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Astrophysicist and Author | Columbia University
6 months ago

Science AMA Series: I'm Janna Levin—astrophysicist, author, and host of NOVA's "Black Hole Apocalypse." Ask me anything about black holes, the universe, life, whatever!

Thank you everyone who sent in questions! That was a fun hour. Must run, but I'll come back later and address those that I couldn't get to in 60 minutes. Means a lot to me to see all of this excitement for science. And if you missed the AMA in real time, feel welcome to pose more questions on twitter @jannalevin. Thanks again.

Black holes are not a thing, they're a place—a place where spacetime rains in like a waterfall dragging everything irreversibly into the shadow of the event horizon, the point of no return.

I'm Janna Levin, an astrophysicist at Barnard College of Columbia University. I study black holes, the cosmology of extra dimensions, and gravitational waves. I also serve as the director of sciences at Pioneer Works in Red Hook, Brooklyn, a non-profit foundation that fosters multidisciplinary creativity in the arts and sciences. I've written several books, and the latest is titled, "Black Hole Blues and Other Songs from Outer Space." It's the inside story on the discovery of the century: the sound of spacetime ringing from the collision of two black holes over a billion years ago.

I'm also the host of NOVA's new film, "Black Hole Apocalypse," which you can watch streaming online now here. In it, we explore black holes past, present, and future. Expect space ships, space suits, and spacetime. With our imaginary technology, we travel to black holes as small as cities and as huge as solar systems.

I'll be here at 12 ET to answer your questions about black holes! And if you want to learn about me, check out this article in Wired or this video profile that NOVA produced.


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PhD|Molecular and Cellular Biology
Moderator of r/science, speaking officiallyScore hidden · 6 months ago · Stickied comment

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380 points · 6 months ago · edited 6 months ago

Hello Janna, thanks for doing this.

I remember reading years ago about a possible black hole scenario:

A rotating black hole would increase its rotational speed as the mass collapsed further in on itself because of conservation of angular momentum, therefore some of the exterior mass would reach "escape velocity?" and would form a ring around the collapsing black hole. If I remember correctly, the overall mass of the black hole would cause a gravitational field that is survivable (no spaghettification) and could be used to either see the other side of the black hole or propel a spaceship to relativistic speeds.

Am I remembering complete gibberish, or is this a plausible cosmic scenario for black holes?

Thanks again!!

EDIT: I believe she replied elsewhere, I'll paste her response here:

Lots of interesting ideas here. A rotating black does twist spacetime like a tornado so that material gets caught in the whirlwind, like cows and trucks and dirt all get swirled around with tornados. Even if you have no angular momentum you will get dragged around the hole and likely fall in. If you have some angular momentum of your own, then yes you could be cast into a swirling disk around the black hole. And many black holes do have bright accretion disks where the matter slowly dribbles in. Think Interstellar. That black hole was very accurately represented with a swirling disk. Separate idea: the bigger the black hole, the more easily you will survive transit through the event horizon and you won’t be speghettified until you’re crushed to death inside the black hole. It’s harder to stand on a basketball (a small sphere) than the Earth (a huge sphere). Perhaps counter intuitively, you notice the curvature less and less the smaller you are compared to the size of the event horizon. 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.

EDIT 2: To expand on the question as u/WholeLot pointed out, I was apparently referring to a ring singularity or Kerr ring. Anything you'd like to add here Janna regarding this ring singularity or Kerr ring?


I think what you're referring to is called a ring singularity, or sometimes a Kerr ring. The ring aspect is not matter but rather the shape of the singularity. It is postulated that a singularity in the shape of a ring may allow one to pass through the center of the ring.

Grad student | Physics | Astroparticle/Dark Matter
9 points · 6 months ago

Probably worth mentioning that this can be visualized very well using a Penrose diagram. Read the intro and look at the "Schwarzschild" (vanilla stationary, non-rotating) black hole part so you can get an idea of how the diagrams work, then scroll down to the Kerr black hole. You'll see that once inside the event horizon, it becomes theoretically possible to pass through the ring and emerge into a new universe through a white hole. Whether this is actually possible or just a fanciful interpretation of the math is unknown, so take it with a grain of salt.

Also, the other black hole they talk about on that page (the Reissner-Nordstrom) is an electrically-charged black hole. These very likely do not exist in nature and may be unstable anyway if they do.

