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String Theory - Mind telling me what's the point?

  1. Apr 22, 2006 #1
    Hi, I saw a documentary of what string is and how small it is. But, what's the point of knowing that there are strings in every atom. I don't know the quantitative ratio of it but the documentary says that if an atom was the size of a solar system, a string would a tree on earth. No experiments can be done to test these, so what's the point of studying them?

    As I am typing this up, my brain is giving me a signal that this is a dumb question :confused: , but what's the point?
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  3. Apr 23, 2006 #2


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    That's like saying whats the point of knowing everything is made of molecules that are made of atoms that are made of smaller particles that are made of smaller particles. Applications of this range from chemistry, to nanotech, to circuitry, and nuclear engineering, and more that we haven't thought of yet.

    We don't really know the benefits of a complete string theory, but it sure would be able to increase our knowledge and development as a human race, and if we did have a finished theory of strings, we would be closer to the grand unified theory, if it exists, and have a greater understanding of the universe possibly leading to developments in artificial wormholes, understanding cosmological phenomena...
  4. Apr 23, 2006 #3


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    There's a controversy over this amongst physicists of different fields, so your brain is wrong, it is not dumb at all.

    There's a lot of criticism (IMO justified) from the part of "other" physicists towards the string and co community, in that what they do is not really physics because not really accessible to experiment for the moment.
    For the moment, say, the last 15 years, the main point towards society of studying string theory is to make popular shows on TV about it - I haven't seen any other "production" coming out of it yet, apart stimulating some mathematical research.
    But, the goal of the whole endeavor is not this - at least in the long term. In the long term, the goal is to have a unified theory of nature which makes it possible to *calculate*, from first principles, the fundamental constants of nature in a unified scheme. So that we can *calculate* the mass of the electron, the electromagnetic fine structure constant, Newton's constant of gravity and so on, and not have to rely on experimental input to do so. For the moment, we don't have any theoretical indication how to calculate these constants: it are free parameters to be determined by measurement.
    But in the 20-30 years of work on it, nothing came out yet in that sense.

    So whether we're being too impatient, and whether one should say that 30 years of work is not long enough yet and that there is progress, or whether we should say that there's something fundamentally fishy about the approach when nothing is on the table after 30 years, is the entire discussion.

    Now, usually, each time when this is done (trying to find the underlying theory to be "finally able to calculate from first principles what we had to measure up to now"), things turn out rather sore, and finally, the best results are still experimental.
    I'm thinking of spectral analysis of relatively complicated molecules (we have the underlying theory which allows us to calculate this in principle - quantum mechanics - but the calculations are terribly complicated when applied to complicated molecules) ; I'm thinking of nuclear processes (the underlying theory is QCD, but I've never seen a calculation that gives us the cross sections of U-235 ab initio)...
    But each time our insight is increased, nevertheless.

    So yes, the day that string theorists can calculate, ab initio, the mass of the electron (even if only approximately, and less accurately than we can measure it), we've made a big step forward, because contact with experimental reality has then been established, and this endeaveour has then entered the long tradition in physics. For the moment, it hasn't.
    Up to now, (and since about 30 years), this is what's promised, but nothing has come of it yet. Is this now time to say it is sufficient, or are we too impatient, is the whole debate.
  5. Apr 23, 2006 #4


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    I guess the basic question for that, vanech, is how do they know how accurate the calculations are based on theory? Isn't that accuracy dependant solely on the accuracy of experimental data, since that's what they're comparing it with to verify the theory?
  6. Apr 23, 2006 #5


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    Compare it to this artificial example. Egyptians (artificial Egyptians invented here by me for the purpose of the story) had an EXPERIMENTAL value for the ratio of the length of the diagonal over the side of a square, in a field. It was called Ohathep's constant, O = 1.41 +/- 0.02.
    So this was a fundamental constant of "square fields".
    They also had another fundamental constant, Shirana's constant, which was the ratio between the circumference of a circle and its diameter. S = 3.15 +/- 0.02.

    And then came along "string theorist" Euclid, and his student Pythagoras (:bugeye: ). And they had a model of "geometry" in such a way that they had a *theoretical* prediction, ab initio, of O: it should be sqrt(2). And also of S: it should be what we call now, pi.
    But next, they had to find a numerical calculation of these theoretical constructs, in other words, find a feasible calculational scheme, with additions and multiplications (the only manipulations we can really do), which produces an approximate result for sqrt(2) and for pi.
    For instance, using a series devellopment.
    And this will then result in a *theoretical prediction that can be verified with the experimental quantity*.
    For instance, sqrt(2) might be found by http://en.wikipedia.org/wiki/Square_root_of_two#Computation_algorithm

    But its accuracy will depend on the quality of the numerical approximation.
    This can be better or worse than the experimental accuracy. Now, in this case, it is not difficult to do MUCH BETTER than experiment. But that's not always the case.

