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What is space?

  1. Jun 25, 2015 #1
    Hello, I'm not an academic but someone who had been watching way too many documentaries and has what is probably a very nieve question? "What is space made of?" It strikes me that in order for gravity or attraction of any sort to occur then there must be something physically linking the objects in question, whether that be energy or what? If that's right then that must be linking everything in every direction to everything else? In my simple mind it seems that if we can establish exactly what space and attraction are, how they actually work rather than just their consequences we might be better equipped to work out the big bang and beyond or is that overly simplistic and wrong? I keep hearing that gravity is a force and how it's measured but not exactly how one object attracts another. The membrane analogy doesn't seem to quite do it for me as it seems to be very 2 dimensional and doesn't adequately seem to translate into a full 3D environment, wouldn't you need infinite membranes eminating in all directive at once?
     
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  3. Jun 25, 2015 #2
    Generally space is considered vaccuum and made of nothing, conpletely empty as gravitation is zero there(considered). But in practical gravitation is zero nowhere, so space is not empty at all. (replied taking space as ordinary sense where space means = place above the atmosphere which is empty. But you can also say space holds stars, planets etc. in big sense!)
     
  4. Jun 25, 2015 #3
    And ofcourse force of gravitation
    F=G##/frac{Mm}{r2}
    When M,m are two masses and r is the distance between them. Even at infinty it is not zero, but the net force maybe zero.
    Compare, net force may be zero but the density is never. It is very less but not completely empty
     
  5. Jun 25, 2015 #4

    phinds

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    It's not. There is no need for anything in order for gravity to work because gravity is just geometry, not a force. You hear it described as a force but that's classical mechanics which works really well on small scales (say a planet) but fails miserably on large scales (billions of light years) and near massive objects such as a black hole or a neutron star. Gravity is the geometry of space-time as explained by Einstein's theory of General Relativity.

    Perhaps it will be easier for you to visualize thinking of magnetism, which IS a force. If you hold magnets close to each other you can feel the attraction or repulsion and that's true whether you do it in a vacuum or in air or in water.
     
  6. Jun 25, 2015 #5

    Further to that, if you think that what might actually carry gravitation then scientists think of graviton which acts as the carrier. But it's mostly quantum mechanics
     
  7. Jun 25, 2015 #6
    Yeah but what I'm struggling with is how does a magnet attract or repel an object, I know you can measure it, predict it and quantify the force but how dies that force actually work at a quantum level? Similarly I've just started reading introductions to quantum physics (for beginners if you like? ) and have read that at those levels there are neutrinos or whatever that seem to communicate or influence reach other even though separated by vast distances? Sorry if I sound dim, I'm not a mathematician, physicist by any stretch of the imagination. But have really been interested by the concepts demonstrated in many documentaries I've seen recently. Many expressing theories to a degree but fail to show exactly how one object reaches out to another c to attract or repel it.
     
  8. Jun 25, 2015 #7
    Doesn't gravity apt to orbits? So surely it is a fierce of sorts even if only at that level? I've seen the common illustration of a planet on a sheet and the mad of the planet causes a depression in the sheet so that nearby objects then rotate around it which is simplistic but surely the sheet has substance to hold the planets up as it were and cause the angle that allows the smaller planet to circle it, I. Each. The membrane or sheet has substance?
     
  9. Jun 25, 2015 #8

    phinds

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    The sheet is a rather poor analogy but somewhat shows the concept of space-time curvature. There IS no "substance", it's all just geometry. Google "geodesic" for further information.
     
    Last edited: Jun 25, 2015
  10. Jun 25, 2015 #9
    Thank you. This is really interesting to me even if I don't know the maths or the language to articulate my interest or thoughts. This awful predictive text doesn't help! It's just that it strikes me, in my simplistic and nieve mind, that whether at the atomic level or the planetary level of one thing orbits another them there is a force, an attraction or indeed a repulsion that holds them often in equilibrium? Something in the space between them that's just strong enough to keep them together or enough apart to maintain the orbit. I'm interested to better understand what is in the space between actually doing this, not what it's called, measured or predicted but physically what is holding the two objects?
     
    Last edited by a moderator: Jun 29, 2015
  11. Jun 25, 2015 #10

    phinds

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    It's a complicated dance. Mass tells space-time how to shape itself and space-time defines the geodesics along which stuff moves. You really need to get rid of this concept of "something in the space". I'll tell you just this one last time: there ISN'T anything there. I get that you are finding this hard to believe/comprehend, but that is the way it is.

    EDIT: by the way, don't feel bad about finding this hard to "get". Newton didn't get it and basically just gave up. He couldn't explain what caused gravity, he just figured out how it works in practical terms. It took Einstein to figure it out.

    Newtonian gravity "makes sense" because it works perfectly on the scale in which humans evolved, so it seems reasonable to us. There was never any survival value in understanding the concepts of General Relativity because they simply are not manifested in everyday life.

    Planets travel in straight lines but the problem with that terminology is that we automatically use Euclidian geometry and "straight line" has a very well defined meaning and it just doesn't look like what a "straight line" in Reimann geometry (which described space-time) is, which is why we call travel in space-time "curved" even though it's really straight (a geodesic) in the sense that matters.
     
    Last edited by a moderator: Jun 29, 2015
  12. Jun 25, 2015 #11
    alasange, I recently found this video on youtube, which I think really helps with visualizing how the warping of space, rather than some active force between two objects, is what "binds" objects with gravity together, including planetary bodies.

    https://www.youtube.com/watch?v=MTY1Kje0yLg

    This should be contrasted with something like electromagnetism, in which force-carrying particles (called bosons) are actually "emitted." The interaction of those bosons is what causes the force between them, whereas gravity merely warps the space around a massive body with no exchange of particles between it and some other massive body.

