Gravity Waves

[The God]

No. If we never directly measure a gravitational wave we still have volumes of indirect evidence. The discussion is always about the sensitivity of the empirical device measuring the waves and how to filter out local natural and unnatural 'noise' which places limits on the sensitivity. Putting them in inertial freefall [jn Space] eliminates the ground 'noise'. It's taken many years to get here. Looking forward to the space experiment. The history of the potential measuring device is pretty interesting.

We have no volumes of indirect evidence of Gravitational Waves, just the certain observations have been speculated to be thus caused.

Secondly, we do not conduct very expensive experiments to discuss about the sensitivity of measureing device, the design of the device is based on the sensitivity parameters (range, tolerance etc) of the measured variable, we ascertain in advance, through simulation and theoreticaly, whether what we are expected to measure falls in the range of the device or not. If so, then only we plan the experiment.

 

[sweetpea]

Secondly, we do not conduct very expensive experiments to discuss about the sensitivity of measureing device, the design of the device is based on the sensitivity parameters (range, tolerance etc) of the measured variable, we ascertain in advance, through simulation and theoreticaly, whether what we are expected to measure falls in the range of the device or not. If so, then only we plan the experiment.

like brucep says, after discussion it is thought best,in this particular case, to place the experiment in space, so as to filter out local and unnatural 'noise' which would mask your chosen range if the experiment was ground based.

The discussion is always about the sensitivity of the empirical device measuring the waves and how to filter out local natural and unnatural 'noise' which places limits on the sensitivity. Putting them in inertial freefall [jn Space] eliminates the ground 'noise'.

http://lisa.nasa.gov/faq.html#science_1

Can LISA science be done from the ground?
No. Both ground motion and time variations in gravity from spurious mass motions on the Earth prevent observations below about 1 Hz on the ground. It is necessary to make measurements in space in order to observe many of the important astrophysical sources throughout the Universe.

My bold. Those 'mass motions' are noise.

 

[paddoboy]

We have no volumes of indirect evidence of Gravitational Waves, just the certain observations have been speculated to be thus caused.
.

We certainly do have evidence that gravitational waves do exist.....
The Hulse-Taylor Pulsar - Evidence of Gravitational Waves
In this current "pre-detection" era it can be difficult to convince those who are not overly familiar with the theory of general relativity that gravitational waves really do exist. Fortunately, the Hulse-Taylor Pulsar (PSR 1913+16) provides firm evidence of a binary system actually emitting gravitational waves!

In 1974 Russell Hulse and Joseph Taylor discovered the signal of a pulsar using the Arecibo radio telescope. The pulsar had a period of 59 milliseconds. Further measurements showed that the orbital period varied in a repetitive manner over a period of 7.75 hours. This meant the the pulsar must be in orbit with another star.

Over the years the period of the pulsar has been measured to high accuracy. General relativity tells us that a binary system will emit energy as gravitational waves and eventually the two objects will inspiral towards each other and merge. As the system evolves towards this merger the period of the orbit will gradually decrease.

http://www.astro.cardiff.ac.uk/research/gravity/tutorial/?page=3thehulsetaylor


Simply put if spacetime can be warped, curved twisted to reveal gravitational effects, there is no reason why it cannot also send out gravitational radiation due to catastrophic collisions.
The hardest part is detection of such phenomena.

 

[Fednis48]

Q-Reeus: Thanks for keeping me honest! Relativity is not my field, but I thought I remembered more about gravitational wave interferometers than I apparently do. For what it's worth, the lecture I saw was Roy Glauber's "Year of Light" talk at the University of New Mexico; it was all about the quantum optics of interferometers, and only passingly about gravity wave detectors specifically. They had a camera set up, but I can't find a video online - maybe they haven't gotten around to uploading it yet.

 

[danshawen]

So was the janitor,

Yes, he was. The janitor provided the ladder and some scaffolding to help Weber attach the piano wires to beams with acoustic ceiling mounts. How did you know?

This is a pretty good survey of the follow-on work on resonant type gravity wave detectors as well as LIGO:

http://arxiv.org/pdf/gr-qc/0501007.pdf

Although cryogenic resonant bars have increased detector sensitivity by a factor of 1000 by reducing thermal noise, this still does not seem to be quite enough. Thorne is hopeful that the upgrades to LIGO will finally pull this off, and so am I.

