Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B What keeps the space-time together?

  1. Feb 14, 2016 #1
    Ever since the proof einstein's theory of gravitational waves the whole world of physics has been literally changed. So I have a question, if space is made of something like a fabric which reacts to gravity; what keeps it together why doesn't it 'tear' if the gravity is too much. Is there a 'thing' that is holding it together? For example a piece of cloth is held together by threads, etc.

    PS. I am just a 9th grader and I know the question is quite stupid. No Hate.
  2. jcsd
  3. Feb 14, 2016 #2

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor
    2017 Award

    Space isn't made of a fabric. That's just an analogy.
  4. Feb 14, 2016 #3


    User Avatar
    Science Advisor
    Gold Member

    Hi Dipto.

    It's not that the question is stupid, but it does illustrate a major issue our minds have when dealing with analogies - we like to run with them a bit too far, whereas they are designed to help us visualise (not even as much as represent accurately!) just a particular idea or set of ideas.

    For example, we use the expression 'fabric of space-time' to help ourselves and others in visualising how relationships between points on a coordinate chart can be affected by gravity or motion. The fabric of textiles, or a rubber sheet, are tangible enough to our minds and experiences so as to allow us to better anchor the idea of stretching and curving of space-time that otherwise would be difficult to convey due to its inherent degree of abstraction.

    But we need to train our minds to stop there, and not try to misuse the analogy for purposes it wasn't meant to fulfil. All the other properties of 'a fabric' that it has have nothing to do with space-time. We did not decide to use the phrase in order to indicate that space should or could tear, or that it's made of threads, or that it's got thickness, or that it can be touched... and so on.

    As an example of how misplaced it is, consider how we could very well call any space a fabric, because it also can stretch and can be planar like a fabric, and be continuous. But space is just a way to describe relationships between positions of some points. Does it make sense to your mind to ask whether length, height and width, or being 25 km westwards and 5 km north of where you are - can tear, or ask what these are made of?

    Space-time is no different. It simply shows relationships between points in both space and time (i.e. where and when something is).

    In the end, only the abstract, mathematical representation is truthful to the subject matter. All analogies are inherently wrong, and should never by used to derive properties of the real world.
  5. Feb 14, 2016 #4


    User Avatar
    Staff Emeritus
    Science Advisor

    One of the reasons for the "fabric" model of space-time is that gravity waves are transverse waves. This is the same sort of wave when you pluck a string, and at least very close to what happens when a stone falls into the water and creates ripples on a pond. It's also the same as light waves, they are also transverse waves. This is in contrast to longitudinal waves, such as sound waves.

    The analogy between transverse waves from a pebble falling into a pond, and electromagnetic waves inspired the early ideas of the ether, the hypothetical medium whose vibrations are light waves, with the ether playing the role of the "pond" and light being a disturbance or vibration in/on the pond. But when people started looking for evidence of the existence of this hypothetical medium, it turned out to be very elusive. Eventually, after the development of special relativity, it was released that the hypothetical medium didn't result in any actual physical predictions, so people stopped taking it seriously.

    The situation with gravity is similar - we still use the analogy of the pebble dropping into a pond to describe the transverse waves, but we now view it as an easy-to-understand analogy, and don't look for any actual physical medium that vibrates - either for light, or for gravity.
  6. Feb 15, 2016 #5
    But perhaps it's useful to ask what allows any point in spacetime to transfer energy to an adjacent point, in the form of Gravitational Waves?

    When we see mechanical waves propagate through water, we know that the molecules which compose the water are mechanically colliding with each other, and thus transferring the wave's energy allowing to it move along through the water.

    What is the analogous underlying process by which a point in spacetime transfers energy to an adjacent point?

    The characteristics of water and water molecules become relevant in defining boundary limits on how much and how fast energy can be transferred from one point to another. We know that water molecules can vaporize from the liquid state to a vapor state if too much energy is given, forming bubbles.

    How do we know that some threshold limit can't be exceeded when it comes to gravitational wave energy?
    Last edited by a moderator: Feb 15, 2016
  7. Feb 15, 2016 #6


    Staff: Mentor

    There isn't one, just as there isn't one for EM waves. Spacetime is not a material medium. Trying to reason about GWs by analogy with waves in a material medium isn't a good idea.
  8. Feb 15, 2016 #7


    Staff: Mentor

    The OP question is based on a mistaken analogy, as has been explained in the responses. Thread closed.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook