# Need help with a thought experiment about point of view,motion, and gravity.

1. Apr 26, 2012

### TEjedi

Ive been thinking about orbits. We have plenty of points of reference(stars) that help us determine our orientation within the milky way ect. So we can effectively determine orbital paths, rotation, ect. So Ive been thinking about the earth and the moons orbital interaction independently of any other point of reference, including the sun. When I do this, it no longer becomes possible to say whether the earth is rotating, the moon is orbitting(in a path at a velocity) or a combination of both. Following this thought, I wondered about where the momentum is. You could assume that the moon is stationary and that the earth is rotating, right? This gives the earth rotational energy but takes the moons momentum or you can say the moon is completing an orbital path every 24+- with a great deal of momentum and we are only slightly rotating. So does this mean that momentum is only a point of view relative to mass? Using the laws of conservation of energy/momentum the earth moon system has the same energy when compared to a 3rd mass at some other fixed point from just an energy/momentum standpoint indepedently of what the earth and moon are doing internally as a system, as I stated earlier. Now when I put this all into motion in my head it breaks down and I dont see why. If the energy can be moved/transformed( between earth and moon) by changing your point of view, then what keeps the moon from crashing into the earth due to gravity or from being flung off into space by reaching escape velocity? I may be having trouble conveying exactly what Im thinking, so I will state it as plainly as possible now. When examining the earth and moon in a directional-less empty universe, what keeps the earth and moon at the same distance if I choose to change the energy distribution between the two by changing my point of view? I can change my point of view right? Help please.

2. Apr 26, 2012

### Vorde

Because every viewpoint is equally workable. So if the moon orbits the earth in our viewpoint, no matter what other reference frame you are in, the same total movement will happen. Its like if a person in a car passes a person standing on the street. The person in the car in say that the person standing is moving, and the person standing will say that the person in the car is moving, but both will agree that they passed by each other.

3. Apr 26, 2012

### bp_psy

This is not true. In each different frame you will measure different energies and momenta. This does not mean that anything is transferred or lost. The law of conservation of energy and momentum refer to measurements in given frames not some universal quantities that must be the same for every observer. In any given inertial frame energy and momentum will be conserved.These are the actual conservation laws.

4. Apr 26, 2012

### Staff: Mentor

In addition, the laws of physics as you want to use them only apply to inertial reference frames. Therefore, you cannot sit on the moon and say "see! Earth is orbiting around me!". You can measure the acceleration of the earth and moon independent of the system (this is an approximation for velocities much smaller than the speed of light and fine within this context) and measure that the acceleration of the moon is larger (ignore the sun for a moment). Therefore, the moon orbits the earth and not vice versa.

Velocity, momentum, kinetic energy and so on are all frame-dependent. However, acceleration, time and relative distance are the same everywhere (this is wrong in special relativity, but that does not change the message here), therefore you get the same physics in all frames. That physics is the same everywhere is a very fundamental statement, and one of the main ideas which lead to the concept of special and general relativity.

5. Apr 26, 2012

### TEjedi

What Im getting at isnt coming through clearly. According to our position in the solar system, were moving (as a planet) at roughly 18 miles a second. We have alot of kinetic energy as momentum, but only from a reference point. Erasing these reference points and examining the earth and moon as the only matter in the universe, how would they interact and appear from other reference points chosen from differing points of view. Saying it isnt true doesn't change what is true. The earth's gravitational field is strong enough to keep the moon from orbitting the sun right? So when exmining this thought experiment, i think its fair to say I can erase all other influences as being minimal and examine the system in a closed enviroment. When I do this, reference points are not available (just like nature doesnt provide us with a universal up or down or any direction) and there is no way to tell what the moons momentum is or isnt. This being said, if I chose a stationary (in relation to both the earth and moon) point between the earth and moon and could observe them and only them, they would seem to just sit there doing nothing. The internal energy of the system would be measured only from a fixed point but what fixes the point in this case to even make a measurement? My intuitive understanding is that if you added energy( in this case momentum) to the moon it would move further away from the earth, right?(assuming its pushing it in its orbital path) This would indicate that momentum does in fact play a role in why the moon and earth are not pulled together. In a universe without any substantial outside influences I should be able to put a point of reference where ever I like, which happens to be at the afore mentioned point and observe the interaction without fixed positions right? In this thought experiment, the moon and earth are equally attracted and repeled at the same time and are at equalibrium. Without momentum as the counterbalancing force to gravity, what is there? Pardon the spelling, its not my strong point.

Last edited: Apr 26, 2012
6. Apr 26, 2012

### TEjedi

I agree completely with your statement and it almost argues my point( except where the moons acceleration being greater than the earths but I know what you meant). Its completely in line with spacetime as einstien described it. Assuming the degree of curvature to be relative to the mass of the higher acceleration body, and increasing as you got closer, is it the curvature of spacetime pushing at each other that keeps them apart? Or is the moons curvature of spacetime totally overcome by the earths and it is moving in a straight line through curved space? If I follow that thought, then the moons distance and velocity(now) was intially determined by the angle of approach which, with out help, would have transfered some energy somewhere to allow the moon to be "captured" by earths gravity and not just redirect it. Im not speculating on the actual creation or process of the formation of the earth or moon but using them to compare interacting masses of comparative mass. As I understand spacetime, we and everything else on the surface are moving through spacetime that is bent towards the earths center of gravity. If the moon is trapped in this curvature, what keeps it from accelerating due to earths larger gravity as I first posted? Do like gravitational fields repel each other like protons do? In short, I think I have a misunderstanding of one of these concepts. I would think that the acceleration due to gravity from two large bodies would be added togther and the two would spiral into each other that much quicker if nothing was holding them apart.

Last edited: Apr 26, 2012
7. Apr 26, 2012

### bp_psy

What I think you are referring is known as Mach's principle and you might want to look into that.
On the other hand physically, there is no such point as you describe.
.
You will always have a changing distance between your bodies therefore you could in principle determine their dynamics.

8. Apr 26, 2012

### TEjedi

Thanks for the help. Although it doesnt answer all my questions it at least shows me that my reasoning was somewhat sound.

9. Apr 27, 2012

### Staff: Mentor

I think you should try to understand the system with Newtonian motion and gravity, and trust GR that it generates the same results (within the accuracy of Newton's gravity of course) with a curved spacetime (which can be derived, of course).

Take a rope with some mass at one end, hurl it around. At every point in time, you have a strong attracting force between you and the mass (via the rope), but the mass will never hit you.

The moon probably formed by material ejected from the earth, so there was no capture process. But other moons in the solar system might be the result of such processes. In that case, the moon has to transfer a part of its kinetic energy to other objects to get captured, right.

10. Apr 27, 2012

### TEjedi

I appriciate the advice mfb. I do understand newtonian physics and some GR with bits of SR thrown in and can derive the some of the equations(sadly just for fun). The purpose of the thought experiment was the physical mechanics involved. I will keep any personal speculation to myself but I will say that Einstein gave empty space a property and I was unaware that Mach had such an influence. Im not a historian on physics nor am I even practicing physics, its just a passion and a hobby at this point. As far as the the rope trick, your talking some real basic stuff and I wouldnt want to confuse anyone that may read this by stating or discussing otherwise, at least not here. The moon was "probably" not captured, agreed, but there are other theories. I would love to pick the brain of some theoretical physicsts because I do spend alot of my time researching, and learning, in that field and still have many questions.