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Difference between expansion of space and objects just moving apart?

  1. Jul 9, 2008 #1
    I read that the expansion of our universe is the expansion of space itself. Apparently, the phrase, expansion of space itself, is meant to characterize the observed displacements of very large scale cosmological structures as an artifact of the isotropic stretching or expanding or some unknown physical structure called "space" (thus continually creating new space), and to distinguish this sort of intrinsic expansion from an isotropic stretching or expansion of large scale cosmological structures into an already existing space.

    I imagine that the very large scale isotropic expansion (and therefore also the very large scale structure of the universe) is due to kinetic energy imparted during some sort of (extrinsic) Big Bang event and not a function of (intrinsic) wave interaction, and that at smaller scales it is wave interaction intrinsic to our universe (in the forms of gravity, electricity, magnetism, weak force, strong force, etc.) that is the structural determinant.

    My question is, if the ontological status of "space" is unknown, then how is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?
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  3. Jul 9, 2008 #2
    Question: Is it known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium?
  4. Jul 9, 2008 #3
    I don't know, but many apparently knowledgeable people speak as though it is known that "space" is expanding and that large scale cosmological structures aren't just stretching and/or expanding into an already existing medium. That is, they speak as though the apparent large scale expansion is intrinsic (ie., due to wave interaction) rather than extrinsic (ie., due to kinetic energy imparted via Big Bang).
    Is there any way to tell the difference from the available data?
  5. Jul 9, 2008 #4


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    You need to provide some concrete evidence like this: a link to some professional writing by a professional cosmologist where he or she asserts that space is expanding.

    Then we can look at what they actually said and see what is going on. Kmarinas has already challenged you to do this.

    It is not enough to say "many people talk as if they think such and such" because we don't know who they are or what they actually said that gave you the impression.

    Maybe they were just talking popularization in the mass media, from which nothing can be deduced. In pop sci talk people necessarily use figurative expressions because the audience is unfamiliar with the basic math.

    What would be interesting is if you could find us a link to some professional communication between experts which makes this kind of assertion----and backs up your allegation.

    for my own part, I never say space expands.
    I am a big fan of mainstream cosmology--the standard LCDM model. I follow research and scan for new papers of interest on a regular basis. I don't recall anybody saying space expands but maybe I just didnt notice, or forgot.

    I believe in certain things---I accept certain premises as valid. I can tell you the main ones if you are interested.

    A. the CMB (cosmic microwave background)

    B. the concept of being approximately at rest with respect to CMB----i.e. temperature nearly the same in all directions

    C. merely as a mathematical construct---observers in other galaxies also at CMB rest, like us. Our situation is not exceptional, in that respect. However I don't suppose that conscious being actually exist :smile: They may or may not.
    I simply think of the universe as full of such observers so I can consider how things would look from their standpoint as well.

    D. the construction of the present moment. all those observers for whom the CMB is the same temperature as it is, now, for us belong to the present moment. this construction is approximate. it gives an idea of an approximate universal now.

    E. the approximate validity of Hubble Law, from the standpoint of every contemporary observer at CMB rest. It is a kind of Copernican assumption, we arent exceptional. this law as I have always seen it stated depends implicitly on a universal present moment. it says the present distance to a remote object is increasing (at this moment) at a rate which is proportional to the present distance. In other words Hubble Law deals with where an object is now. So it has implicitly built in some of what I was talking about in points A thru D, or the equivalent.

    Maybe one of the professionals or grad students will find something wrong with what I just said, Thomas, which is fine with me. It would be interesting :biggrin:. Or maybe you have some question or disagreement. I have to go now but will check in in a few hours or tomorrow.
  6. Jul 10, 2008 #5


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    As marcus explain, 'the expansion of space' is a phrase used mainly in the populisation of cosmology, rather than 'at the coal face' as such. It certainly doesn't describe a physical theory, i.e. the idea that space is a viscous medium dragging galaxies apart. It is nothing more than a metaphor.
  7. Jul 10, 2008 #6
    I see it frequently STRONGLY IMPLIED in professional cosmology and GR books and papers, as a sort of postulate of General Relativity, that the increase in physical distance between galaxies is a mandatory precursor to any increase in the volume of space "as a whole". In other words, that the empty vacuum we see between galaxies MUST RESULT FROM the movement of galaxies away from each other; and that yes, in effect, the galaxies are NOT moving through a pre-existing volume (finite or infinite) of empty vacuum.

