Something already lost in the twisted passages of history is that the first generation of gravitational wave detectors was of an entirely different design than the current interferometers [1]. It never worked and Weber's claim to have detected gravitational waves from SN1987A in 1987, was widely discredited...
Huh, I wonder if anyone's tried to validate this approach again now that we have LIGO (and presumably more precise equipment?). I know very little about the physics involved here, but the articles I found about Weber bars don't cite disagreement about the theory underpinning the experiment, so I'm curious if we expect a detectable effect with our current understanding?
I also know very little about manufacturing Weber bars, but I could imagine it's cheaper to build 100s or 1000s of these and perform signal processing on them than building another LIGO. Or Weber bars in space?
If you look at the lengths LIGO had to go to in order to eliminate background noise to even be able to theoretically detect gravitational waves, it seems very unlikely that a comparatively crude technology from the 80s could have achieved the same things. Like modern lasers or squeezed light states, which were mostly theoretical back then. If I remember correctly, Weber's device was claimed to show a huge amount of events per year, which would indicate tons of GW sources in our neighbourhood. LIGO and the rest of modern astronomy have since disproved that.
Sometimes noise turns out to be signal (see developments leading to Cosmic Microwave Background [0]). At this stage in gravitational wave detection is it well founded to consider noise as noise instead of information we cannot yet understand?
Noise always has different sources, and when you eliminate noise you should always be conscious of the source. There should be a gravitational wave background analogous to the CMVB. Detecting it would be a sensation.
FYI, within the field, “first generation” now refers specifically to the first version of LIGO (not “advanced LIGO”). It’s not just that Weber bars didn’t work, it’s that they couldn’t work according to most physicists (but Weber obviously disagreed).
Umm, not sure what this clumsy attempt to rewrite science history is about?
The internal nomenclature of those working within a particular approach cannot restart the clock of an entire field? Resonant bars were the first generation of gravitational wave detectors, period.
> they couldn’t work according to most physicists
That certainly requires a reference. There were several teams around the world besides Weber pursuing the approach. You make it sound as if the physics behind these bars was crackpot.
Fyi, you may be confusing physicists with astronomers. The prevailing notion in astro circles was indeed that the known pathways to gravitational waves and with calculations made using GR would be too weak to be detected.
(Incidentally astronomers were notoriously hostile to any GR research anyway. Why spend money in a high risk esoteric field if you can keep doing the same stuff over and over?)
But a new observational window can always throw a surprise. A previously unknown class of sources may hit you in the face despite your primitive instrument. Historically this happened repeatedly with x-rays, radio etc. Or the theory you use to make predictions may be deficient and not telling you the whole story...
In this respect Weber was simply unlucky. The universe is in a sense slightly more "conventional" and less exciting than it could have been.
> Resonant bars were the first generation of gravitational wave detectors, period.
Well, too bad, no one in the field refers to it that way (I’m a LIGO member). Jess is pointing out how the language is used, not saying that it’s correct.
A member of LIGO might be too close to tell what is the first gen.
As a person without any connection to the field I see a paper making a distinction between the first generation of interferometric detectors and those using other means [0]. Likewise a couple of expanded papers from the ScienceDirect page on Gravitational Wave Detectors also cite non interferometric types as early work [1]:
The earliest manmade gravitational wave detectors were based on a simple gedanken experiment: if two masses on a spring are momentarily stretched apart and then compressed by a gravitational wave, potential energy is imparted to the spring, independent of how coordinates are defined. from K. Riles, in Progress in Particle and Nuclear Physics, 2013.
Combined with their second sentence that aims to discredit and erase an entire era / approach as crackpot, the implication is that this is indeed the "correct" way. There may be a lingering fear that the historical false detection claim could still tarnish the entire field but it feels overblown. This thread is about exploring "new ways" and it is very instructive to know and study the "old ways".
Last I checked David Blair's in the field, what with the sapphire clock development and his son being part of the 14 September 2015 first detection event.
IIRC David talked about his niobium bar from 1976 as a first generation gravity wave detector - we certainly referred to it as a gravity wave detector in 1982 and it was talked about as such at the fifth Marcel Grossmann meeting.
Openrisk was commenting on how the language is used by practitioners in the field, for which being a LIGO member is directly relevant. It's a mistake to try and mock him for doing that. It just drives actual experts even further away from HN.
My comment goes directly to how practitioners in the field refer to early gravitational detectors .. unless you're of the opinion that David Blair and son are somehow not practitioners in the field?
