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Comics I Don’t Understand » Gravity

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Gravity

on Dec 11th 2017

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dec11-arlo-gravity.gif

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Uncategorized | 24 responses so far

24 Responses to “Gravity”

  1. Winter Wallaby Dec 11th 2017 at 12:34 am 1

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    This made me (literally) LOL.

    I don’t have an explanation as to why he’s doing it, but in the final panel, she’s saying “You want to try that on the sofa?” meaning “please do that somewhere else,” and he’s interpreting it as “to do a good experiment, you should do that in several different places.” I enjoyed panel 4, but my LOL came from the visuals in panel 3.

  2. Stan Dec 11th 2017 at 12:38 am 2

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    I’m not sure the results would be the same. The sheets in the third panel are not a terrible illustration of gravitational waves, so unless they have a cover sheet on their sofa, the analogy may fall flat.

    As for the joke or, in fact, what on Earth he’s talking about, I have no idea.

  3. larK Dec 11th 2017 at 12:52 am 3

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    Unlike Stan, I think I know what he’s talking about, just not why, and I think Janis doesn’t care what or why, just that if he keeps it up, he’s sleeping on the sofa. Arlo seems to be so into talking about gravity (why?) that he doesn’t even hear the threat in her response, and just assumes she’s as interested in understanding gravity as he is by, as Winter Wallaby suggests, doing another trial in a slightly different environment. But then, if he really is caught up in such a frenzy of scientific enquiry — and missing the bold threat of sleeping in the dog house and no nooky (this is ARLO!) — then he should be open to the possibility that in fact the results won’t be the same…

    Yeah, I don’t get it.

  4. Kevin Dec 11th 2017 at 12:57 am 4

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    He is trying to show how gravity is the deformation of space around a mass (planet, start or black hole). He is not jumping up and down but only jumping once to cause the mattress to deform downward. If they has a foam mattress he could have just stood on the mattress to illustrate his point.

  5. Treesong Dec 11th 2017 at 01:41 am 5

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    In case anybody reading this hasn’t encountered it, the usual gravitational analogy of mass telling space how to curve is a bowling ball on a rubber sheet, causing things on the sheet to tend to move toward the depression caused by the bowling ball. Jumping on the sheet to simulate gravity waves is stretching the analogy to the limit, since tremendous masses in fast motion, as in supernova collapse, are quite rare and irrelevant to the gravitational effects we’re familiar with.

  6. larK Dec 11th 2017 at 03:12 am 6

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    I’ve always found the bowling ball on the rubber sheet a totally useless and circular analogy: to try and grasp what gravity is like, you need to use your intuitive understanding of — gravity! The depression caused by the weight of the bowling ball is supposed to explain gravity, but the depression, and the motion of other balls along the curves of the depression, are all caused by gravity to begin with, so how does balls rolling around a circular depression under the influence of gravity explain gravity in any meaningful way? Apparently gravity is caused by gravity…

  7. Cidu Bill Dec 11th 2017 at 03:26 am 7

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    The comic is indeed here not because of the science, because Why on earth is he doing this?

  8. Kamino Neko Dec 11th 2017 at 06:02 am 8

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    Getting really creative with bedroom roleplay?

  9. Cidu Bill Dec 11th 2017 at 06:13 am 9

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    “And how’s THAT working out for you, Arlo?”

  10. Kilby Dec 11th 2017 at 06:16 am 10

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    @ larK (5) - Having seen a number of good computer animations, I was always able to understand the “rubber sheet” analogy as a theoretical (mathematical) concept, but just a few weeks ago I saw a practical demonstration at a planetarium in Berlin. The circular “sheet” was about 5 feet in diameter, and was made of a very elastic fabric. The guide demonstrated the effects with one or two steel balls as “planetary” weights, and invited the kids to put marbles into “orbit” (as moons or satellites). This worked very well (especially considering the ages of most of the participants), and I was even able to launch two or three balls that convinced me that the “s-curve” orbit (Earth to Moon) could work with this setup.

    P.S. As for the comic, the only credible explanation that I can come up with (changing over to a different sort of “body” for the experimental surface) would take this way to far into Arlo terrritory.

