The search for gravity waves: End in sight?

[paddoboy]

An end in sight in the long search for gravity waves:
5 hours ago by David Blair



Our unfolding understanding of the universe is marked by epic searches and we are now on the brink of discovering something that has escaped detection for many years.

The search for gravity waves has been a century long epic. They are a prediction of Einstein's General Theory of Relativity but for years physicists argued about their theoretical existence.
By 1957 physicists had proved that they must carry energy and cause vibrations. But it was also apparent that waves carrying a million times more energy than sunlight would make vibrations smaller than an atomic nucleus.

http://phys.org/news/2014-02-sight-gravity.html



For the first time we have firm predictions: both the strength and the number of signals. No longer are we hoping for rare and unknown events.
We will be monitoring a significant volume of the universe and for the first time we can be confident that we will "listen" to the coalescence of binary neutron star systems and the formation of black holes.
Once these detectors reach full sensitivity we should hear signals almost once a week. Exactly when we will reach this point, no one knows. We have to learn how to operate the vast and complex machines.
If you want to place bets on the date of first detection of some gravity wave then some physicists would bet on 2016, probably the majority would bet 2017. A few pessimists would say that we will discover unexpected problems that might take a few years to solve.


Read more at: http://phys.org/news/2014-02-sight-gravity.html#jCp

 

[Sorcerer]

An end in sight in the long search for gravity waves:
5 hours ago by David Blair



Our unfolding understanding of the universe is marked by epic searches and we are now on the brink of discovering something that has escaped detection for many years.

The search for gravity waves has been a century long epic. They are a prediction of Einstein's General Theory of Relativity but for years physicists argued about their theoretical existence.
By 1957 physicists had proved that they must carry energy and cause vibrations. But it was also apparent that waves carrying a million times more energy than sunlight would make vibrations smaller than an atomic nucleus.

http://phys.org/news/2014-02-sight-gravity.html



For the first time we have firm predictions: both the strength and the number of signals. No longer are we hoping for rare and unknown events.
We will be monitoring a significant volume of the universe and for the first time we can be confident that we will "listen" to the coalescence of binary neutron star systems and the formation of black holes.
Once these detectors reach full sensitivity we should hear signals almost once a week. Exactly when we will reach this point, no one knows. We have to learn how to operate the vast and complex machines.
If you want to place bets on the date of first detection of some gravity wave then some physicists would bet on 2016, probably the majority would bet 2017. A few pessimists would say that we will discover unexpected problems that might take a few years to solve.


Read more at: http://phys.org/news/2014-02-sight-gravity.html#jCp

2017 I might be able to make, so I hope they get it done on time, the idle villains!

 

[quantum_wave]

There is no reason to doubt that gravity waves are prevalent throughout the universe. As the OP link points out, Einstein predicted them as part of GR. From the link: "The search for gravity waves has been a century long epic. They are a prediction of Einstein's General Theory of Relativity but for years physicists argued about their theoretical existence.
By 1957 physicists had proved that they must carry energy and cause vibrations. But it was also apparent that waves carrying a million times more energy than sunlight would make vibrations smaller than an atomic nucleus."

There are gravity waves emitted, according to GR, whenever objects in spacetime interfere with each other's path. The gravity wave energy is supposed to be emitted as a result of such events. The issue is that gravity waves have a very tiny amount of energy relative to the particles involved in the event, and so are undetectable except for the greatest possible events; events associated with the collision of stars, neutron stars, or the formation of black holes. Hence, the target events of the gravity wave detectors are those massive events.

However, keep in mind that in GR, even the fall of an apple to the ground will emit a gravity wave. Maybe even the motion of an object through any medium will cause tiny gravity waves as particles in the moving object encounter particles in their path.

 

[quantum_wave]

...
However, keep in mind that in GR, even the fall of an apple to the ground will emit a gravity wave. Maybe even the motion of an object through any medium will cause tiny gravity waves as particles in the moving object encounter particles in their path.