 

[brucep]

like brucep says, after discussion it is thought best,in this particular case, to place the experiment in space, so as to filter out local and unnatural 'noise' which would mask your chosen range if the experiment was ground based.


http://lisa.nasa.gov/faq.html#science_1
My bold. Those 'mass motions' are noise.

It's a great experiment. I kinda have viewed the path this way. The ground based experimental path was in preparation for LISA. So I never questioned the value of the experiment to this point. That's me. It also makes a difference to understand the details associated with what's being measured. Tough job.

 

[brucep]

Q-Reeus: Thanks for keeping me honest! Relativity is not my field, but I thought I remembered more about gravitational wave interferometers than I apparently do. For what it's worth, the lecture I saw was Roy Glauber's "Year of Light" talk at the University of New Mexico; it was all about the quantum optics of interferometers, and only passingly about gravity wave detectors specifically. They had a camera set up, but I can't find a video online - maybe they haven't gotten around to uploading it yet.

You can get every detail associated with the attempt to measure gravitational waves at the CalTech site.



I did a project on gravitational radiation some years ago. The project was created by the authors of Exploring Black Holes but it was never added to the text. I'll put in " " whenever I use Prof Taylors words. The following is some comments that help explain what being done.

"The LIGO detector is an interferometer employing mirrors attached to 'test masses' at rest at the ends of an L-shaped vacuum cavity. The length of each arm of the L is 4 km. Detection of the gravitational wave is accomplished by effectively measuring the round trip time delay between light sent down the two legs of the detector."

Here is a simple metric for gravitational waves.

dTau^2 = dt^2 - [(1+h)dx^2 + (1-h)dy^2 + dz^2] , h<<1

"Here h is a function of time and space. It is the measure of the fractional deviation from unity of the dx^2 and dy^2 terms in the metric. The wave leading to this metric is a transverse wave, since h describes a perturbation of space only for directions x and y transverse to the z-direction of propagation."

Think of the experimental zone between the test mass as x and y of the Cartesion grid with the gravitational wave approaching from the z direction.

"Change in the shape of the square grid as the wave implied by the metric passes through in the z-direction perpendicular to the grid. The stretching is differential: the x-axis is stretched while the y-axis is compressed and vice versa. The areas of the grid remains constant"

When you use local proper coordinates to analysize the spacetime event the test mass move. If you use remote bookkeeper coordinates the remote speed of the laser light changes. That's a theoretical comment not a practical idea. I think. LOL

 

[brucep]

like brucep says, after discussion it is thought best,in this particular case, to place the experiment in space, so as to filter out local and unnatural 'noise' which would mask your chosen range if the experiment was ground based.


http://lisa.nasa.gov/faq.html#science_1
My bold. Those 'mass motions' are noise.

Anything that can move the test mass other than a gravitational wave. When you read the details of the actual limitations it's mind boggling the instrument sensitivity required.

 

[brucep]

Yes, he was. The janitor provided the ladder and some scaffolding to help Weber attach the piano wires to beams with acoustic ceiling mounts. How did you know?

This is a pretty good survey of the follow-on work on resonant type gravity wave detectors as well as LIGO:

http://arxiv.org/pdf/gr-qc/0501007.pdf

Although cryogenic resonant bars have increased detector sensitivity by a factor of 1000 by reducing thermal noise, this still does not seem to be quite enough. Thorne is hopeful that the upgrades to LIGO will finally pull this off, and so am I.

Thanks for posting the paper.

 

[brucep]

We have no volumes of indirect evidence of Gravitational Waves, just the certain observations have been speculated to be thus caused.

Secondly, we do not conduct very expensive experiments to discuss about the sensitivity of measureing device, the design of the device is based on the sensitivity parameters (range, tolerance etc) of the measured variable, we ascertain in advance, through simulation and theoreticaly, whether what we are expected to measure falls in the range of the device or not. If so, then only we plan the experiment.

41 years of observational evidence. Just that one reference produces 'volumes. You don't know anything about the evidence anyway. You should shut up.

 

[brucep]

Oh, you read about it, did you? That's nice, brucep. I was THERE.

