Starship Asterisk*

Markus Schwarz wrote:

Sunnysky wrote:
It is quite doubtful about LIGO's claim of having detected gravity wave. If you imagine a tube of 1 light-year radius and 1.4 billion light years long you will have about 17.6 million solar starts inside the tube. For 100-200Hz gravity wave, the wave length would be 1500-3000km which are still much smaller compared with the solar radius of 0.7 million km. So, statistics shows there must be dispersion effect on different wavelength waves; the longer wavelength wave will travel faster than the shorter wavelength one and the shorter wavelength wave will suffer more amplitude damping. Now the challenge is, then, how can LIGO get the gravity waveform exactly the same as the theoretical calculations for two black holes merging without considering any dispersion effects?

I don't understand your argument. Where do you get the 17.6 million stars per "tube" from?

Markus Schwarz wrote:
From the linearised field equations of general relativity in empty space (valid for the propagation of gravitational waves) one obtains that the phase and group velocity of gravitational waves equals the speed of light and that there is no dispersion.

Sunnysky wrote:It is quite doubtful about LIGO's claim of having detected gravity wave. If you imagine a tube of 1 light-year radius and 1.4 billion light years long you will have about 17.6 million solar starts inside the tube. For 100-200Hz gravity wave, the wave length would be 1500-3000km which are still much smaller compared with the solar radius of 0.7 million km. So, statistics shows there must be dispersion effect on different wavelength waves; the longer wavelength wave will travel faster than the shorter wavelength one and the shorter wavelength wave will suffer more amplitude damping. Now the challenge is, then, how can LIGO get the gravity waveform exactly the same as the theoretical calculations for two black holes merging without considering any dispersion effects?

Bruce Mardle wrote:

Accelerate a charge and you'll get electromagnetic radiation: light.
But accelerate any mass and you'll get gravitational radiation

Is it really as simple as that? I know very little about general relativity but, as I understand it, EM radiation is typically emitted mostly by dipoles but there are no gravitational dipoles (since mass only has 1 'sign'), just quadrupoles and higher-order poles.

Accelerate a charge and you'll get electromagnetic radiation: light. But accelerate any mass and you'll get gravitational radiation

alter-ego wrote:

daddyo wrote:I wonder what's the latest theory as to why gravity waves have been so hard to detect.

The predicted spatial distortions are exceedingly small, miniscule fractions of a proton size. Previous LIGO generations have been on the marginal end of picking up extreme mass events, e.g 10⁶ solar mass binary mergers. Combined with event size and required proximity for detection, these are rare events. aLIGO is believed to be sensitive enough to see smaller, more numerous (and closer) events like supernovae:

Click to view full size image

Over a 4-km length, the aLIGO can detect an arm-length change 1000 times smaller than a proton! This is an amazing achievement.

aLIGO represents a perceived turning point in gravitational wave detection. As long as it performs per design, I believe scientists will begin to question the physics if no detection is made over a reasonable (years) time frame.

heehaw wrote:All the good APOD folks should hear the rumor (from good sources): please read the New York Times this Thursday, February 11. No guarantees, but I hear ... this might be IT!

https://www.advancedligo.mit.edu/ wrote:

[img3="12 hours of H1 (Washington) and L1 (Louisiana) "double coincidence" operation with an average neutron star inspiral range of about 75 MPc (244 million light years)."]https://www.advancedligo.mit.edu/graphi ... 402_v2.jpg[/img3]

Advanced LIGO News
LIGO O1 Progress Report
November 2015

<<LIGO's O1 observing run, the first data run of the advanced gravitational wave detector era, began in September 2015. An array of always-on software monitors continues to provide diagnostic information related to detector performance as the run proceeds. For gravitational waves from the inspiral of a pair of 1.4 solar mass neutron stars (the "standard candle" for gravitational wave interferometry), H1 and L1 typically range to an all-sky average of about 250 million light years, roughly a factor of four above LIGO's 2010 sensitivity.

