A Dark, Misleading Force

Certain subsectors of the scientifically-oriented blogosphere are abuzz — abuzz, I say! — about this new presentation on Dark Energy at the Hubblesite. It’s slickly done, and worth checking out, although be warned that a deep voice redolent with mystery will commence speaking as soon as you open the page.

But Ryan Michney at Topography of Ignorance puts his finger on the important thing here, the opening teaser text:

Scientists have found an unexplained force that is changing our universe,
forcing galazies farther and farther apart,
stretching the very fabric of space faster and faster.
If unchecked, this mystery force could be the death of the universe,
tearing even its atoms apart.

We call this force dark energy.

Scary! Also, wrong. Not the part about “tearing even its atoms apart,” an allusion to the Big Rip. That’s annoying, because a Big Rip is an extremely unlikely future for a universe even if it is dominated by dark energy, yet people can’t stop putting the idea front and center because it’s provocative. Annoying, but not wrong.

The wrong part is referring to dark energy as a “force,” which it’s not. At least since Isaac Newton, we’ve had a pretty clear idea about the distinction between “stuff” and the forces that act on that stuff. The usual story in physics is that our ideas become increasingly general and sophisticated, and distinctions that were once clear-cut might end up being altered or completely irrelevant. However, the stuff/force distinction has continued to be useful, even as relativity has broadened our definition of “stuff” to include all forms of matter and energy. Indeed, quantum field theory implies that the ingredients of a four-dimensional universe are divided neatly into two types: fermions, which cannot pile on top of each other due to the exclusion principle, and bosons, which can. That’s extremely close to the stuff/force distinction, and indeed we tend to associate the known bosonic fields — gravity, electromagnetism, gluons, and weak vector bosons — with the “forces of nature.” Personally I like to count the Higgs boson as a fifth force rather than a new matter particle, but that’s just because I’m especially fastidious. The well-defined fermion/boson distinction is not precisely equivalent to the more casual stuff/force distinction, because relativity teaches us that the bosonic “force fields” are also sources for the forces themselves. But we think we know the difference between a force and the stuff that is acting as its source.

Anyway, that last paragraph got a bit out of control, but the point remains: you have stuff, and you have forces. And dark energy is definitely “stuff.” It’s not a new force. (There might be a force associated with it, if the dark energy is a light scalar field, but that force is so weak that it’s not been detected, and certainly isn’t responsible for the acceleration of the universe.) In fact, the relevant force is a pretty old one — gravity! Cosmologists consider all kinds of crazy ideas in their efforts to account for dark energy, but in all the sensible theories I’ve heard of, it’s gravity that is the operative force. The dark energy is causing a gravitational field, and an interesting kind of field that causes distant objects to appear to accelerate away from us rather than toward us, but it’s definitely gravity that is doing the forcing here.

Is this a distinction worth making, or just something to kvetch about while we pat ourselves on the back for being smart scientists, misunderstood once again by those hacks in the PR department? I think it is worth making. One of the big obstacles to successfully explaining modern physics to a broad audience is that the English language wasn’t made with physics in mind. How could it have been, when many of the physical concepts weren’t yet invented? Sometimes we invent brand new words to describe new ideas in science, but often we re-purpose existing words to describe concepts for which they originally weren’t intended. It’s understandably confusing, and it’s the least we can do to be careful about how we use the words. One person says “there are four forces of nature…” and another says “we’ve discovered a new force, dark energy…”, and you could hardly blame someone who is paying attention for turning around and asking “Does that mean we have five forces now?” And you’d have to explain “No, we didn’t mean that…” Why not just get it right the first time?

Sometimes the re-purposed meanings are so deeply embedded that we forget they could mean anything different. Anyone who has spoken about “energy” or “dimensions” to a non-specialist audience has come across this language barrier. Just recently it was finally beaten into me how bad “dark” is for describing “dark matter” and “dark energy.” What we mean by “dark” in these cases is “completely transparent to light.” To your average non-physicist, it turns out, “dark” might mean “completely absorbs light.” Which is the opposite! Who knew? That’s why I prefer calling it “smooth tension,” which sounds more Barry White than Public Enemy.

What I would really like to get rid of is any discussion of “negative pressure.” The important thing about dark energy is that it’s persistent — the density (energy per cubic centimeter) remains roughly constant, even as the universe expands. Therefore, according to general relativity, it imparts a perpetual impulse to the expansion of the universe, not one that gradually dilutes away. A constant density leads to a constant expansion rate, which means that the time it takes the universe to double in size is a constant. But if the universe doubles in size every ten billion years or so, what we see is distant galaxies acceleratating away — first they are X parsecs away, then they are 2X parsecs away, then 4X parsecs away, then 8X, etc. The distance grows faster and faster, which we observe as acceleration.

That all makes a sort of sense, and never once did we mention “negative pressure.” But it’s nevertheless true that, in general relativity, there is a relationship between the pressure of a substance and the rate at which its density dilutes away as the universe expands: the more (positive) pressure, the faster it dilutes away. To indulge in a bit of equationry, imagine that the energy density dilutes away as a function of the scale factor as R-n. So for matter, whose density just goes down as the volume goes up, n=3. For a cosmological constant, which doesn’t dilute away at all, n=0. Now let’s call the ratio of the pressure to the density w, so that matter (which has no pressure) has w=0 and the cosmological constant (with pressure equal and opposite to its density) has w=-1. In fact, there is a perfectly lockstep relation between the two quantities:

n = 3(w + 1).

Measuring, or putting limits on, one quantity is precisely equivalent to the other; it’s just a matter of your own preferences how you might want to cast your results.

To me, the parameter n describing how the density evolves is easy to understand and has a straightforward relationship to how the universe expands, which is what we are actually measuring. The parameter w describing the relationship of pressure to energy density is a bit abstract. Certainly, if you haven’t studied general relativity, it’s not at all clear why the pressure should have anything to do with how the universe expands. (Although it does, of course; we’re not debating right and wrong, just how to most clearly translate the physics into English.) But talking about negative pressure is a quick and dirty way to convey the illusion of understanding. The usual legerdemain goes like this: “Gravity feels both energy density and pressure. So negative pressure is kind of like anti-gravity, pushing things apart rather than pulling them together.” Which is completely true, as far as it goes. But if you think about it just a little bit, you start asking what the effect of a “negative pressure” should really be. Doesn’t ordinary positive pressure, after all, tend to push things apart? So shouldn’t negative pressure pull them together? Then you have to apologize and explain that the actual force of this negative pressure can’t be felt at all, since it’s equal in magnitude in every direction, and it’s only the indirect gravitational effect of the negative pressure that is being measured. All true, but not nearly as enlightening as leaving the concept behind altogether.

