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Photon Energy vs. Red Shift, Inflation Theory, Acceleration

  1. Oct 31, 2007 #1


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    Energy of Photon/Red-shift, inflation, acceleration

    I have a few questions (following a short premise) below.

    I have been reading medieval philospohy recently and also some about the "Big Bang." What I am struck by is number of assumptions made in both fields. There is a large amount of "revealed truth" in both. The difference being in the "revealed truths" (assumptions/laws) change with observation for physics.

    The other thing is that nature abhors a vacuum. Thus, reasons are put forward to explain the unexplained. It seems many times that the jump in reasoning outdistances the observed data. Also, understandably, most theories fit the facts, rather than explain the facts. Thus, when new data comes in, existing theories are stretched until broken and overturned by newer theories. Witness the theories about motion of the planets.

    To the point (and I am sure/hope that many of the points below have already been anticipated), these are some questions that I have not found adequate explanations for:

    The distances to galaxies are determined using red-shift. The information comes to us through light as transmitted via photons. The distances are calculated using the shifts in absorption lines via Hubble's Law of Expansion (v=H[0]D) and the Energy of Photon (E=hc/wavelength) equations. The question is "does the energy of a photon dissipate through time and/or interference with inter-gallactic matter?" If not, how is this known? If so, could that not account for red-shift? If it is true, then how is it KNOWN what the dissipation would be? Obviously, were any of this to be true then location of far-away galaxies could explained other than by expansion of the universe. If energy does not dissipate then a photon in a space otherwise a vacuum should remain at a constant energy level eternally.

    Another thing I noticed was the theory of inflation of space-time which is posited to have occured faster than the speed of light (not measurible or falsifiable!). Supposing this to have occured, and having occured early in the universe's history, why the vast emptiness of space? Surely the expansion would have happened evenly within the primordial clouds of "gas" that existed before the formation of galaxies and clusters rather than in quantized/preferred portions of space to allow that clustering. If this were so, then there would be less space between galaxies and the later clusterings would have been into smaller objects. Or assuming that early galaxies had formed, then the expansion would have happened within them as well rather simply between them. This would stretch the galaxies as well as the intervening space-time.

    Lastly, red-shift is shown to have relativistic effects upon it near a black-hole. Similarly the most distant visible galaxies are receding at near speed of light. Wouldn't the red-shift from those galaxies have a similar relativistic effect upon it? After all, can it be shown that the edge of the visible universe is any different than that of an event horizon? Items near an event horizon undergo an "apparent" time dilation effect due to relative speeds. Would this not be true for galaxies receding nearer the edge of the visible universe? If so, then they should appear to be receding slower than they actually are. Meaning, that galaxies receding from us closer in would appear to be receding faster than they should be relative to the speeds of distant galaxies. This would make the expansion of the universe to appear to have sped up more recently than the long ago expansion. This, if not already accounted for, would go a long way to eliminating dark energy which is used to explain the "repulsion" within a vacuum. It is assumed that the dark energy is constant with volume and that as space time expands it expands with it, thus creating more "pressure" with time. It is not a measurable quantity, but rather just falls out of the equations used with the cosmological constant.

    Thank you.
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  3. Nov 1, 2007 #2


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    I think there were many effects taken into account when processing the galactic red-shifts. A few years ago I asked the above question to Saul Perlmutter when he was giving a talk on the subject. The answer was "Yes, this effect was also taken into account".
  4. Nov 1, 2007 #3


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    This idea has a long history and collectively is known as 'tired light', the idea that redshift is merely due to photons losing energy as they travel. This has been falsified by the observation that we observe time dilation in distant objects at the exact extent predicted by their redshift. In fact you can think of redshift as being a direct consequence of time dilation. If the clocks in some distant object are observed by us to be running at a different rate then the frequency of a photon of light will obviously be different for the distant galaxy and ourselves.

    We also know there are certain events, such as the explosion of a particular type of supernovae, that is the same length of time for all such explosions when measured by observers at the location of the explosion. What we see is that the length of these explosions in distant supernovae get longer precisely by the amount you would predict from their redshift.

    Don't think of redshift as a photon losing energy. Redshift is entirely due to a photon being observed in a different frame from where it was emitted. In relativity everything, such as times, lengths and energy are relative quantities that depend on the observer. The same is true for redshift. Nothing 'happens' to the photon, what matters are the frames of the emitter and the observer.

    I really hate the phrase 'in inflation space expanded faster than the speed of light'. In an expanding infinite universe space always expands faster than the speed of light for sufficient distance between two objects, however there is nothing mystical or strange about this. Inflation (if it happened) did indeed happen everywhere evenly, but as above, there is nothing mystical about expansion that prevents clustering, it merely increased the rate at which the mean energy density decreased. The expansion rate does impact on clustering, but it doesn't prevent it.

