Gravitational waves (GWs) are predicted by Einstein’s theory of general relativity. They are described as ripples in spacetime that propagate at the speed of light through space. They can apparently also be described as “graviton particles” that transfer gravitational energy, similar to photons that transfer electromagnetic energy.
Gravitational waves transfer energy quite differently to the way photons do. GWs are neither absorbed, nor reflected by matter, but pass straight through them, losing only a tiny amount of their energy. Also, while photons are emitted in legions just about everywhere in space, GW-emitting bodies are scarce, at least the ones that emit strong enough GWs to give us any hope of detection. As I understand it, GWs loose amplitude directly proportional to distance traveled, just like light waves.
GW detectors are huge (and very expensive) contraptions, mostly buried below the surface of Earth, e.g., the LIGO facility. LIGO stands for Laser Interferometer Gravitational-wave Observatory. In about 2 years of working at near-full sensitivity, LIGO has registered no GWs. Plans are now to build an even larger and more expensive GW detector in space, called LISA (Laser Interferometer Space Antenna).
GW’s have been detected indirectly as the ‘anomalous’ in-spiraling of two pulsars orbiting each other. The rate of in-spiral matches the predictions of general relativity almost precisely, which predicts that the “lost energy” is radiated away as gravitational waves. Scientist are fairly optimistic that GWs exist and that it is just a matter of having a sensitive enough detector and then wait for an event that radiates GWs and is close enough so that we can detect it.
My question: are GW detection attempts not a huge waste of resources, while there are other more pressing needs?
SL
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).
The work has held up to the scrutiny of observation and analysis by many whose names we don’t know nearly as well.
Jim’s only dispute with gravitational waves is that he doesn’t believe in them. So far, the evidence supporting general relativity also leads the physics community to conclude to the contrary–that gravitational waves ought to exist and merit additional research to test the theories even further.
For those who are interested in a generally non-mathematical history of twentieth century physics, which includes “scientist of the decade” profiles of Einstein (1901-1910) and Hawking (1981-90), please ask your library to carry my Physics: Decade by Decade.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
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.”
Jim wrote: “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.”
You’re surely joking again, Mr. Arnold!
Einstein, Feynman, Hawking, etc. would have been humored!
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
“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.â€
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 energyâ€, 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!â€
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).â€
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.
A true monumental cosmic event would be Jim giving the slightest credence to the possibility that Einstein, Hawking, and many others may be right about the existence of gravitational waves and he may be wrong.
I’m not holding my breath.
BTW, didn’t Scruffy respectfully request that Jim open a new thread rather than hijacking this one?
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
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A great thread, with opportunities to learn about gravitational waves and to discuss political considerations about what we fund and why.
Your summary puts a cherry on the sundae, SL. And Burt’s contributions were on point and excellent.
I tried to be the voice of the other readers who were trying to learn along with me, but the two of you deserve the lion’s share of credit.
Even Jim’s remarks were not completely diversionary, so we don’t need to stick him in the doghouse. However, I don’t plan to get involved in his thread on the existence or nonexistence of gravitational waves, if he takes up your suggestion. Been there, done that!
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
I think we must accept that gravitational wave astronomy is still in its infancy and it will take decades (and billions of $) to get it fully functional and useful. Like many scientific/engineering projects that push the limits (the ‘bleeding edge’), it is late and more expensive and less sensitive than originally planned. That’s life. I believe the sensitivity will eventually be reached (at a price) where useful and critical observations will be made through the medium of gravitational waves.
I was initially worried about the wisdom of upgrading LIGO to “Advanced LIGO” in more or less the same time frame that LISA will be put up in space. Thanks to Burt’s replies on the wavelength issues, I now understand that it is more cost effective to build shorter wavelength detectors on Earth and longer wavelength detectors in space. They are complimentary, not a duplication.
The “LIGO sheds light on cosmic event”, http://www.sciencecentric.com/news/08010708.htm that I reported on earlier may have been a “non-event”, but it has hinted that GWs in conjunction with other observational techniques can be very powerful.
So, thumbs up for LIGO/LISA/GEO!
Barking off – Woof!
SL
Hi Fred.
I think Jim has started to ask more pertinent questions, so perhaps he is on the brink of accepting the evidence… Anyway, I see SL has asked him rather politely to start his own thread on the “non-existence of gravitational waves”, or something to that effect.
Back to the ‘non-event’. I think Andromeda’s distance is inside the limits of the present LIGO’s ability to detect two neutron stars merging, so the non-detection probably points to some other cause of the gamma ray burst (GRB), or it must be coming from much farther out. LIGO claims 15 Mpc (~50 million light-years) range for such neutron stars, but I guess they may overstate that sensitivity a bit! Nevertheless, Andromeda is only some 0.75 Mpc away. That was the crux – there is enough faith in LIGO’s ability to rule out a merger inside Andromeda as a cause of the GRB.