Astrophysicist and Author | Columbia University
Original Poster46 points · 6 months ago · edited 6 months ago

Welcome Everyone! We're on!

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9 points · 6 months ago

How does angular momentum in a black hole work, anyway? You'd have to know the inertia and that depends on spatial distribution and that's just... weird....

Fascinating question - I hope she answers this. By "the other side of the black hole" I assume you mean wherever it pokes out somewhere else in the universe, not simply viewing it from behind.

BS | Physics
7 points · 6 months ago

As far as I understand, black holes aren't necessarily wormholes, as a wormhole would need negative mass. So theoretically, it wouldn't poke into another side of universe. But I study atoms, not blackholes so I'm not so sure .

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Yes, I guess I meant to say "see the other side of some weirdly shaped event horizon" and surviving...

How would we "see" it? Would this scenario allow us to theoretically travel through a black hole and survive? Or would it just vs visible from the outside? Also, would the other side of the black hole have the same rotation/effects?

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I am curious about this myself

Same here

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163 points · 6 months ago

Is it possible our understanding of black holes is flawed? What are some possible ways we might be misinterpreting the reality of the situation?

Astrophysicist and Author | Columbia University
Original Poster119 points · 6 months ago

It is always possible that our understanding is flawed. I would say it is extremely likely that our understanding of big black holes is very very very good. Where we run into our limitations is when black holes are small, microscopic. Then we are pushing the boundaries of quantum gravity where we are still grappling with fundamental ideas. The black hole provides us clues, a playground on which to work out these deep concepts.

Is there any real consideration of entropic gravity in academia? Is Erik Verlinde's proto-theory even remotely reasonable?

I've seen his lectures on it, it seems like he's on to something, but he knows it isn't even a full theory or viable framework. Yet.

13 points · 6 months ago

I am curious about this as well. Specifically, is there some known assumption (due to limited data) or observation bias (again, from limited tools/data) that could fundamentally shift our understanding? Something like "I want to say this is a snake hanging from a tree, but I'm pretty sure that if I wasn't blind and standing right next to it this is could be an elephant".

It was not too long ago Hubble determined there were just a few (10x) more galaxies in the observable universe than previously thought. I imagine a shift in our cosmic understanding of that magnitude had to raise questions elsewhere.

That's always the backbone of any good science. You're never trying to prove you're right. It's about proving what's wrong with your hypothesis. When you can't find any more ways to prove you're wrong, you can begin to accept that your understanding is at least mostly right. There's still that little bit of unknowingness(this should be a word!) that someone can always come along later to point out a flaw, but, again, that's the hole point in science anyways. When no one can point out any flaws, it's an accepted model.

Sorry I'm not OP but I really like these questions because I often think people forget about how the scientific method works after high school. I'm always reminded myself when these questions are asked.

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MPH | Global Health | Infectious Disease
217 points · 6 months ago

NDT brought up an interesting point on The Late Show recently about how the universe will/is expanding beyond the horizon. Do you think we've actually already lost cosmic information previously available to us or is this like a billions and billions of years from now kind of thing?

Astrophysicist and Author | Columbia University
Original Poster226 points · 6 months ago

Yes, as a cosmologist friend of mine says, that’s why we have to do astronomy now. If the universe continues to expand eventually all the other galaxies in the universe will be so far away and dragged away from us so rapidly that the light emanating from them will never get to us. It’s a cosmological event horizon. With many similar features to a black hole event horizon. If the expansion is strong enough, our galaxy will be ripped apart and all the stars will drift across the horizon as well and the skies will go dark. This is already happening so that there are galaxies that have moved beyond our observable universe.

This is incredibly depressing.

69 points · 6 months ago

Wait till you hear about heat death.

Then take some solace in the fact that it's all an unfathomably long time away, even compared to the time between now and the end of the Earth when it's engulfed by the dying sun in its red giant phase.

What will happen first? The heat death of the universe or the cosmological event horizon making everything beyond reach?

16 points · 6 months ago

The cosmological horizon, if you're referring to galaxies outside the local supercluster becoming invisible.

3 points · 6 months ago

Wow thats depressing.

The universe will go dark. And then it will go cold. Only then will you know true loneliness.

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19 points · 6 months ago

Or we get zapped by a Gamma ray, get hit by a big asteroid (Apophis is due to either hit earth or pass us by friday 13th 2029). The supervolcano beneath Yellowstone is also overdue to have an eruption, all of these things could erase us long before the sun burns out.