Harrison's first Longitude clock that worked at global scale was H4 .. the fourth in the series.
The LIGO detector came into being as a direct result of the work on earlier first generation detectors and technology invented there (eg saphire clocks) not only made LIGO possible but there are actual experts that straddle multiple generations of detectors.
It's completely understandable that those that came later and have perhaps only worked with LIGO, count detector generations by LIGO editions. These are not, of course, the same as detector generations.
It's likely a mistake to try and gatekeep others that are aware of the history, as noted that can just drive actual experts away.
That's an interesting personal revelation that fails to address the question of whether or not he's in the field and whether the speakers on Gravitation at the fifth MGM in Perth consider his niobium resonant bar part of the generational development of gravity wave detectors.
Look, there are groups around the world doing lots of things the majority think are foolish. (The DAMA experiment comes to mind.) It's good and healthy that this is allowed, and indeed there is a smooth continuum between "very unlikely but I guess someone should be checking" and "lunacy". But we also shouldn't pretend all attempts are equally promising or reasonable.
> Fyi, you may be confusing physicists with astronomers. The prevailing notion in astro circles was indeed that the known pathways to gravitational waves and with calculations made using GR would be too weak to be detected.
I'm not confusing anything. The people who have the expertise to estimate limits on the strongest plausible gravitational waves are called "astrophysicists" for a reason.
> In this respect Weber was simply unlucky.
An extreme example to illustrate why this sort of lazy "it might have worked" thinking is wrong: If I argue that there is a heretofore unknown law of physics that causes black holes to form at low energies as long as it's in Wilmington NC and it's on the 17th Wednesday of the year, and if I build a special detector in my basement in Wilmington that looks for the black hole signature on such a day, and if I claim to see a positive signal, most people will not say that I "built the first generation of table-top black hole detectors". So saying "it might have worked, who knows?" just isn't enough.
It's not a coincidence that Weber both (1) thought it was worthwhile to look for gravitational waves at a strength that was widely thought to be impossible and (2) deluded himself into thinking he had made a positive identification. If he had only done #1, and if he had been clear that he was looking under the lamp post just in case there was surprising new physics, a defense of Weber would be more plausible. (Not necessarily convincing, but at least plausible.) But the fact that he also did #2 strongly undermines arguments that he was clear-eyed about what he was doing.
So, going from a very narrow frequency band (up to 1000Hz) to a much wider range, which can theoretically encode information (e.g. frequency modulation)... Hmm, I'm wondering if comms over gravity is something a sufficiently advanced civilization might consider using, and should we be looking for that 'Hello, world' in some 'natural frequency' like we're doing for EM radiation?
I'm not sure I see what the advantages of communicating this way would be. The amount of energy required per bit transmitted would be astounding.
I feel like, while theoretically possible, it's pretty much all downsides and no upsides. At least for communication purposes.
However, your comment reminded me of an interesting PBS Space Time episode discussing the possibility of finding alien civilizations via the gravitational waves produced by their massive ships accelerating to near light speed.
The medium is the message. If we detected a modulated signal in gravitational waves, it would be like an Iron Age tribe receiving a 100 foot tall perfectly polished stainless steel statue with ornate inscriptions and pictograms. It would be recognizable and within our conception, but it would also be a demonstration of development and access to resources beyond our imagination. That's the upside. In a galaxy of sparse and sparingly advanced civilizations, the message might be "fear us and stay away" in a way that EM would not convey.
>it would also be a demonstration of development and access to resources beyond our imagination. That's the upside [...] In a galaxy of sparse and sparingly advanced civilizations, the message might be "fear us and stay away" in a way that EM would not convey.
I think you've hit the crux the question. If there are only a few civilizations, I agree, that'd be an awe-inspiring deterrent.
However, if you don't know how many civilizations there are that are similarly advanced to your own, sending out a big "we're here!" message may be quite risky.
In terms of game theory, it's a sequential and incomplete information game. I think the smartest decision is to remain quiet.
You can detect gravitational waves based on the strain (amplitude) directly, rather than relying on intensity like electromagnetism.
Because the amplitude is inversely proportional to the distance, but intensity is inversely proportional to the distance squared, this could allow for communication over longer distances.
Thanks for the PBS reference, interesting!
As to the energy required, well, yes, I guess manipulating huge masses will be costly, but, if there is an efficient way to do this, then gravity waves are a parallel plane of communication.