I read in an interview with Kip Thorne (http://discovermagazine.com/2007/nov/the-man-who-imagined-wormholes-and-schooled-hawking/article_view?b_start:int=0&-C=)
that he reckons there may be some GW events in the 5th science run data still to be analyzed. The detection of GWs is not like an instrument that sees it and then the bells ring and the lights flash! It can take years to fully analyze one year’s worth of recorded data. Since it is government funded, the data will eventually be made available for all to look at and analyze.
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
Burt wrote: “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…”
Yea, I was patiently waiting to summarize the thread… Then I noted an article that may blow new life into this discussion: “LIGO sheds light on cosmic event”, http://www.sciencecentric.com/news/08010708.htm
Interestingly, a failure to detect a possible event is hailed as the ‘first success’ of LIGO! Quote: ‘This is the first time that the field of gravitational wave physics has made a significant contribution to the gamma ray astronomical community, by searching for GRBs in a way that electromagnetic observations cannot.’
The gamma ray burst (GRB) was first thought to be coming from a merger and coalescence of two massive but compact objects, such as neutron stars or black holes in the Andromeda galaxy. LIGO was operating in science mode at the time of the GRB, but detected no gravitational waves. The GRB results then had to be reinterpreted. Interesting stuff.
SL
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…
“Are we seeing the familiar pattern here?”
Most definitely. Given that the gravitational waves are thought to degrade the orbits, not the rotation or binding energy of the pulsars, your comment is just a distraction. If the rotation and binding energies were thought to be degrading, and contributing to the radiation of gravitational energy, you could make a positive contribution by correcting my definition of energy being “lost”, leaving my essential point modified but sustained. As it is, my point remains entirely neglected, and your original argument in support of standard gravitational wave theory unsupported, though obscured.
Again, the degradation of the orbits of the pulsars is a reduction in kinetic energy of the orbits, a reduction of relative energy.
Jim wrote: “But what seems to be “lost†is that “orbital energy†is entirely kinetic, and therefore relative. ”
Are we seeing the familiar pattern here? (When losing an argument, change the standard definition of something…)
The orbital energy of a point mass is the total of its kinetic and potential energies. If not a point mass, rotational energy needs to be added and if it is a large, extended mass, gravitational binding energy also comes into play. Any good textbook will tell you this.
For more on binding energy, see e.g.: http://en.wikipedia.org/wiki/Gravitational_binding_energy, with it’s authoritative references.
Regards,
Burt Jordaan (www.Relativity-4-Engineers.com)
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%5D. 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=articlesu17.html, 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)
Fred: “Perhaps Jim is considering a rare but imaginable situation where some of the translational kinetic energy gets transferred into increasing the rotational energy of each pulsar as they pass one another one at the points of closest approach of elongated ellipses. In that case, they would spin faster as they spiral inward. That would be remarkable and observable”
Rare, imaginable, but AFAIK not what is observed on the binary pulsars. The pulsar periods are remarkably stable, definitely not increasing. 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.
SL
Jim wrote: “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.”
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.
Jim: “According to the standard interpretation, gravitational waves are radiated as a product of the degradation of the orbits.”
Woof! The binary stars ripple spacetime and that robs them of orbital energy (as Burt properly defined before) which is radiated as gravitational waves. The ripples cause the loss, not the other way around!
Jim: “So what is this “orbital energy†that is “lost†as the kinetic energy decreases and (consequently) increases?”
It is the potential energy component of the orbital energy that is decreasing (getting more negative). Read Burt’s calculation above.
Jim: “And what is the primary influence that is degrading the orbit, producing the decrease of “orbital energyâ€?”
Spacetime ripples caused by massive orbiting bodies, resulting in gravitational waves. Ask Einstein. There is no other known mechanism that can do it (not even one that Jim can dream up and defend). And BTW, using a double binary pulsar, the GR prediction has now been confirmed to within 0.1% – that’s the experimental error, not a theoretical error!
I think we have exhausted this line of discussion and answered Jim’s problems as patiently as possible. I want to ask Jim, if he wants to continue, to kindly open another thread on it in his own Blog, so that we can discuss the science of GWs here.
SL
Anticipating Jim, one might claim that the paper Scruffy cites is further evidence that gravitational waves do not exist or do not carry energy.
Of course, that would make the person asserting such a claim a “leaper,” since they would be disputing general relativity.
Following up on Burt’s diversion, Jim has two tactics, one of which Burt pointed out. The other is the claim that we are a “chorus” of people so blinded by the success of general relativity that we can’t see his unique wisdom.
In one thread, I recall asking Jim why no one else in the physics community seems to share his view. His answer was that he had no idea. He never considers the possibility that his theory is either not novel or not supported by the evidence.
Anyway, the circular argumentation has begun, so I’m going to stay away from this for at least a day or two.
Fred Bortz — Science and technology books for young readers (www.fredbortz.com) and Science book reviews (www.scienceshelf.com)
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