Conceptual Questions About Space

There are some concepts, facts, and ideas about space that I cannot wrap my mind around or visualize, so I was hoping that some members here could help me out with the questions that I have.

Gravity- According to Einstein's Theory of Relativity, massive objects bend the fabric of space time, and that is how planets and other stars orbit them. This warping of space time is countered by the "cosmological constant" that keeps the planets from sinking into this warping caused by the star and colliding with it.
[atsimg]http://files.abovetopsecret.com/images/member/15ba5ecb82f0.jpg[/atsimg] The bending of spacetime caused by the massive object is what causes other objects to orbit it, much like putting a coin in one of those things that spins it around until it falls in the hole, however the cosmological constant is what keeps the object from following the curvature of the bend in spacetime and sliding inwards towards the object.

I can't visualize this for one main reason: space is three dimensional. This would make sense in a two dimensional universe, but since there are three dimensions and in space there is no up, down, left, or right, what determines the plane that the planets orbit around? Wouldn't the bending of space time be uniform in all directions as opposed to straight below the object as seen in the above image? And judging by the bend in space time caused by the mass, how do the planets all stay on the same plane despite the fact that the warping of space time is more severe the closer the objects are towards the sun? Does the cosmological constant not only keep the objects from orbiting inwards, but it also keeps them on the same leveled plane as the object?

Also since an object with more mass has more gravity, do very small objects still warp the fabric of space-time, but just very slightly? And since the cosmological constant keeps orbiting bodies from crashing into the objects that they orbit, how can stars form by gas coalescing if this cosmological constant was in effect? Wouldn't it keep the stars from touching each other and instead have them orbiting each other?

Expansion of Space: The discovery was made by Edwin Hubble a while back that there were many galaxies and not just the Milky Way. Hubble went above and beyond and also discovered that the universe was expanding by measuring the redshift of objects. Individual objects aren't expanding; the earth isn't expanding, galaxies aren't expanding, but the space in between them is. One analogy that helped me to understand this concept is a loaf of raisin bread: imagine that you could inject more and more yeast into baking bread, making it grow. While the raisins themselves wouldn't be expanding, the space in between them would.

This is where my mind begins to melt: If the space in between them is expanding, then why are some galaxies moving closer together? How can galaxies clump together in opposition to the expansion of space? How can individual galaxies themselves stay together? Why does the space between the stars not spread them apart?

There's another part of the expansion of space that I can't understand: if the farther out you look, the faster things are expanding, then how does that not mean we are at the center of the universe? I know we're not at the center of the universe obviously, but if the expansion speed is greater the farther out we look regardless of the direction....how does that work? The only way that I can perceive that as being possible is if the Big Bang occured smack in the center of where Earth currently is, expanding the universe from this point so that the farther out we look, the faster it's expanding.

Space is flat?: I'm watching an episode of The Universe, and an experiment was done to measure whether or not the universe was flat. They did this by firing lasers deep into space, and by measuring the angles and finding that they added up to exactly 180 degrees, they concluced that the universe was flat. Now one physicist still maintained his position that the universe was curved like a soap bubble and concluded that the curvature is so slight that we can't detect it because the universe is incredibly big. Assuming that the finding is correct and the universe is indeed flat, how does that work? How can it be flat but also three dimensional? If we can point in any direction and see stars, isn't that by itself proof that the universe isn't flat?

Black Holes: Whenever animated representations of black holes are made, they're almost always two dimensional holes in space that suck matter in. But if they're caused by massive stars going supernova, wouldn't they be three dimensional? How would matter be sucked into a three-dimensional black hole? Because galaxies always seem to have matter orbiting the supermassive black holes at the center in a disk-like formation, what about the black hole determines where the matter is attracted to? And if black holes suck matter in because the gravity is so strong that not even light can escape, then what would the fabric of space-time look like underneath a black hole?

Those are some of the things that I can never wrap my mind around, so I'm hoping some of you guys can help me to understand. I'm sure right after I post this I'll remember another concept that I could never fully visualize, so if that's the case I'll just add it into the thread.

reply to post by TupacShakur

I can't visualize this for one main reason: space is three dimensional. This would make sense in a two dimensional universe, but since there are three dimensions and in space there is no up, down, left, or right, what determines the plane that the planets orbit around? Wouldn't the bending of space time be uniform in all directions as opposed to straight below the object as seen in the above image?

