1. The Arrow of Time Nearly all fundamental laws of Physics are symmetric in time A process which satisfies these laws can be rewinded to give another physically valid process

2. The Arrow of Time Nearly all fundamental laws of Physics are symmetric in time A process which satisfies these laws can be rewinded to give another physically valid process But many macroscopic processes are irreversible, and only valid for a certain time direction

3. The Arrow of Time Nearly all fundamental laws of Physics are symmetric in time A process which satisfies these laws can be rewinded to give another physically valid process But many macroscopic processes are irreversible, and only valid for a certain time direction This problem hasnt been solved by the 2nd Law - it is still an active theoretical issue But there are different views on what exactly needs to be explained

4. The Arrows Psychological flow of time

5. The Arrows 2nd Law of TD, Entropy almost never decreases Psychological flow of time Electromagnetic radiation from a point source

6. The Arrows 2nd Law of TD, Entropy almost never decreases Psychological flow of time Collapse of wavefunction under measurement Electromagnetic radiation from a point source

7. The Arrows 2nd Law of TD, Entropy almost never decreases Psychological flow of time Collapse of wavefunction under measurement Cosmological asymmetry, expansion of the universe Electromagnetic radiation from a point source Asymmetry of black holes, no white holes observed

8. The Arrows 2nd Law of TD, Entropy almost never decreases Psychological flow of time Collapse of wavefunction under measurement Time asymmetry in particle physics, kaon decay Cosmological asymmetry, expansion of the universe Electromagnetic radiation from a point source Asymmetry of black holes, no white holes observed

9. Time Reversal What does time reversal symmetry mean?

10. Time Reversal What does time reversal symmetry mean?

11. Time Reversal What does time reversal symmetry mean?

12. Time Reversal What does time reversal symmetry mean? Big important theorem: Any local field theory satisfying SR is CPT symmetric

13. Time Reversal Big important theorem: Any local field theory satisfying SR is CPT symmetric 1964 Fitch and Cronin discover neutral Kaon violates CP symmetry CP symmetry is broken, so Time Reversal must must be violated to maintain CPT symmetry

14. Time Reversal Big important theorem: Any local field theory satisfying SR is CPT symmetric 1964 Fitch and Cronin discover neutral Kaon violates CP symmetry CP symmetry is broken, so Time Reversal must must be violated to maintain CPT symmetry Cosmological asymmetry between matter and antimatter related to arrow of time?

15. Cosmology and Entropy Why does the early universe have such low entropy? Fraction of states: 1 in 10^10^30 statistical fluctuation?

16. Cosmology and Entropy Thomas Gold: in a contracting universe entropy will begin to decrease with time Why does the early universe have such low entropy? Fraction of states: 1 in 10^10^30 statistical fluctuation?

17. Cosmology and Entropy Thomas Gold: in a contracting universe entropy will begin to decrease with time Why does the early universe have such low entropy? Fraction of states: 1 in 10^10^30 statistical fluctuation? Can an asymmetric universe be shown to be probable? Anthropic Principle?

18. Thermodynamic Time Loschmidts Paradox: Any process leading to increase of entropy can be time reversed Fluctuations of decreasing entropy do occur, but with very small probability

19. Thermodynamic Time Loschmidts Paradox: Any process leading to increase of entropy can be time reversed Fluctuations of decreasing entropy do occur, but with very small probability Fluctuation Theorem (1993):

20. Thermodynamic Time Loschmidts Paradox: Any process leading to increase of entropy can be time reversed Fluctuations of decreasing entropy do occur, but with very small probability Fluctuation Theorem (1993): Parity of reasoning -> Low entropy states evolve from high entropy with high probability! Why do statistical arguments apply in only one direction?

21. Thermodynamic Time Loschmidts Paradox: Any process leading to increase of entropy can be time reversed Fluctuations of decreasing entropy do occur, but with very small probability Fluctuation Theorem (1993): Parity of reasoning -> Low entropy states evolve from high entropy with high probability! Why do statistical arguments apply in only one direction? Interventionism: Random interactions with environment disturb correlated particles These interactions must be time-asymmetric to begin with

22. Quantum Measurement Evolution of QM state is time symmetric But under measurement, state collapses onto eigenspace

23. Quantum Measurement Evolution of QM state is time symmetric But under measurement, state collapses onto eigenspace Decoherence: measured state entangled with state of macroscopic system Interference effects suppressed, as if collapse Quantum arrow related to ordinary TD 2nd Law

24. Quantum Measurement Evolution of QM state is time symmetric But under measurement, state collapses onto eigenspace Decoherence: measured state entangled with state of macroscopic system Interference effects suppressed, as if collapse Quantum arrow related to ordinary TD 2nd Law Branched model of spacetime: Different measurement results correspond to different branches As present time progresses branches are trimmed

25. Any Questions? Savitt, S., ed. 1995. Times Arrows Today. Cambridge. Price, H. 1992. Cosmology, Time's Arrow, and That Old Double Standard. UBC. Coauthor of Fluctuation Theorems website: http://rsc.anu.edu.au/~evans/