    As for your concerns about galaxies being pulled apart by the expansion of space, this is another common misconception brought about by the ill-defined term 'the expansion of space'. I recommended very strongly that you have a read of Expanding Space: The Root of all Evil which is an attempt to overcome these common misconceptions.

    As suggested above the effects you describe are already contained in the calculation of the redshift of distant galaxies. We can observe galaxies that are moving away from us with a 'velocity' that exceeds the speed of light. Again while this may seem odd there is nothing paradoxical about it, it is merely a matter of how you define distances and velocities in Relativity. The reference already given about discusses these issues.

    I hope that helps, you're thinking about this stuff pretty well, I'd strongly encourage you to find out more about cosmology!
  5. Nov 1, 2007 #4

    Chris Hillman

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    Some common misconceptions

    This is a very idiosyncratic use of "revealed truth". If you want to discuss medieval philosophy, the Philosophy forum is the place, but if you move over there you should try to use the term "revealed truth" in the standard sense. If you want to discuss modern cosmology, you are in the right place, but you should use terminology standard in modern cosmology (or else explain your terms very carefully--- but I'd advise against using standard terms in highly nonstandard ways even if you explain your nonstandard usage.)

    Define "explain".

    One of the great insights of physics is that one doesn't need to try to explain what gravitation or matter "really is", but only to try to describe its properties and behavior.

    You should familiarize yourself with "distance ladder" if you want to discuss modern cosmology.

    As Wallace already mentioned, this old idea is known as "tired light" and it won't work. In modern mainstream cosmology, light doesn't not get "tired" in this sense.

    No, you've misunderstood. See discussions of "coordinate speed" versus the infinitesimal light cone at some event.

    Another misunderstanding (but don't be discouraged, these are all common misunderstandings among beginners and with proper instruction are not hard to overcome). Look for discussions of "pennies glued to a balloon being blown up".

    Red-shift is red-shift. In gtr, light signals have world lines which are null geodesics (a special type of "straight line"). The curvature of spacetime manifests itself in various ways, among them this: two initially parallel geodesics may converge or diverge as we run along one of the pair. In particular, consider two initially parallel null geodesics, representing a signal sent by observer O at time T1 and time T2 = T1+one second respectively, as measured by an ideal clock carried by O, and which both intersect the world line of observer O'. If these pass through a curved region of spacetime, they can diverge, so that when received by O', depending on the motion of O' he may consider the time interval between receptions to be greater than one second. If the two "signals" are succesive wavefronts of an EM wave, O' will have measured a redshift. This is essentially how both "cosmogical red shift" arises in Hubble expansion, and also how "gravitational red shift" arises for EM radiation from the surface of a white dwarf or neutron star.

    Same misunderstand as noted above (coordinate speed versus a physical velocity--- the latter is observer dependent and only makes sense for measurements of velocities of nearby particles.)

    No, due to divergence of initially parallel null geodesics as the result of encountering spacetime curvature.

    Almost all of your misconceptions are addressed in the very clear textbook by D'Inverno, Understanding Einstein's Relativity; the last chapter offers a concise but clear summary of FRW models (including, presciently, nonzero Lambda models).
    Last edited: Nov 1, 2007
  6. Nov 2, 2007 #5


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    In reply:

    True and on purpose. It means "argument from authority." There are some very big assumptions (with the attendant math) in cosmology (not that they are wrong) that get from existing theory to "observed" conditions, several of which are dark matter, dark energy, and the inflation theory. So far, none have been directly observed, which of course is one of the reasons such a big push is being made to find or solve them. The point being made was that existing theories which appear to work "pretty" well are being extended rather than reexamined, for the most part. I think this is understandable. It is the way the human mind works: fix before replace. This is true in the past as well....just an interesting parallel.

    This comment was simply in some background to my questions which were not philosophy.

    My point. Science is only correct so far as it is correct. Thus, given, some points, many different curves can be made to fit the data. Science tries to pick the simplest model to "explain" the data at hand. However, higher-order solutions are also possible but are not considered until necessary. Philosophy gets all tangled up in essense vs. existence or why vs. what. Science first answers what.

    This and others are unfortunate analogies, since they don't discuss (one way or another) the expansion within the pennies. I read elsewhere that the gravitational attraction within the objects overcomes the coordinate expansion. As I understand it, the expansion is not proved but exists midway in the cosmological proofs that go from pre-existing theory to current observations. Sort of like: first the big bang happened, "then god blew up the universe", then normal physical laws prevailed. This is not a criticsm, just an observation.

