Strings and loops have me all tied up

  1. Is there someone out there who could explain string theory and loop quantum gravity to me in plain English, please? I have no mathematical ability whatsoever, so I'm looking for an explanation of the concepts. Numbers leave me scratching my head. I am not suggesting that my numerical stupidity is anything to be proud of; it is simply a fact. I have tried many times over two decades to grasp quadratic equations and failed. You see my problem. Pleease help.
    Kate.
     
  2. jcsd
  3. Hi katelynndevere
    Welcome to the boards. I am sure there are many people here who will be happy to discuss with you. I am only answeringthis quickly because I see you are still online.

    Richard
     
  4. Thanks Richard,

    I have no doubt about there being many people who might be prepared to enter into discussion with me; what vexes me is whether I will understand what any of them have to say. I read a post that was helpfully entitled 'Intriduction to Loop Quantum Gravity: Everything you ever wanted to know' and was tantalized, but not quite enlightened by it. I fear I may be a lost cause, but I live in hope.
    Thanks again for your welcome.
    Kate.
     
  5. Kate,

    I am not very talented in mathematics either, but I have a good imagination for three and four dimensional objects. I am sure you are not a lost cause, as long as you have hope. May I ask, what features of the intro string caught your imagination? I am asking because it may help our conversation if I know where your strengths are.

    Richard
     
  6. Hi Richard,

    Now, when you say intro string, do you mean thread? I am a simple soul and easily confused. I will assume you mean thread.

    I came across this site whilst trawling the net for a layperson's intro to strings and quantum doohickeys and this thread appears the most relevant to my line of enquiry. i hope to understand more about these theories because it seems to m that they have the potential, between them, to produce a proper big toe, which fascinates me for the same reason it facinates everyone...
    My strengths? I don't have many, but I can follow a logical argument; I can imagine and visualize three and four dimensional objects, so long as I don't have to mathematically describe them, and I WANT to understand. I am not afraid to ask the same question endlessly, in order to get an answer I can really get my teeth into. I have no science background, though, and have found most 'explanations' of string and LQG to be baffling at best. That's probably not what you wanted to know, is it? Ho-hum.

    Hoping you or someone can help me educate myself,
    Kate.
     
  7. nrqed

    nrqed 3,048
    Science Advisor
    Homework Helper



    Kate,

    As I am sure many others will suggest, the best thing would be to start with reading a couple of books written for the general public (i.e. without any maths). You should probably start by reading The Elegant Universe, by Brian Greene. That would give you a very goood starting point and if you get stuck on anything he says in his book, many people here would be happy to help you understand. It would be hard for anybody here to provide you with a more complete and well written intro to string theory than Greene's book. For loop quantum gravity, the book "Three roads to quantum gravity" by Lee Smolin is a good intro.

    regards

    Patrick
     
  8. "Imagination is more important than knowlege", or so it has been attributed, to Einstein.

    Probably the first thing to understand in the Quantum world is the idea that for any observable or measurable quality, there is a smallest amount. It is debatable whether this smallest amount is "real" in the sense of something material like marbles or bb's, but in anycase it has turned out to be a useful tool in understanding the behavior of objects smaller than atoms.

    So we have a set of quantities called Planck units, after the scientist, Max Planck, who spent a great deal of effort thinking about them. We have the Planck mass, the Planck length, and the Planck time, from which most of the other physical quantities of interest can be derived. The Planck mass is roughly human scale, about the size of a small flea, and it is thought to be the amount of mass necessary, if compacted completely, to make a small black hole. The Planck lenght and the Planck time are very small entities, and are related to each other by the speed of light.

    In physics we usually talk about velocity rather than speed, but it is the same thing really. We measure speed or velocity in miles per hour, or kilometers per second, or in some other pairing of a length measure with a time measure. It is not much math, just a division, as in say 8 kilometers / 4 seconds = 2Km/s. Then we can do all sorts of calculations that tell us about how to expect physical objects to behave.

