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Research on:

GENERAL RELATIVITY

AND

GRAVITY

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A. The Principle of Equivalence

• equivalence principle equates accelerating an gravit! effects

The equivalence principle was Einstein's `Newton's apple' insight to gravitation. His thought

experiment was the following; imagine two elevators, one at rest of the Earth's surface, one

accelerating in space. To an observer inside the elevator no windows! there is no ph"sical

experiment that he#she could perform to differentiate between the two scenarios.

The equivalence principle is a funa"ental la# of ph!sics that states that gravitational and

inertial forces are of a similar nature and often indistinguishable. $n the Newtonian form it

asserts, in effect, that, within a windowless laborator" freel" falling in a uniform gravitational

field, experimenters would be unaware that the laborator" is in a state of non%uniform motion.

 &ll d"namical experiments "ield the same results as obtained in an inertial state of uniform

motion unaffected b" gravit".

• although a si"ple an co""on sense assu"ption$ the equivalence principle has

strange consequences

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 &n immediate conseuence of the equivalence principle  is that gravit! %ens light. To

visuali(e wh" this is true imagine a photon crossing the elevator accelerating into space. &s

the photon crosses the elevator, the floor is accelerated upward and the photon appears to fall

downward. The same must be true in a gravitational field b" the euivalence principle.

• such as$ photons #ill %e effecte %! gravit!$ even though the! have &ero "ass

The principle of equivalence renders the gravitational field fundamentall" different from all

other force fields encountered in nature. The new theor" of gravitation, the general theor" of 

relativit", adopts this characteristic of the gravitational field as its foundation.

• t#o classical tests of general relativit!'

There were t#o classical test of general relativit!; the first was that light should

be eflecte b" passing close to a massive bod". The first opportunit" occurred during a total

eclipse of the )un in ()().

• the first is the eflection of starlight %! the *un+s gravit! as "easure %! the ()()

solar eclipse e,peri"ent

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*easurements of stellar positions near the dar+ened solar limb proved Einstein was right.

irect confirmation of gravitational lensing was obtained b" the -u%%le *pace Telescope

last "ear.

. *o"e of the Preictions of the General Theor! of Relativit!

• )ome preictions of general relativit!  differ significantl" from those of classical

ph!sics, especiall" concerning the passage of ti"e, the geo"etr! of space, the

"otion of %oies  in free fall, and the propagation of light. Examples of such

differences include gravitational ti"e ilation, gravitational lensing,

the gravitational reshift of light, and the gravitational ti"e ela!. The predictions of 

general relativit" have been confir"e in all observations and experiments to date.

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 &lthough general relativit" is not the onl! relativistic theor! of gravit!, it is

the si"plest theor! that is consistent with experimental data. However, unanswered

uestions remain, the most fundamental being how general relativit" can be reconciled

with the laws of quantu" ph!sics  to produce a complete and self%consistent theor"

of quantu" gravit!.

/. 0inings of General Relativit!• The secon part of relativit!  is the theor! of general relativit! and lies on t#o

e"pirical finings  that he elevated to the status of basic postulates. The first

postulate is the relativit! principle- local ph"sics is governed b" the theor" of special

relativit". The secon postulate is the equivalence principle- there is no wa" for an

observer to distinguish locall" between gravit" and acceleration.