    Conversely, I am not aware of ANY body of mainstream cosmology/GR books or technical papers which say that it is the mainstream view that galaxies are merely moving through pre-existing empty vacuum, and that if the scale of the matter/radiation universe is finite, then a vast expanse of empty vacuum is believed to exist beyond the outer "edge" of the matter/radiation universe (assuming it's not infinite).

    There seem to be a variety of reasonable objections to the idea that the movement of galaxies enables new space to "well up" or otherwise come newly into existence. Here's one objection to which I have never heard a satisfactory response:

    If the physical "spreading out" of matter and radiation cause new volume of space to exist, then surely it is not the spreading out of the galaxies themselves that performs this task. Presumably in every quasi-local frame everywhere in the universe, relativistic particles are "spreading apart" more rapidly than any galaxies or other non-relativistic matter are. In a quasi-local frame galaxies are constrained to recede from each other at non-relativistic velocities. Relativistic particles, such as the CMB photons, the pre-CMB neutrinos, or even the light and cosmic rays emitted by galaxies must be "spreading apart" at a dramatically faster pace. Therefore, if the movement of matter/radiation causes new volume of space to exist, an insignificantly tiny percentage (maybe none) of that space we see today was created by the movement of galaxies; the galaxies were preceded "everywhere" by relativistic particles. The galaxies are merely "backfilling" a small fraction of the volume already "created by" the movement of relativistic particles.

    So the simple question is, why can't we observe the enormous volume of space created by the spreading apart of these pioneering relativistic particles? If this new volume of space were "welling up everywhere" between galaxies, at relativistic "recession velocities", then either (a) galaxies in every quasi-local frame would be forced to recede from each other at relativistic recession velocities (obviously this isn't the case), or (b) the newly created volume of empty vacuum would be "flowing past" the slow moving galaxies at relativistic speeds. The latter picture seems absurd, e.g., where is the "flowing" space going? Even if it were true, it would mean that as galaxies recede from each other at locally pokey speeds, they are in fact moving into pre-existing space, they are not causing new space to be created. Because the new space must have arrived before the galaxies did.

    EDIT: It seems unreasonable for a number of reasons to draw a distinction, that the movement of "massive" particles causes new space to be created, while movement of "zero rest mass" particles does not. For example, most cosmic rays are protons, so they are indeed massive particles even if they are spreading apart at very high (but not entirely relativistic) velocities.

    In sum, the more one pursues this whole line of argument, the more absurd it seems. Why shouldn't we just adopt the seemingly reasonable proposition that, regardless of whether the universe is finite or infinite, the increasing volumes of space between galaxies already existed SOMEWHERE before the galaxies moved into that space?

    The emphasis of the mainstream approach strikes me as being based on little more substance than the old paradox: if a tree falls in the forest and no one hears it, was there a sound?

    Last edited: Jul 10, 2008
  8. Jul 12, 2008 #7
    I think sketchtrack really puts his finger on the matter in his post here https://www.physicsforums.com/showthread.php?t=242647&page=10 where he says "I have always been told the universe is expanding faster than C. Isn't the expanding space rather than moving galaxies thing just to satisfy the hypothesis that mass cannot move faster than C?"

    This is backed up by the John Baez Physics FAQ http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html#13 on the question of examples of apparent superluminal motion
    "Expansion of the Universe
    According to Hubble's Law, two galaxies that are a distance D apart are moving away from each other at a speed HD, where H is Hubble's constant. So this interpretation of Hubble's Law implies that two galaxies separated by a distance greater than c/H must be moving away from each other faster than the speed of light. Actually, the modern viewpoint describes this situation differently: general relativity takes the galaxies as being at rest relative to one another, while the space between them is expanding. In that sense, the galaxies are not moving away from each other faster than the speed of light; they are not moving away from each other at all! This change of viewpoint is not arbitrary; rather, it's in accord with the different but very fruitful view of the universe that general relativity provides. So the distance between two objects can be increasing faster than light because of the expansion of the universe, but this does not mean, in fact, that their relative speed is faster than light.

