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LIGO Detects Gravitational Waves for Third Time

341 points by eaq 9y ago

wolfram74 9y ago
Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?
- eaq 9y ago
  While there are many compact binary systems in the universe continuously emitting gravitational waves for a very long time, LIGO is only able to detect the most violent waves emitted by the final coalescence and merger. So, on the astrophysical side, there is some joint probability given by how common these systems are, and how likely they are to merge in a given time frame. (Space based observatories like LISA would be able to see the long-lived inspiral waves though.) On the instrumental side, we've only just reached the sensitivity levels to make any detections in the first place, so it's not surprising that we're not getting a huge number of events (otherwise the previous generation of detectors would've seen something). In addition, each individual observatory has its own "antenna pattern", making us less less sensitive to certain sky locations. This will improve as VIRGO, KAGRA, and LIGO-India come online in the future.
  - grigjd3 9y ago
    It's not 100% accurate that "LIGO is only able to detect the most violent waves emitted" but that LIGO can only detect waves emitted in a certain frequency range (total mass) of binary mergers. For instance, LIGO pretty much cannot detect the merger of supermassive black holes.
- yodon 9y ago
  Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in a reasonable amount of time. If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity and if you're getting 1 event per century you're probably not going to be able to maintain the operating expenses to still be running when the detectable event occurs (and, as critically, none of the people involved will be able to get the data they need in the time they need it to get their PhD's, assistant profesorships, or tenured positions, so you can't get the labor force you need for your experiment to work on it, which is really what sets the acceptable duration of most experiments in practice). It looks like the original estimates were pretty good, so events are coming in at about the rate the experimenters hoped they would see them.
  - mturmon 9y ago
    Hmm...are the rates of black holes per volume well-constrained at all? I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? The truth or falsity of this would seem to have a big effect on rates.
    - privong 9y ago
      > are the rates of black holes per volume well-constrained at all? There are estimates. But two of the three LIGO detections are of black holes that are more massive than we had expected to exist (~ a few tens of solar masses). Previously we had convincing examples of black holes with <~ 10 solar masses and others with >~ 1e6 solar masses. But since we didn't have any convincing observational detections of BHs with ~20-40 solar masses, it's safe to say that the volume density is poorly constrained for that mass range. At the high end we have a reasonable estimate of the volume density, because we think all galaxies with spheroid components have a black hole and that the black hole's mass is linked to the spheroid. > I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? It depends on what you mean by "primordial". Micro-lensing experiments (when a star is brightly made brighter by the gravitational focusing of light from an object passing between us and the star), mostly looking towards the LMC/SMC [e.g., 0] have tried to address this. My recollection is that there aren't enough stellar mass black holes around to account for all of dark matter. Assuming Hawking radiation exists, low-mass primordial black holes should have evaporated by now, leaving only the more massive ones. There's a range in between the two, but I'm not sure if you can fit enough of them in a galaxy to account for dark matter while still being consistent with the sensitivity of the microlensing surveys. [0] https://en.wikipedia.org/wiki/Optical_Gravitational_Lensing_Experiment https://en.wikipedia.org/wiki/Optical_Gravitational_Lensing_... > The truth or falsity of this would seem to have a big effect on rates. Possibly. Though in order to emit GWs, pairs of black holes have to become bound to each other. If black holes make up the dark matter halos, they probably have large velocities relative to each other, which would limit their ability to form bound pairs (though it is possible with 3-body interactions). I am not aware of estimates of the BH pair-formation rate in halos _if_ DM haloes are in fact made of black holes. But the event rate probably can't be extraordinarily high, otherwise we might expect to see dark matter halos becoming less massive as the Universe ages. Though there are many confounding factors that might hide such a signal.
- elorant 9y ago
  I'm not a physicist/astronomer so someone could give you a more detailed explanation. From what I understand, LIGO has been designed to detect collisions of mid-sized black holes, and it looks like those aren't that common. There's also a more paper on this issue which you can find here: https://arxiv.org/pdf/1704.04628.pdf https://arxiv.org/pdf/1704.04628.pdf
- BurningFrog 9y ago
  These waves are extremely small, and we've just managed to build instruments sensitive enough to detect the very biggest ones. Earth bound instruments will no doubt get better, but to get a real jump in quality, you need instruments in the stillness of space: http://www.einstein-online.info/spotlights/eLISA http://www.einstein-online.info/spotlights/eLISA
  - ChuckMcM 9y ago
    The info on eLISA is great. I am just waiting for the $10,000 shielding for high end speakers to keep gravity waves from interfering with the acoustic purity of the sound they produce. :-)
    - AnimalMuppet 9y ago
      I will happily sell you such shielding for $10,000...
    - kevin_thibedeau 9y ago
      You absolutely must keep your record player suspended from glass fiber in a vacuum chamber to avoid any unwanted coloration. Just takes 30 minutes to pump down when you want to flip sides.
  - ahnitz 9y ago
    It's important to know that LISA and LIGO aren't really competing for sensitivity. Rather they complement each other by looking in different frequency ranges. The relationship between LISA and LIGO is analogous to a radio telescope and a gamma ray one. They observe different parts of the spectrum. At the frequencies that black holes merge for example, ground motion is not much of an issue, and other noise source dominate.
- InclinedPlane 9y ago
  These events are actually not rare. There are roughly billions of them in that volume over the current lifetime of the Universe. They are comparatively rarer than many other kinds of events, like supernovae, because they involve the extremely massive stars that are themselves very rare.
mturmon 9y ago
