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Posts posted by geordief

  1. 1 hour ago, MigL said:

    I had considered only an outside observer.
    An infalling observer will see things differently I suppose.
    But if you fell through the EH of a huge BH, along with a much smaller BH, would you be able to see it ?
    It it was ahead of you, I don't think you would, and I have my doubts even if the small BH was alongside you.
    You may be able to see it if it followed you through.

    And even if we don't consider different observers, how long can the two EHs remain distinct?
    They are essentially mathematical constructs denoting a region of extremely curved space-time

    This seems a silly question ,but if two objects follow each other through the event horizon of a  very large BH ,does the second  object see the first  object as it passes the EH? 

    Even if the two  are very close together...


    Alternatively would a  Pinocchio  lose the sight of the end of his nose first   as he passes the threshold?

  2. @iNow don't know .Everywhere?

    That would seem to be an answer in the spirit of "where is the centre of the universe ?"

    (answer : "everywhere")

    Do we know that black holes have "back holes" ,anyway?

    I thought we just knew as far as the event horizon and some have suggested all the matter could actually collect there -with there being nothing inside .

    Would the "back hole" be on the event horizon in that case?

    Obviously ,I know zilch about this

    9 minutes ago, MigL said:

    The geometry of space-time 'starts' at the Big Bang; geodesics extend forward in time, but not backwards.
    Somewhat like lines of latitude and longitude at the North Pole; they only extend in one direction ( to use Markus' analogy ).

    A Black Hole is different.
    The Event Horizon of a Schwarzschild BH ( non-rotating and non-charged ) appears differently to different observers.
    While an infalling observer may note nothing peculiar falling through the EH ( no time slow-down or tidal forces for large BHs ),  a distant observer will note all information transmission from the infalling object slowing on approaching the EH, and finally stopping at the EH.
    To the outside observer it would appear as if time had stopped at the EH, along with geodesics leading into the EH.
    The EH is known as a co-ordinate singularity, and this apparent freezing of time does not happen in the proper time of the infalling observer.

    Are you saying that the idea of the universe having an event horizon corresponding to  what a black hole might is  more or less a non starter ?

    Or maybe you were replying to someone else?

  3. 42 minutes ago, KJW said:

    Considering the Schwarzschild metric, there is no upper limit to mass, and therefore no limit to the size and how close to zero the density can be. However, there may be limits at the cosmological scale. For example, I doubt that a black hole can be less dense than the universe as a whole.

    If the region surrounding a black hole is denser than the black hole, then the total mass of the black hole and the surrounding region would be large enough for the surrounding region to be also inside the black hole


    Is it at all possible that the observable universe is inside a black hole with the event horizon  between the observable and the  non observable regions?

  4. 2 hours ago, KJW said:

    It's worth pointing out that although for a small black hole, the tidal forces may be so intense as to cause "spaghettification" well outside the event horizon, for an extremely large black hole, the tidal forces may be so weak that one might not even be aware that one has crossed the event horizon.

    Also, black holes have a peculiar property compared to ordinary matter: the mass of a black hole increases linearly with respect to radius, in contrast to the cube-power for ordinary matter. Thus, extremely large black holes have very low density. One only needs to accumulate enough matter into the large volume to form the large black hole... one doesn't need to compress anything.



    Is there a limit to how large and how "undense" such an object can become?

    Less dense than the surrounding  region ,for example?

  5. 1 hour ago, John Cuthber said:

    The results are in.
    Apparently 17000 dead as a consequence of HCQ.

    We never got a breakdown**of covid mortality per hourly consumption of  forms of social media vs mainstream  medias vs  party affiliation etc etc. ..by the looks of it.


    Hope we learned something from the Covid period 

    **as suggested earlier in this thread.

  6. 48 minutes ago, Genady said:

    This is a common misconception. In fact, we can and do see light coming to us from sources which recede faster than c . It is so because as the emitted light moves away from its source along the line connecting us to the source, it gets to parts which recede from us slower than the source. Eventually, it gets to parts which recede slower than c .

    Here is a more detailed description:



    ([astro-ph/0310808] Expanding Confusion: common misconceptions of cosmological horizons and the superluminal expansion of the Universe 

    Thanks.Probably a bit too hard for my brain to get around but I will accept it for now

    Not sure how it affects the point I was trying to make.

