Ok this is probably dumb, but can someone explain a flat universe to me?

[no limit]

I was never good at math in school, but physics has been really fascinating me lately. I keep hearing the universe is flat which makes sense to me, if you shoot a really powerful laser out in to space it will remain flat. But what I don't understamd is how gravity plays into this. When I think of gravity I think of the following picture which was always taught to me:



What I don't get is the satellite. If gravity attracts objects in a flat matter as that picture above suggests how can the satellite be above the earth? And how can there be up/down? It would seem that everything would have to be flat. When you look at the stars at night no matter where on the planet you are the stars are everywhere, up, down, left, right. But gravity seems to paint a flat picture which would make that impossible. Is there anyone out there that could explain gravity to me in layman's terms?

[JTG of the PRB]

1. I can't pretend to understand astrophysics...

...but the thing that's important to remember is that space is three dimensional, all over. Pictures like that make sense to display how gravity wells affect the fabric of space by distorting it according to the mass of the object, but the picture can be misleading since that "depression" actually surrounds the entire planet. Gravity doesn't just exist on a flat plane, but rather surrounds us in three dimensions. In that sense, the satellite is not "above" the Earth, since there is no "up" or "down" beyond our linear understanding of the surface on which we stand.

The more massive the object, the more powerful its gravitational force. We just perceive gravity as being "flat" because we're bound to the Earth by it. Out in space in orbit of the Earth, there is less gravity, but it doesn't cease to exist - the international space station is bound to the Earth by gravity, just as the moon is. And in that same sense, the Earth is bound to the Sun through the Sun's pull of gravity.

But, like I said, I'm no scientist. I just like stuff regarding the space/time continuum. :hi:

[RandomThoughts]

2. If the universe is not flat, then it will eventuall intersect.

Only a flat universe can be infinate.

Although most comments on flat universe is about totality and not distortions in space time by gravity.

[no limit]

4. but lets assume the universe is flat since what we've seen from experiments

how does gravity fit in to that? I always see gravity illustrated as a flat plane as pictured above. Is that really the case?

[RandomThoughts]

12. Gravity is an effect within a section of space time

Warping space time in a local area. It is not about what I understand flat universe to be.

Flat universe is the question without gravitational influence 'is the entire univiere flat'.

Your talking two different things when discussing flat versus non flat universe and mass distortions of space time simulating gravity.

[no limit]

13. My questions are probably really dumb and my apologies for that, I just want to understand this.

I'm not sure I understand gravity localized in an area of space/time. If I launch a rocket straight up depending on where I am on the planet I will go different locations. Noth pole I will go north, south pole I will go the other way. But the picture I always see of gravity as illustrated in the OP is flat, so how is up/down possible? And I know my picture of gravity is totally wrong, I'm just pulling at straws to try and understand why.

[Rabrrrrrr]

14. Gravity radiates out in all directions from a mass

It's not unidirectional - so no matter where you go on the surface of the earth, you feel the gravitic effect of the mass of the earth, which is always pulling straight down to the center of the earth.

Also, the strength of a gravity field depends on the mass of the item in question; so the sun, which has a crapload more mass than the earth, has a much stronger gravitational field.

So we could say that the volume of localized space that the sun's gravity distorts in any significant way is a much bigger volume than what the earth distorts in any significant way. And the earth's warped-space volume is waaaaaaaay bigger than the way that your body's mass distorts space-time.



In space, there is no up/down - we experience it here on earth because we're in such a huge gravity field, and we have a very clear sense of up and down in relation to the surface of the earth. But out in space, there is no "up/down". From the perspective of Mars, there is no "up" from earth or "down" from Jupiter. Or "up" in relation to our galaxy.

[no limit]

15. I had to think about it for a bit but I think that makes sense (kind of)

Since earth is a sphere (for the most part) no matter where on earh we are we will be pulled down toward the center. Keeping us "glued" to the planet.

But I can't wrap my mind around gravity being flat out in space as your post seems to suggest. The moon, Mars, saturn, and every other planet is on the same plane as earth (I assume) orbiting around the sun. That assumes that gravity is flat. But then how can we look up in the sky (no matter if we are on the south pole or on the north pole) and see stars all around? Is what we see in the sky a side effect of time? The way I simplify it in my head is if I launch a rocket powerful enough to escape earth's gravitaional field in the south pole it will go "down", and if I launch it in the north it will go "up"; no? But that seems to contradict what I know about gravity.

I hope I am making sense, thanks.

[Rabrrrrrr]

16. See my other post below explaining the difference between a flat universe and a planar one.

Don't think "flat" means "like a pancake" (which is planar, in this conversation) - flat is still infinite in all directions, but it doesn't curve back on itself, versus other possible models which are infinite (perhaps) in all directions but still curve back on themselves.


Also, the planets in this solar system do not orbit in the same plane (though they're pretty damn close from a galactic scale), but even if they were, that wouldn't have any effect on whether the universe is flat or not.