I think the community is either giving me a pass by letting me say that, or the implication of gravity waves being caused by the simple encounter between particles elevates the role of gravity waves in General Relativity beyond what I thought it was. That could be taken to mean that there is no interaction involving particles with mass that does not generate a tiny gravity wave, according to GR. Am I right?

 

[quantum_wave]

I think the community is either giving me a pass by letting me say that, or the implication of gravity waves being caused by the simple encounter between particles elevates the role of gravity waves in General Relativity beyond what I thought it was. That could be taken to mean that there is no interaction involving particles with mass that does not generate a tiny gravity wave, according to GR. Am I right?

No one has any interest in the fact that gravity waves might be emitted when even the tiniest particle interaction takes place, even when a particle is simply accelerated relative to another particle? What about the way GPS works?

The Global Positioning System, GPS, requires very accurate clock coordination in order to pin point locations being triangulated by satellites. I often see people say that the accuracy of GPS requires relativistic adjustments to the clock rates.

I can see why the accuracy of GPS is evidence that identical clocks measure time at different rates when they are in relative motion to each other, and I know General Relativity predicts that kind of time measurement error between clocks in relative motion, and/or between clocks in different gravitational fields.

So what is physically going on when identical clocks that are in motion relative to each other measure time passing at different rates. Is time really passing at different rates, or is there different gravitational wave energy density in the environment that the clocks are functioning in, causing one clock to function slower and/or the other to function faster?

I'm posing the question, if there are gravity waves everywhere, does motion increase the relative energy density of the environment of the moving object? If so, does that slow the rate that accelerated particles function relative to particles at rest? If so, if clocks are composed of particles, wouldn't the accelerated clock's particles function slower than the rest clock's particles? If so, wouldn't the rest clock measure time to be passing faster than the accelerated clock? If so, couldn't that be the answer as to why clock rates in GPS satellites need to have relativistic time adjustments in order for GPS to remain accurate?

 

[quantum_wave]

The OP is encouraging, and my layman level discussion of gravity waves is not of any interest. But there are a lot of things we don't understand about gravity that are becoming clearer with the advances in ways to study the relationship between the galactic structure, the CMBR, and gravitational waves. Who doesn't know that the scientific community is trying to figure it all out, and make all the connections to tie in gravity waves, virtual particles that are said to create zero point energy, vacuum energy density, also known as the cosmological constant, and effects like the anomalous slowing of space probes like the Voyager and Pioneer anomaly, and the clock speed corrections to GPS.

What is that tie in? Most agree that there is a natural phenomenon of energy in space, and according to which theory you look at, that energy can differ by an order of magnitude of over 100 times.

The vacuum energy density (aka Cosmological Constant) though, is described as having an effect on gravity, determining if it is stronger or weaker than dark energy. If the cosmological constant is high (positive), the expansion of the universe will continue to accelerate, and if it is low (negative), it will slow to a stop or turn and collapse (I think that is the way it goes), according to GR.

So If there are gravitational waves permeating space, as GR seems to say there should be, it certainly could be an explanation, or at least an alternative to the concept of zero point energy or virtual particles popping into and out of existence faster than the eye can see, or maybe even to the question of dark energy.

With the progress we are making, something is likely to be discovered that will reconcile the vast order of magnitude error between the various theories. Gravitational wave energy density might fill that bill. There is a chance to discover it and quantify it as a result of the search for gravity waves mentioned in the OP, from massive events that can be detected by the latest apparatuses.

That would be a great confirmation of the GR prediction, and at the same time would be experimental evidence that could lead to a better understanding of the possibility that every particle interaction emits its own super-tiny gravity wave. The energy of the tiny gravity waves from individual particle motion would be quite insignificant compared to the collisions of neutron stars, but then we would have evidence of an alternative explanation for the nature of the hidden source of energy in space.

 

[quantum_wave]

In that last post, I contemplated the fact that dark energy is taken as an opposing force to gravity. Observations tell us that expansion wins out over gravity, i.e. dark energy is stronger than gravity, so as the separation of galaxies increases, variable forces shift to favor expansion, causing expansion to accelerate. The strength of the opposing forces changes over time as they play out.

