Tony Rothman. Deep(est) Paradox of Physics:. Physics is Time Symmetric Nature is Not. Fundamental Theories of Physics Don’t Care Which Way Time Goes. Newtonian Physics Electromagnetism Relativity Quantum Mechanics. Nevertheless, Nature Cares.
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Physics is Time Symmetric
Nature is Not
Black Hole Arrow
Electromagnetic Causes Precede Effects
Wave Functions “Collapse”
But Don’t “Uncollapse”
Black Holes Exist But White Holes Don’t (Apparently)
We live in an expanding universe, not a contracting one.
Would time change direction if universe started contracting?
Kaon decay not time symmetric Ko ππ
In a time-reversed world this particle decays at a different rate than in our world!
Change electric charge of all particles in world (Change all particles to antiparticles);Reflect it in mirror;Run time backwards:
New world is indistinguishable from old
Kaon decay not invariant under CP-reversal
Must also change under T-reversal
Entropy always increases
(second law of thermodynamics)
Hawking: Memory and entropy arrows linked
(Requires energy to read one bit, increases entropy by certain amount)
One arrow arbitrary; only six independent signs.
All particle decay CPT invariant but
Rothman to Rothman:
Entropy can be thought of as waste heat generated in any realistic process
“Disorder” in a system
Only law of nature that exhibits direction of time!
But individual particles obey
Central Paradox:Gas is a thermodynamic system;obeys second law.But if gas particles individually obey time-symmetric Newtonian physics, how canarrow of time arise?
An ordered deck of cards
becomes disordered on shuffling
Second Law is fundamental.
Chaotic systems manifest irreversible
Entropy increase tied up with chaos
Chaotic systems are time-symmetric.
If a system is chaotic into the future, it is
also chaotic into the past
To date no satisfactory explanation for direction of time