    Is this all familiar to you, or would you like to go into any of it in more depth?

    Richard

    ps, Patrick is quite correct, and there are many available books in cheap editions which will describe these things better than I could. R.
     
    Last edited by a moderator: Mar 19, 2005
  9. Hi Patrick,

    Thanks for your suggestions, I will start as you recommend, with 'The Elegant Universe'.

    Kate.
     
  10. Richard,

    None of this is familiar to me, but all makes perfect sense.
    When I say I have no mathematical ability, I do not mean to suggest that simple arithmetic and algebra elude me, so your explanation is accessible and helpful, thank you. I look forward to continuing this discussion, but must sign off now as it is very late here (02:58) and I must sleep (damn!) sometime.

    Bye for now,
    Kate.
     
  11. Ok. Glad to help. I was just thinking in the past few days of starting an intro to quantum thread on another website, and perhaps an ongoing conversation could be useful to both of us, especially here where so many interested persons can be of assistance.

    If you understand the idea of physical velocity, as in L/T, that is a length per a time, then the next idea is that of acceleration, which in physics is L/T^2, where T^2 means Time squared, or T x T. We can write that here in Latex form as [tex]L/T^2[/tex]. That is entered on the keyboard as "[-tex-]L/T^2[-/tex-]" I have added the "-" sign in the latex form to prevent it from being translated into a formula here. To get the formula, type it in as shown but do not include the "-" sign.

    There are at least two types of acceleration, that of an increase in velocity, as by means of stepping on the accelerator in an automobile, and that of gravity, which we feel when standing on the surface of our planet. The first type is covered under Einstein's special theory of relitivity, usually abbreviated on the boards as SR, and the second type is covered by Einstein's theory of general relitivity, which is abbreviated GR.

    The relationship between SR, GR, and QM (quantum mechanics) is the area of interest in Grand Unified Theories (GUTs) and Theories of Everything (TOEs.)

    If the dishwasher doesn't swallow a fork and get a bad case of the hiccups requireing a mechanical version of the Heimlich Manouver, I'll be back with more tomorrow.

    It happens I am on my way to work and will check back in here in the morning, probably around 7am my time, which would be 1 pm where you are. Best wishes, sweet dreams,

    Richard
     
    Last edited by a moderator: Mar 19, 2005
  12. I read the elegant universe when i was preparing for a lecture I had to give in england for school. If you want a quick crash course I have a copy of my lecture here which attempts to condense some of the elegane universe. I would be interested in your opinions if you get a chance to listen to it. I would also be interested in what you would think would be useful for someone just starting out with string theory.

    http://www.quantumninja.com/toe/StringIntroLecture.mp3

    if you want you can follow along with

    http://www.quantumninja.com/toe/powerpoint.ppt
     
  13. Hi Richard,

    I understand velocity as length / time, and acceleration as length / time x time, but i do not understand how gravity can be a form of accelertaion. I do not feel as if I am accelerating when I stand still on the surface of the earth. What am I missing?

    and - what does Latex mean? what is a latex? My dictionary does not offer a definition that makes sense in this context, it only refers to rubber, or milky white fluid produced by plants - that's not what you mean, is it? :confused:

    Sorry if these stupid questions are frustrating for you to answer, and thank you for making time to help me with this.

    Kate.
     
    Last edited: Mar 20, 2005
  14. Hi Tom,

    I listened to your lecture this evening - it is a wonderful introduction! I have a mental picture of our universe as a membrane existing alongside myriad potential other universes. I totally get how strings vibrating at different frequencies could form various different particles or stretch out to produce branes.

    Your lecture has, like all good introductions, set my synapses to firing off questions:

    what precisely are quarks? how do they make up other particles?
    what is the weak force? what is the strong force?
    how does gravity leave our universe?
    where does it go? into another brane universe? can it come back? what happens if it does?

    I need to listen to it again, it's a lot to take in.

