[HN Gopher] Astrophysicists unveil glut of gravitational-wave de...
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Astrophysicists unveil glut of gravitational-wave detections
Author : gmays
Score : 137 points
Date : 2021-11-10 14:58 UTC (8 hours ago)
(HTM) web link (www.nature.com)
(TXT) w3m dump (www.nature.com)
| toss1 wrote:
| The resolution of these instruments is astonishing - and
| literally a new window on the universe.
|
| Obviously, being able to detect amplitudes so small is key to
| this whole project, as the sources are so distant (and presumably
| the inverse square law applies).
|
| This makes me wonder how these phenomena would appear much closer
| to the events - how close would we need to be to perceive with
| our senses the passing of a gravitational wave, and what would it
| look like? I'm guessing some kind of passing tidal forces would
| be felt -- has anyone done modeling to figure out what that might
| be like?
|
| How close and how much amplitude (or would frequency be the
| killer?) would be required to start damaging ordinary material
| objects? Is it so close to the source that you're already doomed
| in the black hole's grip anyway, or would an event at the center
| of our galaxy be perceptible here? Would the waves rip apart
| nearby stars (for what value of nearby), or be noticeable in
| their spectra as some kind of ripple? It'd be cool to get some
| kind of a sense of the scale of these events' affected zone.
| markus_zhang wrote:
| Just curious, if we have gravitational waves, what are the wave
| lengths?
| ww520 wrote:
| That depends the period of the body's movement. The wave is
| generated by the body's movement.
| markus_zhang wrote:
| OK I probably got some wild thoughts. I was thinking, since
| light has different wave lengths and human can't see many
| types of light because of that, maybe dark matter is similar?
|
| Just some uneducated thoughts lol.
| ww520 wrote:
| But it's a very good question. It leads to what's the
| nature of gravitational wave. The frequency is certainly a
| key characteristic. It can lead to how LIGO works.
| bil7 wrote:
| radios or other technology can be used to detect EM
| wavelengths humans can't. No tech can currently observe
| dark matter
| queuebert wrote:
| The orbital period of the merging objects is ~ 1 ms near the
| end, and gravitational waves propagate at the speed of light,
| so I'd estimate that the wavelength would be ~ 3e5 x 1e-3, or
| in the neighborhood of hundreds of kilometers.
| foo92691 wrote:
| There is no upper limit to the wavelength of gravitational
| waves. However, the lowest frequency that we can currently
| detect is around 10-100 Hz. Diving the speed of light (3e8 m/s)
| by (10 Hz) gives 3e7 meters. 30,000 km. The upper end of our
| frequency range is currently around 8 kHz which corresponds to
| 37.5 km if I did the math right.
|
| LISA will be sensitive to much lower frequencies (longer
| wavelengths). NanoGrav also searches for these lower frequency
| signals.
|
| We expect most gw's to be of very low frequency. High frequency
| g.w.s require tremendous mass and acceleration to generate.
| They are only generated in the final moments of black hole
| collisions as far as we know.
| SavantIdiot wrote:
| Do gravitational wave interferometers create a single pixel of
| data, or do they create an "image" of gravitational distortion in
| a 2D region? The first few Google/Wiki hits talk about the
| physics of interferometry, but not the actual resulting output
| from real hardware. I'm assuming there is no image, otherwise
| there would be some associated with the articles, rather than
| artistic renditions?
| foo92691 wrote:
| You can think of each individual detector producing a single-
| channel audio signal. By combining the signals from multiple
| detectors it's possible to determine where the signal is coming
| from. But the output is neither a picture nor a single pixel:
| it's a brief blip of a few seconds of audio-frequency time
| series.
|
| A decent analogy is to think of each LIGO detector not as a
| camera but a microphone.
| phkahler wrote:
| >> it's a brief blip of a few seconds of audio-frequency time
| series.
|
| A bit more time and they should be able to point telescopes
| in the general direction of the event prior to a merger. Not
| sure what kind of directional precision can be obtained, nor
| if there would even be much to see.
| foo92691 wrote:
| This was in fact accomplished and is attempted for many of
| the detections.
| btilly wrote:
| I'm pretty sure that they'd have to be pointed in the right
| direction first.
