Time is a dimension, but not like space
Time’s status as a “dimension” like space is probed from both physics and philosophy, with commenters contrasting lay explanations against the rigorous math of spacetime, manifolds, and Minkowski geometry. Much of the exchange centers on relativity: whether massive objects can meaningfully approach light speed, how time dilation and causality limits work, and why faster‑than‑light signaling would break cause‑and‑effect. Others connect time to entropy and the second law of thermodynamics, or question whether time is a fundamental dimension at all versus an emergent property of change in complex systems.
Definitions of space, time, and spacetime
- Several commenters want rigorous definitions (manifolds, Minkowski space, metric tensors) but note this loses most lay readers.
- Others offer informal definitions: time as the separations bridgeable by cause and effect; space as separations that are not.
- Wikipedia-style summaries of spacetime (4D continuum of 3 space + 1 time) are seen as a workable middle ground.
Pedagogy and “moving at the speed of light”
- Many like the heuristic that “everything moves through spacetime at speed c,” trading spatial velocity against temporal progression.
- Critics argue this has little real content and can be misleading; it’s at best a visualization aid.
- Tension between lay explanations and the need to eventually learn the math is a recurring theme.
Relativity, near‑light travel, and feasibility
- Debate over whether macroscopic objects (e.g., humans) can approach 0.9–0.99c.
- One side: Relativity allows any sub‑c speed in principle; enormous but finite energy and long, low‑G acceleration suffice.
- Counterpoints: practical issues like fuel, radiation, and collisions make it effectively impossible.
- Numerical examples (1g acceleration timelines, relativistic mass/energy) illustrate feasibility “in principle” vs “in practice.”
- Twin paradox is raised; resolution: symmetry is broken by the traveling twin’s acceleration.
Causality and the speed of light
- Multiple comments equate c with the speed of causality: signals faster than c would allow effects to precede causes in some frames.
- Discussion of whether an infinite causal speed or no speed limit would still preserve causality:
- One view: infinite speed → instantaneous but not reversed causation.
- Relativistic view: zero delay in one frame implies negative delay in another unless you abandon relativity; with c → ∞ you recover classical mechanics.
- Clarifications that “FTL” means faster than light in vacuum, not in media like water; refractive slowing doesn’t change the universal limit.
Entropy and the arrow of time
- Several participants link time’s “direction” to the second law of thermodynamics and statistical mechanics.
- Some see entropy as deeply fundamental, others as an emergent property of complex systems and initial conditions.
- Mention of Poincaré recurrence and Maxwell’s demon highlights that microscopic reversibility vs macroscopic irreversibility remains subtle.
Alternative and speculative views of time
- Philosophical takes (Kant, information‑theoretic approaches) are cited.
- Speculative models include the universe as a higher‑dimensional event horizon/black hole, and “time as refresh rate of matter,” though these are recognized as unfalsifiable or incomplete in the thread.