This is an intriguing philosophical question related to the nature of gravity and reality.

Gravity is a force. Are there “particles of gravity” like “photons” of light and if there are “gravitons”, what is their speed of propagation. Is it at the speed of light or is it faster than the speed of light?

Here is a thought provoking article on this:

http://www.gravitywarpdrive.com/Speed_of_Gravity.htm

Edited to remove copyrighted information. We cannot police every one of these posts for copyright infringement, kkwan.

dougsmith—admin

The speed of gravity has been measured for the first time. The landmark experiment shows that it travels at the speed of light, meaning that Einstein’s general theory of relativity has passed another test with flying colours.

Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri in Columbia made the measurement, with the help of the planet Jupiter.

“We became the first two people to know the speed of gravity, one of the fundamental constants of nature,” the scientists say, in an article in New Scientist print edition. One important consequence of the result is that it places constraints on theories of “brane worlds”, which suggest the Universe has more spatial dimensions than the familiar three.

John Baez, a physicist from the University of California at Riverside, comments: “Einstein wins yet again.” He adds that any other result would have come as a shock.

http://www.newscientist.com/article.ns?id=dn3232

Youdontsay is correct on this one kkwan. The speed of gravity is not a mystery. Not sure where you got all that stuff you quoted, but in the world of physics this question has been answered.

I am aware of that experiment, but it is not so clear cut and so easy to measure the speed of gravity, unlike light which we can see (at least, the visible spectrum), produce a coherent beam (laser) which can be directed with lenses and/or reflected with mirrors to a distant object/sensor to measure its speed. We know and understand much less about the nature of gravity. We experience its effect, but we cannot see or produce, direct/reflect it like light to create ‘gravity’ beams. That is the mystery of gravity. Is it a force, a form of energy or is it due to curvature of space time? No gravitons have been detected as yet. How, then can we measure its speed if we know so little of its nature? That is the philosophical question.

Consider these objections to the experiment and what it actually measured:

http://www.lbl.gov/Science-Articles/Archive/Phys-speed-of-gravity.html

Albert Einstein may have been right that gravity travels at the same speed as light but, contrary to a claim made earlier this year, the theory has not yet been proven. A scientist at Lawrence Berkeley National Laboratory (Berkeley Lab) says the announcement by two scientists, widely reported this past January, about the speed of gravity was wrong.

Stuart Samuel, a participating scientist with the Theory Group of Berkeley Lab’s Physics Division, in a paper published in Physical Review Letters, has demonstrated that an “ill-advised” assumption made in the earlier claim led to an unwarranted conclusion.

http://www.space.com/scienceastronomy/gravity_speed_030116.html

Physicists leveled heavy criticism Thursday on a report from last week that claimed the speed of gravity had been determined by observation and was equal to the speed of light.

One physicist called the interpretation of the finding “nonsense”. Others were more diplomatic, suggesting that the experiment, involving observations of the bending of light from a distant galaxy as the light sped by the planet Jupiter, had instead measured other phenomena.

Japanese physicist Hideki Asada published a paper, also in the Astrophysical Journal Letters, about a year ago arguing that Fomalont and Kopeikin would actually be measuring the speed of light, not gravity. That paper has been a thorn in Kopeikin’s side ever since. During the AAS press conference last week, when questioned about Asada’s work, Kopeikin was visibly frustrated and said Asada had made a mathematical mistake.

There remains little doubt that something was measured last September when the largest planet in our solar system fortuitously passed in front of a bright galaxy some 9 billion light-years away. What remains is for physicists to agree on what was seen.

You may want to take a look at this well written article in Wikipedia. I don’t like to use them as an exclusive source but in this case there is no need to reinvent the wheel as this author has done a pretty extensive and balanced job of reviewing this subject.

I will agree that the experiments to prove that the speed of gravity =c are not conclusive, but what evidence we do have seems to agree with what Einstein and Relativity predict. The debate about the Jupiter regression observations is not as one sided as you make it appear. In any event there appears to be no evidence that points to another conclusion, and relativity has a prety good track record of proving itself so far.

The question is obviously still open, but I don’t see how this is aphilisophical question. The answers to this problem are going to come from science, not philosophy.

