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Nonsensical representation of gravity

  1. Jan 16, 2012 #1
    Greetings all,

    I am the newest addition of the PF community and let me express my excitement of finally tracking down this intelectual family and hopefully making myself eventually an equal member of it. Now, I will not waste too much of your time let us get down to this PFbuisness=pleasure, shall we?

    I may be just an enthusiast in the world of astrophysics in general but there is something that disturbs me:

    Most educational literature I have come across persistantly visualises the force of gravity and similar phenomena in insuficient dimentions. A typical example would be a visual representation of the curvature of space-time, inflicted by the gravity of an interstellar body. They simply put down a 2D web, curved by the object that sits upon it. We all know that this is simply an insuficient representation, just as as saying "the sun comes up", which is simply inacurate.

    So what I would like to ask is what is their motivation for disseminating such information in such an oversimplified manner?

    It also may be a possibility that I have simply come across such items which were designed for a low level laiman, in that case I would apreiciate it if you could direct to some "higher-level" materials on the topic of astrophysics, preferably some video please or scientific columns with more accurate information...

    Thank you all and have a nice day!
  2. jcsd
  3. Jan 16, 2012 #2

    Simon Bridge

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    This is, indeed, what has happened. You are referring to descriptions aimed at people without the math to handle the whole model.
    Yu won't get it in video - those are aimed at the layman. The full theory is describable only in mathematics ... so you need text references.

    Any senior or post-graduate work on general relativity would give you the theory you need. It is possible that you will need to upgrade your math first ... but you'll find out quite fast. For the real ultra-recent models for gravity, though, you want a work on quantum gravity ... which requires a different kind of math.
  4. Jan 16, 2012 #3
    If your reason for discussing science, is to make yourself feel superior, then describing things in the most complicated manner possible would make sense. However if you would like for most people to learn the basics, you have to describe it in a way most can understand.

    If you would like to discuss 'higher-level' stuff go here:https://www.physicsforums.com/forumdisplay.php?f=68, and there also is a learning materials section at the very top of the list of forums here at physicsforums.
  5. Jan 16, 2012 #4
    Ugh...:yuck: I cursed the math at school and now I curse the day I snoozed my way through it.:zzz:

    Everytime I did an exam my physics professor asked whats my math grade and i reply: #ehem# flung, #ehem ehem# summer school...:biggrin:! And he grinned everytime, fortunately he was a crackpot and was more interested in my understanding of the matter rather than my scribblings on that piece of paper.

    I'll get it though. My own drooling over the subject will keep me going. I presume I can get everything I need from you fine gents?

    This isn't about me feeling superior in any way, or feeling anything at all, its about my own personal pursuits and expanding the horizons of my understanding, but not in a limited form that standard materielle has to offer.

    I don't intend to use this as a subject of bragging in my conversations, this is for me and me alone and i came here asking for some guidance from you experienced people, there really is no cause to insult me.
  6. Jan 16, 2012 #5

    Simon Bridge

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    Mathematics is the language of physics.
    You cannot go past the lay descriptions without it.

    You will get everything you need from your efforts and resources, we can only help you as you get confused or stuck and only briefly - unless, of course, you want to pay for the tuition?
  7. Jan 16, 2012 #6
    I think Jasongreat was talking about the author of the book, not you. I don't understand exactly what you are proposing though. Do you want a 4 dimensional picture, and if not, then no picture at all.
  8. Jan 17, 2012 #7
    Yes, well, I drifted away from it a little... i roughly understand how a mapping of the distorsion of spacetime around a black hole should look like, but my question is why do the TV series such as "the universe" and "nova" oversimplify such phenomena when they have actual access to physicists who can help them construct a more accurate model?

    They do it the same with Alcubierre's warp drive, gravity in general and all other similar things.
  9. Jan 17, 2012 #8


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    Mainly because there is no way to visualize an accurate model, there is no way to "properly" represent 4 dimensions. You can of course do it using maths, but that would be completely incomprehensible to almost everyone (including most physicists).

    There is nothing wrong with simplified models as long as you keep their limitations in mind.
  10. Jan 17, 2012 #9


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    Beat me to it.

    @Thomas: It's sometimes hard enough to model even 3-dimensional phenomenon using 3 dimensions, let alone 4-dimensional phenomenon. If you can figure out a better way to represent it, we would be happy to hear it. I mean, think about it. You need that third dimension in your typical picture to given an idea of the magnitude of the curvature of the 2 other dimensions. If you were to use the third dimension as an actual space-time dimension, how do you show the magnitude of the curvature?
  11. Jan 17, 2012 #10
    Yes, but they already did it so nicely, such as this image:

    Its a lot more suitable to the matter at hand, all you would have needed to add would be some twist in direction of the earth's rotation and the image would be perfect!

    Instead, what they do is this, which is folly and insuficient:
  12. Jan 17, 2012 #11
    I have seen that second image many times on TV and in books. I always thought it was overly simplistic, but I didn't realize until now that it is wrong. If you compare it to the first image you see that the curvature goes the wrong way. The earth is pushing the lines away rather than pulling them in.
  13. Jan 17, 2012 #12

    Vanadium 50

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    No, because it would still neglect curvature in the 0-direction, which for some circumstances is the most important direction.

    It's an analogy. It is not reasonable to expect an analogy to work in all situations. I recognize the problem that when people come to the limits of the analogy, they cling to it, sometimes even to the point of refusing to believe the correct answer when it conflicts with it. But that's a general problem, not one with a specific analogy.
  14. Jan 17, 2012 #13
    I presume that now you see the problem of attempting to represent this in limited dimentions?

