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Plackian space time

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    • Thread Starter

    What causes 10^-43s to be the smallest time possible, and 10^-35 the distance?
    Any help would be much appreciated?

    As far as I can tell, not very much.

    Well, that's not quite true - often in physics you can create what are often called "characteristic scales" by combining relevant constants in a certain way. As a simple example, suppose you have a pendulum of length  l in a gravitational field of strength g - if you want to know the "characteristic time scale" then what you do is look for combinations of those quatities (l and g) that are times. As it happens, the only one is  T_0 = \sqrt{\dfrac{l}{g}}. That means that a good guess is that the period is about this much (it turns out to be a little bigger, because this isn't an accurate method).

    The point of the exercise is that if the "most natural" time scale is some number, say 10^-43 seconds, then you'd expect not much to happen on any time scale smaller than that, and that on much larger time scales you're not going to get that same level of detail. In quantum gravity, the relevant quantities are h from quantum mechanics.  G form gravity and  c from relativity (it turns out that relativity and gravity are related). So you plug in the numbers and it turns out that there is exactly one characteristic length scale -  l_{planck} = 10^{-35}m and so you expect stuff to happen on about that time scale. The planck time is what you get if you do the same thing for times, and corresponds to the time for light to travel one planck distance.

    On the other hand, people often then say that the universe is pixelated into little Planck times which I don't think is really accurate as far as anyone knows. All we know is that the current laws of physics only work for things significantly bigger than 10^-35m and that anything smaller than that starts getting confusing (fortunately, nothing is that small other than the centres of black holes and the very early universe, so we're usually okay).

    Just would like to add abit;
    The Planck length is so small as it is linked to the uncertainty principle. At this length, the measurements of distance are so uncertain that they become unusable, and therefore the smallest 'real/measurable' distance is the Planck length. (hypothetically speaking. We are nowhere near equipment that could measure this! )
    Below this length, the measurements that are taken actually become so unreliable that they break down. The laws of physics do not apply below this length (string theory/loop quantum gravity are trying to fix this issue).
    The same as above goes with time
    • Thread Starter

    Thank you everyone!

    It's to do with quantum mechanics.

    Plank time is the time taken for a particle to moving at the speed of light to cover the plank length.
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