Wikipedia:Reference desk/Archives/Science/2019 May 5

= May 5 =

Neutron star and white or black dwarf
Can a neutron star “devolve” into a white dwarf? I’m mainly interested in some mechanism like an enormous amount of energy injected into the neutron star by impact or radiation, or some other mechanism that i haven’t imagined, but also, perhaps by Hawking evaporation(if neutron stars are thought to be subject to Hawking evaporation?). Thanks for your time.Rich (talk) 17:46, 5 May 2019 (UTC)
 * Masses of neutron stars usually exceed Chandrasekhar limit. So, the answer is no. Ruslik_ Zero 20:46, 5 May 2019 (UTC)
 * Could you go into more detail on the reasonig in between “exceed the Chandrasekhar limit” and “no”? Also, how would Someguys link below affect or not affect your conclusion?Rich (talk) 04:14, 6 May 2019 (UTC)
 * If a star's mass exceeds the Chandrasekhar limit, it is incapable of being supported against collapse by electron degeneracy pressure, which is what keeps a white dwarf from collapsing. To turn a neutron star back into a white dwarf, you need to both remove mass from the star to get it below the Chandraeskhar limit, and convert its neutrons back to nuclei and electrons. Any impact will add more mass to the star, so that's not going to do it. Hawking evaporation requires an event horizon, which neutron stars don't have. --47.146.63.87 (talk) 02:48, 7 May 2019 (UTC)
 * I’m debating within myself whether you mean neutron stars are unlikely to devolve into white dwarfs, or ‘’’cannot’’’ dvolve into white dwarfs, because your answer may or may not, depending on my lack of understanding, consider the possibility of an enormous amount of energy injected int the neutron star, say by fhe neutron star and a projectile rushing towards each other at 99.999..9% of the speed of light, or maybe laser light from an unimaginably powerful laser operated by advanced human scientists in 1 billion AD.Rich (talk) 03:21, 8 May 2019 (UTC)
 * It's mentioned in, same ref I used above incidentally, that a neutron star that manages to lose enough mass to no longer remain a neutron star is expected to explode through the energy of neutron decay and subsequent nuclear reactions. Someguy1221 (talk) 21:20, 5 May 2019 (UTC)
 * This is probably similar to a type Ia supernova, which occurs when a white dwarf collides with another star. The star undergoes runaway fusion, which releases enough energy to unbind the star in a supernova. --47.146.63.87 (talk) 02:48, 7 May 2019 (UTC
 * Oh, saw this reply of yours to Someguy1221 just now- it seems you consider the devolving of a neutron star into a white dwarf, or at least the partial or complete annihilation of a neutron star, at least possible, even if unlikely?Rich (talk) 03:27, 8 May 2019 (UTC)

Sound from acoustic Guitar
If i put the sound from a (plucked) bottom string (E) of an acoustic nylon string (spanish)guitar via a microphone into a spectrum analyser, what spectral output would i see? I could do this, but dont have a microphone.--213.205.242.157 (talk) 22:49, 5 May 2019 (UTC)


 * A plucked string has a harmonic structure with all integer multiples of the fundamental. The amplitude of each harmonic does not seem to follow any exact law, but generally drops with frequency. I'd expect odd harmonics to be higher than evens, generally. There is a good spectrum analyzer for Android phones, I don't know of one for IOS. Greglocock (talk) 07:07, 6 May 2019 (UTC)
 * So what is the lowest spectral component that i would see, and why?80.2.21.124 (talk) 14:10, 6 May 2019 (UTC)


 * The lowest spectral component you would see is the fundamental frequency of that string, which if you tuned the guitar correctly in equal temperament to a reference pitch of A440, would be E2, or 82.40689 Hz. Also assuming you weren't picking up any background noise of course. As for why, my understanding is that when you pluck the string, it vibrates in a complex pattern that is basically a superimposition of all the pure sine waves of the harmonic series. For a more physics-y explanation, I would refer you to .  Bramble claw  x  16:01, 6 May 2019 (UTC)
 * If you want some of the original science on this, French polymath Marin Mersenne (the one who came up with the rule for finding prime numbers) devised a set of laws for determining the pitch of a plucked string, see Mersenne's laws. -- Jayron 32 16:20, 6 May 2019 (UTC)
 * I know about NOR, but just looked at the output from my acoustic guitar when plucking the open low e string using a spectrum analysis app on my phone.
 * I saw there was very little output at the fundamental frequency of 82 Hz. The highest level harmonic was the second at 164 Hz and this was over 25 dB above the fundamental.
 * So an acoustic guitar doesn't produce a lot of the actual note you play. It produces most output at twice its fundamental frequency.