The analogy I heard once is about tribes communicating with smoke signals, while the air around them is filled with radio waves. Maybe we can't hear anyone out there because we're not listening to the right thing...
Maybe that is the filter. If your society hasn't figured out the tech to do it efficiently, the rest of the galaxy doesn't care about what you have to say.
I've always been a bit confused about the idea that an advanced civilization would be uninterested in us because we've "only" reached the ability to communicate via radio waves. We're either a threat or an ally to these other civilizations, and if you were them and you detected a society that was clearly on course to eventually catch up, it would be in your best interest to treat them like one of those things ASAP so you can have a hand in their development.
To me it feels most likely that our signals just have not had enough time to get to them.
I think the "dark forest hypothesis" can apply here.
The advanced alien civilization may indeed be interested in us, but still not consider it their best interest to act as soon as possible.
If they decide to act, other civilizations (perhaps even more advanced) may decide to intervene in some way. A civilization that decides to reach out (in a friendly or hostile way) also reveals their own location in the universe.
The most risk adverse choice is probably to remain quiet, especially if they are millennia ahead of us technologically.
What would a gravitational wave generator look like? A machine to "wiggle" an asteroid, or say a moon? What if you made a huge array of small machines that "wiggle", say, a bowling ball, in perfect sync.
In principle, almost anything: any system of masses will emit gravitational waves with an intensity proportional to its mass quadrupole moment (which may be 0, as it is for rotationally symmetric systems). But the proportionality constant is extremely small. Realistically you're looking at stellar-mass objects, if not larger.
Nah, asteroid is not enough. you'd need to wiggle a couple or more large black holes in super close proximity.
But who knows, may there be alternatives we are not aware of? Is it Higgs boson that gravitational field carrier particle , similar to electrons for EM field? Maybe there is a way to mass-produce those and modulate gravity waves that way, eh ?
...or maybe there is a lower noise floor for gravitational wave comms?
>The amount of energy required per bit transmitted would be astounding.
Has someone calculated this out? Or is it more of a "well we need an exceedingly sensitive instrument to detect some of the most energetic events in the universe from half-a galaxy away" gut-feel? Any reason something like a phased-array for directional comms / beam forming wouldn't work with gravitational waves?
>Has someone calculated this out? Or is it more of a "well we need an exceedingly sensitive instrument to detect some of the most energetic events in the universe from half-a galaxy away" gut-feel?
I was thinking about the energy required to transmit the gravitational waves, not receive them. Being able to move objects massive enough (stellar mass or more) to create detectable gravitational waves in a quick and precise enough manner to allow for communication would require mind-boggling amounts of energy.
Right, sorry I wasn't clear. If you were more interested in "local" communications, and less interested in broadcasting to the rest of the cosmos, do you still need gigantic amounts of energy at the transmitter? And how much power would you need, even if the energy is relatively high? For low power transmissions, what is the limiting factor on the receiving end? Or why can't you detect really low power transmissions? Can't get your receiver close enough to absolute zero, so thermal fluctuations kill receiver sensitivity? Background gravitation noise floor is too high across the band? Quantum fluctuations are a limiting factor? Can't make an X-ray/gamma-ray interferometer? "Antenna" size scales with length rather than area? Other?
I suppose the ratio of Coulomb's constant to the gravitation constant (or something similar) govern the relative difficulty in using gravitational vs. EM? But that's not obvious to me that it would make gravitational wave communications inefficient in absolute terms.
Oh, yeah sorry, I was thinking more along the lines of inter-galaxy communications!
I definitely do not know enough about the topic to approach answering your questions, but I'd certainly be interested in knowing the answers. I really hadn't thought about it in that context.
Since gravitational waves would still be restricted to the speed of light what would the benefit be?
Would "obstacles" be circumvented? I would think interference would still be possible but instead of line of sight it would be large gravitational distortions (black hole, stars).
Humility clause: I don't know what I'm talking about.
Lots of naysayers but I can think of one very valid reason this might be the case. Our species, and many species on our planet, can see a very narrow band of the radiation spectrum. It’s possible an advanced civilization never developed that but was instead far more intimate with gravitational energy instead. We are talking about the universe after all.
I'm thinking you could also create gravity waves by annihilating matter and then creating matter over and over. If you could get a bit of that wave to reflect back to you and then sync your creation/annihilation, you could build a huge wave over time
If we are going all in on imaginary civilizations, even a very narrow (well, not that narrow) frequency band could be used to encode information, couldn't they?