    Now as far as I know John Baez is not a crank but I am not sure if he qualifies as a "professional writer or cosmologist" in your eyes as I have not researched his credentials. A search in the scholar section of google using the keywords "space itself is expanding"+universe pulled up 23 hits as listed here: http://scholar.google.co.uk/scholar...s+expanding"+universe&as_ylo=2003&btnG=Search The reader is invited to decide for him/herself if any of the papers in the list qualify as peer reviewed papers published in a respectable journal as defined by PF rules.

    The point is that questions about "why superluminal recession velocities do not contradict Special Relativity" is often explained away by suggesting that receding galaxies are at rest with the local space in the large scale "expanding space". If the expanding space is a myth as you suggest then an alternative explanation is required for why superluminal recession velocities do not violate Special Relativity. It is not good enough to say SR is not violated because space itself is expanding and have a subclause (but space is not expanding) in the small print. That is having your cake and eating it :tongue:
  9. Jul 12, 2008 #8

    George Jones

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    Why should special relativity have anything to say about this situation?
  10. Jul 12, 2008 #9


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    Charles Lineweaver does so in "Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the universe". I think he is authoritative enough and this is a very important paper on this topic. Personally I would prefer "expansion of the universe" (that Lineweaver uses also), because one may change coordinates to conformal time and space to have a space-time expanding instead of an expanding space only, but in the usual time coordinate space expands and the usage of the term is justified. I do not see why this should be a term that should not be used.
  11. Jul 12, 2008 #10
    Thanks to all repliers for your comments, suggestions, links, references, etc.
  12. Jul 12, 2008 #11


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    You are welcome, Thomas! I hope the links and discussion have resolved the issue for you. As a reminder of what we are supposed to be talking about, here is your first post.

    This sounds to me as if you were imagining space as something physical in the sense of substance or material---that would require being created so that the amount of it could grow. I don't think that is what cosmologists have in mind. So I asked to see some casewhere a professional cosmologist talking to colleagues (not to popular media) said "space expands"---because we can look at a definite example in context and see what is going on. What is actually meant by the phrase.

    What resulted is instructive. Hellfire came up with an example: a 2003 article by Lineweaver and Davis. They use the phrase "expansion of space" and it is clear from context that it is just a shorthand for increase of proper distance in the conventional model.

    Here is the article which Hellfire pointed to:

    I think you can be reassured that there is no implication here of an "unknown physical structure called space" which is stretching or swelling up or growing new substance. Not in the sense of a physical entity more of which would need to be created as it grew. I will quote a large piece of the context so you can see from context what they mean. I'm pretty sure you will agree that all they mean is simply that a bunch of distances are increasing.

    3.1 Misconception #1: Recession velocities cannot exceed the speed of light

    A common misconception is that the expansion of the Universe cannot be faster than the speed of light. Since Hubble’s law predicts superluminal recession at large distances (D > c/H) it is sometimes stated that Hubble’s law needs special relativistic corrections when the recession velocity approaches the speed of light [App. B: 6–7]. However, it is well-accepted that general relativity, not special relativity, is necessary to describe cosmological observations. Supernovae surveys calculating cosmological parameters, galaxy-redshift surveys and cosmic microwave background anisotropy tests, all use general relativity to explain their observations. When observables are calculated using special relativity, contradictions with observations quickly arise (Section 4). Moreover, we know there is no contradiction with special relativity when faster than light motion occurs outside the observer’s inertial frame. General relativity was specifically derived to be able to predict motion when global inertial frames were not available. Galaxies that are receding from us superluminally are at rest locally (their peculiar velocity, vpec = 0) and motion in their local inertial frames remains well described by special relativity. They are in no sense catching up with photons (vpec = c). Rather, the galaxies and the photons are both receding from us at recession velocities greater than the speed of light.