15 points · 6 months ago

Thought to be fair, it doesn't look like any of those would be very likely to completely wipe out humanity as a species.

A general collapse of technological civilization might be in the cards though! They would all cause significant to severe ecological disruption worldwide.

Asteroids have most likely wiped out or set back evolution many times. It’s not unlikely at all given that time frame.

Well, Apophis in particular isn't that big, and even outside of that specific case I think wiping out humanity completely would be pretty difficult for an asteroid impact to accomplish. I'm not sure what timeframe you're referring to though.

The burnout of the sun in OPs comment. I think we should be worried even with a much narrower timeframe though.

The Tunguska Event happened about a hundred years ago, and was a tiny meteorite. Many even smaller meteorites have hit us since then, most famously in Russia just a few years ago. The actual event is happening time and time again, the rocks just have to be a bit bigger. 60 miles wide and we’re apparently done here.

Also, while we still don’t know for certain, more and more evidence points towards multiple comets or meteors causing the younger dryas (ending of the last glacial period), which most likely set back human civilization and possibly made most of the megafauna in NA go extinct.

And as we all know the most popular theory for the extinction of the dinosaurs is also a meteor impact.

60 miles wide and we’re apparently done here.

I agree, but you might have to start talking about very long timescales to see such an event. Which, to be fair, we are. But as far as we can tell, no impact of that size has occurred for the entire history of life on Earth.

I think you could significantly lower that size from 60 miles and still get something that could wipe out anything on land larger than a mouse. I think another Chicxulub impact (an asteroid around 6-9 miles wide, the one often credited with ending the dinosaurs) probably wouldn't be enough to wipe out humanity entirely, but double that size probably would.

I'm at the edge of my informed estimates though. I'd have to look up studies by others to get enough confidence to narrow things down further.

My main assertion is that it's probably a lot easier to end modern civilization than it is to wipe out humanity entirely.

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30 million tons isn’t big enough for you? Imagine that amount of mass traveling faster than any man made object ever, most asteroids going around 35 km/s. Do the math on that kind of energy, it would be catastrophic.

2 points · 6 months ago

On April 17, 1976, Helios-B made its closest pass of the Sun, at a record heliocentric speed of 70 km/s

just saying

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7 points · 6 months ago

Apop has been ruled out until 2068 FYI.

I just googled both of those scenarios, and you are wrong on both counts. The supervolcano beneath Yellowstone hasn't shown any signs of erupting, just that scientists have shown that a supereruption occurs somewhere on earth every 100,000 years or so, and its been around 630,000 years since Yellowstone erupted

Although fascinating, the new findings do not imply increased geologic hazards at Yellowstone, and certainly do not increase the chances of a 'supereruption' in the near future. Contrary to some media reports, Yellowstone is not 'overdue' for a supereruption.> -Yellowstone Volcano Observatory

And the Apophis Asteroid was on the radar for possibly hitting earth, but was ruled out as a threat in 2004

The chance that there would be an impact in 2029 was eliminated by December 27, 2004.[15] The danger of a 2036 passage was lowered to level 0 on the Torino Scale in August 2006.[22] With a cumulative Palermo Scale rating of −3.2,[2] the risk of impact from Apophis is less than one thousandth the background hazard level>

5 points · 6 months ago

Huh, well I'm not sorry I'm wrong in this case. But there's always going to be an unknown threat lurking in our galaxy. We can't ever say that we're particularly safe as long as we've got the sun on or side.

Very true, and the Gamma ray thing is still an issue, and I also just learned about gravitational waves... Those sound like fun as well.

Gravitational waves produce absolutely no threat to us. Any event close enough to produce strong enough gravitational waves to do us harm (i.e. black hole and neutron star mergers) would itself pose much more of a threat.

3 points · 6 months ago

You sound like a fan of Exit Mundi!

It doesn’t even have to engulf us or burn out. If it gets even a little bit hotter, we’re done. And it will start to do that in a billion years or so.

2 points · 6 months ago

Apophis is due to either hit earth or pass us by friday 13th 2029

Apophis now has a 0 rating on the Torino scale, so I think we'll be okay on that one.

I had to look it up because asteroid/comet impact is one of my biggest fears and hearing that one might hit us in a decade spooked me.

