# Space bent by gravity ?

1. Jun 18, 2009

### AlisonArulia

Space bent by gravity ??

Hi, new here so please bear with me. Got a question so if you can help it would be great.

He is something my son asked me the other day and I have been googleing for days to get the answer but they are mainly too comple for me to understand.

If say you (for example) jumped off a wall, or skied down a slope, or drove car down a hill. (this is going to sound silly sorry) are you moveing or is the ground coming up to you.

The reason I ask is my son is doing (finished now) and he said that we stay still and the world moves along a stright line around the sun - when is bent by gravity.

Sorry but I'm lost with all this stuff.

Thanks for your time

Alison

2. Jun 18, 2009

### diazona

Re: Space bent by gravity ??

Welll... you could think about it either way. You'd usually say that you're the one moving, since it just inherently seems "easier" for one person to be moving than for the entire Earth to be moving in the other direction. But physically, there's no reason you can't consider the ground to be coming up to you.

That's actually the basis of Einstein's theory of (special) relativity, that there's no way to distinguish whether you're moving or everything else is moving - only relative motion counts. (Of course, Einstein was talking about things moving in straight lines in empty space; for real objects on Earth, it's technically more complicated, but the basic idea sort of still applies.)

That is actually true, in a sense. In normal geometry, a straight line is the shortest possible path between two points. But according to the theory of general relativity, a big massive object like the Sun distorts space (and time), so that the shortest possible path between two points is no longer straight. In physics we call these shortest paths geodesics (whether they're straight lines or not). The Earth's path around the Sun is one example of a geodesic. (Maybe you're thinking, it doesn't look like an orbit is any kind of shortest path. Well, that's mostly because you don't see the Earth's "motion" through time. When you take that into account, it becomes clear why the orbit is a geodesic, but there's quite a bit of complicated math involved in that.)

3. Jun 18, 2009

### mgb_phys

Re: Space bent by gravity ??

There are a couple of different questions here.

First wether you move down to the ground or the ground moves upto you depends on your point of view. The physics works out the same for both cases you just choose the simplest.
Suppose you are on an aircraft and walk to the front of the cabin, you t consider that you are moving forward at 2mph relative to the plane, rather than 600mph relative to the ground. Similairly when designers are designing the airflow over the wing they assume the wing is stationary and the air is moving over it at 600mph rather than that the air is stationary and the wing is moving - it's just a matter of convenience.

This actually has a very deep and important meaning for physics. Any experiment you do cannot depend on how you are moving (assuming a constant speed ). If this wasn't true your answers would be different in December when you are moving one way around the sun and June when you are moving in the opposite direction. This leads to the theory of Special relativity.

The straight line around the sun is a slightly different question. General relativity says that gravity isn't really a force pulling objects together - instead objects bend space and other objects travel in a straight line in that bent space. Partly this is just another way of doing the maths, but it does gives some different effects for things like light being bent by heavy objects which we can measure and so we believe the theory is true.

ps. when you jump off a wall you are moving toward the ground but the ground is also slightly moving up toward you, but only a very small amount because you weight rather less than the earth.

4. Jun 18, 2009

### AlisonArulia

Re: Space bent by gravity ??

Thank you for your time. Can I ask a broader question about the same thing

My son said that when I ski down a hill, I am afected by fricton that keeps my stuck to the floor but I am only afected by following the earth's movment along the stright line it is makeing around the sun

sorry but I just don't understand any of this. The problem is that I want to try and help him with his school work but (in the 60's) we did not learn about his.

Is there a web site I can look at the would give me a clear outline of this. Basicaly if I ski down a hill which way am i going. Am I staying still or moving - relative - the the earth or something else

sorry to be new at all this but it really is interesting

5. Jun 18, 2009

### jamesb-uk

Re: Space bent by gravity ??

Gravity keeps you stuck to the floor, friction reduces your acceleration. Gravity would still happen, even if the earth was not moving in relation to the sun. Is that what you were asking, I'm not quite clear?

You are moving relative to the earth. What if there were 6 skiers on the planet, going down 6 hills, spread out at the poles, and around the equator. If it was the earth moving as much as that, it would have to swell noticeably. If you jump on the floor, the earth does move slightly, but because its mass is so much more than yours, the equal and opposite forces will have a negligeable effect on it. That has nothing to do with relativity really.
There are lots of good books you could look at. A good place to start would be 'The new adventures of Mr Tompkins'.
A good way to think about things like this is the bowling ball on a piece of rubber analogy. The earth is the bowling ball, and if you put a pea in the curve (you), it will roll inwards. However, if you have two bowling balls, they will both move together, because their masses are similar.

Last edited: Jun 18, 2009
6. Jun 18, 2009

### Nick89

Re: Space bent by gravity ??

If you are skiing down a hill, yes, you are moving relative to the earth. But you could also say the earth is moving relative to you. It's the same thing!

Consider yourself in a spaceship in deep space. There are no planets around that can be seen, perhaps maybe distant stars. There is also another spaceship nearby, and it appears to be moving with constant speed toward you.
The question is, "which ship is moving"? Is your ship moving towards the other ship? Or is his ship moving towards yours? Or are you both moving towards each other?

The answer is again, either one. The answers are all equivalent. In fact, Einstein even goes so far to say that no single physical experiment can be done to determine who is moving. Motion is relative! You simply can not distinguish between the different cases, they are all the same.

