Space time fabric

Discussion in 'Physics & Math' started by Chatha, Mar 9, 2006.

  1. Chatha big brown was screwed up Registered Senior Member

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    Studing QM lately but need some help on some issues

    No physics expert I am but I understad one thing, according to Einstien the phenom gravity is a result of space time fabric. This menaing the Sun's mass is exerting so much pressure on its surroundings that earth is pulled inwards from this pressure. However the fabric is not a two dimensional linear matress but in all directions, therefore why is the earth pulled in one direction towards the sun? Its like putting a ball inside completely inside water and another smaller ball next to it, how can the big ball cause so much significant force that the smaller ball is drawn towards it? This greatly confuses me. The inertia of the earth should also mean the earth is approaching the sun, is this true?
    Anyway If black holes exert so much mass in all directions this multiple direction vectors should cancel out and light should be able to escape but light doesnt, which can only mean that spacetime fabric is flexible and one direction experiences the most force force compared to the rest, which is why light cannot escape from that direction. Even this is hard to imagine because in a huge mass like a black hole its possible that all electrons are at stable state and hence no change in state means so relase of energy/ photons and no observable light. Make any sense?
     
    Last edited: Mar 9, 2006
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  3. Poincare's Stepchild Inside a Klein bottle. Registered Senior Member

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    Well, I am not an expert on gen. rel., and it has been quite a while, but I will give this a shot...

    Einsteinian space-time is 4 dimensional (3 space and 1 time) and is being curved in yet another dimension by the presence of mass. This makes it very hard to visualize. That is why the rubber-sheet model is used so often.

    Objects which are only under the acceleration of gravity are moving along the geodesics of this curved space...that is, objects in free fall and light beams travel in "straight" lines (I know, confusing. You have to have a little knowledge of differential geometry to understand geodesics.) The direction of this curvature is always toward the mass, which is why the acceleration due to gravity is toward the center of mass.

    In the case of a black hole, inside the event horizon the geodesics can only approach the event horizon asymptotically. They never reach the event horizon. That is why light can not escape.

    Hope this helps.
     
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  5. Chatha big brown was screwed up Registered Senior Member

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    I guess if we are talking about a vacuum that will immidiately make sense but what about high pressured air, shouldn't that also create another dimension in itself without the presence of a body of mass? May I say this gives M-Theory a little more credibility. Anyway I understand the concept of space-time gravity but as with many aspect of quantum mechs you need a bucket load of imagination. I guess my problem is geodesics because I can't just imagine that space-time, a 4 dimensonal object, creates a curve or path in one particular direction. Its like putting a ball inside a bucket of Jell-o and putting a ball on top of jell-o, there are two different events and the ball on top of the jell-o is more likely to generate an attractive force on its immidiate sorroundings than the one inside the jell-o. Are you with me?
     
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  7. cato less hate, more science Registered Senior Member

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    I don't follow this statement:
    air is a massive body.

    the rubber sheet analogy is a very poor one, and that may be why you are confused. you have to remember that the rubber sheet is in every direction, not just the horizontal (as commonly shown). for example, say the rubber sheet creates a downward slope when a mass is on it (the downward slope is equivalent to an attractive gravitational force). now make a sheet perpendicular to that sheet, it too is warped by the mass. now another that is angled between those, and another, and another, ect.

    so, the point is, no matter what angle you are approaching a mass from, you can consider yourself as being on that one sheet that is commonly shown (horizontal), and as you approach the mass, the slope magnitude (gravity's force) increases.

    does that help?
     
  8. Chatha big brown was screwed up Registered Senior Member

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    Got it Thanks. My imagination derserted me. Maybe its from reading about the many theories of QM. But consider that the only place this attractive gravitational force works best is in a vacuum like atmosphere, because air mass may influence the geodesics of a traditional vacuum. That is, imagine if the atmospherc pressure around the sun and earth(space) was larger than imaginable then its impossible to have a geodesic gravitional force, if any at all. In fact with enough atmospheric pressure both the sun and earth will actually explode. This gives me an idea. If the sun with its heat is creating enough pressure in the surrounding solar space it may be what causes stars or galaxies to collapse. When you boil water after considerable heat the lid of the pot opens and with enough energy it may explode. However this calls for a closed system and as it is the universe is not closes or is it?
     
