Because motion is generally regarded as relative. In other words, you cannot tell whether sth. is moving or not without a reference object. However, since in the bucket experiment the dynamical effect of rotation and no rotation is distinct without any reference object, he introduces absolute space so motion is no longer relative, but absolute.
Also i read that in modern physics (<PERSON>) the accelerated motion of the water in the bucket is explained by the action of gravitational field.
This is wrong. Despite the name "relativity", <PERSON>'s theory is about the absoluteness of spacetime. Rotation is still absolute; something either rotates or not, without reference.
Other sources say that the accelerated motion remains a dilemma even for modern physics.
I don't know any dilemma in this.
Response to <PERSON> answer :
<PERSON>'s principle and general relativity
It is true that <PERSON>'s principle inspired <PERSON> to discover his famous general relativity, but <PERSON>'s principle is in fact incompatible with general relativity. <PERSON> himself rejects general relativity for it admits absolute spacetime.
The corollary of <PERSON>'s principle about the bucket experiment is that the centrifugal force water experiences results from the dragging of distant star swirling around. 562
This poses a serious question: how does the motion of stars light-years away affects the water instantly?
In addition, the well-known result of <PERSON> has shown that while rotating shells (as a model of distant stars or a big and massive enough bucket that drags the water with gravitation itself) do have effects the same as centrifugal forces and Coriolis forces, they also exert axial forces with the same order of magnitude, without a fictitious force counterpart.
Fictitious forces and gravitational field
The famous equivalence principle states that fictitious forces and gravitation are indistinguishable locally. This gives people false impression that fictitious forces are actually a product of gravitation. But no, while "true" gravitational field is manifestation of spacetime curvature, fictitious forces are a result of non-zero spatial curvature depending on how you slice spacetime into spatial and temporal part. No matter how you rotate the bucket and choose the coordinate system, a flat spacetime is a flat spacetime, and no gravitation is present.
Holographic principle
Holographic principle is more of a conjecture than an established theory. It is "a property of quantum gravity and string theories" as said by Wikipedia. And it is well-known that none of quantum gravity and string theories have ever been experimentally verified and they are constantly evolving, unlike general relativity. So while it is certainly an interesting way to think about bucket experiment, the holographic perspective is by no means the accepted "modern view". 651