Gravity can be thought of as an attraction between objects with mass which is stronger in close quarters and diminishes as distance increases, while dark energy could be thought of as a force driving objects with mass apart, which would get stronger relative to gravity as the distance increases. Thus they would be opposing forces that were not perfectly balanced.

What set of conditions would allow that kind of cosmology? The Cosmological constant, which is a concept of vacuum energy density, is based on there being one big bang, and everything in the universe being causally connected to that one event. That requires that the concept of vacuum energy density be modeled as a feature of that one Big Bang, and everything driving it must be contained within the space occupied by and connected to our Big Bang. We don't yet understand how that could work.

Wouldn't it be easier to understand dark energy if the Big Bang did not create space, and was not the beginning of the greater universe, but instead occurred in preexisting space?

When I think about what kind of force dark energy could be, it occurs to me that it could be the tendency for wave energy in space to seek an equilibrium state, where the wave energy density equalizes across mixed density space; a condition like that would exist in the preexisting space scenario of the Big Bang event. If so, the force of dark energy might be explained by the fact then high density wave energy in space from our Big Bang would spread out into low energy density space surrounding it, causing the energy density differential to begin to equalize by high expanding into low.

The result would be just what we observe, a trend toward energy density equalization which would result in the accelerating expansion and cooling of the hot dense Big Bang as it intrudes into the colder and less dense surrounding greater universe.

In that version of cosmology, the process of energy density equalization could be the cause of the initial expansion of our observable universe, and could explain the mysterious dark energy.

 

[quantum_wave]

Cosmology is all about gravity and expansion, basically. Explain those two observations and you have the basis for a pretty good cosmology of the universe. That is why I brought up the "greater universe" scenario as a possible explanation of dark energy. High gravitational wave energy density intruding into low gravitational wave energy density resulting in the expansion that we observe in the universe; just the opposite of how high concentrations of matter increase the gravitational field.

Gravity is easy to see and feel. We are physically attracted to the ground, and if we jump, we fall right back. But what isn't easy it to know is the mechanics of gravity. The thing that keeps General Relativity and the curvature of spacetime from solving the mechanism question is that it doesn't address the "how" matter curves spacetime. The math is close to giving us a perfect quantification of law of gravity; we know how to predict the motion of objects, whether in classical situations like humans jumping, or in relativistic situations like calibrating the clocks in GPS satellites, but the theory stops short of just how matter reaches out to other matter.

This thread highlights the fact that GR does predict gravity waves, and recent observations of the polarity of the CMB have given physicists more evidence on which to examine the very early gravitational nature of our observable universe. There is no doubt in anyone's mind, I don't think, that when there is matter, there is gravity, and when there are huge amounts of matter in one place, gravity is stronger.

Scientists have proven the possibility of particles far more massive than protons or neutrons, called the Higgs particle. Theorists have predicted that even the Higgs particle has more massive origins as they theoretically back track from the environment where Higgs particles decay, to points before that, when the universe was opaque and photons couldn't escape.

The high density and correspondingly extreme temperatures in the first few hundred thousands years after the Big Bang are thought to have hosted more massive particles than the Higgs, but we probably won't soon have the capabilities to duplicate those particles in colliders even though most agree that the theory is sound.

As the energy density of an environment increases, gravity increases, and on the other hand, as the expansion of the energy density occurs, less dense environments allow for the decay of massive particles like the Higgs. There is a theory called the Big Rip that even predicts the decay of protons into photons as the energy density approaches zero. Though no one predicts that zero energy density is possible, it is a mathematical limit that can continually be approached if the expansion of our universe plays out that way.

This thread mentions several things that are current avenues of thought when it comes to the mechanism of gravity. One was mentioned in the OP, and that is the idea that any event altering the path of a particle on it geodesic path is predicted to cause a gravity wave according to GR, and that is what scientists hope to discover by looking for gravity waves emitted by massive collisions in space.