I was at a talk by Janna Levin, astrophysicist and author of a book Black Hole Blues that describes LIGO. (E.g., https://www.nytimes.com/2016/04/18/books/review-black-hole-blues-recounts-the-quest-to-find-the-cosmic-kazoo.html https://www.nytimes.com/2016/04/18/books/review-black-hole-b...) She gave a neat analogy between GWs, as sensed by LIGO, and an electric guitar. In the sense that a distant pluck on the string is transmitted as a wave down the string to the pickup, which senses a little wiggle in the string and amplifies it. I thought it was a poetic analogy that gives a second meaning to the word "instrument" in this context.
- ehsankia 9y ago
  Except imagine you're embedded on the string itself and cannot actually sense the string "moving through space". The way you have to measure it is by sensing tiny changes in distance between the left and right side of the string as it wiggles around. The actual detail of the experiment and the precision they reach is quite fascinating. Veratisium has a pretty good video explaining it in more laymen terms [0] [0] https://www.youtube.com/watch?v=iphcyNWFD10 https://www.youtube.com/watch?v=iphcyNWFD10
eaq 9y ago
The paper describing the event is available to the public at https://dcc.ligo.org/LIGO-P170104/public https://dcc.ligo.org/LIGO-P170104/public The instrument data of this event is also available to the public at https://losc.ligo.org/events/GW170104/ https://losc.ligo.org/events/GW170104/
- nonbel 9y ago
  Thanks >"GW170104 was first identified by inspection of low-latency triggers from Livingston data [15–17]. An automated notification was not generated as the Hanford detector’s calibration state was temporarily set incorrectly in the low-latency system. After it was manually determined that the calibration of both detectors was in a nominal state, an alert with an initial source localization [18,19] was distributed to collaborating astronomers [20] for the purpose of searching for a transient counterpart. About 30 groups of observers covered the parts of the sky localization using ground- and space-based instruments, spanning from γ ray to radio frequencies as well as high energy neutrinos [21]." https://dcc.ligo.org/LIGO-P170104/public https://dcc.ligo.org/LIGO-P170104/public Regarding the earlier detection: >"At 11:23:20 UTC, an analyst follow-up determined which auxiliary channels were associated with iDQ’s decision. It became clear that these were un-calibrated versions of h(t) which had not been flagged as “unsafe” and were only added to the set of available low latency channels after the start of ER8. Based on the safety of the channels, the Data Quality Veto label was removed within 2.5 hours and analyses proceeded after re- starting by hand." http://ligo.elte.hu/magazine/LIGO-magazine-issue-8.pdf http://ligo.elte.hu/magazine/LIGO-magazine-issue-8.pdf So both times humans had to take special action for the detection to "count". I really wonder about whether the null model they are using is appropriate/relevant here. Also, the other thing I have been concerned about is the lack of any corroborating evidence that these signals are truly generated by inspiraling black holes(gamma ray bursts, etc). Apparently, in this case the above-mentioned miscalibration has impeded that effort: >"The event candidate was not reported by the low-latency analysis pipelines because re-tuning the calibration of the LIGO Hanford detector is not yet complete after the holiday shutdown. This resulted in a delay of over 4 hours before the candidate could be fully examined. We are confident that this is a highly significant event candidate, but the calibration issue may be affecting the initial sky maps. We will provide an update in approximately 48 hours which may include an improved sky map." https://gcn.gsfc.nasa.gov/other/G268556.gcn3 https://gcn.gsfc.nasa.gov/other/G268556.gcn3 I can't tell from that text file whether they got corroborating evidence or not. IANAP though.
shortstuffsushi 9y ago
Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?
- lnx01 9y ago
  Black holes have mass, just like any other object in the universe - like a star, or a planet, or the sun. Consequently, they follow orbits just as any other mass would. In some cases, they're the local most massive object and any other masses move more in response. Other times they are near other black holes, and they orbit one another until they collide and merge. What makes black holes different is their density. The mass a black hole has is confined in a point of zero height, width and length called a singularity. The consequences are as we know, not fully understood by current models of physics. edit: spelling They do indeed move. It's conceivable that a black hole of mass X could be observed orbiting a red super-giant star of mass 100X. I don't think this happens much though, but the universe is big so who knows.
  - shortstuffsushi 9y ago
    I guess this was the biggest question answering piece to me. When I think of a black hole, I assume it has a gigantic mass, enough that it's always the most massive local object, and subsequently pulls in all other things. I didn't realize that might not be the case. As black holes "absorb" everything that "falls" into them, do they continue to build mass then?
    - lnx01 9y ago
      Yes. Whether or not something is a black hole is dependent on its total mass and its radius. So anything can mathematically become a black hole if you compress it enough. The earth could be a black hole if it's total mass were compressed to something like 'less than the diameter of a grapefruit'. At that point space itself cannot contain the mass, physics breaks down and you get a singularity. The moon would continue to orbit as it always did, since the moon's centre of mass is still exactly the same distance from the earth's centre of mass as it was before you pressed the 'compress button' on the north pole. It takes incredible amounts of energy to cause this compression however. And this is why only the biggest stars become black holes. As the outward pressure of fusion diminishes because the hyrogen/helium/lithium/berylium/etc fuel runs out, the sheer gravitational pull of all that mass suddenly takes over and that inward momentum from all directions is enough to cause a singularity.
gjem97 9y ago
> These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time. Astounding, especially given that these are happening at regular intervals in our "neighborhood".
- lnx01 9y ago
  It's like the difference between an explosion of TNT and a atomic bomb, but on a much larger scale. Stars like our sun spend ~10billion years turning a portion of their mass into energy. Most stars are like ours, small, dim and weak in power output. Our sun will not go supernova and will not collapse into a black hole when it dies, it will simply go nova and end up as a dwarf star in a nebula. But, now imagine two black holes each a billion times as massive as the sun turning all their mass into energy in a couple of seconds. 10billion years to convert 99% of the mass of the sun to energy versus 10 seconds to covert 2 billion times the mass of the sun to energy. Now it makes sense that the power output is more in one second than the whole universe put together. Solar fusion is on the cosmic scale a very slow way to convert mass to energy. It's so slow that we humans have been 'on the brink' of harnessing it for power generation for decades. Now imagine if we could build two nano-black-holes and let them collide....
