How Does Time Flow? Physics and Philosophy Perspectives

The Arrow of Time: From Newton to Quantum Gravity

1. Newtonian Absolute Time

Time as a universal, uniform, one-way river—independent of matter, motion, or observers. All clocks tick in unison.

2. Special Relativity: Relative Time

Einstein (1905): Time dilates with velocity. Moving clocks run slower. Simultaneity is frame-dependent. No universal "now."

3. General Relativity: Gravitational Time Dilation

Einstein (1915): Gravity curves spacetime. Clocks run slower in stronger gravitational potentials (near massive objects). Verified by GPS satellites.

4. Quantum Gravity & the Problem of Time

In canonical quantum gravity (Wheeler-DeWitt equation), time disappears—it's not a fundamental variable. Proposals: emergent time from entanglement, thermal time hypothesis, causal sets, loop quantum gravity. Time may be approximate, not fundamental.

5. Cosmological Time

Big Bang (13.8 Gya) defines a cosmic arrow. Expansion gives a preferred foliation (comoving time). But is the arrow fundamental or thermodynamic?

6. Philosophical Views

• Presentism: Only "now" exists.
• Eternalism / Block Universe: Past, present, future equally real.
• Growing Block: Past and present exist; future not yet.

No consensus unifies physics and philosophy. Time's flow remains a profound mystery.