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Does energy have momentum?

  1. May 29, 2006 #1
    I was wondering: From the point of view of an observer who is in motion relative to the sun, the sun has a certain measurable amount of momentum. However, since the sun is constantly turning mass into energy via a nuclear reaction, yet its velocity remains more or less stable, that momentum should be very slowly decreasing. That would violate conservation of momentum.

    The only way I can see around this is if the energy being produced has momentum of its own. So can energy have momentum? Does anyone know if it's possible to measure the momentum of, say, a beam of light?
  2. jcsd
  3. May 29, 2006 #2


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    Gold Member

    Light has momentum, definitely.

    [tex]p_{photon} = \frac{h}{\lambda }
  4. May 29, 2006 #3


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    Staff: Mentor

    Why would that violate conservation of momentum? If the mass is changing, the momentum must also.
  5. May 30, 2006 #4

    First off momentum IS conserved. Keep in mind that all the mass thrown off as radiant energy and solar wind is going someplace. It didn't just disappear. And from a conservation of momentum point of view you have to count that. When you do you'll see that momentum is in fact conserved.

    Secondly the Sun's movement through the cosmos isn't determined ONLY by its momentum. It's not an isolated system. It's being pulled on by everything else in the universe. The more massive and closer things are the more effect they have, but even the tiniest sub-atomic particle on literally the other side of the universe has at least some effect on the Sun's movements, its "speed".

    Just for example, consider this - if momentum was the only force acting on the Sun then it'd be traveling in a nice straight line instead of orbiting the center of mass of our Galaxy. That right there should tell you that gravitation pull from other objects in the universe have more effect on the Sun's path then its momentum.

    Lastly, from a simple accounting viewpoint, if you don't want to count the mass of the Sun lost to radiant energy and the solar wind as being part of "The Sun" proper, the star being observed, then yeah, the momentum of what you're calling "The Sun" IS dropping exactly in line with the amount of mass it's loosing.

    Thing is though, that loss of mass is nearly infinitesimally small in comparison to the Sun's overall mass (consider, it's been losing mass like that for billions of years, and billions of years from now it's STILL going to be losing mass). We're talking about a water balloon here with one incredibly small leak in comparison to its size. And, just as the math'll tell ya, the Sun's change in speed due to that fact is going to be equally tiny, all but unnoticeable.

    So yeah, the Sun's speed, and mass, and momentum, ARE changing. But it's no more noticeable than the Sun's *drop* in mass compared to it's enormous *overall* mass.

    Picture a lead bowling ball rolling down an ally. Every ten feet a piece of the bowling ball the size of a half a grain of sand flies off it. You know, and I know, that under those conditions the mass and therefore the momentum of the lead bowling ball is changing. But how noticable is that change? Not very. What's being lost is too small in comparison to the total mass of the bowling ball, so you see very little change in the bowling ball's mass and momentum. The change IS there, it's just a very tiny change.

    That make more sense?
    Last edited: May 30, 2006
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