Wednesday, June 3, 2020

Relativity: The Special and General Theory by Albert Einstein Book PDF free download




The purpose of mechanics is to describe how bodies change their position in space with "time." I
should load my conscience with grave sins against the sacred spirit of lucidity were I to formulate
the aims of mechanics in this way, without serious reflection and detailed explanations. Let us
proceed to disclose these sins.

It is not clear what is to be understood here by "position" and "space." I stand at the window of a
railway carriage which is travelling uniformly, and drop a stone on the embankment, without
throwing it. Then, disregarding the influence of the air resistance, I see the stone descend in a
straight line. A pedestrian who observes the misdeed from the footpath notices that the stone falls
to earth in a parabolic curve. I now ask: Do the "positions" traversed by the stone lie "in reality" on a
straight line or on a parabola? Moreover, what is meant here by motion "in space" ? From the
considerations of the previous section the answer is self−evident. In the first place we entirely shun
the vague word "space," of which, we must honestly acknowledge, we cannot form the slightest
conception, and we replace it by "motion relative to a practically rigid body of reference."

The positions relative to the body of reference (railway carriage or embankment) have already been
 defined in detail in the preceding section. If instead of " body of reference " we insert " system of
co−ordinates," which is a useful idea for mathematical description, we are in a position to say : The
stone traverses a straight line relative to a system of co−ordinates rigidly attached to the carriage,
but relative to a system of co−ordinates rigidly attached to the ground (embankment) it describes a
parabola. With the aid of this example it is clearly seen that there is no such thing as an
independently existing trajectory (lit. "path−curve" 1) ), but only a trajectory relative to a particular
body of reference.

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Relativity: The Special and General Theory by Albert Einstein Book PDF free download 


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