    Another thing: discussions of the isostatic expansion of the universe which state we are not at a preferred place in the universe never take the next step of asking what the universe would look like at the furthest observable point ("straight line" or null geodesic, as you have it). Now if this were repeated would an edge finally appear, or would space fold back on itself (negative curvature, I believe) and we would eventually see the same things over? I believe this is unknown.

    Anyway, I appreciate the time taken in the response. It has provided some answers and directions where to find things. The dicussions I have read up to now have not used the "tired light" term. Searching the internet for that brings up some interesting sights, some against and some strongly idiosyncratically in favor!
  7. Nov 2, 2007 #6


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    This is the first use of the term I had heard. I was able to find numerous references on the web about it. Unfortunately the wikipedia entry for red-shift did not mention it as a previously examined and discarded explanation for red-shift.

    This assumes that the universe is in fact infinite and that inflation did happen, so the definition of inflation with the additional assumption of even expansion, it would happen everywhere by self-definition. Note that there are a series of tautologies in cosmology. The red-shift is shown by time-dilation and vice versa. Only with the addition of the standard candles is the circularity broken. Much of philosophy suffers from this (apparently without the practioners noticing this).

    The inflation theories with dark energy are reminiscent of Hubble solution for a static universe with matter "popping" into existence. Is it possible that this dark energy if it actually exists rather than simply being the result of expanding coordinates could interact with matter and be in fact converted into matter?

    Anyway thanks for the respones.
  8. Nov 2, 2007 #7


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    I have read that article now. Interesting. It doesn't postulate why space might be "expanding" other than that was and will continue. It does say that local conditions overwhelm inflation. The section on "is everything expanding" is sort of pointless since it talks about a something that no internal forces would expand just as the surrounding space. Of course, this something would by definition only be a bounded piece of that space due to no internal forces.

    But anyway thanks for the referral.
  9. Nov 2, 2007 #8

    Chris Hillman

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    Tugging gtw back from the precipice?

    gtw, take heed, I think you are venturing further into the realm of the kind of anti-scientific polemic often seen at creationist websites.

    That may be what you mean by this phrase, but this is completely different from the dictionary meaning. You should never use a standard term with an idiosyncratic meaning quite different from the standard definition without warning the reader. IMO, doing this "on purpose" is like standing on the streetcorner yelling "I killed my wife, and I'm glad I did it!" It's not likely to win admirers.

    I have referred you to textbook computations, but you should not assume that this implies that I have not verified these computations; I have. IMO, the imputation of alleged "arguments from authority" to myself and others here is absurd. It's perfectly appropriate to suggest that you study some cosmology textbooks in order to overcome some misconceptions you evidently are suffering from about what modern cosmology actually says about our universe.

    You do cosmologists a disservice, I think, if you assume that they are not aware of multiple philosophical issues which arise in this subject. IMO you are not likely to make a valuable contribution to the philosophy of space and time unless you read some more recent contributions, and you will not be able to "critique" modern cosmology unless you really understand this vast subject correctly (in the sense of not misunderstanding the math or its geometric interpretation, and in the sense of not misunderstanding the aim and methods of physics). Unfortunately, this forum has seen quite a bit of "caricatures of cosmology" from would-be "philosophical" critics lately.

    I advise you to read some good books providing some background, particularly Sklar, Space, Time, and Spacetime.

    Did you overlook my comments about the hierarchical nature of the theories used to tie together the observations into one more or less coherent picture? I pointed out that it would be a serious mistake to assume that if a particular speculation currently touted in the research literature (and therefore discussed on popular science forums) should be decisively rejected in the future, that this would immediately invalidate everything in cosmology. That's simply not true.

    It would be better to say that's the way science works. Any "hard" science consists of a complicated network of experimental knowledge tied together by a hierarchy of theories, some fundamental, others derived or even "phenomenonlogical". There are good reasons why such scientific pictures tend to modify foundational theories only when irrefutable evidence requiring such modifications are needed. Clearly, this is just how we should expect a complicated network to react to "stresses" from possibly dubious evidence which doesn't seem to "fit"; the weakest links break and are repaired first. New links and new bricks of knowledge are constantly being added to the edifice. History suggests that on the whole the structure of science tends to grow stronger over time, not weaker.

    Speaking of phenomenological theories, in a classic paper published many years ago in Science, the famous mathematician Mark Kac compared two phenomenological theories of oxygenation in the bloodstream and showed that they both fit the data quite well, but that taken in isolation neither would be obviously prefered on the basis of "Occam's razor". The kicker was that just one makes sense in the light of the broader knowledge of molecular biology.

