3 ms·

What is Galilean invariance? You are below deck in a ship moving uniformly on a smooth sea. No experiment can tell you if the ship is docked or if it has motion

by nyc111 9y ago

What is Galilean invariance? You are below deck in a ship moving uniformly on a smooth sea. No experiment can tell you if the ship is docked or if it has motion relative to the see. Assume that the ship is docked. In that case you say that the ship is at rest. And you attribute to the ship some properties because it is at rest. But is the ship at rest absolutely? Not at all. The ship is moving with the earth. So do you deny that the earth is moving? I assume not. If so how can you claim that the ship is at rest? Motion is absolute means that all is in motion. This is an axiom. When you say motion is not absolute, can you give an example of an object which is at rest, which does not move with the earth and does not move with the galaxy? There is no such object.

dragonwriter 9y ago
> Motion is absolute means that all is in motion That's not a usual use of “absolute”; “motion is universal” would be the normal way of expressing that idea. > This is an axiom. Well, you could have an axiomatic system in which that was an axiom, but I don't see why you would want to.
- nyc111 9y ago
  Can you give an example of a system which is not based on axioms? There is none. There may be hidden assumptions but there will always be axioms. Axioms are simply definitions that are kept constant. In your case you make the assumption that absolute rest exists. The axiom of the universality of motion is a good one because it agrees with the observations.
  - cgmg 9y ago
    > In your case you make the assumption that absolute rest exists. This is a straw man. No one is making that assumption. Something is in motion or at rest with respect to a reference frame. For some reason, you keep misinterpreting this statement and ranting about "absolute motion" and "absolute rest".
cgmg 9y ago
> If so how can you claim that the ship is at rest? The ship is at rest with respect to its own reference frame. > So do you deny that the earth is moving? I assume not. The earth is at rest with respect to its own reference frame. It is moving with respect to other things, such as the sun, or the center of the galaxy. Motion and rest are relative. This is the key concept, and I don't understand why you're having such trouble with it.

by nyc111 9y ago

dragonwriter 9y ago

> Motion is absolute means that all is in motion That's not a usual use of “absolute”; “motion is universal” would be the normal way of expressing that idea. > This is an axiom. Well, you could have an axiomatic system in which that was an axiom, but I don't see why you would want to.

nyc111 9y ago
Can you give an example of a system which is not based on axioms? There is none. There may be hidden assumptions but there will always be axioms. Axioms are simply definitions that are kept constant. In your case you make the assumption that absolute rest exists. The axiom of the universality of motion is a good one because it agrees with the observations.
- cgmg 9y ago
  > In your case you make the assumption that absolute rest exists. This is a straw man. No one is making that assumption. Something is in motion or at rest with respect to a reference frame. For some reason, you keep misinterpreting this statement and ranting about "absolute motion" and "absolute rest".

cgmg 9y ago

> If so how can you claim that the ship is at rest? The ship is at rest with respect to its own reference frame. > So do you deny that the earth is moving? I assume not. The earth is at rest with respect to its own reference frame. It is moving with respect to other things, such as the sun, or the center of the galaxy. Motion and rest are relative. This is the key concept, and I don't understand why you're having such trouble with it.