But I fear we are stuck with it. Cosmologists talk about negative pressure and w all the time, even though it’s confusing and ultimately not what we are measuring anyway. Once I put into motion my nefarious scheme to overthrow the scientific establishment and have myself crowned Emperor of Cosmology, rest assured that instituting a sensible system of nomenclature will be one of my very first acts as sovereign.

326 Comments

326 thoughts on “A Dark, Misleading Force”

  1. John,

    In reply #99, what’s your point?

    Light/radiation as negative mass? It would seem to be the opposite in many ways. Gravity contracts/light expands. Also when gravity reaches critical density, it ignites and releases energy as light. As Mead describes light as primarily as waves, possibly when it grounds out/detected as a particle/photon, that it is the first stage of it gravitationally collapsing. Resulting in a larger cycle.

    Uh, no. Light has zero mass (as near as we can tell: at the very least, the mass of the photon is absurdly small). Furthermore, to say that “gravity contracts/light expands” is just an absurd statement to make. Gravity is a force, while light is an electromagnetic wave. The two are completely different sorts of phenomena. What you’ve done here would be rather like drawing a comparison between the behavior of, say, ice cream and trees: the two are in different categories altogether.

    And no, wave function collapse has nothing to do with gravitational collapse.

  2. It is strange that Carver would say that quantum mechanics is all about waves and not particles, since he worked with Feynman. St Rich pretty much said that the quantum world was more about particles than waves. I am not a partisan particularly of either viewpoint, though the wave function in quantum mechanics is really not ontologically real, but is more of a functional distribution which defines what is probable under a measurement at different regions of space. It does have to be pointed out that that measurements usually involve a localization of a wave function into a particle-like sense. The waves are not measured generally, but inferred from the statistics of many measurements

    A paper by Akoury, Kriedl , … may represent the first measurement of quantum waves. They bombarded a single H_2 molecule with X-rays. This photo-ionized the molecule and the electrons of course fly away free. However, the two “sluggish” protons act as a most basic two slit system, and in a quantum mechanical version of the Young experiment in classical wave optics results in a interference-entanglement of the two electron wave functions.

    As for photons and mass. The wave equation for the photon is

    p^2Y – &^2Y/&t^2 = 0,

    for Y the wave function. If the photon has a mass then the zero is replaced with an m^2c^4, which is the Proca wave equation. As a quantum result the propagator for this wave equation is 1/(p^2 – m^2), for p^2 the momentum. Now if the photon had mass then sinse the momentum is p = h/L, for L the wavelength and h the Planck constant, then different frequencies of light would propagate differently and light with different wavelengths of light would exhibit a dispersion. Yet light from the most distant galaxies exhibit no such dispersion. The lower bound on the photon mass is less that 10^{-60}electron volts — essentially zero mass.

    As for so called wave function collapse, the best approach to understanding this is with decoherence of Zurek et al IMO. Roger Penrose does think there is an objective reduction of quantum states and that gravity or quantum gravity actually plays a role. I think in spite of his previous stellar career that he has probably gone off into lala land on this.

    Lawrence B. Crowell

  3. Lawrence, earlier I was too irritable with long-suffering and mostly helpful Greg Egan about attempts to avoid “collapse” problems, like multi-worlds and decoherence, and just generated heat out of it instead of much light – I can’t really blame him, but the issue wasn’t really cleared up. (What bugged me was the use of terms like “illusion” or “appears to collapse” which aren’t really rigorous scientific terms anyway, are they?) As I read about those things in my middle-brow way, it looks like they cheat: they slip the idea of “probability” into the very attempt to explain the collapse, but probabilities involve collapses already being taken for granted to generate the probabilities! Really, if there is no specific “interventionistic” style collapse mechanism (such as objective collapse), waves just *stay waves* forever! That is the nature of the mathematics of “waves”, and their being equivalent to anything about probability is what we are trying to explain to begin with, not something to sneak into the beginning stages of trying to explain the very same thing. Do you think the transactional approach of John Cramer might help either?

    Not only that, I hear that decoherence and maybe even MW can’t really handle other issues like the reallocation of waves occasioned by Renninger-style negative measurements, and I still haven’t heard decent explanations of how unreliable detectors are supposed to affect WFs (really, we can’t base the physics on mere “displays” of results, they can be wrong!)

  4. Wow, this touches on a number of things. The problem is that I could spend several hours writing on this and generate a huge post here. The matter of wave function state reduction has a bearing upon this matter of cosmology.

    I think that quantum information is preserved, and in fact I think the total quantum information in the universe is very small, or maybe zero. Information we observe in the universe is due to EPR pairs separated by horizons, such as the cosmological horizon or those with black holes. The conditional entropy across horizons can be negative, which in principle nullifies the information we observe — if we can wait around long enough and have a quantum computer record of all EPR pairs which enter a black hole, or in the earliest moments of the universe More ordinary measurement I think involves a diffusion of entanglement phases into every larger environments or reservoirs of states which prevents a Poincare recurrence of states.

    One of my stipulations is that there is really only one “universe.” So called other universes or cosmologies are amplitudes which are einselected out. Different amplitudes in the cosmological path integral correspond to different cosmologies. If there exist other cosmologies this leads to an open question if one were to say that there exist different cosmologies with classical content. For the state or grand path integral

    Psi[g, Y] = int &g&Y exp^{iS[g,Y]}

    a standard rule is that |Psi[g, Y]|^2 = 1 — the whole shebang as a quantum amplitude is unity. The decoherence between different metric configurations or amplitudes between different cosmologies D[g’, g” ] ~= delta_{g’,g”} means that the reduced wave function(al) is

    Psi{g, Y] —> Psi[Y] =~ int &Y exp^{iS[g’,Y]},

    where the metric “g” here is a classical spacetime. We would still have that |Psi[y]|^2 = 1 and the amplitudes for the other cosmologies are relegated to tiny amplitudes which might manifest themselves on a larger scale in the classical cosmology as metric fluctuations. So to have universes or different cosmologies with classical content this would appear to require relaxing the unitary rule |psi|^2 = 1 when it comes to quantum cosmology.

    So this is in part my take on the einselection of states in quantum cosmology which leads to a classical cosmology. The we might of course ask, “Why this cosmology and not some other?” We might imagine in the Many Words perspective that our observable world is just one “eigen-branch.” Yet I prefer to stay away from quantum interpretations which to my mind have too much metaphysical content.

    I think gravity is an einselector “machine.” The Weyl curvature distorts the spherical spatial distribution of a set of particles into an oblate spheroid as the cloud of particles falls to a gravitating body. This has a distortion effect on something called the Wigner function for quantum systems. This has a physics similar to decoherence, and it suggests that gravity might play a role in einselecting a few quantum states as classical variables. Also with some calculations I have done I have found that his has some frame dependencies to it which I thought indicated a problem with it. Then came along Don Page who wrote very recently

    http://arxiv.org/PS_cache/arxiv/pdf/0712/0712.2240v1.pdf

    where he advances something very similar — an observational dependence to the state selection process.