A significant portion of the range improvement comes from the gain that LIGO has realized at low frequencies. The Initial LIGO interferometers were insensitive to astrophysical signals at 40Hz. Currently, both L1 and H1 operate with a displacement sensitivity of better than 10^-19 meter per root Hertz at 40Hz. When a post-O1 round of commissioning begins in January 2016, LIGO will continue to push the 10^-19 m design sensitivity benchmark closer to 10Hz.

Currently LIGO has spent 47% of time in O1 with L1 and H1 running together, just below the O1 target of 50%. The improved stability of the interferometers has led to some very long lock stretches, including a 60(+) hour lock stretch on L1 that began on November 4. Once a lock loss occurs, the path to re-locking sometimes is lengthy. Roughly 15% of available time has been used attempting to lock, a figure that LIGO aims to reduce. LIGO's new technique for locking the long arms independently, named arm length stabilization, has proven to be rapid and robust. Locking the optical cavities near the beam splitter [the DRMI] and bringing the DRMI and the long arms under unified control requires more time and oversight. When locked, H1 and L1 demonstrate significantly better immunity to environmental effects than did Initial LIGO. The combination of out-of-vacuum hydraulic pre-isolators, in-chamber active isolation systems and multi-stage optic suspensions are delivering as expected. The sensing range of the detectors has become much more insensitive to local fluctuations in natural and anthropogenic seismicity. Unsurprisingly, large earthquakes and other disturbances that elevate ground velocities to levels above a few microns per second usually cause lock losses.

A locked detector provides a full set of control signals that enable a vigorous seismic defense. When an environmental disturbance pushes the detector from its operating point, a number of these signals disappear, and their absence creates challenges for detector operators on the path back to lock acquisition.

Of course glitches in the interferometers can masquerade as signals of interest. Detector commissioning and engineering teams, working closely with the Detector Characterization group of the LSC, continue to identify glitch sources, other noise sources, and instabilities in the detectors. Some sources receive mitigation from the control room. Others will require configuraton changes that will occur during the next commissioning break. LIGO plans to continue O1 through mid-January 2016.>>

Chris Peterson wrote:

Ron-Astro Pharmacist wrote:How does one make a half-silvered mirror? Use half the amount of silver or apply the regular amount to half the surface...

Yes, that's exactly how you do it. The reflectivity is controlled by the thickness of the deposited material- usually aluminum, but other materials may be used depending on the wavelength of the light the beam splitter is designed for. It is also possible to make prism beam splitters that depend on internal reflection rather than a reflective metallic coating.

Boomer12k wrote:Gravity, is an effect created by a mass body, where spacetime is indented, and warps down inward to the Center of the Mass Body. A Gravitational Wave...is supposed to be an outward going wave in the medium of space time....not necessarily the same thing, or effect...and I personally think is in a bit of error. IT is not the same as a boat on a lake. Gravity makes things fall INWARD back to the center of the mass body. I don't think anything goes "out" very far...my personal opinion.

If YOU were walking in your house, you are not creating a gravitaional wave that goes out to the rest of the room...your sphere of influence may warp the space around you, making things TEND to fall toward you as you bend spacetime inward towards you....you do not make a "wake" therefore. Those "waves" would fall back into you, OR, the spacetime would calm back down very quickly...

The Earth's Tides are not detecting the Moons Gravitational Waves....but the warped spacetime effect of normal gravity...an INWARD falling toward the Moon...Not the same disturbance of Gravitation waves by orbiting bodies....my opinion...

geckzilla wrote:
David, I was confused about that as well, and as far as I can gather the detectors within each of the arms are currently 2km long while the arms themselves are each twice that length, thus capable of housing detectors twice as long.

• 1) a 4 km long Michelson interferometer and
  2) a 2 km long Fabry–Pérot interferometer.

https://en.wikipedia.org/wiki/LIGO wrote:
<<The LIGO Hanford Observatory houses one interferometer, almost identical to the one at the Livingston Observatory. During the Initial and Enhanced LIGO phases, a half-length interferometer operated in parallel with the main interferometer. For this 2 km interferometer, the Fabry–Pérot arm cavities had the same optical finesse, and thus half the storage time, as the 4 km interferometers. With half the storage time, the theoretical strain sensitivity was as good as the full length interferometers above 200 Hz but only half as good at low frequencies. During the same era, Hanford retained its original passive seismic isolation system due to limited geologic activity in Southeastern Washington.>>

Jim Leff wrote:

alter-ego wrote: aLIGO represents a perceived turning point in gravitational wave detection. As long as it performs per design, I believe scientists will begin to question the physics if no detection is made over a reasonable (years) time frame.

