# Janna Levin on Reddit (u/Janna_Levin)

Part of Scientists on Reddit, researched with Autumn (https://www.autumn.ai) from public sources. Updated 2026-11-07. Interactive version: https://www.autumn.ai/resources/scientists-on-reddit/janna-levin

Janna Levin: Cosmologist at Barnard College.

- Reddit account: [u/Janna_Levin](https://www.reddit.com/user/Janna_Levin/) (AMA only)
- How we know it's theirs: [The 2018 r/science AMA, posted from this account](https://www.reddit.com/r/science/comments/7pwibg/)
- Where they post, in the records we retrieved: r/science (53), r/readwithme (2)
- Known activity in the posts we found: 2018 to 2020
- Wikipedia: https://en.wikipedia.org/wiki/Janna_Levin

## Moments

### u/Janna_Levin in r/science, Jan 12, 2018 · Explains things

Replying to a redditor:

> Science AMAs are posted early to give readers a chance to ask questions and vote on the questions of others before the AMA starts. Guests of r/science have volunteered to answer questions; please treat them with due respect. Comment rules will be strictly enforced, and uncivil…

> All black holes, rotating or not, allow you to see behind them because even light travels along the curved spacetime. If you were to shine a flashlight directly “behind” (can’t say which is the front or the back) a black hole, some of the light will fall in, some will spray out and its path will curve around the black hole so that those standing on the other side will see your flashlight. The lesson: Don’t hide behind a black hole. They can still see you.

(Excerpt.)
Full thread: https://www.reddit.com/r/science/comments/7pwibg/comment/dsktlmt/ · 6 points as of Nov 7, 2026

### u/Janna_Levin in r/science, Jan 12, 2018 · Explains things

Replying to a redditor:

> How would you explain expansion inside that gigantic black hole?

> There is no expansion inside a gigantic black hole. The expansion of the universe has a domain of domination, the largest scales. The black hole overpowers that expansion locally, on the scale of the interior of the black hole. As do we. We are bound so well by atomic forces that we are not expanding with the expansion of the universe. We expand mostly by eating too much.

Full thread: https://www.reddit.com/r/science/comments/7pwibg/comment/dsku2xu/ · 113 points as of Nov 7, 2026

### u/Janna_Levin in r/science, Jan 12, 2018 · Explains things

Replying to a redditor:

> If Black Holes are “rips” in space time, at the Planck length, what is on the other side of space time?

> If only we knew. Here the predictions of general relativity become unreliable. Like a dying man scrawling in the dirt, relativity is telling us something as it breaks down. The singularity is a clue not a genuine prediction, a clue that some other law of physics takes over at these Planck scales. That theory will predict for us what is on the other side. For now, we can conjecture. Maybe a big bang? The birth of a new universe?

Full thread: https://www.reddit.com/r/science/comments/7pwibg/comment/dskufgn/ · 27 points as of Nov 7, 2026

### u/Janna_Levin in r/science, Jan 12, 2018 · Explains things

Replying to a redditor:

> It has been said that the directions of space and time “reverse” within the event horizon of a black hole - the singularity effectively becomes your unavoidable “future”. Does this mean that there is some degree of freedom to move through time or receive information from the…

> Yes, I love this. Think of a black hole as inducing a rotation of space into time and time into space for an infalling astronaut relative to a space traveller at a safe distance. At the event horizon the rotation is complete. All of the infalling astronaut's time has been rotated away and they appear to freeze at the horizon, time effectively stopped. However, from the astronaut's point of view, time ticks completely normally and she falls across the horizon in short order. Inside the black hole, the rotation overshoots. What the distant space traveller called a spatial direction has become a time direction and the infaller sees the singularity not in the center of a sphere, but in her future as inevitable as the next second is for us. There is no more freedom afforded. The singularity can no more be avoided than the conclusion of this hour.

Full thread: https://www.reddit.com/r/science/comments/7pwibg/comment/dskuok6/ · 61 points as of Nov 7, 2026