I read the wiki article before I posted the topic. From the article, consider this strange effect of gravity which Newton could not reconcile:

Isaac Newton’s formulation of a gravitational force law requires that each particle respond instantaneously to every other massive particle irrespective of the distance between them. In modern terms, Newtonian gravitation is described by the Poisson equation, according to which, when the mass distribution of a system changes, its gravitational field instantaneously adjusts. Therefore the theory requires the speed of gravity to be infinite.

The criticisms of what was measured was also mentioned:

Several physicists, including Clifford M. Will and Steve Carlip, have criticized these claims on the grounds that they have allegedly misinterpreted the results of their measurements. However, Kopeikin and Fomalont continue to vigorously argue their case. (See the citations below for the details of the arguments pro and con.)

The wiki referred to Clifford M. Wills’s website:

http://physics.wustl.edu/cmw/SpeedofGravity.html

The question is:

Does this tell us anything about the speed of propagation of gravity? The consensus among relativists is NO!

How can we really measure the speed of propagation of gravity?

* If we could measure the effects on the Shapiro delay to order (v/c)^2, then we could test the speed of gravity. But these effects would be at the thousandths of a picosecond level, hopelessly small.
* The real way to measure the speed of gravity is to detect and study gravitational waves. By comparing the arrival of a gravitational-wave signal with that of an electromagnetic signal from an astrophysical source, one could compare the speed of gravity to that of light to parts in 10^(17).

In other words, it means it will be very difficult.

My first post referred to Tom Van Flandern’s website which was also mentioned in the wiki:

http://metaresearch.org/mediaandlinks/press/SOG-Kopeikin.asp

The physical meaning of “the speed of gravity”

Although gravitation and relativity are technical subjects, the mistake made by Kopeikin is not unlike measuring the speed of a falling apple and claiming that is the speed of gravity. The following section provides the background to understand what actually was done wrongly.

All gravitational phenomena unique to Einstein’s general relativity (GR), such as light bending, gravitational redshift, perihelion advance, and Shapiro delay of radio or radar signals, arise in a static or near-static gravitational potential field, also sometimes called in various contexts by the names “the gravitational field”, “the space-time medium”, “the light-carrying medium”, “aether”, or “elysium”. Disturbances of this potential field or medium are called “gravitational waves”. According to GR, such waves propagate at the speed of light, as do all other phenomena associated with the potential field that propagate at all. This gravitational wave speed has been confirmed directly by measures of the aforementioned phenomena unique to GR, and indirectly by binary pulsar observations. There is no current dispute about this, and no expectation of any other result for the propagation speed of gravitational waves. However, the name notwithstanding, “gravitational waves” have nothing to do with gravitational force. They are ultra-weak disturbances of the potential field or space-time medium due to the acceleration of bodies. So far, they have proved too weak to detect directly in any laboratory or astrophysical experiment. They are certainly far too weak to have any observable influence on any macroscopic body in their path.

By contrast, gravitational forces are large, readily detected, and control the dynamics of most of the visible universe. Gravimeters easily detect the gravitational force from, and motion of, a person entering a room, for example.

A common thought experiment asks: “What would happen to the Earth’s orbit if the Sun suddenly ceased to exist?” The answer is now clear. The usual relationship “force is the gradient of the potential” would instantly end. The Sun’s potential field would then begin to dissipate, taking 8.3 minutes to dissipate out to the distance of the Earth’s orbit; so effects such as light-bending and clock-slowing would persist for that long. But the Newtonian component of gravitational force, the force that keeps Earth in its orbit, would cease almost instantly, and Earth would fly off along a straight line like a weight on a spinning merry-go-round that broke free from its moorings.

There is also the vexing problem of how to combine Einstein’s concept of gravity with quantum gravity:

http://en.wikipedia.org/wiki/Quantum_gravity

Much of the difficulty in merging these theories at all energy scales comes from the different assumptions that these theories make on how the universe works. Quantum field theory depends on particle fields embedded in the flat space-time of special relativity. General relativity models gravity as a curvature within space-time that changes as a gravitational mass moves.

At present, one of the deepest problems in theoretical physics is harmonizing the theory of general relativity, which describes gravitation, and applies to large-scale structures (stars, planets, galaxies), with quantum mechanics, which describes the other three fundamental forces acting on the atomic scale.

