Answer, if u can...

Discussion in 'Physics & Math' started by rohIT, Feb 26, 2012.

  1. rohIT Registered Senior Member

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    76
    This is not a challenge as the title might suggest. I had a few doubts, mostly related to String Theory, so, if anyone can, please help me understand

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    1) Firstly, the experiment with the two metal plates to demonstrate Casimir Effect. Why can't we think it to be just the cohesive force between them? And how is the phenomenon of repulsion explained?

    2) If each elementary particle corresponds to a string (an electron string, a proton string, a neutron string, etc.), how is the degeneration explained? And, where do quarks fit in?

    3) If all forces are transmitted through specific particles whose properties are determined by the string vibrations, what exactly is energy (I previously thought light was pure energy)?

    4) How is observed speed of light same for all the observers in relative motion?

    5) In "The Brief History Of Time", Stephen Hawking explained the Spin Quantum Number as the number of times you'd have to rotate a particle so that it looks exactly like it looked prior to the spin. Then how come the spin of graviton is 2 and how is any other spin possible at all?

    6) I did not understand how String Theory (ST) treats distances. It says that if one observer determines by experiment, that the distance was x and another found it to be 1/x, it was exactly the same! How would anything fit in the universe then?

    7) ST says that the dimensions we do not observe are curled up. I did not understand where, and why. What could make any dimension curl up in the first place?

    8) I do not understand why Bohr's Model was kicked away because of the implications of Uncertainty Principle. Even if we could not determine experimentally the exact position of a particle, what makes the theoretical determination incorrect?

    9) How is radiation explained in ST? I mean, if each particle is a string and if each messenger particle is also a string, how can a sting give strings?

    10) How heavy is the string (theoretically of course)?
     
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  3. AlphaNumeric Fully ionized Registered Senior Member

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    1. It isn't a cohesive force. It's to do with the conductivity of the plates, not their interactions. Interactions like Van Der Waal's forces or Hydrogen bonding are something different.

    2. What do you mean by degeneration? And quarks are other string oscillations. All particles are string oscillations.

    3. Formally energy is the conserved quantity associated to translation in time. Energy in string theory is like energy in anything else, it's associated to motion of things and potential between different components.

    4. Do you mean how is it known, ie by experiment, or do you mean how does it follow from physical principles?

    5. Actually the number of times is 1/n where n is the quantum number. Electrons have spin 1/2 and they would need to 'rotate' twice. It's not really rotation in the usual sense, that's just a way of understanding the mathematical structure. Since it's just an analogy you shouldn't take it too far.

    6. What you're referring to is something quite different. There's an \(R \leftrightarrow \frac{1}{R}\) symmetry but it isn't the thing you're thinking it is. Distances in string theory are described in just the same way as in other areas of physics, using a metric. The \(R \leftrightarrow \frac{1}{R}\) thing is about relating to different string theories. Given a string construction on a toroidal space with radii R if you invert the size of the toroidal radii and reformulate the string fields you end up with a second, entirely equivalent string construction. Apriori you would not expect that, you'd expect the result thing not to behave in the same way. Why should two completely different structures in space-time behave in exactly the same way? It's because of what is known as T duality.

    7. They are everywhere. It's like saying "What is up?" At every point in space you can draw little arrows which point up or down or left or right or backwards or forwards. If you could see things on the Planck scale you'd be able to draw 6 other arrow directions. The analogy used in lay person books is about how a hosepipe seen from a long way away looks like a line but when you look very closely you see it actually has thickness and depth. Why they are curled up and other directions are not is an open question.

    8. The theory doesn't take into account all parts of quantum mechanics so it is inconsistent with experiments if it says precisely where a particle is and how fast it is going. It was constructed in the early days of QM, before all the implications were understood.

    9. Strings can split and join, shrink and grow.

    10. A string's rest mass depends on the oscillations on it, just as energy can construct to the mass of systems. In bosonic string theory the lightest string mass is tachyonic and inconsistent. In supersymmetric string theory the lightest string mass is 0.
     
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  5. camilus the villain with x-ray glasses Registered Senior Member

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    Thank you.
     
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  7. indianmath Rajesh Bhowmick Registered Senior Member

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    Do u have any idea abt the spin numbers of quarks?
    If not then get some.

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    regards.
     
    Last edited: Feb 27, 2012
  8. rohIT Registered Senior Member

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    sure, thanks

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  9. rohIT Registered Senior Member

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    @AlphaNumeric
    1) I understand H bonding and Van Der Waal's forces. I want to know why cant the coming loser of the plates be attributed to any of these (of course, H Bonding wont come into the picture, i am speaking about cohesive force and if you say they could use different metals and end up with the same result with different magnitude of force, why couldnt that be attributed to adhesive force?)

    2) If quarks are string oscillations and protons are also string oscillations, how come three quarks make up a proton? are these quark-strings superimposed to form protons and other particles?

    3) In fusion reactions, mass is converted into energy and heat and light are given off. If pure energy is given out, how are the messenger particles formed? And, how much energy is required to create strings?

