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AonSao
2009-Jan-14, 09:32 AM
I've heard GR explained as "a bunch of pool balls sitting atop a trampoline". Also, every time I see pictures of galaxies, they appear to be pretty "flat".

Are all of the celestial bodies in a galaxy really lined up along the same plane, or are they more "3D" than oversimplified explanations have led me to believe?

Tog
2009-Jan-14, 10:12 AM
You'll get a lot better answers in a bit, but I can actually cover some of this one.

The "pool balls on a trampoline" is only talking about the effects of gravity on spacetime. Spacetime is the trampoline, and the mass of the ball represents the gravity generated by the objects. This creates a curved effect to spacetime which was predicted, an later observed during a solar eclipse. Distant satrlight passing close to the sun was observed to be in a different spot that it should have been. The sun's gravity "warped" the space around it and altered the path of the light.

Galaxies themselves are not located in a single plane. The best analogy I can think of is to take hundreds of coins and pile them on a blanket. Get a few people to yank the blanket tight and launch thr coins into the air. As this is happening, take a picture of the coins. What we see in the sky is that picture. Some galaxies are edge on (m104) while others are face on (m51) Some, like Andromeda and Triangulum are very close, whie others are so far away, they are only faint smudges on even the deepest Hubble images.

They can also be found in every direction. If you have objects to the north, south, east, and west, they could all be in the same plane. When you also have similar objects straight above, a plane is no longer possible.

Be sure to check back for any corrections, and further detail from others more knowledgeable than myself.

AonSao
2009-Jan-14, 10:59 AM
Thank you for the response ^.^


The "pool balls on a trampoline" is only talking about the effects of gravity on spacetime.

Isn't the effect of gravity on spacetime the main thing that determines their position though? ... so they would be in a line in that case?

The coins on a blanket seems like a good analogy. The coins wouldn't be lined up in a straight line in the air, but the overall shape of the coins would be horizontal. My mind has this image of the black hole at the center determining the plane that the galaxy sits upon, and the variances (perpendicular to the plane) are due to gravity from the bodies within the galaxy acting upon each other (also their individual direction/speed).

Maybe it is easier to ask if our own solar system lies upon a flat plane? There are far fewer variables involved in that scenario.

Tog
2009-Jan-14, 11:32 AM
Isn't the effect of gravity on spacetime the main thing that determines their position though? ... so they would be in a line in that case?
The effect is a curve, so it's very obvious close to the object, but much less so the further you get. Technically, the gravity from a single grain of rice can be felt throughout the universe, its just that effect is so small as to be insignificant.

Each "trampoline" is only a small representation of the area around the body, not the entire universe or galaxy. The sun's "trampoline" might only extend for a light year or two. Beyond that, the plane just sort of blends in with everything else.

If there is something bigger and closer, it will overwhelm the weaker gravity being generated by a more distant object. Sirius is big. Much bigger than our sun, but the gravity from the sun dominates our solar system. The gravitational effects from other stars are next to meaningless as far as Earth is concerned.

When you get up to galaxies, they do clump together because of their own gravity. Galaxy clusters, and even superclusters are known. Back to the blanket analogy, let's say that 5 coins came off with nearly identical starting values in all variables. They would travel together for quite a while. If we scale that up to galaxy size, those 5 coins might get far enough away from everything else to essentially ignore the gravity from the other galaxies, but form a cluster of those 5, which are gravitationally bound. How that cluster eventually arranges itself would depend on gravity and inertia, as well as the constant expanding of spacetime.


The coins on a blanket seems like a good analogy. The coins wouldn't be lined up in a straight line in the air, but the overall shape of the coins would be horizontal. My mind has this image of the black hole at the center determining the plane that the galaxy sits upon, and the variances (perpendicular to that plane) are due to gravity from the bodies within the galaxy acting upon each other (also their individual direction/speed).

But the coins in the blanket won't all have the same starting values. Those closer to the middle will have more speed and a more vertical direction than those closer to the edges. This might end up making something like a hemisphere, or dome. Now, if we take that step further, and wrap that blanket all the way around so that when it's pulled tight, it basically inverts itself into a sphere, the coins will plas out in all directions. another way to think of that might be a bomb or grenade, where all the galaxies are shrapnel. Some may have started off quickly and are further away, while others were slower to get moving and are close in. Either way, the result is a "cloud" with a lot of empty space and galazies suspended in it.


Maybe it is easier to ask if our own solar system lies upon a flat plane? There are far fewer variables involved in that scenario.

It depends on what you mean by flat. Most of the planets orbit the sun within a few degrees. Pluto has a 17 degree inclination to the Ecliptic. The Ecliptic is the line the sun makes as it passes through our sky. Or, from the point of view of the sun, the Ecliptic is the only plane in which the Earth will allways be found.

Now, the best idea for the formatio of the solar system was a spinning cloud of gas, that eventually formed into the sun and planets. Because of the way things formed, it's going to end up on a plane more or less.

If we assume that other solar systems formed the same way, they will be on a plane as well. These two planes, do not have to be the same, or even similar. Our plane can be anywhere from 0 to 90 dergrees relative to the planes of other systems in our galaxy. This is becaue, even though the gravity of our galaxy can has an effect on out solar system as a whole, that effect is overshadowed by the gravity of the sun.

Spiral Galaxies look similar to soalar systems, and do have a central plane that runs through them, but that "plane" can be several thousand light years thick, with stuff crossing from the north to the south and back again over hundreds of millions of years. So again, it depends on what you mean by a "plane". In the true geometic sense, no. Bodies in a galaxy are not on a plane. They wander above and below it, and some might do so at very steep angles. In our own night sky, the band of cloudy light seen from dark skies is the "Milky way". This is the plane of out galaxy. The rest of the sky is filled, fairly uniformly, with other stars. If everything were on the same plane, that wouldn't happen.

Cougar
2009-Jan-14, 04:07 PM
I've heard GR explained as "a bunch of pool balls sitting atop a trampoline".

As Tog said, the trampoline is analogous to spacetime. But it is a projection of 3D spacetime onto a 2D surface. It is an attempt to portray how mass curves spacetime. This is easily portrayed in 2D, but not so easy to visualize in 3D... hence the need for an analogy.

mugaliens
2009-Jan-14, 11:11 PM
Also, every time I see pictures of galaxies, they appear to be pretty "flat".

Those are just the pretty ones. ;)

They come in all shapes and sizes (http://en.wikipedia.org/wiki/Galaxy#Types_and_morphology).