Clearly, Mr. Potter, literacy, like fame, isn't everything.

?. LOL.

 

[The God]

Can anyone claim that he can convince an ordinary person about what is spacetime ? It drags but it does not resist, it has curvature but its no circle or sphere of anything, it gets ripple but it is no waterline or liquid like or foam like or solid like or air like or plasma like. Its is no matter but has certain mind boggling properties.

Coming to GW, after the grand success of GP-B, where the dragging of spacetime was measured, it was the matter of the time only that the GW was measured directly.

Its a different thing that GR is in principle a single body solution, it miserbaly fails in (our abilities to solve, may be) tackling multi body distortions in the spacetime, even to solve the Mercury precession, the poor Mercury was approximated to nought, and GW is not a single body phenomenon (unlike GP-B), it requires two to create the acceleration and thus the GW. Lets see.

 

[paddoboy]

Can anyone claim that he can convince an ordinary person about what is spacetime ? It drags but it does not resist, it has curvature but its no circle or sphere of anything, it gets ripple but it is no waterline or liquid like or foam like or solid like or air like or plasma like. Its is no matter but has certain mind boggling properties.

For someone who claims to be god and has no peer, or at least has never acknowledged any peers, the above is quite an ignorant statement to make, even for a non scientific god/lay god/person or whatever.
Most scientists and students of science know that science creates models to match what we observe. If a lay person needs to be taught this than so be it.
GR covers all aspects so far and as they have been evidenced and observed.
Gravitational waves are well supported for the reasons I have stated, and spacetime curvature and twisting have been measured.
Most students of science know that...Lay people like yourself may have a problem.
http://www.einstein-online.info/elementary/generalRT

 

[paddoboy]

The following in my opinion explains it rather simplistically well, even for a lay person........
http://www.einstein-online.info/elementary/generalRT/GeomGravity
Einstein's geometric gravity

The key idea of Einstein's theory of general relativity is that gravity is not an ordinary force, but rather a property of space-time geometry. The following simplified analogy, which substitutes a two-dimensional surface for four-dimensional space-time, serves to illustrate this idea.

Imagine empty space - in our case, a two-dimensional plane - with no forces acting between the bodies floating around. If there are no forces, then classical mechanics and Einstein's mechanics of special relativity are in agreement: Under these circumstances, bodies move along the straightest possible lines (which in this case are just straight lines in space) with a constant velocity. In the following image, this is symbolized by the straight paths of two particles A and B:

In particular, particles that start to move along parallel trajectories (as in the above image) will never meet, but are fated to remain forever at a constant distance from one another.

In the world of classical physics, if particles diverge from this behavior, it must be because there is a force acting on them. Forces accelerate particles, causing them to leave the straightest possible paths and follow curved trajectories instead. In our two-dimensional example, look at the following picture,

in which the particles A and B start out in parallel, but are then accelerated towards one another. In Newton's theory of gravity, gravitation is a force which could cause such an effect. For instance, the reason that the two particles in the above picture accelerate toward each other and then meet could be that they are both attracted gravitationally by a massive body located at the point of their meeting.

However, there is another possibility in which the same situation (where two particles that start out in parallel converge and finally meet) could arise. The two particles could still be moving on the straightest possible lines - not in the plane, but on a curved surface! The following image shows an example:

In that situation, there is no force making the particles deviate from the straightest possible lines; the mere fact that the particles are moving on a sphere means that, even if they still move as straight as possible, their paths will converge.

Einstein's theory is exactly analogous to this. In Newton's theory, gravity makes particles leave their straight paths. In Einstein's theory of general relativity, gravity is a distortion of space-time. Particles still follow the straightest possible paths in that space-time. But because space-time is now distorted, even on those straightest paths, particles accelerate as if they were under the influence of what Newton called the gravitational force.

 

[paddoboy]

http://www.einstein-online.info/elementary/gravWav

Gravitational waves

With space and time not as rigid background structures, but as dynamical objects (changing as the world changes in and around them), general relativity predicts fundamentally new phenomena. One of the most fascinating is the existence of gravitational waves: small distortions of space-time geometry which propagate through space as waves!

