„What it takes to produce a gravitational repulsion is a negative pressure. According to general relativity, it turns out… both pressures and energy densities can produce gravitational fields, unlike Newtonian physics, where it's only mass densities that produce gravitational fields.“

—  Alan Guth

Lecture 1: Inflationary Cosmology: Is Our Universe Part of a Multiverse? Part I.
The Early Universe (2012)

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Alan Guth photo
Alan Guth photo
Willem de Sitter photo
Gerald James Whitrow photo
Jayant Narlikar photo
Lee Smolin photo
Albert Einstein photo

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

—  Albert Einstein German-born physicist and founder of the theory of relativity 1879 - 1955

Statement of the equivalence principle in Yearbook of Radioactivity and Electronics (1907)
1900s

William Crookes photo

„It is curious that the popular conceptions of evil and malignant beings are of the type that would be produced by increased gravitation“

—  William Crookes British chemist and physicist 1832 - 1919

toads, reptiles, and noisome creeping things — while the arch fiend himself is represented as perhaps the ultimate form which could be assumed by a thinking brain and its necessary machinery were the power of gravitation to be increased to the highest point compatible with existence — a serpent crawling along the ground. On the other hand, our highest types of beauty are those which would be common under decreased gravitation.
The "daughter of the gods, divinely tall," and the leaping athlete, please us by the slight triumph over the earthward pull which their stature or spring implies.
Address to the Society for Psychical Research (1897)

Otto Lilienthal photo

„All flight is based upon producing air pressure, all flight energy consists in overcoming air pressure.“

—  Otto Lilienthal German aviation pioneer 1848 - 1896

Der Vogelflug als Grundlage der Fliegekunst (1889); English edition: Birdflight As The Basis of Aviation (1911).

Albert Einstein photo
Richard Feynman photo

„We can deduce, often, from one part of physics like the law of gravitation, a principle which turns out to be much more valid than the derivation.“

—  Richard Feynman, livro The Character of Physical Law

chapter 2, “ The Relation of Mathematics to Physics http://www.youtube.com/watch?v=M9ZYEb0Vf8U” referring to the law of conservation of angular momentum
The Character of Physical Law (1965)
Contexto: Now we have a problem. We can deduce, often, from one part of physics like the law of gravitation, a principle which turns out to be much more valid than the derivation. This doesn't happen in mathematics, that the theorems come out in places where they're not supposed to be!

Marshall McLuhan photo

„To say that a body or its gravitational field 'bends in space' in its vicinity is the discuss visual space in acoustic terms.“

—  Marshall McLuhan Canadian educator, philosopher, and scholar-- a professor of English literature, a literary critic, and a communicatio… 1911 - 1980

p. 40
1980s

Willem de Sitter photo
Willem de Sitter photo

„Both the law of inertia and the law of gravitation contain a numerical factor or a constant belonging to matter, which is called mass.“

—  Willem de Sitter Dutch cosmologist 1872 - 1934

We have thus two definitions of mass; one by the law of inertia: mass is the ratio between force and acceleration. We may call the mass thus defined the inertial or passive mass, as it is a measure of the resistance offered by matter to a force acting on it. The second is defined by the law of gravitation, and might be called the gravitational or active mass, being a measure of the force exerted by one material body on another. The fact that these two constants or coefficients are the same is, in Newton's system, to be considered as a most remarkable accidental coincidence and was decidedly felt as such by Newton himself. He made experiments to determine the equality of the two masses by swinging a pendulum, of which the bob was hollow and could be filled up with different materials. The force acting on the pendulum is proportional to its active mass, its inertia is proportional to its passive mass, so that the period will depend on the ratio of the passive and the active mass. Consequently the fact that the period of all these different pendulums was the same, proves that this ratio is a constant, and can be made equal to unity by a suitable choice of units, i.e., the inertial and the gravitational mass are the same. These experiments have been repeated in the nineteenth century by Bessel, and in our own times by Eötvös and Zeeman, and the identity of the inertial and the gravitational mass is one of the best ascertained empirical facts in physics-perhaps the best. It follows that the so-called fictitious forces introduced by a motion of the body of reference, such as a rotation, are indistinguishable from real forces. ...In Einstein's general theory of relativity there is also no formal theoretical difference, as there was in Newton's system. ...the equality of inertial and gravitational mass is no longer an accidental coincidence, but a necessity.
"The Astronomical Aspect of the Theory of Relativity" (1933)

Kenneth N. Waltz photo

„External pressure seems to produce internal unity.“

—  Kenneth N. Waltz, livro Man, the State, and War

Chapter V, Some Implications Of The Second Image, p. 149
Man, the State, and War (1959)

Willem de Sitter photo

„Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Etiam egestas wisi a erat. Morbi imperdiet, mauris ac auctor dictum.“