    May I ask - why would the earth push the lines out if gravity pulls stuff towards the gravitating body? Doesn't it contract space?
    (I'll doublecheck... now where's that ebook...)
    Last edited: Jan 17, 2012
  15. Jan 17, 2012 #14
    Actually, I've changed my mind about both images. If the lines are meant to be geodesics, then they should be pushed out away from the earth. You can see this in the following correct image:

    Graviational lens

    Therefore, if the lines are meant to represent geodesics, then the 3D image is completely wrong. The 2D image is slightly better, but it is only correct in 1 dimension, north to south. The other 2 dimensions still show the earth pulling the geodesic lines in. What's more, if a literally 2 dimensional image were created, it could show the effect perfectly well because of the spherical symmetry.
    Last edited: Jan 17, 2012
  16. Jan 17, 2012 #15
    That 3D image is just like they put a box around the earth and pushed in the sides, which is still not an accurate representation of gravity in the 3 spatial dimensions. As was said before me: If you keep the limitations in mind with the 2 dimensional image it serves as a perfectly well representation of gravity and is something that is very simple for everyone to conceptually grasp.
  17. Jan 17, 2012 #16
    The light isn't being pushed away from that massive body, the way I read that image. The rays approaching that body are divergent, in straight lines, as they come from the distant galaxy pictured, then they are redirected into convergence by the massive body's gravity and focused onto the earth, creating the illusion of a different source location, as depicted by the larger orange arrows. The light rays are definitely being deflected toward the massive body. Extend the original lines as they emerge from the distant galaxy, and you'll see that, if they didn't encounter the gravity lens, they wouldn't hit the earth at all.
  18. Jan 17, 2012 #17
    I don't see how you're characterizing one as 3d and the other not. They both look 3d to me. The 'traditional' second one has always looked completely non-sensical to me: a globe sitting on a rubber sheet distorting it by its 'weight'.
  19. Jan 17, 2012 #18
    Simply compare the curved lines in my link to the curved lines in the 3d image. In my link, the gravitating body is at a focal point of the hyperbola. That is to say, inside the curve. Just think about the path of a comet as it nears the sun. It doesn't pass in front of the sun, it goes behind it. Thus, the sun is inside the curve, not outside of it.
  20. Jan 17, 2012 #19

    Simon Bridge

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    The "2D" image is the "rubber sheet" model and only intended to work on a single plane through the planet - the pic of the planet is supposed to show where it is, not how space-time bends around it. This is why the 2D one looks sort-of OK as a representation of the geodesic. Remove the cute pic of the planet and put x-y-x axes on it instead.

    Of course - the representation could be anything - none of the pics have labelled what the grids are supposed to be: we are just guessing. Perhaps it's a spacial representation of the chance of finding intelligent life at the coordinates?

    The point of this sort of picture is to give an idea of how geometry can lead to something like the behavior a layman would expect .... but it's way off and can lead to the idea that there is a sort-of meta-gravity "outside space-time" pulling objects into the 3D hollows pressed into it by planets.

    But lets face it - the primary purpose of a TV show is not to provide accurate information and imagery, the primary purpose of a TV show is to [strike]make money[/strike] be entertaining enough so people will watch it ... creating an audience, with somewhat predictable demographics, to advertise to. The purpose of the show is to sell advertising. The station's profit is the difference between the cost of bringing you the show and what they can sell the advert slots for.

    The common graphics are used because they are popular with the target audiences.
    Last edited: Jan 17, 2012
  21. Jan 17, 2012 #20
    Yes, excelent, now we're getting somewhere!

    However, I would like to stress that the lines posted previously in my upper image are supposed to represent the spacetime web distorting on the gravitation of a single body, in this case: the earth!
  22. Jan 17, 2012 #21
    #Thomas#'s contention, though, and I agree with him, is that this illustration actually confuses the target audience. It's used over and over without actually being popular or enlightening.
  23. Jan 17, 2012 #22

    Simon Bridge

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    What does "space-time web" mean? Do they tell you?
    I agree it confuses the target audience.
    The question was "why is it used when experts are available" (paraphrase)
    We know why:
    It is not cost effective to get it right - for the kinds of reasons referred to by others.
    Anyway - TV stations are not in the business of making hard ideas clear to people.

    Now if that was in college text book...
  24. Jan 17, 2012 #23
    It is. I just walked three blocks down to the public library and found it in Halliday/Resnick/Walker, which is a well respected, widely used text. I also found it in a 'self teaching guide' called "Advanced Physics Demystified, sort of a "for dummies", but intended to be rigorous rather than to entertain.

    It's also been discussed here, at PF, as a proper depiction (if you understand it properly):

  25. Jan 17, 2012 #24

    Simon Bridge

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    Then that wasn't the question I was responding to and you'd have to look very carefully at what is being claimed, exactly. And I stand by my reply in post #2 ... it is a simplification for people starting out to get an idea without getting the idea.

    The challenge is to improve on that description at the level that text is speaking to.

    I'll bet the same text does a bit of SR but no GR and reinforces the planetary model of the atom and other simplifications? Would you have more accurate versions of these also added to the same text?

    There are constraints on college texts too.

    (Personally I'd be disappointed if anything more than a 101 course is taught from that book. Yeah it causes confusions but what can you do? The trick is to try for a better description at the level, and remember to apply the same standard to the other sections as well.)

    The link you post explicitly spells out that the rubber-sheet picture is an analogy.
    To be useful it has to be understood properly - like any analogy.
    (Or any idea at all really.)
    What's wrong with that?
  26. Jan 17, 2012 #25
    Study linear algebra, differential and integral multivariable calculus and differential geometry and then you're ok to understand how the model really works :wink: Do you see now why these theories appear simplified?
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