|
| There is very little time between the start of something
| detectable, and its finish. But in the case of a neutron
| star merger, we were able to point other telescopes and see
| the resulting magnetar for several hours.
| secondaryacct wrote:
| Or a 3 pixels camera whose pixel color changing is what
| matters :D
|
| The only difference between a camera and a mic is the number
| or vibrating thing it cares about (mic only cares about the
| vibration of its single membrane, cameras create millions of
| membranes sensitive to photon vibration on a grid)
|
| LIGO 3 interferometers care about the time-variation of the
| difference of distance measure in 3 groups of 2 mirrors. So
| it's more than a single mic, and it's a derivative of 2
| distance measures, in time. It would be like a 3-pixel video,
| with white as a baseline for the 3 pixels, and it would
| varies towards green or blue depending of the negative or
| positive difference (random colors) between the mirror
| distances.
|
| Or yeah, a 3 channel sound :S
| foo92691 wrote:
| The microphone analogy is particularly apt, because the
| signals are also audio-frequency. You can listen to them.
| Sometimes in the control room we play the output on a
| loudspeaker. It can help in tuning the instrument (most of
| what you hear is noise).
| phreeza wrote:
| They measure the total distortion over the baseline, so in that
| sense it's a single pixel. But I think measurements from
| several interferometers can be combined to give some very
| limited spatial resolution.
| ww520 wrote:
| LIGO is a huge milestone. Turning it on is like the moment
| Galileo pointed his telescope to the moon. This is a new class of
| instruments observing the universe in a medium that was never
| utilized before. Using gravitational waves can observe things
| that cannot be seen before, like stars behind the dust clouds.
|
| It opens a new window to the world. We might finally be able to
| "see" dark matter. May be able to see the gravitational imprint
| from before the Big Bang, the gravitational leak from extra
| dimension or other universes.
| sdenton4 wrote:
| Here's one (negative!) result on trying to detect dark matter
| with LIGO:
|
| https://www.ligo.org/science/Publication-O3DarkPhotons/
| ww520 wrote:
| I thought LIGO is kind of crude, i.e. it's being used to
| measure in the scale of blackhole level gravity. The dark
| matter experiment is trying to measure the gravity of a
| photon? May be too optimistic?
| metalliqaz wrote:
| I hope to live long enough to see the evolution of
| interferometers that will be to LIGO as HST was to Galileo's
| telescope.
| foo92691 wrote:
| This is what you're waiting for: https://en.wikipedia.org/wik
| i/Laser_Interferometer_Space_Ant...
| dylan604 wrote:
| I thought LIGO was the HST to Galileo's telescope.
| ww520 wrote:
| Galileo's telescope and HST all work on electromagnetic
| energy. LIGO work on gravitational wave. LIGO is the
| beginning of a completely different class of instruments.
| netcraft wrote:
| the Gravitational Observatories are so impressive to me - I can
| remember reading about them being theoretical well before LIGO
| ever started construction, and the fact that theyre here and
| working just like we expected is so amazing. I can't wait until
| we have LISA in space!
|
| But its so strange when we shut it off to do upgrades and stuff -
| like I totally understand why we have to do it, but its like we
| finally turned on a microphone and could hear things that were
| always happening but we could never observe before, and then we
| turn it off for a little while - the thought that there are
| events that are going on right now that we will never be able to
| detect because we arent listening gives me major FOMO.
| omegalulw wrote:
| > the thought that there are events that are going on right now
| that we will never be able to detect because we arent listening
| gives me major FOMO.
|
| I can relate to the sentiment but keep in mind that human
| timescales are downright puny compared to cosmological
| timescales. And there's lots of stuff going on all the time
| (lots is an understatement) so I would say you won't lose much
| turning off the detector for a year.
| netcraft wrote:
| I totally understand this from a rational standpoint, but if
| you take that to its logical conclusion, you expect that
| nothing will happen in that year that is very interesting or
| rare, so why look at all?
|
| Sure, our timescales are nothing, making what we have even
| more valuable, no? We've missed out on a whole lot of
| observations - we have a lot of catching up to do!
