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Space-time Fabric: how what and why it warps?

  1. Dec 31, 2007 #1
    I would like to ask another 3 question,

    1.Gravity causes warps in the space-time fabric in general Relativity what cause this gravity? i was told in grade school it was with the planets rotation around its axis, but now, trying to do my own research in General Relativity i have found nothing of that sort...but that doesn't mean its wrong...just maybe its general understood?? But if thats the case how?

    2. what are these warps in the fabric really?

    3. and i came across this description and i can't make much sense of it...if you could maybe help explain it;


    "When a mass is present in the above space-time it distorts it so that whilst it remains true that traveling through space causes you to travel through time, traveling through time now causes you to move (accelerate) through space. In other words just by existing, you are compelled to move through space - this is gravity."


    please help clarify!


    And thank you
     
  2. jcsd
  3. Dec 31, 2007 #2

    jcsd

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    Newtonian gravity models gravity in terms of force fields, General relativity models gravity in terms of spacetime curvature. Both these models are primarily mathetical ratehr than ontological in nature.

    In special relativity we use flat spacetime to model the motion of objects, etc. Special relativity is more a kinematical theory that doesn't model gravity. But when we start to use a wider varitiety of spacetimes (i.e. don't stick to just flat spacetime) we find we can model gravity too using spacetime.

    What I'm trying to say is general relativity deosn't really try to answer questions like 'what causes gravity?' and 'what ware these warps in the fabric of spacetime really?', it just provides a mathematical model that makes predictions that can be tested.

    Okay that may seem rather dry, but really that's all that general relativity does, any philosophicla implicatiosn are thigns that we read into the theory.
     
  4. Dec 31, 2007 #3
    Thank you,

    I was, technically, looking for an answer that didn't exist.
    That makes looking for the answer frustrating haha

    so thank you for saving me the further aggravation ^_^

    BUT

    do you know where i can find any philosophical implications i could look at on my own?
     
  5. Jan 1, 2008 #4

    A.T.

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    When in free fall, you move straight ahead trough space-time. When the space-time is curved, the direction of this straight line changes in regard to the dimensions of space and time. When the space component of this direction changes, your speed trough space changes.

    To get a picture download this and look at chapter 2. (Don't worry: no math but a lot of pictures)

    To see what happens locally in a strongly simplified (!) way look here (flash player needed). Set the "initial speed" to 0, the "gravity" > 0 and press "play". You see that the rocket initially at rest, starts moving trough space by moving straight through space time. Note that the engines are off.

    For a more global picture around a spherical mass look here (flash player 9 needed).

    You can get the latest flash player here:
    http://www.adobe.com/products/flashplayer/
     
    Last edited: Jan 2, 2008
  6. Jan 1, 2008 #5
    I'm unfamiliar with the math of general relativity, but I have a pretty fair understanding of it. It starts with the equivalence principle, which states that the force you feel if you are accelerating is the same as the force of gravity. For example, if you were in a coma and woke up in a spaceship, you wouldn't have any way of knowing if the 1G force that was pulling you to the floor was the force of gravity, or just the force of acceleration you feel because the spaceship is firing its rockets.

    One example of this equivalence you can experience in everyday life. If you hold a helium balloon you are aware that it automatically moves the opposite direction of gravity. If you hold the same helium balloon in a car, when the car accelerates you feel yourself pushed into the back of the seat, but you'll see that the balloon moves toward the front of the car.

    So if the two forces are equivalent, what does that mean? Imagine you are in an accelerating rocket ship. If you have a flashlight, light emitting from it will appear to bend toward the floor, because the spaceship moves upward from its original reference frame toward the beam of light. If the equivalence principle is right, this answers the age old question of whether light bends under the force of gravity.

    Now, how do we go from this to warped space and time?

    Time is the easiest one for me to understand. Say I aim my flashlight toward the ceiling of my accelerating spaceship. If somebody above me looks at the light, it will be red shifted. This is because he is accelerating away from the place where the light was when it was first emitted, because the entire spaceship is accelerating. Because of this, each wave of light is going to have a larger gap between it by the time it reaches him than when it was emitted from me.

    Now say he is looking at a clock instead of a flashlight. There are only so many light waves emitted from my clock each second. But if he is seeing red shifted light, that means the time between the waves is larger. The only way he can see what is going on with my clock is by seeing the light emitting from that clock. So if there's only a certain number of waves for each tick of my clock, and he sees more time between those waves, then he sees more time between ticks of the clock as well. As a result, my clock is ticking slower than his.

    Similarly, if I look at his clock, I'm accelerating toward the light from his clock, so the light from his clock is blue shifted, the time between waves is reduced, the time between ticks is reduced, and his clock ticks faster than mine.

    By the equivalence principle, if this happens in an accelerating spaceship, it happens in a gravitational field. So gravity warps time.

    Gravity also warps space. This is harder to explain, because you have to consider the fact that gravitational pull is different at different distances from the earth. However, like I already said, by the equivalence principle, a beam of light has to bend in a gravitational field. (But just like in the example of the accelerating spaceship, the light isn't really "bending", but rather, our reference frame is accelerating to make it look like the light is bending.)

