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

Calculating speed of spacetime

  1. Feb 7, 2012 #1
    I've been reading up on the topic of gravitomagnetism, and how a gravitomagnetic field is created by a particle with mass moving through space the same way how an electromagnetic field is created by a particle with charge moving through space.

    Now I have been reading up on this topic, and obviously this would be a good way to warp the contents of spacetime. But I'm a bit confused on one thing. How do you calculate the speed at which the effect causes spacetime to travel? I have been playing around with the equations for days but have been unsuccessful in determining the velocity of spacetime. Can anyone help with this?
     
  2. jcsd
  3. Feb 7, 2012 #2

    Dale

    Staff: Mentor

    Spacetime doesn't have a velocity AFAIK.
     
  4. Feb 8, 2012 #3
    I'm not really sure what you mean "the velocity of spacetime." The Maxwell-esque gravitomagnetic equations (which are a weak field approximation of general relativity) can be used to calculate the speed of gravitomagnetic waves, and it turns out they travel at c. I don't know if this is what you had in mind.
     
  5. Feb 8, 2012 #4
    Here is what I meant. Since gravity distorts spacetime, how would the a spinning ring through gravitomagnetism cause spacetime to distort? I'm assuming it would cause spacetime to move one way through the center, and the opposite direction outside the edges. The question is how quickly would it cause spacetime to move? Or what is the rate of spacetime distortion with respect to time?
     
  6. Feb 8, 2012 #5

    Dale

    Staff: Mentor

    A spinning ring would be a dipole source, and the minimum for gravitational radiation is quadrupole (i.e. a pair of orbiting masses). There is some evidence that gravitational radiation from a pair of orbiting masses travels at c.
     
  7. Feb 9, 2012 #6
    Perhaps I should rephrase the question. If a gravitomagnetic drive system, such as one studied by the ESA, were to run at a fixed power, would it keep going faster and faster or would it reach a certian speed as a function of power being fed to the device? Or is it a little of both? And how do I calculate what that is?
     
  8. Feb 9, 2012 #7

    Bill_K

    User Avatar
    Science Advisor

    You may be thinking of ion propulsion drives, often used for planetary missions. These are not gravitomagnetic. The gravitomagnetic force does not "push spacetime", it pushes matter, so you would still need reaction fuel. And even if you circulated neutron star matter around the ring at relativistic speeds, the reaction force you'd get would be far too small.
     
  9. Feb 10, 2012 #8
    I'm not talking about ion engines. Ion engines work by ionizing particles and ejected through screens given an electrical charge that pushes them out the back at a low mass rate but at a high speed.

    What I'm talking about is rotating masses that affect gravity. And it's entirely enclosed.

    Tell you what. Allow me to rephrase the question again. How much force would be created at a variable distance from the device? That and how much would the effect locally change the speed of light? (I've heard about how gravity affects light. An example of this would be black holes.)
     
  10. Feb 10, 2012 #9

    Dale

    Staff: Mentor

    Please provide a reference for this.
     
  11. Apr 24, 2012 #10
    Ok, I'll rephrase the question. There is the analogy of how when an electric curent creates an electromagnetic field? I need to know the strength of a gravitomagnetic field caused by a moving mass. Such as a stream of matter being spewed from a quasar or something. Als I would like to know how strong the gravitomagnetic field is as a function of the radius from the source.
     
  12. Apr 24, 2012 #11

    Dale

    Staff: Mentor

  13. Apr 25, 2012 #12
    I'm having a hard time understanding the material. Could you provide a worked example? Such as a given mass, a given speed, a given radius from the moving mass, and how quickly the mass is moving.
     
  14. Apr 26, 2012 #13


    It is possible to follow up on CCatalyst’s question about the velocity of spacetime? In particular I wonder what the speed of time is and how we calculate it. I’m thinking of the conclusion derived from different observations and the premise that light is constant, which implies that time for moving observers is slower than time for stationary observers.

    Speaking of the speed of time doesn’t sound odd to me if we think one person’s time moves slower than another person’s time.

    Also, what does "AFAIK" mean?
     
  15. Apr 27, 2012 #14
    its still not a clear question. are you referring to centrifugal force and how it could bend lights speed and direction?

    someone please correct me if im wrong because i hardly know anything about the topics in this forum but...

    you got it backwards. it wouldnt be a moving observer if you were experiencing it first hand. time is relative to the first person so to observers, the one with motion is moving through time faster than one who is stationary. obviously for these effects to become apparent youd need to have enough mass to bend spacetime.

    there is no speed for time. time is just t and our way of measuring it is through seconds, hours, weeks, years, etc. time does not travel, just like a line, x-y graph or x-y-z graph also dont travel. spacetime can bend but not travel

    AFAIK means as far as i know
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook