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Thread: Ep. 102: Gravity

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  1. #1
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    Ep. 102: Gravity

    You seem to like a nice series, so here's a new one we've been thinking about. Over the course of the next 4 weeks, we're going to cover each of the basic forces in the Universe. And this week, we're going to start with gravity; the force you're most familiar with. Gravity happens when masses attract one another, and we can calculate its effect with exquisite precision. But you might be surprised to know that scientists have no idea why gravity happens.

    More...

  2. #2
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    Sounds like a great one to me. Do experts not know why gravity happens, how gravity happens or both?

  3. #3
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    I look forward to every episode...
    I found your podcast at the end of May and listened to the 90 show backlog until mid-June when i caught up.
    Going a whole week without hearing you two was rough.
    I started going through the old episodes like a drunk getting the last drops out of old bottles
    You two crack me up sometimes (i lost it at 20:30 to 20:50 this episode)
    My favorite quotes from this episode:

    Pamela: "No. no. no. no. no. not gonna happen."
    Frasier: "ok fine. ok fine!"

    yeah Frasier ... better stay away from that line of questioning
    and you guys are serious about it too

    [edit: forgot my orignal reason for posting]
    When gravity in space is described like a rubber sheet ... well it seems misleading.
    If you consider the sheet ... well, it just takes me away from (what seems to me) the natural multidimentional aspect of gravity.
    I guess it is to help visualize the concept, but ..
    Don't most people think of gravity pretty much as it is described by its effects?
    Isn't it a better analogy to just say everything pulls on everything else, like magnets, but with no North or South poles?
    Or is there an aspect that is better imagined by using the 'rubber-sheet' analogy, and I am missing it?
    (I get the idea , i think, but then i have to think .. the rubber sheet is in every direction .., and that just fills up my brain )
    Last edited by plc222; 2008-Aug-21 at 10:41 PM. Reason: Forgot my original comment

  4. #4
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    Quote Originally Posted by plc222 View Post
    I look forward to every episode...
    I found your podcast at the end of May and listened to the 90 show backlog until mid-June when i caught up.
    Going a whole week without hearing you two was rough.
    I started going through the old episodes like a drunk getting the last drops out of old bottles
    You two crack me up sometimes (i lost it at 20:30 to 20:50 this episode)
    My favorite quotes from this episode:

    Pamela: "No. no. no. no. no. not gonna happen."
    Frasier: "ok fine. ok fine!"

    yeah Frasier ... better stay away from that line of questioning
    and you guys are serious about it too

    [edit: forgot my orignal reason for posting]
    When gravity in space is described like a rubber sheet ... well it seems misleading.
    If you consider the sheet ... well, it just takes me away from (what seems to me) the natural multidimentional aspect of gravity.
    I guess it is to help visualize the concept, but ..
    Don't most people think of gravity pretty much as it is described by its effects?
    Isn't it a better analogy to just say everything pulls on everything else, like magnets, but with no North or South poles?
    Or is there an aspect that is better imagined by using the 'rubber-sheet' analogy, and I am missing it?
    (I get the idea , i think, but then i have to think .. the rubber sheet is in every direction .., and that just fills up my brain )
    Hey there Plc222,

    The rubber sheet analogy is fine, as far as it goes. Scientists call them 'embedding diagrams' and they can teach us interesting stuff about gravity and space and time, more so than just the basic 'rubber sheet' job.

    And yes, you can just say gravity pulls things, if you choose. That's basically what classical Newtonian gravity tells us, and was good enough to get astronauts to the moon and back. Apparently NASA mostly uses Newton's gravity to this day.

    Try Kip Thorne's Black Holes and Time Warps : Einstein's Outrageous Legacy. I got it from my local library and couldn't put it down. It starts with basic gravitation (lots of embedding diagrams) and goes on to more advanced gravitation, using the history of black hole research to tell the story. There's lots of great stuff on how physics got done, like Oppenheimer, The Bomb, The Iron Curtain... riveting stuff. Only the last chapter was a bit hard going (wormholes and other super advanced stuff - ouch!).

    Another great read is Michio Kaku's Einstein's Cosmos : How Albert Einstein's Vision Transformed Our Understanding of Space and Time. Basically it's Kaku's homage to Einstein's life and work, and tells the story of how relativity grew from the work of Newton, Maxwell et al. including some poignant stories about how WWII and Nazism affected Einstein and those around him. Also from my local library.

    Thorne and Kaku are both noted physicists in their own right. Hope you like 'em.

  5. #5
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    The rubber sheet analogy is indeed misleading. If you use gravity to bend the sheet, then there is no reason for the small ball to roll down to the large one. You cannot use gravity; you have already expended that to bend the sheet.

    There is something else about this episode that bothers me. Einstein told us that gravity is not a force, but rather a change in the shape of distance and time. Gravity would be a reduction in spacetime induced by proximity to a large mass. We have tried for almost a century to prove him wrong, but in failing to do so, we have made his pronouncement stronger. Yet this episode, like everyone else talking about gravity, continues to call it a force.

  6. #6
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    I have a couple of dumb questions that I am hoping that someone could help answering. One has plagued me for a while and the other just came up with this last episode.

