Gravitational Waves - useful or wasteful?

http://www.scienceblog.com/cms/clear-and-interesting-newpaper-summary-gr...

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)

Fred, when you say:

"My argument rests on the success of a nearly a century of physical theory and observation that supports general relativity." (emphasis added)

you neglect to emphasize “nearly a century”, or to repeat your frequent enumeration of famous scientists, which you’ve employed elsewhere to support your position. That would have been a more pertinent response to my response to you, although admittedly, it would have undermined your argument. And you might have explicitly conceded that in your more recent message you’ve acknowledged that my position is more than “only a dispute based on not believing in gravitational waves” (a paraphrase), as if I'm taking some sort of religious position. All of this is just to clear away the peripheral objections you’ve raised that might discredit my argument without addressing it. Let’s address it.

You claim that I’m “disputing arguments that have such powerful and long standing observational support.” There is observational support for the interpretations of GR that derive from its characterization as a geometric distortion or curvature of spacetime. There is no unambiguous observational support for the existence of energy-bearing gravitational waves.

You say I’m “in the position of challenger and thus needs to cite observations that demonstrate errors or at least problems with the theory if [I] expect to be taken seriously.”

I’ve done that. I’ve pointed out the truism often lost on mathematical physicists, that mathematics can be ambiguous in its relation to the physical world; I’ve said I can see no rational explanation of a physical transition from a theory of gravitation as a geometric distortion or curvature of spacetime to force, energy, and/or the production of energy-bearing waves; and I’ve claimed that no one has offered a coherent description of such a transition except by reference to abstract and ambiguous equations. That’s the problem. You can list all the famous scientists that share your expectation, you can count all the years they’ve been confidently expecting verification, but no one has explained how curvature can produce energy, rather than merely redirect it.

“Of course that interpretation will be subject to change if the theory is overthrown by clear evidence. But at this point, no such evidence exists.” What sort of evidence could prove something doesn’t exist if there’s already no evidence that it exists?

“Why is it so disturbing to you that people demand evidence when you are challenging a theory with so much observational support?” Again, there is no unambiguous observational support for the aspects of GR that are based on the association of gravitation with force and/or energy. You’re demanding evidence that energy-bearing gravitational waves don’t exist? So far the lack of evidence for their existence has only resulted in efforts to re-calibrate expectations. How long can that continue?

Unless someone can specifically show how the field equations require that gravitation as a geometric distortion or curvature of spacetime produces energy-bearing waves – without introducing force “through the back door”, there is neither evidence nor theoretical support for gravitational waves. And no amount of ridicule or invoking time-honored authority can make 0 > 0.

Fred has written:

Jim attacks with ad hominem nonsense:

"When all else fails your arguments rest on Famous Names, not reason."

My argument rests on the success of a nearly a century of physical theory and observation that supports general relativity as described by Einstein (hence his fame) and elaborated on by Hawking (whose fame rests on his own impressive body of work and his remarkable biography).

My point has eluded you, and therefore strikes you as nonsense. In modern science, it doesn’t matter if your argument rests on a thousand years of physical theory and the opinion of every physicist who’s ever lived, famous or not. A modern scientific argument rests on observations and interpretations, not its longevity or pedigree. One person can undo the interpretations of millennia, and of countless brilliant predecessors. By your standards, as expressed in your repeated references to the time the idea of energy-bearing gravitational waves has prevailed, and the preeminence of those who’ve supported the idea, physics would still be in lock-step with Aristotle and Ptolemy.

More from Fred:

Jim's only dispute with gravitational waves is that he doesn't believe in them.

Again, my point, my dispute has eluded you. (My dispute is available for anyone to see by pressing and holding the up-arrow.) Evidently, you sometimes (at least) read challenging arguments inattentively.

A bit ironic that someone defending the paradigm would see me as unwilling to question what's correct or wrong.

When all else fails your arguments rest on Famous Names, not reason.

I've pointed out that there doesn't seem to be a continuity between the transformation of potential to kinetic energy and changes in the distribution of mass - or even (possibly) the loss of relative energy - and a non-tidal gravitational wave. Why should anyone accept a leap as an answer, or a reason for self-doubt?

I wasn't expecting to convert you or "scruffy" anyway. You'll wait for official guidance. But it's a public forum, apparently with lots of silent readers.

And I'm done here. Laugh lots. Rest easy.

"Einstein, Feynman, Hawking, etc. would have been humored!"

Maybe so. Maybe they'd stop laughing long enough to explain how "orbital energy", i.e., kinetic energy, would become "gravitational energy", i.e., non-tidal re-distributions of curvature.

Meanwhile, I'll continue to be astounded (it's too wasteful and unnecessary to be funny) until thinking like the following is finally abandoned:

"... a monumental cosmic event occurring in a nearby galaxy should have generated gravitational waves that would be easily measured by the ultra-sensitive LIGO detectors. The absence of a gravitational wave signal meant GRB070201 could not have originated in this way in Andromeda. Other causes for the event, such as a soft gamma ray repeater or a binary merger from a much further distance, are now the most likely contenders."