Now, if the ships are accelerating, then it becomes a slightly different story (and also a much more difficult story!)

7. Jun 18, 2009

### Klockan3

Re: Space bent by gravity ??

You guys don't get the question, you are just talking about the Galilean relativism. The deal with GR is that the earth's surface is accelerating upwards while the person jumping is following a straight line and thus it is the surface accelerating towards the jumper, no matter which reference frame you got. Assuming no air resistance of course. Also note that this applies for all sides of the earth at the same time, so the whole of earth's surface is constantly accelerating outwards.

And as for everything else, if you want a crash course just read the wikipedia article:
http://en.wikipedia.org/wiki/General_relativity

8. Jun 19, 2009

### AlisonArulia

Re: Space bent by gravity ??

Sorry to not understand (everyone on this site is very clever) but would not this mean the world will just keep getting bigger. Or am I missing something basic here ?

9. Jun 19, 2009

### denisv

Re: Space bent by gravity ??

Physicists are mathematicians who are unencumbered by trivialities like signs or convergence :)

10. Jun 19, 2009

### AlisonArulia

Re: Space bent by gravity ??

I am sorry, I don't understand that answer

11. Jun 19, 2009

### denisv

Re: Space bent by gravity ??

We are accelerated inwards, yes.

12. Jun 19, 2009

### A.T.

Re: Space bent by gravity ??

No. Even in classical mechanics accelerating towards a point, doesn't mean you move closer to it. See http://en.wikipedia.org/wiki/Centripetal_force" [Broken]. Similarly in General Relativity acceleration away from a point doesn't mean you move further away from it.

In General Relativity acceleration is defined as deviation from the straight path (geodesic) in spacetime, which is traversed by free falling (force free) objects. So you have to accelerate to deviate from free fall and keep a constant space coordinate in a gravitational field. The earths surface needs to be accelerated outwards in order for the earth to keep a constant diameter.

Last edited by a moderator: May 4, 2017
13. Jun 19, 2009

### denisv

Re: Space bent by gravity ??

I think you're confused. Gravity is accelerating us inward, and molecular forces are keeping it from collapsing into a point.

14. Jun 19, 2009

### diazona

Re: Space bent by gravity ??

This statement is true in a rather twisted sense... but that's general relativity for you

Here's a way to think about it. Let's say you get dropped down to Earth from far out in space, in a box with a window in the bottom. You're going to be in free fall, which means you're effectively weightless - sure, you're being pulled down by gravity, but so is the box and everything else in it, all at the same rate. With some simple physics experiments you can convince yourself that the law of inertia holds within your box; for example, if you throw a ball in the box, it flies perfectly straight without speeding up or slowing down at all. So as far as you're concerned, you're in an inertial frame of reference, just like being in deep space where there is no gravity.

Now, imagine looking out the window of your box. You'll see the Earth rushing up toward you at ever increasing speeds. Now, you might be falling - or you might really be in deep space, and maybe the Earth you see is just a humongous beach ball with pretty paint and a rocket on the back, accelerating toward you. (Sounds ludicrous, I know, but as far as physics is concerned it's perfectly possible) The point is that you can't possibly do any sort of physics experiment within your box to tell whether you're falling toward the real Earth or just being shot at with a giant rocket-propelled beach ball. The two situations - gravity and acceleration - are exactly the same physically. And, in a sense, that's why falling (or sliding down a ski ramp) is equivalent to the ground accelerating up at you.

Technical note: when you're falling toward the Earth, you are following a geodesic. That's probably where the idea that you're moving straight while the Earth accelerates up toward you came from (although that idea is grossly oversimplified, if not actually wrong).

15. Jun 19, 2009

### A.T.

Re: Space bent by gravity ??

That's the Newtonian model where gravity is a force interaction. The OP asked about "bend space" so it was explained to him terms of General Relativity, where there is no force interaction inwards. Things fall down due to their inertia and have to be accelerated outwards in order not to fall.

16. Jun 19, 2009

### denisv

Re: Space bent by gravity ??

I'm not convinced. You seem to be suggesting that the mass of the earth is accelerating radially out, whereas in the picture diazona suggested the entire earth is taken to accelerate towards you. Surely the two are not equivalent?

17. Jun 19, 2009

### A.T.

Re: Space bent by gravity ??

He is talking about coordinate acceleration (dv/dt) of the earth in the free fallers frame. I'm talking about proper acceleration (what an accelerometer measures) of the earths mass outwards, which is absolute (the same in every frame).

18. Jun 20, 2009

### cragar

Re: Space bent by gravity ??

gravity does not bend space , mass or energy bends space and gravity is the effect of the bent space.

19. Jun 20, 2009

### A.T.

Re: Space bent by gravity ??

To be even more exact: Gravity is not the effect of curved space but of curved spacetime. Curved space alone would not affect objects which are at rest in space. But gravity does.

20. Jun 20, 2009

### AlisonArulia

Re: Space bent by gravity ??

Thank you all for your very good answers. I only wish I could fully understand it all. I will take some time and read up on all the answers and try and understand a little better.

Just one little point
I am a "her" - not a "him"

Thanks again

Alison

21. Jun 20, 2009

### A.T.

Re: Space bent by gravity ??

If you want to understand how gravitation is modeled in general relativity I would suggest to start with some visualizations:

http://www.relativitet.se/spacetime1.html
http://www.physics.ucla.edu/demoweb..._and_general_relativity/curved_spacetime.html