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Urban legends in authoritative astronomy

  1. Mar 18, 2008 #1

    Ken G

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    Every now and then we come across a deeply ingrained falsehood or half-truth in places that otherwise seem authoritative, and we wonder how that came to be. I think they spread in a manner very similar to "urban legends", where each authority takes a previous authority as its basis, without independently checking the conclusion. When a slight variation is included each time, this process can quickly lead to outright falsehoods.

    My favorite is the explanation often given for why high-mass main-sequence stars are so much more luminous than low-mass main-sequence stars. The typical story is that nuclear fusion is highly sensitive to temperature, and massive stars have higher temperature cores so that the pressure can support the greater mass of the star, all leading to the high luminosity. The logic of that causation is completely false, the sensitivity of nuclear fusion to temperature only tells you an estimate of the core temperature, the luminosity then follows from the least sensitive dependences on core temperature-- the overall force balance, which sets the stellar radius, which sets the luminosity, which refines the core temperature. The usual logic is exactly backwards-- things that are sensitive to temperature are not set by temperature, they set the temperature.

    I can only imagine that the many authors and websites that give the false argument are taking it from each other. What other examples of particularly unfortunate effects of this process can people cite?
     
    Last edited: Mar 18, 2008
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  3. Mar 18, 2008 #2

    Wallace

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    1) Almost every time someone uses the phrase 'expansion of space' to describe the expanding universe
    2) That The Big Bang implies all the material in the Universe was initial at one point in space that 'exploded'

    Actually there are plenty of examples to do with the Big Bang and expansion of the Universe, they are just a couple of biggies. They get perpetuated all the time, often by people who should know better!
     
  4. Mar 18, 2008 #3
    I was just going to say those - The Big Bang "explosion" is my favorite.

    The size of the Universe. There's crazy confusion here if you try to look it up using Internet sources, some of which really should know better.
     
  5. Mar 18, 2008 #4

    Ken G

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    Yeah, I second those, especially "expansion of space". I even see authoritative sources say that "new space is constantly filling in between the galaxies", as if space was a fluid with physical attributes that quantify its "amount". Perhaps it someday will be, but I haven't seen it so far-- and indeed, a lot of relativity seems predicated on that not being the case.
     
  6. Mar 18, 2008 #5

    Wallace

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    Here's a pedantic point of nomenclature that I find somewhat annoying; that it came to be that the current value of H is dubbed 'the Hubble constant', when a better term would be something like 'the current value of the Hubble parameter'. Not really an 'urban legend' as such, but a confusing use of terminology none the less.
     
  7. Mar 18, 2008 #6

    DaveC426913

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    I see a difference between the OP and the subsequent posts.

    My impression (and correct me if I'm wrong) is that the first example seems to be one of a bit of knowledge held as dogma by otherwise very-informed people within the science.

    The others seem to be more of the garden-variety, armchair amateurs with a simplistic knowledge of the science.

    It's one thing if Joe the blacksmith thinks the world is flat, it is entirely another if Galileo himself declares that it's flat.


    Or am I reading too much into this?
     
  8. Mar 18, 2008 #7

    kdv

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    Howo would you describe the expansion of the universe?
     
  9. Mar 18, 2008 #8

    turbo

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    One glaring example of an urban legend in cosmology/astronomy is the notion that structure formed in a hierarchical manner since the BB and that the metallicity of astronomical objects evolves over time as generations of stars go supernova.

    Unfortunately for these concepts, the inverse-square law for absolute magnitudes requires that the black holes powering the most distant quasars (z~6.5) be extraordinarily luminous, and according to conventional models they would have to consist of black holes of several billion solar masses cannibalizing host galaxies of at least a trillion solar masses. These observations turn the hierarchical model of structure formation on its head. How could such monsters have formed in only a few hundred million years? Also, if structures continue to accrete gravitationally, where are the later, closer analogs of these monsters? Nobody knows.

    This has been pointed out by the SDSS team, and here is a nice presentation to the Space Telescope Science Institute by Michael Strauss, the SDSS science spokesperson.

    http://www.stsci.edu/institute/itsd...ScienceColloquiaFall2005/MichaelStrauss110205

    Other anomalies include the observation that the SDSS quasars from z~5.7-6.5 all exhibit Solar or super-Solar metallicities. How can this be if they reside at a distance corresponding to a BB age of ~800 M years? Where did they get the time to accrete materials from generation upon generation of supernovae to achieve these metallicities? Also, against all theories, there is no evolution in either the absolute nor relative metallicities with redshift.