Also, the GPS clock rate adjustments that are required because the clocks in the GPS satellites are in relative motion to each other. The presence of gravity waves and the effect that they have on the rate that particles function as they are accelerated or as they pass through gravitational fields can be associated with the concept of gravitational wave energy density. The hypothesis is that the gravitational wave energy density affects the rate at which particles function, and the rate that particles function affects the rate that clocks measure the passing of time, hence the need to adjust clock rates in GPS.

In addition, the concept of vacuum energy density was tied into gravity from the perspective that it represents a force of expansion which has the opposite effect as the gravitational wave energy density has on the motion of objects.

So we have important observations and interesting hypotheses that address the mechanics of gravity. Nothing is likely to do much better than Einstein's Field Equations at predicting the motion of objects, but the mechanics of the motion of objects is more and more being described by the gravitational wave energy density of the environment through which particles and objects are moving.

 

[quantum_wave]

I was just reading about how Penrose trumps the Big Rip that I mentioned in the last post which takes gravity and dark energy to their completion; complete entropy in what is referred to as the "heat death" of the universe.

Roger Penrose trumps that with his new cosmology which he explains in his book, "Cycles of Time". I have to get it and read it, but I watched his Ted-Talk and read some articles discussing it. They say he vaguely implies that once the universe is reduced to nothing but photon energy, i.e. no matter and no gravity, there is a correlation between that and the initial conditions of our Big Bang. Could the Big Rip then lead to another Big Bang? Anyway, that is taking theoretical physics beyond the layman level, so I only mention it because it shows that hypothesis and speculation are part of theoretical physics at Penrose's level. He is an expert on GR, and a well known mathematician, physicist, and philosopher.

Here are a couple of links about his thinking. The second one is from the author of what I think is his blog called Not Even Wrong, but I could be wrong about that. Anyway he is not too keen on Penrose's ideas or his latest book.

Check out this paper.
http://accelconf.web.cern.ch/accelconf/e06/PAPERS/THESPA01.PDF
Check out this book review:
http://online.wsj.com/news/articles/SB10001424052748703730804576317072124312488

Right now the consensus cosmology is dealing with a recognition that particles have wave-particle duality, and the macro and micro realms are far apart in their quantification of the amount and nature of wave energy in space.

I think there is a general consensus on the wave-particle duality nature of matter, with the exception of the most hard line Copenhagen interpretations of quantum mechanics. They go as far as to suggest that maybe the particle does not just have its states superimposed when it isn't being observed, but maybe instead, the particle is not even there, unless measured or observed. They must view wave-particle duality as optional.

Either way, according to the OP, GR predicts that gravity waves are there at all times waiting to be detected, and so space carries them somehow, from one source event like a particle or object interaction, to another.

Gravitational waves, which some say are not the same thing as the gravity waves the article in the OP says they are searching for, may be emitted and/or absorbed by matter as part of the mechanics of the motion of objects. That would seem to be taking the concept of gravity waves in GR to its max, i.e. that would mean mere relative motion causes gravitational waves. The consensus in the community may not yet be taking that as part of GR.

All a layman can do is just keep contemplating the nature of the universe and follow along until the answers are provided by the scientific community.

 

[quantum_wave]

Sorry to keep adding to this, but I've gotten interested in the impending discovery of gravity waves emitted by objects. Until now, they seemed pretty vague in GR, but now the popular media is pushing them.

Just to clarify what I brought up in the last post about the title of the article in the OP, about gravity vs. gravitational waves. The following SpaceDotCom article indicates they must have meant gravitational waves in the article linked in the OP.

http://www.space.com/25089-how-gravitational-waves-work-infographic.html
"Einstein's general theory of relativity suggests, among other things, that masses in space distort the geometry of spacetime. In addition, moving objects emit waves of gravitational radiation that carry energy away into space."

