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Thought experiment on movement

  1. Apr 15, 2015 #1
    Question: If there were only a single elementary particle in the entire universe - could it move, and if so, how would its movement be measured?
  2. jcsd
  3. Apr 15, 2015 #2


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    The only reference frame to measure any movement would be its own, in which by definition it cannot move.

  4. Apr 15, 2015 #3


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    Well, any constant motion would be undetectable. But if it were possible for the particle to accelerate, that could (probably) be detected, because if this particle had electric charge, acceleration would cause it to radiate. A change in the acceleration would cause it to emit gravitational waves.

    That said, if there wasn't some other particle around, there'd be nothing to cause it to accelerate (or change its acceleration), so it'd just have constant motion which is undetectable and indistinguishable from being stationary.
  5. Apr 16, 2015 #4


    Staff: Mentor

    If there is only a single elementary particle in the universe, what measuring device would be used to measure its movement?
  6. Apr 17, 2015 #5
    lol it takes physics to do physics!
  7. Apr 21, 2015 #6
    Well, since my post says it is a "thought experiment" I'm sure you could think of something.............
  8. Apr 21, 2015 #7


    Staff: Mentor

    No, I can't. That's why I asked the question. It's your thought experiment. If you can't think of something either, what does that tell you?
  9. Apr 21, 2015 #8


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    While thought experiments do allow for highly impractical parameters ("...imagine I flew my starship to the centre of the sun"), those parameters have to be ones that aren't directly relevant to the thought experiment.

    In your scenario, the concept of measuring movement is directly relevant: movement is not an absolute - it must be relative to something (Einstein taught us that) - requiring at least two particles.

    So, I guess you have answered your own question.
  10. Apr 22, 2015 #9
    Thank you for your CONSTRUCTIVE comment. I see your point and thank you for taking the time to write something that is informative.
    Last edited: Apr 22, 2015
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