  11. Powers Dec 11th 2017 at 10:14 am 11

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    larK: The “rubber sheet” analogy is intended to allow people to analogize 4-dimensional warping of 3-D space using a 3-dimensional warping of the 2-dimensional sheet. Gravity is an inherent part of the demonstration. The lower-dimension model cleverly leverages the Earth’s gravitational field to simulate the effects of gravity on planetary bodies at a small scale.

  12. Soup Dragon Dec 11th 2017 at 02:40 pm 12

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    Lark: Have you ever seen a plastic ball-and-rod molecular model? You might object that the plastic in the model was also made from atoms and molecules, making them useless as a model. But we can still use the model to illustrate some aspects of chemical bonding.

    Imagine a large sphere (”the sun”) lying on the rubber sheet, making a deep depression. Toss a small sphere (”a comet”) onto the sheet, so it will be rolling not directly against the sun. Rather than go in a straight line, the comet will trace an elliptical curve around the sun (if thrown with the appropriate speed and angle).

    Now imagine the sheet is invisible. Actually, you never saw it.

    As you would only see the elliptical curve, you might be tempted to think there is an attractive force working between the two spheres. Indeed, that is how we have explained these phenomena for centuries. The point of this model is to show that the object (comet) simply follows a “banked road” through space, the banking (curvature) being caused by the large object. There is no attractive force working between the two spheres!

    No model is perfect. Apart from using-gravity-to-explain-gravity, you must also ignore the fact that the comet needs to roll (Hi, Keera!), and to roll without friction, for this experiment to work.

  13. Winter Wallaby Dec 11th 2017 at 02:42 pm 13

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    larK #6: I understand your concerns about the rubber sheet analogy, but consider the alternative: https://xkcd.com/895/

  14. larK Dec 11th 2017 at 03:24 pm 14

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    Thanks for the xkcd Winter Wallaby!

    To al the others: I was a physics major back in undegrad; I came to realize I visualize/understand things in a minority thinking style — when it works, it allows me to deeply and intuitively grasp concepts and play with them and come to other conclusions about them in what seem to me to be “obvious” ways, but when it doesn’t work, I simply don’t really understand something. I don’t “intuitively” understand equations, I need to tediously work my way through equations, and translate them into my way of thinking, but it is a lot of work. Since my learning /understanding style is a minority style, most teaching is catered for the majority style, and I suffer. Since I am not a genius, it takes me a lot of work to translate the standard stuff to my way of thinking, so I just can’t keep up, and it’s not fun. This is why I gave up physics.

    This rubber sheet model has always bothered me, because it seems to be in my style of thinking, but it is really not — I can’t manipulate this model to intuitively understand anything. The very fact that it requires gravity in a fundamental way to explain gravity (unlike the example given of molecules being used in models of molecules — that is not fundamental to the model, as I can draw the model on a computer screen, or even just visualize it in my mind and manipulate it there, no use of molecules needed) makes it meaningless to me. I’m better off in these instances slogging my way through the equations.

    ( An example of how I came to this realization was the Coriolis force — in class the professor derived a bunch of equations, and then answered questions by plugging into the equations — and I could manipulate the equations like anyone else, but I had no intuitive overview of what was happening; it was basically a black box — plug in one thing, get out another. And yet, quite apart from the equations, I had my own fundamental understanding and internal model of the Coriolis force that allowed me to “see” the interactions, and I could plug in inputs and intuitively tell you what the output would be — not the exact to the decimal place answer, but immediately tell you which way the forces would point in which hemisphere under which rotation. I spent a lot of time with the professor that day after class and we worked our way through my way of understanding and mapping it to the equations, and in that instance, yes, I got a nice full understanding, I knew then intuitively which parts of the equations did which things my mental model did, and how they inter-related, but it was a lot of time and effort — and I could already before all this answer practical questions about the Coriolis force! I already had mastery of the subject, I just needed to tediously map it to another way of solving the problem, the majority way. (And yes, I could of course just plug in numbers to an equation and grind them out, but you will not become a very good physicist just doing that.)

