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I've been wondering about the Planck length recently, but it is not observable. What is the smallest actually observable structure in the universe?

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    $\begingroup$ are photons observable enough? $\endgroup$ – aaaaaa May 8 '15 at 2:47
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    $\begingroup$ The smallest observed scales are given by the highest particle energies measured, so far, which would be in the 1e20eV range, far below the Planck scale. The latter is a very questionable concept, anyway, because as long as relativity holds the effective interaction energy in a collision event is observer dependent, i.e. one observer would see the Planck scale probed, while another would not. Both can, of course, not be true at the same time, so if relativity is a fundamental symmetry that hold on all scales, there is no such thing as a "smallest structure". $\endgroup$ – CuriousOne May 8 '15 at 3:13
  • $\begingroup$ You might get some answers here, scroll down a little to detailed list under sub-atomic. en.wikipedia.org/wiki/… It's Wiki, so I can't swear by it's accuracy. $\endgroup$ – userLTK May 8 '15 at 7:24
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What is the smallest actually observable structure in the universe?

The smallest structure that I have seen is the electron cloud around an atom.

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  • $\begingroup$ Broken link?$ $ $\endgroup$ – jinawee May 8 '15 at 13:10
  • $\begingroup$ @jinawee The link is repaired. $\endgroup$ – LDC3 May 8 '15 at 13:18
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It depends on what you mean by observable. The smallest confirmed particle is the electron neutrino. Observation/measurement is another discussion in itself.

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Your question comes with an extra difficulty: the meaning of distances and sizes at the quantum level.

From the current understanding of particle physics, all particles are point-like and hence volumeless. Of course this is an interpretation of the theory and this statement would require the experiments to probe arbitrarily small distances, which does not happen.

Insofar the shortest distances probed are around $10^{-16}$ cm, while the LHC is expected to achieve $10^{-17}$ cm. But these distances do not immediately correspond to structures.

If we refer to smallest structures to volume occupying entities in nature, then the smallest known will have to be mesons (quark and anti-quark pair, like the pion $\pi$), whose volume and shape is still a matter of research, but you can take an upper bound to be of order of the size of the nucleus of Hydrogen (a proton): $10^{-12}$ cm.

References: https://www.ias.edu/articles/large-hadron-collider http://en.wikipedia.org/wiki/Orders_of_magnitude_(length)#1E-15

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