Most readers will have encountered several wave phenomena in everyday life. Sound waves, for instance: a small region of air is compressed, and the fact that its inner pressure is a bit higher than that of neighbouring regions leads to its expansion. This expansion leads to compression nearby, and in this way, the slight surplus in pressure propagates further and further. Such pressure waves are produced when we talk: our vocal cords compress the air around them, sound travels as waves, and these waves are absorbed by our ears when we hear them. In Einstein's case, the situation is somewhat different, but the basic principle is the same: a slight distortion in one region of space distorts nearby regions, and in the end, there is a moving distortion which speeds along at the highest possible speed (the speed of light). Such travelling distortions of space geometry are called gravitational waves.

http://www.einstein-online.info/elementary/gravWav/rhythm
The rhythm of geometry

Distortions of geometry: what does that mean? First of all, distances shrink and expand in a certain coordinated way. That's the main mechanism by which gravitational waves act on the rest of the world: they rhythmically distort distances between freely falling objects.

For the simplest case of a gravitational wave, the consequences are shown in the animation below. Imagine that we are, once more, in empty space, far away from all sources of gravitation. On the floor of our spaceship, we create a mandala, painstakingly constructed by arranging coloured grains of sand:

Note that all the sand particles are free, so they float weightlessly near the cabin floor.

A simple gravitational wave traversing this mandala would change the distances between the sand particles as shown in the following animation. The wave moves from behind the computer screen towards the reader.

[Animation size 118kB; please allow time for loading]

The coordinated dance of stretching and shrinking - stretching in one direction, while distances shrink in the perpendicular direction - is a general property of gravitational waves, as is the fact that all distortion takes place in a plane perpendicular to the direction in which the wave travels.


http://www.einstein-online.info/elementary/gravWav/sources
Making waves

In our universe, gravitational waves are produced in many different ways. Almost every occasion in which masses are accelerated leads to the generation of travelling space distortions, be it two heavenly bodies orbiting one another or stellar matter jettisoned into space in a gigantic explosion.

However, all of the gravitational waves that reach us from the depths of space are very weak, since as such a wave propagates away from its source, it spreads out farther and farther, and the distortions get weaker and weaker. In order for our detectors to measure a gravitational wave, on the other hand, it must be comparatively strong (and that will be the case only for waves generated in some of the most violent situations our universe has to offer).

Promising situations include two orbiting neutron stars, or a neutron star orbiting a black hole, or even two black holes orbiting one another. Such objects (which will be described further in the following chapter, Black holes & Co.) are extremely compact (i.e. for objects of theirmass, they are of extremely small size). It is this compactness that makes these binariesexcellent sources for strong gravitational waves.

While gravitational waves have not been directly detected so far, there is strong indirect evidence. The smoking gun is a system of orbiting neutron stars with the catchy namePSR1913+16. Einstein's theory predicts that gravitational waves carry away energy. For a system of orbiting stars, such a decrease in total energy leads to an ever faster and closer orbit. Over decades, radio astronomers have monitored the time that it takes the stars of PSR1913+16 to complete each successive orbit, and lo and behold: this orbital period decreases over time exactly as predicted by general relativity. This is strong evidence that the speed-up is indeed due to the radiation of gravitational waves, and the reason Russell Hulse and Joseph Taylor were awarded the Nobel prize for physics for the year 1993.

With orbiting objects drawing nearer and nearer (as in the case of PSR1913+16), the end is inevitable: There will be a collision, and if neutron stars or black hole collide, a huge amount of energy is radiated away in the form of gravitational waves. The following simulation by scientists of the Max-Planck-Institute for Gravitational Physics shows the spatial distortions effected by these waves as expanding, coloured regions. In this example, the collision and merger involves two black holes.

[Image: W. Benger AEI/ZIB. Animation size 93 kB; please
allow time for loading.]

Supernovae (violent explosions of dying stars, in which huge amounts of energy are freed and huge amounts of matter ejected into space) are also promising gravitational wave sources.
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::

I hope those simply explained articles, answers the OP's questions.

 

[Q-reeus]

Q-Reeus: Thanks for keeping me honest! Relativity is not my field, but I thought I remembered more about gravitational wave interferometers than I apparently do. For what it's worth, the lecture I saw was Roy Glauber's "Year of Light" talk at the University of New Mexico; it was all about the quantum optics of interferometers, and only passingly about gravity wave detectors specifically. They had a camera set up, but I can't find a video online - maybe they haven't gotten around to uploading it yet.