  15. James Pollock Dec 11th 2017 at 03:46 pm 15

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    The biggest challenge of physics is how many of the fundamental rules don’t match the intuitive understanding of them, because we evolved at the bottom of a gravity well near the boundaries of the liquid/solid components and the gaseous.
    This leads to things like intelligent, educated people lecturing Robert Goddard that rockets can’t possibly work in outer space.

  16. Cidu Bill Dec 11th 2017 at 05:14 pm 16

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    Goddard should have relished the chance to say “And yet it moves.”

  17. Brian in STL Dec 11th 2017 at 05:46 pm 17

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    Well, no. Goddard’s papers regarding space flight were theoretical. He had not produced a rocket that came anywhere close to leaving Earth’s atmosphere. There was considerable opposition to the idea of the rockets operating in vacuum due to a fundamental failure by many to understand Newton’s action/reaction law. The common thought was that rockets worked by pushing against the atmosphere. There was a famous NY Times editorial lambasting some of Goddard’s ideas. He regretted even mentioning some of the ideas, like sending a rocket to the Moon, because people focused on that rather than the things he really wanted to do.

  18. James Pollock Dec 11th 2017 at 06:10 pm 18

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    “Well, no.”

    To what are you objecting, exactly?

  19. Kilby Dec 11th 2017 at 06:37 pm 19

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    @ larK (14) - Even though I ran into them in a number of undergrad courses, I never understood “Green’s functions”, so I was hopelessly unable to actually do any work with the stupid things. Later, I had the glorious pleasure to watch a professor who managed to explain them in a single lecture hour of a grad-school mathematics course, and managed to give me an intuitive feel for what they could be used for. Alas, the feeling was short-lived, but I still remember that I did understand them at least for that brief moment.

  20. Mark in Boston Dec 11th 2017 at 09:14 pm 20

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    This is interesting because I just read an article in Communications of the ACM entitled “Metaphors We Compute By” (Alvaro Videla, CACM October 2017, Vol. 60 No. 10, page 42). “Consider the following example. Say you could program a computer to command other computers to perform tasks, respecting their arrival order. This description is already difficult to understand. On the other hand, you could describe the solution by describing a queue server that assigns jobs to workers based on a first come first served queue discipline.”

    Additionally, in software engineering it is very important to specify how a proposed system should behave, not how it should be implemented. The rubber sheet model does not claim to show how gravity is implemented; it shows how it behaves. Note that the behavior of the balls conforms to Newton’s laws just as well as it conforms to Einstein’s laws. I admit that it is a closer analogy to curved space-time than to Newton, but I also point out that it does absolutely nothing to show or explain differences observed from different frames of reference, like time dilation or the Fitzgerald contraction.

  21. Meryl A Dec 13th 2017 at 03:07 am 21

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    How about the “guest” room - surely son’s room is still there and set up for use when his family visits?

  22. Terrence Feenstra Dec 14th 2017 at 09:43 am 22

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    I would be “bouncing” on Janis. Sorry, but this is an Arlo.

  23. Terrence Feenstra Dec 15th 2017 at 09:46 am 23

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    “Spacetime tells matter how to move; matter tells spacetime how to curve.”

    John Wheeler - theoretical physicist. One smart fellow.

  24. by jiminy Dec 28th 2017 at 11:51 pm 24

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    I think what Arlo is trying to demonstrate is not gravity, but gravity waves. If he was just talking about gravity is a deformation of space, he could stand still. By jumping and sending out ripples, he is demonstrating how gravitational waves are sent out through space from massive changes. It’s a topic that’s been in the news lately because gravitational waves have been detected.

    As for why it’s funny, that’s another matter. Personally I would say it’s not that funny, but it’s pleasant. Arlo is really into the subject and Janis isn’t, so there’s a little relationship humor; he doesn’t understand how someone could not share his obsession, so when she asks him to leave, he assumes it’s to do the demonstration on another surface. And some character humor, in that he is being silly and thinks it is deadly serious. I have no problem withe the strip, but I think it’s more an homage to a great physics discovery than a hilarious joke. And that’s fine.

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