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Constituent of a string

  1. Jun 29, 2009 #1
    1) When a guitar string vibrates, the molecular structure in the strings compresses and expands. Is there something in quantum strings that compresses/expands to allow vibration?

    2) What keeps a string from moving into a different dimension?
  2. jcsd
  3. Jun 29, 2009 #2
    The strings in string theory are fundamental components of matter, and so they are not made of smaller pieces. Furthermore they don't compress or expand while they vibrate, they have a fixed length that does not change.

    I'm not sure exactly what your asking. Imagine 4-d spacetime embedded in a higher dimensional space, so for the purpose of illustration 4d spacetime looks like a flat sheet. Now imagine strings with open endpoints, i.e. not a closed loop, and require the endpoints to always stay in contact with the 4d sheet. This is one way to model matter. Gravitons are always closed strings, so they cannot be confined to the 4d sheet, they are free to move through the higher dimensions which is part of why gravity is so weak for us.
  4. Jun 29, 2009 #3
  5. Jun 29, 2009 #4
    It seems a segement at a peak of occilation would be longer than that same segment durning the neutral phase. If a string is not made of smaller pieces what change allows vibration?[/QUOTE]

    Maybe I should've said: what's the difference between a convex portion of a string and a concave protion?
  6. Jun 29, 2009 #5


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    Talk of strings is just a guiding mental image, not a direct claim about physical reality.

    It is the mathematical properties of a "string-like entity" that are being illustrated. Many kinds of physical realities could perhaps fit the mathematics.

    For example, I find it more understandable to think of strings as trapped resonances rather like solitons or standing waves. You have vibrations focused at locations. Self-organised harmonics.

    Of course, this intuitive picture then leads me to dislike certain mathematical extensions to string theory - like the idea of little thready strings attached to branes at one end, free at the other.

    But I think my soliton picture does help explain the issues that worry you. There would be a medium in which strings are free to wiggle, yet remain confined to locations. Strings would be just emergent harmonics produced at a location by a top-down weight of context.
  7. Jul 2, 2009 #6
    Not so!!! Strings DO combine and break into pieces. And of course they expand and compress when they vibrate, as in when energy is added. Their are NOT in gneral of a fixed length. The likelyhood of them rearranging is called the string coupling constant; otherwise the strings may pass through each other.

    Take a string on the horizon of a black hole, for example. From time to time a loop of string will extended from the horizon due to quantum jitters....sometimes the loop will break off..a small loop will break free....(a piece of a string) ......a distant observer will see that as an emitted particle of radiation...this is the string theory explanation of Hawking radiation!!

    Finally, think of strings as elongated streams of energy...it's hypothesized some of these may be so big as the span the universe, although to date no such gargantuam strings have been experimentally observed.

    The above reflects explanations from Leonard Susskind, THE BLACK HOLE WAR, 2008

    quantum jitters...a fundamental instability even a "zero" energy levels.
  8. Jul 2, 2009 #7
    I suspect strings don't like such confinement: remember that when things are confined in very small spaces they get very energetic....wavelengths must get small to fit the space and via E=hf that means energy increases...quantum jitters go crazy!!!
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