Albert Einstein

We shall, therefore, assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system.

The mass of a body is a measure of its energy content.

The most beautiful fate of a physical theory is to point the way to the establishment of a more inclusive theory, in which it lives on as a limiting case.

Es ist bequem mit dem Einstein. Jedes Jahr widerruft er, was er das vorige Jahr geschrieben hat.

Geh Recht viel spazieren, dass Du Recht gesund wirst und lies nicht gar zu viel sondern spar Dir noch was auf bis Du gross bist.

...to the question whether or not the motion of the Earth in space can be made perceptible in terrestrial experiments. We have already remarked... that all attempts of this nature led to a negative result. Before the theory of relativity was put forward, it was difficult to become reconciled to this negative result.

Lieber Habicht! / Es herrscht ein weihevolles Stillschweigen zwischen uns, so daß es mir fast wie eine sündige Entweihung vorkommt, wenn ich es jetzt durch ein wenig bedeutsames Gepappel unterbreche... / Was machen Sie denn, Sie eingefrorener Walfisch, Sie getrocknetes, eingebüchstes Stück Seele...?

The state of mind which enables a man to do work of this kind is akin to that of the religious worshiper or the lover; the daily effort comes from no deliberate intention or program, but straight from the heart.

By a clock we understand anything characterized by a phenomenon passing periodically through identical phases so that we must assume, by the principle of sufficient reason, that all that happens in a given period is identical with all that happens in an arbitrary period.

We may assume the existence of an aether; only we must give up ascribing a definite state of motion to it, i.e. we must by abstraction take from it the last mechanical characteristic which Lorentz had still left it. ... But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.