Detector Watch S6 V1

Indeed! I'll have a crack at it!! :-)

Yes, such an effect has been seen. Tidal movements of the Earth's continents has been factored into a program that slightly adjusts the interferometer mirror positions on a continuous basis to account for arm length changes that are induced. The ground under the interferometers actually changes shape to a degree that corrections are needed, or the interferometer would lose lock within a short period of time ( about an hour I think ). So if anything we don't want the interferometer to 'see' the Moon! Actually I think there are GR studies of the moon, but they are using laser ranging ( bouncing off arrays left during Apollo times ).

Hasn't been tried but I guess could be. I think they've accepted that it will take a space array ( LISA ) to look at that lower frequency band and below rigorously. The more noise there is in a band then the longer are the 'integration times' required to pick up a given signal. In which case you'd need longer un-interrupted stretches of interferometer lock, with correspondingly more analysis work ( that translates to the work units here at E@H ). For the type of search here at E@H ( which is only one aspect of LIGO work - hunting for continuous waves from pulsars ) most of the signal searching is a 'matched filtering' technique. This requires data that remains coherent in phase - you don't miss any wavelengths - for extended periods. A typical data segment could potentially contain tens of thousands of gravitational wave cycles of interest, at some frequency, and the analysis method will return a low statistic ( sort of a signal to noise ratio ) if there's even a half wavelength shift over that time. Indeed because E@H does an all sky search, the signal templates we try to match with the data are of many varieties corresponding to sky position, source frequency and frequency change per Doppler shifting. Because the Earth rotates and orbits around the Sun, in addition to whatever motion the source has way out there. The point is there is already an enormous calculation load, which is why E@H exists, for the better frequency reception bands. I haven't read of anyone getting especially excited or expecting detections from LIGO below about 50Hz, say.

Not exactly. But for a good laugh I'll tell you of what they have been up to with testing the seismic instrumentation system ( from this article ):

Nice work if you can get it! Depth charging the local flood retarding pond. The 'gravity' wave mentioned here is older unrelated terminology from hydrology. It's simply the bulk up/down motion of the water.

Yep, see above.

The Sun doesn't burp in a way that would be realistically detected. Or if it did, we would all be in mortal strife. This is a deep issue that goes to the heart of the difference between gravity and the other forces. If I grab some charge and push it around I can generate emission of waves easily. Indeed we are communicating now using technology based upon that. For gravitational wave production it is not simply enough to accelerate some mass ( which is the gravitational 'charge' ). The pattern of movement must have particular geometric characteristics, related to conservation of mass/energy/momentum/angular-momentum and that there are no negative masses ( or anti-gravity if you like ). While the Earth-Sun system may have such characteristics, the effect is small due to relatively low masses and speeds. There is also another issue of distinguishing/detecting effects in the 'near' and 'far' fields. General Relativity is non-linear, so close in to the source there are all sorts of 'higher order' effects which rapidly diminish with distance - like an inverse cube law say. Whereas we ( hopefully! ) see waves from distant sources because the strength in the far field goes like a simple single inverse power.

Indeed one goal of this GW detector business is to verify whether any of our above understandings of the consequences of GR are true. I am currently re-reading a book called "Traveling at the Speed of Thought : Einstein and the Quest for Gravitational Waves" by Daniel Kennefick. It outlines quite well the history of GR research. The themes of all of your questions were/are/will-be the object of discussion amongst researchers. In the end experiment will decide and guide ( Hello? Anyone doing string theory listening? ). The most dramatic result from GW experiments would be we looked, no waves were found, and if they were there we would have heard them.

Nowhere near enough mass/energy/velocity to achieve that. Unless possibly if the tsunami was real close to the interferometer. It'd be along the lines of the holding tank above. But then there'd be other issues.

This is the key point. Order of magnitude calculations rule out any method of generating waves here on Earth. You have to have mass/energy/velocities far out of even solar system scale to even begin to wiggle spacetime significantly. That's a result of high speed of light ( or that to higher powers actually ) and the slightness of G. The frequency band that a space array would detect refers to really, really big things like multi-million stellar mass black hole mergers ( ie. colliding galaxies ), whole universe oscillations from the ( well, alleged ) inflationary epoch etc.

Cheers, Mike.

Ah, Feng Shui for LIGO! Well done. :-)

[ The astronomers keep raving about the solar system barycentre. The place to be. It's inside the Sun .... ]

Cheers, Mike.

The barycenter is the centre of mass - a computed position based upon our knowledge of positions and masses of bodies in the solar system. There isn't anything unusual in space or time at the barycentre on account of it being that. If I choose a different set of objects to calculate for I'll get another answer. Given that it varies, that suggests a number of possibilities like inaccuracy in our knowledge, or influences unaccounted for. Note that the Sun also is in orbit too, though we often think of it as fixed. Being rather larger than other bodies it just doesn't move as much. Indeed the extrasolar planet detections have been based upon the slight apparent wobbles in motions of stars due to the presumed/deduced presence of planets.

