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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? Sh

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?

raverbashing 8y ago
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.
kevin_thibedeau 8y ago
The other detectors in the group provide different aspects. They all see the same events.
magicalhippo 8y ago
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-detection-of-gravitational-waves-produced-by-colliding-neutron-stars/ http://public.virgo-gw.eu/ligo-and-virgo-make-first-detectio...
magicalhippo 8y ago
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 https://twitter.com/ligo/status/601848954645401601 [2]: https://www.ligo.caltech.edu/page/vibration-isolation https://www.ligo.caltech.edu/page/vibration-isolation
- magicalhippo 8y ago
  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.
- jessriedel 8y ago
  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!).
  - magicalhippo 8y ago
    But having three arms on the surface still makes the detector blind to the third dimension.
    - granos 8y ago
      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.
      - magicalhippo 8y ago
        Right, that's basically what they do now by building multiple detector facilities. I was talking about multiple arms in a single location.
jloughry 8y ago
Build one near the Equator.
deleted 8y ago
[deleted]

raverbashing 8y ago

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.

kevin_thibedeau 8y ago

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

magicalhippo 8y ago

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-detection-of-gravitational-waves-produced-by-colliding-neutron-stars/ http://public.virgo-gw.eu/ligo-and-virgo-make-first-detectio...

magicalhippo 8y ago

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 https://twitter.com/ligo/status/601848954645401601 [2]: https://www.ligo.caltech.edu/page/vibration-isolation https://www.ligo.caltech.edu/page/vibration-isolation

magicalhippo 8y ago
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.
jessriedel 8y ago
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!).
- magicalhippo 8y ago
  But having three arms on the surface still makes the detector blind to the third dimension.
  - granos 8y ago
    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.
    - magicalhippo 8y ago
      Right, that's basically what they do now by building multiple detector facilities. I was talking about multiple arms in a single location.

jloughry 8y ago

Build one near the Equator.

deleted 8y ago

[deleted]