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Gravity waves?

  1. Aug 15, 2008 #1
    can someone explain gravity waves to me? if gravity is only in the presence of mass then how can gravity waves propagate through space in the absence of the presence of mass? Have gravity waves been detected or proven or is it only theory so far?
  2. jcsd
  3. Aug 19, 2008 #2
    For a time the idea of gravity waves was akin to that of dark matter and energy, i.e a mysterious thing we saw the effects of but did not see it directly. The biggest reason for this was that gravitational waves themselves are rather minute in how they affect things. That is to say that for example when the first nuclear bomb was dropped by the US during testing, the gravitational wave produced by it was so small that even if you were standing directly under the tower that dropped the bomb you would only be stretched a tiny fraction of an atomic diameter (by the gravitational wave of course, not the blast).

    A gravity wave itself has been understood to be a very much predicted thing in that Albert Einstien's equations actually predicted them. Since it's been found that almost everything is composed of a field this means that gravity itself is a field and therefore is subject to everything a field is. In a field such as a electromagnetic field where electrons are coursing up and down a transmittion tower you have electrons jumping from states of higher energy to lower and vis versa and going this way and that essentially making for a jittery environment. Apply this to a gravitational field emitating from a source like a supernova you now have gravitons flowing back and forth like ripples in a pond from the origin point. In gravity's case though these ripples are not just particles undulating through space, they are rather space-time itself warping and curving. This is a predicted thing that occurs in Einstien's theory of General Relativity and makes a great deal of sense.

    I hope that answered your question.
  4. Aug 19, 2008 #3
    Sorry I forgot to mention that we have begun work on detecting gravitational waves. The Laser Interferometer Gravitational-Wave Observatory (LIGO) being run by the California Institute of Technology and the Massachusetts Institute of Technology with generous funding by the National Science Foundation is going forward with this project. The idea is that by being able to measure the extremely small perturbations created by gravitational waves we will be able to detect their presence.

    The project consists of two hollow tubes each 4 kilometers long and a bit over a meter wide which are arranged in a giant L. When laser light in simultaniously shot down the vacuum tunnels in each tube, and reflected back by highly polished mirrors which measure the relative length of the tunnels, they will give the length at a very, very precise measurement.

    The idea is that if a gravitational wave were to roll by then then one of the tubes would be warped by an amount able to be measured by the laser measurement devices. The reason for the length of the tubes is because the longer, or more massive something is the more amplified the gravitational warping will be and easier it will be to measure the warping. Also the laser beams are bounced back and forth more than a hundred times for each run which increases accuracy and redundancy.

    So far this is still in the testing phase as they fine tune the device and increase its sensitivity. Once its put online for observations I imagine they will find the presence of gravitational waves rather quickly.
  5. Aug 19, 2008 #4
    Interesting! so it would be a ripple in the dimensions of space time without the need for an object with mass to move along with it. Would gravity waves move at the speed of light?
  6. Aug 19, 2008 #5


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  7. Aug 19, 2008 #6
    Pretty cool eh :) Based on Einstien's Theory of General Relativity these ripples actually move at the same speed of light which means for example that if our Sun were to just vanish in a split second we would not only see all light fade out 8 minutes after it happened but we would also feel the gravitational affects that exact same 8 minutes later when the light went out since thats the time it takes light to travel 1 AU.

    The next question is what underlying symmetry do light and gravity share to cause this to happen? I have theories but I've already rambled enough... :P
  8. Aug 19, 2008 #7


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  9. Sep 25, 2008 #8
    With respect to the discussion of gravity as a wave...can that wave be altered...intensified or magnified?
  10. Sep 25, 2008 #9
    Considering that our knowledge of them is so rudimentary since they are so hard to actually measure I would venture the guess that yes they would have to be. If you take the analogy of ripples in a pond converging at a point, the individual waves will create a larger and thus altered wave. The gravitational waves traversing the fabric of space-time would theoretically have to operate in a similar manner. The sum of individual waves throughout space still might not be amazing as you've spread the energy out across a decidedly larger distance than the aforementioned pond.
  11. Sep 25, 2008 #10
    A single wave in a pond disturbs the area around it...dispersing energy and creating other waves...or ripples. I envision gravity waves a little differently...perhaps traveling in unison..."stacked" on top of each other as they move farther from the point of origin...maybe flattening as they move farther?
  12. Sep 25, 2008 #11
    Interesting concept but what provides the boundary for these waves to "stack" would they not intersperse and become the sum of their parts?
  13. Sep 25, 2008 #12
    I guess that would be the "magnification" I'm interested in finding.