  - onebigbug 9y ago
    > But, now imagine two black holes each a billion times as massive as the sun turning all their mass into energy in a couple of seconds. Sure, lemme just take off my "good at socializing with apes and running for long periods of time after antelope" hat and put on my "Cosmological scale" hat. Huh, I seem to have misplaced that one. And the one I'm currently wearing is oddly well affixed.
  - danbruc 9y ago
    I think your numbers are a bit off. I am not sure about the exact numbers but I think the sun will only burn about half or so of its mass over its lifetime. Also the black holes we observed merging are stellar black holes with masses on the order of tens of solar masses, not galactic black holes with millions or billions of solar masses.
cletus 9y ago
The numbers here are just staggering: - Black hole merger occurred 3 billion light years away - Two solar masses were converted to energy - Briefly 10^34 megatons of energy were released every second This is hard to intuitively wrap your head around because we think of space as constant. Something like this can distort space itself. Amazing stuff.
- kstrauser 9y ago
  > - Briefly 10^34 megatons of energy were released every second That quote caught my eye too. What's the full unit on that? Is that literally the "m" you'd plug into E=mc^2, or was there an elided "...of TNT", like we'd use to describe nuclear weapons?
  - Cerium 9y ago
    It appears it is "... of TNT". Wolfram Alpha converts "10^56 ergs to imperial megatons" to a value of: 1.095×10^23 long megatons (using E = mc^2). While it converts "10^56 ergs to megatons" to a value of "2.39×10^33 megatons of TNT" which is much more like that number. Peak luminosity from https://losc.ligo.org/events/GW170104/ https://losc.ligo.org/events/GW170104/ Query: https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+megatons https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+megato... Query: https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+imperial+megatons https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+imperi...
  - deleted 9y ago
    [deleted]
  - Florin_Andrei 9y ago
    It must be TNT equivalent. One solar mass is 1.99 × 10^30 kilograms, and we know that 2 solar masses were converted in total, so 4 x 10^30 kilograms, which is far less than the "megatons" mentioned, in terms of pure mass. I wish folks would avoid mixing military units and general relativity units like this, it's confusing.
    - nategri 9y ago
nurettin 9y ago
How does LIGO separate vibrations caused by a nearby truck from whatever reading that is required for gravitational waves?
- fogleman 9y ago
  Well, for one, there are two detectors, one in Louisiana and another in Washington.
- CodeCube 9y ago
  This is a complete guess, as I don't have any firsthand knowledge ... but I can only imagine that they have a whole array of seismographs on the premises, which they can use to clear noise from the main LIGO readings they are interested in. Curious to know if this is the case from anyone who happens to know one way or the other :) edit: the more I think about it, the more I think that random vibrations from passing trucks would be irrelevant ... it doesn't detect vibrations, it's measuring the speed of light between two points
  - netcraft 9y ago
    vibrations from traffic, seismic events and even thunderstorms can be detected. If the mirrors or lasers move, it makes a difference. The sensitivity of these instruments are such that its akin to measuring the difference in the width of a human hair in the distance between here and alpha centauri. On the actual scale theyre measuring the slightest movement of the a fraction of a width of a proton.
  - lnx01 9y ago
    Actually, I think it's measuring the intensity of light between two points. It has two perpendicular lasers of some wavelength which then interfere with one another. If they interfere perfectly you measure a zero, if the interference is off by some amount you measure a deviation in intensity from the i^ or the j^ direction. You can eliminate passing trucks, earth tremors, mining, asteroid impacts etc simply by applying a band-pass filter that excludes measurement frequencies outside of the range predicted by the equations. No expert though, just guessing.
netcraft 9y ago
Great veritasium video about this latest wave: https://www.youtube.com/watch?v=NVKO7UCIlgs https://www.youtube.com/watch?v=NVKO7UCIlgs What is involved with increasing sensitivity I wonder? Is it purely lengthening the arms? or are there other advancements required? Hopefully one day we can have these things in space, isolated from noise and curvature of the earth and no need for vacuum equipment.
- russdill 9y ago
  Secret signals? What secret signals?
- grigjd3 9y ago
  Sensitivity is about picking out a signal from the noise. The way you increase sensitivity is to decrease the noise. Effectively this means isolating the environment of the LIGO experiment and a great deal has been done on this. If you search arXiv for LIGO over the past two decades, you'll find plenty of articles on this, but I warn you, you may be reading about injecting null energy modes into the system in one paper while reading about mirror design and reflection results in the next paper.
- greeneggs 9y ago
  From that video: "If we improve our detector sensitivity, by say a factor of two or three, the rates will go up from, you know, seeing one every month or every two months, to seeing one every day or every week." - David Reitze, Executive director of LIGO I don't know what their uptime is, but it sounds like they probably have a number of as-yet unreported observed events.
- sleavey 9y ago
