Relative Motion (A Thought Experiment)

An orbit involves acceleration. Although relativity ensures you can never know whether you are moving or not (or, more profoundly, all fixed points are equal) you can still know about acceleration. On either the 'earth' or 'moon' you could know the other must be moving because it does not fall into you due to gravitational effects.And what are they moving relative to? Wherever you choose as a fixed point.In a very real sense, this is the same situation we have in the universe. Nothing is staying still, so we simply choose our fixed point and work from there - if we're dealing with the solar system we tend to pick the sun as the fixed point. If we're dealing with the motion of stars in the Milky Way, we pick it's centre as our fixed point.

Yes, you can.Acceleration can be deduced without a frame of reference, orbits are accelerational in nature.

Imagine you're in a car, cruising along the motorway (freeway in the US, I think?) at 85 mph. Although you're thumping along, you don't feel any motion. Now imagine you have to brake suddenly (some twat in a Passat cuts you up), you'll be thrown forward in your seat by the deceleration, and then as you get up to speed again when the idiot in the Passat gets out of your way you'll be pressed back in your seat.Now, imagine doing the same thing but being blind-folded. Suppose for a second the car's so good you can't hear any wind or road noise, or fell any vibrations or bumping through the suspension. There's now no way you can tell the difference between travelling at 85mph, 40mph and being stationary. However as soon as your speed changes you can feel the acceleration acting on your body.Einstein's theories extend this principle. There is no experiment you can perform that can distinguish between being stationary, and travelling at speed but you can tell the difference between constant speed and changing speed.The same applies for your planets, although the forces produced will be tiny - they are still (theoretically) measurable.You'd also be able to tell you're in orbit because it's the only way two gravitationally attracted bodies could be arranged such that the could appear not be approaching one another.

It is impossible to find evidence of motion. You can only decide that you are moving next to a point you have decided to consider fixed.What the earth/moon inhabitants can find is acceleration. Wrong. It can be said to be rotating. It is only constant linear motion that you cannot talk about. Constant linear motion produces no forces.(And don't worry about it; relativity makes no sense, much like Quantum theory).This message has been edited by Mr Jack, 07-28-2004 09:44 AM

Doesn't matter. If the other body appears stationary compared to yours, and there is nothing holding it there, you must be in orbit. You don't have to see the motion to deduce that. If the two bodies were actually stationary they would accelerate towards one another and collide.