No problem Fednis48. As you said, there appears to be nothing uploaded yet from that talk, but it lead to watching an anecdotal by Glauber which was of some interest:

If GW's are detected, and it would not be GR's variety, the one thing that can be guaranteed is no quantum aspects of such will ever be observed. Unlike with light.

 

[The God]

If GW's are detected, and it would not be GR's variety, the one thing that can be guaranteed is no quantum aspects of such will ever be observed. Unlike with light.

That is right, in case we want to keep GR alive. Truthfully there is nothing like GR variety GW, its preposterous, the biggest hindrance to QGT/unification is the inability of key people to shake off GR, so IMNSHO GW of non GR variety will be detected, it makes sense, and then a very big hard look will be taken on GR and soon after we will have a workable sound unified theory. A decade maximum, before 2025..It will become untenable to sustain GR, if we do not get GR variety GW, so some fireworks and nasty stuff would be there.

Recall that question some popscience guy asked about Laura Mercini paper...then wtf we were looking for if no BH...

 

[danshawen]

Any 'shrinkage or expansion' would be RELATIVE, but to what? The source of the gravity waves? (That's a 'yes'). The dimensions and/or time dilation of the object receiving energy from the gravity wave before, during, or after it passes? What hard record of its passing would you have to compare it to? That would be a "NO". You wouldn't notice local changes if you were in free fall relative to the source of the gravity wave, any more than you notice the time dilation or length contraction associated with our relative motion with respect to red shifted galaxies at cosmological distances from us.

To be able to do that would require comparison of our time dilation or length contraction with that of something at rest or between us and those distant galaxies, or in the case of a passing gravity wave, with respect to another object not currently affected by the wave. It can be done. It can even be done relatively easily by observation of micro shifts of stellar spectra. But you don't get such differences USING ONLY ONE OBSERVER. For all their sophistication and complexity and expense, gravitational wave detectors as they are presently constructed, or even practical to construct, are ONE OBSERVER based instruments. No current designs will ever detect a gravity wave for that reason alone.

Relativity states that no experiment may be performed that would allow measurement of an absolute velocity, particularly with respect to 'spacetime.' This includes the current crop of gravity wave experiments, which is one reason why so many of them resemble the Michaelson-Morely experiment in physical implementation.

It is not I who misunderstands relativity or gravity waves.

Prove me wrong.

 

[sweetpea]

Anything that can move the test mass other than a gravitational wave. When you read the details of the actual limitations it's mind boggling the instrument sensitivity required.

About LISA, with such a high required sensitivity, I was wondering how they 'get around' solar light pressure moving the probes out of alinement with each other?
My very non-expert guess...their 'looking' for a rhythmic pattern in the observation and not a constant movement...like I said, I'm no expert.

 

[origin]

Any 'shrinkage or expansion' would be RELATIVE, but to what? The source of the gravity waves? (That's a 'yes'). The dimensions and/or time dilation of the object receiving energy from the gravity wave before, during, or after it passes? What hard record of its passing would you have to compare it to? That would be a "NO". You wouldn't notice local changes if you were in free fall relative to the source of the gravity wave, any more than you notice the time dilation or length contraction associated with our relative motion with respect to red shifted galaxies at cosmological distances from us.

To be able to do that would require comparison of our time dilation or length contraction with that of something at rest or between us and those distant galaxies, or in the case of a passing gravity wave, with respect to another object not currently affected by the wave. It can be done. It can even be done relatively easily by observation of micro shifts of stellar spectra. But you don't get such differences USING ONLY ONE OBSERVER. For all their sophistication and complexity and expense, gravitational wave detectors as they are presently constructed, or even practical to construct, are ONE OBSERVER based instruments. No current designs will ever detect a gravity wave for that reason alone.

Relativity states that no experiment may be performed that would allow measurement of an absolute velocity, particularly with respect to 'spacetime.' This includes the current crop of gravity wave experiments, which is one reason why so many of them resemble the Michaelson-Morely experiment in physical implementation.

It is not I who misunderstands relativity or gravity waves.

Prove me wrong.

Laser interferometers will be able to detect gravity waves.

Prove me wrong.