Fabric of space

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itsawonder

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When I have seen depictions of the "Fabric" of space-time it is always shown as a "grid" with objects such as planets, stars, etc showing an indentation on this grid. The size of the indentation is relative to the mass of the object, with a black hole culminating in a huge "gravity well" ending in a "point (like a funnel)", which I assume represents the singularity.<br /><br />My question is as follows: Is this representation accurate? It implies that a black hole has a definite "top" and "bottom". It would seem to me (bear in mind as a lay person) that a black hole would appear to have a gravity well regardless of your orientation to it and the "top" and "bottom" exists in all directions at all times.
 
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derekmcd

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Yes, it is accurate in a 2d world. The intent is not to imply a top and bottom, but a representation of the strength of the warp of the fabric of space surrounding said object. Little difficult to represent in 3d. <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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plutocrass

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I have a few drills with subheaded exercises where I ask the students to imagine what is really being represented by the space-time fabric grid thing with the bowling ball in the middle that causes that funnel shape. These types of exercises are good to get your mind thinking creatively in different ways. I'll list one drill, and one exercise and five questions.<br /><br />Drill#1) Imagine how gravity affects someone in 2d land. This is because the gravitational force acts in a 3d manner. <br /><br />An example of one exercise in this drill is to picture the land as a 3d shape, (but it is perceived as 2d by its 2d occupants you may picture as dots). First, picture a 2d piece of fabric (paper), and call this 2d land. Now make it 3d by curve the land in a c-shape so that the edges point upwards. Finally, touch the edges together so that the structure forms an "O"-shaped tube, and lie it on its side. Make the occupants live inside the tube, moving from side to side. <br /><br />Questions: <br /><br />1)When moving side to side, will they experience gravity? If yes, how will they perceive it?<br /><br />2)How would they perceive your finger if you poked it through the edge of the tube?<br /><br />4)How would they experience gravity if you sent the tube rolling down the hill?<br /><br />5)Bonus: a square is 2feet by 2feet. You have two cubes. Cube A is 2meters long by 2meters wide by 2meters high, and CubeB is 2meters long by 2meters wide by 8meters high. Which cube holds more squares?
 
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derekmcd

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answers:<br /><br />1) no. In a 2d world, there would be no mass, therefore, no gravity.<br />2) As a dot... similar to the layers of a cat scan.<br />4) see answer 1<br />5) both would be infinate... a square has no thickness. <div class="Discussion_UserSignature"> <div> </div><br /><div><span style="color:#0000ff" class="Apple-style-span">"If something's hard to do, then it's not worth doing." - Homer Simpson</span></div> </div>
 
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siarad

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I don't think it's entirely true, you need to rotate it 360 degrees to make a solid of revolution.<br />If the bowing ball was crushed to a marble's size the dent would be much steeper creating a greater gravitational <b>effect</b>.<br />More knowledgeable people here will no doubt be able to talk about horizons or something & there being an up & down from which jets escape. <br />If you mean up & down coz external gravity is used to create the dent then it's not right as gravity is being used twice <img src="/images/icons/rolleyes.gif" />
 
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