# Curved or Flat

Hi there.

I was wondering if the univers is curved or if it is flat.

By the way, one other question.
When a person lifts an object the object gains Ep. Is the energy given to object by the person or does it gain energy by the Gravitational Field? even though i know that Forces (Gravity) cannot give or gain energy.

If a person was born on top of a mountain where would the Ep come from? Would it be from Gravitational Field?

HallsofIvy
Homework Helper
If the universe were totally flat, there would be no gravity at all! Since there is gravity, the universe is curved. (Although, since gravity is NOT terribly strong, compared with the other forces, the universe is not VERY curved.)

Potential energy is always relative to some fixed value. It doesn't really make sense to talk about Ep without saying relative to something. If an object magically appeared on a mountain top, it would have potential energy relative to the base of the mountain simply by virtue of its position.

However, things don't just "magically appear". If a person is born on a mountain top then his/her mother had to get on that mountain top! Work had to be done to get her to the mountain top.

arildno
Homework Helper
Gold Member
Dearly Missed
Plus:
Energy (chemical potential) given through the food brought up to the mother will be expended to develop the foetus..

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bayan said:
Hi there.

I was wondering if the univers is curved or if it is flat.
It appears to be flat.
HallsofIvy said:
If the universe were totally flat, there would be no gravity at all! Since there is gravity, the universe is curved. (Although, since gravity is NOT terribly strong, compared with the other forces, the universe is not VERY curved.)
When cosologists speak of the universe being flat they are invariably refering to the 3-D space. I.e. the universe is "spatially" flat. This means that the Riemann tensor for 3-space is zero. But even for spatially flat universes the spacetime is not flat.
bayan said:
When a person lifts an object the object gains Ep. Is the energy given to object by the person or does it gain energy by the Gravitational Field? even though i know that Forces (Gravity) cannot give or gain energy.
I don't know what "gain energy by the Gravitational Field" means. Work done by a force gives rise to a "change" in the potential energy of the object.
If a person was born on top of a mountain where would the Ep come from? Would it be from Gravitational Field?
If something simply, with mass, appeared out of nowhere then it would violate the principle of the conservation of energy, even in the absense of gravity. E.g. if it appeared at rest in frame S then in a frame S', moving relative to S, the object would have a finite and non-zero kinetic energy. Thus, to an observer in S, energy appeared out of nowhere and thus the total energy of this closed system would increase from zero to a finite value - thus violating the conservation of energy.

If the person was born in the usual sense of the term then the total potential energy of child + mother would remain constant, i.e. it'd have the same before and after birth - i.e. Ep(Pregnant Mother) = Ep(mother) + Ep(child). Same would hold during the 9 moths. An increase in the mass of the mother must come from a decrease in the mass of the enviroment - e.g. mother eats fruit off trees at top of mountain - mother gains mass - tree loses mass. Etc.

Pete

1. Flat or curved, recent studies had shown that mass of the universe is not enough to curl up the space time arround itself. But it is not still 100% sure bcoz in the calculations only visible structures are taken into account. And it is belived that if the discovery of the MACHOs and WIMPs might affect the calculation, as they did in the rotation of galaxies.
2. i support answer by Arildno

Curved and finite

The Universe is curved, since there exists gravity. Scientists even have concluded that the universe is not infinite, this due to curvature! They have proceeded to measure its dimensions. As a 2D surface can be curved in a 3D space, the 3D space is curved in the 4Dimensional time-distance space. The earth surface is an example of a curved 2D surface. You go and go around and eventually may return to the same point. This is the case with the universe. Now, if there had existed a (imaginary) powerful enough telescope, if you looked far away you might eventually see your back, actually yourself gazing at the distance...! This would have occured due to the bending of rays as they move through a curved 3D space.