Dear Ludwig,

Leaving aside davidf's observation for the sake of the exercise I reiterate my earlier point;

“There are not multiple truths as you assert just plenty of close approximations that suffice as truths to make life easier until something absolute is required.”

There are no inconsistencies with your latest post.

To say the picture is absolutely still might not be considered a 'close approximation' but it is designed to make life easier. And yes relative to the wall it is stuck on the picture is still and thus able to be described in this manner. However if we require an absolute truth about the stillness or otherwise of the painting then the explanation about its trajectory in space is required.

On the subject of the being stationary (or not):

Here's some thought-provoking stuff from Brian Greene in "The Fabric of the Cosmos".

On gravity and acceleration being equivalent:

"...Right now you feel gravity's influence. If you are standing, you feel the floor supporting your weight. If you are sitting you feel the support somewhere else....you probably think that you are stationary--that you are not accelerating or even moving at all. But according to Einstein you actually are accelerating....Since gravity and acceleration are equivalent, if you feel gravity's influence, you must be accelerating..."

(Greene earlier used a character called "Barney" jumping into an evacuated airless shaft as an example)

"When Barney jumps from his window into the evacuated shaft, we would ordinarily describe him as accelerating down toward the earth's surface. But this is not a description Einstein would agree with. According to Einstein, Barney is "not" accelerating. He feels no force. He is weightless....He provides the standard against which all motion should be compared...From Barney's perspective as he freely falls by your window...you and the earth and all the other things we usually think of as stationary are accelerating upward.....

Clearly this is a radically different way of thinking about motion. But it's anchored in the simple recognition that you feel gravity's influence only when you resist it. By contrast, when you fully give in to gravity you don't feel it. Assuming you are not subject to any other influences (such as air resistance) when you give into gravity and allow yourself to fall freely, you feel as you would if you were freely floating in empty space--a perspective which, unhesitatingly, we consider to be unaccelerated.

In sum, only those individuals who are freely floating, regardless of whether they are in the depths of outer space or on a collision course with the earth's surface, are justified in claiming that they are experiencing no acceleration."

In physics, “experiencing no acceleration” means you are moving with an inertial system (a frame of reference within which bodies are not accelerated unless acted upon by external forces). By physics I mean classical physics or special relativity; the case is more complicated in general relativity.

This is different from "being stationary", a term that (in physics) makes sense only in the sense of stationary WITH RESPECT TO something, some frame of reference. There are many inertial systems that are not stationary with respect to each other.

The motion of any object in the universe is seen differently according to the frame of reference chosen. The difference may be too small to measure, but the motion that we see differs with each observer. If we assume the frame of reference is ourselves an object that we see at rest is at rest with respect to ourself. That does not mean we have stated a truth. We merely have chosen a particular frame of reference. If we assume the frame of reference is the sun the earth and all on it are moving through space around the sun with the additional motion that objects may be moving with respect to the earth. We can assume any point in space is a frame of reference. Some comments on this string have confused the choice of a particular frame of reference with a truth.

If we use the earth as a frame of reference we can say the sun goes around the earth. At one time people did, and a complicated path of the sun could be calculated from that assumption.

http://en.wikipedia.org/wiki/Deferent_and_epicycle tells about the path computed under that assumption. However, we choose to use the sun as the frame of reference and assume that the earth goes around the sun. If we do that the path of the earth's motion is much simpler. It is an ellipse, and we can compute the motion using the Newtonian laws of motion and the Newtonian law gravity so we say that the earth goes around the sun. In doing so we ignore the fact that our solar system is part of a galaxy and the entire solar system is moving with respect to the center of that galaxy. It is reasonable to do this since the sun is by far the most massive body close to the earth, and the effects of the other celestial bodies on the earth's motion is very small with respect to that of the sun.

However, there are not multiple truths merely multiple frames of reference which can be chosen.

Dear david f,

Nicely explained, I think, except for

>>We can assume any point in space is a frame of reference.<<

I think you mean “any position” or something like that, since a frame - fixing a coordinate system with respect to which you do your measurements and calculations - is obviously not given by just a point but about the point plus a triple (quadruple in case of special relativity) of linearly independent vectors.

In classical (and that of special relativity) mechanics there is a privileged class of frames of reference, called inertial, with respect to which physical laws of mechanics are mathematically simple and formally the same irrespective of which one such frame (coordinate system) you use do express them in.

This is theory, and you have shown that in practice the frame associated with our sun is much closer to being an inertial system than the one associated with our earth.

Of course, “truth” (whatever one means by that in theoretical physics) has nothing to do with the choice of a frame of reference or coordinate system.

Dear George,

One problem with this discussion is that truth is a word with many meanings. You got the point of my post which was the arbitrary choice of reference point which you made clear has to be supported by other data has probably nothing to do with the truth which the originator of this string talked about. So I ask RandomGuy who started this string, "What do you mean by truth?"