# Space is flat

## Main Question or Discussion Point

space is flat what the hell does this mean. It must be at least six foot thick as thats how tall I am? how thick is space it must be pretty freakin thick.

Is it flat like a turkish bread (ie sort of flat but thick as well) or flat like a balloon where the skin of the ballon is some lightyears thick ?

I mean the flatness of space compared to the macro object we observ, planets, suns, black holes, atoms you might as well say space isnt flat as flatness has no relevance on small objects such as these

## Answers and Replies

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Hootenanny
Staff Emeritus
Gold Member
When one says that "space is flat", one doesn't mean that the universe is flat like a "skin of a balloon" as you say, rather "flat spacetime" refers to what is know as Minkowski space.

In relativity, the three spacial dimensions are combined with a time-like to create the four dimensional spacetime, which in special relativity is described by Minkowski space. However, one can only use Minkowski space to describe the universe where it is locally flat, that is where there is not significant gravitation. Where there is significant gravitation, we say that spacetime has become curved. Four dimensional space is very difficult to visualise since we are intuitively only aware of the three dimensions, however one can 'draw' a 3D projection of spacetime (see http://upload.wikimedia.org/wikipedia/commons/2/22/Spacetime_curvature.png" [Broken]).

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space is flat what the hell does this mean. It must be at least six foot thick as thats how tall I am? how thick is space it must be pretty freakin thick.

Is it flat like a turkish bread (ie sort of flat but thick as well) or flat like a balloon where the skin of the ballon is some lightyears thick ?

I mean the flatness of space compared to the macro object we observ, planets, suns, black holes, atoms you might as well say space isnt flat as flatness has no relevance on small objects such as these
"flat" is a topology term meaning euclidean.

HallsofIvy
Homework Helper
Even the usual use of the word "flat" has nothing to do with thickness so I suspect this whole thread is a troll.

robphy
Homework Helper
Gold Member
Even the usual use of the word "flat" has nothing to do with thickness so I suspect this whole thread is a troll.
Well.. there is the every notion of "flatten".

"space is flat" means that, loosely speaking, "parallel lines don't converge or diverge"....

Flat Spacetime

Disclaimer: I'm not a physicist. Experts, am I wrong?

Rab: If your question means why is space called "flat" on small scales then I don't know what you're talking about, but if you mean flat on the largest of known scales then here's my attempt at an answer. My understanding is that space can't be discussed independently of time since it takes time for anything to move through space. So instead of "space" I'll say "spacetime". The average curvature of the whole universe has been found by something called the WMAP (google it) to be flat, which just means it's on the average just about as 'round' as it is 'inverted'. Think of 'round' like the earth where a line on its surface ultimately comes back on itself, and think of 'inverted' as like a saddle-shape, where if you follow the curvature the lines never end up meeting again. Total universe is neither of those, but is still 'curved' here and there according to Einstein, hence ON THE AVERAGE "flat".

Now for the abstract bit. As I understand, the universe's 'shape' isn't like a real shape, but corresponds to the average distribution of energy in spacetime throughout the universe. So a 'flat' universe doesn't correspond to flatness like a flat surface, but instead means that on the average the energy distribution throughout a "flat" universe is almost the same everywhere when you look at the whole universe. To help you understand this, think of how light REALLY curves as it passes a black hole, which is REALLY dense. In fact, it is said light gets 'trapped' within black holes, which is a consequence of this curvature, which is itself a consequence of high energy density. On the average, the whole universe is not curved because the density everywhere is about the same. So the bottom line: flat spacetime on a universe size scale really refers to a pretty uniform (meaning everywhere about the same) energy density (meaning anything causing spacetime to curve) distribution throughout the universe. Incidentally, the WMAP also found the age of the universe is about 13.7 billion years, which I thought was neat. If any of this is wrong, somebody please correct me so I'm less ignorant.
-Gerrit

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