    4) No, i mean that if the observers conduct an experiment to determine the speed of light they will observe it to be the same, irrespective of their frame of reference. How is that possible?

    5) Hawking wrote that electron is like a two sided arrow, so you need to rotate it by 180 degrees (half rotation) just as you need to rotate that arrow so that it looks exactly the same. I remember it was half-rotation and not 2 rotations.

    6) Thanks

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    I get this now

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    7) I don't get it. You can draw the 3 lines anywhere in space because these 3 dimensions are expanded. It is like, given a graph sheet, you can draw the axes anywhere, because the paper exists in those dimensions (of course i am not taking the third dimension into consideration). But if they exist everywhere, at small enough scales, things could travel in those dimensions as well. And if the size of the dimension is so small that you can return to the original position in that dimension with the slightest movement, you keep travelling through other dimensions so many times if you consider them to be present everywhere. Can you please give me a link where i can read more about this?

    8) Uncertainty Principle restricts our ability to know accurately the momentum and position. How does that imply that a particle does not have a definite momentum and position?

    9) Ok. How much energy is required for this? And i the stings can either split or grow when the energy is supplied, what determines which one happens?

    10) If i remember correctly, the rest mass of the particles depend on the string oscillations, not the mass of the string.

    And, thanks for your answers

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  10. Brett Nortje Amateur stripper Registered Senior Member

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    [1] I guess it is the force then? If you were to throw a ball, or throw a brick, it is still the same force 'throwing' the materials.

    [2] Three quarks make a proton because they have up and down quarks, with two up quarks for each down quark. This means they have have balance and form a body with these values, and only through these values find balance.

    [3] The messenger particles are formed the same way they form in your mind - they would carry information along a neural web, in this case the energy line, from the energy given off. Think of a piece of fluff with wind behind it.

    [4] Well, light is three dimensional. They will find light refracting off of each three dimensional source, but, if they were to view it from head on, they will not be able to see the other dimensions, only height and width.

    [7] They curl up into a parcel, like a compressed thing.

    [8] I would say it does. A particle takes up space, but we don't know how to understand where it is.

    [9] It gorws until it splits because it is too big, like water droplets.
     
  11. AlphaNumeric Fully ionized Registered Senior Member

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    6,702
    Those are classical forces. If you turn off quantum mechanics you can still have classical electromagnetism, which predicts those effects. The C.F. is due to the way in which quantum mechanics allows certain things to be integers only, rather than continuum values. Plus quantum mechanics predicts the value of the force the plates experience and it's experimentally supported.

    You can combine oscillations to make more complicated oscillations.

    There's another way of viewing hadrons, via gravity/gauge duality. Given a brane you can attach a string to it by the string ends, like a handle struck on a sheet. To someone living on the brane the ends of the string look like point particles and depending on how the gauge field of the string/brane behaves the particles can look like quarks. The two particles are linked by a string and so keep close to one another. Plus if you pull the particles apart then the string can snap, the ends join the brane and now you have 2 pairs of quarks, just like how pulling a quark out of a hadron makes more quarks. Using this you can examine confinement and colour superconductivity, as well as the QCD mass gap. Depending on the properties of the brane, it's dimensions, it's charges, whether other branes are in the space-time, you can do lots of interesting things. You'd be surprised how difficult it is to use a 7 dimensional brane with strings attached, orbitting a stack of 3 dimensional black holes, to describe technicolour (an alternative to the Higgs boson)

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    What is 'pure energy'? Energy is a property of things. Things move faster or new particles, such as light photons, are made. The energy needed to make a string depends on its oscillations. If the oscillations represent a photon then the string mass will be zero but it'll have the corresponding photon energy.

    Because relative motion means that two different people measure distances and times differently, in such a way that the velocity of light is unchanged even if other velocities are.

    It's 2 rotations because the electron is a spinor. Rotating by 360 degrees with spinors picks up a factor of -1, so doing 720 gives you (-1)(-1) = 1.

    I think there's a "Simple introduction" Wiki page on string theory.

    Pretty much yes. Initially people said "Well obviously the particle has a definite position and momentum but our measuring devices aren't precise enough!" but after the development of Bell's inequalities and experiments confirming them that concept, known as Hidden Variables, took an arrow to the knee.

    It depends on the particular dynamics of the system. It's not entirely understood fully but as a general idea strings carry tension, like normal strings, so to stretch them costs energy just like stretching a rubber band takes energy. The precise motion of snapping involves quantum fluctuations so you can't so with absolute precision when it'll happen but its influenced by the properties of the string.

    Yes. The string ends move at the speed of light so the string as an object in and of itself is massless. However, when you add in the energy of the oscillations you now have something which doesn't behave like a completely massless thing. That's not quite literally true but the whole method of going from the string action through to the effective theory of the string oscillations and their mass tower, along with supersymmetric corrections to kill a tachyonic state in bosonic theory, makes up a sizable chunk of most introductory string theory lecture courses and even for the postgrads which sit them some of the things are a little eye brow raising at first glance :bugeye:.
     
  12. rohIT Registered Senior Member

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    guys, thank you! will get back to you with more questions to ask. Thanks

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