Imagine it as if it were bent down into a hole (Like the coin thing you were speaking of)

and then imagine another bend into a hole right above it but this one is like going a hole through the roof/ceiling above it instead of below it.

(Equals)= it self out

I'm not 100% sure but that's just my thoughts on it.



Just try and imagine these two pictures super-imposed on each-other

[atsimg]http://files.abovetopsecret.com/images/member/0e5e5c9e27bb.jpg[/atsimg]

[atsimg]http://files.abovetopsecret.com/images/member/90e631a9be5e.jpg[/atsimg]

reply to post by TheUniverse


I assume you're referring to blacks holes, so I tried to visualize what you are saying and put it in the form of a paint drawing. I warn you though, this thing is a masterpiece, you might cry from the incomprehensible beauty of my artistic capabilities: [atsimg]http://files.abovetopsecret.com/images/member/461d06f41ade.jpg[/atsimg] If the blue line is normal space-time, then would the red holes be sort of what the black hole does to space-time? Any if that's the case, why would it only be in two directions? Why does it not extend like that 360 degrees?

Ah I see you edited it and you were actually referring to gravity of bodies in general. You're explanation makes sense, but the issue of space being three dimensional still makes my mind say "No way Jose". I would imagine space-time as being bent 360 degrees around the object, not just above it and below it. Maybe the poles of the object have something to do with it? The orientation of the magnetic field could determine where space-time is bent at.

reply to post by TupacShakur


I'm not certain but i added pictures to visualize it better maybe someone with more physics intellect can chime in

I'm not a scientist.

S & F

I think gravity works more like if you jumped in a pool and instead of displacing water it just compressed around you, changing the local viscosity.

Planets orbit like gyroscopes and like to migrate into the same plane as the suns rotation. if something were stationary it would fall straight into the sun, but we move at the right speed to stay in one place. Look as the moon is slowly moving away from us.. if it was slower, it would get closer and faster until it merged with earth.

If you can imagine being on a raisin in a really big loaf of bread, like the size of a house, it wouldn't matter where you were located as long as it wasn't near the edge.

reply to post by 4chi11e

If you can imagine being on a raisin in a really big loaf of bread, like the size of a house, it wouldn't matter where you were located as long as it wasn't near the edge.

Yeah I can understand that part, but what I can't understand is how solar systems, galaxies, and clusters of galaxies stay together. If the space in between them is expanding, then I don't understand how they instead resist the expansion of space and even get closer together. I would assume the mutual gravitational attraction has something to do with it, but I don't know for sure.

It's almost like we would need 4 spacial dimensions to be able to describe space.
Also the universe can't be flat. Are we back in the 14th century? if the universe was flat we couldn't have 3 dimensions.

Originally posted by TupacShakur

Gravity

General Relativity is a complicated mathematical representation of space-time in 4 dimensions. The bending of a 2-dimensional fabric is just a handy (albeit very misleading) visual illustration. Space, in GR, is 4-dimensional, including the 3 familiar dimensions of space and 1 dimension of time. All 4 end up being "bent" by the presence of mass. However, "bent" is also a misleading term. Space-time is not being bent... it's been warped. That is, compressed, or expanded. Let's say you had a balloon in a container of water. The container is sealed and completely filled with water (there is no air inside), so there is nowhere for the water to go. You then add air to the balloon, and its expansion causes the water around it to become denser (that's not quite what would really happen, but let's say it is). If you take air out, the surrounding water gets less dense. This is what is meant by the "bending" or "warping" of space-time. And this also happens in the 4th dimension (time), which is what causes time to run slower in a gravitational potential.
No matter the size of the object, the result of its presence on space-time is this "warping" effect.

The math is much to complicated to get into... but the result of this warping is the tendency for mass to attract other objects (gravity).

The cosmological constant has very little to go with gravity, itself... Einstein only added it to his gravitational field equation to allow for an expanding universe. It plays no part in the orbital dynamics of the planets. What keeps them from crashing into the sun is their angular momentum, which matches the inward pull of gravity, keeping them at a constant distance.

Expansion of Space

There are not many universes... there are many galaxies.
Locally, the expansion of the universe is very small (it takes hundreds of thousands of light-years before it becomes even mildly significant). So, local expansion rates are easily overtaken by a given galaxy's proper velocity. Just because the space between two galaxies is increasing doesn't mean they can't be moving faster than that expansion and, overall, be getting closer.

