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I was wondering what is the smallest unit of measurement possible. I found from google that it is Planck length but it was mentioned that any microscope couldn't see it. Then why is it even a unit of measurement? And how did they discover it or made it?

Qmechanic
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Dingus45191
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Though you can see phrases as "the Planck length is the smallest length that can exist" or "the smallest length that can be observed", none of these are experimentally proven.

Though there are many speculations about the significance of the Planck length, none is proven in any currently accepted theory. It is expected, though, that quantum gravity effects become definitely non-neglegible at the energy/distance scale set by the Planck length, so it provides a heuristic scale at which we should not expect our current theories to make accurate prediction.

Is the Planck length the smallest length that exists in the universe or is it the smallest length that can be observed?

What we can say, is what really gives significance to the Planck length, is that at the energy/scale distance set by it, quantum gravity effects become non-negligible.

In quantum gravity, geometry with the usual rules doesn't work if the (proper) distances are thought of as being shorter than the Planck scale.

Does the Planck scale imply that spacetime is discrete?

In other words, if in our calculations we wanted to use lengths smaller then the proper distances, our currently used geometrical rules don't apply.

So the answer to your question is that the significance of the Planck length has more to do with quantum gravity effects, and it does not mean at all that it is the smallest unit of measurement possible, nor the smallest length that can ever be measured or observed, and to our current knowledge, spacetime might very well be continuous allowing for even smaller units of measurement.

Árpád Szendrei
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The value of a unit is not necessarily in having the size of anything measurable, even though this is how historically many known units came into existence. Specifically Planck units are defined in terms of universal physical constants, i.e. their value does not depend on the units that one uses to measure these constants. See here

Roger V.
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Imagine for a second that space itself was made up of pixels. Each pixel is a Planck length long, a Planck length wide, and a Planck length high (space is three-dimensional). Further, the size of the pixels is dictated by the very laws of physics, and so is a fundamental property of the universe itself.

Wouldn't we want to know the size of those pixels?

TheEnvironmentalist
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    relation with Planck length? – pasaba por aqui Nov 24 '20 at 17:03
  • @pasabaporaqui This is, with a bit of handy oversimplification, the definition of the Planck length. No smaller distance is physically meaningful, and while arguments about the quantization of spacetime are a bit out of scope for the question, this is effectively the universe in which we live – TheEnvironmentalist Nov 24 '20 at 17:51