Bad Astronomy article here.
What strikes me is that the detectors are only 2-dimensional. They have an X axis and a Y axis. Why don’t they have a Z axis? Drilling a hole 4 km deep is nothing new. Is a Z axis not actually useful?
I probably should not WAG here but here is my WAG:
Remember that LIGO is actually two observatories. As such their 2D planes are oriented in different directions which would give them some amount of detection on the Z-axis I would think. (Think of two sheets of paper flattened along their axis…since the two are separated far apart on the globe those two planes will not align perfectly but will be angled to each other in the third dimension.) So you get your detection in on the third axis without the cost and added complexity of digging a deep hole.
4km would make it as deep as the deepest mine shaft in the world. And they’ve been digging down since 1981. It may be worth the cost if you are literally digging up gold along the way, but it would be a ridiculous cost for a research project.
There are deeper boreholes like the Kola Superdeep Borehole, but these are very narrow shafts - the Kola is 9 inches in diameter. The vacuum chambers at each ends of LIGO are somewhat larger.
The point of the detectors isn’t to see the shape of the wave but determine the point in the dome of the sky where the source is located. The 2 LIGOs give a good view and as you add more detectors like Virgo the region of the sky gets more definitive.
Example here - APOD: 2017 September 28 - LIGO Virgo GW170814 Skymap
Oil well shafts are routinely longer, with the deepest being over 10km. and they can be 1 metre or more in diameter.
Well yes, but would not the third dimension help massively with both?
Also - 3 arms (XYZ) at one location may help determine the direction of the gravitational wave, but it’s not a very accurate way to do it. You’d be relying on the relative signal strength between the 3 pairs of arms. But the signal is so weak that the difference in signal strength has a huge error bar.
The timing, on the other hand, can be measured very accurately. So using detectors at 3 locations, the direction can be measured very accurately by measuring the time delay between 3 locations. (LIGO currently only has 2 locations, but is now coordinating with Virgo which makes it 3.)
I would think the diameter would need to be a lot bigger. Presumably the scientists would need access to the equipment to perform maintenance and calibrations and whatnot.
Maybe, but the cost to build a 4 km shaft capable of installing/servicing and replacing the instruments would be a waste when you can just build another detector on the other side of the planet.
1 meter is nowhere close to enough. See the link I posted above showing the size of the vacuum chamber that would need to be at the bottom of the hole.
And working at the bottom of the hole is a completely different matter. From this article:
But you don’t need people there.
The equipment at the end of the arm is large and complicated. It would be incredibly expensive to design it so that it can be installed and operated remotely, without anyone ever going down there.
I get the feeling you might not understand what the equipment looks like and what is required to attend to its installation/servicing and replacement needs.
Here’s a pdf for Advance LIGO presented in 2003. Run through it and look at the type of equipment they were discussing back in early 2003
But again, why build a z-axis when you can build another one, possibly maybe 12,600 km away?
Yeah, a lot of things make this a non-starter. One does need access to the full extent of the arm; the equipment at the end is particularly complex; the optics get complicated if you try to split/mix a third arm; all the equipment is engineered for operation at the surface with gravity pointing down, so you’ll have to develop entirely different seismic, thermal, electrical, etc. isolation and control techniques for the third arm; because of this, certain common-mode noise cannot cancel; digging a 4km shaft is not feasible*; etc.
And as has been noted, you can just build another 2D one on another part of the earth to get that different view of things, which is in fact what we have (especially with LIGO+VIRGO).
[sub]* This isn’t poking a drill bit down. This would need to be a fully excavated, structurally supported, and habitable shaft.[/sub]
People, people, you are thinking in the wrong direction–the third arm needs to be a tower 4km tall!
You’re right, I don’t.
It seems that this is the simplest and most cost-effective solution.
Now, what we really need is about a dozen of these things, scattered widely across the globe. Sure, other countries can build their own if they want, and that works too, but we already have one design (the LIGO design) which we know works, and which has all of the R&D already done. Building another one would cost a lot less than half the cost of building the first two.
Last I heard, there were in fact plans to build another LIGO in India, and Japan is working on one called KAGRA that will (probably) be even better than LIGO (but we can’t be sure because it isn’t actually finished yet). And Italy is continually improving VIRGO, based on what we’ve learned at other detectors. All of that is a good start, but there’s a ton more science you can do with even more of them. For starters, with enough locations, you could do your signal detection just by coincidence matching, without having to resort to templates (which make it almost impossible to detect unexpected sources).
As an example, the test masses are hung at the ends suspended from wire. This makes it very easy for them to move horizontally (actually, along a circular arc whose tangent is horizontal, but the movements are small enough that the distinction is unimportant), but they won’t so easily move vertically. To do a vertical equivalent, you’d need to hang it from springs, and then you’d need to worry about perfectly characterizing the elastic and other material properties of those springs (which would probably change with time, even).
No, it isn’t. Setting aside all of the difficulties of building the device to fit into an underground borehole where it cannot readily be accessed for maintenance or upgrade, the fact is that such a cavity would have to be bored, reinforced and isolated against tectonic movement, and all other manner of problems that would come along with it. If there were any advantage to constructing an interferometer with arms in three orthogonal axes, it would be easier (though far from trivial) to construct it upward where it can at least be accessed, although the maximum construction height and fidelity of measurement would be limited.
As scr4 and Chronos note, it would be better (as well as cheaper and easier) to simply build more detectors at different parts of the globe which would allow a measurement of direction and estimate of distance. Better yet would be to position interferometers in different solar orbits, increasing the precision of measurement by orders of magnitude as well as signals at much far lower frequencies (in the mHz range instead of tens of Hz) and the ability to look at gravitational waves emitted by cosmic-scale structures.
Stranger
Oh, and an aside about those wires they’re hanging from: As with everything else on the project, the engineers calculated the necessary properties for those wires to within a how-do-you-do… and then discovered that folks had been making wires with the necessary properties and to the necessary precision for centuries. It’s just standard off-the-shelf piano wire. I’m sure it’s comforting, among all of the 40-kg flawless sapphire crystals and 4-km long deep-vacuum chambers and so on, to find something that’s just off the shelf.
I was referring to multiple sites, not the 3D one.