    That didn't make much sense to me, but I guess you are trying to re-express in your own words what I said about an important insight: physics need not try to explain what "matter" is, only to explain its properties and behavior. Likewise for "radiation", "quanta", "spacetime", "the EM field", "the gravitational field", &c. &c.

    Did you miss the point? The pennies represent gravitationally bound objects (galaxies), so they don't expand.

    By "coordinate expansion" I assume you mean "Hubble expansion". If so, the nature of the Hubble expansion appears to be very well established; I think you probably misunderstood papers like this which discuss in more detail the issue of what it means to say that a gravitationally bound system doesn't expand as per the Hubble expansion, just gets further away from other gravitationally bound systems as per the Hubble expansion.

    No, it's a caricature. This does not describe even approximately what contemporary cosmologists believe about the early universe.

    Don't you mean homogeneous? As for isotropic (not "isostatic"), that means absence of preferred directions.

    The simplest, most idealized, and best known cosmological models are the FRW dusts (and FRW radiation fluids modeling for the "radiation dominated epoch"), which are isotropic and homogeneous. But there are many known simple cosmological models which are homogeneous but anisotropic, such as the Kasner dust, and others which are inhomogeneous but time-varying spherically symmetric, such as the LTB dust, and still others which are lack any symmetries. Many of these are perturbations of simpler models such as the FRW models.

    A geodesic curve is the analog of a straight line in a Lorentzian manifold. A null geodesic curve is a very special type of geodesic curve; roughly speaking it can be taken to model "the world line of a photon" (or better say "laser pulse" because carelessly mixing up quantum phenomena with classical physics adds an unwanted and probably unneccessary level of complication here).

    There is no notion of "furthest observable point"; probably you misunderstood something you read about "domains of influence". That is, we can fix some event in a Lorenzian manifold and solve the geodesic equations to find all null geodesics radiating from that event. This gives a "light cone in the large", which can in general have a very complicated shape, typically developing self-intersections and so on. In the simplest cases it is roughly conical and its interior consists of the events which are "causally influenced" by the original event.

    No, no. First, in the context of discussions of a Lorentzian manifold such as a cosmological model, "space" doesn't make sense without qualification (e.g. by defining a family of spatial hyperslices which are Riemannian three-manifolds). Second, you might be thinking of "light cones in the large" developing self-intersections. Third, generally speaking, it is possible that there might be multiple "light paths" connecting two world lines, so generally speaking some observers may see multiple images of some object (c.f. gravitational lensing). Most likely you are thinking of the possibility that the spatial hyperslices which are everywhere orthogonal to the world lines of the dust particles in an FRW model are compact, in which case one could look for subtle "periodicities" in the appearance of the night sky (this possibility is being pursued by Cornish and Weeks and some other groups).

    I highly recommend that you study the textbook by D'Inverno in order to master the true geometry of the simplest cosmological models, the FRW models (plus de Sitter lambdadust).

    There should be a special PF button which produces a warning message: the InterNet is full of misinformation about modern science. If you want to know what modern science really says, obtain and study a standard textbook--- such as the one by D'Inverno.

    Careful, this is not self-consistent as written. You probably meant to write that "in places, modern cosmology comes perilously close to tautology". Which would sound good in a polemic (and it increasingly seems to me that your intention is to post a polemic) but in my opinion is potentially misleading.

    There is no "Hubble solution"; you mean the FRW models. Matter cannot just "pop into existence" in any solution to the EFE, since gtr obeys the law of conservation of energy. (There are some pedantic quibbles which I would ordinarily mention here, but I sense that you already confused, so let's slur over such fine points.) You are probably thinking of the "steady-state universe" model proposed by Hoyle et al., which many decades ago was believed to be a viable competitor of the original "Big Bang" model proposed by Gamow et al, but which was decisively rejected in the sixties.
    Last edited: Nov 2, 2007
  10. Nov 2, 2007 #9


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    As if "tired light" doesn't require additional assumptions? :rolleyes:
    Didn't you just complain (incorrectly, but still...) that the standard model contains too many assumptions?
    None of those are assumptions: they are predictions of the theory. It appears you are misunderstanding the scientific method itself. I agree with Chris. It seems that you have either a basic misunderstanding of or ideological objection to the very concept of science. Until you deal with that, you will not ever be able to understand and accept specific scientific theories.
    Last edited: Nov 2, 2007
  11. Nov 2, 2007 #10


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    Assumptions. No actually I don't buy any of that. Haven't seen them anyway.

