# Can LIGO Detect Gravity Waves from Directly Above or Below?

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• Bern1937
In summary: The middle dot does move about an inch, but that's because the distance between the two dots has changed. The length of the arms doesn't change, just the distance between the dots.
Bern1937
TL;DR Summary
LIGO operation, for gravity waves
As I understand, the gravity wave detection system is possible due to the change in length of the two arms, as a wave passes, thus changing the distance the light beams travel. As this distance changes, the time of flight will change in relationship to each other. This difference is then measured, after combining, and monitored, for the changing phase angle, caused by a passing gravity wave.

My question: If the source of the wave is located directly above or below LIGO, will the gravity wave be detected?

Thank You,

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Point of language: LIGO detects gravitational waves. Gravity waves are a type of surface wave on water.

The directional sensitivity of a LIGO detector is quite complex. The "stretch and squish" effect is perpendicular to the direction of travel of the waves. So a single LIGO instrument is actually most sensitive to waves coming from directly above or below with polarisation parallel to its arms, since one arm is stretched and the other one is squished. It is blind to waves coming in the same direction with polarisation at 45° to its arms because they don't stretch either arm. It is less sensitive to waves coming in at other angles even if the polarisation is optimal. The velocity of the source also matters since this can make some component of the stretch and squish parallel to the detector's timelike direction, to which it is also blind.

All of this is why it started with two detectors in different orientations, and why they have added and plan to add more.

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There's a nice diagram at Wikipedia showing how a ring of free floating masses react to a gravitational wave passing through perpendicular to the plane of the image. You can see the stretch-and-squish in action.

LIGO doesn't have a ring of free floating masses. It has just the topmost and rightmost dots and another one in the centre, which would be unmoving. If you ignore everything except those two dots and the middle of the ring, you can see that one arm lengthens as the other shortens. But that's if the wave is polarised conveniently. If the wave happens to come in polarised so that LIGO is at 45° (so its arms point north east and south east on the diagram) then you can see that those dots don't move at all.

## What is LIGO and what does it do?

LIGO (Laser Interferometer Gravitational-Wave Observatory) is a scientific experiment used to detect and measure gravitational waves. It consists of two identical detectors located in the United States, which use laser interferometry to detect tiny ripples in space-time caused by the collision of massive objects, such as black holes or neutron stars.

## How does LIGO sense the direction of gravitational waves?

LIGO uses two perpendicular arms, each 4 kilometers long, to measure tiny changes in the length of space caused by gravitational waves. By comparing the time it takes for a laser beam to travel down each arm, scientists can determine the direction and strength of the gravitational wave.

## Why is the direction of sensing important in LIGO?

The direction of sensing is important because it allows scientists to pinpoint the location of the source of the gravitational wave. By combining data from multiple detectors, they can triangulate the source and confirm the detection of a gravitational wave.

## How accurate is LIGO in sensing the direction of gravitational waves?

LIGO is incredibly accurate in sensing the direction of gravitational waves. It has the ability to detect changes in length as small as one thousandth the diameter of a proton, allowing it to pinpoint the direction of a gravitational wave source within a few degrees.

## What are some potential applications of LIGO's direction sensing capabilities?

LIGO's direction sensing capabilities have the potential to revolutionize our understanding of the universe. By detecting and studying gravitational waves, scientists hope to gain insights into the nature of gravity, the formation of black holes, and the origins of the universe. LIGO's data could also be used to test Einstein's theory of general relativity and potentially lead to new technologies, such as more precise navigation systems and improved methods for detecting and monitoring earthquakes.

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