The Direction of Time

• The fundamental laws of physics are time symmetric; i.e. events can happen backwards in time and still satisfy the law. The laws are:
• Electromagnetics - Maxwell's equation
• Gravity - Newton's or Einstein's general relativity
• Quantum mechanics
• The second law of thermodynamics is not time symmetric.
• Definition:
1. the entropy of an isolated system never decreases. The entropy of living organisms decreases over time; however, this doesn't violate the law because they are not isolated.

Entropy: disorder; thermo equilibrium.
2. Heat flows spontaneously from higher temperature to lower temperature.
3. You can't break even in energy conversion.
• It's a statistical law, not a fundamental law.
• Direction of time is observed in physical phenomenon. The mathematical formulation of phenomenon doesn't preclude time to pass in the other direction.

Now, Past, and Future

• Time sequence 'A occurs before B' is meaningful in physics.
• 'A occurs at clock time t₁' is meaningful in physics; 't₁ is now' is by convention. There is no reason to choose one instant over another as 'now' in physics, just as there is no static reference in space such as ether.

Einstein's Special Relativity

1. The speed of light is constant in vacuum
2. The mathematical formula of fundamental physical laws are the same for all inertia systems.

• Principle of equivalence: motion is relative, not absolute.
• Simultaneity is used to replace the concept of 'now'.

• For observer A in the middle of the train, lightning struck the east end of train first.
• For observer B in a train moving west, lightning struck the west end of train first.
• For observer C on the platform, two lightning struck simultaneously.

• Which event happens first is not universal. Each observer can have his own 'local time'.
• This doesn't allow causality to be violated. The time sequence of two physical events can vary according to observers only if they are apart in space-time so that they can't be causal.