Ask HN: Time loop and partial blackhole?

TEXT START: Hello. I am a 9th grade student from India.

The Dissection

This is not an article. It's a 9th grader in India asking genuinely good questions about thermodynamics and General Relativity, with the intellectual honesty to say "identify flaws in my understanding" rather than defending a position. That's rare at any age. No propaganda here, no copium, just a kid trying to learn.

Q1: Entropy and Time's Arrow

Your understanding is mostly correct.

The Second Law doesn't prove time can't run backward, but it explains why we never observe it. Here's the key distinction:

• Thermodynamic time's arrow points in the direction of increasing entropy. A reversed-time video would show entropy decreasing — broken eggs reassembling, heat flowing from cold to hot. We never see this because it requires spontaneous, statistical unfathomable coincidence. The math isn't "impossible in principle" (in a low-entropy configuration), but in practice, the probability is so close to zero that it's functionally never happening at macroscopic scale.

• Microphysical laws (Newton, QM) are largely time-symmetric. An electron falling doesn't "know" time is moving forward. The asymmetry comes from boundary conditions — the universe started in an extraordinarily low-entropy state (Big Bang), and we are moving toward maximum entropy.

Flaw to correct: You're thinking of entropy as a "rule" that forbids backwards time. It's better understood as a statistical fact about what overwhelmingly probably happens given initial conditions. The universe could theoretically re-collimate, but the odds make it irrelevant.

Time loop / closed timelike curves: These exist in some GR solutions (rotating black holes, wormholes), but they require conditions we don't observe in nature — and they're considered unstable under quantum effects. The thermodynamic arrow doesn't forbid them mathematically, but it makes them irrelevant to anything you'd experience.

Q2: Your Mental Model of Time Dilation

You're close, but there's a subtlety.

Your model — "things slow down in strong gravity" — is the right instinct but describes it imprecisely. Here's the correct picture:

• Clocks tick slower in strong gravitational fields. True. But it's not that atoms "vibrate slower." If you were holding a clock, it would feel completely normal to you. Time for you passes normally. The dilation only shows up when you compare your clock to one in weaker gravity.

• What actually curves: Not time and space independently — spacetime itself. Gravity is geometry. Time slows down because spacetime is more "stretched" near mass. Think of it like: a grid made of rubber. Heavy objects warp the grid. Moving "across" that warped grid takes longer in the time direction, even though locally you experience normal ticks.

• Your model isn't wrong — it's an approximation that's useful. Physicists sometimes use "gravitational redshift" (light losing energy climbing out of a gravity well, which looks like time slowing) as an intuitive anchor. Your atomic-vibration framing gets at the right phenomenon but describes the mechanism slightly wrong.

The deep point: In GR, there is no "universal clock." Time is local and relative. The "flow" of time isn't something moving — it's woven into the geometry of the universe.

The Verdict

You've got better instincts than most college students. Your questions are well-posed. The entropy question is the more philosophically profound one — it's actually the core of why time feels like it's moving.

The flaws: (1) entropy isn't a law that forbids reversal, it's statistical behavior; (2) time dilation isn't atoms slowing down, it's geometry changing.

ScienceClic is good. So is minutephysics and Veritasium for building intuition. Keep questioning like this.