Here's the Wiki link to both and you can see it must be "gravitational waves":
http://en.wikipedia.org/wiki/Gravity_wave
http://en.wikipedia.org/wiki/Gravitational_wave

As a layman, I find things in the popular science media, like SpaceDotCom, that could be reinterpretations, misrepresentations, or just passed on wrong. A case in point is the second sentence from the quote from SpaceDotCom article, "In addition, moving objects emit waves of gravitational radiation that carry energy away into space."

Does GR really say that? Is it in the equations like that or is that a popular media misinterpretation.

Maybe GR does include the fact that moving objects emit gravitational waves that carry energy. It might depend on what the "squeeze and stretch" means in the SpaceDotCom article, which goes on to say that, "gravity is the weakest of the fundamental forces, and the effect of gravitational waves are also weak. It is said that the waves squeeze and stretch space as they pass, but the effect is sub-atomically small."

If GR says that, then as a layman can I use the analogy of send and receive to equate to "moving objects emit waves of gravitational radiation that carry energy away into space", and "the waves squeeze and stretch space as they pass, ..."?

If moving objects emit or send energy/information into space, then do other objects receive and use that energy; does it provide physical information to tell distant objects "how to move" and provide the energy to make that move?

 

[quantum_wave]

Good morning cruel world.

A layman like me might think that the waves would squeeze and stretch objects microscopically as they pass, and that maybe that is the phenomenon that the detectors are trying to measure. When you squeeze something it takes on a form of energy, potential energy, and when that something stretches as a reflex response to the squeeze, it expends that energy. The "squeeze" came as a wave, and the reflex "stretch" sends a wave back into space.?

Therefore, a gravitational wave that comes in directionally from a major wave emitting event like the distant collision between neutron stars, might be absorbed as a directional "squeeze" of an object in space, the squeeze corresponding to the direction from which the wave arrived. The reflexive stretch would then emit a new spherical wave spreading out in all directions, I'm assuming. Any reflexive stretch wave would then go on to be felt by other objects in space in all directions, transmitting some information corresponding to the original spherical gravitational wave from the collision event, maybe even having a tweak in its own motion as a result of "feeling" the original event?

Maybe that is what the gravitational wave detector apparatus is set up to detect. The OP article references multiple detectors that can triangulate the source and determine the location in space where it was emitted. If the detector can "feel" the squeeze and "stretch" of the wave as it traverses space, does it follow that all objects in space could also "feel" its squeeze, and would re-emit the energy absorbed by the squeeze with a reflexive "stretch"?

 

[brucep]

Good morning cruel world.

A layman like me might think that the waves would squeeze and stretch objects microscopically as they pass, and that maybe that is the phenomenon that the detectors are trying to measure. When you squeeze something it takes on a form of energy, potential energy, and when that something stretches as a reflex response to the squeeze, it expends that energy. The "squeeze" came as a wave, and the reflex "stretch" sends a wave back into space.?

Therefore, a gravitational wave that comes in directionally from a major wave emitting event like the distant collision between neutron stars, might be absorbed as a directional "squeeze" of an object in space, the squeeze corresponding to the direction from which the wave arrived. The reflexive stretch would then emit a new spherical wave spreading out in all directions, I'm assuming. Any reflexive stretch wave would then go on to be felt by other objects in space in all directions, transmitting some information corresponding to the original spherical gravitational wave from the collision event, maybe even having a tweak in its own motion as a result of "feeling" the original event?

Maybe that is what the gravitational wave detector apparatus is set up to detect. The OP article references multiple detectors that can triangulate the source and determine the location in space where it was emitted. If the detector can "feel" the squeeze and "stretch" of the wave as it traverses space, does it follow that all objects in space could also "feel" its squeeze, and would re-emit the energy absorbed by the squeeze with a reflexive "stretch"?

FYI

A bit about the physics of the propagating gravitational wave. Real simple metric. Just the kind I like.


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 [will be in the 2nd edition]. I'll put in " " whenever the quote is directly from the project author.

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."

Draw a square on a piece of paper and divide it into 16 sections [a grid]. Set left corner y->up and left corner x->right. Set z orthogonal to the grid representing the direction of a propagating transverse gravitational wave.