    The first question is, “Why do objects tend to orbit on a plane?” Shouldn’t a round object generate an equal force of gravity in all directions?

    The other question is that if gravitons exist shouldn’t we be seeing a change in the object creating them? What I mean is that with the sun it is constantly sloughing off energy in the way of light and heat over time it will eventually run out of energy. If a rock out in space is also sloughing off energy in the way of gravitons wouldn’t we see a change in mass or something? You can’t just hemorrhage energy and not have some sort of side effect, right?

  7. #7
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    Black holes and fundamental forces

    It's often stated that light (as mediated by photons) cannot escape black holes, but why is the same not true of gravity (if we describe it as mediated by gravitons)? Observationally, they do exert gravity and do not emit light, but what accounts for that?

    Now imagine I have collected a bunch of positive ions, sneak up to the event horizon of a black hole, and throw them in. Does the black hole 'now' have a positive electromagnetic charge? Even though photons cannot be exchanged?

    I imagine the answers to these and similar questions require precise relativistic framing (and the questions themselves need to be framed more precisely no doubt). I think I once heard an answer for my first question, but I don't remember how that goes now.

  8. #8
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    Quote Originally Posted by TSemple View Post
    It's often stated that light (as mediated by photons) cannot escape black holes, but why is the same not true of gravity (if we describe it as mediated by gravitons)? Observationally, they do exert gravity and do not emit light, but what accounts for that?

    Now imagine I have collected a bunch of positive ions, sneak up to the event horizon of a black hole, and throw them in. Does the black hole 'now' have a positive electromagnetic charge? Even though photons cannot be exchanged?

    I imagine the answers to these and similar questions require precise relativistic framing (and the questions themselves need to be framed more precisely no doubt). I think I once heard an answer for my first question, but I don't remember how that goes now.
    Hi there

    I'm sure you'd love Kip Thorne's book as much as I did - see last post.

  9. #9
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    Quote Originally Posted by Shibboleth View Post
    I have a couple of dumb questions that I am hoping that someone could help answering. One has plagued me for a while and the other just came up with this last episode.

    The first question is, “Why do objects tend to orbit on a plane?” Shouldn’t a round object generate an equal force of gravity in all directions?

    The other question is that if gravitons exist shouldn’t we be seeing a change in the object creating them? What I mean is that with the sun it is constantly sloughing off energy in the way of light and heat over time it will eventually run out of energy. If a rock out in space is also sloughing off energy in the way of gravitons wouldn’t we see a change in mass or something? You can’t just hemorrhage energy and not have some sort of side effect, right?
    Maybe try looking up gravitational waves - they are predicted by relativity, and although never detected directly, have been indirectly observed in the orbits of binary neutron stars, whereby radiating gravitational waves causes the neutron stars to slow a bit (lose energy) so that they inspiral towards each other. That might be a bit of a rough description 'cos I haven't got any of the books with me at the moment - but it'll set you in the right direction...

    Advanced LIGO is a ground experiment that is seeking to directly observe gravitational waves, and LISA is the space version that hopes to 'see' the big bang directly.

    EDIT: some one might explain this better, but gravity is equal in all directions. If you're thinking about how planets tend to form in a plane (like here in the solar system) I think it's all about the initial conditions. The solar system formed from diffuse gas that began to clump under it's own gravity, the clump tends to spin to conserve the angular momemtum of the infalling gas, like a skater pulling her arms in, and the clump tends to 'bulge' and flatten out. Even so, Pluto does not orbit in the plane of the other planets. (Pluto is more like an asteroid than a planet anyway.)

    That's so rough an explanation I hesitate to even put it up. So take it with a grain of salt. But it'll at least get everyone thinking while we try to improve it!

  10. #10
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    Try this from the Astronomy Department at Cornell University:

    Q."Why do all the planets orbit in the same plane? I am a novice astronomer and have been enjoying learning about the stars and the local universe. I have noticed that the orbits of the planets are relatively planar (with the exception of Pluto). I'm curious why this is. Are the orbits planar because of the gravitational pull the planets have on each other (I envision the creation of the solar system with erratic orbits all dampening to the center of mass). I don't know if there is a "proven" answer to the question so I am interested in some of the prevailing theories."

    A."The orbits of the planets are coplanar because during the Solar System's formation, the planets formed out of a disk of dust which surrounded the Sun. Because that disk of dust was a disk, all in a plane, all of the planets formed in a plane as well. Rings and disks are common in astronomy. When a cloud collapses, the conservation of angular momentum amplifies any initial tiny spin of the cloud. As the cloud spins faster and faster, it collapses into a disk, which is the maximal balance between gravitational collapse and centrifugal force created by rapid spin. The result is the coplanar planets, the thin disks of spiral galaxies, and the accretion disks around black holes."

    Dave Kornreich. Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California

    So I'll give myself a pat on the back - my amateur effort wasn't too bad.

    I even wanted to mention centrifugal force, but wasn't sure.