    Is the gravity particle made of strings which have closed up into loops? Is this what quantum loop gravity is all about? (or am I way off the mark here?)

    Your lecture is very accessible - are you a teacher?
    Thanks very much.

    Kate.
     
    Last edited: Mar 20, 2005
  15. selfAdjoint

    selfAdjoint 8,147
    Staff Emeritus
    Gold Member

    If you jump off anything you accelerate; your downward speed increases at the rate of an additional 32 feet per second every second. If you just stand you have weight. Weight is a force; the amount of weight is determined (up to a close approximation) by Newton's force equation: F = ma, where m is your mass and a is that acceleration. So that is how acceleration acts when you are not falling.

    Latex (properly spelled LaTeX) is a way to enter mathematical and chemical expressions so they come out like in the books. There is a sticky thread on how to use it up I think in the general physics forum.
     
  16. SelfAdjoint,

    Thanks for your quick response.

    So, if i am standing still, my acceleration is length (0) x time (also 0 if I am standing still) x 32 (feet per second)

    So I'm actually always falling downward at a rate of 32 feet per second and it is only the physical surface of the earth that stops me moving?
    That's how gravity makes me stick to the surface of the earth?

    So why is gravity on earth 32 feet per second? I gather this is related to the earth's mass? what is the earth's mass?

    Kate.
     

  17. Hi Kate.

    Sorry about the misspelling of LaTex. I should have explained it.

    If you are standing still on the surface of Earth, you are still accelerating at 32 feet per second per second. This is indeed due to Earth mass. Einstein thought that the acceleration of gravity (GR) and the acceleration of an object on, say, a rocket in space (SR), should be the same thing. Imagine yourself in a room in a rocket ship which is accelerating under the thrust of a rocket moving toward Alpha Centauri, our nearest star other than Sol. If the rocket thrust is 32 feet per second per second, it is pushing the floor of the ship toward Alpha Centauri, and the floor pushes your feet which pushes on the rest of you. You can stand on the floor of the ship just as you can stand on the floor of your room at home. If you drop something, it will fall to the floor just like it would in your livingroom.

    What Einstein said was that if you are in a room without windows or any other way to look outside, you cannot tell if you are standing on the surface of a planet or on the floor of a rocketship. Acceleration, in both cases, is what you feel, and you cannot tell the difference between the acceleration of gravity and the acceleration of a rocket thruster, except by refering to outside objects that are not affected by the thrust. This is where he started when he wrote about his General Theory of Relitivity, back in 1915.

    In the rocket ship, everything in the room around you is also being accelerated by the same thrust, so it moves in the same frame of reference as you do, in other words, you would say that you, and the chair on the floor next to you, are at rest, or not moving.

    The same thing is true on the surface of the planet. Everything you see around you on Earth is accelerating just the same, in the same frame of reference, so you do not experience any sense of movement. That is what relitivity means. Movement is always relitive to something. If two objects are in the same frame of reference, they do not appear to move, relitive to each other. Only when one of them is accelerated differently from the other does motion occur.

    Say you are holding a rock and you let it fall. It moves relitive to you. We might say the rock is in free fall, which means that, while it is falling, it is not feeling the acceleration of the Earth as you are. For the time that it falls, it is in a rest frame. When it hits the ground, it once again becomes accelerated.

    What is the weight of a falling rock? It hasn't any. In free fall, things are weightless. If you go up in an airplane and jump out (be sure to use a parachute) you will be in free fall, and you will feel weightless. Until you hit the ground of course. Then you will feel your weight again. This is because weight is not the same as mass. Your mass does not change in free fall, only your weight does. Weight is the force of the acceleration of gravity acting on your mass. In physics, we say F=ma, force equals mass times acceleration.

    In a sense, when you are standing on the surface of the Earth, you are not falling downward because the surface is pushing up on your feet, just as the rocket thruster pushes up on the floor of the rocket. The ground pushing up on your feet prevents you from falling any further, and at the same time, gives you the feeling of weight.