  There are loads of possible ways to increase sensitivity, but none of them are easy or cheap given that the low hanging fruit was all picked off in previous generation detectors. Increasing arm length is the "easiest" but definitely the most expensive option. Try finding a 40km L-shaped area that's seismically stable and free from significant anthropogenic activity. There may only be a handful of places in North America. However, 4km is already on the cusp of being long enough that gravity misaligns the two mirrors are each end of each arm due to the curvature of the Earth. Going to 40km would prompt the need for static corrections to mirror alignment, which will increase the amount of seismic noise that couples into the longitudinal direction in which gravitational waves are sensed. There are other problems such as the need to either refocus light at points along the arms (very susceptible to alignment and thermal noise) or use much, much bigger mirrors. The Advanced LIGO mirrors are already ~40kg, ~30 x 15cm cylinders of the purest fused silica known to man circa ~2012. There is talk of increasing the mirrors to 200kg and ~50 x 25 cm, and no facility is currently capable of producing pure enough fused silica at that size. An "easier" option is to increase the laser power. This gives diminishing returns, and leads to an increase in high frequency sensitivity at the expense of low frequency sensitivity (due to photon pressure pushing the mirrors around noisily). However, the challenges are to make stable lasers that are also powerful - very tricky - and to mitigate the effect that laser absorption has on the mirrors within the interferometer - as you increase laser power, things heat up. Hot mirrors can lens the light, misaligning it and creating extra loss (i.e. reducing sensitivity). It's trickly to mitigate. Another effect of higher laser power is the introduction of parametric instabilities, where the mechanical body modes of the mirrors are amplified by the high laser power, leading to huge spikes of noise at narrow frequencies which are difficult to damp out. Another is to use a different interferometer topology: instead of an L-shaped Michelson interferometer, suggestions have been made for Sagnac interferometers which possess an interesting property called quantum non-demolition, which can potentially reduce the limiting noise source in Advanced LIGO which directly increases sensitivity. Research into this is at a very early stage and will not be seen in detector facilities for decades, if ever. So, the short answer is: there are lots of potential methods to increase sensitivity, but all of them are challenging and require significant R&D and money.
mudil 9y ago
Here's what I don't understand. In the first detection, they mentioned that two black holes collapsed, emitted gravitational waves, and the resulting combined mass was less than then sum of two previous masses because energy was spent on gravitational wave generation. Hence it means, that due to gravitational interactions, objects leak mass. Now, we know that every object in the universe is gravitationally related to every other object, plus universe is expanding hence objects are constantly in flux with each other. The question is where all the leaked mass goes? Can this leakage account for dark matter? What about the space-time, does it function as a storage medium for this energy that now came from the leaked mass? Please explain...
- iaw 9y ago
  E = mc^2 "Mass" in a blackhole is not the same as mass here on earth. The likely answer to your question is that the mass was converted to energy in the form of gravitational waves.
- lnx01 9y ago
  Objects do no leak mass due to gravitational interaction, but gravitational interaction always expends some energy. The earth orbiting the sun expends some energy in the form of gravitational waves but it's miniscule. It is enough though that the earth's orbit decays measurably. This experiment is designed to detect the waves carrying away that lost energy, but you need a cataclysmic event like the collision of two black holes for the event to be energetic enough that you can measure it and measure it on astronomical scales. This is the reason why LIGO must be so sensitive, we hope not to experience a nearby black hole collision for as long as we're alive, so we must measure a distant one. Mass and energy are interchangeable but they both must be conserved. They didn't leak mass, the missing mass became energy in the form of emission (photons and some kinetic) and gravitational waves, which until recently we could not detect. The equations suggested they were there though, and that's why these experiments were funded, a way to find out if the uncertainties in the standard model were true or not. At this point, our model seems to predict what we track in reality with this experiment. Of course, there are gaps in the standard model and they must all be tested. LHC is also looking at the gaps, and confirming/invalidating them.
- 9y ago
smortaz 9y ago
FYI - if you want to check out the data, the code, even an audio of the wave checkout: https://notebooks.azure.com/roywilliams/libraries/LIGOOpenScienceCenter https://notebooks.azure.com/roywilliams/libraries/LIGOOpenSc... It's a Jupyter notebook that anyone can clone and run. [edit: updated link]
- japhyr 9y ago
  It's just awesome to see these notebooks released. I'm not going to play with them, but I love that people are able to. It makes me much more confident in the results that are announced, and I hope this approach to doing science becomes the norm.
  - jxramos 9y ago
    I second the wider adoption of notebooks. I'd really love to see government budgeting offices and what not begin to make their spending and analyses transparent through these documents. We need government on github and jupyter notebooks :D
castis 9y ago
When a gravitational wave hits the earth, does the planet oscillate in place for the duration, or is our position in the cosmos displaced, or something else altogether?
- marcosdumay 9y ago
  The planet gets shorter, then longer, then shorter again... All that in a single direction, while the size on the other two directions stays the same.
  - aroberge 9y ago
    That is incorrect. What you describe is a dipolar oscillation. The simplest gravitational waves woudl have a quadrupole moment: stretch in one direction while contracting in the direction perpendicular to it. Have a look at https://en.wikipedia.org/wiki/Gravitational_wave https://en.wikipedia.org/wiki/Gravitational_wave where there is an animation illustrating the simplest case.
    - marcosdumay 9y ago
      Right. There are no dipolar gravity oscillation because of conservation of momentum, thus the waves must be quadrupolar. Didn't think about that before.
- dwaltrip 9y ago
  The actual space in which the planet resides stretches and shrinks as the gravitational wave passes through it. The fabric of space itself is the medium that the wave travels through. However, the affect is incredibly tiny, even though it was generated by two black holes colliding. The size of the distortion experienced here on Earth is 1000x smaller than the width of a proton! It's mind boggling. I think I remember hearing that there is immense distortion in the area immediately around the collision, but I'm not certain.
  - asmithmd1 9y ago
shawkinaw 9y ago
This is awesome. I spent a summer in high school at LIGO Hanford (Washington), it's so cool to see positive results starting to come out of it.
nsxwolf 9y ago
Does the calculation of how long ago this event occurred account for the speed at which the universe is expanding? Does it need to?
- ahnitz 9y ago
  We do account for the expansion of the universe in fact. We estimated that this source was at about z ~ 0.2 (see https://en.wikipedia.org/wiki/Redshift https://en.wikipedia.org/wiki/Redshift). Roughly speaking this means there'll be only ~20% effect as the scale of the universe (a) has increased by (1+z) over the time it has traveled.