    Since the pull of gravity is different at different distances from the earth, the light will "curve" by different amounts. Now normally, if you draw one line, and then you draw two lines perpendicular to that line, you have two parallel lines. That's what happens in flat space. But what if I draw a line from the center of earth into outer space? And then what if i fire two laser beams perpendicular to that line? They will appear to curve toward the earth, due to the equivalence principle. But since the gravitational acceleration is stronger closer to the earth, the beam of light will bend more if it closer to the earth. As a result, even though both lines are perpendicular to the first one, they are NOT parallel lines! They are moving away from one another. This is not possible on a flat piece of paper. It is only possible in curved space. So space is warped.

    Here is another way of looking at it. Like I said, the equivalence principle states that the force of gravity is actually an imaginary force, the same force we feel in an accelerating car. Therefore, while you sit in your chair, you are not actually sitting still. You are accelerating away from the center of the earth at 9.8 meters per second per second. And objects that are in free fall do not feel the force of gravity, they are weightless. So objects that are in free fall are not accelerating toward the surface of the earth. The surface of the earth is actually accelerating toward objects that are in free fall! And yet it is quite clear that the earth is not expanding. If the surface of the earth is not expanding, and yet it is accelerating away from the center of the earth, the only possible explanation is that time and space are warped in such a way that everything remains "at rest."

    All of this sounds really weird but it has been experimentally verified. The universe we live in really is this strange.
     
  7. Jan 2, 2008 #6

    paw

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    While the rest of your post seems to be resonable this part is just plain wrong, that is unless I misunderstand you.

    A helium balloon does NOT move in the 'opposite direction of gravity'. A helium balloon responds to gravity the same way all matter does. The reason a helium balloon rises is that it is being acted upon by a stronger force; buoyancy.

    A helium balloon in a car will move to the back under acceleration the same way your body does. It does NOT move toward the front of the car. Air resistance and friction might make it hard to see but that is in fact what happens.
     
    Last edited: Jan 2, 2008
  8. Jan 2, 2008 #7
    Buoyancy acts in response to gravity. Yes, of course a helium balloon is being pulled toward the center of the earth and yes, in an accelerating car it is being "pulled" toward the back of the car. But the result is that the balloon moves away from the center of the earth and the balloon "moves" toward the front of the car.

    Try it. The "gravity" generated by the acceleration of the car alters the direction of buoyancy in the car. I'm not making this up. If you have a bunch of helium balloons in the back of the car and you accelerate rapidly, they will all fly toward the windshield even as you are pushed into the back of your seat. I've seen it myself. Then try slamming on the breaks. The balloons will move to the back of the car when you lean forward into your seatbelt. This baffled me when I first saw it. It's pretty cool.
     
  9. Jan 2, 2008 #8
    Well.. there is something to be said about this. A planet rotating on its axis does warp space because of it's rotation in addition the the warp caused by its mass. The majority of the force we measured is due to the warping from the mass, however, the rotation also exerts a very subtle effect.
     
  10. Jan 3, 2008 #9

    paw

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    Ok, I see what you are saying now. The balloon is responding to buoyancy as a secondary effect of gravity or acceleration. In your original post it wasn't completely clear (at least to me).

    I'm a bit skeptical whether you'd see the balloon move forward in a car. I mean the air density isn't going to change much under normal acceleration. I'd have thought air resistance and friction etc. would hide the effect. I'll give it a try one day. Meanwhile I'll have to take your word for it.
     
  11. Jan 3, 2008 #10
    In response to :

    1. The concept of what is gravity is really still in the air. Most believe that the answer will come when physicists figure out how to connect Quantum Gravity with Classical Gravity. As far as gravity goes on a macroscopic level it is simply understood that it comes from mass, why still escapes us. As stated above the mass imparts a much greater "warp" than rotation.

    2. The best analogy that I can give is to think of the warps in space-time as being similar to placing an object of mass into a fluid. Any object placed into a fluid will cause a warp in its behavior. The tricky part from this analogy is that the flow of a fluid on earth is generally asymmetrical, flowing in one direction; i.e. a river. Space-time on the other hand is symmetrical meaning that any "flow" we might think of actually happens in all 360 degrees at once.

    3. I don't really like that explanation, especially the part about moving through time causes you to accelerate through space. According to Einstein's time dilation the faster you go the slower time gets and the slower you go the faster time gets. At the speed of light Time" supposedly stops. This means that when you travel through time you are also traveling through space but not accelerating. If you accelerate in time you decelerate in space and vice versa. It's kinda like you only have so much "energy" that can be used with motion through space or time, as you increase one you equally decrease the other without adding or subtracting from your initial "energy" allotted.

    My best guess is that the quote is meant to describe the relationship of space-time, I would not, however, attribute that relationship to gravity itself. Not yet at least...
     
  12. Jan 4, 2008 #11

    A.T.

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    You forgot the part about distortion of space-time. What I think is meant: "movement through time" combined with "distortion of space-time" causes "acceleration through space".

    You need both:
    - Without distortion of space-time you would keep you initial direction in space-time: moving only trough time and at rest in space.
    - Without the inital movement through time you would not be affected by the distortion of space-time
     
    Last edited: Jan 4, 2008
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