Science: How the “facts” change Fred Raab LIGO Hanford Observatory LIGO-G 020518 -02 -W Science: How the "facts" change
Science relies on facts, just as a builder may use stones to construct a house. But a pile of stones is not a house and a collection of facts is not science. -sentiment ascribed to H. Poincaré Let’s explore the role of facts and process in the study of gravity. . . LIGO-G 020518 -02 -W Science: How the "facts" change 2
Gravity experienced by the earliest humans least well understood of all the forces of nature today a puzzle that has given mankind some of its key insights into the origin and structure of our universe not a smooth intellectual ride… LIGO-G 020518 -02 -W Science: How the "facts" change 3
Aristotle’s Gravity Fact: heavy objects fall faster than light objects • Evidence: clearly heavy objects are dense (earthlike) and more strongly attracted to earth • Explanation: heavy objects have more earth and light objects more air; each element seeks out its own kind LIGO-G 020518 -02 -W Science: How the "facts" change 4
Galileo’s Gravity Fact: all objects – heavy or light – fall the same when wind is not a factor • Evidence: rolling objects of different composition down an incline allows one to carefully time the falling motion; observed “law of falling bodies” is a purely mathematical relation independent of composition of object • Explanation: hmm… LIGO-G 020518 -02 -W Science: How the "facts" change 5
Newton’s Gravity Fact: all objects – heavy or light, on earth or in the cosmos – fall according to the same laws, with no dependence on mass or composition • Evidence: equivalence of periods of pendula made with different object in “bob”; detailed laws governing everything from projectiles to the motion of the moon • Explanation: universal relation between inertia and strength of gravity, unlike in any other known force (e. g. , electric and magnetic behavior of different substances is different) LIGO-G 020518 -02 -W Science: How the "facts" change 6
WOW! Newton’s theory made the domain of science a much, much bigger place. For the first time in history, the laws of earth and the laws of the cosmos were linked. Even more astounding, they appeared to be the same laws! LIGO-G 020518 -02 -W Science: How the "facts" change 7
Einstein’s Gravity Fact: gravity is a manifestation of the structure of space and time • Evidence: orbit of Mercury; bending of starlight; gravitational redshift; pulsar timing; timing by global positioning system, etc. • Explanation: space and time are warped by the presence of matter ( or energy); matter moving through warped space and time produces the effects we ascribe to gravity; the reason all stuff falls at the same rate has nothing to do with the stuff, but depends on the space and time they fall through LIGO-G 020518 -02 -W Science: How the "facts" change 8
Warping of a 2 -D Sheet of Space Like a flat sheet of paper LIGO-G 020518 -02 -W Like the curved surface of a globe Science: How the "facts" change 9
The New Wrinkle on Equivalence Not only the path of matter, but even the path of light is affected by gravity from massive objects A massive object shifts apparent position of a star Einstein Cross Photo credit: NASA and ESA LIGO-G 020518 -02 -W Science: How the "facts" change 10
Matter in Motion Makes Gravitational Waves Gravitational waves are ripples in space when it is stirred up by rapid motions of large concentrations of matter or energy LIGO-G 020518 -02 -W Rendering of space stirred by two orbiting black holes: Science: How the "facts" change 11
WOW! Einstein’s General Relativity is ultimately a theory of space and time. Newton’s equations gave astronomers the key to measuring the expanse of space. Einstein’s equation gave astronomers the key to unlocking time. For the first time, the laws of physics were extended over all of space and all of time. Thus began the new science of cosmology: the study of the evolution of the universe. LIGO-G 020518 -02 -W Science: How the "facts" change 12
The Big Bang Universe Fact: the entire extent of the universe originated from a tiny, hot, region of space at the beginning of time, approximately 14 billion years ago • Evidence: relative abundances of light elements, detection of relic light (CMB) from big bang in 1964, subsequent measurement of the CMB spectrum; images of universe at the time of atom formation • Explanation: follows outline of Friedman’s solution of Einstein equation, resulting in an expanding universe; some alterations (e. g. , inflation) made, some in progress LIGO-G 020518 -02 -W Science: How the "facts" change 13
Big Questions for 21 st Century Science Images of light from Big Bang imply 95% of the universe is composed of dark matter and dark energy. What is this stuff? The expansion of the universe is speeding up. Is it blowing apart? WMAP Image of Relic Light from Big Bang (~2003) There are immense black holes at the centers of galaxies. How and when did they form? What was it like at the birth of space and time? LIGO-G 020518 -02 -W Hubble Ultra-Deep Field (2004) Science: How the "facts" change 14
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