341 points by eaq 9y ago

Anyone familiar with this branch of astronomy want to explain why one detection in a volume on the order of 27 billion cubic light years is reasonable? Are they still processing data and will find more events? Is the sensitivity highly anisotropic so the detection volume is significantly smaller? Or are events like this just really conveniently rare that we get about 1 every data gathering interval?

eaq 9y ago
While there are many compact binary systems in the universe continuously emitting gravitational waves for a very long time, LIGO is only able to detect the most violent waves emitted by the final coalescence and merger. So, on the astrophysical side, there is some joint probability given by how common these systems are, and how likely they are to merge in a given time frame. (Space based observatories like LISA would be able to see the long-lived inspiral waves though.) On the instrumental side, we've only just reached the sensitivity levels to make any detections in the first place, so it's not surprising that we're not getting a huge number of events (otherwise the previous generation of detectors would've seen something). In addition, each individual observatory has its own "antenna pattern", making us less less sensitive to certain sky locations. This will improve as VIRGO, KAGRA, and LIGO-India come online in the future.
- grigjd3 9y ago
  It's not 100% accurate that "LIGO is only able to detect the most violent waves emitted" but that LIGO can only detect waves emitted in a certain frequency range (total mass) of binary mergers. For instance, LIGO pretty much cannot detect the merger of supermassive black holes.
yodon 9y ago
Events big enough to be detected are quite rare, but the phenomenon you are asking about is more a financial reality than a "convenient coincidence" as you put it. There is a pretty steep curve connecting sensitivity and cost, so when the team that built LIGO was designing it, they used the best available models of colliding black hole event rates to estimate the sensitivity required to deliver a conclusive result in a reasonable amount of time. If you're getting 10 events/second with a device like this, you probably overpaid for sensitivity and if you're getting 1 event per century you're probably not going to be able to maintain the operating expenses to still be running when the detectable event occurs (and, as critically, none of the people involved will be able to get the data they need in the time they need it to get their PhD's, assistant profesorships, or tenured positions, so you can't get the labor force you need for your experiment to work on it, which is really what sets the acceptable duration of most experiments in practice). It looks like the original estimates were pretty good, so events are coming in at about the rate the experimenters hoped they would see them.
- mturmon 9y ago
  Hmm...are the rates of black holes per volume well-constrained at all? I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? The truth or falsity of this would seem to have a big effect on rates.
  - privong 9y ago
    > are the rates of black holes per volume well-constrained at all? There are estimates. But two of the three LIGO detections are of black holes that are more massive than we had expected to exist (~ a few tens of solar masses). Previously we had convincing examples of black holes with <~ 10 solar masses and others with >~ 1e6 solar masses. But since we didn't have any convincing observational detections of BHs with ~20-40 solar masses, it's safe to say that the volume density is poorly constrained for that mass range. At the high end we have a reasonable estimate of the volume density, because we think all galaxies with spheroid components have a black hole and that the black hole's mass is linked to the spheroid. > I was under the impression that it's a possibility that dark matter consists prominently of primordial BHs? It depends on what you mean by "primordial". Micro-lensing experiments (when a star is brightly made brighter by the gravitational focusing of light from an object passing between us and the star), mostly looking towards the LMC/SMC [e.g., 0] have tried to address this. My recollection is that there aren't enough stellar mass black holes around to account for all of dark matter. Assuming Hawking radiation exists, low-mass primordial black holes should have evaporated by now, leaving only the more massive ones. There's a range in between the two, but I'm not sure if you can fit enough of them in a galaxy to account for dark matter while still being consistent with the sensitivity of the microlensing surveys. [0] https://en.wikipedia.org/wiki/Optical_Gravitational_Lensing_Experiment https://en.wikipedia.org/wiki/Optical_Gravitational_Lensing_... > The truth or falsity of this would seem to have a big effect on rates. Possibly. Though in order to emit GWs, pairs of black holes have to become bound to each other. If black holes make up the dark matter halos, they probably have large velocities relative to each other, which would limit their ability to form bound pairs (though it is possible with 3-body interactions). I am not aware of estimates of the BH pair-formation rate in halos _if_ DM haloes are in fact made of black holes. But the event rate probably can't be extraordinarily high, otherwise we might expect to see dark matter halos becoming less massive as the Universe ages. Though there are many confounding factors that might hide such a signal.
elorant 9y ago
I'm not a physicist/astronomer so someone could give you a more detailed explanation. From what I understand, LIGO has been designed to detect collisions of mid-sized black holes, and it looks like those aren't that common. There's also a more paper on this issue which you can find here: https://arxiv.org/pdf/1704.04628.pdf https://arxiv.org/pdf/1704.04628.pdf
BurningFrog 9y ago
These waves are extremely small, and we've just managed to build instruments sensitive enough to detect the very biggest ones. Earth bound instruments will no doubt get better, but to get a real jump in quality, you need instruments in the stillness of space: http://www.einstein-online.info/spotlights/eLISA http://www.einstein-online.info/spotlights/eLISA
- ChuckMcM 9y ago
  The info on eLISA is great. I am just waiting for the $10,000 shielding for high end speakers to keep gravity waves from interfering with the acoustic purity of the sound they produce. :-)
  - AnimalMuppet 9y ago
    I will happily sell you such shielding for $10,000...
  - kevin_thibedeau 9y ago
    You absolutely must keep your record player suspended from glass fiber in a vacuum chamber to avoid any unwanted coloration. Just takes 30 minutes to pump down when you want to flip sides.
- ahnitz 9y ago
  It's important to know that LISA and LIGO aren't really competing for sensitivity. Rather they complement each other by looking in different frequency ranges. The relationship between LISA and LIGO is analogous to a radio telescope and a gamma ray one. They observe different parts of the spectrum. At the frequencies that black holes merge for example, ground motion is not much of an issue, and other noise source dominate.
InclinedPlane 9y ago
These events are actually not rare. There are roughly billions of them in that volume over the current lifetime of the Universe. They are comparatively rarer than many other kinds of events, like supernovae, because they involve the extremely massive stars that are themselves very rare.

