Glos­sary

Focus-Science and Technology - - Guide to the Cosmos -

LIGHT CLOCK

A type of clock where light bounces be­tween a pair of mir­rors. These pro­vide a use­ful way to think about Ein­stein’s Spe­cial The­ory of Rel­a­tiv­ity, which says a mov­ing clock will run slower than a sta­tion­ary one.

TWIN PARA­DOX

The puz­zle that two iden­ti­cal twins should age at dif­fer­ent rates depend­ing on how they move. There’s ac­tu­ally no para­dox – Ein­stein’s Spe­cial The­ory of Rel­a­tiv­ity ex­plains why this is true.

NEUTRON STAR

These as­ton­ish­ingly dense dead stars have a mass roughly equal to the Sun, but squeezed into the size of a city. Spin­ning neutron stars emit pulses of ra­dio waves, which can be used by as­tronomers to test Ein­stein’s the­ory of grav­ity.

PRIN­CI­PLE OF REL­A­TIV­ITY

The idea that there is no way to de­fine ‘at rest’ in any ab­so­lute sense. In other words, all mo­tion is rel­a­tive.

SPACE-TIME

Mod­ern physics com­bines the three di­men­sions of space and the one di­men­sion of time into this sin­gle, four- di­men­sional en­tity.

GRAV­I­TA­TIONAL WAVE

A mov­ing rip­ple in the fab­ric of space-time that causes lengths to change and clocks to tick at dif­fer­ent rates as it passes by.

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