Yes, but not efficiently (it will be very slow) and will be hard to detect because of noise (the article mentions the 'cacophony' of gravity noise).
This is similar to EM radiation, so, you do frequency modulation (or amplitude, but that seems harder with gravity - you'll need to modify mass...), so you have a base high frequency on top of which you add lower frequency. The substraction later gives you a clean signal (or, Gravity Radio).
I'm not sure "slow" matters that much. If (and that's a big if) there is advantage to announce yourself to other civilizations using gravity waves, instead of other means, then it probably does not matter too much if transmitting a reasonable amount of information takes a decade or so. You'd put it on repeat of course, and hope that interested civilizations don't have too short of a lifespan to still make sense of it.
Some of it, definitely. But if the purpose is to seek others and announce yourself (like we did with Voyager 1 and 2), then you'll want it to be plain.
Also - Public radio transmissions are not encrypted (as your purpose is to have as many listeners as possible... You monetize on ads).
Just a plug that you can tour the LIGO facilities for free! I visited a couple years ago and got a tour of the Hanford facility, included a lecture beforehand as well. Really awesome people and got to tour the entire facility, even going to the control room.
I’m amazed there was no mention of LISA [0] — a space-based gravitational wave detector using 3 satellites flying in formation 2.5 million Km apart! Seriously cool engineering, planning to launch in 2035.
> ...Researchers are now working on several next-generation LIGO-type observatories, both on Earth and, in space, the Laser Interferometer Space Antenna;...
> LISA was first proposed as a mission to ESA in the early 1990s.
I remember reading about LISA when I was a little kid. Back then it was projected to launch in the far future of 2015. Now I would be surprised if it actually launches in 2035.
The university I studied at had a Gravitational Wave center under the Physics dept, and all the professors would joke about how LISA had been "less than a decade away" for 20 years.
I just learned of a new proposal to use an already planned probe, as a gravitational wave detector. Maybe I am missed it, but this does not appear to be covered in TFA.
> Bridging the micro-Hz gravitational wave gap via Doppler tracking with the Uranus Orbiter and Probe Mission: Massive black hole binaries, early universe signals and ultra-light dark matter
Probably a dumb question, but... is it basically proven then that gravity doesn't exist (it's effects are just a result of spacetime's geometry?). Because it seems like these gravitational waves experiments show that spacetime exists and has measurable geometry. Yet every time quantum mechanics comes up everyone talks about how we haven't found the gravity force carrier yet which doesn't make sense to me if gravity doesn't exist and is a consequence of the geometry of spacetime.
These experiments confirm the classical theory of gravity, which is Einstein’s general relativity, just as Maxwell’s equations are the classical theory of the electromagnetic field. Just as Maxwell’s equations are perfectly adequate for waves of macroscopic intensity, GR is perfectly adequate for astrophysical gravity waves.
A whole separate question is what is the quantum mechanical theory that has general relativity as its classical limit? For electromagnetism, quantum electrodynamics (understood in the 40’s) is the quantized version of Maxwell’s and predicts that electromagnetic energy measurement outcomes come in “chunks” (photons). But, although much is known about features of “quantum gravity” (like that gravitons will be massless, spin 2), there is famously no consensus yet about the precise theory.
As to how to reconcile the force carrier picture with spacetime picture — even classically one can consider an “overall” spacetime background geometry such as that created by the whole earth. Then consider little ripples perturbing this background. Gravitons are these little ripples turned on a quantized amount (heuristically). How the overall background itself gets formed as an “enormous pile of gravitons” will depend on the precise theory of quantum gravity. String theory does have a partial answer to this so can model such things as black holes quantumly.
So I want to try to answer what I can, despite being a layman on this.
Gravity exists, it manifests as the warping/geometry of space. This is in contrast to the other fundamental forces which get explained via Quantum Field Theory. That's the very high level difference of the two, our current understanding of gravity does not work the same way as the way everything else does, and so far we can't find a provable theory (yet) that makes the two work together at all scales.
String theory purported as a way to create a quantum theory of gravity and explain everything else, but my understanding is that it's fallen out of favor because it mostly turned into a tunable mathematical framework that could just change to fit any observations that were made, so it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything). I believe this is where predictions about a possible gravitational force carrier generally come from, aka the graviton.