“Low earth orbit (LEO) satellites experience drag from the atmosphere, slowly robbing them of orbital energy (lost as heat that is radiated away) and they spiral downwards, increasing their orbital velocities. There is never any indication of their orbital velocities decreasing and then increasing. Ask NASA. The same thing happens with the inspiraling binary stars - no orbital velocity decrease, just an increase.”

You’re treating “orbital energy” as if it’s a unique form of energy, something other than momentum. The atmospheric drag is a drag on “orbital energy”, not momentum, i.e., not on the velocity component of the momentum, i.e., not the kinetic energy?

“The binary stars ripple spacetime and that robs them of orbital energy… which is radiated as gravitational waves. The ripples cause the loss, not the other way around!”

The “ripples” in spacetime are commonly defined as changes in curvature produced by changes in the distribution of mass. Cyclic changes in mass distribution can produce tidal effects, but there’s no legitimate PRESTO beyond that to justify some exotic non-tidal “gravitational energy” being radiated outside the system.

“It is the potential energy component of the orbital energy that is decreasing (getting more negative).”

You’ve finally identified something specific about your “orbital energy.” The problem is, potential energy is relative. It’s relative to kinetic energy. There’s nothing absolute about it, like rest-mass-energy. If there’s a net decrease of kinetic energy to the system, then the rest of the universe is merely gravitating relative to the system, i.e., ithe expansion of the universe is being slowed by an infinitesimal amount. I don’t know if that’s plausible, but in any case it’s essentially no different than a tidal effect.

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Burt quoted me: "… the degradation of the orbits of the pulsars is a reduction in kinetic energy of the orbits, a reduction of relative energy." Then wrote: "Surely You're Joking, Mr. Arnold!.... Ironically, as the orbital energy of the neutron stars reduces, the kinetic energy increases, in complete opposition to what Jim seems to hold."

It’s not so funny if you have to actually define “orbital energy” as something other than lost kinetic energy. And the kinetic energy issue isn’t exactly black and white, is it. There has to be a reduction in the kinetic energy between the pulsars to reduce their mutual orbit. As the distance between them decreases, potential energy converts to kinetic energy, partially stabilizing the orbit. According to the standard interpretation, gravitational waves are radiated as a product of the degradation of the orbits. So what is this “orbital energy” that is “lost” as the kinetic energy decreases and (consequently) increases? And what is the primary influence that is degrading the orbit, producing the decrease of “orbital energy”?

Jim, my comment corrected what was a gross "mis-definition" of orbital energy: You wrote: "...“orbital energy” is entirely kinetic, and therefore relative". Do you recall that statement?

You then wrote: "Again, the degradation of the orbits of the pulsars is a reduction in kinetic energy of the orbits, a reduction of relative energy."

Surely You're Joking, Mr. Arnold!

Ironically, as the orbital energy of the neutron stars reduces, the kinetic energy increases, in complete opposition to what Jim seems to hold.

In order to make a positive contribution, as Jim requested, consider a simple Newtonian scenario with circular orbits and where the spin remains constant. If the orbiting neutron stars lose one unit of orbital energy very gradually, they will settle into a closer, faster circular orbit where the kinetic energy is 1 unit higher and the potential energy 2 units lower, i.e., -2 + 1 = -1 units. The calculations are very simple for the Newtonian case, coming from: E = 0.5mv^2 - GmM/r and v^2 = GM/r. The same principle holds in the relativistic case, it's just a little more complex and there are gravitational waves to consider!:))

Regards,

Burt Jordaan (www.Relativity-4-Engineers.com)

PS: Scruffy, sorry for this diversion, but you know how it is when arguing with Jim. I'll rest my case and let you continue with the main thread...

What a chorus, you three. Whatever the cause(s) of the orbital decay of the Hulse-Taylor binary system – tidal, magnetic, or some combination, and initiated or accelerated, or not, by some external collision – the orbital decay is undisputed. But what seems to be “lost” is that “orbital energy” is entirely kinetic, and therefore relative. No one is denying what GR predicts, that “changing mass distribution can create ripples in spacetime” [Daniel Sigg, in Burt’s reference to http://www.ligo.caltech.edu/docs/P/P980007-00.pdf]. But the idea of energy-bearing waves presumes a “force” of gravity, a “gravitational energy” which would be absolute, not relative.

Hi SL, you wrote: "There must be a tiny decrease in pulsar spin rate due to tidal gravity, which works against the inspiral effect, but I have no idea of its magnitude. Perhaps Burt knows."

I didn't, but I found a comment here:
http://relativity.livingreviews.org/open?pubNo=lrr-2006-3&page=articlesu..., talking about the binary pulsar B1913+16.

