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Relativity Questions

  1. Mar 6, 2008 #1
    Ok im pretty new to the relativity thing. Im not at university, and I have started studing relativity by myself out of interest - so dont laugh at these stupid questions pleeease :)

    1. Biggun - the fabric of space time - is this purely theoretical - to help us understand gravity, or is it a real thing? Furthermore, why and how are we able to think of space and time in this way?

    2. Light is effected by gravity. Does light have mass then?

    3. From what i understand of the fabric of space time, it is like a weight on a taught pice of cloth. anything coming towards towards the wieight will be attracted to it, and begin circling it? Why doesnt it just fall into it?

    4. What, very broadly is the schism between Quantum, and Relativity?

    Very VERY broad questions... sorry

  2. jcsd
  3. Mar 6, 2008 #2


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    What is your definition of "real"?
  4. Mar 6, 2008 #3
    broad questions... very broad, general answers

    To understand anything you will need a foundation from which to view from:

    Be sure you understand Classical Physics well: before tacling Relativity and Quantum Mechanics.

    Science in broad strokes has three fundamental Theories:

    Classical: Ok as it is for day to day “normal” use but proved itself to be inadequate to handle two situations. The solution to those problems were not fixable within the classical view but required two new and different theories to address those situations.

    General Relativity: Built to go beyond classical relativity options like Special Relativity and Space-time Concepts into a independent theory that addresses the Very Large (as in size distance and speed) and the Very Heavy (even if very small in some cases). Astrophysics needs this as Classical cannot successfully address those issues.

    Quantum Mechanics: Built from classical frustrations with quantum issues into an independent theory that has been imagining successful in predicting our “reality”. As a fundamentally theory it allows us to understand the very small with tiny size (as in masses much smaller than a single atom). Important to note there are many different “interpretations” of QM. It defines Particle Physics and the Standard Model that goes with it.

    Your number 1 and 4 go together:

    Of course gravity is real you feel it every day!
    The real question is what kind of “Thing” is it. And that puts you right into how QM and GR are incompatible with each other.
    For QM to work it needs gravity to be the result of particles (gravitons, others) interacting between items of mass. Simple and fundamental to the theory, no positive proof yet that’s why scientists work on the theory.
    That is totally incompatible with the simple and fundamental foundation to GR where gravity is the result of curvature of space and time caused by masses in that space. A very different “thing” than particle interactions.
    What is that “thing” – sorry way past a general answer to a general question! Plus if GR could really answer that question it would be better than QM. Another reason why we don’t know which of the three theories is “correct”. (although everyone figures it is not Classical based on the success of the other two)

    As you read about GR which I assume is your primary focus you will find it helps to understand some about QM as well. Neither has been able to show itself completely correct by showing the other to wrong. The two do find some areas that overlap.

    Like light having no mass, but consists of “Energy”. All three theories can work with the SR principle that Energy has a certain equivalence to mass to allow effects as if it had mass. Thus it should bend when passing a large mass as it follows the curve on your “tight piece of cloth” depending on it starting direction. And when a photon disappears into an atom as it is absorbed the atom gains a tiny bit of mass.

    Finally “tight piece of cloth” is not exactly a “space-time” analogy and it is not a very “good” description of what GR gravity is. Consider it an “ok” description and as good as you can find until you get beyond the broad answers. I.E. lots of reading, math (matrix math), and thinking to go.

    As to “Why doesnt it just fall into it?”
    No one says it won’t – it just a hard thing to have happen by accident. Anything on your “tight piece of cloth” will only go straight in if it initial motion is straight at the center. How many other directions might it randomly be moving if you divide that up into degrees, minutes and seconds? And your only dealing with a flat two dimensional sheet (the curve is only for gravity) instead of three dimensions.
    And the odds of finding something that is not already moving is pretty much impossible.
  5. Mar 7, 2008 #4


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