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I've always been more disturbed by the idea that a vacuum metastabilty event already started and is coming towards us and the only warning we would get is as all the earth's particles basically dissolve.

x2. Like staring at the sea/ocean, unsure if the horizon is a wall made of water coming faster than you can process.

The Big Rip would be more "fun".

With a metastability event, we'd never even see it coming.

The Big Rip, due to the Universe's constantly accelerating growth at every point, would give us a few month's notice or so as we see galaxies start winking out and then the stars in our galaxy starting to wink out, and then finally every planet and object in our solar system flying away from each other until they simply all come apart and every atom basically just flies apart at the same moment.

We have very different meanings for the word fun. ;)

We'll have found a way to leave earth by then

If we (or some sufficiently directly descended civilization) are still around, then almost certainly, yes. But it's still something that you might be sad about, like the idea of distant galaxies becoming invisible.

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If you think that's depressing you should google 'entropic heat death'. Basically the theory that every star and every source of heat in the universe will eventually burn out. The universe will end as a cold dark nothing. No sources of energy. No light. But don't worry!! You won't be around to see it. Neither will your children. Or their children. None of this will happen for Billions of years. Humans will likely be extinct LONG before any of this happens.


That reminds me of a great short story by Isaac Asimov: The Last Question

Such a haunting and wonderful story. Great read!

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Even if not extinct almost definitely not recognisable as humans.

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That’s why now is known as ‘the golden age of astronomy’. Millions of years later some constellations will begin to vanish.

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PhD | Chemistry | Synthetic Organic
35 points · 6 months ago

Thanks for taking the time for this AMA!

After watching Nova the other night, the early development of black holes in the young universe seems to have been critical for the establishment of galaxies. This early universe was a smaller place due to the expansion of the universe, could super-massive black hole has been formed by black hole combining in a smaller volume of universe? If so, what would the effects of the gravitational waves generated be on the structure of galaxies? It's a simple, and inaccurate analogy, but if you watch the surface of a pond, small floating bits of leaves and what not tend to be driven together by the motion of the surface. Would these gravitational waves actually empart force on matter?

Astrophysicist and Author | Columbia University
Original Poster37 points · 6 months ago

All good questions. Yes black hole formation in the early universe may well have skipped the whole star formation followed by death throes. Gravitational waves are agonizingly weak, as is gravity in general. They can't bounce around big masses, which is why LIGO needed to suspend the mirrors so delicately so they could bob on the wave. The mirrors in LIGO move by a ten-thousandth the width of a proton.

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With the size of Sagittarius A what kind of star or other event is capable of creating such a large black hole? Or did it simply absorb enough mass to obtain its size? And how long would it take something of its size to fizzle out due to Hawking radiation?

Astrophysicist and Author | Columbia University
Original Poster78 points · 6 months ago

Luckily for our prospects for employment as scientists, there are many great questions to left to ask. And this is one of them. That’s the real fun. To ask questions we don’t yet know the answer to and to struggle and sweat to try to answer them.

We don’t yet know how black holes got so big. Very likely some black holes formed in the earlier universe without becoming stars first. There may have been enough mass swept together to directly collapse to big black holes which then merged and grew bigger and took down all the debris and stars and dust and gas available to them in the densest regions that became the galactic center to get bigger still.

It could take somewhere near 10100 years for supermassive black holes to Hawking evaporate. For comparison, the universe is only 1011 years old, which is 10 billion years (more accurately the universe is not quite 14 billion years old). The future is much longer than the past.

“The future is much longer than the past.” May this be true for us all.

9 points · 6 months ago

The show actually went into this. The theory is that these black holes are Direct Collapse Black Holes. In the early universe, there were MASSIVE clouds of hydrogen gas. The clouds began the usual spin as gravity pulls it together just like in star formation. But because there was so much mass there the immense gravitational forces caused the direct formation of a massive singularity rather than a star.

32 points · 6 months ago

What advances in science do you see being made within the next ten years?

Astrophysicist and Author | Columbia University
Original Poster71 points · 6 months ago

I'm better at predicting phenomena on the scale of many billions of years. Apologies. We all have our ranges.

Ok then, what advances in science do you see happening in the next billion years?

Ok then, what advances in science do you see happening in the next billion years?

Economical Fusion Power Plants will definitely be in the next billion years or so.

All of them!

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I feel like this response is something Siri would say.

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 future in the moments before you reach the singularity? I’m assuming a really huge black hole where tidal forces may be survivable near the event horizon?