80.2.20.124 (talk) 18:43, 6 May 2019 (UTC)
 * Any explanations?80.2.20.124 (talk) 19:42, 6 May 2019 (UTC)
 * First thing to check would be the bandpass of your phone + spectrum analysis app setup. It might well fail to capture low frequencies (because it is too small compared with the sound wavelength; same reason that headphones reproduce low-frequency sounds much better than earphones). (And beyond physical limitations, I suspect the market for free spectrum analysis phone apps is tilted towards "have a shiny user interface" rather than "give scientifically accurate results with uncertainty margins and whatnot").
 * Second would be the guitar's amplification factor via the chamber. For the same reason, I would expect to amplify much better higher-frequency harmonics since the wavelength of sound at 82Hz (about 4m) is much bigger than the guitar.
 * Finally, is it even sure that the fundamental mode (of a guitar-chord-outside-a-guitar) is supposed to contain more energy than the higher-frequency harmonics? (Sure, very high frequency harmonics must contain lower and lower energy, but there is no mathematical reason for the spectrum to be decreasing all the way.) (That is not a rhetorical question, I actually do not know.) Tigraan Click here to contact me 10:27, 7 May 2019 (UTC)
 * I checked the specs on my mobile phone, and it appears that the microphone response is very flat all the way up from 20Hz to 15kHz or so.How they do it, I dont know.213.205.242.157 (talk) 17:34, 7 May 2019 (UTC)
 * Is it possible that there is no ( or very low level) fundamental actually emitted by the body of the guitar?80.2.21.157 (talk) 14:36, 7 May 2019 (UTC)
 * I'm not sure I follow your question: The fundamental frequency is just the lowest frequency soundwave emitted for any particular musical note. For any note, it has a fundamental frequency.  It's a defining characteristic of a musical note, not an optional one.  -- Jayron 32 14:41, 7 May 2019 (UTC)
 * I mean here that there is negligible out put from the body at the fundamental resonance frequency of the open low E string. The body is only excited at twice the frequency of the open string. in fact i believe that none of notes on the guitar produce much body resonance at the string's vibration fundamental.--213.205.242.157 (talk) 17:30, 7 May 2019 (UTC)
 * Here's a book, available online at zero cost, or in print at low cost: Physical Audio Signal Processing, from the Stanford Center for Computer Research in Music and Acoustics. This book is all about the physics and human-perception of sound, and the ways we can model it.
 * Your human ear probably discerns the fundamental frequency by estimating the spacing of the harmonics, not by estimating the lowest spectral peak, nor the one with maximum power. This means that it's possible for you to "hear" a low E even if there is zero power near 82 Hz.  This is a great perceptual trick for the digital audio synthesizer engineer who has to make music play via a poor-fidelity, low-cost speaker; or for the audio compression engineer who has to figure out how to encode perceptually-identical sounds using the fewest bits.
 * The spectrum looks like a power series of sinusoidal peaks, with bandwidth around each peak. The width, shape, and time-variance of the peak is what formally-trained musicians call the Timbre or "color" of the sound.
 * Additionally, the spectrum has noise at most frequencies. Some of this noise is parasitic and detracts from the quality.  Some of the noise is a good thing that makes you perceive a positive, natural quality to the tone; purely sinusoidal signals do not sound like guitars - at best, they sound like flutes, or beeps, and so on.  You're playing a nylon guitar, and not a steel string; and so the noise and timbre - the spreading of the energy away from the peaks - will probably be relatively wider-band.  This is the normal and natural character of this instrument type.
 * Additionally, there is a reverberation, or time-varying decay. Simple reverberation is a uniform exponential decay over time; but real reverberation is complicated.  Non-idealities in the reverberation characteristic will make the relative distribution in the power spectrum change over time: the note is not only quieter, but also changes its "tone color" over time after you pluck it.  Additionally, the bandwidth of the spectral notes may widen or narrow, depending on the Q factor or "resonance" of your instrument.  Despite its name, this is a technical detail of your instrument, and not specifically a measure of the "quality" of its construction.  Expensive, artisan stringed-instruments don't necessarily have high Q - not unless their designer wanted them to sound like mass-produced electronic synthesizers.
 * If you plucked (or struck or hammer-off'ed or finger-picked or finger-muted, or any of the other elaborate skilled techniques used to play a guitar), the characteristic sound of the guitar will be convolved with the impulse of your plucking. There are a lot of details here, but this implies an abrupt high-bandwidth, impulse-like signal at the start of the note.  Conventionally, this is modeled with four numbers (for simplicity): the attack, delay, sustain, reverb parameters.  Modern and high-fidelity physical models use many more numbers, or estimate the sampled waveform; because it's hard to reduce a realisitic musical instrument to only four parameters.  This is why your nylon guitar sounds different from my nylon guitar; and both sound very different from my acoustic steel guitar, and both sound very very different from my Gibson SG; and if we're listening to any of these through an amplifier, a microphone, or any digital system, the sound is further modified by lots of signal conditioning electronics and/or software.
 * If you're really interested in musical and acoustic spectrum analysis, and you're sufficiently mathematically inclined, the second book in the series is Spectral Audio Signal Processing (also zero-cost online, or at a small nominal printing fee for paper copies). This book goes into the gory details of how to analyze the spectrum of your guitar, using the methods of Fourier transforms and the methods of the special type of calculus we use for digital systems, the infamous z-transform.
 * Nimur (talk) 14:47, 7 May 2019 (UTC)
 * I think the above post has illuminated the question very well. Ther is more to this business than the wiki pages say on acoustic stringed instrument sound production. I think they dont work like everyone thinks they work. Maybe the missing fundamental effect is in play?213.205.242.157 (talk) 17:30, 7 May 2019 (UTC)
 * I haven't read all of what Nimur wrote, but I think s/he did not mention that the sound of a guitar quite strongly depends on where you pluck the string. I don't have a spectrum analyser available (not even an acoustic guitar), and I would be grateful if somebody could check up on this. I assume 80.2.20.124 above plucked their guitar above the hole (as one would normally do), i.e. about a quarter string length away from the bridge. Try plucking halfway along the string (above the 12th fret), this results in a smoother, deeper sound. By contrast, plucking close to the bridge gives a harsh, trebly sound. This is easily heard and should be reflected in the spectrum of the sound. It has to do with the initial conditions: just before you release the finger or the plectrum, the string has a roughly triangular shape. If you pull in the middle of the string, you strongly excite the base note: half a wavelength along the string. Plucking over the hole enhances the first harmonic, which has an entire wavelength along the string. You can easily suppress the base note by lightly pressing above the 12th fret. Getting closer to the bridge moves the spectrum to higher harmonices, without suppressing the base entirely, which is why it's still recognisable. And then there's of course all the complications that Nimur described. --Wrongfilter (talk) 17:51, 7 May 2019 (UTC)
 * You might re-read my post - I specifically described that your "plucking" technique is quite important. I also linked to ... not one, but two books, with entire chapters on this math, and more math, and more math...
 * I am very happy to spread free knowledge, but there is a lot more knowledge out there beyond what I have time to summarize.
 * This is, after all, only a free encyclopedia; and this knowledge is really quite valuable.
 * I might also say that there's a lot of room for the artistic musician who appreciates these details in their own way. Whether we are mathematically-minded or not, the world really does work this way and we need the math to describe the details.  If you're specifically interested in the details of spectrum analysis (as opposed to, shall we say, the "qualia" of the music), you will find the math helps enhance the appreciation of the phenomena.  Nonetheless, it all works out for the best when we can harmonize the art and the science.
 * Nimur (talk) 17:57, 7 May 2019 (UTC)
 * My statement is based on maths: the Fourier transform of the initial shape of the string depending on where you pluck the string. --Wrongfilter (talk) 18:26, 7 May 2019 (UTC)
 * I shall try re plucking my guitar's open low e string at its mid point. Watch this space for results.80.2.21.28 (talk) 22:27, 7 May 2019 (UTC)

