# Bending of space and time, is it true?

1. Mar 28, 2015

### AshUchiha

I've heard many theories like object with mass bends the space-time and causes object to fall for it. If that's true then the Earth should end up being colliding with the Sun right? , because the Earth's mass is 1/1 million of that of Sun according to my knowledge.

If we take Earth's mass as m then Sun would be 1mil m, so would m's gravity really matter to 1mil m? I don't guess so, I dont think the theory explains quite of it. There is also another question I would like to put up, I'm sorry its not in the heading but, can this statement be true "Gravity can neither be created nor be destroyed". I've tried searching it in google and everywhere else but I wasn't able to get a satisfactory answer.

2. Mar 28, 2015

### FactChecker

The distortion of space and time is another way of looking at gravity. The Sun bends space and time in such a way that the Earth is really traveling in a straight line as it orbits the Sun.

3. Mar 28, 2015

### Cruz Martinez

You cant conclude that the earth should be falling and eventually colliding with the sun just because space-time has non-zero curvature. Freely falling particles follow inertial paths through space-time, one possible inertial trajectory is to move straight towards the massive object. Another possible inertial path is the one earth follows.
Which particular inertial path is followed by a freely falling particle is determined from initial conditions.

4. Mar 28, 2015

### pvk21

I don't think earth will fall into sun because space-time bending.if that was the case it would have fallen way before.instead due to sun's space-time bending earth rotate around the sun in elliptical path.

5. Mar 29, 2015

### Drakkith

Staff Emeritus
There is only one theory that posits this. It's called Einstein's General Theory of Relativity, or General Relativity for short. GR explains that space and time are joined together into space-time and that gravity is a geometrical effect of mass and energy. In other words, mass and energy 'bend' space-time in a way that is (slightly) analogous to putting a bowling ball on a trampoline or rubber sheet. The weight of the bowling ball bends the surface, and if you roll a smaller ball you will see that this bending of the surface causes its path to curved instead of continue on straight. Of course, the rubber sheet analogy is just that. An analogy. Space-time is a 4-dimensional construct and the way it 'bends' is much, MUCH more complicated and difficult to grasp.

If you perform the analogy above you will notice that you can roll the smaller ball in such a way as to make it 'orbit' the larger ball a few times before it loses enough speed to fall into the larger ball. This happens because there is friction slowing the ball down. The Earth orbits the Sun in a near-vacuum so there is practically no friction. This allows the Earth to continue to orbit the Sun without falling in. In order to fall into the Sun the Earth would have to slow down by a significant amount.

Eh, I don't really like the question to begin with. Gravity, as a general term, simply refers to the fundamental interaction known as Gravitation. This interaction, as a whole, cannot be destroyed and it already exists, so it cannot be created again. Also, in simple terms gravity can be understood as a value at any point in space representing the strength of the gravitational force at that point. In other words, if we measure the gravitational force at a point in space that is 10,000 km directly above the north pole of the Sun, we will get some value that represents the strength of the force felt at that point. If we move away from the Sun this value gets smaller, and if we move closer this value increases. This value can change without you moving if other objects move by, but it is a smooth process. You can't have a value of, say 100, and then have it instantly jump to 200 or 0 or anything else. It increases gradually in response to the movement of other objects. So in that sense you can't destroy or create gravity. You can only change the value at any point in space by moving objects around. More

So I guess the short answer is no, gravity cannot be created or destroyed.

6. Mar 29, 2015

### Bandersnatch

Have you got Guth's Inflationary Universe on hand? He's got a neat thought experiment there that shows how it can happen. In short - imagine an empty, spherical shell of matter collapsing under its own gravity. What is the gravitational acceleration outside and inside the original shell radius before and after the collapse?

7. Mar 29, 2015

### Drakkith

Staff Emeritus
I do not.

Offhand I'd guess that the acceleration is the same outside the original shell radius, and a lot more inside since it was zero originally. That's why I don't like the question "can gravity be created or destroyed". The answer kind of depends on what you mean by 'created' and 'destroyed' in this context.

8. Mar 29, 2015

### Bandersnatch

I understood it as going from zero to any positive value. Which, as you said, is what happens inside the shell.

9. Mar 29, 2015

### Drakkith

Staff Emeritus
That's they way I understand it too.

10. Mar 29, 2015

### AshUchiha

Drakkith, Actually "Gravity can neither be created nor be destroyed". What I really meant was, you see Mass can neither be created nor be destroyed right? And we know that each object with mass would have its own gravity, so as mass cannot be destroyed or created. Their gravities can neither be created nor be destroyed too right? And Bandersnatch, that spherical thing you are saying can you explain it a bit less complicated?, atleast so that an Highschool kid can understand it

11. Mar 29, 2015

### Bandersnatch

I'm going to assume you have learned about Newton's Law of Gravity already.