    Consider this...the potential for the greatest sum of mass (hence gravity) anywhere in the universe should be a straight line to infinity (through all of the mass along that line) in any direction?

    If that's reasonable, then if the force could be focused/magnified along that line...well I've long wanted to further develop the theory.
  14. Sep 26, 2008 #13


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    In the dispersal of gravity waves would be along the time dimension. Take as example the case of two neutron stars orbiting close to one another:

    As one neutron star passes through a particular point in space, it causes an increase in the gravitational curvature of that area, which equates to a "peak" in the gravity wave pattern. Once that object has passed through that area, the "absence" of a neutron star passes through the same area, causing a decrease in the gravitational curvature of that same area, this corresponding to a "trough." This peak and trough immediately begin traveling outward from their sores and lightspeed, so that the next occurrence of the presence of a neutron star forms a second peak that is separated from the first both in space and time.

    Given these conditions, it seems highly improbable that they could be "stacked."
  15. Sep 26, 2008 #14
    Let's think outside the box for a moment...I envisioned this long ago...so close your eyes and consider this...

    What if...

    What if you could focus on a specific point in space...and somehow gather all of the gravity waves flowing along that line...like a magnifying glass does for sunlight...or maybe like a tuning fork?

    Next, re-focus/aim the device onto a cluster of mass (in the general area of where the waves are headed)...and again...magnify the gravity waves.


    From the point you focused the pull (magnification) of the waves...you should achieve PUSH/PULL...as an alternative to trying to push...basically hitch a ride on the gravity wave from the middle point towards the (cluster) object of re-focus.

    If it would be possible, and if the waves travel at the speed of light (hmmm)...you should move pretty fast?

    Last thought...before someone tells me...friction should be reduced because everything riding the wave would be moving...right?

    Don't be TOO mean in your responses...I'm not a scientist...just a guy with some ideas he can't explain mathematically..
  16. Sep 27, 2008 #15


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    Sorry, didn't see the rest of the post!
  17. Sep 27, 2008 #16
    Actually, your description of the "wake" left behind was very interesting...thanks.
  18. Sep 27, 2008 #17
    Here's a question that you might want to address...another of my concepts.

    If you had a small spacecraft (pod, capsule or satellite with no propulsion system) in space...not in orbit...and you could extend/unfold a very lightweight framework...like a big umbrella...coated with a very thin and lightweight material/membrane...like a metal film (or even some of the new NanoSolar material printed on a film (for power))...and open this "umbrella" in front of the spacecraft VERY wide...maybe miles wide (don't forget we're in space...lots of room and no weight)...

    1.) How (large would it have to be and how) close to a large mass (star, planet, black hole, asteroid?) would the "umbrella" need to be in order to be affected by the gravitational pull of that object in such a way as to be pulled by that body through space?

    2.) How fast could the spacecraft move (given the width) before friction slowed/crushed/burned it...I'm assuming acceleration would increase over time?

    3.) If it was possible, and you could steer towards one object, then re-align towards another like swinging on ropes in the trees...would the "trough" that you described push or pull our spacecraft as it passed the last body of mass on it's way towards the next body of mass?

    Don't worry, I'm almost out of these ideas.
  19. Oct 14, 2008 #18
    1. You don't need an umbrella. All things are attracted towards a large mass (unless there is a larger mass closer; then the attraction is towards a balance point based on your squared distance and direction from all of them). An umbrella won't change the amount this occurs. For example only speed prevents the moon from heading straight for us.

    2. There is no friction in space due to your falling towards a large mass. This is because all things fall with your ship even if they are moving at a different speed. You might find some resistance with those moving things themselves but gravity will not change this by any amount. This is different to falling through atmosphere because the atmosphere can't fall as it already pressing (pressure) against the planet.

    3. There would not be any troughs. Even if there were such things as gravity waves the massive objects would have to change direction to make the gravity 'fluctuate' (like binary stars). Even then you will not feel these changes in your spaceship. You feel weightless at all times. Whether the gravity increases or decreases you will still feel weightless. It is not like sea waves where waves combine to create higher waves or lower troughs and you can definitely feel them. You do not feel gravity 'crests' and 'troughs'.
  20. Oct 14, 2008 #19
    Wouldn't the larger surface area of the "umbrella" yield a stronger gravitational pull(s) and thus increased acceleration?
  21. Oct 14, 2008 #20


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    No. Gravity acts on mass. Period.

    If your umbrella weighs, say, 1000kg, then it would be pulled exactly as if you used a 1000kg cube only 1m on a side.
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