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Extra Dimensions

  1. Oct 18, 2015 #1
    Large Extra Dimensions

    I watched a video with Brian Greene explaining string theory and he said that extra dimensions may be curled up very tiny and that's why we don't see them.

    Is it possible that some extra dimensions may be very large but we don't see them or are all of them very tiny?
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  3. Oct 18, 2015 #2


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  4. Oct 18, 2015 #3
    “Using universal extra dimensions to explain dark matter yields an upper limit on the compactification scale of several TeV.”


    “to explain dark matter” is exactly what I was thinking, but I don't understand the compactification thing, it sounds like the LED thing doesn't work or something is that right?
  5. Oct 18, 2015 #4


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    Large respect to the Planck scale, of course still small respect to the world, abut hopely big enough to alter the exponent of a inverse-square force.
  6. Oct 18, 2015 #5


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    If it was a Brian Greene video then I can predict with some confidence that, whatever it does, it does not explain string theory. That doesn't mean there's anything wrong with it, just that you have to understand its limitations - you won't learn any physics from it, you'll just learn how people who do understand the physics talk about it to people who want an idea of what the physics is about without actually learning it.
    No. At the level of precision and rigor that you're getting from a pop-sci video, we would wave our hands and say that the dimensions we are aware of are already as large as any dimension can be - and that's why we're aware of them. The most accessible real answer that I know of is given as part of an example in Hartle's textbook on general relativity; it's pitched at a level appropriate for an undergraduate physics major.
  7. Oct 18, 2015 #6
    Nugatory, do you think that Brian Greene, Lawrence Krauss, Carl Sagan, Jacob Bronowksi, Neil deGrasse Tyson, Michio Kaku and other reputable scientists making documentaries are doing the wrong thing? Even if I don't really understand all the technicalities at least I can learn the history and as far as me not really learning any physics I have at least learned enough to understand how profoundly amazing our universe really is. I imagine if I did understand all of it, it would just get even more amazing.
  8. Oct 18, 2015 #7


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    I do not; I said that there's nothing wrong with it as long as you understand the limitations of what they're doing. If we didn't have popularizations we'd be stuck in all-or-nothing world where people would have nothing if they didn't have the time and inclination to put years into earning the mathematical price of admission. Even then, some popularizers are better than others - Feynman's "QED: The strange theory of light and matter" is a classic, and although Einstein's popular writings are now sadly dated they are striking for clarity without oversimplification.

    You are so right.... It's the difference between looking at a photograph of a delicious meal in a cookbook, and cooking the meal for yourself, feeling and tasting it.
  9. Oct 22, 2015 #8
    There are good and bad teachers at every level. There are good and bad elementary school teachers. There are good and bad junior high school teachers. There are good and bad high school teachers. There are good and bad physics professors at the undergraduate level. There are good and bad physics professors at the graduate level. Being a popular science writer is like being a teacher, in that your goal is to educate the audience, and like other teachers, some are good, and some are bad. However, there is a huge difference between a popular science writer, and the other types of teachers I listed. All of the other types of teachers I listed, require that the teacher has a specific college degree, meet a variety of requirements, be actually hired by a school, after which their performance is constantly evaluated. On the other hand, anyone can call themselves a popular science writer. Anyone can write what they claim is popular science, without having the slightest clue what they are talking about. In other words, unlike the other types of teachers I listed, there is no built in mechanism to weed out the really bad ones. Surely, the student, in this case, the member of the public wanting to learn, without any scientific background themselves, has no way of evaluating the comparative quality of a given popular science writer. Also, another thing, is often, the intended audience, does not actually, want to learn. In a college class on quantum mechanics, the professor or the textbook writer, is trying to make quantum mechanics seem as intuitive as possible. On the other hand, a popular science writer intentionally tries to make quantum mechanics seem as counter intuitive as possible, because they are catering to a specific niche audience that enjoys being freaked out by supposed quantum weirdness.

    Lastly, in regards to the cooking metaphor, but the world class chef and the restaurant patron obtain equal benefit to eating the meal, specifically the enjoyment of the taste and the nutritional value. However, the physicist working on string theory, and a member of the public watching a tv show about it, do not obtain equal benefit from a recent paper on a string theory, which is their understanding of the Universe. Not knowing how a meal was made does not reduce your benefit from it, but not knowing how physics theory was made, does greatly reduce your benefit from of it.
  10. Oct 22, 2015 #9
    What is the purpose of your post?
  11. Oct 24, 2015 #10


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    Depends on what you define as "wrong"... misguiding someone can be wrong...
  12. Oct 24, 2015 #11
    Do you think that Brian Greene, Lawrence Krauss, Carl Sagan, Jacob Bronowksi, Neil deGrasse Tyson and, Michio Kaku has misguided me?
  13. Oct 24, 2015 #12
    What got me started thinking about this was a lecture by Neil deGrasse Tyson where he said one of the ideas that he thought was cool was that what we call dark matter could be the effect of another universe in the multi-verse influencing our universe. So if there were a brain next to ours that could influence our universes gravity without being a part of our universe that could be the source of the gravity effects that we see in our own universe that we call dark matter.

    That got me thinking about large extra dimensions, what if one or more large dimensions of the eleven dimensional universe exists that it could be influencing what we see in our visible universe. Since gravitons can exist in all eleven dimensions maybe there is one (or more) unseen large extra dimension that is somehow creating the gravitons that we see the effects of in our visible universe.
  14. Oct 25, 2015 #13
    Could dark matter not be a substance?

    What happens if we can't find a dark matter particle?
  15. Oct 25, 2015 #14


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    Yes... Or more precisely, the place they've guided you to isn't what you think it is.
  16. Oct 25, 2015 #15
    Wow, so would you say that people in the general public that support and are interested in science don't understand what's really going on at all?
  17. Oct 25, 2015 #16


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    No, I would not say that. Good popularizations provide a reasonable sense of what's going on, and they are invaluable for people who are interested but not interested enough to put in the full-time no-kidding hard work that it takes to be a player on the field instead of a spectator in the stands.

    What they do not do is provide a foundation on which you can build new ideas. For that, you need the complete mathematical formalism.
  18. Oct 26, 2015 #17
    I have been shocked by some of the inaccurate and incorrect things that Kaku has said in popular video's. He has done a real disservice to the uninitiated who are really trying to learn correct physical concepts. I have found that Tyson tries to stay much more accurate and avoid going over the edge.

  19. Oct 28, 2015 #18
    This is very much outside the mainstream view! The vast majority of physicists say that the dark matter is a variety of particles, within this universe, not yet identified, perhaps, but not limited to, the lightest supersymmetric particle. When you said "brain", you meant "brane", which is short for membrane, which you could imagine as a universe, within higher dimensional space, called the bulk. This is the problem I references earlier, where the popular science writers try to say something that sounds as shocking and outrageous as possible, because they are catering to a specific audience that wants that feeling of "Wow! Gee Whiz! That's amazing! Who would have thought?" which is a different than someone who actually wants to understand the physics. If you were really trying to explain dark matter, you could talk about the rotation curves of galaxies, gravitational lensing, and suggest some reasonable mainstream explanations, saying it's probably particles we have not yet identified, perhaps supersymmetric particles, and you would not say "dark matter could be the effect of another universe in the multi-verse influencing our universe". That's what you would say if you were trying to amaze the audience that wanted to be amazed but not what you would say if you were trying to teach the audience that actually wanted to learn the truth.
  20. Oct 30, 2015 #19


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    I second all of that.
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