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Science: How the “facts” change

Fred Raab

LIGO Hanford Observatory

Science: How the "facts" change

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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...

Science: How the "facts" change

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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…

Science: How the "facts" change

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Aristotle’s Gravity

Fact: heavy objects fall faster than light objects

• Evidence: clearly heavy objects are dense (earth-like) and more strongly attracted to earth

• Explanation: heavy objects have more earth and light objects more air; each element seeks out its own kind

Science: How the "facts" change

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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…

Science: How the "facts" change

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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)

Science: How the "facts" change

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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!

Science: How the "facts" change

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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

Science: How the "facts" change

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Warping of a 2-D Sheet of Space

Like a flat sheet of paper Like the curved

surface of a globe

Science: How the "facts" change

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The New Wrinkle on Equivalence

Not only the path of matter, but even the path of light is affected by gravity from massive objects

Einstein Cross

Photo credit: NASA and ESA

A massive object shifts apparent position of a star

Science: How the "facts" change

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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

Rendering of space stirred by two orbiting black holes:

Science: How the "facts" change

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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.

Science: How the "facts" change

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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

Science: How the "facts" change

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Big Questions for 21st 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?

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?

WMAP Image of Relic Light from Big Bang (~2003)

Hubble Ultra-Deep Field (2004)