Now, consider equally-spaced dots on the surface of a balloon (yep, the balloon again). You choose one dot to represent Earth... let's called it dot E. If you blow up the balloon and, from E, measure the distance to the surrounding dots, they will appear to be moving away from E faster and faster the further out the dots are. It doesn't matter which dot you pick to be E... the observation will be the same.
The same goes for the expansion of the universe. The expansion is the same throughout the universe, but, relative to the Earth (which always appears stationary to us), the expansion increases linearly with increasing distance. As you noted, it doesn't mean we're at the center of the universe... what it means is that the expansion of the universe is uniform.

Space is flat

"Flat" is a subjective term. A dot is flat. A line is flat. A square is flat. A cube is flat. The inside of a sphere is flat. The surface of a sphere is not.
Any space that isn't bent in any way is "flat".

Black Holes

Black holes are spherical (generally). The disk of matter surrounding them is formed by their rotation. Black holes rotate very fast, and this centripetal force causes matter to accumulate in a disk shape (just like the rotation of the Earth causes it to be slightly wider than a perfect sphere at the equator).

In response to the last paragraph of gravity.

Gases are made up of atoms which can be thought of as tiny masses. Well sun formation takes a huge period time relative to human existence. So, the masses one by one begin to combine. The constant mentioned does not keep two objects from combining it just keeps objects in motion from "rolling" straight into other objects. See if you can find some photos of two binary stars "colliding". The collision takes a very long time as the weaker sun will stretch along the plane of its orbit into the stronger sun. This also occurs with very small masses but there is no stretch if they are solid. So ( you may be able to find video) when say salt is in a gravity free vacuum it tends to clump. It doesn't all clump together just bits will clump together. You can translate this down to the atomic level and then multiply it out for millions of years and you have star formation. Think of planets as one of two things, leftover pieces of the gas that originally made up our solar system or an object from another location in the galaxy ( possibly from a separate galaxy) that has been fought by the gravitational force of the sun. Also, if an object is already headed straight for the sun then it can collide with the sun at which point it will be vaporized. If it passes close enough it is "tugged" on and its path is altered but it may not have been caught.

I enjoy watching the salt and water particle videos that NASA has. They help visualize larger processes very well. You should Google or YouTube search for them.

Since space is expanding, causing everything to move away from everything else, technically everything is a center of the universe.

Space if flat because if you shine a laser beam into space it stays a straight line. If space were curved, the beam would arc slightly and eventually, in theory, end up hitting you in the back.

Originally posted by deanGI5
Since space is expanding, causing everything to move away from everything else, technically everything is a center of the universe.

Objects within a solar system get closer together. Solar systems within a galaxy also move closer to the center of the galaxy, however when dealing with the largest scale, galaxies expand outward from one another and away from a specific point. This point is the point that everything is expanding from but as it expands outward the localized masses will become gravitational bound to one another. Still expanding outward but collecting with what's closest to itself.

Couldn't the expansion of the universe be an illusion caused by masses collecting within galaxies. Then again I guess the change in distance wouldn't be noticeable if this was true.

reply to post by CLPrime

General Relativity is a complicated mathematical representation of space-time in 4 dimensions. The bending of a 2-dimensional fabric is just a handy (albeit very misleading) visual illustration. Space, in GR, is 4-dimensional, including the 3 familiar dimensions of space and 1 dimension of time. All 4 end up being "bent" by the presence of mass. However, "bent" is also a misleading term. Space-time is not being bent... it's been warped. That is, compressed, or expanded. Let's say you had a balloon in a container of water. The container is sealed and completely filled with water (there is no air inside), so there is nowhere for the water to go. You then add air to the balloon, and its expansion causes the water around it to become denser (that's not quite what would really happen, but let's say it is). If you take air out, the surrounding water gets less dense. This is what is meant by the "bending" or "warping" of space-time. And this also happens in the 4th dimension (time), which is what causes time to run slower in a gravitational potential.
No matter the size of the object, the result of its presence on space-time is this "warping" effect.

The math is much to complicated to get into... but the result of this warping is the tendency for mass to attract other objects (gravity).

Ah that makes a lot more sense than a two-dimensional representation of a four-dimensional process. Still, it's hard for the mind to imagine something that it barely understands.

The cosmological constant has very little to go with gravity, itself... Einstein only added it to his gravitational field equation to allow for an expanding universe. It plays no part in the orbital dynamics of the planets. What keeps them from crashing into the sun is their angular momentum, which matches the inward pull of gravity, keeping them at a constant distance.

