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Comment by deftnerd | Hacker News Reader

The detectors are a big L-shaped pipeline that lasers bounce around in. Doesn't that just measure the X and Y axis of gravitational waves? What about Z-axis? Shouldn't they drill down as far as they can and add another arm?

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Yes, each detector in itself cannot detect a wave going perpendicular to the two arms.

However, the two detectors that make up LIGO are oriented differently. In addition there's the VIRGO detector in Italy and more detectors being built.

With multiple detectors you can not only measure in different orientations, but also consider the difference in detection time between the detectors. So by combining data you can really narrow it down.

You can see an example of that in the first image in this article[1]. The light green band is the most probable event area using LIGO data only. The dark green circle is the combined VIRGO + LIGO data. Note how the LIGO only data can't tell which hemisphere the signal came from, but the addition of the VIRGO data allows them to figure that out.

The blue bands/circle are the optical measurements.

[1]: http://public.virgo-gw.eu/ligo-and-virgo-make-first-detectio...

I forgot to add, the reason they don't drill down is because it's hard and expensive. The arms are 4km, and to get the required sensitivity they bounce the light about 300 times before detecting it.

Drilling down 4km would be quite expensive, especially since you need a large mirror assembly[1] and vibration isolation system[2] down there. The mirror assembly and vibration isolation systems would also have to be of a different design, further increasing cost and complexity.

[1]: https://twitter.com/ligo/status/601848954645401601

[2]: https://www.ligo.caltech.edu/page/vibration-isolation

The immediate follow up question would be "but why don't they make the arms shorter?"

As far as I know the problem then is you'd have to bounce the light even more to compensate, and for each bounce you lose some light. Say you drilled "just" 1km down. To compensate you'd have to bounce the light 1200 times. Say the mirrors reflect 99.9% of the light. After 300 bounces you still have 74% of the light left, but after 1200 bounces that's down to 30%. And that's only the reflection loss, then you got beam spread etc.

So why not just increase the laser power to compensate then? AFAIK, increasing laser power means increasing laser noise, so that would ruin your signal to noise ratio.

In addition, the shorter arm means you'll be even less sensitive to longer wavelength gravitational waves, regardless of the number of bounces. The LISA mission was supposed to use satellites flying 2.5 million km from each other, to be able to detect really low frequency waves.

It would be waaaay easier to put additional detector 90 degrees away on the surface of the Earth than it would be to drill 4km into the ground (and service mirrors there!).

But having three arms on the surface still makes the detector blind to the third dimension.

Imagine a giant cube around the entire earth that touches in 6 places -- one on each cube face. Pick 3 faces and place a detector where each touches the earth -- on the surface. You can rotate the detectors around on the surface so that you get an arm in each axis relative to the center of the earth.

Right, that's basically what they do now by building multiple detector facilities. I was talking about multiple arms in a single location.

This isn't necessary since the wave looks the same at all locations on Earth.

It can come directly from above, in which case you can't discriminate between from above or from below as it'll hit all arms at the same time if they're at a single location. You'd also have poor directionality.

Wouldn't the Spacetime Cube be just as vulnerable to the perpendicularity problem, though? It might have more sensitivity in near-perpendicular cases, but that's simply because there are more detectors.

As mentioned, if multiple detectors are not spatially coinciding, then one can use the time difference between detection events as well.

A time cube...

The other detectors in the group provide different aspects. They all see the same events.

Build one near the Equator.

You don't need 2 dimensions to detect a wave in 2-d (for example ripples on a lake). It's the same here.

There are angles where the detector are less effective, but your aim is not to have perfect coverage, it is to detect them.

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