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Name: Kevin
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According to some sources I have seen the smallest unit of length is defined as a Planck length, 1e-35m. A proton as designated is classical physics has a dimension of 1e-15m, and an electron has a "size" of 1e-22m. With this said, the smallest unit of measure I can think of is the subatomic distance between adjacent particles. However due to the force between 2 adjacent atoms, I believe it is called the strong force, 2 things never actually come into contact with each other. Since I feel that this is the smallest unit of measure, I do not understand how a length of 1e-35m to exist. An electron is 1e13 times larger than this predetermined length, so even if there is something that forms electrons, that is a significant difference is size.

Hi Kevin, there are a lot of unresolved questions in this field, so it's easy to become confused. In fact, I am quite confused by your question. It sounds like you are questioning the existence of the Planck length -- is this what you are asking? The Planck length is simply a unit (like a meter or a mile). However, if you are questioning the practical definition you provided (that it is the "smallest possible length"), then I would agree with you. I am not sure that calling it "the smallest possible length" is quite right -- perhaps a better way of phrasing it might be that classical theory becomes meaningless at lengths smaller than the Planck length. So it would not be useful to envision two protons as two hard spheres that approaching each other to a "Planck length" distance because quantum effects at that size scale would dominate this classical view. I am not sure if I have answered your question, so please respond back to clarify.

Hope this helps,

Burr Zimmerman


First, protons and electrons are not the only particles. A particle that interacts much less that a proton or electron, and can therefore get much closer to other particles, is the neutrino. Electrons and protons are "fermions": two of the same particle cannot exist in the same place at the same time. A neutrino has no electric charge and is a "boson": the number of neutrinos that can exist on top of each other, that can even pass through each other, is unlimited.

The Plank length is the smallest distance that can be measured. This does not mean that the distance between two objects cannot be less, but that any distance you measure will be so uncertain that nobody will know for certain that the distance is less. Many of the limits within quantum physics apply more to measurement than to existence. If no distance measurement less than 1e-35m can ever be measured, then there is no way to verify whether or not such a distance can exist. You could say it is more a limit to what we can know rather than what can be.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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