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String theory? sounds like a loose thread

  1. Nov 13, 2003 #1

    jimmy p

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    In sunny England, there is a 3 part tv programme type thingy on 'The Theory of Everything' in which string theory stars. Im one for embracing new ideas, but string theory sounds like a load of old hats to me. It may be the wishy-washy way that they present it with tons of special effects and that goddamned quantum cafe

    ("Can i have an orange juice please?")
    ("I'll try")

    Is there a more widely accepted string theory because the programme said there are about 5 accepted theorys. If so, where can i find such a theory which isnt in a complex language?

    Also the programme said that the strings were first proposed as a cut elastic band yet the graphics depicted them as loops...which one is correct?

    And finally, a string is supposed to be like a billion times smaller than the quark...so why is a quark so big? Does it stop being a particle, so is it just a probable area for where the string will be? And would that mean factors such as length, or (dunno bout this) frequency of the string affect charge and mass?
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  3. Nov 13, 2003 #2


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    I think the confusion comes from the fact that the string theory is combination of alot of amaller ideas that many people have contributed and literally stitched together. In the case of relativity you had the singular brilliant mind of Einstein and the equivalence principle as a guiding light. I thought a guy named Ed Witten at Princeton had found a way to unite the five string theories that were considered distinct? I am not a phycicist so I'd let them answer the particulars. Just my simplistic view of this absolutely fascinating field!
  4. Nov 13, 2003 #3


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    Here is a nontechnical essay in which a physicist who was involved in the development of string theory explains his story. He tells how the five original string theories came to be, and how they were unified by dualities. He is quite frank about the problems that come up due to the fact that no good experimental testing is available.
  5. Nov 13, 2003 #4
    All 5 of them are actually unified. The theory that is supposed to unify them is called M-theory, but it hasn't been fully worked out.

    Both. You can have both open and closed strings.

    No. A quark is a string (or rather, a string vibrating in a particular way).

    Yes, those determine what kind of particle a string acts like.
  6. Nov 13, 2003 #5
    Re: Re: String theory? sounds like a loose thread

    Isn't that only in Type 1 string theory?
  7. Nov 13, 2003 #6
    Re: Re: Re: String theory? sounds like a loose thread

    Yes, but the point is, it's possible to have both open and closed strings in string theory.
  8. Nov 13, 2003 #7
    "The Elegant Universe" originated over here in the states, Public Broadcasting System TV station WGBH, for their "NOVA" series.

    link --->

    The production is slick. Program host Brian Greene is a string physicist, best-selling author of the book with the same name as the program, and now something of a media darling, apprearing on nightime TV shows ,interviewed and publicized in magazines of wide circulation. He is recapitulating the glory of Carl Sagan in the role of science promoter, whose series "Cosmos" was broadcast in 1980. The style of this program shows it. Behind all the glitz and FX ("effects") lies an extremely sophisticated scheme of mathematics. The program featured several of the bright ones, pioneers and adepts, of string theory. The program does have flaws. Still, I think it is worth viewing several times.

    The Quantum Cafe is an attempt to reduce 5,000 words about the smallest level of physical reality to a scene, without having to explain details like the uncertainty principles and instability. Maybe it doesn't work so well, but its importance is to enhance the point of the wide difference between the realm of the very small and the very large. It should be coupled with the scene at the bottom of the elevator shaft (a very Cosmos-like touch IMO) with those quantum fluctuations boiling up everywhere (but not disturbing the elevator itself, somewhat like Dr. Who's tardis). The important contrast is with the top of the elevator shaft, where serene cosmic smoothness of spacetime rules. This is the background for any theory for unifying the small with the large, not just string theory itself.

    The 5 theories are actually 5 mathematical models that all satisfy the requirements that were set out for a string theory. There was little to guide a proper choice back in the 1980 era of string theory. But, by mid 1990s, the duality concepts of Witten and others tied these 5 models together in one supertheory, "M".

    By the way, I'll wager that "M-theory" originally simply meant "Membrane Theory". The word "membrane" was shortened to "brane", similar to the way that "n-tuple" from the theory of relational databases was shortened to "tuple".