    Seems to have gotten more attention than was intended. Simply I was referring the number of places where I have read certain things that are assumed (perhaps to be proved later) and withtheories progressing from there. Also things I have read gloss over a number of issues, doubtlessly because they are established science, but still I wondered about a couple things and thus looked for some answers. Hence I discovered the physicsforum...

    Absolutely, science and really anything should be conservative and kick the tires before heading off in a new direction. Hopefully, not so resistent to change to simply not consider new postulates.

    Interestingly, I just finished reading Copplestone's treatment of WIlliam of Ockham. Twice today I read this usage wrt cosmology.

    No I didn't. Simple analogies like this typically omit the part about "gravitationally bound objects" overcoming "Hubble expansion" which engendering some of my original questions. That's all.

    Of course, that's not really true. Otherwise science would have stopped long ago when it had "good enough" answers. First the why is asked, then the what is answered, then the why is asked, then scientists look to find an underlying what and the cycle repeats.

    IMO I am not likely to make a contribution to the philosophy of space and time. IMO there need not be one. The science either describes what is or doesn't.

    I picked that out of an article about Zwicky (which mentioned Hubble) and tired light without looking closely enough, remembering from long ago.

    Perhaps a little strong. It more a comment on the probably more popular presentations I have read in wikipedia.com which due to the nature of the audience and space gloss over "assumptions" perhaps and don't detail the efforts that go into proving them at least in agreement with the data, since clearly there is much that is not understood.

    Of course not, one assumes that it is based upon observation (which improves with time). Similarly, cosmology is a process of refinement.

    Dark energy via the Cosmological constant "pops into existence" (from what I read) due to the expansion of the universe to create the "negative pressure." I.e. space has a "fundamental energy" or it is "the cost of having space." (wikipedia's horrors way of saying it.) Surely, this must violate some law. Given that it is thought to be more than 70% of the mass energy balance of the universe, it's not a small thing.

    For example, dark enegy and cosmological constant. Again from wikipedia (understandably, not the best source):

    There is a certain amount of self definition in the above.

    Anyway, it's been en(ter)lightening, having spent the day looking at various articles. Undoubtly, this has caused entropy to spike somewhere in the universe! (perhaps in my head)
  12. Nov 2, 2007 #11


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    Not mine, Zwicky's.

    Not a compaint. Given the unknowns, obviously some working hypothesis is needed.

    Now these remarks of yours are assumptions. Or are they predictions?

    None of dark energy or matter things have been directly "seen." They are postulated by data that doesn't fit observed matter and energy. They are created to explain excess local gravity and missing non-local gravity (acceleration of expansion), to loosely use the terms. Given they were not seen earlier and new data violated earlier models, new models were/are created to explain the data. Thus, in some sense they are not predicted by the theory: they are the theory. It is not the same as when Mercury's position was found. The theory then preexisted the data. It is different this time. It is very different. These theories are reactive not predictive (they go between existing theory to existing data, not from existing theory to new data). So much for understanding scientific method. I have no problem with that, by the way.

    Most of the comments I have seen have been interesting, despite the random gratuitous remarks peppering the responses I see.

  13. Nov 2, 2007 #12

    Chris Hillman

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    Hi, mtw,

    If it helps, I am not assuming that you have assigned yourself the mission of antiscience polemics. But you should know that such things have happened before at PF so it shouldn't be too surprising if that experience colors some of our responses. If it doesn't apply to you, don't take it personally, respond in good faith, and the atmosphere should clear.

    But enough of that--- that is a meta-issue concerning historically rooted forum concerns, and I sense you wish to avoid meta-discussions, as I do.

    I wrote:

    You responded:

    We may be talking past one another here. Some of your comments remind me of frequent PF posts by a number of amateurs who feel that if for example physics provides no "underlying physical mechanism" for how an EM signal can traverse an "vacuum", then it has failed. I hope you do not belong to this group.

    I appreciate your comment to the effect that you do indeed wish to ask about the reliability (or not) of certain notions bruited about in the popular physics literature, rather than philosophy (a word which in discussions among mathematicians or physicists often is inflected with a certain sniff).

    But as a reader of philosophy I feel that you ought to be see why the quoted assertion is open to doubt! :wink: At the very least you'll have to describe how a scientist is supposed to know "what is", except by measuring something, in which case, it seems to me that we are back to what I said about physics only needing to describe the behavior and properties of an EM field, not what it "is".

    I should have said that in my own reference to "Occam's razor" (spellings of this term vary, in case anyone was wondering) I was referring to what modern scientists tend to take him to have said rather than to what he actually said, which can probably only be fully understood in the context of his times. (If you haven't read Zoe Oldenbourg's novel The Cornerstone, this is a fine way to establish some context for the Medieval world.)