    Does it mean there isn't  an unobservable universe  beyond  the  observable  universe?


    8 minutes ago, swansont said:

    Where did you find it? Is it a credible source?

    I think I found it here in physics stackexchange (by googling)

    https://physics.stackexchange.com/questions/552306/does-time-require-matter-to-exist#:~:text=In other words%2C GR states,without realistic physical initial conditions.


  7. 2 hours ago, Airbrush said:

    I was wondering if time can exist without matter?  This is what I found:

    "GR states that spacetime is the field produced by matter just like the electromagnetic field is produced by charges. Vacuum solutions are unphysical, they don’t exist in reality."

    Does that mean that if the big bang has a finite size, the outer 3d limits of the expansion could be expanding at ANY speed (assuming there is no infinite speed), and beyond that limit could be a region of no matter and therefore no spacetime, just "space" if you will?  No more spacetime, just space, until you encounter another big bang coming from another direction?

    I thought one way to explain an accelerating expansion of the universe is our observable universe is located inside a great void, like a bubble surrounded by unimaginably great masses pulling our region of spacetime apart from all directions.



    Don't really know what to think.

    But I don't think the observable universe is surrounded by a great void as in my understanding it is surrounded by "more of the same"  but that we have no idea how much of this "more of the same" there or-or what it is doing. 

    The only reason we don't  see this "more of the same" is that it is receding from us faster than the speed of light and so has become invisible to us.


  8. 1 hour ago, Genady said:

    No, that would be an extrinsic curvature

    I am confused.Wouldn't it be extrinsic curvature  if the boat fell off an  imagined edge but intrinsic curvature  if it just dropped out of view when it reached the horizon?

    Or does the light ray  with which we see the boat disappear exist  in  an embedding 3rd dimension making the surface extrinsic after all?

  9. 29 minutes ago, TheVat said:

    So, if I'm following any of this, curvature is defined in a way that is intrinsic to the manifold and not dependent upon its embedding in higher-dimensional spaces.  Like, if I'm a flat bug on the shore of a calm lagoon I can measure a curvature of the "flat" water as flat bug-ships drop over the horizon, without any reference to an earth interior or global topology?  

    Yes ,you draw  a triangle  on the flat water and measure the 3 angles.

    If they add up to 180 it is flat .

    Less and it is a positive curvature ;more and it is negative (like a saddle)

    And ,less obviously you can do the same in 3d(perhaps using a 2d surface embedded in the 3d in the same way)

    Not sure how you would do it in  4d spacetime.

  10. 3 minutes ago, Genady said:

    No. "Local" is space-time volume where the metric can be approximated by Minkowski's one with vanishing first derivatives.

    OK .So can subsets of  the physical universe (eg the solar system)  be described as "local" or "global"?


    Does "local" only describe the model and not the physical objects it attempts to model?


    And are we talking about spacetime curvature or the topology of the physical (subsets of the) universe?


    I think I was talking about just topology in that last post(even if I used terminology  that applies normally to spacetime curvature) ie is there such a thing as local topography versus global topography?

  11. 1 hour ago, Genady said:

    We see the light coming from very distant sources in the observable universe. It appears to be very homogenous and isotropic.

    very distant= still local ?

    That wouldn't  be evidence of a global topology, would it?

    The observable universe is still  "local" isn't it?



    8 hours ago, MigL said:

    You're having problems imaging the topology of 4D space-time by comparing it to 3D or 2D topologies ?
    Welcome to the club.

    Mathematics, however, has no such limitations.

    Does the maths you are referring to make any physical predictions or is it entirely " theoretical"? ("theoretical" in layman's speak)

  12. 5 hours ago, Markus Hanke said:

    Actually, it doesn’t - they are separate concepts. GR determines only local geometry, but not global topology. For example, the maximally extended Schwarzschild metric could describe both two separate, singly-connected regions of spacetime, or a single multiply-connected spacetime. Geometry is the same in both cases, but the global topology isn’t

    How do we know that a global topology exists at all and that we don't just have a patchwork of local topologies?

  13. 6 minutes ago, Genady said:

    2D sphere is a geometrical model, not a physical model of the 3D space of our universe. I've suggested it as a tool to help in understanding how it is geometrically possible to be edgeless and bounded. It was not to suggest that our space is a 2D sphere.