The use of the barycentre is to convert our observations/results to that reference frame. It is probably the closest to an inertial frame ( un-accelerated ) that we would have sufficient knowledge of here-abouts. Alot of the workload at E@H is to unwrap the effects of Earth's accelerated motions- the varying Dopplering of the signals due to rotation about axis and orbit around Sun. As your diagram shows any change in this barycentre frame is very slow, so only low acceleration and hence nearly inertial.

The Earth's surface is not inertial, witness the Coriolis effects on weather systems. I have found that even light aircraft need to account for non-inertial effects when travelling significant distances. So if you go Northward, in the Southern Hemisphere, then effectively the ground under you is moving faster towards the East the more North you go. Zero at the South Pole and about 1000 mph at the equator. It'd go like the cosine of latitude, I think. The aircraft's course may seem to deviate to the West without even trying! So you have to point the aircraft more eastward than you would otherwise calculate, to ensure that you 'catch up' with the earth and nullify the effect. But that depends upon the basis of navigation one uses. I think these types of effects were first discovered with cannonballs!! :-)

I'm not aware of any conjecture of the energy production or transport within the Sun being affected by planetary positions. But that may well be my ignorance. One wonders what planetary rhythm(s) would produce eleven-ish year cycles. That's the right order of magnitude for the gas giants' orbital periods.

LIGO and LISA are distinct efforts. Probably the LISA people are taking an interest in the current GW efforts. There is quite a range of possible GW sources to 'view'. Roughly speaking things bigger in size and mass go to lower frequencies. So the various methods are going to be better ( generally ) at picking up different types of objects. Ultimately all of the stellar binary variants are going to be searched for.

One particular method I rather like is the use of high precision pulsar timings over many years. This may detect a subtle change in the rate of time passing here on Earth compared to the field of pulsars. So our atomic clocks may slow, say, due to a very low frequency gravity wave passing by. We would see that as the pulsars speeding up. So low in GW frequency that it takes years to do so, it's wavelength is humungous.

Cheers, Mike.

( edit ) Darn the difference b/w 'centre' and 'center' spellings. Divided by a common language we are! :-)

( edit ) And that leftward deviation DownUnda gives the North - > West and South -> East rule for here. If you go south then the ground is travelling less fast towards the east and hence you will 'overtake' it comparatively. This behaviour is the converse in the Northern Hemisphere where southward movement puts you to the west of where you expected, and northward movement to the east. Just think of it like a cannonball, a ballistic movement that begins with a certain eastward velocity from the point of firing. That is : what is the eastward speed of the ground in the direction you are firing towards - is it greater than or less than where you began? So more generally expressed : movement away from the equator makes you go eastward, and toward the equator you go westward. [ We know the earth rotates toward the East 'cos the Sun rises there! ]

After some further thought about the barycenter, it occurred to me that the barycenter is not truly inertial. And thus that can't be the reason for it's use. If I bung a small test mass in there it will fall inward toward the Sun's centre. To keep it from moving away from the barycenter I'd have to apply some other force to stop that - hence breaching the concept of an inertial frame. Or put another way, the centre of mass is not the point of force equilibrium in an inverse square field *. For instance the point where the attractions from the Sun and Jupiter are equal ( and opposite as vectors ), say, is going to be way out near Jupiter and nowhere near the barycenter.

So I looked up what the International Astronomical Union has to say and came up with the definition of barycentric radial-velocity measure that includes this phrase :

note the 'zero gravitational potential' component. All in all it seems the idea is to construct a fictitious reference frame and convert various different observations to it, for the purpose of comparison between the research efforts. So when a pulsar Doppler measurement ( 'radial velocity measure' ) is made by a radio astronomer, say, we may compare that with some gravitational wave Doppler measurement by transforming both to this standard view. They no doubt would have made their measurements at quite different circumstances of the Earth's motion. It's fictitious because the gravitational potential at the barycenter is certainly not zero - that's why a test mass would actually wander off fairly quickly. So you want a motionless observer ( no velocity with respect to the barycenter ) which is at the barycenter, but ignoring the actual gravitational forces there! And we are ignoring things like being evaporated by the high temperature of course! :-)

It would appear that the value in doing this is to separate those Doppler effects caused by distant behaviours - at the source, stuff like lensing on the way, cosmic expansion, whatever - from what are obviously 'local' ones. And I'm guessing that because the position of the barycenter varies relatively slowly, one can make a reasonably sensible comparison of observations when they are transformed to the barycenter if the time of said observations is suitably close.

I suspect there are further subtleties however, and of course I may be spouting rubbish! :-) :-)

Cheers, Mike.

( edit ) * - except in the special case where one has only the two masses, and they are equal. Or generally, the centroid of an equilateral polygon of equal masses .....