    Having said that, and also clarifying that I'm also no expert at all (just an interested layman, like yourself), I believe there is a competing theory explaining gravity. While gravity can be explained exclusively as the warping of space-time, some (I believe) claim that gravity actually is a force just like electromagnitism, etc. If this is the case, objects with mass would "emit" something called a graviton, which is also a boson like in electromagnetism, and that's what creates the force between two objects with mass.

    I don't know if this is still a viable theory in physics, or if it's even in opposition to theories about warping space-time (both theories might be fully reconcilable with each other, for all I know).

    Interestingly, the effects of gravity from a massive body "travel" at the speed of light. For example, if the sun were to disappear right now, we would not experience it for several minutes -- the same length of time it takes light to arrive from the sun. Likewise, the gravitational wave emitted from the black hole at the centre of the galaxy that keeps us bound to it is not an ongoing and immediate thing, but rather the gravity that's influencing our solar system at this very moment is actually tens of thousands of years old.

    To explain this from the force-carrying graviton perspective, it's a predictable conclusion that gravitons travel at the speed of light. For the warping of space-time theory, it could be explained by space-time warping at a certain speed. Much as in the video above, removing a marble from the flexible sheet does not return the space to its regular, flat shape immediately, but takes time. So too would adding or removing a massive body require time for space-time to return to normal (a change in space-time requires a change in both space and time -- that seems reasonable to me). This latter explanation is fully my own thought-process on the matter, not something I'm aware of being argued by scholars of the field.

    Anyway, I hope that at least gives some food for thought, though I'll again stress that I'm no expert in this.
     
  13. Jun 25, 2015 #12
    Yeah i looked up geodesics,again little on the how or as you say how mass tells space time how to be or how spacetime defines geodesics. Lots of how to measure etc. I'm being too feel that no one actually knows the actual mechanics of these or the actual mechanics of forces like electromagnetic interaction and how they actually happen, how they attract or repel. Lots of, it's a force, an exchange of particles or how to quantify it. I've read in discussions on electromagnetics that some believe it's photons in high quantities, acting in my simple terms almost like a body of water between objects? I'm going to look at maxwell equations next and go back to my books, but even they state they don't know for instance at the quantum level how particles seem to communicate or effect each other at distance, hence my starting my journey to ask questions of others who might have an understanding. Thanks for your comments though
     
  14. Jun 25, 2015 #13

    A.T.

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    Better forget it again. It's a bogus analogy, discussed many time here. For example:
    https://www.physicsforums.com/threa...visualization-of-gravity.726837/#post-4597121

    The warping of space doesn't do that. You need to consider space-time.



     
  15. Jun 25, 2015 #14
    Thanks, A.T. -- those are really useful videos!
     
  16. Jun 25, 2015 #15
    Great thanks it has, but raised so many other questions!!!! To my mind space time must have substance of some sort even if only at quantum level of it can be bent? If the bending of spacetime causes orbits then Why then do some objects move out of orbit despite the lack of obvious influence I. Each. Larger object? Example video is good but rather linear and 2D to my mind and doesn't explain how or why all systems seem to be flat in nature rather than a swirling ball of orbiting objects? Then I started thinking of black holes and wondering if they are indeed holes? All I've read seem to imply you'd fall into one and only be able to see a shaft of space as you liked over your shoulder? To my mind it makes more sense that they are 3 dimensional objects like planets but of infinite density and being drawn to one you'd still see the same view of space you had etc. My head hurts now! Lol
     
  17. Jun 25, 2015 #16

    phinds

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    The reason we call the center of a black hole a "singularity" is because "infinite density" is non-physical and indicates a place where or math models no longer describe reality. You certainly will see "infinite density" in pop-science expositions, but that's just 'cause they think "math model breaks down" is too hard to explain.
     
  18. Jun 25, 2015 #17
    Do you think that for some reason space can only tolerate a certain amount of density, that at a certain point it explodes causing a big bang? Totally divergent from my initial post I know. On that being totally uneducated in all of this and in my simplistic world view I take the mechanical view. A winch on a land rover pulls a car out of mud, the cable connects them and is what is used to pull it out. Similarly a ram rod is used to repel objects, it's the rod that carries the force to move / push the object. That said I'm trying to uneerstand what the mechanics of attraction / forces actuality are (where that be magnetic,electro magnetic or whatever). Everything I've read tells how to measure or predict such forces but not what is actually happening, are particles of some sort connected, how is the magnet attracting the steel (I read they are swapping electrons, oppositely charged etc. But if you could magnify it or see in av different spectrum or whatever, what would you actually see, how or what connects them? )
     
  19. Jun 25, 2015 #18

    phinds

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    As far as I am aware there is no reason to think so

    No one knows WHAT caused the beginning, which is why it is called the "big bang singularity". Again, "singularity" does not mean "point" it only means "the place where the math models show unphysical results and we don't know WHAT is going on".

     
  20. Jun 25, 2015 #19
    This is where the physicists point out that physics is, in fact, about predicting not explaining. I know, not very satisfying is it? Actually a lot can be done by "mere" prediction. And really if you think about it hard enough, all "explanations" demand further explanations, it seems ultimately you get to a point where it is "turtles all the way down."
     
  21. Jun 25, 2015 #20
    Actually, beyond a certain density, space collapses into a black hole. Or, if the density is high enough everywhere, the universe collapses into a big crunch. Those are the standard answers. Anything more gets you into highly speculative territory.
     
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