    Now I suspect that the einselection principle for the universe we observe, and not a whole gemish of other amplitudes for other cosmologies, is a maximal complexity conjecture. In other words the cosmology which has a classical content is one which permits some maximal amount of complexity in local regions, or some large number of information states which are einselected as classical information. This might be a more operative physics for issues such as Polchinski-Busso theory on 7-brane (dual to 4-brane of the universe) cancellations of a bare cosmological constant which has an Anthropic Principle interpretation.

    The matter of wave function collapse has buggered quantum physics ever since Bohr laid down the Copenhagen interpretation. That works very well, and for most of what people do is operative. But it is unsatisfying on a deep level. Now the matter is being taken up in decoherence theory, einselection and what I see as a matter of quantum gravity. This is not to say I think that Penrose’s idea of Planck scale fluctuations or collapses as inducing state reductions, but more that the classical variables which exist in the universe are einselected in quantum gravity/cosmology. If the deoherence and einselection approach are on the right track then the classical world is really a sort of approximate reality, where really there are still quantum waves underlying everything which we don’t see under a coarse graining.

    Lawrence B. Crowell

  5. Now I suspect that the einselection principle for the universe we observe, and not a whole gemish of other amplitudes for other cosmologies, is a maximal complexity conjecture.

    Lawrence, I like your ideas a lot, and was delighted to come across your book (Quantum Fluctuations of Spacetime) recently while thinking about Hamming codes in M theory. What do you think of the Machian holographic principle (category and number theoretic: ie. not written in terms of stringy classical geometry) as a maximal complexity constraint? This avoids the ‘einselection-looks-like-collapse’ semantics (which I think is a problem but you may disagree) by rephrasing quantum causality in terms of a (categorical) pure quantum logic. The maximal complexity results from the need to use the whole n-cat heirarchy to reach the correct emergent classical picture.

  6. Lawrence,

    While Feynman was a colleage and friend, they were in disagreement on this and this book was Mead’s argument. Suffice to say the math is beyond me. His argument is essentially that quantum mechanics is a digital description of an analog reality.

    As to the earlier point about quantum entanglement, it does make sense as a wave. If you and I were twenty miles to either side of a radio station, that we were both tuned to, it wouldn’t be any mystery that we heard the same sounds at the same time.

    Theoretical physics is to applied physics what design is to engineering. If I want to know what can be concieved, I’d ask a designer. If I want to know what works, I’d ask an engineer. Given Mead has been a seminal figure in the computer revolution, I wouldn’t personally dismiss his experience when compared to academics who’ve spent their careers figuring what happens at the edge of a black hole. Remember it is because of applied physics that society funds theorectical physics to the degree it does, so it is ultimately wise not to dismiss everyone whom you may disagree with, without considering the entire picture.

  7. Neil,

    What bugged me was the use of terms like “illusion” or “appears to collapse” which aren’t really rigorous scientific terms anyway, are they?

    We’ve gone over this. Of course the appearance of collapse is rigorous scientific terminology. In fact, it is only the appearance of collapse that we can ever conceivably measure. Given two theories, one with collapse and one without, the one without must necessarily have the observed appearance of collapse within it to be valid. From there we have to things that need to be done to decide between the theories:

    1. Occam’s Razor tends to be a good guide for deciding which is more likely, and thus useful for guiding future experiments.
    2. The use of experiments to distinguish the two theories.

    The difference between quantum mechanics with collapse and quantum mechanics with only the appearance of collapse is that there is one less axiom in the latter. Thus, by Occam’s Razor, it seems rather likely that the latter is more likely. Furthermore, quantum decoherence is a phenomenon that does not happen suddenly, but instead quite gradually. As a result, it can be (and has been) observed quite directly in the laboratory.

    Thus it turns out that the many worlds interpretation of quantum mechanics is not in any way an attempt to avoid any “collapse problems.” Instead, it’s an explanation of the observation of wave function collapse that requires no additional assumptions. As a result, we can be pretty confident that it’s accurate.

    Lawrence,

    I think gravity is an einselector “machine.”

    This description could just as easily describe the other three forces we know of too, though, as well as any other quantum mechanical interactions.

  8. LC:
    ABSTRACT: Don N. Page; December 13, 2007…last sentence…

    “SQM also suggests the possibility that past steps along our evolutionary ancestry may be so rare, that they have occurred no where else in the past history of the universe that we can observe”.

    It is almost certain that life in the universe is a common phenomenon. It is also almost impossible that beings identical to ourselves exist anywhere else. One does not have to be a quantum theorist to fathom that! However it is interesting that certain quantum theories point to such an obvious conclusion about our uniqueness in the universe.

    For similar reasons, related to genetic inheritance and environmental upbringing, each human individual who has ever lived is also unique. Cloning people doesn’t produce robots, just individuals with identical genetic inheritance- as identical twins. However, no pair of identical twins live exactly identical lives.

    This comment in Don’s abstract has profound implications…

  9. Jason,

    Uh, no. Light has zero mass (as near as we can tell: at the very least, the mass of the photon is absurdly small). Furthermore, to say that “gravity contracts/light expands” is just an absurd statement to make. Gravity is a force, while light is an electromagnetic wave. The two are completely different sorts of phenomena. What you’ve done here would be rather like drawing a comparison between the behavior of, say, ice cream and trees: the two are in different categories altogether.

    Presumably mass can be converted to energy and vice versa. Obviously all the connections are not apparent, but for those of us not tracking down all the details, it is a logical conclusion that some form of cycle exists.

    Thus it turns out that the many worlds interpretation of quantum mechanics is not in any way an attempt to avoid any “collapse problems.” Instead, it’s an explanation of the observation of wave function collapse that requires no additional assumptions. As a result, we can be pretty confident that it’s accurate.

    While it may not be apparent, I am generally the go along to get along type of person. But when presented with comments like the above, especially when I’ve been expected to swallow a series of other incongruities, from the universe being 13.7 billion years old, which happens to be how far light appears to travel, even though an inflation stage blew it up to many times its visible size, then that there is lots of dark matter causing galaxies to spin faster than Newton’s laws account for and, oh wait, 70% of the universe is actually invisible energy because these darn galaxies are flying away from each other much faster then predicted. Of course I should be softened up enough to buy the many worlds, other universes, etc.. After a while though, the average person is going to start scratching their head. At some point it starts looking like cult behavior.

    Would Einstein buy this? Or would he be going back and reviewing past assumptions. Frankly I’m no smarter then I was at twenty, but I’m alot wiser and much of that has come about from having many of the certainties I was taught break down and having to pick up the pieces and start over again.

