Why does time pass?

Philosophical vs. Physical Perspectives:

• Human experience views time as a perpetual present moving unidirectionally into the future.
• Physicist John Archibald Wheeler defined time as that which prevents all events from occurring simultaneously.
• Many experts consider the fundamental question of "why" time passes to be a matter of philosophy rather than pure physics, as the concept of "happening" is inherently temporal.

Einstein's Special Theory of Relativity (1905):

• Demonstrated that time is relative to the observer's speed; it passes at different rates depending on motion.
• Albert Einstein showed that high velocities mix space and time, making the distinction between them relative to the measurer.
• Time dilation occurs for travelers moving near the speed of light, causing time to slow down for them relative to stationary observers.

Development of General Relativity (1915):

• Hermann Minkowski first proposed that space and time form a single four-dimensional continuum called spacetime.
• Einstein initially dismissed the spacetime concept as purely mathematical but later adopted it to incorporate gravity.
• General Relativity posits that massive objects warp spacetime, creating the curvature experienced as gravity.
• Proximity to massive bodies (e.g., black holes, Earth, Sun) slows the passage of time; this effect is proportional to gravitational pull.

Practical Verification of Relativistic Time Dilation:

• GPS satellite systems must account for relativistic time dilation to maintain accuracy.
• Without corrections for gravity and velocity effects, GPS would generate significant positional errors.

The Arrow of Time and Entropy:

• Fundamental physical laws (including Einstein's equations) are time-symmetric and work equally well if time runs forward or backward.
• The observed unidirectionality of time is defined by Arthur Eddington as the "arrow of time."
• This arrow is driven by the Second Law of Thermodynamics: entropy (disorder) increases over time.
• Statistical probability dictates that systems evolve from order to disorder (e.g., a building decaying), making the reverse process highly unlikely.

Time Travel Possibilities and Constraints:

• Travel to the future is theoretically possible via relativity; for example, spending one hour near a black hole could result in seven years passing on Earth.
• Einstein's equations theoretically allow for wormholes (shortcuts in spacetime), but practical implementation faces two major hurdles:
  • The immense mass and energy required to create a wormhole.
  • The inability to keep a wormhole open long enough to traverse it without it collapsing.
• Current consensus suggests wormholes are likely forbidden by the laws of nature, despite not being explicitly ruled out by equations.
• While theory does not strictly forbid backward time travel, practical and physical evidence suggests the arrow of time cannot be reversed.