    In special relativity, redshifts arise directly from velocities. It was this idea that led Hubble in 1929 to convert the redshifts of the “nebulae” he observed into velocities, and predict the expansion of the universe with the linear velocity-distance law that now bears his name. The general relativistic interpretation of the expansion interprets cosmological redshifts as an indication of velocity since the proper distance between comoving objects increases. However, the velocity is due to the rate of expansion of space, not movement through space, and therefore cannot be calculated with the special relativistic Doppler shift formula...

    Now we've got a definite example of someone using the phrase, let's see if there is any confusion about what they are talking about. Or if anybody has questions.

    Other people may disagree but the way it looks to me is that the main trouble in the original post is with the interpretation ("Apparently the phrase... is meant to characterize...") Judging from the example that Hellfire turned up, the phrase is not meant to characterize what you say.
    Last edited: Jul 12, 2008
  13. Jul 12, 2008 #12
    Special Relativity has something to say because if a distant galaxy is moving through space it should be subject to time dilation. Modern cosmology does not include SR time dilation so therefore modern cosmology does not consider distant galaxies to be moving through space but to be moving with space.

    In SR an object is constrained to move at less than c relative to the observer while in GR the velocity of objects are constrained to move at c relative to the local space and that includes velocities that are greater than c relative to a distant observer. For example if a distant galaxy that is locally at rest with the CMB but moving at 5c relative to us on Earth, then any object or photon local to that galaxy is constrained to move at 4c<v<6c relative to us on Earth. Some might argue that motion is constrained to +/- c relative to other local objects but the fact is that objects the same distance as the our galaxy from Earth are constrained to 4c<v<6c even if they are in a void away from any other objects. That suggests that the constraint on velocity is the local space and not local objects or distant observers.

    A further complication is this. If a value of zero for Omega(mass) and a value of zero for Omega(Lambda) is plugged into the FLRW metric then the answers are very different from those of SR even though those values suggest no mass or cosmological constant. Shouldn't SR apply to universe with no mass or gravity or cosmological constant? What exactly is dragging a photon outwards in the early stages of its voyage in a universe with no gravity or cosmological constant?

    Distant galaxies are at rest locally with respect to what? I could guess they mean that the galaxy is at rest with the local CMB but I am sure the CMB has no physical effect on the motion of the galaxy and the CMB is not an aether that objects move with respect to. So my final guess as to what the galaxy is at rest with respect to is the local space and the local space is not at rest with our local space. This gives a physical quality to space. The local space near the galaxy is moving relative to our local space and presumably the space inbetween is expanding.

    So there we have it. A professional cosmologist saying "the velocity is due to the rate of expansion of space, not movement through space". That clearly implies that the velocity of a receding galaxy is due to it being dragged along by the expansion of space and that the velocity of a receding galaxy is not due to its own momentum taking it through space. He also suggested that the galaxies are at rest locally with the space. In other words the distant galaxies are stationary objects embedded in space and it appears to be moving relative to us because the space it is embedded in is moving relative to us. This is not just a philosophical difference in interpretation because the maths is different. If the object is embedded in comoving space then there is no SR time dilation in the calculations. Modern cosmology does not include time dilation in the redshift calculations so there is no motion relative to space.
  14. Jul 12, 2008 #13


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    You have your own way of thinking about it.
    It is probably useless to try to communicate my point of view to you but I'll make a brief try.
    For me distances exist. But space has no physical objective reality (several einstein quotes to this effect).

    For me, the Hubble Law describes the regular increase of distances. (on average largescale pattern) To call that "expansion of space" is merely figurative, a metaphor, a figure of speech----as I said before a shorthand expression. Professionals occasionally use it, but context tells us what they actually mean.

    I try not to say "expansion of space" because it gives noobs the impression that space is a THING with some kind of objective physical reality. The mistaken impression that, as you said, space is so substantial that it can drag stuff along :biggrin:.