I will also say don't think of gravity as flat - that's a concept that has no meaning at all. Gravity is always in the direction of the mass. So, for example, in our solar system, the moon is mostly affected by the earth - there's a gravity vector from the moon toward the center of the earth (and from earth to the center of the moon, though that's much smaller, but it's enough to pull ocean water around and make tides). But, everything with mass has gravity, so there is also a gravity vector from the moon toward the sun (and vice versa), from Jupiter to the moon (and vice versa) and to and from Mars, to and from Saturn, to and from every asteroid and comet, and to and from every galaxy and other object in the universe.

Gravity is not flat - the universe is flat.

gravity is always an arrow from one center of mass to another.

Gravity, however, curves space, which is one way of thinking how stuff stays in orbit: one can think of the moon as always moving in a straight line (from it's perspective), except that the earth's strong gravity has curved space just enough that the moon's straight path becomes an orbit around the earth. Same with the earth's orbit around the sun, our solar system's orbit around the galactic core, and our galaxy's orbit around the supercluster it's in.

Mind-rackingly bizarre, I know. It took me a year or two before really "getting it".

[no limit]

21. Thank you for your patience, I appreciate the explaination.

So how did our solar system end up basically on the same plane? It doesn't matter if you look at the solar system, our moon, or saturn's rings or its moons:



Everything always seems to fall on the same plane (or am I assuming this wrong?). But in physics there seems to be no requirement for that to happen if I understand you correctly. Some of Saturn's moons could orbit it from the south to the north poles while other moons could orbit it from east to west poles. It's just in the case of Saturn or even our solar system everything seems to fall on basically the same plane but physically it doesn't have to? Do I understand that right?

Thanks again.

[Rabrrrrrr]

28. Things tend to orbit in near-identical planes because of the spin of the thing being orbited.

The sun rotates, so all the planets are in close to (but not entirely) the same plane that is perpendicular to the axis of rotation of the sun. Saturn rotates, so it's rings and moons are in close to the same plane, also perpendicular to its axis of rotation.

Think of it like a blob of pizza dough that you toss up into the air and spin - it flattens out in a plane perpendicular to the axis of rotation. It's the conservation of angular momentum. Physics at work!

Fascinating thing about Saturn's rings: they max out at about 30 feet in width. In terms of orbit around a planet that large, that is, for all intents and purposes, orbiting in the same plane! Probably no more than a few milliseconds of arc between surfaces.

[Twillig]

8. No, both an open universe and a flat universe will expand forever.

a closed universe will collapse back into a big crunch.

I got at least that much from Lawrence Krauss's lecture. I don't know if I could answer in any respect the OP's questions other than to say that space is curved by mass, or General relativity would be wrong.



For the atheists: http://www.youtube.com/watch?v=7ImvlS8PLIo

For everybody else: http://www.youtube.com/watch?v=huYvSvN05mQ


Roughly the same lecture in both cases!

[Xipe Totec]

3. Why don't you post the question in the science forum?

:popcorn:

[no limit]

5. We have a science forum? My bad.

[Xipe Totec]

7. ...

http://www.democraticunderground.com/discuss/duboard.php?az=show_topics&forum=228

[Xipe Totec]

6. posted reply to wrong thread - deleted. nt

[Rabrrrrrr]

9. It's flat in the sense that it isn't curved, not flat in the sense of being planar.

Not that that probably helped you very much.

But think of the universe as flat, like a playing card, except that the edges are way off into infinity... now, picture that playing card, which is really thin, as being instead infinitely thick. That's 3-dimensional flat. And that's what the universe is - as opposed to the universe being, say, a giant sphere, or the surface of a giant sphere, or a hyperboloid, or a parabola, or a saddle... and so on.

All it means to say that the universe is flat is that one 'edge' of the universe doesn't meet any other 'edge' (as it would if it were any other shape).

But, within that flatness, gravity does its space-bending magic in local areas.

That probably isn't real clear, but it's hard to describe in words - would be easier if you were standing in front of me.

Though there is also much thought that universe might be the surface of a large sphere - like the latex in a blown up ballon, in which instead of air in the middle of the universe (like in a balloon), the interior is literally non-existent, because it's not part of the universe, even though it's enclosed by it, because the universe is just the surface.

[uncommon]

25. ^^ This is a very good answer.

[pokerfan]

10. There's a pretty good article on wiki

Basically we can't visualize four dimensions so we have to use three dimensional analogies which compress space down to two dimensions which can be curved into the third dimension....



http://en.wikipedia.org/wiki/Shape_of_the_Universe

[madinmaryland]

11. So you are saying we are all repubs and see the the world one dimensionally??

:wtf:

:hi:

[pokerfan]

17. Conservatives deny Relativity

E=mc²? Not on Conservapedia

Religious believers have quite the love/hate relationship with Albert Einstein. You'll often hear them quoting the physicist's comments about God not playing dice with the universe to support their own views – despite the fact that Einstein himself said, "I do not believe in a personal God." One young-Earth creationist site even uses an Einstein quote in a diatribe against evolution. Now the pendulum is swinging over to hate as Einstein goes the way of Darwin, becoming an unlikely Christian enemy.

