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Dimentions of string theory

  1. Jul 5, 2009 #1
    I've heard it said that to include a quantum theory of gravity in string theory that the model must include extra dimensions to explain the apparent weakness of gravity. First off, why? couldn't you just as easily say the universe is bigger than we thought and that's why gravity is so weak? And if you label the three dimensions of space and the dimension of time as dimensions, why isn't temperature a dimension of an objects existence, or any other fundamental unit of an object such as the SI base units of measure?
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  3. Jul 5, 2009 #2


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    There is a fair more bit to it than simply gravity being weaker than expected. It's a bit more involved than can be explained in a thread.
    It is not necessary to define the temperature of an object to be able to pinpoint its location uniquely in spacetime.
    SI units of measure are nothing but a yardstick. You use them to measure distances in the three physical dimensions. They are equivalent to metres or light seconds.
  4. Jul 5, 2009 #3
    You have asked a complex question. IF you really want a good read, I suggest THREE ROADS TO QUANTUM GRAVITY, by Lee Smolin...I have a 2001 edition and the following comes from there:

    (There are a myriad of quantum theories and string theories so specific examples of each might have some different component relationships. You might also be interested that the Holographic principle appears to be a fundamental principle in BOTH approaches.)

    Extra curled up dimensions in string theory are necessary to make string theories mathematically consistent, not any desire to marry string theory with other theories. In other words, no one to my knowledge manipulated hidden string theory dimensions to achieve some goal with gravity. (But not all extra dimensions yield conditions like those in our universe.) In fact the reverse happened: It was accidentally discovered that consistent string theory included a "gravity particle"...that was NOT an intent of early string theory. String theory was NOT originally developed to study gravity!!!

    Gravity derives from matter and energy in the universe and on large scales it's pretty uniform; so local gravity does not derive from the size of the universe but mostly from local mass and energy. The universe is getting bigger every day, anyway, due to cosmological expansion. Another way to think about this is that gravity from the cosmological horizon, for example, because it's so far away, is negligible in our galaxy.

    There are three space dimensions and one of time in classical relativty, four in total. That's because Einstein's equations related those to the effects of gravity; his equations did not tease out temperature as a fundamental entity. But his equations do show that gravity is effected by energy,mass even pressure: more energy, more mass leads to more gravity, so a hot brick has more "mass" than a cold brick.
  5. Jul 5, 2009 #4
    The reason for extra dimensions is not so ham-handed as to "explain the apparent weakness of gravity." When we work out the mathematical constraints for a relativistic quantized superstring, the only way to solve the constraints is to pick a specific high dimensional spacetime, often D = 10.

    No, the size of the universe is not a parameter that can be used to solve the superstring constraints.

    These are only dimensions in the generalized mathematical sense where a dimension is any parameter, temperature, pressure, insect population, momentum, etc, in some abstract "space." In superstring theory the D = 10 spacetime dimensions can be rotated and transformed into each other, in the normal way that special relativity works in our ordinary 3 + 1 dimensional spacetime, to relate the coordinate systems of various observers in relative motion.
  6. Jul 5, 2009 #5
    ok, do you know of an online source where i could research to get a better understanding of this, im only 15 so its hard to get college level information when your in high school
  7. Jul 5, 2009 #6


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    Wiki is not a bad start. There are references for further reading at the bottom.
  8. Jul 5, 2009 #7
    thanks guys :)
  9. Jul 8, 2009 #8
    I agree with the above answers to your question though I would like to add my comments. I have heard of suggestions that gravity is weak because it has to work in multiple dimensions thus diluting its effect in our three / four dimensional world. I think that Liz Randal from MIT is one proponent of this.

    However, this is not the reason for the extra dimensions in string theory. The extra dimensions in string theory would appear necessary to remove inconsistencies which otherwise appear. The strength of gravity is related to the amount of matter in any locality and a universal constant. As the universe expands gravity will get weaker on the large scale because matter will be less dense but gravity's strength compared to the other forces is more dependent on the universal constant(s) of gravity - plural here since there are Newton's & Einstein's constants but they are related.

    As to labelling dimensions, you can label any variable as a dimension mathematically but in string theory we are talking about actual extra dimensions of space. Also, in the sub-atomic world temperature would be just a measure of a particles energy / momentum. What we measure as temperature in our world is just an average of particle energies.
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