Then there's theories like Loop Quantum Gravity, where the way it works is that space-time itself is quantized and that's how you get things to mesh because you can now use the same wave-function style of things that all other quantum theories use. Though I think this doesn't predict much about a quantum field for gravity on it's own.
I believe one of the other things that runs into everything being difficult is that with relativity you end up with a lot of infinities in the equations and results and so there's a "new" kind of math for it that gets called "renormalization" that prevents them from coming out but it also has issues when translating between quantum theories and relativity.
> it doesn't have the same kind of predictive power that people want (i.e. too much freedom so it can be used to explain anything, not just everything).
That's the popsci version that's been disseminated, yes. It's not exactly wrong, but it's misleading.
First a bit of background. Quantum field theories like the standard model are effective theories, not fundamental ones. We know we don't know the real high-energy physics, so we treat it as a black box and loosely speaking "average it out" as a new free parameter. This is analogous to how an engineer designing a bridge can ignore the fact that iron has a crystal structure and treat it as a continuum with bulk properties like tensile strength. In reality this having a particular tensile strength is a state, not an intrinsic property, and you could end up with a different tensile strength if you melted the iron and let it resolidify (I'm not a metallurgist, substitute some other material if that's not true for iron), but we can build bridges without knowing that.
In the same way, Standard Model is a particular form of "solidified" string theory. It's true that there are many, many, many others, but they're not free parameters in the same way. You can write down perfectly reasonable looking quantum field theories that string theory can't produce, and if our best effective theory was one of them then we would have good reason to reject string theory. But it's not.
So the situation we're in is that we have some solid material, and we want to know what a single molecule of it looks like, but we can't see anything other than the bulk properties. What the "string theory is unfalsifiable" crowd is demanding is that whatever molecule we predict have only a few possible crystal structures. And maybe it does. That would be convenient. But sometimes nature inconveniences us: it might be some crazy carbon allotrope. It might be glass.
I believe they're referring to the idea of a gravitational force carrying particle, like photons for electromagnetism, W/Z bosons for the weak force, and gluons for the strong force. Typically this gets called a graviton but they've never been observed and the theories predicting them as far as I know don't predict that we can detect them in any meaningful/practical way right now which is also one of the problems for issues with a quantum theory of gravity.
I've always wondered (but not done the research/reading) on how that would mesh with black holes since you'd need gravitons to escape to mediate the curvature of space-time but that'd seemingly (to me) require them to be able to either ignore the curvature of space-time or travel faster than the speed of light in order to do so. And I believe that those two options there are actually mathematically equivalent as far as the consequences of things go.
This question isn’t actually anything to do with quantum mechanics. One can already ask, for GR, can a gravity wave generated by a wiggling object that has already passed the event horizon propagate back out through the event horizon? The answer from gr is unambiguously no. Effectively, all complicated goings-on inside the black hole are washed out into just a couple parameters for an outside observer: total mass, total charge, total angular momentum. All the matter contained by the black hole does still make its gravitational effect felt on outside objects through these three parameters.
This observation in fact is what inspired wheeler to ask his grad student bekenstein what then happens to the entropy of a cup of tea thrown into the black hole — how to reconcile with 2nd law of thermo. Which in turn was the start of the very long story of black hole thermodynamics.
That would suggest space is "made of" something, so what would that be? If it's made of something, how is that "carrying" the wave, causing it to distort in the presence of mass?
- "Physicists Have Figured Out a Way to Measure Gravity on a Quantum Scale" with a superconducting magnetic trap made out of Tantalum (2024) https://news.ycombinator.com/item?id=39495482
I just want to add something about how sensitive LIGO is. The gravity waves it is detecting are equivalent to measuring the distance from Earth to Alpha Centauri with a variation the width of a human hair.
Anyway, these techniques are aimed at detecting different types of gravitational waves, not necessarily about simply increasing sensitivity. I don't know what dictates the frequency of a gravitational wave.
Truth be told, I still don't get what expanding space or gravitational waves really are but then again I'm just an idiot who doesn't understand tractor calculus [1].
If one takes the width of the plow or seeder one is tractoring and divide the length of the field by that width one will get the number of times the tractor goes back and forth across the field to completely seed or plow it.
If the tractor travels at constant velocity V_tractor m/s and multiplies by the width of the plow or seeder and the time in seconds the tractor took to plow or seed the field one gets the total area of the field expressed in meters^2. This can be extended to a tractor traveling at V(t) if infinitesimal units dt of area are summed.
