APOD: Advanced LIGO: Gravitational Wave... (2016 Feb 07)

[RocketRon]

And how do they shield/compensate LIGO from the scale of the moons gravity changes ?

[monarshist]

Was it inentional to put the main building exactly on a fault?

[Chris Peterson]

I don't think you generate gravitons, nor gravitational waves....just pushing air....making air waves....swish....
Drop a pebble in at one end of lake Superior...and try to detect that on the other side...an awful lot going on inbetween.

But drop a lot of pebbles, at regular intervals, and you might detect that all the way across Lake Superior. We can detect oscillating signals that are deeply buried in noise.

Most, if any, ripples in spacetime go back down very quicky... Like radio waves...they don't go out fifty light years intact...only around 1.5 ly...then they are so dispersed they are inditiquishable from the background radiation.... I think gravitational waves might be similar.

Their energy drops off according to the inverse square law. At some point the signal becomes significantly less than the noise. But that doesn't mean the signal is gone. Nor does it mean the signal is theoretically undetectable.

To borrow a concept.... If it hasn't been detected....it doesn't exist....

I don't think existence depends on our ability to detect something.

[Chris Peterson]

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. But some confounding signals will only be eliminated by finding them on one instrument and not others.

[RocketRon]

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

Where would one find such details/discussions/calculations.

[Chris Peterson]

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

Gravimeters used for geological surveys have to compensate for the Moon's pull. GPS stations around the Earth detect the changing tidal direction and magnitude of the Moon. Probes in orbit around the Moon have mapped its gravitational field with very high resolution.

So yes, measuring the gravity of the Moon is accomplished many different ways.

Again, however, don't mistake a gravitational field for gravitational waves. The Moon's gravitational field is trivially detected and measured, but the Moon doesn't produce gravitational waves that we are capable of detecting.

[Heavy Man]

https://www.youtube.com/watch?v=X5EzjZynKbQ

[neufer]

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]

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.>>

Exactly. The signal is tuned out by controlling the arm length. This is possible because of the very low frequency of the tidal signal. This is a basic control system.

[DavidLeodis]

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

I would be grateful if someone could please clarify for me what is the length as I'm confused. In the explanation it states "Pictured here are the four-kilometer-long arms of one such detector: the LIGO Hanford Observatory". The image is also in the information brought up through the "Pictured here" link where in the information (but dated October 20 2000) with the image it states "The Washington two-kilometer interferometer, WA2K as it is called, has long optical cavities that span the two kilometer distance from a "mid-station" on each arm of the observatory to the corner station building...Currently, the WA2K interferometer is the largest precision optical device in the world. Yet that title will soon be surpassed when the Louisiana four-kilometer (LA4K) interferometer comes on line in Livingston. Then will follow Washington's four-kilometer interferometer, currently being installed next to WA2K at Hanford”. Is the "four-kilometer-long arms" in the explanation the combined length of 2 arms each 2km long?

[geckzilla]

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.

[DavidLeodis]

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.

Thanks geckzilla for your response, which is appreciated . That does seem to be the likely answer.

[Ron-Astro Pharmacist]

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

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.

[Jim Leff]

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?

[Chris Peterson]

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

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.

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?

An interferometer doesn't depend on any exotic quantum principles.

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. :|

The LIGO instrument is extremely simple and well understood on a theoretical level. All the magic lies in the engineering required to maintain stability and reject ground-based vibrations.

[Chris Peterson]

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?

Because it is also measuring a variety of other tiny vibrations. It can be calibrated. We know with a high certainty what its sensitivity to spacetime contractions is; if they aren't detected, it strongly suggests that those gravitational waves aren't there, or are much weaker than theory predicts. Either way, it would mean people would start looking at alternative theories.

[neufer]

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.

The Hanford arms are 4 km long and capable of housing two separate interferometers:

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

<<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.>>

[Boomer12k]

Gravity...is not necessarily....Gravitational....

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...

:---[===] *

[Chris Peterson]

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.

But it does. Like a ripple in a lake, a moving mass produces an outward propagating ripple in spacetime.

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...

No, you produce a gravitational wave that propagates away from you at the speed of light. Of course, it's an extremely weak wave (or if quantized, as described above by Art, only a rare graviton emission), but that's not important. You are treating it as if there is some kind of damping mechanism, and GR doesn't (as I understand it) provide one. A gravitational wave no more "calms down" than does an electromagnetic wave. Either one simply diminishes in intensity with distance according to an inverse square law, and for the same reason.

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...

That is certainly true.

[Ron-Astro Pharmacist]

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.

Well the hunt for light dark matter goes on. Thanks for your insight regarding interferometers - they are a ingeniously simple concept that tells us much about the world we live in. Of course I keep trying to bend the universe to see it from a different angle.

[neufer]

[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.>>

[Jim Leff]

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!

Looks like you win this year's science gossip leak award!

[Ann]

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.

What a timely observation!

Ann

[Bruce Mardle]

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.

[neufer]

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.

1) An unaccelerated charge has electric field lines sticking out of it that all point back to the charge.
2) When accelerated those electric field lines have a kink in them that propagates out as an electromagnetic wave.
3) When combined with an attracting opposite charge: dipole EM radiation is produced.
4) When combined with a repulsing similar charge: quadrupole EM radiation is produced.

1) A unaccelerated mass has gravitational field lines sticking out of it that all point back to the mass.
2) When accelerated those gravitational field lines have a kink in them that propagates out as a gravitational wave.
3) When combined with an attracting similar mass: quadrupole gravitational radiation is produced.