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I What are the units for the Heisenberg Uncertainty Principle?

  1. Jun 5, 2017 #1
    To my understanding(correct me if I am wrong), one consequence of the Heisenberg Uncertainty Principle is that energy levels can fluctuate by some amount, e, for a short time, t. As long as e x t does not exceed h/4pi (where h= planck's constant). My quarry is that what are the units for the time and energy. I assume it is in joules and seconds but the book did not specify so I am unsure.

    P.S this energy fluctuation is so weird, it kinda disproved the law of the conservation of energy.
  2. jcsd
  3. Jun 5, 2017 #2


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    Gold Member

    you assumed correct ...
  4. Jun 5, 2017 #3
    thanks for clarifying
  5. Jun 5, 2017 #4


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    Staff: Mentor

    When you look up the value of Planck's constant, you will see that it is stated with some units: For example, the first Google hit says that ##h=6.62607004\times{10}^{-34} m^2 kg / s##. So if you are measuring your distances in meters, your masses in kilograms, and your times in seconds that's the numerical value of Planck's constant that you'd plug into the calculation. When you do, you'll find that your answer for the energy comes out in units of ##kg\cdot{m}^2/s^2##, the unit that we call a Joule. So yes, Joules are right, but that's not something you have to assume - you chose to have the answer come out in Joules when you chose to write Planck's constant in that form.
    It does not, but with quantum mechanics you do have to be much more careful and mathematically precise in the way that you state and use the law. We have many other threads on this topic, but if you can't find one that explains it to your satisfaction, feel free to start a new thread asking that question.
  6. Jun 5, 2017 #5
    Thank you for the reply.
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