[Harri (OP)]

A two black hole merger occurred in 2015 and LIGO observed waves from the event. Wouldn't the waves from such an event be observed at LIGO for quite a considerable amount of time? Or does such a massive event only produce a wave/s capable of being observed for a very short length of time?

[Halc]

LIGO has a frequency range to which it is sensitive, about 10 hz to 10k hz, no so different from the range of sound detectable by people. So if the massive objects orbit slower than a 5th of a second, LIGO isn't going to pick it up. Near the merger, the frequency rises very rapidly, resulting the characteristic 'chirp', after which the signal is no longer detectable.
It apparently takes under a second for an orbit to drop from a 5th of a second down to at least the photon-sphere, after which the wave signal fades away.


The waves do 'last forever' in the sense that they keep traveling past Earth out forever, always getting weaker by distance per the inverse square law.

[geordief]

LIGO has a frequency range to which it is sensitive, about 10 hz to 10k hz, no so different from the range of sound detectable by people. So if the massive objects orbit slower than a 5th of a second, LIGO isn't going to pick it up. Near the merger, the frequency rises very rapidly, resulting the characteristic 'chirp', after which the signal is no longer detectable.
It apparently takes under a second for an orbit to drop from a 5th of a second down to at least the photon-sphere, after which the wave signal fades away.


The waves do 'last forever' in the sense that they keep traveling past Earth out forever, always getting weaker by distance per the inverse square law.

Do they travel as as expanding sphere?

If so ,is the total  energy  level  on each concentric ring of the sphere the same no matter the distance from the source measurements are made?

[Halc]

Do they travel as as expanding sphere?

Gravitational waves travel in all directions at light speed, so yes in that sense. A given wave isn't spherically symmetric (rings as you put it) any more than a propeller in the air creates spherical waves. It creates more like spirals, strong in the orbital plane and weakest along the axis of rotation.

If so ,is the total  energy  level  on each concentric ring of the sphere the same no matter the distance from the source measurements are made?

There are not concentric rings, and the energy is most concentrated in the orbital plane. If you had an instrument capable of measuring Earth's gravitational waves, it would be stronger out by Neptune's orbit compared to the same distance but along the rotation axis of our solar system.

[geordief]

Do they travel as as expanding sphere?

Gravitational waves travel in all directions at light speed, so yes in that sense. A given wave isn't spherically symmetric (rings as you put it) any more than a propeller in the air creates spherical waves. It creates more like spirals, strong in the orbital plane and weakest along the axis of rotation.

If so ,is the total  energy  level  on each concentric ring of the sphere the same no matter the distance from the source measurements are made?

There are not concentric rings, and the energy is most concentrated in the orbital plane. If you had an instrument capable of measuring Earth's gravitational waves, it would be stronger out by Neptune's orbit compared to the same distance but along the rotation axis of our solar system.

Thanks,I  think I understand that now.

Still, I am still  wondering if we can still say  that these very asymmetric  "rings" carry away their  energy without  any loss of power at all  as they encounter obstacles in their path.

So would a neutron star or another black hole absorb their energy?

Or indeed just any  object of any appreciable mass?

Or does the gravitational wave go through these objects as if they were not there?

[Halc]

I am still  wondering if we can still say  that these very asymmetric  "rings" carry away their  energy without  any loss of power at all  as they encounter obstacles in their path.

A spiral is still symmetric, just not spherically symmetric.
By time-symmetry, if matter (say a binary star) can emit gravitational waves and lose energy in doing so, then such a system can hypothetically absorb them, gaining energy in the process, but the passing wave is not going to be focused in such a way that any measurable effect will occur. It would have to look like the time-reversed wave spiraling in, which isn't how it looks when it was emitted from afar.

So would a neutron star or another black hole absorb their energy?

A lone mass like a  neutron star which seems no more capable of absorbing them as emitting them. Any mass will however deflect the waves, breaking the symmetry like water waves crossing a shallow spot. A black hole must absorb the energy as there is no worldline through them. They're also the only things that absorb dark matter.

Or does the gravitational wave go through these objects as if they were not there?

Like dark matter, it goes through them like they were transparent, but still deflected by the gravity.

[evan_au]

does such a massive event only produce a wave/s capable of being observed for a very short length of time?

It takes millions of years for two black holes to merge, and they radiate gravitational waves all that time. However:
- The frequency is too low for LIGO to detect
- The power level is too low for LIGO to detect
- A proposed space-based gravitational wave detector would be able to measure much lower frequencies, and so may be able to detect hundreds of pairs, all slowly getting closer to each other, months before they move into the frequency range detectable by LIGO.
See: https://en.wikipedia.org/wiki/Laser_Interferometer_Space_Antenna

From another angle, some of the observed events are billions of light-years away. So any observers with similar equipment to ours would have been able to detect them long ago.  See the list here (1 MegaParsec=Mpc = 3.2 Million light-years).
https://en.wikipedia.org/wiki/List_of_gravitational_wave_observations

is the total  energy  level  on each concentric ring of the sphere the same no matter the distance from the source measurements are made?

If you add up the energy in each expanding sphere, it will stay the same, no matter how far you travel.
- Apart from some loss due to refraction from black holes, etc
- Which would not be much, because a stellar-mass black hole is perhaps 10km across, while the wavelength of gravitational waves is 1,000 km or more. A wave is not much affected by anything smaller than 1/4 wavelength. But a black hole in the center of a galaxy would have more of an effect on the higher-frequency gravitational waves.

always getting weaker by distance per the inverse square law.

As I understand it, our current generation of gravitational wave detectors do not measure the energy of the gravitational wave directly, but measure the strain of the spacetime distortions. The energy is proportional to the square of the strain.

This means, of all the methods used by astronomers for exploring the universe, this one does not suffer from the inverse-square law, but degrades as an inverse law.
- If you increased the area of an optical telescope by a factor of 4, you would be able to see twice as far, and explore a volume 8 times larger (inverse square law).
- If you increased the sensitivity of a gravitational wave detector by a factor of 4, you could see 4 times as far, and explore a volume 64 times larger (inverse law).

[Colin2B]

@Harri
I’ve changed the title of your question from gravity wave to gravitational wave.
A gravity wave is one that uses gravity as the returning force eg water waves.