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Effect such as gravitational waves?

  1. May 18, 2012 #1
    I'm not a physics student (I was a long time ago) but I find the General Relativity subject extremely interesting. I'm very interested in the way the (I don't even know how to call it) "fabric" (the another dimension used to represent objects' gravity/energy) behaves.

    For example when I searched what happens if two immense planets almost collide when moving fast towards each other (and their gravity fields collide), they create sort of ripples (of gravity) in that "fabric". What happens when something drastically changes it's speed all of a sudden or drastically gains energy, is the fabric "elastic"? Does a concept similar to "G" force exist when changing energy quickly?

    If anyone can point me at reading material regarding these questions or give me a short description.. Even if I'm viewing it all wrong. I couldn't find anything that answers these questions.

    Thank you very much!
  2. jcsd
  3. May 18, 2012 #2
    Welcome to PF!

    Keep in mind space isn't literally a 'fabric', that is an analogy. It's not like the curvature of space exerts a force on an object, it's that it changes its natural path through spacetime.

    To get a feel for this, imagine you have a large, flat piece of graphing paper. Now, you draw a straight line on it. Intuitively, you know what it means for something to be straight on a flat surface - that if you use that line three times to make a triangle, its angles should add up to 180°. However, this is only true for Euclidean, or flat, surfaces. To simulate the curvature of spacetime, alter the lines on the graphing paper. About half-way up, say, the lines of the graph curve slightly to the right as they go up. Now draw a straight line on this.

    In this situation, a straight line can't be thought of as Euclidean - you must remember that it means that it is the shortest distance between two points. So, to draw a straight line on this curved graph, you must curve it to right once you get to the point the graph changes.

    This is how gravity works in GR. Space, like the graphing lines on the paper, takes on curvature. Objects simply try to follow the straightest possible path, which is what leads their paths to appear curved and attracted towards other objects.

    Gravitational waves are changes in the curvature of space - imagine that an enormous star appeared 1 light year away from earth. It is so enormous, that according to Newtonian gravity, it should begin to attract the earth instantly. However, since nothing can move faster than light in relativity, the gravitational force can not reach the earth for 1 light year. This is the idea behind gravitational waves - they are the changes in the curvature of spacetime that propagate at the speed of light.
  4. May 18, 2012 #3
    Thank you Mark, for both the welcome and the reply (:

    I'll try to rephrase my question a bit so that I would perhaps get an answer closer to what I was aiming for. I might be wrong on several parts but after what I've been reading and seeing, this is how I get it. I apologize in advance, I don't mean to waste your time but I'm really curious about it.

    Although I read about it in several places, in the movie "Einstein and Eddington" (ttp://www.imdb.com/title/tt0995036/) they choose to simulate the curvatures in the way I liked the most. They were using the analogy of a table cloth, 2 people we're holding a table cloth at 4 corners, butting a something in the middle (e.g. Bread loaf) to create the curvatures and then taking an orange and throwing it around the cloth so it would circle the loaf.

    What I called the fabric (is The table cloth):
    The table cloth symbolizes the "new" axis induced by energy. This axis which is a non-negative axis. 0 symbolizes no energy/no gravity/no mass/no speed or heat (possibly theoretical non-reachable limit) and a high positive number symbolizes a concetration of mass / high temperature / velocity or any other form of energy. The same axis which is used to whenever demonstrating a black hole - without using too many words "where time moves slower (e.g. inside the black hole)" relatively of course.

    What my question aims to clarify for me is the properties of that axis. "What happens when strong gravitational fields collide", "what happens if a gravitational field is created extremely fast" are there any side effects? (This is where I gave the example of the gravitational waves). I have a pretty good mental image of how it looks statically. I'm looking for some simulation of How would it look like? How would It react? when it is going through changes.

    Again thank you for helping (:
  5. May 18, 2012 #4
    That example with the cloth is good, but the reason I tried to explain things in terms of geometry is because the analogy has some problems. For example, the cloth is curved because the loaf of bread already has a gravitational attraction to the earth, but in GR space is curved around an object. Also, the analogy is a two dimensional surface, but space is three dimensional. I see you understand, I just wanted to point that out.

    Rapid or large changes in gravitational fields are what cause gravitational waves - take for an extreme example, a collision between black holes. Enormous amounts of gravitational waves are emitted in the collision. Large astronomical events such as black hole collisions are how LIGO wishes to detect gravitational waves.

    Also, gravitational waves function as a normal radiation that give off energy over time. Similar to the way an object with an enormous electrical charge will radiate electromagnetic waves, an object with a large gravitational field slowly emits gravitational waves.

    Some searching on the internet got me this, which may give you visual you are looking for.
  6. May 18, 2012 #5
    Amazing example of gravitational waves. I'm glad I managed to explain myself correctly and thank you for looking into it.
    This example however is a bit more complex and demonstrates a lot of rules combined altogether.

    I'll follow other similar videos in this manner like you did.

    My original question still remains - Where can I find documentation of how the 'spacetime' - how this fabric behaves. Is gravitational waves the only effect visualize-able?
  7. May 18, 2012 #6
    Oh, so you mean general relativistic effects? There are plenty of those.

    For example, gravitational frame-dragging is the 'twisting' of spacetime (See for Visual.). This was confirmed by Gravity Probe B. For information on it, see here.

    There is also gravitational lensing, the curving of light around a cluster of matter. See here .
  8. May 18, 2012 #7
    Amazing! Thank you so much. I will now have a lot of reading to do :)
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