    As someone else stated, there are open strings and closed strings. The theories that postulate breadsliced membranes for the regular 4D universe prefer open strings rooted on the slice, while gravity's gravitons seem to be closed. This was illustrated by the jelly vs. cinnamon-sugar scene in the program.

    Something that is missing from all this is discussion of fields. Do these exist all around those vibrating strings? The size of these particles in physics comes from the measurement of interactions between particles, scattering and absorption patterns. From a higher scale, a vibrating string might act like a solid massive particle. Some of these, like electrons and quarks, don't seem to come apart.

    Some other flaws in the program:
    1. No mention was made that there are rival approaches to the problem of unification.
    2. Einstein's work, illustrated in the first part of the program, really bears little relationship to the way unification is done since 1960. I guess he gets credit for trying to do it at all.
    3. The planet rolls around the lip of the sink to illustrate unforced gravity by the sun. Sir Arthur Eddington loved this illustration too. But, properly, the planet should be a flat dot in the surface itself. The same thing is true of the sun. TV demands that one show something bigger, so the bodies are shown atop the surface.
    4. The program doesn't really try to explain why there are 10 or 11 dimensions.
    5. Related to this last point, the account of history is selective. In the 1970 period, when string theory was an orphan child, theoretical physicists borrowed the aspect called "supersymmetry" for an ordinary pointwise field theory called "supergravity". There they got used to extra dimensions- as many as 26. Part of the big raveup for string theory in the 1980 period was that fewer dimensions were required, and also something called "renormalization" became unnecessary by switching to strings instead of sticking with points. The program didn't get into any of this. Insufficient time, I reckon.

    Despite this, I recommend viewing the program again (pour a nice pint of bitter; remember, it is entertainment). I recommend reading Greene's book, as much of it as you can stand. And I recommend coming back frequently to Physics Forums Strings, Branes & LQG topics to get the latest foibles and flummery of high-class thought, the leisure of the theory class.
    Last edited: Nov 13, 2003
  9. Nov 13, 2003 #8
    You can do string theory coupled to background fields, but those fields are supposed to be derivable as a coherent state of strings; everything comes from strings (or branes).

    Are there any competitors left? Kaluza-Klein mostly died off after Witten.

    Actually, string theorists independently invented supersymmetry about the same time as, or slightly before, supergravity. (I've seen some competing accounts of this, probably boiling down to what you mean by "inventing supersymmetry".) Supergravity tops out in 11 dimensions; 26 came from (non-supersymmetric) string theory. That superstring theory uses fewer dimensions was not a real selling point for the theory; if you're going to accept more than 4, it doesn't matter too much how many you use.
  10. Nov 13, 2003 #9
    Thanks for your corrections.

    Ambitwistor: everything comes from strings (or branes)

    How does fermionic string A send off messenger bosonic strings to interact with another fermionic string B, and vice versa? Is it accidental at first, but subsequently both fermionic strings start mapping the distances, ranges and motions of the other in their own vibration patterns and become increasingly deliberate about it?

    Or else, is this kind of information passed between them through a continuum of normally inactive strings located everywhere around?

    Ambitwistor: Are there any competitors left?

    I must reject my item 1 as my mistake. Quantum gravity theories are not really unification theories if they don't actually explain the existence of other fundamental particles and interactions. I stand corrected.

    "Loop quantum gravity makes no claim to be other than the quantum theory of gravity, and in fact appears able to incorporate equally well a wide variety of matter fields and interactions. So while loop quantum gravity can easily incorporate the standard model of particle physics, it, at least so far, makes no claims to explain any features of the standard model."

    "For example, non-commutative geometry appears in both [string theory and loop quantum gravity],[...]"

    "For example, supergravity can be considered to be a limit of string theory, although a few purists might want to insist that there still may be a quantization of supergravity which is not a string theory."
    quoted from Leo Smolin review article "How far are we from the quantum theory of gravity?", dated March 19, 2003, arXiv:hep-th/0303185v2

    Penrose twistors don't count either, I suppose.

    Ambitwistor: (I've seen some competing accounts of this, probably boiling down to what you mean by "inventing supersymmetry".)