Measuring a gravitational wave passing through the experimental model [grid] at time intervals.

t = 0 the grid is undistorted

t = 1/4 the grid is stretched horizontialy [horizontal rectangle]

t = 2/4 the grid is undistorted

t = 3/4 the grid is compressed vertically [verticle rectangle]

t = 4/4 the grid is undistorted

As the gravitational wave passes through the grid in the z direction the stretching of the grid is diffential: the x-axis is stretched while the y-axis is compressed and vice versa. The areas of the grids remains constant."

"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."

The first indirect evidence for gravitational waves was revealed during the research on PSR 1913 + 16. Binary Pulsars.

http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm

Joseph Taylor Nobel Lecture. Part VI 'testing for gravitational waves'.

http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/taylor-lecture.pdf

 

[quantum_wave]

FYI

A bit about the physics of the propagating gravitational wave. Real simple metric. Just the kind I like.


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 [will be in the 2nd edition]. I'll put in " " whenever the quote is directly from the project author.

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."

Draw a square on a piece of paper and divide it into 16 sections [a grid]. Set left corner y->up and left corner x->right. Set z orthogonal to the grid representing the direction of a propagating transverse gravitational wave.

Measuring a gravitational wave passing through the experimental model [grid] at time intervals.

t = 0 the grid is undistorted

t = 1/4 the grid is stretched horizontialy [horizontal rectangle]

t = 2/4 the grid is undistorted

t = 3/4 the grid is compressed vertically [verticle rectangle]

t = 4/4 the grid is undistorted

As the gravitational wave passes through the grid in the z direction the stretching of the grid is diffential: the x-axis is stretched while the y-axis is compressed and vice versa. The areas of the grids remains constant."

I'll draw that grid and try to follow the wave in the z direction at the time intervals. Can I make the z direction go along the x axis, or does it have to be perpendicular to x and y? At t=1/4, is the horizontal stretch shown by making the horizontal lines of the grid curve up along the y axis, and is the t=3/4 "squeeze" shown by making the vertical lines curve along the x axis? I am getting a picture of a wave that is like the transverse EM wave, but that wouldn't be the shape of a gravity wave that propagates spherically through space from an event like the collision of massive objects, I wouldn't think.

Isn't a physical gravitational wave going to be a continuous wave of the curved plane wave type, essentially starting spherically and beocming a flatter plane wave by the time it reaches the detector because the radius would be so great?

"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."

Can we assume that until a measurable wave comes along, the distances measured on the L shaped array by the laser will be constant in both directions. Then when the measurable wave comes through it will make the two distances slightly different as the stretch and compression (or squeeze) occur, setting off the alarm in the control room.

The first indirect evidence for gravitational waves was revealed during the research on PSR 1913 + 16. Binary Pulsars.

http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm

Joseph Taylor Nobel Lecture. Part VI 'testing for gravitational waves'.

http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/taylor-lecture.pdf

Interesting.

 

[brucep]

I'll draw that grid and try to follow the wave in the z direction at the time intervals. Can I make the z direction go along the x axis, or does it have to be perpendicular to x and y? At t=1/4, is the horizontal stretch shown by making the horizontal lines of the grid curve up along the y axis, and is the t=3/4 "squeeze" shown by making the vertical lines curve along the x axis? I am getting a picture of a wave that is like the transverse EM wave, but that wouldn't be the shape of a gravity wave that propagates spherically through space from an event like the collision of massive objects, I wouldn't think.

Isn't a physical gravitational wave going to be a continuous wave of the curved plane wave type, essentially starting spherically and beocming a flatter plane wave by the time it reaches the detector because the radius would be so great?
Can we assume that until a measurable wave comes along, the distances measured on the L shaped array by the laser will be constant in both directions. Then when the measurable wave comes through it will make the two distances slightly different as the stretch and compression (or squeeze) occur, setting off the alarm in the control room.
Interesting.

