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

Gravity and FTL

  1. Jan 25, 2004 #1
    The earth has a gravitational pull that attracts things to it correct? If we launced a rocket from Earth and said rocket was
    traveling from the ground to space at, oh let's say, 100 miles per hour (or kilometers - it really doesn't matter). Given the slow speed of the rocket, it would never leave orbit correct? If that is the case, can we say that the gravitational influence of the earth negatively exceeds the speed of the rocket, or in other words, exceeds the speed of the rocket in the opposite direction? Obviously, the Earth's gravitational influence is not as great as something larger (such as the Sun) or something physically different (such as a black hole). Using the same idea, light being the rocket and a black hole being the Earth, can it be said that the black hole's gravitational influence negatively exceeds the velocity of light? I'm sure it's not that simple, but what I would like to know is if the speed of gravitational influence (given a large enough object) would be considered FTL?

    Jeremy
     
  2. jcsd
  3. Jan 26, 2004 #2

    turin

    User Avatar
    Homework Helper

    I would (caveat: rather ignorantly) say no. I have heard that it is believed that the speed of gravitation is c. I have heard that the supporting evidence is the upper limit on the mass of the graviton, but I have also heard that the graviton has not yet been observed, so I don't know what that would mean. At any rate, the reason I say no, is that, in the case of a BH, space-time still looks like it always does in geodesic coordinates. And that's what it comes down to: the choice of coordinates. Just because something looks wierd in a particular coordinate system (i.e. light cannot escape) doesn't mean that it is a good representation, or even true. It is just that, near a BH, space-time gets so messed up that you can't extend your flat coordinate system out very far and you realize the curved-ness more easily.
     
  4. Jan 26, 2004 #3

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I don't know what you mean by "negatively exceeds." I also don't know what you mean about a 100 mph rocket that would "never leave orbit" -- a 100 mph rocket would never reach orbit.

    - Warren
     
  5. Jan 26, 2004 #4
    By negatively exceeds, I mean exceed the speed of the rocket in the opposite direction. Example: If a rocket is traveling east at 100 miles per hour and the gravitational influence (or just gravitation pull) of some object keeps it from continuing east or better yet, pulls it back west, could you say that the gravitational influence of that object exceeds the speed of the rocket - slowing it down to the point where the rocket stops and begins traveling in the opposite direction.

    I used the 100 mile per hour rocket as an example because it would not leave earth and never reach orbit. In other words, would the earth's gravitational influence appear to be greater than the speed of the rocket? Sorry, I probably should have clarified that.

    Thanks for responding.

    Jeremy
     
  6. Jan 26, 2004 #5

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, general relativity predicts that gravitational disturbances travel at the speed of light. Recent experiments seem to confirm this prediction, but the case has by no means been totally closed on the issue.

    In any event, the Newtonian idea that gravitational disturbances propagate everywhere instantaneously is at odds with quite a bit of modern, experimentally-supported physics and is almost assuredly incorrect.

    - Warren
     
  7. Jan 26, 2004 #6

    russ_watters

    User Avatar

    Staff: Mentor

    However, with a source of thrust able to provide a steady 100mph speed perpendicular to the surface, it would leave earth's gravitational influence eventually.
     
  8. Jan 26, 2004 #7

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That's true, but that's a pretty degenerate "orbit."

    - Warren
     
  9. Jan 26, 2004 #8

    turin

    User Avatar
    Homework Helper

    I was just thinking about this. Good point. If the rocket goes 100 mph, then it goes 100 mph (I'm assuming at sea level?). As it rises higher and higher, with the same amount of thrust, it will actually increase its speed (I'm of course assuming that all of this discussion is wrt the earth's surface). Maybe jgravatt can clarify what is meant by this 100 mph rocket, specifically why it couldn't escape the earth's gravity if it continued at 100 mph (wrt the surface of the earth).
     
  10. Jan 26, 2004 #9
    Sorry about the confusion. I was ignorantly thinking that something traveling at such a slow speed would never break the earth gravitational pull. Is there an equation that I could use to determine what minimum velocity would be required to break free of an objects gravitational influence?

    Jeremy
     
  11. Jan 26, 2004 #10

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    To leave the Earth completely (and be able to turn off the engine and not immediately fall back), a spacecraft would have to reach escape velocity, defined here:

    http://hyperphysics.phy-astr.gsu.edu/hbase/vesc.html

    At the surface of the Earth, escape velocity is ~11 km/sec.

    At a distance of 100,000 Earth radii, 637 810 000 000 m (4.26 AU), escape velocity is only ~35 m/s.

    If the craft travelled at 100 mph (44.7 m/s) for about 450 years, it would be free of the Earth's gravitational influence.

    (back of the envelope calculation)

    - Warren
     
    Last edited: Jan 26, 2004
  12. Jan 26, 2004 #11

    chroot

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Update: more precision. At 100 mph, the craft would be free at a distance of 2.66 AU. It would take the craft 283 years.

    - Warren
     
  13. Jan 26, 2004 #12
    Dang! There goes my super-charged Chevy experiment. Too much gas for the tank
     
  14. Jan 26, 2004 #13

    russ_watters

    User Avatar

    Staff: Mentor

    Yeah, I'm not saying its a good idea or anything.

    And thanks for the escape velocity calculation - I was thinking about it, but you know about me and math and the restraining order thing...

    Today I was talking on the phone with a contractor and I subtracted 2.9 from 10 and got 6.1.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Gravity and FTL
Loading...