    Finally, it is expected that the great column densities of the highest-redshift quasars would greatly increase the odds that they would be lensed. Not a single one of the z>5.7 quasars is lensed. Watch Strauss' presentation with an open mind. He is not some wild-eyed radical, but is the science spokesperson for the most eminent observational astronomy consortium in existence.

    There is some revolutionary stuff on the table, boys and girls. Perhaps some of the quandries can be resolved if we consider that quasars might not be at the cosmological distances implied by their redshifts, but that concession does not resolve all the problems. There are other underlying assumptions that need to be re-examined if we are to make sense of the SDSS data.
     
    Last edited: Mar 18, 2008
  10. Mar 19, 2008 #9

    Ken G

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    Yes, that leads to a lot of confusion, as does "dark matter".
     
  11. Mar 19, 2008 #10

    Ken G

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    The difference I see is that the OP is a real misconception held by the authorities, while the others have more to do with misconceptions promoted by the authorities in the interest of "dumbing down" an actual understanding. But in some cases the authorities themselves may hold the misconception to some degree, it isn't clear. I would count it a "half-truth" instead of a "falcity", there's ambiguity there.
     
  12. Mar 19, 2008 #11

    Ken G

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    Just like that.
     
  13. Mar 19, 2008 #12

    Ken G

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    Is it possible that this is related to the reason quasars "turn off"-- they deplete their "food supply"? In other words, the hierarchy might describe the potential for mass sources, but the actual mass sources can be depleted by previous history. That would be contrasted to the "trickle down" hierarchy of a turbulent cascade, which is in equilibrium so cannot run dry.
    Lost in the noise perhaps? Metallicity varies greatly from place to place. Also, it is hard to infer metallicities at high Z except in bright objects that might not track the prevailing conditions-- maybe the problem is that the theory is overgeneralized but not completely wrong?
    Small-number statistics? Or is there really something fundamentally amiss here?
     
  14. Mar 19, 2008 #13

    DaveC426913

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    Yes, well said. That's what I was trying to say, badly.
     
  15. Mar 19, 2008 #14

    Ken G

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    I understood what you meant-- and it's a valid point.
     
  16. Mar 19, 2008 #15
    turbo
    I see the quasar as a huge black hole forming
    from two or more smaller but still massive black holes
    as they orbit eachother perhaps very close
    they would move thru each others matter disks
    and or disrupt the orbits of the disk matter

    this would allow more matter to interact with the hole faster
    then a non moving BH with infalling matter as that does not look like it would allow the massive show nore the jets
    once there is only one massive BH the show is over
     
  17. Mar 20, 2008 #16

    Chronos

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    Metallicity in the early universe is not a difficult pill to swallow. Massive pop III stars were going nova all over the place in the first few hundred million years. Black holes [quasars] were also doing their part in churning out metallicity during that epoch. I do not view that as a serious argument agains the redshift - distance relationship.
     
  18. Mar 20, 2008 #17

    kdv

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    So if you were teaching cosmology and a student would ask "what do you mean by the universe is expanding"? you would say "I mean....that it is expanding." and not say anything else?

    Do you agree that the proper distance between points in a FLRW universe is increasing with time? This must mean that space is inflating.
     
  19. Mar 20, 2008 #18

    Hurkyl

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    Out of curiousity, how are you making sense of an absolute notion of position? And how are you making sense of proper distance without specifying a path?
     
  20. Mar 20, 2008 #19

    Ken G

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    No, I would say "the distances between galaxy clusters are increasing as the universe ages. There are many ways to picture how that happens, but the most elegant one is to imagine that space itself is expanding and carrying those clusters "along with it". This picture emerges from the way we choose to coordinatize cosmological space, which brings me to the Cosmological Principle...". Now, isn't that much better than telling students that space is raisin bread? That tends to replace old misconceptions with new ones.
    I'm not sure what you mean by proper distance (I see it as a proper time), but the various ways we use to measure distance do yield increases with proper time.
    How do you figure that? There isn't a theory for the action of space.
     
    Last edited: Mar 20, 2008
  21. Mar 20, 2008 #20

    kdv

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    I am not using any notion of absolute position. I am not sure which of my statement refers to an absolute notion of position. I am talking about relative distance between galaxies.

    I mean the following: if you install a mirror on a remoet aglaxy (far enough so that it is carried away by the Hubble flow) and you send a beam of light to that mirror and wait until it comes back, don'tyou agree that it takes a longer time for the beam of light of come back to us as time passes (on our clock)? If I send two signals one day apart, they won't come back one day apart, no?
     
  22. Mar 20, 2008 #21

    kdv

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    fair enough. If I was a student, the sentence "...emerges from the way we choose to coordinatize space" would leave me with the impression that maybe the whole thing is not "real" and just an artifact due to a choice of coordinates. But that's probably just me.
    At least we agree that there is an increase of distance with proper time.