This thought experiment relies on something that Newton first proved a while back, and what you might have been told in school without much explaining - that gravity far away from a spherically-symmetric mass is the same as if all the mass were concentrated in its centre. That's why you can calculate the acceleration due to gravity on the surface of Earth as if all its mass were in its centre, 6700-odd km away.
It's what is called the 'shell theorem', and it can be understood if you know some calculus, or purely on geometrical terms - look it up on Wikipedia.
The other thing the theorem shows is that the gravitational field inside a spherically symmetric, empty shell of matter is always 0.

Imagine a set-up like this:

You've got a hollow shell of mass M represented by the grey circle. The shell contracts under its own gravity until it is smaller - here represented by the black circle. It doesn't matter how far it had actually contracted when we look at it again. The point is, it's smaller than before.

The gravity field (i.e., acceleration due to gravity, or $F/m$) outside this shell (at point A) is the same as if all the mass were concentrated in the centre of the circle and given by $g=\frac{GM}{R^2}$.

The field inside the grey circle before it contracted is zero throughout the volume, including points B and C.

Now lets look at it after it contracted:
-the field outside the original radius (outside the grey circle, point A) is still the same, because we can still treat it as if the mass were concentrated in the centre;
-the field inside the new radius (inside the black circle, point C) is still zero, because this volume is still surrounded on all sides by spherically-symmetric shell of matter;
-the field between the two radii (point B) is now equal to $g=\frac{GM}{R_B^2}$, whereas before the shell contracted, it was zero.

So what you've got, is gravity appearing in a volume of space where once it wasn't there.

This works the other way around as well. Imagine a star exploding (for simplicity, let's say it doesn't leave anything behind - like a supernova type Ia explosion).
Before it explodes, a planet can be in orbit around it due to the gravity it feels from all that mass.
After it explodes, the gravity that held the planet in orbit disappears as soon as the 'shrapnel', i.e. the envelope of matter ejected by explosion, passes the planet's orbit. Now the planet is inside an expanding, spherically symmetric and hollow shell of matter, and the value of the gravitational field it experiences is zero.

Last edited: Mar 29, 2015
12. Mar 29, 2015

### pvk21

So you mean to say that gravity can be created or destroyed

13. Mar 29, 2015

### Bandersnatch

Like Drakkith, I dislike such words, as they suggest agency.
I mean what I said - you change the arrangement of some mass, and you get a positive value of the field where it used to be zero (and vice versa).

14. Mar 29, 2015

### Drakkith

Staff Emeritus
Things are even more complicated than they seem. In GR the curvature of space-time is not zero inside the shell, so even though you wont accelerate in any direction if you were inside the shell, you would be experiencing time dilation compared to an observer outside and far away from the shell.

15. Mar 29, 2015

### phinds

Mass cannot be destroyed?

What do you think goes on at CERN every day? Mass is converted to energy.

What do you think goes on in an atom bomb? Mass is converted to energy.

How about the interior of the sun? Mass is converted to energy.

16. Mar 29, 2015

### Drakkith

Staff Emeritus
Mass is not destroyed in any of those situations. Matter is converted into energy, but this energy has mass.

In other words, if you measure the mass of the system prior to this conversion, and then measure the mass of the system after the conversion but before any energy leaves the system, you will find that the mass of the system is identical in both cases. Once this energy leaves the system, perhaps by being radiated away, the mass of the system is reduced according to Einstein's equation.

Einstein's equation doesn't really tell us that mass can be 'converted' to energy, it tells us that if you track where Y amount of energy goes, you will find that X amount of mass accompanied it, and if you move X amount of mass around, it took Y amount of energy to do so. That's how I understand it at least.

Last edited: Mar 29, 2015
17. Mar 29, 2015

### phinds

Yeah, I know the mass/energy total remains the same but the matter that goes into the energy is no more.

18. Mar 29, 2015

### Drakkith

Staff Emeritus
Your posts confuse me, good sir.

19. Mar 29, 2015

### phinds

Hm ... seems clear to me so I must be missing something. My understanding is that although mass/energy total is conserved in a closed system, the matter being converted to energy means less matter (less mass) and more energy.

20. Mar 29, 2015

### Drakkith

Staff Emeritus
You replied to my earlier post while I was editing it. Have you reread post 16 since I edited it? If not, that may make it a little clearer what I mean.