    There's an important point here which I have struggled to convey here and in some other very recent threads. It keeps coming up but is hard to get across to members of the general public who have read articles in New Scientist (insert vomit icon here) on "dark energy" or some other phenomenological speculation. It has to do with the fact that the body of theory in physics is hierarchical, building derived theories on well-established foundations, and having tentative recourse to dubious theoretical speculations while trying, as it were, to fit a new brick into place. Another aspect has to do with the distinction between phenomenological notions (dark matter currently qualifies, and I would say dark energy does too--- although some might squawk; to some extent its a matter of taste where one draws the phenomenological line) and notions ultimately founded in some fundamental theory.

    Don't misunderstand--- phenomenological physics can be good physics (one might say that classical thermodynamics is the sine qua non of a phenomenological theory). I'm not talking about distinctions as simple-minded as "good" and "bad", but ones which relate to the strength of the edifice of physics and to trying to guess which bricks will still be firmly in place a hundred years from now. As you yourself said, the edifice of science is constantly being added to and even renovated (but never, it seems restored to an earlier state!). Many of the things you are talking about are analogous (in my image) to workers on scaffolding near the top trying to add some bricks, but all that stuff might be cleared away in a dozen years and replaced by a floor of a completely different design by architects whose names are currently unknown to the NS crowd.

    Ur... Neptune?

    I think you are recalling the story of how Adams and Leverrier studied the motions of the planets, using the perturbation theory established in the context of Newtonian gravitation earlier in the nineteenth century, and independently deduced the existence of another major planet, which was soon discovered in the place they had predicted. This was justifiably regarded as a triumph for Newtonian physics and generated tremendous popular interest.

    I'd add another example of something which can probably be held to be dissimilar to the current situation of "dark energy" and "dark matter" in the grand edifice of physics: Einstein predicted the existence and properties of gravitational radiation, according to gtr. Later researchers worked out a perturbation theory suitable for making relativistic corrections which eventually became good enough to apply in the case of binary neutron star systems. In such cases, gtr predicts that gravitational radiation should very slowly carry off energy from the system, with the consequence that the orbit of the binary should gradually tighten to make up the lack. In the seventies, Hulse and Taylor found such a binary and showed that it exactly matched the gtr prediction. This is an indirect confirmation of the existence of gravitational radiation (which has not yet been directly observed) and was regarded as so important that it resulted in a Nobel Prize.

    Well, one of the really fun things about cosmology is that sometimes completely unexpected things are discovered. This has happened repeatedly: the multiplicity of galaxies, quasars, gamma ray bursters, and more recently indirect evidence for things we are provisionally calling dark matter and dark energy. One thing physicists do when they are confronted by something completely surprising is to try to figure out what they can say about X without having a very good idea what X might "be". That typically involves what I've been calling "phenomenological speculations".

    My own sense is that the evidence for dark matter, while indirect, is rather convincing and I expect future work will confirm its existence in new ways. I agree that a genuine theory of solid appearance which makes testable predictions, ideally concerning phenomena we can test in a physics lab, will be neccessary before one can have confidence in "dark matter" comparable to the confidence we probably feel that "electrons" exist. But not to worry: it's early days yet. This is a wholly unexpected development and you need to give physicists some time to figure out what might be going on!
    Last edited: Nov 2, 2007
  14. Nov 3, 2007 #13


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    What level of math is assumed in D'Inverno relativity book? Tensor calculus or below?

    No, Mercury. It was the first real world proof of Einstein's theory.


    Tests of Einstein's general theory of relativity did not provide an experimental foundation for the theory until well after it was introduced in 1915. Physicists accepted the theory because it correctly accounted for the precession of the perihelion of Mercury, a phenomenon which had long baffled astronomers and physicists, and because it unified Newton's law of universal gravitation with special relativity in a conceptually simple way. (Einstein has been famously quoted as describing what his reaction would have been if his theory had not been confirmed by Eddington and Dyson in 1919: "Then I would feel sorry for the dear Lord. The theory is correct anyway."

    Somehow my thoughts have become tangled in the internet ether.

    "What is", but not in the existential way.

    So far, I am struck what BS most philosphy is. The need to close questions leads to huge jumps. There is in inability to simply state "I don't know" rather than invent things whole cloth. Ockham (and some others) made a huge leap and said some things you just have to accept on faith. I consider this at least honest. I was also struck by some loose comments in my reading of cosmology that showed certain parallels, which led to some of my questions. (Did Plato, the philosopher, ever catch the irony of calling Pythagoras a sophist? Wasn't he? It's in the word! Different sense of course) This diversion into cosmology has certainly impacted my time travels in philosophy, I am stuck at the end of the middle ages still.