    (What I meant was that if you install a mirror on a remote galaxy (carried away by the Hubble flow) and you send two light signal to it one day apart, they will come back with a time delay superior to one day because the second light signal will have needed to travel a longer distance)).

    we agree that the distance increases with proper time. And I think we agree that this is not because the galaxies are moving away in already existing space. So I don't see why we can't conclude that there is more space to travel through as time increases (unless you say that if something has to travel more distance it does not apply that it travelled through more space. Then it becomes an issue of terminology. I guess I take this as a definition: that if there is more distance between two points, there is more space between them.)
     
  23. Mar 20, 2008 #22
    I certainly wouldn't be very tolerate to anybody that demanded this wasn't so. Yet fundamentally how do we know this is the case? Is there any empirical differences in this and the Hubble gauge in Weyl geometry approach?

    This would be a nice experiment but in no way can it be said that this has ever even in principle been done. The redshift alone cannot be construed as a round trip light signal in proper time. I give a simple thought experiment here to demonstrate the effect;
    https://www.physicsforums.com/showthread.php?p=1647178#post1647178

    I would agree that your interpretation is very likely. However, I will object to calling it factual so long as empirical determinate exist that we have yet to measure. If you think that some empirical measure has already determined it factually I would love to hear about it. I expect that empirical data will be available within a few years.
     
  24. Mar 20, 2008 #23

    Ken G

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    So yes, you are talking about proper time, applied to a light signal. And yes, they will not come back one day apart. But the "reason" for that is coordinate dependent-- and one cannot state that "space is expanding" as if that was a physical principle. One can only say "in one particularly useful way of coordinatizing space, this is consistent with imagining that space itself is expanding." That is what I'd say, even to a nonscience person, because I don't believe that creating an illusion of understanding is a good replacement for not fully understanding.
     
  25. Mar 20, 2008 #24

    Ken G

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    That is why I would use it as a springboard to address the Cosmological Principle, which is the only thing that makes it not purely an artifact of the coordinatization. If that still left you wondering, it would hopefully prompt you to ask, "does that mean it's all just an artifact of the choice of coordinates", which would then usher in the next key lesson about what are invariant observables and what are arbitrary concoctions in our minds. In short, it spurs the process of learning relativity and cosmology at the same time, rather than shutting off that process, the way saying "space is expanding and carrying everything with it" does.
    I'd have to stop you there-- what is "already existing space"? How do you construct a physical meaning for space? There isn't even a unique means to establish the distance between two events.
    But if you apply your definition to our galaxy and the LMC, you have a problem. You would clearly say that our galaxy and the LMC are "actually moving", but which one is the one that's moving? We can reference it to the CMB, but that's just a chosen coordinatization, that's what I'm talking about-- most people use "comoving frame coordinates", but relativity tells us that is not an absolute, it's just an arbitrary choice. The concept of "space" acts differently in different chosen coordinatizations (and I use that word instead of "reference frame" because I think of a reference frame as a local property of an observer, whereas a coordinatization is a global mathematical set of instructions for labelling events).

    The invariants are things like the time on your clock it takes your light signal to get back-- but many different "pictures" may be used to correctly calculate that number. This is the core lesson of relativity, I would say: the best any observer can do is identify an equivalence class of descriptions for what is observed. Why would we teach that in relativity, and contradict it in our first breath of cosmology?
     
    Last edited: Mar 20, 2008
  26. Mar 20, 2008 #25
    Even if we accept Cosmological Principle at face value it is nonetheless an arbitrary inertial frame we simply chose for convenience of parameterization. Defining our solar system as having a sun in the middle is also a convenience of parameterization. To then call that more than a convenience, whether solar system or Universe, "not purely an artifact of the coordinatization" flies in the face of everything we know about relativity.

    Yes ask yourself if you have subverted your own question, "does that mean it's all just an artifact of the choice of coordinates", by some assumption about the Cosmological Principle. Convenient yes, absolute or more than an artifact of the parameterization no.


    In general no, with respect to a particular observer yes. In infamous words it is what we measure. This is essentially the operational definition kdv defined soon after. I therefore fail to see your objection to kdv using "already existing space" in reference to something he denies can be characterized that way.

    Now you are taking issue with "actually moving", yet the very definition of "actually moving" is to move wrt something, in this case one of the galaxies which he clearly stated. Would not every observer in the Universe agree that the galaxies are moving wrt each other?

    So the invariant you speak of here assumes the very operational definition that you denied kdv. Except he used space whereas you used its inverse, time.

    ETA: Removed a modifier "not".
     
    Last edited: Mar 20, 2008
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