Really? I was sure that I learned he included it to counter the inward pulling force of gravity, but I'm probably wrong. The angular momentum matching the inward pull makes sense, because isn't that how satellites are put into orbit?

There are not many universes... there are many galaxies.

Epic typo on my part, I meant galaxies.

Locally, the expansion of the universe is very small (it takes hundreds of thousands of light-years before it becomes even mildly significant). So, local expansion rates are easily overtaken by a given galaxy's proper velocity. Just because the space between two galaxies is increasing doesn't mean they can't be moving faster than that expansion and, overall, be getting closer.

That makes sense.

Now, consider equally-spaced dots on the surface of a balloon (yep, the balloon again). You choose one dot to represent Earth... let's called it dot E. If you blow up the balloon and, from E, measure the distance to the surrounding dots, they will appear to be moving away from E faster and faster the further out the dots are. It doesn't matter which dot you pick to be E... the observation will be the same.
The same goes for the expansion of the universe. The expansion is the same throughout the universe, but, relative to the Earth (which always appears stationary to us), the expansion increases linearly with increasing distance. As you noted, it doesn't mean we're at the center of the universe... what it means is that the expansion of the universe is uniform.

OK so what you're saying is that the expansion really isn't faster the farther out in space you go, that's just an illusion caused by our perspective in space? I actually got a balloon, put a bunch of dots on it, but it was hard to watch the dots while inflating it, so I inflated it, picked a dot as earth, and ran it in reverse and I think I saw what you were talking about. Then it popped in my face

"Flat" is a subjective term. A dot is flat. A line is flat. A square is flat. A cube is flat. The inside of a sphere is flat. The surface of a sphere is not.
Any space that isn't bent in any way is "flat".

I'm still having a hard time visualizing this....when I think "flat", I think of a piece of paper. The only way i can see it as flat is if the universe was a sheet of paper, I don't know why I think that way but that's how I see it when I hear a flat universe.

Black holes are spherical (generally). The disk of matter surrounding them is formed by their rotation. Black holes rotate very fast, and this centripetal force causes matter to accumulate in a disk shape (just like the rotation of the Earth causes it to be slightly wider than a perfect sphere at the equator).

That makes sense. Does the matter also get sucked into the area around the disk instead of the top, bottom, and so on because of the centripetal force? Or despite the fact that the matter congregates around it in a disk, does it get sucked in anywhere around the black hole?

Originally posted by TupacShakur
reply to post by CLPrime

 

... Still, it's hard for the mind to imagine something that it barely understands.

Indeed it is. The math is complicated enough (ever tried Tensor Calculus?), it's conceptually almost impossible for the mind to visualize... especially since it works in 4 dimensions, and our minds just don't do 4 dimensions.

... I was sure that I learned he included it to counter the inward pulling force of gravity, but I'm probably wrong. The angular momentum matching the inward pull makes sense, because isn't that how satellites are put into orbit?

The cosmological constant does counter the inward force of gravity... but it doesn't do it where gravity is present. The cosmological constant (now associated with dark energy) only becomes significant in the vacuum. It's what causes the expansion of the universe (by exerting an outward force)

OK so what you're saying is that the expansion really isn't faster the farther out in space you go, that's just an illusion caused by our perspective in space? I actually got a balloon, put a bunch of dots on it, but it was hard to watch the dots while inflating it, so I inflated it, picked a dot as earth, and ran it in reverse and I think I saw what you were talking about. Then it popped in my face

That's exactly what I'm saying. And not just me...every cosmologist, astrophysicist, and astronomer will say that, too
Also, I claim no responsibility for any injuries incurred by the physical demonstration of my thought experiments

I'm still having a hard time visualizing this....when I think "flat", I think of a piece of paper. The only way i can see it as flat is if the universe was a sheet of paper, I don't know why I think that way but that's how I see it when I hear a flat universe.

That's how the word "flat" is typically interpreted. But, in physics, "flat" applies to any number of dimensions. It's just something we have to get used to.
You could look at it this way: if you pile a whole bunch of pieces of paper on top of each other, you get a cube. Each piece of paper was flat, so the resultant cube can also be said to be flat (3-dimensionally flat).

Does the matter also get sucked into the area around the disk instead of the top, bottom, and so on because of the centripetal force? Or despite the fact that the matter congregates around it in a disk, does it get sucked in anywhere around the black hole?