    You can look up the Earth's mass and read about how it is calculated. Try looking in Wikipedia.com, a free online encyclopedia. I use the number 1.38E33 Mass units, where a mass unit is about the mass of a small flea. The number shown is read "one point three eight times 10 raised to the 33 exponential power", and can be written as 138000000000000000000000000000000.00 fleas. I'll try converting this to kilograms for you later, if someone else does not beat me to it, but right now I am overdue at supper.

    Be well,

    Richard
     
  18. I'm not sure I understand. Surely the rock that I let fall to the earth IS still accelerating due to gravity? (I understand that it is weightless but not without mass whilst falling) What really confuses me is how the rock can be said to be 'in a rest frame' whilst in free fall. Surely it is still moving as it falls to earth? I'm sure I'm missing something obvious, sorry to be such a duffer.

    Kate.
     
  19. The rock has a rest frame, the frame in which the rock is at rest.

    Suppose you jumped out of an airplane with the rock, would it be falling relative to you? Neglecting air resistance, you and the rock fall at precisely the same rate so it is not moving relative to you.

    In the rest frame of the rock you dropped, you are moving away at 9.8 meters per second per second, and the earth is moving towards it. Do you see how this works?

    Einstien's idea was to view gravity as an accelerated frame of reference. (Similar to the way that centrifugal force is due to your acceleration, not any actual force.)

    If you want to learn modern physics, you have to spend some time learning classical physics. I don't recommend spending any time thinking about string theory or loop quantum gravity, which are not even actual theories at this point.
     
  20. Thank you Crosson

    Thanks for your help with acceleration and rest frames and your suggestion that i concentrate on classical phyics. Whilst i am sure that what you say is correct, I am driven by an irresistable desire to understand strings and loops, because of the 'parallell universe' possibilities inherent within such ideas (even if they are not strictly theories at this point, due to being unproveable). I'm afraid I must allow my heart to lead my head, right or wrong, so I will go on trying to learn only as much classical physics as I need, to be able to grasp string theory and LQG.
    Thanks again, all the same.
    Kate.
     
  21. Original Post





    Let me answer your last question first...
    No I am not a teacher, I am only a high school senior, this is something I should make very clear from the start. Everything I put into the lecture is from things I have learned from other people... the way I interpt it. I am just saying this because I am not 100% correct all the time, not that anyone is but I just wanted to make that clear.

    Going on to your other questions...
    it will take me awhile to answer them fully but let me give you some short answers.

    Quarks are subatomic particles that until recently were thought to be among the smallest constiutients of matter. They come in 6 varieties... Up, Down, Strange, Charm, Bottom, Top. These quarks can combine in differnt ways to create hadrons. The most common reference of what quarks make are protons and neutrons

    The weak and strong force are among the four fundamental forces in our universe that we know of. (The other two being gravity and electromagnatism) The weak force is the force between leptons and quarks. It is the force that allows quarks to tranform so that and down quark could become a up quark.The strong force is responsible for hodling a nucleus together against the repulstion of the protons. However it is a very short range force. Its range could be [tex] 10^{-15} [/tex] m where the range of the weak is [tex] 10^{-17} [/tex]. Also in comparision to the strong the weak force is [tex] 10^{-5}[/tex] the strength

    This is only theorized possible with the existince of gravitons. Gravitons are believed by some to be closed loop strings which wouldn't be bound to our dimensions and would be free to travel into higher dimensional space, unlike open loop strings which are theorized to be responsible for the other forces we see. This could be one explanation to why gravity is so much weaker than the other forces. Some believe that maybe gravity is just as strong as the other forces, but that we don't see that because the graviton responsible for the force is only in our dimensions for such a short period of time. After the intial idea of leaving our dimensions what exactly the graviton would do is a matter of much debate.
     
    Last edited: Mar 21, 2005
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