Two obsessions are the hallmarks of Nature's artistic style:
Symmetry—a love of harmony, balance, and proportion
Economy—satisfaction in producing an abundance of effects from very limited means#984•
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John Wheeler had a knack for inventing striking phrases to describe physical ideas. "Black hole" is a memorable Wheelerism, as is "Mass Without Mass," which we'll make use of later. Wheeler had a poetic way of describing the essence of Einstein's theory of gravity, general relativity, which we can build on:
Matter tells space-time how to curve.
Space-time tells matter how to move.#5904•
How does space-time instruct matter to move, exactly? Its instruction, according to general relativity, is very simple: Keep going as straight as you can!#5901•
Energy-momentum tells space-time how to curve.
Space-time tells energy-momentum what straight is (in space-time).#5900•
Yin is the yielding principle, associated with earth and water (matter). It "does what comes naturally" (Oklahoma!) or "follows the force" (Star Wars), following the path of least resistance—the geodesic.
Yang is the animating principle, associated with sky (space-time), light (electromagnetic fluid—see below!), or other driving forces.#5914•
Our Core Theories center on the interplay between lightlike space filling fluids (yang) and substances (yin) they both direct and respond to.#5925•
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Time translation symmetry is the wisdom of Ecclesiastes:
What has been is what will be,
and what has been done is what will be done,
and there is nothing new under the sun.
What Shakespeare lamented, here,
If there be nothing new, but that which is
Hath been before, how are our brains beguil'd,
Which, labouring for invention, bear amiss
The second burden of a former child#5919•
David Hume, taking off from Berkeley, produced more sophisticated arguments for radical skepticism. Hume saw no way to justify the assumption that physical behavior is uniform across time. But without that assumption, no prediction is secure—not even, for example, the prediction that the Sun will rise tomorrow.
Yet uniformity of behavior is simply, according to Hume, an irrational leap of faith.
Bertrand Russell encapsulated Hume's analysis in a memorable joke:
The man who has fed the chicken every day throughout its life at last wrings its neck instead, showing that more refined views as to the uniformity of Nature would have been useful to the chicken.#5908•