In some physics theories like string theory we have notion of spatially extended quantum objects - strings, membranes etc. Assuming that such objects exist, how would they appear in experiments ? More precisely are there any thought experiments for determining whether a quantum thing being observed has spatial extension or not, and if it has spatial extension then what is its dimension, topology etc.
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Possible duplicates: http://physics.stackexchange.com/q/15/2451 and links therein. – Qmechanic Dec 10 '12 at 19:08
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2@Qmechanic concerning the duplicate I dont agree, this question is rather asking about how ST could be proved instead of disproved – Dilaton Dec 10 '12 at 20:47
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More precisely are there any thought experiments for determining whether a quantum thing being observed has spatial extension or not,
You are in effect asking if there exists a theoretical program to check whether the proposed strings have any experimental signatures that verify their existence unequivocally, which verification would also include the fact that the string is different than a point particle.
This is the whole project of string theory and string phenomenology as a "thought experiment" and most of the experimental program of the LHC tries to determine which phenomenological string model is physically realized, if at all.
Suppose that the LHC and its successor, possibly the ILC find a specific string phenomenological model is verified by the measurements, i.e. the new resonances found, the branching ratios and symmetries observed fit very well with one of the proposed models, then the model will be validated, and the premises of the model will be validated, i.e. the supposition of string dimensions in that model.
One has to realize that the grand majority of our knowledge of the behavior of the microcosm comes from measurements in the macrocosm that verify the hypothesis of the theoretical models. That we all accept that an electron is a point particle does not come because we have taken a ruler and measured its dimensions. It comes because the hypothesis that it is a point particle fits perfectly the accepted models for the microcosm at the moment, which is the so called standard model of particle physics. If a hypothesis with strings fits all new and old data better then the strings are as real as points, as far as the microcosm goes.
The resonance and scattering spectra at high energies are the tools to see the dimensions of objects in the microcosm through the use of strict mathematical theories.
and if it has spatial extension then what is its dimension, topology etc.
It will be the dimension, topology etc assumed in the model that will be validated, in the future, and of course if there exists such a validated outcome. Otherwise new theories will be pursued.
In other words our "rulers" have become extremely layered and complex.
anna v
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At the currently available energies, string will look like point particles. This is essentially by design: all the currently know elementary particles look like points so the next generation theory needs to predict that they will look like points.
Actually they needed to look like points at the energy scales of the 1970s and 80s, but they turned out to be so point-like on those scales that their extent is still unaccessible.
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4I thought the question is aking about by what phenomenological observations they could be distinguished if the Planck scale (or energies about 3 orders of magnitude smaller) were accessible. There exist studies investigating such questions and I think the OP is interested in these... – Dilaton Dec 10 '12 at 20:16
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@Dilaton you must be thinking of something like this: http://www.news.wisc.edu/his would check the extra dimensions over our four, not the point versus one dimensional object. It would again be a tool to validate string theory, but imo too inaccurate to distinguish between models. see my answer. – anna v Jan 10 '13 at 07:58
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@annav the link does not work :-/. I like your answer (+1). Appart from such (indirect ?) phenomenology I was thinking about direct stringy effects which are (with our current technologies at present not accessible) assumed to kick in at the quantum gravity scale, such as the effects of directly exciting higher vibrational modes of the fundamental strings, observations of some kind of Regge trajectories for elementery particles instead of mesons, excite particles with unusual fractions of charges which would be related to winding modes, etc ... – Dilaton Jan 10 '13 at 10:22
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I am not sure if such effects would allow to distinguish between different models, but they would definitely proof that fundamental strings exist. Of course (as said) these things are just thought experiments at present. – Dilaton Jan 10 '13 at 10:25
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Thanks @annav, now I could read it. Seems they want to distinct between different shapes the extra dimensions can have by looking at the cosmic micro wave background. I think this would be a similar indirect hint to strings as for example trying to infere the shape of the extra dimensions from measuring the spectrum of Kaluza-Klein particles or the detection of supersymmetry. Some time ago I was thinking about writing something about the more direct effects I had in mind but this is now no longer needed since this question has settled down to an accepted good answer ;-). – Dilaton Jan 10 '13 at 17:23