  10. Presumably mass can be converted to energy and vice versa. Obviously all the connections are not apparent, but for those of us not tracking down all the details, it is a logical conclusion that some form of cycle exists.

    No. Mass is energy. Now, it is possible to convert between mass energy (the energy in any internal degrees of freedom) and kinetic energy. But conversion between mass and energy is a nonsensical statement.

    And no, there is no logical conclusion to be had until you define your terms properly, which you haven’t done.

    While it may not be apparent, I am generally the go along to get along type of person. But when presented with comments like the above, especially when I’ve been expected to swallow a series of other incongruities, from the universe being 13.7 billion years old, which happens to be how far light appears to travel, even though an inflation stage blew it up to many times its visible size, then that there is lots of dark matter causing galaxies to spin faster than Newton’s laws account for and, oh wait, 70% of the universe is actually invisible energy because these darn galaxies are flying away from each other much faster then predicted. Of course I should be softened up enough to buy the many worlds, other universes, etc.. After a while though, the average person is going to start scratching their head. At some point it starts looking like cult behavior.

    So? When we enter new areas of observation we should expect things to be weird and counter-intuitive. The statement that a theory describes things as being weird is in no way evidence against that theory. Until you have some real, solid evidence that scientists are being misled, your claims of cult behavior are baseless.

    Would Einstein buy this?

    Irrelevant. The progress of science is independent of any one person.

  11. Machian holographic principle: The holographic principle came about because of the duality between Anti-deSitter spacetime and conformal field theory. This duality was illustrated by Maldecena as a way in which the AdS spacetime has Minkowski spacetime at conformal infinity.

    The AdS spacetime, a five dimensional spacetime with signature [-,-,+,+,+], with effectively two “time variables,” has at conformal infinity a timelike flat spacetime. As such it can’t be determined by a spatial data set without some additional boundary conditions. The deSitter spacetime is related with signature [-,+,+,+,+], where one of the time directions in the AdS is Wick rotated to a spatial variable.

    In an 11-dimensional “bulk” there is a 10-brane of supergravity. There is a topological result called Poincare duality in algebraic topology, which is that in any n dimensional space a p-dimensional “chain” or cocycle (p —-> |out>. BTW, string theory has its origin with Veneziano amplitudes and S-matrix theory.

    Smolin wrote his three paths to quantum gravity, and I should point out that the LQG is probably just a view of quantum gravity through a keyhole on another door. I suspect that string theory and LQG, both approaches have their warts and problems, are just different approximate perspectives on the same problem.

    Lawrence B. Crowell

  12. Sam Cox on Dec 20th, 2007 at 1:50 am
    LC:
    ABSTRACT: Don N. Page; December 13, 2007…last sentence…

    “SQM also suggests the possibility that past steps along our evolutionary ancestry may be so rare, that they have occurred no where else in the past history of the universe that we can observe”.

    It is almost certain that life in the universe is a common phenomenon. It is also almost impossible that beings identical to ourselves exist anywhere else. One does not have to be a quantum theorist to fathom that! However it is interesting that certain quantum theories point to such an obvious conclusion about our uniqueness in the universe.

    ———————–

    I worked out a chaos problem with the stability of stellar systems. I looked at the range of extrasolar systems identified and estimated the Lyapunov exponent for a planet at 1AU around these G-class stars. The “Jovian” planets for the most part perturb these putative earths in unstable orbits and with some Bayesian statistics I found that our galaxy may only contain a few hundred Earth-like planets which might permit biology. In other words biology might be a process inherent to the universe under the right conditions, but those conditions are comparatively rare. Observership in the universe is maybe far more rare as well. We share this planet with other intelligent life forms, cetacians and cephalopods, such as octopi — which are invertebrates and some have basketball sized brains, but they lack some faculties required (social behavior, or hands or … ) to be a cosmological observer.

    Don Page’s idea here is commensurate with Stephen J. Gould’s comments as well. Evolutionary biology does not have an apriori attractor point or “goal” for some progress. Biological evolution may select for an increased complexity in organisms, as measured by tissue types, or organizational structures, but the range of possibilities seems to literally explode. Of course this is a geo-centric view of biology — who knows what another bio-planet has going on. I just published a book on sending probes to such planets BTW. Yet biological evolution does not appear to operate with some goal of generating cosmological observers.

    This tends to make my cast a suspicious eye on cosmological principles. If you think about it the only thing we human beings really make at the end is garbage. If there is any identifiable long term function our species serves it is to tear up this planet and convert everything into trash. We are doing a bang up job of this!

    Page’s main thesis is that einselection appears to have an observer dependent content. It suggests a sort of frame dependency which might indicate a reason for some confusion we have. It might also be a pathway to a commonality between freme dependencies with geodesics and the rest with general relativity.

    Lawrence B. Crowell

  13. Jason, yes we’ve been through this in the sense that you nor Greg E. never defined (and certainly haven’t defended same) what you think “appearance of” means in this context. Just what is an “appearance” instead of the “real thing” here anyway? I keep asking because you keep saying nothing in the answer (nothing that satisfies describing what a collapse is, versus what an appearance is, etc.) For example, it is absurd to talk of “appearance of collapse” being “the only thing we can measure” when the measurement is the very definition and probable cause of collapse. (If you and other empiricism end-runners are now meaning some tricky way of talking about appearance and reality, don’t pretend that is just “science” business as usual.) The wave has to really collapse because the detection at some point means it can’t be anywhere else. It isn’t about which description posing as a “theory” is simpler, it’s about which description is what we find happens, and then trying to explain *that*.

    Since we started out with dark energy, here’s another issue and question, about energy conservation and gravitational waves. I asked this at Backreaction (where they are more appreciative of my questions and less into too-clever-by-half metaphysical manipulations),
    at http://backreaction.blogspot.com/2007/12/indirect-detection-of-gravitational.html:


    At 2:19 PM, December 19, 2007, Blogger Neil’ said…

    I was browsing Penrose’s latest big opus, The Road to Reality, and saw discussion about gravitational radiation and energy. I got the impression, there is no way to assign energy to traveling gravity waves like there is in E&M radiation (E and B amplitudes squared leading to energy density.) Well, maybe there’s proper justification, but: how can you get energy conservation if the radiating bodies lose energy, but there’s no energy “out there” to make up the difference?

    And BTW, shouldn’t criticisms or alternative perspectives (like what Al refers to) get some open-minded attention? I wouldn’t know, just asking.

    At 2:30 PM, December 19, 2007, Blogger Bee said…

    Hi Neil’,

    I don’t mind ‘alternatives’ and it’s nice people are looking into various things etc blahblah, but given that GR works pretty damned well I’d like to see really good reason before I consider replacing it with something else.