| pinko wrote:
| LIGO collaboration member here: we focus on maximizing the
| cumulative number of detections over time, and it turns out
| that by turning the detector off for a year to upgrade it, we
| can increase the sensitivity so much that a year after that we
| will have more total detections than if we had left it running
| continuous for two years. So you also have to think about all
| the distant/faint signals we would never be able to detect
| because we kept listening at too low a sensitivity...
|
| (We do make sure that we have a dramatically less sensitive
| sister detector in Germany, called GEO, listening whenever
| we're not so that we'll see something really close and loud,
| like a galactic supernova, even when LIGO is offline.)
| foo92691 wrote:
| Also the experimentalists get very antsy when they/(we) can't
| touch the instrument. :-)
| ars wrote:
| That makes me think of the "Wait calculation" in https://en.w
| ikipedia.org/wiki/Interstellar_travel#Wait_calcu...
| netcraft wrote:
| Thanks for the info! I expected it was something like this,
| that you expect the increased sensitivity is worth it. I just
| wish we had more observatories running in different
| sensitivities - we've just got to get the cost down, thats
| all :)
|
| I didn't know about GEO, thanks for that!
| wpietri wrote:
| I sincerely love that your reply to FOMO was to point out how
| his FOMO should be even worse. This is exactly how my brain
| works when I'm optimizing something: I have to balance one
| obsession with all the other things I need to obsess about.
| jaspax wrote:
| Why would you have FOMO? Why is any particular gravitational
| wave important enough that you'd worry about missing it? I'm
| not trying to hassle you here, I'm genuinely perplexed by this
| attitude.
| wpietri wrote:
| I used to work in a library and we all had a similar feeling
| about books. They were all valuable and to be preserved!
| Objectively, that's not practical. But that base irrational
| feeling is still useful and important.
| TheOtherHobbes wrote:
| Technically we've missed at least four billion years of
| gravitational wave events, so a week of shutdown for an
| upgrade isn't going to make a huge difference to that.
| wpietri wrote:
| Yes, that's where the practicality comes in. But I'm sure
| there are people who have FOMO over all that data
| collection we missed over the last 14 billion years. And
| that's good! Maybe somebody will come up with something
| clever that is better than nothing. E.g., all the CMB
| work:
| https://en.wikipedia.org/wiki/Cosmic_microwave_background
| techbio wrote:
| I miss those days four billion years ago, before modern
| worries and before days.
| dylan604 wrote:
| only for Earthlings. I'm sure (hopefully) someone on a
| rock around a different star might have had those worries
| in their day too.
| DrBazza wrote:
| Close by, interesting astronomical events are rare.
|
| The last decent naked eye supernova was the crab nebula in
| 1054.
|
| It would be a real shame to miss the next one in this galaxy.
|
| Other "rare" (non-gravitational wave) events I can think of
| are: the Shoemaker Levy comet hitting Jupiter, the Carrington
| Event in 1859, Betelgeuse dramatically dimming (last year).
| t849846224 wrote:
| Last 'decent' naked eye supernova was surely Kepler's
| Supernova of 1604. And the recent fading of Betelgeuse
| wasn't all that unusual for a fairly typical red supergiant
| pulsating star.
| netcraft wrote:
| I replied to a sister comment similarly, but the one resource
| that we are severely limited by is time. There were events
| that were far more common in the early universe that we may
| never see now. Rare things could happen at any time.
|
| The reality is that we can't observe 100% of the time for
| resource constraints and that the cost/benefit of upgrading
| is totally worth it in the long run - rationally speaking its
| the right move. I will just always wonder what we are missing
| out on that we might never have the opportunity to observe
| again - or maybe not in our lifetimes.
| jliptzin wrote:
| How do they know it's black holes colliding and not just large
| stars?
| foo92691 wrote:
| Extensive computer modeling is used to predict the expected
| waveforms, and then we fit the observed waveforms to the
| expected ones.
| jliptzin wrote:
| Cool thx
| still_grokking wrote:
| Isn't it more like you fit actually noise to expected forms,
| and out comes the expected form?
|
| At least someone explained it like this to me.
| skulk wrote:
| Maybe electromagnetic signals can disambiguate the two
| phenomena?
| mrfusion wrote:
| Could we be seeing warp drive signatures?
