. I think you have put your finger on the heart of the question I was trying to ask: does the passing of a GW cause enough relative movement between localised elements of matter to trigger hearing? Clearly some relative movement happens, because "the interformeter tube gets shorter" ... this means that two mirrors moved closer to each other, and presumable further from other points in line with them. Similarly with hairs... – GreenAsJade Feb 17 '16 at 00:11
  • ... the a secondary answer has been suggested, which is that the GW might also induce 'ringing' or 'vibration' in the structures it passed through. This is an interesting thought, but definitely not the "physics question' I was trying to ask. I was trying to ask about whether GWs induce relative motion - that in this thought experiment might trigger hearing. – GreenAsJade Feb 17 '16 at 00:13
  • @GreenAsJade I agree that theoretically there must be some differential motion but in the case of hearing, the hair must move enough to open a physical pore to allow certain ions to pass through. I think the differential motion on a scale of 1 mm or so would not accomplish that! – John Fistere Feb 19 '16 at 07:17
  • I chose the amplitude of the GW in the "thought experiment" to match my understanding of the amplitude of typical sound waves. In principle, the question is "if the GW amplitude matches sound wave amplitude, would you hear it?" – GreenAsJade Feb 19 '16 at 07:26
  • @GreenAsJade - I agree that conceptually there is some differential motion as a gravity wave passes but the differential that exists over a fraction of a millimeter would be to small to trigger a response in the inner ear. – John Fistere Feb 23 '16 at 07:54
  • In what way does this differ from sound? The amplitude of "normal" (not ridiculously loud) sound waves is (as I understand it) fractions of millimeters... yet it triggers a response... – GreenAsJade Feb 23 '16 at 21:50
  • @GreenAsJade - I think the difference is this: Imagine a boat tied with a short line to a floating dock. If you could imagine the water sloshing back and forth the boat would would be pulled back and forth. The flowing water is analogous to the motion of air flowing past the hairs in the ear. There would be forces on the line analogous to the bending of the hairs , and sound being heard. – John Fistere Feb 25 '16 at 07:33
  • On the other hand if there simply waves, or swells, that would be analogous to gravity waves, the boat and dock would move together, and there would be no change in tension on the line, and continuing the analogy, no sound would be heard. – John Fistere Feb 25 '16 at 07:33
  • Yes - I think this is at the heart of one aspect of this question. One difference between GW and air waves is that GW obviously bends the body too. So to be head, the gradient of displacement needs to be high enough. However, we have established that this is not determined by the wavelengh, due to transverse probagation. So that leaves the question: what is the gradient of the displacement? – GreenAsJade Feb 26 '16 at 04:09