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

Gravitation due to acceleration

  1. Nov 11, 2009 #1
    The speed at which the earth is orbiting the sun increases it's mass due to relativity. Does centripetal force also increase the mass of the earth? Is the increase in mass enough to be measured? If so, then, the gravitation must also increase. Wouldn't galaxies also have more or less gravitation/mass due to their movement speeds?
     
    Last edited: Nov 11, 2009
  2. jcsd
  3. Nov 11, 2009 #2
    I like this and have similar questions. As long as we're measuring velocity of galaxies and stars, where is the reference point and how do we make sure that the reference point isn't moving in relation to the rest of the universe? Is there a way to measure velocity of the reference point and remove that from the rest of the equation? Why would or wouldn't centrifugal rotation add mass (I don't know what its called, but I'm thinking the answer is no due to the reactive force applied by the opposite side of the object. I think its kinda like putting both feet on a door and then pulling on the handle trying to open it, or pulling yourself up by your bootstraps...). HOWEVER, if centrifugal rotation does increase mass, or gravitational pull (either or), then would there be a measurable difference between the plane of rotation and the axis poles? Using the earth as an example, would there be more mass or gravitational pull at the equator than at the north or south pole? I'm inclined to say no, after all, one of the reasons that NASA launches from Florida is because of the inherent centrifugal force from the earth's rotation helps "throw" the rocket/ shuttle away from the earth.
    However, lets say that this does happen. How fast would something have to rotate before the increase in mass/ gravitational pull become measurable? Relativistic speeds? As far as black holes go, this sure would help explain a lot. Lets say that a black hole rotates at near c for example. Now, how much of its pull is due to actual material mass and how much is due to relative mass? Is this possibly why x rays seem to escape from the poles of a black hole and not the rotational plane? Is it possible that black holes rotate fast enough that much of their pull is from the rotation and not from the compacted material? Is the material even more compacted due to rotation and length contraction? IF the massive gravitational pull DOES come from rotation, does that mean the black hole is rotating at c+ to capture light? Would a black hole not be a black hole if it didn't rotate fast enough? These questions will surely be shot down as soon as someone says no, rotation does not increase mass, no matter how fast something rotates, and then they prove it. And by prove, I don't mean throw math at me, I just mean point me to the person/ people/ theory that says it doesn't happen so that I can read it. I don't do so well with higher math.
     
  4. Nov 11, 2009 #3
    It really doesn't. Way back in the 1950's, there was an author (it may have been George Gamow) that tried to explain the effects of relativity by talking about a speed object increasing it's mass. It turns out that this is not a good way of thinking about what is going on.

    This is why thinking about relativity as "increasing mass" is not a good way of thinking about what's going on. It leads to all sorts of confusion. It turns out that when an object is traveling fast it *doesn't* increase it's gravitational pull, which is why the idea of fast objects increasing in mass is not an accurate description of what is going on.
     
  5. Nov 11, 2009 #4
    I also like this because once people start asking questions like this it tells that they are thinking about what is going on, and it turns out that the reason things are confused is that thinking about a fast moving object as "increasing mass" is a bad way of thinking about what's going on.

    What does happen is that relativity predicts is that as you get closer to the speed of light, it becomes harder and harder to accelerate. Someone back in the 1950's talked about this effect as "increasing mass" but it turns out to be not a great way of thinking about it because then you start asking whether this "increased mass" should result in more gravity, and it doesn't.

    A better explanation is that one fact of the universe is that light in a vacuum is always moving at the speed of light. So no matter how fast you move toward a beam of light, it is always moving away from you at the same speed. So you can never catch up to it.
     
  6. Nov 11, 2009 #5
    Part of the energy used to propel the object closer to the speed of light is converted to mass right? Then wouldn't a planet gain mass as it gets closer to the speed of light? Also, could you give a more detailed explanation of the way we should be thinking about the energy/mass relationship? Thanks
     
  7. Nov 11, 2009 #6
    Or is the mass of the original object increased because of moving near/at the speed of light?
     
  8. Nov 11, 2009 #7
    Is this Hawking radiation?
     
  9. Nov 11, 2009 #8
    No. As I mentioned if you think of things as gaining mass, then things become very confusing.

    You start from the weird fact about the world which is that light in a vacuum always travels at the speed of light. You run toward a beam of light. It's still traveling away from you at the speed of light. Nothing to do with mass. It's a weird property of the universe.

    So what happens is that the equation used to calculate energy changes to take into account this weird fact.
     
  10. Nov 12, 2009 #9
    No it's not Hawking radiation. It's produced when material is being accreted into a black hole, basically from infalling material crashing into other material that is piling up around the hole in what's called an Accretion Disk. The disk is also why the radiation is seen coming from the poles - the disk gets in the way of light being emitted in the same plane.
     
  11. Nov 12, 2009 #10
    longitudinal and transverse relativistic masses are different.
     
  12. Nov 13, 2009 #11
    So, is this assumed that the running observer is holding the flashlight? Or whatever is projecting the beam of light. From the way I'm interpreting this, you're saying that the speed of light is relative to the observer. Meaning that, an observer running parallel to a beam of light could run 99% the speed of light and that light beam (the beam is NOT being projected by this quite out-of-breath observer) would be moving away at 100% the speed of light relative to the observer. So a nonmoving observer would see a beam of light moving at 199% the nominal speed of light. we know that is not true, so umm... could you try again to explain that? Sorry, I lack the math background to understand it that way and i always get hung up on phrases like "due to the effects of this-guy or this-other-guy's-law-or-theory". Not to call myself stupid, just undereducated I guess. Ow, even that stings.
     
  13. Nov 18, 2009 #12
    I'm not quite sure what the question is but the speed of light is constant from every reference point. Light can only move at 200% speed relative to another photon moving in the opposite direction.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Gravitation due to acceleration
Loading...