    Either that or current theory (luminosity/mass) needs some work or perhaps the "estimates" of galactic mass need work. As I understand they are based upon huge extrapolations of solar (sol) luminosity. I used to read that it was thought the Milky Way had 100 billion stars and was larger than Andromeda. Now I read it has 400 billion stars and is smaller. Sometimes I see one and sometimes the other. So certainly there is a rather large range out there. But I imagine tightening those estimates is very important as they impact what dark matter might be (be it postulated WIMPs, dead stars, rouge planets, or what-not).
  15. Nov 3, 2007 #14


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    I think that blanket statement needs a little elaboration. These philosopher's were clearly bright people. They lived in a period where experimental science and that ethos were just forming. Also mainly they were clerics of some sort. That said, they were taking Greek philosophy forward. Early Greek philosophers made some important insights without the benefit of controlled experiment. Starting with Plato, and with growing strength, Aristotle, Plotinus, and others built a metaphysical edifice which though differing in some details expounded and explained a spiritual world. Each made changes to the previous person.s ideas but accepted the basic premises. If the Greeks had stopped at the sophist level which presumed the unknowability of some things, that would have been best. The Middle Ages philosophers picked up Plato and later philosophers and plugged their formulations into their faith. Since they were already used to accepting certain things upon faith, the philosophers' ideas, so long as they didn't dramatically contradict their faith, were accepted and argued about, accepting their main premises. It is the break from that tradition that marks the Renissance.

    I first used "revealed truth," in my first post to mark the accepted mysteries which generated a little heat. They serve as placeholders where direct knowledge is missing. Both the philosophers and scientists try to fill these in. Philosophy argues through language constructs and insights and gets tangled up in that through syllogisms and circular definitions of words often creating distinctions without a difference. Science should and does constantly re-examine premises. Through this process additional insight is gained. However, there is a lot to learn and in that process a certain amount of suspension of disbelief is required to digest all of that. One can not question everything along the way. The downside is, of course, some things are left uneximined by individuals. Thus certain things are accepted on faith, as it were. The diffence between science (in this limited sense) and religion is who you trust. (The unlying premise of questioning and accepting is opposite of course.) So there are certain parallels in their development and jumps in logic along the way. So the statement stands as self-evident. For me, the real question is "can you explain dinosaurs?"
  16. Nov 3, 2007 #15

    Chris Hillman

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    Tensor calculus is employed freely but is everything you need is developed in early chapters. I'd say that the prerequisites are junior year undergraduate math and some physics.

    Physical theories are never "proven true", they can only be shown to be in
    disagreement with experiment or observation. Thus, we speak of "ruling out" competing gravitation theories. Those which remain (this select group currently includes but is not limited to gtr) are termed "viable".

    There should be a special PF button which prints a warning message: Wikipedia is an unstable and unreliable source of "information". Cite the Britannica, or cite a textbook, please.

    About Mercury: astronomers noticed in the nineteenth century that after accounting for the perturbing influence of the major planets (particularly Jupiter), which causes the perihelion of Mercury to precess, there is a small remaining precession which cannot be explained by Newtonian gravitation. This term turns out to be perfectly accounted for by the so-called geodetic precession effect in gtr. Indeed, gtr predicts such an effect for Venus, Earth, Mars, asteroids, binary neutron stars, and so on, and wherever astronomers have looked, the gtr prediction has been verified. The solar system observations of this effect comprise one of the four "classical solar system tests" of gtr.

    Strictly speaking this success was not a "prediction" but a "postdiction". A second of the four classical tests, the Einstein light bending formula, was a genuine prediction since noone had tried to measure such an effect before Einstein came along. This formula has also been rather precisely confirmed in various solar system observations/experiments (the best tests use radio waves from distant quasars, not optical light, which at the right time may happen to pass near the Sun on their way to Earth).

    Maybe you meant to write "extra-Newtonian precession of the perihelion of Mercury" rather than "position of Mercury"?

    Sorry, I still don't know what you mean.

    I keep urging budding philosophers to stop arguing over the same old spacetime stuff and to turn their artillery on something important: statistics. For some reason this advice is robustly ignored :rolleyes:

    A big ball o' rouge? Would that be Mars in drag? :wink:
  17. Nov 4, 2007 #16


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    Do you of any books with solutions for D'Inverno? The amazon site says it is too bad the problems are not solved. Not an inexpensive book.

    True, but a good place to start, but not end. People try to make it better. And many of the articles have links to stable data.