    Regarding your question, I’m not sure I know what you mean with “out there”, but I will have a try. It is of course correct that there is no well defined stress-energy-tensor of the gravitational field itself, and esp. that of grav. waves vanishes (well, it’s a vacuum solution). One can however very well talk about energy transport towards infinity, e.g. like the energy loss of the source discussed above. That usually requires a gauge-fixing. I think Weinberg discusses this very nicely in his book. Best,

    B.

    OK, but I still want to know *how* and “in what” the lost energy is stored, to comp. for the radiating system having less, if not in gravitational waves per se?

  14. Neil,

    The appearance is just that: when one performs a measurement under certain conditions, one sees collapse. Of course, this is only the classical limit of the phenomenon. Quantum decoherence predicts a very specific, gradual transition to where the system appears to be fully collapsed dependent upon the interactions that the system undergoes.

    For example, it is absurd to talk of “appearance of collapse” being “the only thing we can measure” when the measurement is the very definition and probable cause of collapse.

    Nope. It’s the interactions that we make use of to perform the measurement that cause the appearance of collapse. That there is a person there to read out the results is irrelevant.

    The wave has to really collapse because the detection at some point means it can’t be anywhere else.

    Not in the least. The problem is that we are not separate from the system we are measuring. The reason that we see collapse is because we are described by the exact same quantum mechanics that describe the system we are measuring, and in order to see a measurement we have to interact, however indirectly, with the system we are measuring. Quantum decoherence is, after all, about components of the wave function losing information (and the ability to interact) with other components of the same wave function. Thus we see the detection at some point (in the example of a spatial detection) not because it really is there, but rather because that is simply the component of the wave function which “we” happen to be (the other components would also be the same persons, of course, just with different outcomes).

    As far as your gravitational wave question, that’s a pretty good question. This paper claims to have the answer:
    http://arxiv.org/abs/gr-qc/0102072

    I’d read through and try to get a basic understanding, but my plane is boarding, so I have to go.

  15. Jason,

    No. Mass is energy. Now, it is possible to convert between mass energy (the energy in any internal degrees of freedom) and kinetic energy. But conversion between mass and energy is a nonsensical statement.

    And no, there is no logical conclusion to be had until you define your terms properly, which you haven’t done.

    The gravitational field of a galaxy extends out several times its visible size, so presumably it’s drawing matter in from that area. Galaxies are visible from 10+ billion lightyears away, so presumably they radiate light out over that area. Are you arguing that the matter being drawn into a galaxy has no causal relationship to the light radiating away from it?

    So? When we enter new areas of observation we should expect things to be weird and counter-intuitive. The statement that a theory describes things as being weird is in no way evidence against that theory. Until you have some real, solid evidence that scientists are being misled, your claims of cult behavior are baseless.

    When I was young, I saw patterns in the clouds. Now that I’m a good bit older and pushed the horizon of reality a few times, I see clouds in the patterns. Few things seem weird to me anymore, just degrees of organization, which modern cosmology has issues with, since its loose ends have been increasing much more rapidly(96% invisible universe?)then they have been decreasing. As for science being misled, how can the increase of the energy density of the universe by such a massive amount not cause anymore soul searching then it did? Can you think of any other field where prediction could be that far off and not have serious repercussions?

    Irrelevant. The progress of science is independent of any one person.

    The progress of science is independent of any group of people as well. BBT can’t paint itself much further into the corner.

    Lawrence,

    And in fact some biblical creationists have put their cross hairs on big bang and cosmology and I have seen references to these plasma cosmology ideas as “evidence” of God’s recent creation of the universe.

    Actually the reverse is true; Hannes Alfven, one of the developers of plasma cosmology; “I was there when Abbe Georges Lemaitre first proposed this theory [Big Bang]. Lemaitre was, at the time, both a member of the Catholic hierarchy and an accomplished scientist. He said in private that this theory was a way to reconcile science with St. Thomas Aquinas’ theological dictum of creatio ex nihilo or creation out of nothing. There is no rational reason to doubt that the universe has existed indefinitely, for an infinite time. It is only myth that attempts to say how the universe came to be, either four thousand or twenty billion years ago.”

    Monsignor Georges Henri Joseph Éduard Lemaître (July 17, 1894 – June 20, 1966) was a Belgian Roman Catholic priest, honorary prelate, professor of physics and astronomer.
    Fr. (later Msgr.) Lemaître proposed what became known as the Big Bang theory of the origin of the Universe, although he called it his ‘hypothesis of the primeval atom’.
    He was a pioneer in applying Einstein’s theory of general relativity to cosmology: suggesting a pre-cursor of Hubble’s law in 1927, and then publishing his primeval atom theory in the pages of Nature in 1931. At the time Einstein believed in an eternal universe and had previously expressed his skepticism about Lemaître’s original 1927 paper. A similar solution to Einstein’s equations, suggesting a changing radius to the size of the universe, had been proposed in 1922 by Alexander Alexandrovich Friedman, as Einstein informed Lemaître when he approached him with the theory at the 1927 Solvay Conference (Friedman had also been criticized by Einstein), but it is Lemaître, with his proposed mechanism, that made the theory famous for several reasons according to historians. First, Friedman was a mathematician who was not working with astronomical data or concerned with the math as a description of physical reality. Secondly, Friedman died young and could not further work on his ideas. Thirdly, Lemaître worked with astronomers and made his theory in accord with observations and had consequences which could be tested. Fourth, Arthur Eddington made sure that Lemaître got a hearing in the scientific community.
    Lemaître also proposed the theory at an opportune time since Edwin Hubble would soon release his velocity-distance relation that strongly supported an expanding universe and, consequently, the Big Bang theory. In fact, Lemaître derived what became known as Hubble’s Law in his 1927 paper, two years before Hubble. However, since Lemaître spent his entire productive life in Europe rather than emigrating to America, American publicity machines have preferred to stress the contributions of scientists such as Hubble or Einstein who can be claimed to have a US connection.(from wikipedia)

  16. State reduction is an open problem in physics, and we should not be to quick to make hasty assumptions. A complete reduction of a state is within the Copenhagen viewpoint. A wave function psi = sum_n C_n phi_n that gives a measurement O_i of the observable O reduces the state to phi_i, for some i in the summation index over the state space. In Bohr’s Copenhagen interpretation the classical world imposes a projector operator P_i on the wave function to project out that state. Projector operators are not unitary, and are idempotent, which puts them outside standard quantum formalism. But Bohr tells us not to worry because the imposition of a classical world or the detector necessitates this. So here a measurement is thought to reduce a wave function to a “point,” or some spectific eigenstate corresponding to our measurement.