Astrophysicist and Author | Columbia University
Original Poster59 points · 6 months ago

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.

This is fascinating, but so confusing. I guess it's really hard to describe, but you're saying the astronaut perceives time as space? What would that even look like, do you know? Like a 3 dimensional area you progress through at a linear unchangeable speed? I would love to wrap my head around this in some basic way; thanks for the concept.

8 points · 6 months ago

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.

My understanding has been that an infalling explorer could look over their shoulder and see incoming light, compressed into an ever-smaller aperture above them, as they approach the singularity. Your description sounds very, very different, but maybe I'm overthinking it?

My understanding is that this is impossible - you can't shift your perspective away from the singularity. Don't ask me to explain it - it's just something I remember reading. All directions come back to the singularity.

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My understanding of it is that an in-falling observer wouldn't actually be aware of when they crossed the event horizon from a visual perspective. Or, at any rate, it wouldn't be an obvious boundary, like crossing the finish line of a race. From the in-falling observer's perspective, they would be continually falling towards the event horizon and never actually "crossing" it.

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I’ve heard this mentioned before, the to an outside observer. An object or person falling into a black hole would appear to “freeze” before hitting the event horizon. I always assumed this was just due to the warping of time caused by the black hole’s gravity, and unless I’m confusing the singularity with the event horizon, that if we had a long enough time period to observe, albeit potentially infinite, we would observe the falling in.

I just wanted to help point out, in case you weren't completely satisfied with the answer. Time and Space don't actually "reverse". Without getting too complicated, take a look at this Penrose Diagram. At face value, it's a little jarring to interpret but in practicality, it's fairly simple. By the way, this is very well covered in this episode of PBS: Spacetime.

Anyways, the best way to view this diagram is to picture the x-axis always moving in the y-direction, as in, up. This is because, as we know, time moves in one direction, forward. "Up" on the y-axis is illustrating time moving forward. This is also called, being uni-directional, time moves in one direction. The diagonal lines that form the boundaries (where each corner meets) are considered the singularities of a black hole. This is important because that's how the the "reversal" happens. I'll use the same example as the video above. If someone were to cross the event horizon, say, at the top left diagonal line, the diagram you currently see would be mirrored across that line (I tried to make a picture but i'm terrible with Photoshop and can't really find one). If you can visualize this, that means the "new" x-axis is now time and the "new" y-axis is space. In this new case, the y-axis is still uni-directional, but is now the space component, which makes sense because you're "falling" uncontrollably towards the singularity.

To finally get to your question, you could in theory, get information from the past or future but to do so, you would need to accelerate (along one of those curved lines) to "reach" that information and in doing so, you actually approach the singularity faster as well. So yes, you can get information but pursuing that information is actually just quickening your eventual death (if that's what actually happens). Hope this helped explain the "reverse"ing of time and space a little bit for you.

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Saw Nova on Wednesday. Entire family enjoyed it.

Is it possible that our entire galaxy or even the observable universe is inside the event horizon of a gigantic super massive black hole? How can we detect that this is or is not the case?

For such a large black hole, is it possible that objects can escape out of its event horizon through cosmic expansion of space?

Astrophysicist and Author | Columbia University
Original Poster42 points · 6 months ago

Excellent. Great to hear. We can map our cosmic spacetime pretty well and what we observe is an expanding universe full of other galaxies. There are as many galaxies in the observable universe as there are stars in our own Milky Way. That is a very different spacetime description from the interior of a black hole, which drives all matter to crush together in a future singularity. However, we do see a cosmological event horizon, a region beyond which no light can get to us. So galaxies that are too far away from us can beam light our way in vain. That light will never reach us.

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19 points · 6 months ago

How would you explain expansion inside that gigantic black hole?

Astrophysicist and Author | Columbia University
Original Poster113 points · 6 months ago

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.

Next time I'm polishing off an entire pizza I'll be sure to shake my fist in obstinance at The Universe to remind it that I expand on my own terms.

Yeah, why is it so easy to eat an entire pizza alone? The crust thickness is irrelevant to quantity consumption.

That is true so long as you believe in yourself.

Cheese 1, Expansion of the universe 0

This idea is part of the Holographic Principle. It's an actual working theory and has considerable math behind it. You can read this article as well for some more information.

