I do know what the difference is but what I am trying to understand is how an object knows its speed is changing and/or how space knows an object is accelerating. The particular thing I am interested in is the fact, as I understand it, that a charged particle that moves at changing speed generates E/M radiation whereas that same particle at rest or moving with constant speed does not.
At any given instant, the particle is either actually not moving or is moving at a constant speed — how does a charged particle "know" what its previous speed was or how does "space" know this? Does this not imply some sort of "memory." At a macro level, a large object accelerating deforms but what about a very simple particle, like an electron which I am told lacks internal structure — how does an electron "know" it is accelerating?
EDIT: To what extent could the following explanation be true (even if only remotely so):
An electron undergoing acceleration has a field around it and as with macro objects, pushing it causes the field to deform. This deformed field then "expresses" photons. However, if the acceleration was constant, why would you get a continuous stream of photons? I would imagine that only at the time that the field changed in shape would a photon or photons be expressed. On the other hand, if the acceleration changed so that the field kept changing shape, it would be at the time of the change that the photons would be emitted — an electron undergoing constant acceleration in one direction would not be expected to emit a stream of photons. But I think it nonetheless does emit photons and if that is so, what is the triggering event for a photon to be emitted? Is it after a certain amount of time accelerating and if so, how does the electron measure this time.
Best Answer
The difference is that acceleration is absolute whereas velocity is relative. In other words, the local laws of physics are exactly the same for two objects moving with a constant velocity relative to one another, so there is no local experiment that can determine whether one is moving and the other is stationary. On the other hand, if two objects are accelerating relative to one another the local laws of physics that determine their behaviour are different, so it is possible to find their absolute acceleration. How is the state of zero absolute acceleration defined ? See Mach’s principle.
So if a charged particle emits EM radiation then it is accelerating; if it does not then it has zero acceleration.
Obviously an electron does not “know” whether or not it is accelerating - its behaviour is determined by the laws of physics. Indeed, you could say that the laws of physics are the fundamental global entities here and an electron is just a local manifestation of those laws which happens to have certain local properties such as charge, mass and spin.