Tell any 3 differences between action and reaction
Answers
This question sounds simple and boring, where in fact it is fundamental, important and subtle. Say someone throws a brick at your window-God forbid, and the window was broken. Then the throwing is action and the breaking is reaction. So it is simple- the action comes first and the reaction is the result. This is also connected to the idea of causality.. that is you can’t have the window breaking and you don’t understand why, later you see a brick going towards the window and you say AH.. I know now why.. No this is impossible according to our experience.. it never happens but we really don’t know why.
Think about it more, and you find it is much more subtle than that. First Newton says that for every action there is an equal and opposite reaction. But he doesn’t say if the two should be simultaneous or at two different times. When you think about it, you find it is instantaneous when the two- the body causing the action and that receiving it, are next to each other.. but when they are apart, the process must take time.. it is all because of Einstein.. he says nothing can go beyond the speed of light and it is true as per every experiment. If you like, it is like Newton’s cradle.. one ball hits and after few microseconds (or whatever) the last ball flies away.. giving time for the stress pressure wave to move across the group of balls. This is the speed of sound in steel. In empty space it is the same but at the much higher speed of light.
So when the moon moves, the tide does not follow immediately.. the action in this case is the new force from the moon-that is after it moved. This force has first to travel at c to reach earth, then affect water, then water takes time to accelerate. This is how things happen and that is why we believe in an inevitable causality. When you apply this to the sun earth situation or any of the other stable orbit, you discover a new fact. If there is a delay between action and reaction and this resulted in the forces not being along the line joining the two, the orbit can’t remain stable. This was realized as far back as Euler and he decided that for this reason, gravity must move at infinite speed. Everybody believed him, and who dares challengin him- he was using equations to prove it. This result killed the subject of finite speed for gravity for hundreds of years till Einstein came and found that his theory seem to suggest that gravity moves at c only.
The solution to the problems came from the retarded potential integral. If you take a static inverse square force orbit, be it due to an electrostatic or gravity forces and you assume that action moves at c only, you find that the magnitude of the force between the earth and sun is retarded ok as per the speed of light and as per the distance between them, giving about 8 minutes. But surprisingly the direction of force is found to point to the ‘instantaneous’ position not the retarded one. Yes the instantaneous position.. due a magical cancellation of the of the change of direction of the force by the velocity effects. see and the math pages article below. These give the equations for EM, and a similar treatment can produce the gravito-magnetic equations- the equivalent to GR. So magically, when we look at the sun we see it where it is now, although any event on it including its pull or any change in its intensity for example, can’t reach us before 8 minutes.