    Here's a link to the site:
    http://curious.astro.cornell.edu/index.php

  11. #11
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    Here's some more good stuff from Dave Kornreich:

    Q."Have gravitational waves been proven to exist 100%? Or are astrophysicists only about 99.9% sure that they are real? I read in a book by Timothy Ferris that some people from Princeton proved pretty much beyond a shadow of a doubt that they exist, but I wanted to make sure. Thank you for your time."

    A."Gravitational waves are predicted by General Relativity, which is one of the most supported theories in science. Nevertheless, they have never actually been detected, mostly because of the difficulty involved in the endeavor. Astronomers do watch the orbits of close binary pulsars, which are predicted to emit gravitational waves, and find that the orbits decay at exactly the correct rate predicted by the theory of gravitational radiation and have no other explanation available. So that is the best evidence that gravitational waves exist, and is probably what you were reading about. It's pretty good evidence. My money is on their existence. I also want to point out that nothing in science is proven 100%, beyond a shadow of a doubt. The best you can do is to say that something is "so well supported that it would be perverse to deny it." Even such "facts," however, have been falsified from time to time. All scientific findings are tentative by their very nature."



    Q."In 1991 I posed the following question to David Schramm while we were on a solar eclipse trip. He did not blink an eye in answering but I really could not follow his rationale:

    If a photon is the carrier of the electromagnetic force and a graviton is supposedly the carrier of the gravitational force, how can the gradient (force) of the gravitational well of a black hole be sensed if neither photons nor gravitons can escape? If the 'information' of how massive the black hole is gone forever, how does a body know how strongly to be attracted to the hole? If I cannot feel any gravitons from the black hole why would I be pulled into orbit around it if I approached from a safe distance? Why would spacetime warp if it does not know a massive body exists next door? These questions have bothered me for 19 years now. I hypothesized that there would be such a thing as a "Fermi age/length" (not its value) when a teenager so I know that I am both very smart and very dumb but this one perplexes me."


    A."This is a very insightful question. I'd be interested to know what Dr. Schramm told you about it. In any case, the answer is somewhat subtle. I'll treat the gravitational case here, but the same general ideas apply to electromagnetism. In General Relativistic terms, the force of gravity is represented solely by the curvature of space-time around the black hole. The black hole is just a very curved portion of space-time. There is no need to "communicate" this information to the rest of the universe, because it is already there, lying in wait for a passing test particle. The field can also be thought of (in classical mechanics) as a "fossil field," generated by the continuously collapsing star which, to an outside observer, never actually crosses the event horizon due to time dilation.

    Quantum mechanically, the situation is a bit different. Note that at present no quantum theory of gravity exists, but we do know some of the properties that it must have. In the quantum universe, forces are mediated by virtual bosons, like photons and gravitons. The key here is that these particles are virtual, not real. They exist of course, so in that strict sense they are real, and en masse can be detected and measured. However, they are the physical manefestations of the Uncertainty Principle, and as such live for only a very short time and cannot be detected individually.

    You may have heard about how space is a "frothing sea" of elementary particles, with particles and antiparticles continuously popping into and out of existance. That would be another manifestation of virtual particles. Virtual particles are essentially allowed to do anything at all short of violating causality during their lifetimes. They necessarily violate conservation of energy just by existing, and they're also allowed to violate many other physical laws before they disappear. One of these is the restriction on travelling slower than the speed of light. So the virtual gravitons and photons speed away from the singularity faster than light until they're beyond the event horizon, from whence they spread out into the universe to carry on their virtual business. Various physical processes conspire to insure that no actual information is transmitted superluminally; that is, if the black hole were to instantly disappear, its gravity would still be felt outside a radius ct from the singularity."



    This last one is from Lisa Wei, on the same site.

    Q."What is a graviton? Where can it be found?"

    A."According to Weisstein's world of Physics, a graviton is "a theoretical particle having no mass and no charge that carries the gravitational force." In quantum mechanics, we know that light comes to us in massless packets, or particles, called photons, which carries electromagnetic force with it. Because this worked so well and is essential to the theory of quantum mechanics, people hypothesized the existence of a similar particle which carries the gravitational force. As of today (June 4, 2003) we have not found proof that the graviton exists. Because these particles carry very little energy, they are very hard to detect - but we're still looking!"

    Lisa graduated from Cornell in May 2004 with a Bachelors in Astronomy. While here she studied frost streaks on Mars and the substructures in the Virgo Cluster. She is now a graduate student in Astronomy at the University of Maryland.

    Sorry for such a long post, hope y'all don't mind, but I thought some of you might be interested.

  12. #12
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    If the universe was a piece of graph paper, and expansion is forever making the lines move further apart, is the "space" I take up as a person (a microscopic dot on the paper) expanding at the same rate? Are planets and galaxies expanding? I can't imagine that is true, as the expanding universe would just be filled with equally expanding matter for no noticeable change.

    What is it that keeps the space I inhabit from expanding? Gravity? Chemical bonds?

    Also, as it's understood the universe expansion is accelerating. In my imaginary graph paper universe, is the rate at which two adjacent lines move apart accelerating, or is it the net effect of many lines of graph paper's expanding that generates an overall impression that distant objects are moving away faster than closer objects in space/time?

    Thanks for your time!

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