mturmon 9y ago

I was at a talk by Janna Levin, astrophysicist and author of a book Black Hole Blues that describes LIGO. (E.g., https://www.nytimes.com/2016/04/18/books/review-black-hole-blues-recounts-the-quest-to-find-the-cosmic-kazoo.html https://www.nytimes.com/2016/04/18/books/review-black-hole-b...) She gave a neat analogy between GWs, as sensed by LIGO, and an electric guitar. In the sense that a distant pluck on the string is transmitted as a wave down the string to the pickup, which senses a little wiggle in the string and amplifies it. I thought it was a poetic analogy that gives a second meaning to the word "instrument" in this context.

ehsankia 9y ago
Except imagine you're embedded on the string itself and cannot actually sense the string "moving through space". The way you have to measure it is by sensing tiny changes in distance between the left and right side of the string as it wiggles around. The actual detail of the experiment and the precision they reach is quite fascinating. Veratisium has a pretty good video explaining it in more laymen terms [0] [0] https://www.youtube.com/watch?v=iphcyNWFD10 https://www.youtube.com/watch?v=iphcyNWFD10

eaq 9y ago

The paper describing the event is available to the public at https://dcc.ligo.org/LIGO-P170104/public https://dcc.ligo.org/LIGO-P170104/public The instrument data of this event is also available to the public at https://losc.ligo.org/events/GW170104/ https://losc.ligo.org/events/GW170104/

nonbel 9y ago
Thanks >"GW170104 was first identified by inspection of low-latency triggers from Livingston data [15–17]. An automated notification was not generated as the Hanford detector’s calibration state was temporarily set incorrectly in the low-latency system. After it was manually determined that the calibration of both detectors was in a nominal state, an alert with an initial source localization [18,19] was distributed to collaborating astronomers [20] for the purpose of searching for a transient counterpart. About 30 groups of observers covered the parts of the sky localization using ground- and space-based instruments, spanning from γ ray to radio frequencies as well as high energy neutrinos [21]." https://dcc.ligo.org/LIGO-P170104/public https://dcc.ligo.org/LIGO-P170104/public Regarding the earlier detection: >"At 11:23:20 UTC, an analyst follow-up determined which auxiliary channels were associated with iDQ’s decision. It became clear that these were un-calibrated versions of h(t) which had not been flagged as “unsafe” and were only added to the set of available low latency channels after the start of ER8. Based on the safety of the channels, the Data Quality Veto label was removed within 2.5 hours and analyses proceeded after re- starting by hand." http://ligo.elte.hu/magazine/LIGO-magazine-issue-8.pdf http://ligo.elte.hu/magazine/LIGO-magazine-issue-8.pdf So both times humans had to take special action for the detection to "count". I really wonder about whether the null model they are using is appropriate/relevant here. Also, the other thing I have been concerned about is the lack of any corroborating evidence that these signals are truly generated by inspiraling black holes(gamma ray bursts, etc). Apparently, in this case the above-mentioned miscalibration has impeded that effort: >"The event candidate was not reported by the low-latency analysis pipelines because re-tuning the calibration of the LIGO Hanford detector is not yet complete after the holiday shutdown. This resulted in a delay of over 4 hours before the candidate could be fully examined. We are confident that this is a highly significant event candidate, but the calibration issue may be affecting the initial sky maps. We will provide an update in approximately 48 hours which may include an improved sky map." https://gcn.gsfc.nasa.gov/other/G268556.gcn3 https://gcn.gsfc.nasa.gov/other/G268556.gcn3 I can't tell from that text file whether they got corroborating evidence or not. IANAP though.

shortstuffsushi 9y ago

Somewhat naive questions, as I know very little about astronomy. Do black holes "move?" How is it that they could merge if they're stationary, unless they're pulling each other in I guess? If black holes are indeed pulling in everything, does that mean the whole universe would eventually be one giant black hole?

lnx01 9y ago
Black holes have mass, just like any other object in the universe - like a star, or a planet, or the sun. Consequently, they follow orbits just as any other mass would. In some cases, they're the local most massive object and any other masses move more in response. Other times they are near other black holes, and they orbit one another until they collide and merge. What makes black holes different is their density. The mass a black hole has is confined in a point of zero height, width and length called a singularity. The consequences are as we know, not fully understood by current models of physics. edit: spelling They do indeed move. It's conceivable that a black hole of mass X could be observed orbiting a red super-giant star of mass 100X. I don't think this happens much though, but the universe is big so who knows.
- shortstuffsushi 9y ago
  I guess this was the biggest question answering piece to me. When I think of a black hole, I assume it has a gigantic mass, enough that it's always the most massive local object, and subsequently pulls in all other things. I didn't realize that might not be the case. As black holes "absorb" everything that "falls" into them, do they continue to build mass then?
  - lnx01 9y ago
    Yes. Whether or not something is a black hole is dependent on its total mass and its radius. So anything can mathematically become a black hole if you compress it enough. The earth could be a black hole if it's total mass were compressed to something like 'less than the diameter of a grapefruit'. At that point space itself cannot contain the mass, physics breaks down and you get a singularity. The moon would continue to orbit as it always did, since the moon's centre of mass is still exactly the same distance from the earth's centre of mass as it was before you pressed the 'compress button' on the north pole. It takes incredible amounts of energy to cause this compression however. And this is why only the biggest stars become black holes. As the outward pressure of fusion diminishes because the hyrogen/helium/lithium/berylium/etc fuel runs out, the sheer gravitational pull of all that mass suddenly takes over and that inward momentum from all directions is enough to cause a singularity.

gjem97 9y ago

> These are collisions that produce more power than is radiated as light by all the stars and galaxies in the universe at any given time. Astounding, especially given that these are happening at regular intervals in our "neighborhood".

