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I Dark energy question...

  1. Feb 13, 2017 #1
    Here is a question that has been bugging me for a while…

    When a star is moving through space it has momentum equal to its mass times its velocity. As the star converts hydrogen into helium it loses mass as that mass is converted to energy (light) and neutrinos. Both of these escape the star in all directions. The loss of this mass should not impact the momentum of the star since any mass/energy radiated in one direction is countered by the same mass/energy radiated on the opposite direction. So the net result is a loss of mass with momentum conserved. In classical Newtonian physics, momentum must be conserved so, to balance the (momentum = mass x velocity) equation, one would require the star to increase its velocity. If you include the mass lost by coronal mass ejections which also occur in all directions... the loss of mass is even more pronounced...

    (momentum = mass x velocity) vs (momentum = slightly less mass x slightly more velocity)

    Now the acceleration of stars over time has in fact been observed. Reference the type 1a supernova study done. But the increase in velocity was attributed to “dark energy” isn’t this classical Newtonian explanation a far better way of explaining the increase in velocity observed rather than relying on “dark energy”?
     
  2. jcsd
  3. Feb 13, 2017 #2

    Drakkith

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    Actually it was a thought experiment similar to this that Einstein published in his paper "Does the Inertia of a Body Depend Upon Its Energy Content?"
    From this thought experiment he concluded that the mass of the radiating object must decrease as a consequence of the conservation of energy and mass. So the short answer is that yes, the mass of the star must decrease.

    The expansion of the universe, and its acceleration, applies to the galaxy cluster scale and above (clusters, superclusters, etc), not to individual stars trapped within the confines of their galaxies by gravity. The stars within the Milky Way are not expanding away from us.
     
  4. Feb 13, 2017 #3
    well looking at galaxy clusters, they also will radiate matter and energy. should that cause them to increase in velocity in order to conserve momentum?
     
  5. Feb 13, 2017 #4

    Drakkith

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    Nope. Momentum is conserved when a body radiates energy in all directions. The radiation emitted in the direction of the original motion is blueshifted and thus has more energy (and momentum) than radiation emitted in other directions. Momentum is conserved and the object's mass decreases while its velocity remains the same.
     
  6. Feb 13, 2017 #5

    to me that makes sense when talking about light but when talking about matter like neutrinos or a larger coronal mass ejection, if the matter leaves the cluster in all directions at the same velocity relative to the cluster, how does it conserve momentum?
     
  7. Feb 13, 2017 #6

    Drakkith

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    The material ejected in the direction of motion has a higher velocity and more momentum than the object while material ejected in the opposite direction has less (as measured from a frame of reference in which the object is moving). Momentum is again conserved.
     
  8. Feb 14, 2017 #7

    Chronos

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    Relativistic mass creates paradoxes to which the universe appears allergic. Science tends to unravel paradoxes. I would hazard to guess our universe is ultimately logical and causally connected.
     
  9. Feb 14, 2017 #8

    Bandersnatch

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    You're an ice skater standing on ice. You throw two identical balls - one forwards, one backwards. You now have less mass than before the throw. Has your velocity changed?.
    You're a cyclist riding a bike, when alien abductors use their 'quantum-laser' hoists [TM] to lift you off the bike. There is now less mass in the system on the ground - should the bike accelerate or continue at the same speed?
     
    Last edited: Feb 14, 2017
  10. Feb 16, 2017 #9
    what about if you are a star traveling through space and you lose some mass that was converted in to energy? how can the conservation of momentum equation be balanced? if momentum is conserved and you have less mass afterwards... what if the lost mass didn't take any momentum with it, then the remaining mass still has all of the momentum but with less mass... sorry if I am being dense here, by the way... I'm just having trouble visualizing how this would work...
    and thanks for all your responses so far. :-)
     
  11. Feb 16, 2017 #10

    Drakkith

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    The radiation or matter ejected from the star has momentum and is conserved in the same manner as I explained above.

    The key is that the matter and radiation must have momentum. There is no way for the star to lose mass without also losing momentum.
     
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