It seems that the folks at Conservapedia – a sort of conservative alternative to the more familar online encyclopedia Wikipedia – are not fans of Einstein's most famous theory, general relativity. In fact, they view it as a far-reaching liberal conspiracy.

http://www.newscientist.com/article/dn19303-emc2-not-on-conservapedia.html

[HopeHoops]

18. If you haven't read "Flatland", do so.

It was written in the mid 1800's and explains different dimension universes pretty well (with illustrations).

[dixiegrrrrl]

20. I second the rec to read Flatland.

Also, if you read your question, you will see the concept of "up" and "down" is OUR perspective,
not the universe's. to the universe, stars are not "right" "left" "up" or "down". They are just
"there" AND "somewhere else"
so the question might well be: "why do humans see in only 3 dimensions?"

[no limit]

23. Thanks, I read the first 7 chapters this morning and do plan on finishing it later

:toast:

[Orsino]

19. The picture you're looking at mixes what we think of as 3-D space...

...with a two-dimensional analogue that shows how matter stretches that space--because showing space stretched in three dimensions would be too much of a mess.

Imagine, though, walking across that concentric grid in the picture. To get from one side to the other, the distance you'd travel is not the same as if the Earth weren't there. You would also be deflected from the as-the-crow-flies "straight" path.

Now imagine tipping that concentric grid any way you want in three dimensions, and realize that it doesn't change. With the Earth's mass there, space is stretched and curved, increasing the effective distance and bending things like light and your own motion.

So what we call the force of gravity is more properly thought of as the space-stretching (and time-stretching) effect of matter. These effects aren't just theory. They are taken into account by your GPS receiver, for instance, and the principle was demonstrated as early as 1919, using the Sun's greater gravity well. Curved spacetime also nicely accounted for weird-ass observations of Mercury's orbit.

[no limit]

22. I see, so that 2D grid can be tipped any which way depending on the object...

...that is traveling on that grid? So really there could be millions of different grids if you have millions of different objects? Do I understand that correctly?

I really do appreciate you trying to explain this to me, hopefully it's not a fruitless effort :). I did read such books as why e=mc2 and why we should care as well as watched many documentaries on this but the explaination of gravity I always seem to run in to (the way I understand it) is that it's in a sense flat, which I'm sure is wrong and I think you clarified that for me a bit assuming I'm understanding you correctly.

[Orsino]

27. Yeah, that's it.

A perfectly spherical and uniformly dense mass would deform space the same way in every plane, from every direction. In real life, there are parts of the planet that are more or less dense than the average, and some satellite surveys have to take this into account.

[mcollins]

24. From NASA

The shape of the universe is determined by a struggle between the momentum of expansion and the pull of gravity. The rate of expansion is expressed by the Hubble Constant, Ho, while the strength of gravity depends on the density and pressure of the matter in the universe.

The density of the universe also determines its geometry. If the density of the universe exceeds the critical density, then the geometry of space is closed and positively curved like the surface of a sphere. This implies that initially parallel photon paths converge slowly, eventually cross, and return back to their starting point (if the universe lasts long enough). If the density of the universe is less than the critical density, then the geometry of space is open, negatively curved like the surface of a saddle. If the density of the universe exactly equals the critical density, then the geometry of the universe is flat like a sheet of paper. Thus, there is a direct link between the geometry of the universe and its fate.

The simplest version of the inflationary theory, an extension of the Big Bang theory, predicts that the density of the universe is very close to the critical density, and that the geometry of the universe is flat, like a sheet of paper.

The WMAP spacecraft can measure the basic parameters of the Big Bang theory including the geometry of the universe. If the universe were open, the brightest microwave background fluctuations (or "spots") would be about half a degree across. If the universe were flat, the spots would be about 1 degree across. While if the universe were closed, the brightest spots would be about 1.5 degrees across.

Recent measurements (c. 2001) by a number of ground-based and balloon-based experiments, including MAT/TOCO, Boomerang, Maxima, and DASI, have shown that the brightest spots are about 1 degree across. Thus the universe was known to be flat to within about 15% accuracy prior to the WMAP results. WMAP has confirmed this result with very high accuracy and precision. We now know that the universe is flat with only a 2% margin of error.

[spinbaby]

26. It's my understanding it's balanced on the backs of four elephants

which stand on a giant turtle.

[Deep13]

29. It's when someone leaves the cap off the universe ...

...and all the dissolved carbon dioxide escapes so it isn't fizzy anymore.

[mysuzuki2]

30. Well for one thing,

in a flat universe the playmate of the month just wouldn't be the same.

[Dr Morbius]

31. I submit it's very important to remember one thing about gravity:

I don't think we completely understand it yet. I am convinced there's a connection between gravity and the expansion of spacetime; it's the only answer for the "dark matter" problem I can see. There are relationships between gravity and space, only some of which we comprehend.

I'm still working on this, though.

[Generic Brad]

32. I can explain a key feature of a flat universe

No bras. No breast augmentation. At first glance it is hard to distinguish males from females.