Curved spacetime or gravitons - or both?
What causes gravity?--213.205.242.157 (talk) 23:09, 5 May 2019 (UTC)
 * See Gravity. ←Baseball Bugs What's up, Doc? carrots→ 23:32, 5 May 2019 (UTC)
 * You can't see Gravity: it's invisible! {The poster formerly known as 87.81.230.195} 2.122.2.132 (talk) 18:06, 6 May 2019 (UTC)
 * As of right now, we have no usable theory of quantum gravity. All quantized forces operate within some kind of field; for example in quantum electrodynamics, photons are quantized waves within the electromagnetic field.  The problem with quantized gravity is that we have no good theory of quantized spacetime, since spacetime is the field gravity operates in.  That's basically what general relativity implies: the gravitational field is spacetime itself.  That's because an electric charge causes a warping of the electric field, while a mass causes a warping of spacetime which means that spacetime is the gravitational field.  See Gravitational field, to wit "The fields themselves in general relativity represent the curvature of spacetime."  Any such theory that attempts to quantize spacetime quickly produces paradoxes and results that contradict observations.  In the parlance of physics, what we say is that spacetime cannot be renormalized, but that's just a fancy mathematical way of saying that you can't quantize spacetime.  As far as we can tell, spacetime is continuous, which means that it cannot be quantized, which means there is no meaningful "quantum of gravity" or graviton, as yet.  If spacetime can be quantized, it's quantum would be the graviton.  -- Jayron 32 18:16, 6 May 2019 (UTC)
 * That's a very good answer and just the sortt of answer I was looking for. Thanks. Ron.
 * 80.2.20.124 (talk) 18:48, 6 May 2019 (UTC)