This is known as the Fundamental Theorem of Tractor Calculus and is the basis of Tractor Field Theory;)
A more niche but nonetheless interesting method that I was hoping to see discussed was magnetism. Gravitational waves are expected to decay into photons in intense magnetic fields. Or so I was told by one of my physics professors back in the day. I did understand the math somewhat back then, but it is beyond me now. It does however seem as though some people are still exploring this avenue [0].
It's mentioned both in the article, and in the comments here where someone else thought it isn't mentioned.
They didn't use the acronym ("LISA"), but instead spelled out the entire thing.
>Researchers are now working on several next-generation LIGO-type observatories, both on Earth and, in space, the Laser Interferometer Space Antenna; [...]
Any physicists in here that could layout a path of going from rudimentary first year level physics knowledge to being able to understand on a deeper level topics such as gravitational waves?
These articles are interesting but are very abstract when you do not have knowledge from first principles.
I’m a scientist who works on LIGO and next generation detectors and I don’t have a deep knowledge of the physics behind gravitational waves either! But realistically you should pick up some lecture notes on general relativity: https://www.damtp.cam.ac.uk/user/tong/gr.html
You can't aim our current grav wave detectors. In a very simplified form, it's two very long tubes with laser in it, you aren't pointing these buildings like you could do with even very large telescopes.
The detectors have different sensitivity depending on the direction of the waves, so if a wave comes from a "bad" direction (perpendicular to both arms, e.g. directly from above), a particular detector might not detect anything, even if the wave is strong.
This is why (amongst many other reasons) it's important to have multiple detectors around the world (e.g LIGO has two locations in the US, you also have Virgo in Italy, and they do collaborate), this way you can assure in theory that you can "see" every wave, no matter which direction it's coming from.
Having only two detectors was always a sketchy affair given the tremendous amount of filtering done by the teams. Now with three detectors, if the correlataed event count between pairs of detectors doesn't diminish from one detector events with the same ratio as from 2-3 detectors (high school statistics), then we will know that event detection is real.
> The detectors have different sensitivity depending on the direction of the waves, so if a wave comes from a "bad" direction (perpendicular to both arms, e.g. directly from above), a particular detector might not detect anything, even if the wave is strong.
Unless there's such a thing as polarized gravitational waves, https://www.ligo.org/science/Publication-O1StochNonGR/ which might exist but are hard to discern with current detectors from what I understand. It'd be really cool to learn that there's such a thing as vector and scalar polarized gravitional waves.
> LIGO can't point to specific locations in space
Since LIGO doesn’t need to collect light from stars or other objects or regions in space, it doesn't need to be round or dish-shaped like optical telescope mirrors or radio telescope dishes. Nor does it have to be steerable, i.e., able to move around to point in a specific direction. Instead, each LIGO detector consists of two 4 km (2.5 mi.) long, 1.2 m-wide steel vacuum tubes arranged in an "L" shape (LIGO's laser travels through these arms), and enclosed within a 10-foot wide, 12-foot tall concrete structure that protects the tubes from the environment.
> A mirror at the vertex of the arms splits a single light beam into two, directing each beam down an arm of the instrument
Mirrors at the ends of the arms reflect the beams back to their origin point where they are recombined to create an interference pattern called 'fringe
Intuitively, it would seem that one wouldn't 'aim' a gravitational wave detector. The inputs are omnidirectional, and direction-finding would be accomplished by triangulation based on an array of detectors.
Gravitational waves detectors, just like any kind of antennas have anisotropic sensitivity. By having multiple detectors with different orientations you can determine the source of the signal. But you can't aim them, no, at least not any current or planned ones. They are simply too big (many kilometers) and require extreme stiffness.
Everybody knows of LIGO, but it's actually three detectors that work together, LIGO, Virgo, and KAGRA.
There is also a project called GEO600 in Germany that is collaborating with LIGO (and therefore also Virgo/KAGRA). This detector is much smaller though.
A gravitational wave moves at the speed of light. So if you compare arrival times from several widely-spaced detectors, you can get a pretty good idea of what direction the wave came from.
Even a single detector has two arms at 90 degrees to each other, which can give you a rough idea of where in the sky it came from. But now that we have multiple detectors online we get better and better about spotting the origin.
It's a triangulation problem. As long as the two detectors are perpendicular with each other you can triangulate the source of the signal received by both of them.
[1] https://en.wikipedia.org/wiki/Weber_bar