lnx01 9y ago
It's like the difference between an explosion of TNT and a atomic bomb, but on a much larger scale. Stars like our sun spend ~10billion years turning a portion of their mass into energy. Most stars are like ours, small, dim and weak in power output. Our sun will not go supernova and will not collapse into a black hole when it dies, it will simply go nova and end up as a dwarf star in a nebula. But, now imagine two black holes each a billion times as massive as the sun turning all their mass into energy in a couple of seconds. 10billion years to convert 99% of the mass of the sun to energy versus 10 seconds to covert 2 billion times the mass of the sun to energy. Now it makes sense that the power output is more in one second than the whole universe put together. Solar fusion is on the cosmic scale a very slow way to convert mass to energy. It's so slow that we humans have been 'on the brink' of harnessing it for power generation for decades. Now imagine if we could build two nano-black-holes and let them collide....
- onebigbug 9y ago
  > But, now imagine two black holes each a billion times as massive as the sun turning all their mass into energy in a couple of seconds. Sure, lemme just take off my "good at socializing with apes and running for long periods of time after antelope" hat and put on my "Cosmological scale" hat. Huh, I seem to have misplaced that one. And the one I'm currently wearing is oddly well affixed.
- danbruc 9y ago
  I think your numbers are a bit off. I am not sure about the exact numbers but I think the sun will only burn about half or so of its mass over its lifetime. Also the black holes we observed merging are stellar black holes with masses on the order of tens of solar masses, not galactic black holes with millions or billions of solar masses.

cletus 9y ago

The numbers here are just staggering: - Black hole merger occurred 3 billion light years away - Two solar masses were converted to energy - Briefly 10^34 megatons of energy were released every second This is hard to intuitively wrap your head around because we think of space as constant. Something like this can distort space itself. Amazing stuff.

kstrauser 9y ago
> - Briefly 10^34 megatons of energy were released every second That quote caught my eye too. What's the full unit on that? Is that literally the "m" you'd plug into E=mc^2, or was there an elided "...of TNT", like we'd use to describe nuclear weapons?
- Cerium 9y ago
  It appears it is "... of TNT". Wolfram Alpha converts "10^56 ergs to imperial megatons" to a value of: 1.095×10^23 long megatons (using E = mc^2). While it converts "10^56 ergs to megatons" to a value of "2.39×10^33 megatons of TNT" which is much more like that number. Peak luminosity from https://losc.ligo.org/events/GW170104/ https://losc.ligo.org/events/GW170104/ Query: https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+megatons https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+megato... Query: https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+imperial+megatons https://www.wolframalpha.com/input/?i=10%5E56+ergs+to+imperi...
- deleted 9y ago
  [deleted]
- Florin_Andrei 9y ago
  It must be TNT equivalent. One solar mass is 1.99 × 10^30 kilograms, and we know that 2 solar masses were converted in total, so 4 x 10^30 kilograms, which is far less than the "megatons" mentioned, in terms of pure mass. I wish folks would avoid mixing military units and general relativity units like this, it's confusing.
  - nategri 9y ago

nurettin 9y ago

How does LIGO separate vibrations caused by a nearby truck from whatever reading that is required for gravitational waves?

fogleman 9y ago
Well, for one, there are two detectors, one in Louisiana and another in Washington.
CodeCube 9y ago
This is a complete guess, as I don't have any firsthand knowledge ... but I can only imagine that they have a whole array of seismographs on the premises, which they can use to clear noise from the main LIGO readings they are interested in. Curious to know if this is the case from anyone who happens to know one way or the other :) edit: the more I think about it, the more I think that random vibrations from passing trucks would be irrelevant ... it doesn't detect vibrations, it's measuring the speed of light between two points
- netcraft 9y ago
  vibrations from traffic, seismic events and even thunderstorms can be detected. If the mirrors or lasers move, it makes a difference. The sensitivity of these instruments are such that its akin to measuring the difference in the width of a human hair in the distance between here and alpha centauri. On the actual scale theyre measuring the slightest movement of the a fraction of a width of a proton.
- lnx01 9y ago
  Actually, I think it's measuring the intensity of light between two points. It has two perpendicular lasers of some wavelength which then interfere with one another. If they interfere perfectly you measure a zero, if the interference is off by some amount you measure a deviation in intensity from the i^ or the j^ direction. You can eliminate passing trucks, earth tremors, mining, asteroid impacts etc simply by applying a band-pass filter that excludes measurement frequencies outside of the range predicted by the equations. No expert though, just guessing.

netcraft 9y ago

Great veritasium video about this latest wave: https://www.youtube.com/watch?v=NVKO7UCIlgs https://www.youtube.com/watch?v=NVKO7UCIlgs What is involved with increasing sensitivity I wonder? Is it purely lengthening the arms? or are there other advancements required? Hopefully one day we can have these things in space, isolated from noise and curvature of the earth and no need for vacuum equipment.