    The Copenhagen interpretation has gotten a lot of criticism by some folks, but it first has to be pointed out that it works very well. The main problem with it is that we all know that the classical world is built up from quantum systems, atoms, electrons, nuclei etc. And so this dichotomy is something artificial. The decoherence approach says that a quantum system with its entanglement phase looses that phase to a reservoir of states corresponding to the environment. This is not too different from the Fermi golden rule for spontaneous emission. There excited states emit photons in a vast number of possible occupation states (vacuum states) and a grand sum over these leads to a spontaneous irreversible emission of photons. Similarly a quantum system with some density matrix is reduced to a diagonal matrix and the off-diagonal terms of the form C_m^*C_n|n>

  17. My last post on state reduction was cut off. So before I talk about Lemaitre here is the rest of them:

    (state overlaps or entanglements) are lost to the reservoir of states. The entanglement is preserved but just scrambled up in the reservoir of states. The now diagonal density matrix then just gives a classical-like set of probabilities for the outcome. Decoherence does not tell us which state obtains, but it does tell us how under a measurement the observed outcome is reduced first to a classical probability problem. We still have to look at the dice to see the outcome.

    Einselection is a bit more on decoherence, for it attempts to indicate how a wave function is “squashed” into a sharp peak, or a near delta function. If this is completely reduced to a delta function corresponding a measured outcome this recovers the Copenhagen ad hoc state reduction (collapse). Yet to be true to QM we can’t completely reduce it that far. We might come very close, but maybe not completely.

    So we might find that the classical world is something which exists FAPP, or as itself a good approximation. In the same light a measurement of a quantum system is an approximation. And of course realistically measurements are never exact: first we have the uncertainty principle, then we have instrumentation tolerances, shot noise in electronics and so forth. So the prospect that we don’t completely reduce a quantum state might not be so horrid. We are giving up a security blanket more than anything.

    As for Many Worlds and other interpretations my main problem with these ideas is that they are fundamentally untestable IMO. There does not appear any way around the Bell inequality result to peek behind the curtain to tell which of these, MWI, Bohm etc, are really operative. This is a problem I think exists with the Meade idea with QED.

    As yet the problem is still open. Some progress has been made on this, but the problem has yet to be satisfactorily solved and produce results which can be tested and not rely upon “things” behind the quantum curtain.

    ——————-

    I was referring to more recent times. I have seen some Creationist stuff in recent times which reference “plasma cosmology” as evidence for a recent origin of the universe — recent as in 6000 years ago.

    Lemaitre also admonished the Pope that this should not be taken as evidence of God’s creation. He did say that the universe came about from the “radioactive decay” of a primeval atom. In a way inflationary cosmology of Guth is a more precise statement of this: An unstable vacuum with a Higgs particle that adjusts the vacuum state to a physical vacuum and inflates a spacetime from a near Planck scale to the large.

    As for light emitted by galaxies influencing gravity, or visa versa, I suspect that you are not quite on track there. Gravity provides the pressure inside a star to maintain nuclear fusion, which is the energy source for light emitted by stars. But there is no causal relationship between light and stars as you might be thinking. Black holes are the epitome of a non-emitting object and they have a gravity field — one hell of one close up.

    Lawrence B. Crowell

  18. LC:

    1.) “Yet biological evolution does not appear to operate with some goal of generating cosmological observers.”

    GOOD point, in my opinion…and I appreciate the second interjectory paragraph too, but then in the third paragraph below, you correctly return to discuss frame dependency as a possible reason and explanation for this conundrum….

    2.) “This tends to make my cast a suspicious eye on cosmological principles. If you think about it the only thing we human beings really make at the end is garbage. If there is any identifiable long term function our species serves it is to tear up this planet and convert everything into trash. We are doing a bang up job of this!”

    3.) “Page’s main thesis is that einselection appears to have an observer dependent content. It suggests a sort of frame dependency which might indicate a reason for some confusion we have. It might also be a pathway to a commonality between freme dependencies with geodesics and the rest with general relativity.”

    When the universe is observed from a particulate 4D frame of reference, and because what we observe is the result of accelerations, gravitational and relativisitic effects, we are not seeing the universe the way it actually cosmologically exists.

    Yes, our existence is tied in the reality we observe- to evolutionary developments which could not be exactly duplicated anywhere else in the universe. Yes, the world we live in is so real, technologies can be developed on the basis of careful observation and measurement of the cosmic processes and universe we observe.

    Yet, in a phylogenically developing quantum quasi-static universe, the requirement for its very existence, observation in ever higher levels of complexity, may in a sense, be like a few diamonds processed from huge amounts of cosmic “ore”.

    It is important, I think, to remember that inorganic and most likely, organic complexity in the universe are actually very widespread. The proportions and behavior of specific sets of similar particles and atoms are global phenomena. For observational complexity to exist in the universe, particles must not only be particles and atoms, atoms, but the behavior of all this inorganic matter, resonances at stellar temperatures, valence relationships and the like are and must be strictly constrained.

    All of this is tied to the nature of gravity itself, the very specific density necessary at the big bang and the nature of the grand proportion…the way energy becomes energy densities…matter…and the behavior of energy as scale in the universe varies.

    I keep coming back to Hawkings observation that the universe “just is”. From our frame, we place value judgments on what we observe and try to ascertain “purpose”, “worth” or “worthlessness”. However, however low our opinon of the quality of existence, existence…the observation, measurement and appraisal of our 4D reality is all we have…it is all there IS. I should stress that the universe emphasizes diversity…human culture, conformity. The reason for this difference is the reality of existence VS the way we collectively observe from limited frames.

    A relative of mine passed on recently, and the day before she died of congestive heart failure resulting from cancer, some of us visited her. She was quite lucid. One of the relatives expounded on the religious viewpoint regarding death…very passionately!

    When he finished, she turned to me and said: “Sam, what do YOU think? I told her that if death was non-existence, there wasn’t much she- or we- could do about it, so worry under that circumstance was unnecessary.

    However, I also told her we live in a universe which exists only as it is observed and measured, so while I felt non-existence was possible, it was the least likely answer to her question. I told her that reincarnation from my point of view is untenable because we exist as individuals ONLY because of our unique genetic and environmental experiences.

    I told her, that from her frame, and the frame of any and all living, I felt there were only two really viable possibilities. 1. Exeriencing existence in geometric inversion and decreasing entropy, followed by reemergence into the same life she already experienced, or 2. A gradual re-awakening after death, as a child into her same existence…she would be chasing her kid sister around the house some day, and suddenly stop and look in a mirror with surprise…wondering why she was young…then continuing to chase her sister around the house- and into eternity.

    If life ended at death, it is virtually impossible we would be existing “now”.

    I might add that I do not regard existential problems as philosophical issues. To my mind, such things are at their heart matters for scientific inquiry and ultimately factual solution and understanding.

  19. Lawrence,

    As for light emitted by galaxies influencing gravity, or visa versa, I suspect that you are not quite on track there.

    As for the vice versa, assuming that’s gravity influencing light, I thought that was what provided the original proof of Relativity, the experiments in 1919 showing the light from stars being bent by the gravity of the sun, during a solar eclipse!