The gravitational wave propagates through four dimensions of spacetime. The metric, and grid example, describe a two dimensional slice of the propagating wave. The example for the changing geometry of the grid are exaggerated for pedagogical purpose. This is what the author of the project writes. "Think of a gravitational wave as a very small tidal force that varies with time and position." "It changes some measure of the separation between two test mass floating near one another in a local inertial frame." For example the laboratory frame of the LIGO experiment. The distance between the test mass is metered by the 'laser beam'. It's pretty interesting, and informative, to read details about the LIGO experiment. Difficult task to detect something so small as the perturbation caused by a passing gravitational wave when conducting this experiment on the surface of the earth.

 

[quantum_wave]

The gravitational wave propagates through four dimensions of spacetime. The metric, and grid example, describe a two dimensional slice of the propagating wave. The example for the changing geometry of the grid are exaggerated for pedagogical purpose. This is what the author of the project writes. "Think of a gravitational wave as a very small tidal force that varies with time and position." "It changes some measure of the separation between two test mass floating near one another in a local inertial frame." For example the laboratory frame of the LIGO experiment. The distance between the test mass is metered by the 'laser beam'. It's pretty interesting, and informative, to read details about the LIGO experiment. Difficult task to detect something so small as the perturbation caused by a passing gravitational wave when conducting this experiment on the surface of the earth.

Though this thread is talking about the existence of gravitational waves that are predicted by Einstein, and therefore we would be talking about waves propagating through spacetime, it seems that the design of the LIGO experiment would be capable of detecting a gravitational "squeeze"/"stretch" if it propagated through spacetime, or if it propagated through three dimensional space. Is it possible, from the data that will be collected, to distinguish between a wave propagating through spacetime vs. through 3D space?

 

[brucep]

Though this thread is talking about the existence of gravitational waves that are predicted by Einstein, and therefore we would be talking about waves propagating through spacetime, it seems that the design of the LIGO experiment would be capable of detecting a gravitational "squeeze"/"stretch" if it propagated through spacetime, or if it propagated through three dimensional space. Is it possible, from the data that will be collected, to distinguish between a wave propagating through spacetime vs. through 3D space?

It propagates in the four dimensions of spacetime. Always.

 

[quantum_wave]

It propagates in the four dimensions of spacetime. Always.

Gravitational waves then, are theory specific? I would call them gravitional waves in any theory that features them.

 

[brucep]

Gravitational waves then, are theory specific? I would call them gravitional wave in any theory that features them.

Theories predict stuff. That's why we write them down and investigate the predictions. Based on the experimental evidence if your theory of gravity doesn't predict gravitational radiation it's most likely wrong. It's either natural phenomena or it isn't natural phenomena. Quantum gravity predicts the graviton. A gravitational wave is the classical analog for the graviton. So gravitational radiation can be specific to classical models of gravity and quantum models of gravity.

 

[quantum_wave]

Theories predict stuff. That's why we write them down and investigate the predictions. Based on the experimental evidence if your theory of gravity doesn't predict gravitational radiation it's most likely wrong. It's either natural phenomena or it isn't natural phenomena. Quantum gravity predicts the graviton. A gravitational wave is the classical analog for the graviton. So gravitational radiation can be specific to classical models of gravity and quantum models of gravity.

Spacetime is theory specific, but the theory of GR evolves as advances are made and as new discoveries are dealt with by the scientific community. It will likely continue to be updated as the physical nature of spacetime is sorted out over time. Gradually, a physical medium filling space could deliver the mathematical characteristics of spacetime that govern motion through space. No one is saying we are going to have to abandon GR in order to talk about a physical medium of space being aether like, are they?

This Wiki, http://en.wikipedia.org/wiki/Aether_theories, is a good update and history, and an example of how the peer reviewed changes find their way into the layman level popular scientific media, long after the scientific community begins dealing with them.

 

[brucep]

Since a measurable aether 'doesn't exist' and isn't required to describe the physics of this universe. It's a theoretical dead end at this point in the history of the literature. That's my analysis. Other folks can have an analysis. Hopefully they would defend the physics during the discourse. Using physics.