Sir Isaac Newton: Absolute Reference Frame As opposed to Einstein, did Sir Isaac Newton believe in an absolute reference frame?
 A: Newton's first law is usually said to be an assertion about the existence of the so called inertial reference frames.
But let's get back a little bit first and discuss the idea of Galileo Galilei about reference frames. What Galileo noticed and explained by means of a quite nice story is that there is one entire class of reference frames on which the laws of mechanics are exactly the same.
This class of reference frames are exactly the ones which are not accelerated and let me explain the whole point: the idea is basically to try to detect motion by means of experiments on mechanical phenomena.
The point is: one knows in a certain frame of reference that the laws of mechanics hold in a certain way. In particular, the law of inertia holds good: a body in rest stays at rest unless acted upon by a force.
Now imagine, that someone in another reference frame, cannot see "the outside" and wants to be able to detect whether he is in motion or not. As Galileo suggested, imagine that someone is locked up below deck of a ship, without possibility of looking outside, and wants to be able to detect if the ship is moving or not.
One possible way to detect that would be to perform the same experiments of mechanics performed in a "stationary frame" and try to analyze deviations.
Galileo noticed then that if the ship is in uniform motion (that's to say, no acceleration), no deviation is found. Observing the experiments on a frame at rest or at constant velocity one will reach the same conclusions. In other words the observed law of Mechanics is the same in this class of frames.
Now noticed that there are two frames at play here: the first one, which we used to formulate the laws in the first place and the second which we test.
How does one detect the first frame then? There's where comes Newton's first law and its existence assertion.
Newton's First Law asserts that there is a frame of reference where bodies at rest stays at rest unless acted upon by forces and bodies moving uinformly stay so unless acted upon by external forces. This devises a criteria for what frames to use on Mechanics an furthermore, asserts that there is one such frame.
Galileo's analysis then tells us that any other frame moving uniformly with respect to some frame like that is equivalent from the point of view of Mechanics.
In that sense, from the point of view of formulating Mechanics, for Newton it was already known that there was no absolute frame, since there was one entire class of frames of reference that could be used: the inertial reference frames which are the ones discussed above.
What Einstein did was to purpose one extension: he had reasons to believe that not just the laws of Mechanics should hold in the same form in those frames, he believe that all the laws of Physics should have the same form in such frames.
So, again: for Newton there's no absolute frame from the point of view of Mechanical phenomena while for Einstein there's no absolute frame from the point of view of all Physics.