Quote: "Because the orbital separation is large compared to the neutron stars’ compact size, tidal effects can be ignored."

and:

"The consistency among the constraints provides a test of the assumption that the two bodies behave as “point” masses, without complicated tidal effects, obeying the general relativistic equations of motion including gravitational radiation."

Makes sense. I guess neutron stars are so compact and gravitationally "solid" that they are not easily deformed by a relatively "distant" companion.

[Edit] I checked the last paragraph numerically and got the following: For 1.35 solar mass (each) neutron stars, orbiting at about one solar radius from one another, each star's surface gravity is about 180 Mega-g. That's how hard a neutron star squeezes itself.

The tidal gravity stretch/squeeze caused by the companion (one solar radius away) over the ~20 km diameter of the neutron star is only some 2 milli-g. Negligible.

Regards,

Burt Jordaan (www.Relativity-4-Engineers.com)

Jim writes:

The positing of gravitational energy, and its radiation in gravity waves, is a residue of the association of gravitation with force.

My understanding is that the waves are a consequence of general relativity, which doesn't necessarily associate gravity with a force. In any case, the potential energy that Jim speaks of is -- TA-DA -- gravitational, not "some exotic energy."

But regardless of whether I'm understanding Jim or misinterpreting his comments, the LISA detector will test the hypothesis that gravitational waves exist. So let's wait for results instead of going in circles about this.

Also, I'm not in touch with the observation of in-spiraling pulsars, but what is the likelihood that we are catching them in a suborbital dance that is the final stages of a collision? It would seem to me that such a collision event would last a very small fraction of a double-pulsar system's lifetime.

Thus on probability grounds, I find the gravitational wave hypothesis to be much more persuasive than the not-quite orbital collision hypothesis--especially since general relativity is continuing to pass all observational tests to date.

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)

SL, you're a leaper: "Tidal gravity makes things spiral away from each other (like the moon-earth combination)."

I wrote that the inspiralling behavior of the pulsars can be explained as the conversion of potential to kinetic energy. Massive bodies approaching each other with trajectories just short of orbital will spiral toward a collision. Your "woofs" seem always to be "goofs."

Back to the question of gravitational waves:

Burt quoted Wikipedia: "...in many cases real matter and energy can be approximated as test particles, situations arise where their appreciable mass (or equivalent thereof) can affect the background gravitational field in which they reside."

This is a description of a second-order effect which, if gravitation is a geometric effect, would seemingly be geometric too.

Consider two pulsars in a stable mutual orbit. Is there energy in their geodesics, like a static charge? I can understand their fields being more intense than would be expected by the sum of each of their particulate masses, due to second-order effects. But why would the geodesics of composite particles produce a "gravitational energy" that would be radiated if, for example, a collision between one pulsar and an asteroid knocked it into an inspiral toward the other pulsar?

The positing of gravitational energy, and its radiation in gravity waves, is a residue of the association of gravitation with force. It's often described as analogous to the electromagnetic radiation produced when an electric charge oscillates along an antenna. But orbiting pulsars are not oscillating - except in the visual field of an astronomer - they are orbiting, each following its geodesic.

There is no reason to posit the existence of some exotic energy, and its radiation, when the conversion of potential to kinetic energy serves as well with the inspiralling of pulsars as it does with any other gravitational approachment.

Far be it from me to judge whether NASA or the NSF or foreign scientific research organiztions ought to be budgeting money at this level of detail. Let the researchers make their case, as they usually do, in the form of proposals to funding agencies.

The only test of his theories that Jim has proposed is his claim that, contrary to presently accepted theory, gravitational waves should carry no energy at all.

The fact that LIGO is underway indicates that the physics community deems it important to evaluate the important prediction of general relativity that gravitational waves exist and are detectable (which means they carry energy). I'm surprised that Jim doesn't agree that these measurement are worth doing.

I think we need to look into that project to decide where this discussion ought to go, but my understanding is that the observation of two collding supermassive black holes gives the best opportunity so far to study the supposed ripples in spacetime.

Fred Bortz -- Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)

The standard interpretation of the effects of the inspiralling pulsars is an excellent example of the pre-relativistic legacy still weighing on gravitation-theory.

If you think of the inspiralling pulsars as each moving along its geodesic in mutual orbit, there is no force involved, and the "lost energy" is just a conversion of their relative potential energy to kinetic energy. "Gravitational waves" are propagated due to the periodic changes in their combined gravitational field as each orbits the other, whether they are inspirally or not, and in either case there is no reason to believe the waves carry energy - any more than the orbit of the moon propagates energy-bearing waves toward earth.

The periodic variations in the gravitational field the pulsars exert here on earth is infinitesimal. Their gravitational field itself is infinitesimal here. So yes, the efforts to detect the waves are wasteful, and what's worse, theoretically unjustified.