Relativistic effects are only symmetrically relative when observing inertial (or non-accelerating) frames of reference. The Twin on Earth does not accelerate or change his frame of reference at any point. The travelling twin experiences at least two frames of reference - moving away from Earth, and moving towards Earth - and therefore experiences acceleration (realistically he experiences more frames than this since I am taking the simplest example of instantaneous acceleration).

The are a couple of stages to my response to the Gravity/Acceleration comment. 1/. I'm pretty sure that the original premise of the Twins Paradox was as a thought experiment. The presence of a planet was just an indication of one twin staying still. A more appropriate description might be - one twin at rest in a vacuum and the other twin accelerating away from, then towards this brother/sister. This is a closed environment, there are no other effects or objects acting on the twins. 2/. The result of Gravity affecting a mass is acceleration. Acceleration is not a force, but a natural consequence of a Force ( f = ma - newtonian number 2 law I think, could be 1 though...it's been a few years). But yes, in a real world example both twins would be experiencing acceleration - from quite a number of sources I would guess, not just the planets Gravity. The only difference is that one twin is experiencing centripetal acceleration, and therefore his speed does not increase (even though his velocity changes), while the other twin is experiencing acceleration that pushes his speed close to that of c. The difference between the two effects means this can be simplified to the closed environment situation of the thought experiment while only losing extremely minor outside influences (comparatively). The acceleration itself isn't the direct cause of the time dilation effects. The travelling at a significant proportion of the speed of light causes those. I'm not sure I've explained the above very well, it has been far to many years since I last looked at special relativistic effects (it's not the sort of thing I come across in every day life *8-).

The answer is validating that is observed Experimentally that the time expands for the stationary twin (but why does that happen ? !!). Does this mean that acceleration is the cause of Time Dilation and not the velocity ? Because as the question goes, the time difference can happen both ways. Can anybody tell me how accelerating from the so-called staionary twin cause the time of only stationary twin to expand wrt the other

Absolutely. Special Relativity predicts that the travelling twin appears to have aged less time according to the stationary twin. ... and vice versa. However, Special Relativity only deals with constant motion in a single direction ... it can't deal with the acceleration that must occur if the travelling twin ever returns home. Suppose the travelling twin travels in a circle at close to the speed of light and returns this way to his starting point. We no longer really know who is stationary or not, but lets see what happens: The "stationary" twin predicts that the clock of the other twin has slowed down, due to the time dilation. The "moving" twin however, is aware of what appears to be a gravitational field pulling him inside his ship towards the wall away from the centre of the circle ... this is actually the ship's acceleration. When he calculates the clock of the other twin ... the one he thinks is moving ... he finds that the other's clock seems to speed up due a general relativity effect of motion through a gravitational field. Thus when both meet up again, both agree that the traveller is younger. Thus everything is relative, but you have to take gravity into account too.