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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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Presentation Transcript
deep est paradox of physics
Deep(est) Paradox of Physics:

Physics is Time Symmetric

Nature is Not

fundamental theories of physics don t care which way time goes
Fundamental Theories of Physics Don’t Care Which Way Time Goes
  • Newtonian Physics
  • Electromagnetism
  • Relativity
  • Quantum Mechanics
roger penrose lists seven ways in which nature distinguishes past from future seven arrows of time
Roger Penrose lists seven ways in which Nature distinguishes past from future,seven “arrows of time”
seven arrows of time

Seven Arrows of Time

Memory Arrow

Electromagnetic Arrow

Quantum Arrow

Black Hole Arrow

Cosmological Arrow

Kaon Arrow

Entropy Arrow

1 memory arrow
Memory only

works backwards

(We don't remember

the future)

1. Memory Arrow
2 electromagnetic arrow
2. Electromagnetic Arrow

Electromagnetic Causes Precede Effects

3 quantum arrow

3. Quantum Arrow

Wave Functions “Collapse”

But Don’t “Uncollapse”

4 black hole arrow

4. Black Hole Arrow

Black Holes Exist But White Holes Don’t (Apparently)

5 cosmological arrow

5. Cosmological Arrow

We live in an expanding universe, not a contracting one.

Would time change direction if universe started contracting?

6 kaon arrow

6. Kaon Arrow

Kaon decay not time symmetric Ko ππ

In a time-reversed world this particle decays at a different rate than in our world!

cpt theorem

CPT theorem


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

princeton 1964 experiment

Princeton 1964 Experiment:

Kaon decay not invariant under CP-reversal

Must also change under T-reversal

7 entropy arrow

7. Entropy Arrow

Entropy always increases

(second law of thermodynamics)

question are arrows independent

Question: Are arrows independent?

Hawking: Memory and entropy arrows linked

(Requires energy to read one bit, increases entropy by certain amount)

rothman to penrose one arrow arbitrary only six independent signs
Rothman to Penrose:

One arrow arbitrary; only six independent signs.

Sudarshan to Rothman:

All particle decay CPT invariant but


Particle-decay arrow.

Rothman to Rothman:

Temperature dependent?



Entropy can be thought of as waste heat generated in any realistic process

“Disorder” in a system

second law entropy of a system always increases under realistic irreversible process

Second Law:Entropy of a system always increases under realistic (“irreversible”) process

Only law of nature that exhibits direction of time!

thermodynamic system system with lots of particles gas obeys second law

“Thermodynamic system”:System with lots of particles(Gas)Obeys second law

But individual particles obey

Newtonian physics


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?

boltzmann goes to probabilistic theory

Boltzmann goes to probabilistic theory

An ordered deck of cards

becomes disordered on shuffling

prigogine s solution
Prigogine’s Solution:

Second Law is fundamental.

Can’t derive.

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