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?
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?
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.
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.
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.
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.
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?
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.
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.)
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.
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 :)
> 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.
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!
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.
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.
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).
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.
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?
From the talk by the Caltech professor leading LIGO, the theory was good on paper but the engineering problems were very tough. The platform to make measurement has to be absolutely still. But the Earth always has some movements due to seismic activities. For years they just couldn't get the passive vibration isolation tech stable enough for the measurement to work. The project was almost canceled. Finally they went back to the drawing board and partnered with some companies to build new active vibration isolation technologies. It took years to get it working.
Their active vibration isolation technology is insanely good. It basically detects tiny seismic movement of the Earth far away and actively compensates the stable platform. This is one instance of scientific project spawning off new technologies, which will have many other uses in the future.
My head instantly went to fusion experiments[0]. There are planned experiments in that space which, if they go ahead, won't produce results until the 2040s.
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.
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?
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...
Some quantum-gravity theories predict additional polarization modes that general relativity doesn't, so such measurements may start ruling particular theories in or out.
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).
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.
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/
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?
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.
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).
Wikipedia lists S200114f (on page https://en.m.wikipedia.org/wiki/List_of_gravitational_wave_o...) in a way that to me makes it sound like there’s substantial uncertainty about the cause of it; is that maybe because Wikipedia is out of date and people figured out the (not unusual) cause since, or is maybe just wrong, or maybe I misunderstand what you mean, or I misunderstood what the people who labeled the entry for it on Wikipedia meant?
Readit News
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?
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.
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.
:)
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.
https://www.ligo.org/science/Publication-O3DarkPhotons/
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.
(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.)
I didn't know about GEO, thanks for that!
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.
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!
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).
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.
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?
Their active vibration isolation technology is insanely good. It basically detects tiny seismic movement of the Earth far away and actively compensates the stable platform. This is one instance of scientific project spawning off new technologies, which will have many other uses in the future.
[0] - https://en.m.wikipedia.org/wiki/List_of_fusion_experiments
https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Ant...
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.
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?
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...
Seriously, impressive cutting edge technology!
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Some quantum-gravity theories predict additional polarization modes that general relativity doesn't, so such measurements may start ruling particular theories in or out.
So having another as far away as Japan should improve triangulation substantially.
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/
https://lisa.nasa.gov/
note: I worked on the OG LIGO at Hanford in grad school.
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.
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The most exciting thing would be to observe an unexpected signal.
What they're picking up now is events with titanic energies, things like black holes merging and neutron stars colliding.
These are many, many of orders of magnitude more energetic than even supernovae!