    PS: the "historical account" is highly inaccurate :smile:
  7. Apr 23, 2006 #6
    The whole point is that it explains away the inconsistencies of General Relativity and Quantum Theory. The whole point isn't to find something useful but rather, to explain away the contradictions of the current theory.

    And until we can explain away contradictions, we cannot be sure that we are exactly on the right track, or totally off it altogether.

    Another big pull (and probably one of the ambitions of physicist everywhere) it to find the most fundamental set of rules and phenomena that can act as the basis of all physics, much like how mathematics is based off only a handful of axioms.
  8. Apr 24, 2006 #7
    Most of you might not have realised, that string theory already has a strong evidence to back it up. It predicts the existence of gravity! In the string theory documentary, Professor Schwarz encountered a massless particles which arises naturally from string theory, and turns out to be the particle of gravity, I think that is the main reason why string theorists are convident they are on the right track.
  9. Apr 24, 2006 #8
    I feel that is very hard to understand the sting's nature without pass throught QFT... and I have the impression that we have no deep comprension on QFT too! Ceers :rolleyes:
  10. Apr 24, 2006 #9
    String Theory—what's the point?

    Here and here is what's the point.
  11. Apr 25, 2006 #10
    Maybe String theory is really a theory which will change physcis in the future. The ideas behind it are really good and so one has to think that at least some points behind this idea are right.

    But isn't the Heim Theory a thing physicsits should give a long look at? At least it is predicting things! There is a mass formula which is able to calculate with a minimum of input every particle yet known. No other theory has achieved this until now.

    Maybe it has something to do with fear. Sounds weird but just think:
    1st scenario: You are working quite alone at a theory your whole life and it tourns out that all this work was for nothing because the theory is false.
    2nd scenario: You are a string theorist. Thousands of other physicists and you are trying to find a unified formula. But as bad as the world is it tourns out - string theory is false. (pff... it will take some time until you can even prove this theory wrong)

    So and now, which scenario will be the worse? I would say the 1st. And this in my opionion is one reason why so many physicists are working on this theory without knowing if it will lead to something - it's just better than achieving nothing in your life alone.

  12. Apr 25, 2006 #11


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    Ah, you're talking about this "one-dollar experiment establishing string theory's first prediction", where Schwarz drops his pen, and says: "look, it falls down !" (guess his pen was not a Mont Blanc :biggrin: )

    This is indeed impressive, and what got string theory off the ground: the natural occurence of a spin-two particle.
    True, it is indeed something impressive to start with, but it is not *sufficient* to prove the endeavour fail-safe.

    I'm thinking of another example where something similar happened, and where things turned out, in the end, to be wrong: SU(5) GUT.

    With a lot of work, the standard model was build up: first QED, with the U(1) gauge group. Next, the "most natural" extension, to SU(2), by Yang and Mills, failed - at first. Until some ugly fiddling around with spontaneous symmetry breaking and the Higgs mechanism (something nobody would have come up with if the data didn't push people into doing so to reproduce the Fermi weak interaction with a heavy boson) produced U(1) x SU(2).
    Next, SU(3) also got a false start: first it was applied to flavor (the eight-fold way by Gell-Mann) which turned out not to be anything fundamental, but got fished up in QCD as the color gauge group. Great, we now had a huge bricolage of U(1) x SU(2) x SU(3) in a twisted representation, and that's our famous standard model. Pretty ugly, doesn't seem to make much elegant sense.
    And then, people realised that this was possible to obtain from SU(5): it was the simplest and most beautiful scheme to "naturally" generate the "ugly" standard model group.
    Guess what ? It turned out to be wrong because it predicted the decay of the proton at a rate which was experimentally not observed. And then people started thinking of other ways to do the same thing. They're still thinking, and it became much less elegant, minimalistic and "obvious"!

    Have a look at the involved history of guts:

    So, it is not because SOME aspects are sometimes naturally predicted with a theoretical idea, that this idea is also correct. It surely is promising, but this is not a proof of "correct".
  13. Apr 25, 2006 #12


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    Funny though how that bricolage keeps reappearing in higher mathematical contexts. SU(5) was just the first one noted. Unless there's a newer one I guess Baez's reduced holonomy group of a certain Calabi-Yau manifold is the latest. Not one of them is guaranteed to apply to low energy physics, but it's all one in the eye for the view you expressed of the theory being an arbitrary cock-up. And remember the Coleman-Mandula theorem that says without tricks like strings and supersymmety U(1)XSU(2)XSU(3) is fundamentally all there is, folks.
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