| pwned1 wrote:
| Why would anyone downvote this? I love this type of out of the
| box thinking.
| queuebert wrote:
| Aliens are trolling us by doing burnouts near our planet.
| foo92691 wrote:
| So far all of the signals observed by LIGO are from sources
| that we expected would exist.
|
| The most exciting thing would be to observe an unexpected
| signal.
| baq wrote:
| did you model how an alcubierre drive would look like on the
| waveform? (only half joking ;))
| 2bitencryption wrote:
| question: according to Wikipedia, LIGO was built between
| 1994-2002, and didn't detect gravitational waves until 2016.
|
| I never heard about LIGO until the discovery in 2016, so for
| almost 20 years it was off my radar, so to speak.
|
| What multi-decade experiments are being created today, which will
| be ready to produce amazing results in 20-30 years? What's
| currently under construction, but I'll never hear about it until
| 20 years from now, when it makes an amazing discovery?
| mabbo wrote:
| The article mentions the new KAGRA detector in Japan joining the
| group. Does anyone know: how does the accuracy improve as more
| detectors come online?
|
| Will we see a day where we have 20, 50, 100 detectors around the
| globe and events are near-certain because so many detectors see
| them? Or is the diminishing returns, and 4 detectors is already
| too many?
| ww520 wrote:
| The next stage should be putting them in space. Vibration on
| earth is a huge problem. Space has a much more stable
| environment. Also the distance between the laser detectors can
| be far, greatly enhancing the magnifying power.
| djd3 wrote:
| Check out LISA.
|
| https://lisa.nasa.gov/
|
| _note:_ I worked on the OG LIGO at Hanford in grad school.
| pinko wrote:
| In terms of the science, land-based and space-based GW
| detectors are complementary, as they detect GW waves at very
| different frequencies. One doesn't replace the other.
|
| There are also serious (if obviously longshot) efforts by
| colleagues of mine to propose moon-based GW detectors:
| https://indico.ego-gw.it/event/263/
| ww520 wrote:
| Isn't to detect different frequencies it's a matter of
| varying the distance between the detectors? Space based
| detectors can be placed arbitrary close or far, moved at
| will. Ground based is fixed. Also I remember isolation from
| Earth's vibration was a huge if not the biggest engineering
| challenge. There's no such problem in space.
| akuchling wrote:
| With three detectors, it becomes possible to measure the
| polarization of gravitational waves.
| https://ligo.org/science/Publication-O1StochNonGR/index.php
|
| Some quantum-gravity theories predict additional polarization
| modes that general relativity doesn't, so such measurements may
| start ruling particular theories in or out.
| BurningFrog wrote:
| The current ones are Louisiana, Washington state, and Italy.
|
| So having another as far away as Japan should improve
| triangulation substantially.
| pinko wrote:
| You might be interested in this talk on the future of GW
| detectors and the resulting science prospects:
| https://www.youtube.com/watch?v=iet6pS4gxCk (esp. from ~25:40
| to the end).
| mabbo wrote:
| Thanks, that was very intersting!
| pinko wrote:
| Additional detectors improve our ability to detect somewhat
| (assuming they're of similar sensitivity -- otherwise they can
| actually hurt our overall network sensitivity!). But the _real_
| advantage is in source localization to guide multi-messenger
| (optical/gamma/neutrino) followup, which is where many of the
| most important discoveries will come from. It's like
| triangulation.
|
| Given that an observatory costs on the order of ~$1B to build
| and operate for a few decades, we probably won't see more than
| five current (second generation) instruments (2x LIGO + Virgo +
| KAGRA + LIGO India).
|
| There are also two proposed but not yet funded 3rd-generation
| ground-based instruments ("Cosmic Explorer" and the "Einstein
| Telescope"), one planned space-based instrument ("LISA"), and
| early efforts at proposing a future moon-based detector (the
| Gravitational-Wave Lunar Observatory for Cosmology, the Lunar
| Gravitational-Wave Antenna, and the Lunar Seismic and
| Gravitational Antenna).
|
| To get to tens or hundreds of detectors, someone will have to
| invent a fundamentally different technology that can be
| produced at dramatically lower cost. Maybe next century...