The essential idea is, we know that black hole event horizons, in our universe, have a 2-sphere shape, that is, a two dimensional plane is some curved shape (while also having some global topology differences). So, in our 3D world, a black hole event horizon has a 2D surface. One could hypothesize that in a 4D universe, a black hole would have a 3D event horizon. Additionally, a black hole "consumes" all the information (matter) that "falls" into it. The holographic principle says that information from higher dimensions can be encoded on a lower dimensional boundary, like watching a 3D movie. We all know it's a 2D screen but the movie's world appears to have length, width, and depth. Melding these two ideas together, that would mean a black hole in a 4D universe is consuming the information of that universe and encoding the information on to a 3D event horizon. We're inside that black hole's event horizon and the universe we see is actually that 4D encoded information in 3D.

I guess to answer your actual question, this is also what explains the expansion. As more information is encoded, the expansion continues. There's some issues with conservation that I don't know enough about but i think this is the gist of the idea.

We’re actually living in a 3D event horizon in a white hole! That’s why we’re expanding space time. I like it.

Edit: changed last sentence...seemed accidentally snarky.

The volume increases as a function of time. From the outside, that is fixed at the event horizon, from the inside it is continually expanding towards the singularity.

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2 points · 6 months ago · edited 6 months ago

Our physics breaks down at the singularity, right?

Like a sausage grinder, what plops into Universe 2 might not look like a White Hole, but might be, from observers inside Universe 2, look like the Cosmic Background Radiation, observable in any direction, from when parent BH formed 13.3 Billion years ago?

Space-time added from the not-White-Hole-but-Faint-Dim-Sphere-In-All-Directions just evenly inflates Universe 2, everywhere at once?

There's actually a theory similar to this which is that every black hole actually contains its own universe, which implies that our universe likely exists inside the black hole of another universe.

My limiting understanding says no you can't escape, that there's no "wall" out there somewhere that separates the universes and if you could cross it, you'd pop out the event horizon and into our "parent" universe. You could travel at 99.99% the speed of light in any direction until the heat death of our universe and you still wouldn't find such a wall.

Here's my pet theory, and if Mrs. Levin sees this I'd be very curious as to her thoughts on how ridiculous it is:

Time and space are tied together in a fundamental way. I believe this suggests even our very timeline is distinct to our universe; that the parent universe is running on a different clock, so to speak. In fact, I believe it to be a perpendicular timeline.

That is to say, everything that ever happened across the entire history of our parent universe all happened at the instant of the Big Bang in our universe, t=0. Everything from the parent universe that ever fell into the black hole that is our universe all got inserted in that one brilliant moment we call the Big Bang. The entire history of our parent universe came and went in the moment of the birth of our universe. Our timeline then begins, perpendicular to the parent universe's timeline.

If we shared the same timeline, there would be new matter frequently falling into the black hole, where does it all go? We aren't being bombarded by exotic matter flying in at us from the edges of the universe, at least not as far as we can tell. On the contrary, our universe seems stable and all indications are that it's a closed system.

7 points · 6 months ago

That is a very interesting idea.

I want to point out that the new timelines do not have to be perpendicular to the parent ones.

They can be in line with them but infinitly faster. So they point in the same direction but eternity in the parent universe only lasts for a single moment in the new one.

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I was confused by some data related to LIGO's observations of gravitational waves. Say you have two black holes, each weighing 20 Solar masses. They merge together, the resulting black holes weighs say 30 Solar masses, while 10 were radiated away as energy of gravitational waves. Now, if nothing can leave black holes, how was that huge amount energy able to leave them anyway?

Astrophysicist and Author | Columbia University
Original Poster29 points · 6 months ago

This is indeed very subtle. Black holes are nothing. They could be formed as part of the initial conditions of a spacetime. So when we say they have "mass" we mean they have some gravitational energy associated with them that is a mass equivalent. When too black holes merge the new black hole must be bigger than each, but amazingly doesn't have to be bigger -- or even as big -- as the sum of the two. And in fact the merger releases some of their gravitational energy without releasing any matter. It's a good question.

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First of all, I’m sorry for any english mistakes I could make , I’m italian.

Gravitational waves are deformation of space-time. When the two black holes that orbit around each other are distant they emit gravitational waves, but they are too weak to be detected. The orbit shrinks beacuse of the energy lost by grav. waves (and other interactions, maybe with stars, etc.) so they start to orbit faster around each other and the energy of the GW increases, until they merge, and you detect the max value of the gw’s energy. Once they mergerd, the new black hole can still emitt a bit of gw untill it form a stable sphere and the emission stops.