  9. Chatha big brown was screwed up Registered Senior Member

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    Why is Einstiens relativity theory not compatible with black holes?
     
  10. Poincare's Stepchild Inside a Klein bottle. Registered Senior Member

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    In what way?

    Schwartzchild's solutions to Einstein's equations for general relativity predict the existence of black holes.

    Cato...Thanks for the help explaining the rubber-sheet analogy. I do so much work where you have to project higher dimensions down to something low dimensional where you can see it, that I often forget most people haven't developed that skill.

    Chatha...As to whether the universe is an open or closed system, well, it depends on what model of cosmology you use. The classic model is closed, but some of the new ideas say that it may be open
     
  11. Chatha big brown was screwed up Registered Senior Member

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    Just one more question to clearify my thought. Why is it that the faster a particle goes the more mass it gains. Thats against the laws of classical physics. Photons for example gain to mass, please explain to us idiots
     
  12. cato less hate, more science Registered Senior Member

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    well, the simple answer is that special relativity multiplies the rest mass of a particle by the factor gamma. gamma gets larger as the velocity WRT (with respect to) an observer increases. a photon has no rest mass.

    thats a simplified version, I recommend wikipedia for more information.
     
  13. DaleSpam TANSTAAFL Registered Senior Member

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    Cato's answer above is correct, however, I would like to point out that most physicsts today will not talk of relativistic mass and will only refer to a body's rest mass when they talk of mass. Instead, the usual idea is to talk about relativistic momentum. This makes a little more sense because both classical and relativistic momentum increase with velocity. With relativistic momentum the only difference is that the increase is not a linear function of velocity (except at low velocities).

    Although it is not necessarily wrong to speak of relativistic mass (and many on this forum like the idea), it is not an essential concept to understand SR. The reason that the idea is not used by physicists is that a body's mass is proportional to the magnitude of its four-momentum, and it is therefore a frame-invariant quantity. This frame-invariant mass is the rest mass, not the relativistic mass.

    -Dale
     
  14. marv Just a dumb hillbilly... Registered Senior Member

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    It depends on whether you talk about gravitational mass or inertial mass. They're not the same.

    The bullet from a .223 rifle round weighs 55 grains if measuring the gravitational mass on Earth. When fired on Earth at near sea level, it transfers 55 grains of gravitational mass and a couple tons of inertial mass to the target. Inertial mass is only stored kinetic energy. Nothing more.

    Gravitational mass is relative to the gravitational field in which the object is measured, and inertial mass is relative to velocity within the medium traveled. For instance, the metrics would be quite different from Earth if measured on the Moon or Mars!

    In short, Einstein was full of shit.

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    Now all Einsteinites are welcome to flame on...

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    Last edited: Mar 14, 2006
  15. Zephyr Humans are ONE Registered Senior Member

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    You could say so ... but we usually define mass to be an invariant property and multiply that by gravitational acceleration to get weight which is simply the force an object exerts due to gravity. Weight is properly measured in Newtons but laypeople confuse the terms since bathrooms scales give the result in kilograms (a scale would obviously give the wrong mass on the moon).

    And as you can probably guess from the name 'Newtons' - this was long before Einstein

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    Last edited: Mar 14, 2006
  16. marv Just a dumb hillbilly... Registered Senior Member

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    ...which is fine for measuring gravitational mass, but not the inertial mass which strikes at the heart of so much of Einstein's work.
     
  17. Poincare's Stepchild Inside a Klein bottle. Registered Senior Member

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    marv...

    Gravitaional mass m is what you get when you solve G = g*m*M/d^2

    Inertial mass m is what you get when you solve F = m*a

    In common situations, these are going to be the same. Your assertion that a bullet masses tons while moving is completely wrong. The mass of the bullet, both gravitational and inertial, is the same.