| prox wrote:
| I can donate my raspberry Pi if you need it ;)
|
| Seriously, impressive cutting edge technology!
| [deleted]
| Invictus0 wrote:
| Interesting how, even with three LIGO observatories, there is
| still a 15% false positive rate. How many more observatories are
| needed to reduce this to a negligible number?
| raymondh wrote:
| GW fanboy here. IIRC, additional detectors provide better
| ability to locate the GW source so that other telescopes can
| also capture an event. They also improve polarization
| measurements. The number of detectors doesn't determine the
| false positive rate which is just a user selected point on a
| receiver operating characteristic curve. In general, we get
| more pay off from improving detect sensitivity than from having
| more detectors (that's why GEO 600 was more useful for the tech
| it developed rather than its most recent observations).
| pinko wrote:
| LIGO collaboration member here: this is a tunable knob,
| independent of the number of detectors, that we've set very
| intentionally based on feedback from astronomers who want to
| follow up with optical/gamma/neutrino instruments following our
| BNS detections. We could reduce our false positive rate to a
| negligible number today, at the cost of _not_ reporting many
| likely discoveries.
|
| Also, a minor point but there are only two LIGO detectors
| online at the moment, with a third sister detector in Italy
| (named Virgo), and a fourth coming online soon in Japan (named
| KAGRA). There does in fact exist a third LIGO instrument, but
| it's currently mothballed, awaiting construction in India.
| [deleted]
| gus_massa wrote:
| Do you publish some number about how sure you are?
| pinko wrote:
| Yes, we carefully explain the statistical confidence of our
| detections in each discovery paper, and there are dozens of
| methods papers that go into excruciating detail on those
| techniques.
|
| Prior to our first detection, the overwhelming prime
| directive of our collaboration was _not to make a false
| detection_ and we went to insane lengths to avoid one;
| e.g., we had a small team of people secretly injecting
| false signals -- "blind injections" -- into our data, so
| that we all expected to be regularly seeing them and
| wouldn't be tempted to gin the analysis to find a detection
| where there wasn't one. (Amusingly, because of this, it
| took weeks for many of us to believe the first detection
| was real, even after the blind injection team swore it
| wasn't one of theirs. In the end, we charged an independent
| team to do a forensic analysis of everything from the
| security cameras and seismometers in the observatories, to
| every computer and disk the data passed through, to
| convince ourselves this couldn't possibly have been
| maliciously injected by hand by a rogue scientist. It was a
| wild few weeks!)
|
| Now that it's clear gravitational waves exist, however, we
| focus on optimizing the false alarm rate for astronomers
| (who want it well above zero so they don't miss anything)
| rather than optimizing for ~zero false detections.
| matt123456789 wrote:
| Are your security cameras sensitive enough to pick up
| rogue sophons as well? (Three body reference, sorry.)
| im3w1l wrote:
| That sounds hilarious, but you must be quite unique in
| having this approach right? Or are the particule physics
| people playing similar games?
| pinko wrote:
| Unique as far as I know, at least in a scientific
| collaboration like ours. But we were unusually sensitive
| to the risk of false detections given (a) skepticism of
| LIGO's prospects from other physicists, and (b) the
| lingering stain of a false detection claim, from a bar
| detector, a few decades earlier.
|
| We believed that if we had announced a discovery that
| turned out to be wrong, it would probably have meant the
| end of our experiment, and in practice the end of the
| field, at least for a long time. It was fortuitous that
| the first detection turned out to be gold plated and
| unambiguous, otherwise we would have probably published a
| bunch of "we saw something interesting but can't claim it
| as a GW discovery" papers on the next few weaker events
| before we would have felt comfortable making a confident
| claim.
| airstrike wrote:
| Thank you for all these replies and for the fantastic
| work you and the team do.
|
| This is now my favorite AMA
| dekhn wrote:
| Has LIGO ever responded to Sabine Hossenfelder's claims?
| http://backreaction.blogspot.com/2019/09/whats-up-with-
| ligo....
| DrBazza wrote:
| I thought that 2019 article is now "answered" by the fact
| that we have multi-messenger observations of
| gravitational wave events that confirm fading light
| curves.