Circular, no? How can we know it's been performing per design if nothing's found?

Ron-Astro Pharmacist wrote:How does one make a half-silvered mirror? Use half the amount of silver or apply the regular amount to half the surface... :ssmile:

I know there are different ways to make them but, if all is not known at the quantum level, how can we be sure all the results from such a surface (which has been relied on for so many years) tell the entire story?

Not trying to cast doubt on enormously well proven physics (I wouldn't claim to know) but it does seem we have placed a lot upon this one devise. I suspect it's the best technology for what we have actual physical and theoretical knowledge. :|

alter-ego wrote: aLIGO represents a perceived turning point in gravitational wave detection. As long as it performs per design, I believe scientists will begin to question the physics if no detection is made over a reasonable (years) time frame.

geckzilla wrote:David, I was confused about that as well, and as far as I can gather the detectors within each of the arms are currently 2km long while the arms themselves are each twice that length, thus capable of housing detectors twice as long.

RJN wrote:

narodnik wrote:Four-kilometer arms, not two!

Yes, this error has been corrected on the main NASA APOD. The arms are four kilometers long. We apologize for the error.
- RJN

neufer wrote:

Chris Peterson wrote:

RocketRon wrote:
So can LIGO measure the gravity changes from the moons orbit. ?
Never assume we can run before we can walk.

And the scale and units of those forces ?

The effect of the Moon's gravity on the arms of LIGO is many orders of magnitude greater than the effect of passing gravitational waves. But LIGO isn't detecting gravitational waves from the Moon, it is being directly stretched by the Moon's gravitational field. Just one of many confounding signals that have to be compensated for. Some, like the Moon, are simply tuned out by adjusting the arm length- easy since the frequency is much, much lower than the signal being sought.

Reminiscences: A Journey Through Particle Physics
By Adrian C. Melissinos

(paraphrased from) The Earth Tides, p. 163:

<<LIGO mirrors are mounted on the non-rigid surface of the earth which is deformed by tidal forces. Arm deformations cannot be accurately predicted because the elastic coefficients of the earth vary at different locations. For LIGO's 4 km arms an asymmetric tidal signal of up to 0.2mm far exceeds the dynamic range of the control system. Such tidal effects must be corrected by a predictive feed-forward system to an accuracy sufficient to allow the control system to keep the LIGO phase locked.>>

Chris Peterson wrote:

RocketRon wrote:
So can LIGO measure the gravity changes from the moons orbit. ?
Never assume we can run before we can walk.

And the scale and units of those forces ?

The effect of the Moon's gravity on the arms of LIGO is many orders of magnitude greater than the effect of passing gravitational waves. But LIGO isn't detecting gravitational waves from the Moon, it is being directly stretched by the Moon's gravitational field. Just one of many confounding signals that have to be compensated for. Some, like the Moon, are simply tuned out by adjusting the arm length- easy since the frequency is much, much lower than the signal being sought.

Reminiscences: A Journey Through Particle Physics
By Adrian C. Melissinos

(paraphrased from) The Earth Tides, p. 163:

<<LIGO mirrors are mounted on the non-rigid surface of the earth which is deformed by tidal forces. Arm deformations cannot be accurately predicted because the elastic coefficients of the earth vary at different locations. For LIGO's 4 km arms an asymmetric tidal signal of up to 0.2mm far exceeds the dynamic range of the control system. Such tidal effects must be corrected by a predictive feed-forward system to an accuracy sufficient to allow the control system to keep the LIGO phase locked.>>

RocketRon wrote:So are there instruments that have detected and calculated the strength of the moons gravity ?

RocketRon wrote:So can LIGO measure the gravity changes from the moons orbit. ?
Never assume we can run before we can walk.

And the scale and units of those forces ?