    I had read somewhere that the proto-stringers first exploited the supersymmetry concept and this got picked up by Freedman, Nieuwenhuizen and others. That is what I meant by "borrowed" (not "invented"). Supergraviters liked their own theory because it was more like a standard quantum field theory: to the "point". They were just as excited about it as the stringers became when their time had come.

    I can't find who actually first came up with the idea of supersymmetry. It was actively developed by several different groups (Schwarz's, Ramond's, etc.) in the early 1970s, so say Haber and Kane ("Is Nature Supersymmetric", SciAm, June 1986).

    Ambitwistor: if you're going to accept more than 4, it doesn't matter too much how many you use.

    Normally I would agree: after 5 or 6, why not thousands of dimensions? But I keep thinking about the task of working toward specific solutions. Is it harder to classify and categorize solutions in 26 dimensions instead of a mere 10 or 11?

    Thank you.
  11. Nov 13, 2003 #10
    There isn't such a thing as a "fermionic string", at least in the sense of not having bosonic properties. In non-supersymmetric string theory, there are bosonic strings; in superstring theory, there are superstrings, which combine both bosonic and fermionic properties. (Well, a caveat: some people refer to superstrings as "fermionic strings", but that can be misleading, since they do have bosonic degrees of freedom.)

    I don't know. People haven't put as much work into classifying the solutions of bosonic string theory. If the classification of superstring solutions is easier, it's not necessarily because of lower dimension; supersymmetry can make things easier too.
    Last edited: Nov 13, 2003
  12. Nov 13, 2003 #11
    There isn't such a thing as a "fermionic string"

    I meant that in the sense of instantiating a quark or electron. I'll ask it more specifically.

    How does the string representing electron A send off messenger strings representing photons to interact with another string representing electron B, and vice versa?

    Anything to do with existence of a field?

    Thank you.
  13. Nov 13, 2003 #12
    What do you mean, "how"? In perturbative string theory, there is a finite probability for a string to spontaneously break. I'm not sure what to say beyond that.
  14. Nov 14, 2003 #13

    jimmy p

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    ok, that hasnt really explained amazingly much to me but thanx anyway! man i feel dumb!

  15. Nov 14, 2003 #14

    I'm sorry for turning around again and again on the shortcomings of my replies.

    Do view the show again if you get a chance.
    Do check out Brian Greene's book "The Elegant Universe".

    Best wishes,
  16. Nov 14, 2003 #15

    jimmy p

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    Hey thanx, i have 2 out of the 3 shows recorded, the 3rd is this sunday, ill watch em again soon, though im not really sure if it will help to much...i think im in too far over my head anway!
  17. Nov 15, 2003 #16
    quartodeciman wrote:

    "How does the string representing electron A send off messenger strings representing photons to interact with another string representing electron B, and vice versa?"

    The string representing the electron has conserved quantities such as mass, charge, spin, and momentum all incorporated in its vibrational mode.

    What is "vibrating", you might ask. Is it the shape of the string vibrating through space under tension like guitar string? Or is it some function whose value changes with time along the string while the size and shape of the string itself does not matter? As I understand it, these conserved quantities are conserved independent of how the size and shape of the string itself might change with time. These conserved quantities are an integration of some function along the length of the string. What is that function being integrated along the string? I don't know, maybe it is a variation in the tension along the string, maybe it is some background potentional, maybe it is some probability density function. I'll let someone more informed on these issues reply to that.

    In any event, a string representing an electron has a vibrational mode that incoporates the mass and momemtum of the electron, amongst other things. And if that electron string splits off into an electron string and a photon string, then the mass properties remain with the electron string, while some of the previous momentum is transfered to the photon string that has split off. The original electron string then looses some momentum, which is carried off by the photon string.

    And it seems obvious that the photon string cannot split off if all the properites of the photon are evenly distributed along the entire length of the original electron string. It must be that the properties of any photon that might split off must exist along some continuous portion of the original electron string before it even has a chance of splitting off into an individual particle. So it must be that some vibrational modes of the original string with given conserved quantites must be expressible as a string with a continuous portion that has splinter particle properties.

    We are probably interested in what the probability is that vibrational modes can exist with a given set of conserved quantities that can accumulate along a continuous portions of the length of the string. For example if that continuous portion of splinter particle properties is circulating around the original string, then the faster it circles the more probable it will be to split off.
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