    Are there any physical models that do that with posited versions of an actual universe?

    Are there any models that picture such a universe "from  a bird 's eye view"?

    Or is that just an illogical question to ask?(the  "bird"/observer  being part of the universe could not take a "bird's eye view"  of itself as a part of the whole)

  14. 1 hour ago, Genady said:

    This does not affect geometry or topology.

    It is not an analogy. It is a model.

    2D sphere is not hollow. 2D spherical surface in 3D space, is.

    Can we populate the surface  of that 2d  model of a sphere  with 3d objects of an imaginary universe which is posited to be edgeless and bounded?

    How would I see in the model that the objects were 3d if they were embedded in the 2d surface?

  15. 30 minutes ago, Genady said:

    I think so.


    For an edgeless but bounded space imagine a 2D version, a sphere.

    For an edgeless and unbounded space - I don't see a problem. It's just like a number line but in 3D.

    Is this a space with no time component?

    A space that is ready to be populated with objects?

    I think I am more familiar with a space that is created by objects reconfiguring themselves(in an overall expansionist way as per observations)

    I think that is the orthodox view even if I am not up to speed with it.


    35 minutes ago, Genady said:

    For an edgeless but bounded space imagine a 2D version, a sphere

    It still seems difficult for me to imagine  the 3d universe existing on the 2d surface of the sphere

    There doesn't seem to be room for the 3 dimensions.

    Is it just an analogy?

    And the universe is not hollowed out ,is it?)unless the "hollow" is somehow the past history-surely not that)


  16. 28 minutes ago, Genady said:

    I don't know.

    Absolutely. Metric determines curvature. It does not determine topology.


    Depends on other properties, e.g. homogeneity and isotropy.

    I don't know.

    The difficulty I have is of picturing  a topology for a universe that has no edge or boundary.

    It seems others don't have that problem or perhaps they have enough of a mathematical understanding of the possibilities   that they don't need to have a mental picture

    Would I be right to think that a topology  doesn't require any particular shape  but it just describes the way parts of the whole connect with each other?

  17. 3 hours ago, Genady said:

    Take a 2D flat surface, like a flat sheet. Its intrinsic curvature is identically 0 everywhere. Roll it into a cylinder. You get a different topology, but it has the same, 0 everywhere, intrinsic curvature. 

    How many different kinds  of topology are possible for the universe?

    Would these topologies depend on more than the spacetime curvature?

    Are they all expanding ,contacting ,steady state?

    Any other ways to distinguish between different candidate topologies?



  18. 8 hours ago, Markus Hanke said:

    Newtonian gravity has nothing to say about massless particles, so strictly speaking it makes no prediction here. However, if one assumes that photons have a very small but finite mass, then one can use Newtonian gravity to work out how they are deflected around massive bodies. Turns out that deflection angle doesn’t depend on the exact mass of the photon, so long as it is much smaller than that of the central body.

    The result you get is off by a factor of 2 compared to actual observations - to get the correct angle, one must use GR.

    Interesting, though that apparently Laplace and Mitchell (1815?) considered the idea of a Black Hole.They must have thought light had some mass,I suppose.

  19. 7 hours ago, MigL said:

    This is an aspect of the topology of the universe, and has nothing to do with bending of light by gravity.
    If the universe is 'curved', light has no choice but to follow the 'curvature; it cannot travel 'outside' as there is no outside.

    is the topology not a function of spacetime curvature? (perhaps the "curvature" description of the topology gave me the wrong idea)

      What might cause different topologies to arise?

  20. 13 minutes ago, MigL said:

    Yes that is what 'closed' implies.
    A positive curvature will always close on itself.
    This can be seen on the surface of a globe, where a ray of light following the curvature will come up behind itself.
    Different distances to the 'back of your head', however, may be seen in different directions depending on topology.

    Did/does Newtonian physics predict something similar?(light returning to sender)

    Wasn't there someone before Einstein who also predicted that light would be affected by gravity?

  21. 11 minutes ago, Bufofrog said:

    A new poll came out that said 7 out of 10 republicans are morons.  Specifically, the poll said 67% of the republicans say Biden's election was not legitimate, which is the same thing as saying they are morons. 

    It is understood that the 7th out of every ten gave as a reason that number 6 had told them so.

    It is not known who was the original source  of the information.

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