    I understand that. Only tautologies can be "proven" true. But "prove" is a little shorter to type than "so far as showing no statistically significant disagreement with currently best theory involving fewest number of postulates" every time I want to say that. Perhaps a button next to the Quote button by the message box would help.

    Perhaps, one day, physicists will decide to simply restate matter as some sort of energy form that emits gravitational radiation and talk about it in that way.

    Partially, anyway, since they did not have the data of where Mercury would be exactly until Eddington did his observations. But, yes, now I remember the light bending experiment.

    I read that the Newtonian approximation of Mercury's position was calculated using the effects of the other planets upon it. Was the sun's motion around the galaxy also considered? (I know it sounds like I have a problem with gtr. I don't. I see the competing theories so far ultimately don't work and are MORE complicated as well, failing your Occam's razor test)

    Has the radiation (light) pressure of galaxies themselves (rather than on single stars) been calculated or considered? This would create a force pushing outward in all directions. If at high enough pressures, then isostatic expansion would occur (at lower and lower amounts of pressure of course unless more stars light up over time), I would think. If the pressure fallout were at a lower power than gravitational falloff then, of course, that would increase repulsion over time, though. Perhaps though the pressure is too low. Also, I read that gravity should pull galaxies together. However, it also pulls them apart if there are more on an already expanding shell. (I now see something called BlazeLabs/URL] has looked into EMRP plus an exchange of attitude a physicsform thread)

    I said that so that someone would give me the third that physics doesn't describe what is, etc. Also, philosophy constantly, so it seems to me, worries about "what is", i.e. what is "real."

    The philosophy reading is just part of larger project to read foundation western (not the cowboy type) literature. I popped over to the big bang stuff, which I see now and then in Science News, and starting reading a bit and was just thinking about a few things instead of just simply accepting them and going on.

    You should check out a photograph by Jacques-Henri Lartigue in 1913 . Search for 1913 (or scroll down and see a distorted car (kind of like the distortion/rotation caused by near light speeds.

    I'd like to thank you guys. This forum provides quite a service, taking time to answering inquiries of all sorts! I really need to stop these non-mathematical musings.
    Last edited by a moderator: May 3, 2017
  18. Nov 4, 2007 #17

    Chris Hillman

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    Suggest everyone try to use good judgement in seeking information on the web

    Oh dear!--- gtw, I was about to answer your questions when I noticed that you went out of your way to mention a website which promotes various cranky theories, immediately after I had warned you not to rely on websites for "information" about science but rather to study a good textbook. You hinted that you regard textbooks as too expensive, but I take this as an indication that you probably aren't really very interested in the subject at hand. This killed my interest in this thread.

    If you have more questions after you've had a chance to study D'Inverno, feel free to start a new thread, but if you continue to mention cranky websites, don't expect answers from me. My BS detector is set more sensitively than the PF moderators's, but you might want to review the [thread=5374]PF forum rules[/thread], which includes this sentence:
  19. Nov 5, 2007 #18


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    I simply remarked I was surprised at the expense. I was not expecting it to be a textbook when first directed there. Also I mentioned I read that it did not have answered questions. It is currently in my amazon basket along with Newton's Principia Mathematica , etc. It been a while since I have had to deal with college math at Liberty University:wink: Just joking.

    My grandfather went out and studied gtr in the '20s soon after it came out.

    I searched for "radiation pressure universe" and found a number of websites, most of which were simply Harvard abstracts. Seaching for "radiation pressure galaxy" simply brings up discussion about cosmic dust being blown in halos. The mentioned website had some discussion. Is this a website that has an agenda? I have no idea if it is valid or not. The questions came after the thoughts, not from them. I have no agenda to push. Knowing where and how a theory breaks down shows some of the underlying physics. (The philosophic remarks, if you notice are in response to responses to why I came to be looking at this site, not simply grandstanding for effect, despite appearances.) So, I feel the comments are unjustified.

    ... upon further searching I see that there are quite a lot of websites pushing this or that theory.... I had no idea people had so much vested in these alternate "theories..." This site must get deluged with them. There is Steven Byers website, physicsmyths vs facts (which looked good until I started reading.) Etc...

    The question was simply if galactic radiation pressure has been calculated and what percentage (if any) of the postulated strength dark energy it was. Perhaps there is a paper on this. Any pressure would push in all directions. Clealy I am anticipating through simple thought some of the "alternate theories" you complain out. Once a name is put upon them one can find a number of people out there pushing their pet ideas.
  20. Nov 5, 2007 #19


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    Thats <crank link deleted>.
    Last edited by a moderator: Nov 5, 2007
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