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B Can there be any acceleration without mass?

  1. Mar 11, 2018 #1
    So, we know that force equals mass times acceleration. A force is needed to cause an acceleration. I am wondering though, is mass required for accelerations to happen? Why or why not?
     
  2. jcsd
  3. Mar 11, 2018 #2

    Dale

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    I would say yes. Anything without mass must move at c at all times. It cannot accelerate.
     
  4. Mar 11, 2018 #3
    I don't think that this is a valid argumentation. Constant speed doesn't mean that there is no acceleration.
     
  5. Mar 11, 2018 #4

    sophiecentaur

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    It does. If the 'entity' only exists at velocity c then when would it be accelerating? It would emerge from whatever reaction/ interaction generated it at c. Slower than c and it would not be in existence.
     
  6. Mar 11, 2018 #5
    Just a little hint: speed is constant for v·a=0.
     
  7. Mar 11, 2018 #6

    sophiecentaur

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    But for 'rectilinear propagation'?
    Though I must say I had ignored motion in a circle. o:)
     
  8. Mar 11, 2018 #7
    Of course linear acceleration is not possible with constant speed. However, there is Shapiro delay.
     
  9. Mar 11, 2018 #8

    sophiecentaur

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    But the speed, measured at any point would still be c (??). Isn't that the basis of GR?
     
  10. Mar 11, 2018 #9
    We can discuss the motion of a point under various conditions and constraints without reference to any mass at all. It makes perfect sense without reference to either force or mass.
     
  11. Mar 12, 2018 #10
    Yes, the locally measured speed of massless objects is always c.
     
  12. Mar 12, 2018 #11

    sophiecentaur

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    So where does this take the thread? :smile:
     
  13. Mar 12, 2018 #12
    This seems like a nonsense statement. Suppose the point is at rest?
     
  14. Mar 12, 2018 #13
    Which point are you talking about?
     
  15. Mar 12, 2018 #14

    sophiecentaur

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    Under what circumstances could the object be at rest? What would be a 'stationary' photon be like?
     
  16. Mar 12, 2018 #15
    If there is no mass, then all that exists there is a geometrical point. That is point of which I am speaking.
     
  17. Mar 12, 2018 #16
    The adjective "massless" doesn't make much sense with a geometrical point. I am talking about objects that are subject to E²/c² = m²c² + p². Such objects can only be at rest with m>0 and they always move with c (locally measured) with m=0.
     
  18. Mar 12, 2018 #17
    The adjective "massless" applied to a geometrical point makes perfect sense. What mass do you think Euclid ascribed to a point? No, the thing that is a stretch is the idea of a mass point. The latter is a useful fiction, but it really does not make rigorous sense.
     
  19. Mar 12, 2018 #18
    No, it doesn't because geometrical points never have mass. In theory you can have a point size object with mass located at a geometrical point but not a geometrical point with mass. Therefore "massless geometrical point" is a tautology.
     
  20. Mar 12, 2018 #19
    By all means, have it your way. This thread seems pretty pointless anyway.
     
  21. Mar 12, 2018 #20

    sophiecentaur

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    Also, a photon is not a point particle. It has no defined extent so it is pretty meaningless to assume you could use a stopwatch and push the button when it goes past. Using a very mechanical model is just not appropriate.
     
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