russdill 9y ago
Secret signals? What secret signals?
grigjd3 9y ago
Sensitivity is about picking out a signal from the noise. The way you increase sensitivity is to decrease the noise. Effectively this means isolating the environment of the LIGO experiment and a great deal has been done on this. If you search arXiv for LIGO over the past two decades, you'll find plenty of articles on this, but I warn you, you may be reading about injecting null energy modes into the system in one paper while reading about mirror design and reflection results in the next paper.
greeneggs 9y ago
From that video: "If we improve our detector sensitivity, by say a factor of two or three, the rates will go up from, you know, seeing one every month or every two months, to seeing one every day or every week." - David Reitze, Executive director of LIGO I don't know what their uptime is, but it sounds like they probably have a number of as-yet unreported observed events.
sleavey 9y ago
There are loads of possible ways to increase sensitivity, but none of them are easy or cheap given that the low hanging fruit was all picked off in previous generation detectors. Increasing arm length is the "easiest" but definitely the most expensive option. Try finding a 40km L-shaped area that's seismically stable and free from significant anthropogenic activity. There may only be a handful of places in North America. However, 4km is already on the cusp of being long enough that gravity misaligns the two mirrors are each end of each arm due to the curvature of the Earth. Going to 40km would prompt the need for static corrections to mirror alignment, which will increase the amount of seismic noise that couples into the longitudinal direction in which gravitational waves are sensed. There are other problems such as the need to either refocus light at points along the arms (very susceptible to alignment and thermal noise) or use much, much bigger mirrors. The Advanced LIGO mirrors are already ~40kg, ~30 x 15cm cylinders of the purest fused silica known to man circa ~2012. There is talk of increasing the mirrors to 200kg and ~50 x 25 cm, and no facility is currently capable of producing pure enough fused silica at that size. An "easier" option is to increase the laser power. This gives diminishing returns, and leads to an increase in high frequency sensitivity at the expense of low frequency sensitivity (due to photon pressure pushing the mirrors around noisily). However, the challenges are to make stable lasers that are also powerful - very tricky - and to mitigate the effect that laser absorption has on the mirrors within the interferometer - as you increase laser power, things heat up. Hot mirrors can lens the light, misaligning it and creating extra loss (i.e. reducing sensitivity). It's trickly to mitigate. Another effect of higher laser power is the introduction of parametric instabilities, where the mechanical body modes of the mirrors are amplified by the high laser power, leading to huge spikes of noise at narrow frequencies which are difficult to damp out. Another is to use a different interferometer topology: instead of an L-shaped Michelson interferometer, suggestions have been made for Sagnac interferometers which possess an interesting property called quantum non-demolition, which can potentially reduce the limiting noise source in Advanced LIGO which directly increases sensitivity. Research into this is at a very early stage and will not be seen in detector facilities for decades, if ever. So, the short answer is: there are lots of potential methods to increase sensitivity, but all of them are challenging and require significant R&D and money.

mudil 9y ago

Here's what I don't understand. In the first detection, they mentioned that two black holes collapsed, emitted gravitational waves, and the resulting combined mass was less than then sum of two previous masses because energy was spent on gravitational wave generation. Hence it means, that due to gravitational interactions, objects leak mass. Now, we know that every object in the universe is gravitationally related to every other object, plus universe is expanding hence objects are constantly in flux with each other. The question is where all the leaked mass goes? Can this leakage account for dark matter? What about the space-time, does it function as a storage medium for this energy that now came from the leaked mass? Please explain...

iaw 9y ago
E = mc^2 "Mass" in a blackhole is not the same as mass here on earth. The likely answer to your question is that the mass was converted to energy in the form of gravitational waves.
lnx01 9y ago
Objects do no leak mass due to gravitational interaction, but gravitational interaction always expends some energy. The earth orbiting the sun expends some energy in the form of gravitational waves but it's miniscule. It is enough though that the earth's orbit decays measurably. This experiment is designed to detect the waves carrying away that lost energy, but you need a cataclysmic event like the collision of two black holes for the event to be energetic enough that you can measure it and measure it on astronomical scales. This is the reason why LIGO must be so sensitive, we hope not to experience a nearby black hole collision for as long as we're alive, so we must measure a distant one. Mass and energy are interchangeable but they both must be conserved. They didn't leak mass, the missing mass became energy in the form of emission (photons and some kinetic) and gravitational waves, which until recently we could not detect. The equations suggested they were there though, and that's why these experiments were funded, a way to find out if the uncertainties in the standard model were true or not. At this point, our model seems to predict what we track in reality with this experiment. Of course, there are gaps in the standard model and they must all be tested. LHC is also looking at the gaps, and confirming/invalidating them.
9y ago

smortaz 9y ago

FYI - if you want to check out the data, the code, even an audio of the wave checkout: https://notebooks.azure.com/roywilliams/libraries/LIGOOpenScienceCenter https://notebooks.azure.com/roywilliams/libraries/LIGOOpenSc... It's a Jupyter notebook that anyone can clone and run. [edit: updated link]

japhyr 9y ago
It's just awesome to see these notebooks released. I'm not going to play with them, but I love that people are able to. It makes me much more confident in the results that are announced, and I hope this approach to doing science becomes the norm.
- jxramos 9y ago
  I second the wider adoption of notebooks. I'd really love to see government budgeting offices and what not begin to make their spending and analyses transparent through these documents. We need government on github and jupyter notebooks :D

castis 9y ago

When a gravitational wave hits the earth, does the planet oscillate in place for the duration, or is our position in the cosmos displaced, or something else altogether?

marcosdumay 9y ago
The planet gets shorter, then longer, then shorter again... All that in a single direction, while the size on the other two directions stays the same.
- aroberge 9y ago
  That is incorrect. What you describe is a dipolar oscillation. The simplest gravitational waves woudl have a quadrupole moment: stretch in one direction while contracting in the direction perpendicular to it. Have a look at https://en.wikipedia.org/wiki/Gravitational_wave https://en.wikipedia.org/wiki/Gravitational_wave where there is an animation illustrating the simplest case.
  - marcosdumay 9y ago
    Right. There are no dipolar gravity oscillation because of conservation of momentum, thus the waves must be quadrupolar. Didn't think about that before.
dwaltrip 9y ago
The actual space in which the planet resides stretches and shrinks as the gravitational wave passes through it. The fabric of space itself is the medium that the wave travels through. However, the affect is incredibly tiny, even though it was generated by two black holes colliding. The size of the distortion experienced here on Earth is 1000x smaller than the width of a proton! It's mind boggling. I think I remember hearing that there is immense distortion in the area immediately around the collision, but I'm not certain.
- asmithmd1 9y ago

shawkinaw 9y ago

This is awesome. I spent a summer in high school at LIGO Hanford (Washington), it's so cool to see positive results starting to come out of it.

nsxwolf 9y ago

Does the calculation of how long ago this event occurred account for the speed at which the universe is expanding? Does it need to?

ahnitz 9y ago
We do account for the expansion of the universe in fact. We estimated that this source was at about z ~ 0.2 (see https://en.wikipedia.org/wiki/Redshift https://en.wikipedia.org/wiki/Redshift). Roughly speaking this means there'll be only ~20% effect as the scale of the universe (a) has increased by (1+z) over the time it has traveled.