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Time and matter, is there a smallest unit?

  1. Aug 5, 2014 #1
    dear friends, I have always wonder: is there a smallest unit for time and matter? or in other words, can we continuesly devide time and matter into smaller pieces non-stop?
    ive had this question since young when I imagined myself cutting a piece of cheese. I just can't help stop cutting it and it kinda troubled me.
    Anyone please help me?
  2. jcsd
  3. Aug 5, 2014 #2


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    Electrons are considered "point particles" and you can't get any smaller than that. Otherwise, quarks are the smallest we know of.

    Time is not considered to be quantized although there ARE theories that it is/might be.

    This topic is beaten to death here regularly so I suggest a forum search.

    HINT: every thread has a list at the bottom of the page of similar threads. That's a good place to start.
  4. Aug 5, 2014 #3


    Staff: Mentor

  5. Aug 5, 2014 #4
    Yes. They're called Planck length and Planck time. The Planck length is 1.61619926 × 10-35 meters, Planck time is the time it takes light to cross a Planck length.

    From wiki:

    "Simple dimensional analysis shows that the measurement of the position of physical objects with precision to the Planck length is problematic. Consider the following thought experiment. Suppose we want to determine the position of an object using electromagnetic radiation, i.e., photons. The greater the energy of photons, the shorter their wavelength and the more accurate the measurement. If the photon has enough energy to measure objects the size of the Planck length, it would collapse into a black hole and the measurement would be impossible. Thus, the Planck length sets the fundamental limits on the accuracy of length measurement."
  6. Aug 6, 2014 #5


    Staff: Mentor

    Unfortunately, the dimensional analysis of the wiki is wrong. A photon cannot collapse into a black hole regardless of its energy/wavelength.

    At this point, I would say that we don't know. We have no reason to believe, based on current measurements, that there is a smallest unit. But our current theories are not expected to work on Planck scales and our current measurements are nowhere close to that level.
    Last edited: Aug 6, 2014
  7. Aug 6, 2014 #6


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    'Smaller and smaller = more and more difficult' doesn't necessarily imply an actual limit.
  8. Aug 6, 2014 #7


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    No, the universe is not limited to them as any sort of "smallest unit". It does seem to be true that there would not be a way to measure smaller amounts, but that doesn't mean that there aren't smaller amounts.Because we don't have any specific names for smaller units, I can't say "consider the blumpf" but I CAN say "just think about a length of 10E-40 meters" or consider a length of 1/100th of a Plank length". There is nothing yet known that constrains the existence of such lengths in reality.

    In practical terms it's reasonable to think of them as the smallest possible units since you can't measure anything smaller, but if you want to move away from the theoretical to the practical, then it seems more reasonable to say that in practice we will likely never be able to measure things anywhere near as small as they are so they are smaller than the minimum units. Using current technology we can't measure anything down below about 20 orders of magnitude larger than one Plank length, but of course that's likely to get better with new technology, but 20 orders of magnitude seems unlikely. The 20 by the way is just my recollection and might be off but some, but not many, orders of magnitude.
  9. Oct 7, 2015 #8
    Many people and scientists say the Planck length is the minimum length possible. the same they say about the Planck time. but so far I have not read the reason for this answer. It is not a matter of measurement, is conceptual. if they say that the Planck length is or has been the minimum, does that mean the universe was that size in the BB? I do not think so
  10. Oct 7, 2015 #9


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    That's a common misconception - we even have an Insights article: https://www.physicsforums.com/insights/hand-wavy-discussion-planck-length/

    As this is an old thread and we're working from a misconception, I think we can close the discussion here.
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