    That is a mistake: it is not a material. It is not a thing. It cannot expand because it does not exist. It cannot drag galaxies around (as you suggested) because it does not exist. What exists are events and relations between events, such as distance. These are things which we can observe.
    Distances can increase.

    but I see no reason why you shouldn't talk to your heart's content about space as if it were a material and capable of expanding and dragging. You use whatever turns of phrase you like. what I am saying is that I try not to use that phrase, even as a shorthand or metaphor, because it seems to mislead people. that is my personal choice.
  15. Jul 13, 2008 #14


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    I find your view very reasonable marcus. But I thnik that one can also interpret it in another way: space or spacetime is a metric, which in turn represents distances, and this turns "space expands" into "distances expand" that should be more inline with your view.
  16. Jul 13, 2008 #15

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    No, the answers are the same as SR. It just that in the no mass no cosmological constant scenario, standard cosmological coordinates reduce to unusual (but valid) coodinates for a patch of Minkowski spacetime. See the last few posts in the thread

  17. Jul 13, 2008 #16

    A metaphor is 'a figure of speech in which a phrase is used to stand for something else '. So says my dictionary. The issue here seems to be that in this case the nature of the 'something else' confuses many clever folk. You have company, Thomas T. It incudes popularisers (say Brian Greene) and, as pointed out in this thread, John Baez and Charles Lineweaver, not to speak of others in this thread.

    I'm not very clever, and have been severely confused for quite a while by what is written about the ongoing global change that seems to be a feature of our universe. I think that Wallace's advice is sensible. I take it to be: assume that the 'expansion of space' is a metaphor for change that is difficult to comprehend, and leave it at that. But some folk, like Marcus here, have well-founded opinions that go beyond accepting a metaphor. And I also have a bit more to say about the 'something else':

    Expansion is a familiar phenomenon that can be directly measured with laboratory apparatus or radar on a local scale; examples are the expansion of a heated rod or a gas, the expansion of the Earth-Moon axis (3.3 cm/yr) and the just-discovered contraction (negative expansion) of the planet Mercury. Sadly, we cannot so directly establish that the distant parts of the universe are 'expanding' away from us, or receding kinematically .

    What cosmologists have done is to invent a description of how the universe behaved in the past, as if time ran backwards so that expansion becomes contraction. This quantitative invented description is firmly based on the red-shift, as described by GR. It suggests that the universe was once hot and dense, and that relic radiation from this era must exist. Because the CMB does exist it is widely accepted that charge in the universe is indeed something like 'the expansion of space'. There is of course a mountain of supporting evidence for this view.

    But claiming that space itself does expand, just like a familiar local-scale phenomenon, is going too far. It is rather like saying that space is curved, which is a metaphor that (esoterically) stands for the shape of geodesics in the spacetime of GR. But the metaphor does not stand for the shape of sufficiently rigid measuring rods. Such rods do not have to partake fully of the distortions of spacetime mandated by gravity, since their shapes are ruled by much stronger interactions of nature.

    The smoke and mirrors here is that if such distortion is uniform expansion, the requirement of rigidity falls away. This is why the metaphor of "space expanding' is flawed.

    But I agree that it's about the best we can do, Wallace. Perhaps one should leave the mysteries to the philosophers.
  18. Jul 13, 2008 #17
    You raise an interesting point here. How do we distinguish between the moon moving away from us at 3.3cm/yr and the moon being stationary while the distance between the moon and Earth increases (expands) at 3.3cm/yr? At the end of the day is it all just about semantics?

    There are some reasons cosmologists prefer the expanding space rather the receding kinematically picture. One is that the luminosity distance of some objects appears to be as much 47GLyrs while the age of the universe is around 13.7 Gyrs. This does not seem compatible with a model of static space where recession velocities would be limited to the speed of light. However it should be noted that redshift does not directly tell us recession velocities. You can only calculate a recession velocity when you have first chosen a model. It is probably not widely recognised that Special Relativity predicts a dimming of luminosity that makes objects to appear to be much further away by a factor of (1+z)^2 than their proper distance at the time of emmision bringing distances more in line with comoving expanding space model. I personally believe that if you add in a large inflation factor at the beginning, you can dispose of the expanding space and accelerating expansion but I am still working on the maths to prove that.
    Last edited: Jul 13, 2008
  19. Jul 13, 2008 #18
    Hi kev and oldman,
    The inability to distinguish which of two fundamentally opposed theories is valid and which is invalid cannot reasonably be described as semantics or philosophy. If one theory is valid, the other is invalid; they can't both be valid at the same time, and one of them is fundamentally wrong. No philosophy or careful wording can change that.