    It’s not that light influences gravity, obviously the gravity has the local advantage, but if gravity affects light, it would seem the symmetry would imply that there is some very minor counter-effect? “For every action, there is an equal and opposite reaction.”

    Gravity provides the pressure inside a star to maintain nuclear fusion, which is the energy source for light emitted by stars. But there is no causal relationship between light and stars as you might be thinking. Black holes are the epitome of a non-emitting object and they have a gravity field — one hell of one close up.

    There has been much discussion recently of jets from galaxy cores;

    http://www.space.com/scienceastronomy/071217-death-star-galaxy.html

    http://news.xinhuanet.com/english/2007-12/18/content_7272144.htm

    Here is a clear picture of a jet from a galaxy core;

    http://apod.nasa.gov/apod/ap000706.html

    Just like the radiation from the stars surrounding the black hole, this is another form of energy escaping from galaxies. According to the first link above, it may well provide the source for future galaxies;

    Turns out that the “death ray” may not be all bad news for the victimized galaxy, at least theoretically, as such a massive influx of energy and radiation could help form new stars and solar systems by compressing gases.

    “In the end [3C321] may be the source of new life in that distant galaxy,” said Martin Hardcastle, an astronomer at the University of Hertfordshire, in the United Kingdom. Hardcastle explained that the jet will continue to pour out of its parent supermassive black hole for about 10 to 100 million longer — plenty of time to squeeze otherwise inert gas together into new star systems.

    “Jets can be highly disruptive … but [create] stellar nurseries,” Tyson said. “It’s a fascinating sort of duality about how these high-energy phenomena influence the environments in which they’re embedded.”

    If you are willing to consider that the mass falling into the galaxy is the original source of this energy, then here is a version of the cycle.

    As I recall, at the center of a normal gravitational body, such as the earth, the gravitational effect balances out, so that while the pressure is enormous, the gravity is neutral. If this was applied to galaxies, it might explain why they shoot these jets out from the poles.

  20. This posts answers multiple posts above:

    In Sam Cox’s response he quotes me in, “This tends to make my cast a suspicious eye on cosmological principles. ” I meant to say anthropic cosmological principles.

    My point in mentioning this is two fold. The weak anthropic principle (WAP) is really a sort of question. Bethe challenged a WAP statement that the laws of physics had to conform to an ancient Earth, but gravity compression models of solar energy predicted a sun lasting only a few 10’s of thousands of years. So Bethe figured out a nuclear fusion model of solar energy, and viola the WAP problem was removed. I think something similar is afoot here. The WAP implication of D4-brane and D7-brane duality and the cancellation of the bare cosmological constant by Polchinski-Busso suggests strongly that there is some other “non-anthropic” physics, or quantum gravity, or cosmological principle waiting in the wings.

    The second objection is that we humans have a prejudice to see the universe as existing around us, or that it was created with us as its purpose. Religion is largely fashioned around this idea. Yet to be honest I find these things as intellecually vacuous, though maybe I am somewhat disposed to the Flying Spaghetti Monster 🙂 But you do indicate that the universe does permit a vast array of diversity, which is in line with my hypothesis that the cosmology einselected from the grand path integral (analogous to a grand partition function of “everything” in a Euclidean measure) is somewhat in line with this. Yet I think that we humans are just some flash in the pan, even if we appear to be cosmological observers.

    This is not exactly the forum to write about it, but I can offer some pretty strong reasons to see us humans as really dysfunctional life forms, or as some sort of terminator species that is hopelessly out of balance. At no point in the natural history of this planet do I think there was ever a species of animal our size and dietary requirements that numbered at one time 6.6 billion. Then consider we drive cars and run machines and use far more energy. We are just billions of ground apes exponentially rampaging out of control. We also tend to put some of the most insane or lunatic of our kind in positions of great power — my country elected GW Bush twice! And GWB IMO is a dim-witted loony.

    As for J Merryman, I also saw the astrophysics of the galactic jet spraying a second galaxy. This involves lots of complicated astrophysics of accretion disks around black holes, in this case a galatic black hole of 10^6 or larger solar masses. The gravity pulls in surrounding matter and heats it up. This enormous heating up of material is what generates these spectacular processes. It involves lots of plasmas heated up by gravity accretion, magnetic fields getting wound up in the plasma which ejects ionized high energy material into these jets. I am not particularly that learned in these astrophysical complexities, where you get the geniuses who run complex differential equations on computers to simulate or predict these things.

    Gravity waves and energy — Gravity waves are type N solutions in the Petrov classification. This classification involves Killing eigenvectors of the Weyl curvature. The easiest way to understand gravity waves is with weak gravity waves, so called pp-waves. These obey a differential equation of the form

    &_c&^c h^{ab} = k T^{ab}

    where h^{ab} = g^{ab} – 1/2 h_{ab} tr(g) and h_{ab} the flat Minkowskian metric. Here & = 4-dim partial, k = 8piG/c^4. The momentum energy term on the right is the source of the gravity wave. This is a wave equation similar to that for a Maxwellian electromagnetic wave with a source. In this weak case matters of energy conservation don’t appear. Weak gravity waves are all we are likely to ever detect from distant astrophysical sources, such as colliding black holes and other violent events.

    This apparent energy conservation appears because this is a bimetric theory, which departs in subtle ways from the general covariance of general relativity. In a way we are imposing an implicit energy conservation onto the problem, which works sell enough in this weak field domain. For strong gravity fields we are not quite so fortunate in being able to pin down energy conservation — the time variation in the field prevents a global timelike Killing field.

    However, as Feynman pointed out if you have two beads on a stick a gravity wave will cause them to move. If these rub the stick they generate heat, or if we were to put magnets on the beads and fix solonoids around them we will get a current. So from some basic physical arguments it can be seen that gravity waves carry energy or impart it on systems they interact with.

    Lawrence B. Crowell

  21. Lawrence,

    So from some basic physical arguments it can be seen that gravity waves carry energy or impart it on systems they interact with.

    That is essentially a description of my understanding of gravity as a collapsing spacetime geometry. Not that it is energy, but that the release of energy results in a collapse of the system. This may not be entirely correct, as there may be a gravitational attractor, but that if it is part of a larger cycle, there is a siphoning effect, where systems that are radiating energy also draw in material. What might propel this system is the existence of more energy then the space can hold in solution, so that it keeps clumping and precipitating out, until it heats up to the point of breaking down and radiating away. Since this released energy is hotter then the existing background radiation, it displaces it, so that the cooler radiation starts to clump and precipitate out and so on. Necessarily this cycle means that the universe is infinite space already occupied by energy and therefore cannot expand, as this would result in the loss of energy and eventual dissolution to the level of cold radiation, which the space can hold in solution, below 2.7K.

