In String Theory, how can a string as infinitesimally small as the Planck length, manifest itself as a much larger and massive particle?
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String theory is a model and in any model you can do whatever you want. It's not even clear, though, that string theory can even generate the fields we see, so the question is a little premature. – CuriousOne Jun 11 '16 at 05:50
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Duplicate of "Particles from String theory" http://physics.stackexchange.com/q/82992 Similar : "How can particles being closed strings in String Theory create solidity in objects?" http://physics.stackexchange.com/q/24215 "How exactly are the different motions of only one kind of fundamental string assumed to give rise to the spectrum of elementary particles we observe?" http://physics.stackexchange.com/q/44584 – sammy gerbil Jun 11 '16 at 22:52
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The particle is not larger nor more massive.
We normally describe particles using quantum field theory, and in QFT particles do not have a size. This is discussed in the answers to Why do physicists believe that particles are pointlike? and it would be worth reading through them.
To say that a particle is a point is a bit of an over simplification. We generally determine the size of a particle by scattering something (e.g. another particle) off it and studying the angular distribution of the scattering products. What QFT tells us is that for an elementary particle as we keep increasing the scattering energy the apparent size of the particle keeps decreasing. In the limit of infinite scattering energy the size would go to zero. What string theory tells us is that in the high energy limit the scattering is described by string theory rather than QFT, and the apparent size goes to the string length scale rather than zero.
As for the mass, in string theory all the particles we have seen in our accelerators are massless and they correspond to excited modes of a massless string. The mass comes from the Higgs mechanism, which operates at a much, much lower energy than the string scale.
John Rennie
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