    Our present inability to rule out either of them is frustrating for us, but by no means is it the end of the story. If we're still doing astronomy and physics in future decades, eventually we should expect to develop the technological and analytical capability to rule one of them out.

    If the Hubble flow of galaxies causes new vacuum space to "well up" between galaxies, then don't we need to inquire into the physical mechanism which allows that to occur?

    At what scale of granularity does this "new" vacuum insert itself between the existing vacuum and particles? Above or below the Planck scale? Why doesn't the "new" vacuum appear between gravitationally or electromagnetically bound particles that periodically move slightly apart, thereby preventing them from subsequent returning to their former, closer spacing?

    At the instant "new" space is created, can we categorically exclude the possibility that any adjacent electromagnetic radiation wave will physically extend into the "new" space -- and if so, doesn't this require a modification of the Heisenberg uncertainty principle?

    Must we conclude that only Hubble motion causes expansion of space, while peculiar motion does not? If so, then what physical aspect is fundamentally different as between Hubble motion and peculiar motion? Do the same concepts of momentum and inertia apply to both kinds of motion? Do we try to answer this question through quantum mechanics? Or do we content ourselves with the knowledge that GR spacetime geometry precisely describes the difference mathematically, and just leave the physical causation of the difference to forever remain a metaphysical mystery?

    Last edited: Jul 13, 2008
  20. Jul 13, 2008 #19


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    Who says that new vacuum space wells up between galaxies?

    As I understand the Hubble flow, nothing new is created. It simply refers to distances changing in a certain way.

    It seems to me that you are thinking of space as a thing. Involved in a physical process of new vacuum inserting itself. Vacuum pushing vacuum apart.

    I don't understand the role of the quote-marks. Who is being quoted? None of this makes sense to me.

    Who says that new space is created? Why attribute reality to something where there is no evidence of it?

    All we have in the Hubble flow is a case of distances changing between stationary objects. General relativity tells us this is natural and to expect it. Change is just something that distances do (in accordance with the field equation, initial conditions, and the influence of matter).

    Einstein got over this hangup already in 1915 when he said the principle of general covariance deprives space and time of the last shred of physical reality.

    I wish you could apply your considerable rhetorical skills to helping others get over the (materialising of space) hangup, rather than making it worse.
    Last edited: Jul 13, 2008
  21. Jul 13, 2008 #20
    Hi Marcus,
    The terminology about space "welling up" is from one of your and my favorite https://www.physicsforums.com/archive/index.php/t-175900.html", on this subject, "Expanding Space: the Root of all Evil?" 7/07 by Francis, Barnes, James & Lewis"
    They go on to conclude:
    I have no problem with the ambiguity of the conclusion, since we don't know the answer, but I think it's worth focusing on what physical mechanism the reference to new space "welling up" is intended to invoke. If the answer is that none was actually intended, fine, but then why don't we just explain the FLRW model as "a cloud of self-gravitational gas and matter expanding through space as a result of something which simultaneously imparted a range of initial momenta to its constitutent particles?" Why the mystical, metaphysical references to new space being created? I fail to see how such references are helpful to either amateurs or professionals if indeed they are intended to be gratuitous.

    I'm trying to avoid coming across as merely disagreeable. But at the end of the day, space itself either is, or is not, expanding physically. We can help clear away a lot of hocus-pocus pixie dust from the general understanding of cosmology if we simply describe both alternative physical theories, and explain what the physical implications are of each. On the other hand, in my opinion, using the concept of expanding space as a teaching tool in some contexts while denying any reliance on it in other contexts is confusing at best, and disingenuous at worst. And saying that "it doesn't matter" fundamentally defies my sense of scientific curiosity.

    However, I can't disagree that of late this has been the most "politically correct" course.

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