    But you do indicate that the universe does permit a vast array of diversity, which is in line with my hypothesis that the cosmology einselected from the grand path integral (analogous to a grand partition function of “everything” in a Euclidean measure) is somewhat in line with this. Yet I think that we humans are just some flash in the pan, even if we appear to be cosmological observers.

    Here is an interesting biological hypothosis from this plasma site; http://www.holoscience.com/synopsis.php?page=10

    It seems that when a dwarf star or gas giant planet “gives birth” to a rocky satellite, parent and child usually remain closely bound. Our solar system, with its widely spaced orbits and chaotic features, appears to be the result of a recent cosmic “traffic accident”. This seemingly wild conjecture is supported by the global stories of prehistoric planetary encounters. So to use our situation as a measure of a normal planetary system will give wildly misleading ideas of how life begins and estimates of the likelihood of life elsewhere in the universe. The most benign situation for life in an Electric Universe is inside the electrical cocoon of a brown dwarf star. Radiant energy is then evenly distributed over the entire surface of any planet orbiting within the chromosphere of such a star, regardless of axial rotation, tilt, or orbital eccentricity.

    The exceedingly thin atmosphere of such stars has the essential water and carbon compounds to mist down onto planetary surfaces. The reddish light is ideal for photosynthesis. Such a model provides one reason why the Search for Extra-Terrestrial Intelligence (SETI) project is unlikely to succeed. Any advanced civilization on such a planet will be unaware that the universe exists outside its own stellar environment, and radio communication through the glow discharge of the star is impossible!

  22. Well, now that I’m back online, I’ll go ahead and respond to this.

    John,

    The gravitational field of a galaxy extends out several times its visible size, so presumably it’s drawing matter in from that area. Galaxies are visible from 10+ billion lightyears away, so presumably they radiate light out over that area. Are you arguing that the matter being drawn into a galaxy has no causal relationship to the light radiating away from it?

    The gravitational field is infinite, actually. But that doesn’t mean it draws in matter from that far away.

    However, there is some interesting physics to be had here. Within the galaxy, much of the matter (most, I think) is not yet collapsed into stars, but is instead in a diffuse gas that permeates the galaxy. As these gas atoms zip around the galaxy, they radiate, due to the acceleration due to gravity, and due to interacting with other gas atoms (note that the gas is more than hot enough to be ionized, so the particles that make up the gas are charged, and the acceleration of charged particles induces radiation). This thermal radiation causes them to lose energy: as they radiate, they cool. This loss of energy is seen in that the cloud as a whole, as it radiates, slowly collapses inward.

    So the collection of matter really doesn’t do anything to the radiation. But once the matter has been collected, it will radiate and its orbit will decay as a result.

    As I recall, at the center of a normal gravitational body, such as the earth, the gravitational effect balances out, so that while the pressure is enormous, the gravity is neutral. If this was applied to galaxies, it might explain why they shoot these jets out from the poles.

    The physics is the same. It’s just that the Earth is neutrally charged (to an extreme degree), so its orbit doesn’t decay as a result of its acceleration due to gravity. The solar system has also largely been cleared out, so its orbit doesn’t decay significantly due to interactions with other matter in the solar system. If either was the case, then the Earth would lose energy (largely through radiation, though in the case of interactions, some of the energy would also be transfered to the interacting particles), and its orbit would decay as a result.

  23. Gravity waves are analogous to electromagnetic waves. With electromagnetic waves the oscillation of charges on a dipole, or higher moment, will result in changing electric and magnetic fields which propagate out in waves. Gravity waves are similar, but they are quadrupole or higher. The reason for that is a dipole moment is mx, m = mass, x = displacement and a time variation of that would be m dx/dt, which is zero by momentum conservation. Also gravity waves have to directions of polarization, instead of one for EM waves, which is tied to something called helicity which is 2. For this reason the putative quantum particle of a gravity wave, called the graviton, is said to have an intrinsic spin of two.

    A static gravity field, say one around a star produces no gravity waves. It requires the orbit of one body around another. Further, the orbit has to be elliptical, or rather it can’t be circular. A noncircular orbit has a quadrupole moment which can produce gravity waves — distrubances in spacetime which radiate out as waves. I can’t recall the man’s name at the moment, but a Nobel prize was given to him for finding two neutron stars in a mutual orbit that decayed, or the pulsar frequency measure of the orbit increased, as the two neutron stars spiralled in and lost energy to weak gravity wave production.

    As for extrasolar systems, most of them have so called torch Jovians, or large gas planets that orbit tightly around the parent star. Even those which don’t most often have a Jovian (the identifiable planet by Doppler wobbling of the star) in an orbit 1 or 2 astronomical units from the star. A terrestrial planet in such a system would not exist in a stable orbit that kept conditions proper for the evolution of life.

    I do finally have to point out that gravity is cancelled out in general relativity. A freely falling body experiences nothing strange if the frame is small enough. The conditions are equivalent to being in distant space with little or no gravitating bodies nearby. This is the Einstein equivalence principle. There are some other aspects to this, but I will leave it at this. A solid body resists the inward drop of gravity through material forces, mostly electromagnetic in the buld properties of cool matter, and anything at rest on the surface of this body experiences an upward force from this geodesic deviation from free fall. In this way one measures a gravity force. As it is said, it is not the fall that kills you, but the sudden stop and the end.

    Lawrence B. Crowell

  24. I need to give a bit more on extrasolar planets. A gas giant around a star in an orbit comparable to Jupiter, with a distance ~ 5 AU and orbital frequency 11.8 years, would be hard to detect. The Doppler wobbling is smaller and the frequency of this is very low. So the statistics against extra-solar systems comparable to our solar system may well be skewed by this.

    On a more general note, it has been my experience with elementary students that teaching physics involves “unteaching” in many cases. Sometimes students have all sorts of strange ideas about physics, and teaching physics can often mean stripping away these mistaken ideas.

    The internet is to my mind a mixed blessing. When it comes to physics and astronomy there are many sites out there with bogus notions. I am an amateur astronomer and in Sky & Tel or Astronomy there was a full page advert for a so called theory and a book on this. I went to the website for this and quickly deduced this was nonsense. Just a few days ago I got an email spam for some relativity site. I looked at it and found it to be completely ridiculous. The internet has a democratic aspect to it, which I fully support and disagree with schemes against so called equal access, but Science is not really a democracy, and wrong ideas are simply discarded, no matter how much the upholders might scream unfair. At the end of the day the final arbiter is nature or the universe, and no amount of wanting things to be a certain way can change that verdict.

    Lawrence B. Crowell

  25. Pardon me if already answered (there’s so much here), but I still am not clear how energy is conserved if gravity waves can’t actually “hold” energy as Penrose, Bee, and some of you here tell me. As the energy of the rotating system decreases, energy has to increase “somewhere”, right? But how?

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