The hunt is on to discover the ‘messenger particle of gravity’, the graviton:

http://en.wikipedia.org/wiki/Graviton

In physics, the graviton is a hypothetical elementary particle that mediates the force of gravity in the framework of quantum field theory. If it exists, the graviton must be massless (because the gravitational force has unlimited range)

There is also the ongoing search for the Higgs boson, which give mass to every elementary particle:

http://en.wikipedia.org/wiki/Higgs_boson

The Higgs boson particle is one quantum component of the theoretical Higgs Field. In empty space, the Higgs field has an amplitude different from zero. This is also known as a “non-zero vacuum expectation value”, and illustrates the concept that there is no such thing as a completely “empty” vacuum. The existence of this non-zero vacuum expectation plays a fundamental role: it gives mass to every elementary particle which has mass, including the Higgs boson itself.

The exploration of ideas and concepts of reality which are not ‘definite knowledge’ lies in the realm of philosophy and virtually all mentioned above are of that nature.

kkwan - 20 June 2008 10:08 AM

The exploration of ideas and concepts of reality which are not definite knowledge lies in the realm of philosophy and virtually all mentioned above are of that nature.

I can’t disagree with this statement more. There is no such thing a “definite knowledge”. All knowledge is simply theory with a greater or lesser degree of empirical evidence to support it. There is no black and white line between that which we “know” to be true and that which is speculation. Your definition would put any new theory in the realm of philosophy.

In some ways we are all philosophers. The difference between scientist and philosophers is scientists demand proof and rigorously pursue it. You seem to be willing to say such proof can never be found so lets just speculate and not worry about such things. I guess only time will tell who is right, but I for one hope never to see the day when defeatism sets in and physics is left to the philosphers.

Let’s venture into this website on the philosophy of nature. It is written by Prof Ian Thompson, a physicist:

http://www.generativescience.org/books/pnb/prospects-philosophy-nature.html

In the Twentieth Century, despite the advances of modern science, we are no longer sure what the world is made of. We are confident that it is made out of parts of various kinds, but it has become mysterious what these parts are made of.

This means that if we want to know what these ultimate substances might be like, we have to turn to philosophy rather than to physics.

Contrast this with the views of Mary Midgley, a fiercely combative English moral philosopher who is concerned that a purely scientific approach to reality is reductionist, dogmatic, self-serving, narrow-minded and will diminish our humanity:

http://en.wikipedia.org/wiki/Mary_Midgley

Midgley strongly opposes reductionist and scientistic philosophies, and is particularly concerned with attempts to make science function as a substitute for the humanities, a role for which she claims it is wholly inadequate.

She argues against reductionism or the attempt to impose any one approach to understanding the world as the only right way to see things. She suggests that there are “many maps, many windows"[12] on reality and argues that “we need scientific pluralism - the recognition that there are many independent forms and sources of knowledge - rather than reductivism, the conviction that one fundamental form underlies them all and settles everything,”[13] and that it is helpful to think about the world as “a huge aquarium.

kkwan please refrain from copying other websites wholesale onto this forum. If you need to refer to something on another site, please copy a few sentences at most, and provide a URL. Or summarize in your own terms. Much of the material you are copying here is itself copyrighted. You’ve been warned before about this.

Thanks much.

My apologies. In future, I will either summarize in my own words or provide a URL with the minimum of copy and paste. I sincerely have no intention to get this forum entangled into copyright issues.

However, I do note that wikipedia articles are copyleft:

http://en.wikipedia.org/wiki/Wikipedia:Copyrights

The license Wikipedia uses grants free access to our content in the same sense that free software is licensed freely. This principle is known as copyleft. Wikipedia content can be copied, modified, and redistributed so long as the new version grants the same freedoms to others and acknowledges the authors of the Wikipedia article used (a direct link back to the article is generally thought to satisfy the attribution requirement).

Some other websites are not specific on the right to copy either partly or fully. An example is this website:

http://www.generativescience.org/index.htm

kkwan

Right, I understand kkwan. But it’s troublesome for Mods to have to police each lengthy cut-and-paste individually and track everything down. Easier simply to limit cut-and-paste to ‘fair use’, i.e. a few sentences and no more.

Thanks for the understanding.

Point taken. Thanks for the tip.