| dnautics wrote:
| I believe we still only have one multi-messenger
| observation. One observation has infinite variance =P.
|
| They're ALSO not independent observations, though. The
| LIGO observation was made, and then the directive was "go
| look for correlating events". That's a dependent
| observation.
|
| To be truly independent, they would have to inject
| synthetic data (say 3x) into the observation reports and
| make sure that the multi-messenger results don't
| correlate with the synthetic data, much like GP says they
| did "at the beginning of LIGO" with internal (non-
| multimodal) signals.
|
| This would have the side benefit of making the analysis
| pretty simple to do (just a chi-squared analysis) and you
| don't have to have a PhD in signals analysis with a
| specialization in the filters used by LIGO to believe
| with a high confidence that 1) GWs exist and 2) LIGO is
| actually measuring GWs.
| ars wrote:
| When I see LIGO graphs they always appear linear - the
| change in frequency is linear with time.
|
| But with time dilation as black holes near each other,
| shouldn't the frequency change be exponential? Or does it
| cancel out - the frequency goes up as they get closer,
| and then time dilation lowers it back as it dilates into
| infinity?
| nharada wrote:
| LIGO blows my mind whenever I hear news about it. The sensitivity
| alone is insane -- it can detect a change in distance between its
| mirrors 1/10000th the width of a proton. This is the equivalent
| of measuring the distance to the nearest star (4.2 light years)
| to an accuracy of _1 human hair_. Those are bonkers numbers. How
| the hell did we even come up with this thing?
| queuebert wrote:
| Interferometry has a long history in experimental physics. The
| Michelson-Morley experimental design scaled up to large
| distances and using modern technology, like lasers and
| computers, gets you LIGO.
| foo92691 wrote:
| And resonant cavities, the best mirrors ever made, and a huge
| amount of cleverness and work over 40+ years.
| jjoonathan wrote:
| Speaking of which, I can understand how interferometry gets you
| to, say, 1/1000th of a wavelength, but the wavelength is
| 1000nm. How do they go from 1nm to 1/10000th the width of a
| proton? What's the trick?
|
| Is it an integral transform thing, like how spectrum analyzers
| can claim super low noise floors if you sort of gloss over the
| "noise is proportional to badwidth" part and look in a tiny
| bandwidth without normalizing?
| tobinfricke wrote:
| Great question! The precision is not just better than the
| wavelength of the light. It's also way smaller than the
| surface roughness of the mirrors! How does it work?!
|
| Like you suggest, and adding to what sleavey mentioned above,
| I would say the answer is: averaging over time and space. The
| laser beam is pretty wide, so it averages over a significant
| area of mirror surface. (The optical system also selects one
| spatial mode of the laser beam.) And the stated displacement
| sensitivity ("1/10000 the width of a proton") only occurs
| when you integrate over the sensitive frequency band.
| sleavey wrote:
| Cavities. We trade off bandwidth for peak sensitivity by
| sending the same light back and forth between mirrors in the
| arms of the interferometer hundreds of times. As the
| gravitational wave passes, the same light samples it over and
| over and picks up additional phase shift, enhancing the
| signal. The downside is that we can't see gravitational waves
| at signals far above the cavity pole frequencies at a few 10s
| of kHz, but the most promising sources we aimed at when the
| detectors were designed were considered to be below that.
|
| We also use techniques called power and signal recycling to
| enhance this bandwidth-sensitivity tradeoff even more.
| Combined these techniques give you what remains between your
| 1/1000th wavelength and the actual sensitivity of LIGO and
| Virgo.
| magicalhippo wrote:
| In addition they've been using squeezed light[1] since 2019
| to help increase the sensitivity.
|
| [1]: https://www.optica-
| opn.org/home/newsroom/2019/december/squee...
| colanderman wrote:
| That sounds like a block & tackle [1] for light.
|
| [1] https://en.wikipedia.org/wiki/Block_and_tackle
| gabagool wrote:
| *the second nearest star
|
| :)
| daot wrote:
| shameless plug on a project I did with a High School physics
| student plotting gravitational waves inspired by the "joy
| division" plot: https://moleksak.com/ligo/ ... we'll have to
| update it with the new data!
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(page generated 2021-11-10 23:00 UTC)