    Given the special conditions of Special Relativity and General Relativity, I fail to see how inertial mass "strikes at the heart of so much of Einstein's work."
     
  18. Raphael Registered Senior Member

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    If only the definition of weight were as simple as physicists would have you believe.

    Weight vs. Mass; What's the Difference?
     
  19. DaleSpam TANSTAAFL Registered Senior Member

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    Wow, that is one hell of a rifle. A relativistic mass of 2 tons and a rest mass of 55 grains means that your bullets are flying at .999999999998 c. That's 880986 times the speed of sound. Impressive artillery, perhaps a bit excessive for taking down your average buck. Must leave one hell of a bruise on your shoulder.

    -Dale
     
  20. marv Just a dumb hillbilly... Registered Senior Member

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    Poincare's Stepchild, DaleSpam, okay, so I exagerated a little.

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    But a 225 Winchester weighing 55 grains (.125 ounce) fired at a velocity of 3570fps imparts 1556 pounds of force as kinetic energy to the target. Reference here. The gravitational mass of the 55gr bullet IS STILL ony 55gr at impact.

    Einstein says, via equivalency, that the .125 ounce bullet acquired an additional 24,895.875 ounces of "mass" when accelerated to 3570fps. I maintain that the additional 24,895.750 ounces is stored kinetic energy, not "mass".
    Time to re-examine the contention of C and "infinite mass".
     
  21. DaleSpam TANSTAAFL Registered Senior Member

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    No problem. Btw, you definitely have the most decorative collection of smilies that I have seen on this forum.

    I don't know exactly what your physics background is, but you are mixing all sorts of things together. Specifically, you are confusing energy, mass, impact, and weight. The key error is that you are equating the final 1556 pounds of impact force with the initial .125 ounces of gravitational force or weight. The gravitational force, or weight, w, is equal to w=mg where m is the mass and g is the acceleration due to gravity (9.8 m/s²). On the other hand, during the impact the force on the bullet, by Newton's laws is f=ma. As you know, the bullet decelerates from 3570fps to 0fps in a very short amount of time, in other words a is very large, much larger than g. This large a leads to a greatly increased f without increased m.

    In fact, if you really wanted to do relativistic calculations then at 3570fps the gamma factor is equal to 1.0000000000066. This leads to a negligible increase in "relativistic mass" of 2.3 10^-14 kg. Again, most modern physicists (including Einstein) do not consider "relativistic mass" to actually be mass. Instead they will talk of the relativistic momentum increasing and will only talk of mass as rest mass. Basically, most modern physicsts would agree with your second-to-last sentence (other than the number). They would agree that the increased "relativistic mass" of 2.3 10^-14 kg is stored kinetic energy, not mass. The re-examination that you correctly propose already occured 70 or 80 years ago.

    -Dale
     
  22. marv Just a dumb hillbilly... Registered Senior Member

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    Thanks. I beg, borrow and steal from other forums I post on. I try to make them appropriate. Here are some you can link to w/IMG code.
    None - school drop-out - but avid reader! Please explain to me the following:
    1. It is generally conceded that you cannot accelerate beyond C because "mass" becomes infinite at C.
    2. C is still generally considered to be invariable when lab experiments clearly demonstrate that C depends on the medium through which light is transmitted.
    3. It should take more force to accelerate a 1kg mass from 10mph to 15mph than it would from 5mph to 10mph, following Einstein's equivalency.
     
  23. Poincare's Stepchild Inside a Klein bottle. Registered Senior Member

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    First *...It is more appropriate to say the object must have infinite momentum. For an object with mass it would require an infinite amount of energy to reach the speed of light.

    You might try looking up info on the Higgs field. I don't know if this will help you much, because I don't understand it all that well myself. The Higgs field is supposed to give matter its property of "mass" and is why a particle with mass can not reach the speed of light.

    Second *...The speed of light IN A VACUUM is invariable. Matter interacts with the electrical and magnetic fields of light to slow it down.

    Third *...Yes, it will take more force. However, that difference is so minute that it would not really matter. Newton is still a very good approximation for everyday events.
     

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