# Thread: How large objects curve space time

1. Hi Experts,

I understand that Large objects like planets can curve space time & other smaller objects fall in the curve giving a some explanation of Gravity.

Ques a)

Does space acts like a very low density ocean/medium on which a Star floats and the Star curves the ocean (space) based on its size & mass.
I mean how otherwise an analogy with a fabric is drawn & how the space gets curved.
In space bodies are weightless so how does a star bend space fabric without any weight.

If an object was moving in a straight line & suddenly a curve comes in space, why the object will go in the curve in first place & not in straight line, unless the space itself exerts some force & pushes it in the curve or unless the space was supporting the object in the earlier path as well.

Somehow the bending of space theory by large objects doesn't makes full sense to explain gravity.

Ques b) I read space is expanding. So is space taking along the stars & galaxies with it (like dots on balloons). How is that even possible unless space exerts some force on the stars & galaxies. else the stars & galaxies will stay at same place & space will move beyond.

Regards
Anurag

2. Originally Posted by Anurag
Hi Experts,

I understand that Large objects like planets can curve space time & other smaller objects fall in the curve giving a some explanation of Gravity.

Ques a)

Does space acts like a very low density ocean/medium on which a Star floats and the Star curves the ocean (space) based on its size & mass.
I mean how otherwise an analogy with a fabric is drawn & how the space gets curved.
In space bodies are weightless so how does a star bend space fabric without any weight.
It's not weight, it's mass (actually, mass-energy) that is relevant. The amount of curvature is proportional to mass-energy, so all masses -- even tiny ones -- will curve spacetime. It's just that we don't really notice much curvature unless the mass is very large.

If an object was moving in a straight line & suddenly a curve comes in space, why the object will go in the curve in first place & not in straight line, unless the space itself exerts some force & pushes it in the curve or unless the space was supporting the object in the earlier path as well.

Somehow the bending of space theory by large objects doesn't makes full sense to explain gravity.
The object IS going along a "straight line". It's just that the line is curved. That's gravity.

Ques b) I read space is expanding. So is space taking along the stars & galaxies with it (like dots on balloons). How is that even possible unless space exerts some force on the stars & galaxies. else the stars & galaxies will stay at same place & space will move beyond.
"Space expands" means "the distance between objects increases." No force is exerted. Hard to get your head around, sure, but that's not a maths problem, it's a problem of limited human intuition. Don't forget -- our brains evolved to find food, avoid becoming food, and finding a mate so that our offspring could carry on with the next cycle of those tasks. Expecting the resulting brain also to grasp immediately notions such as curved and expanding space is a bit much. That's why we invented maths and the scientific method. We make models and test them. When they agree with experiment, we continue using them. When they don't, we modify or discard them. Relativity is incredibly well tested, so we rely on it. We'll continue to do so until an experiment tells us not to. That day has not arrived.

3. Dear tk421,

But I am still lost. Unless space moves the stars & galaxies with it, how the galaxies are all moving away & so called space is considered to be expanding with increasing acceleration.
The x y z co ordinates of the galaxies change during space expansion, so definitely some force acted on it to cause that co ordinate change.

If we only say that space expanded between two galaxies moving them apart. That does not adds up.
Unless space has some grip or force acting on the galaxies, how the galaxy move? Galaxies are not mass-less objects. They are massive.

Is that what the scientists are calling Dark energy. Or is the concept of Dark energy something else.

Regards
Anurag

4. Originally Posted by Anurag
But I am still lost. Unless space moves the stars & galaxies with it, how the galaxies are all moving away & so called space is considered to be expanding with increasing acceleration.
The x y z co ordinates of the galaxies change during space expansion, so definitely some force acted on it to cause that co ordinate change.

If we only say that space expanded between two galaxies moving them apart. That does not adds up.
Unless space has some grip or force acting on the galaxies, how the galaxy move? Galaxies are not mass-less objects. They are massive.
The key statement you made is: "the x y z co ordinates of the galaxies change during space expansion". So it is not that the galaxies are being pushed along the x axis, but rather the distance between the divisions of the x axis is increasing. So you could just say it is our measurement of the distance that is increasing. But maybe that is just more confusing, I don't know.

The important point is that nothing is "pushing" the galaxies away, it is just that it is natural for an even distribution of stuff to drift apart over time. (Unless gravity or something else holds them together). So this is just one of those many things in the universe that don't work according to our intuition.

Is that what the scientists are calling Dark energy. Or is the concept of Dark energy something else.
Dark energy is something else. People expected that the expansion would gradually slow down as gravity pulled on things. Or, if not, it would just carry on at the same rate (if there weren't enough gravity to slow it down). Well, it turns out that after slowing for a bit, the rate of expansion started speeding up. This was kind of unexpected (the scientists who spotted it got Nobel Prizes) and no one knows the reason for it. So the reason has been given the name "Dark Energy". Dark because we don't know what it is. And energy because that is one way of describing the behaviour.

5. Originally Posted by Anurag
Dear tk421,

But I am still lost. Unless space moves the stars & galaxies with it, how the galaxies are all moving away & so called space is considered to be expanding with increasing acceleration.
The x y z co ordinates of the galaxies change during space expansion, so definitely some force acted on it to cause that co ordinate change.
To echo Strange's excellent answer, I would say that it is important not to over-generalise intuition that is necessarily built up from one's limited experience. One of the hardest things in science is to develop meta-knowledge, that is, a catalog of where one's knowledge is reliable, and where it is not. You have assumed that force is required to make things move apart just because that's what you've seen, but you have not considered that this is, in fact, a generalisation rooted in a tiny set of "experiments" that you have experienced as a human on our tiny planet.

Einsteinian general relativity takes us well beyond what humans have experienced, so you should expect counter-intuitive, maybe even crazy-sounding, predictions. Einstein himself had difficulty accepting all of the consequences of his own theory, so that gives you some idea of how intuition should not be considered wholly reliable. For example, he wanted his equations to produce an eternally stable universe, so he somewhat arbitrarily added a term to his equations to make that possible. When contrary evidence accumulated, he was embarrassed.

If we only say that space expanded between two galaxies moving them apart. That does not adds up.
Unless space has some grip or force acting on the galaxies, how the galaxy move? Galaxies are not mass-less objects. They are massive.
Again, your objection is based on inuition and an improper generalisation of a limited set of experiences. Your (and my) terrestrially-based intuition is extremely unreliable (i.e., frequently useless) on cosmic scales. Just as Einstein struggled to understand the implications of his own theory (which grew from individually "simple" postulates), you will struggle to understand them. The first step is to discard a firm reliance on your unreliable intuition, and follow the maths. There really is no other way. And the maths show "crazy" things, such as expansion (including at superluminal speeds).

Is that what the scientists are calling Dark energy. Or is the concept of Dark energy something else.
Einstein's equations allow for a contracting or expanding universe, with a static solution being the least probable. Slipher and Hubble's observations showed that it is expanding. Recent observations reveal that it is expanding at an increasing rate. It's this most recent observation that has stimulated the dark energy hypothesis. It is very much an active area of ongoing research.