Matter gets sucked into a black hole solely because of gravity. It accumulates at the equatorial region because of centripetal force. Matter can get sucked in at any location "on" the black hole's "surface", but, for the most part, it will tend to form a disk shape before it reaches the black hole.

reply to post by CLPrime

Indeed it is. The math is complicated enough (ever tried Tensor Calculus?), it's conceptually almost impossible for the mind to visualize... especially since it works in 4 dimensions, and our minds just don't do 4 dimensions.

Unfortunately I haven't got around to trying out Tensor Calculus, but I'm going to major in Astronomy and Physics so maybe the math classes will involve that stuff. We can understand it but we can't visualize it, that's good enough for me.

The cosmological constant does counter the inward force of gravity... but it doesn't do it where gravity is present. The cosmological constant (now associated with dark energy) only becomes significant in the vacuum. It's what causes the expansion of the universe (by exerting an outward force)

I see, and dark matter does the opposite because it's the extra, undetectable matter that keeps galaxies together, right?

That's exactly what I'm saying. And not just me...every cosmologist, astrophysicist, and astronomer will say that, too
Also, I claim no responsibility for any injuries incurred by the physical demonstration of my thought experiments

Don't worry, I suprisingly escaped the popping balloon without any life threatening injuries.

That's how the word "flat" is typically interpreted. But, in physics, "flat" applies to any number of dimensions. It's just something we have to get used to.
You could look at it this way: if you pile a whole bunch of pieces of paper on top of each other, you get a cube. Each piece of paper was flat, so the resultant cube can also be said to be flat (3-dimensionally flat).

That's a good way to put it. You also said that the interior of a sphere can be flat, and that's where my mind went all "Does not compute" on me. I take that as meaning if we sliced that sphere into thin slices, it would be flat on the inside , but if we just look at the exterior of the sphere as a whole it's curved, is that sort of what you're saying?

Matter gets sucked into a black hole solely because of gravity. It accumulates at the equatorial region because of centripetal force. Matter can get sucked in at any location "on" the black hole's "surface", but, for the most part, it will tend to form a disk shape before it reaches the black hole.

Perfect, that's exactly what I was trying to understand.

Originally posted by TupacShakur
reply to post by CLPrime

 

... dark matter does the opposite because it's the extra, undetectable matter that keeps galaxies together, right?

Yep, just like regular matter. Any source of gravity acts contrary to the cosmological constant. That's why galaxies don't expand with the rest of the universe... things that are gravitationally bound are not subject to expansion.

You also said that the interior of a sphere can be flat, and that's where my mind went all "Does not compute" on me. I take that as meaning if we sliced that sphere into thin slices, it would be flat on the inside , but if we just look at the exterior of the sphere as a whole it's curved, is that sort of what you're saying?

Sure is. 2-dimensional people living on the surface of a sphere would say their "space" is curved. 3-dimensional people living inside the sphere would say their space is flat.

Does it help to realize that an orbit is (essentially) on a plane? That third dimension of up/down can be ignored when conceiving of a planet orbiting the sun, or a collection of solar systems and stars orbiting about the center of a galaxy, or a series of galaxies orbiting about each other.

The other part about the expansion of the universe...

The effect on orbits is nominal. Concepts such as orbital decay due to friction, due to other secondary gravitational influences of other bodies, and due to the accretion of mass from asteroid strikes, as examples- those are usually extremely slight in influence. With celestial bodies, sometimes those orbits eventually end in collisions, and sometimes they end with one forever flying away from the other.

That third dimension does come into the calculation of gravity, however-- even though the effect of the force remains on a plane. The gravitational force of the sun on the earth, for example is not merely a calculation of mass and distance, but also the angular size of the earth from the sun (and visa versa):

If the Earth could remain the same distance from the sun and of the same mass, but be twice as large (and so appear larger from the sun), the sun's gravity has more surface area to attract, and so the force of the attraction would increase. As I understand it, that is the only effect of that up/down dimension on orbits-- otherwise, the mathematics can be worked out using points representing the center of the mass of each object-- I think.

Originally posted by Frira
If the Earth could remain the same distance from the sun and of the same mass, but be twice as large (and so appear larger from the sun), the sun's gravity has more surface area to attract, and so the force of the attraction would increase.

Gravity acts on mass, it doesn't act on surface area. If Earth were twice the size but the same mass, the gravitational attraction between Earth and the sun would be the same as it is now.
For gravity to increase, mass has to increase.