What I’m trying to say is that you don’t actually detect the gw from the new bh but from the deformation of space-time caused by the system of the two accelerated black holes.

I hope this will help you :)

4 points · 6 months ago · edited 6 months ago

Reads easier if I make hand gestures at my monitor.

Good description of your idea, better English than my Italian.

Edit: Gravity waves are very weak. The rest of this is moot.

I wonder, if the waves are like ships' wakes, what happens if waves double up or cancel each other out? Like a galactic center, where there may be multiple pairs of BH's circling.

Is there some-where, some-when in the Universe where an unfortunate star under 10 solar masses, that normally wouldn't become a BH, maybe in the middle of it's life, would be induced to collapse into a BH by converging, massive, gravity waves? Heck, even one wave if it's big enough.

Or a neutron star, very, very close to becoming a BH, gets shoved over the brink by gravity wave peaks, but then pulled back by gravity wave troughs? It would look like an X-ray and gravity wave source appearing and disappearing out of nowhere, with no immediate pre-formation Supernova. Right?

Waves hitting a breakwater sometimes double-up or negate each other. Could a double-whammy of ... "negative" gravity waves... tear a neutron star up, even briefly?

I watched your Nova episode, and it was fantastic! Your hosting was excellent.

In the special, it is mentioned that there is a limit to how much material a supermassive black hole can consume in a period of time, due to the photonic pressure of the light emitted by the accretion disk. Does this mean that quasars would actually be more bright if this limit did not exist? Also, would this not suggest that many quasars would be of equal brightness, since they have reached this limit?

My second question is this: It is mentioned that inside the event horizon, an observer (the astronaut who got e't by the black hole) would see the entire future of the universe play out due to time dilation. However, give the funky things that black holes do to the path of light in the vicinity of the event horizon, would the geometry of space-time allow the light entering the black hole to even be coherent enough to extract such an image, let alone see it with one's eyes? I know there is a lot of talk about information being consumed by black holes, but is that information in any kind of coherent form once it is inside the black hole?

Astrophysicist and Author | Columbia University
Original Poster17 points · 6 months ago

Thank you! That means a lot to me. Much appreciated.

Quasars may derive their brightness by twisting up magnetic fields. They form batteries in an electromagnetic engine. To crank up their brightness, I suspect you would have to crank up the magnetic field pinned around the black hole in that accretion disk.

In this limiting format I can say this, the light would be focused at you so that what we really expect is a bright flash of white light. Not to quote myself, but it would be like the bright light at the end of the tunnel in the purported near death experience. Only it's a total death experience.

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This may be a silly, simplistic question with a difficult answer... But what is the current theory for what's in / on the other side of a black hole?

Astrophysicist and Author | Columbia University
Original Poster81 points · 6 months ago

This questions intrigues everyone, including professional scientists. Fact: We don't know. But that's half the fun. It may be that there is the birth of another universe. It also may be that in some subtle way there is no interior to the black hole. The interior is an illusion and instead there are wormholes that map the outside to the inside. Stay tuned for the next 50 or so years as we work it out.

Okay, its been half an hour. Do we know now?

Addendum - Thank you for doing this AmA! Reading your answers is really fun and I am learning a lot!

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In your opinion, When do you think technology will be advanced enough for humans to capture images or footage of an actual black hole?

Astrophysicist and Author | Columbia University
Original Poster24 points · 6 months ago

Soon! The event horizon telescope already has data collected by organizing telescopes around the globe to act as one giant eye. They are staring at our our Milky Way's own supermassive black hole in the direction of Sagittarius. That black hole is 4 million times the mass of the sun, but only a few sun widths wide and is 26 thousand light years away. That will be like resolving a piece of fruit on the moon. But they will do it.

Not OP, but the Event Horizon Telescope (EHT) is in the process of doing this. It is taking pictures of the black hole at the heart of the Milky Way (Sag a*). In fact, it has already theoretically taken enough data to be able to see it, it just needs to process it all right now.

This article was posted the other day and does a great job explaining the process for capturing an image of a black hole.

Can we create mini black holes and use them for technology? Are there certain attributes limiting the mini black holes we create from being usable?

Astrophysicist and Author | Columbia University
Original Poster72 points · 6 months ago

In theory, we should be able to create mini black holes, much the same way we create other subatomic particles, by smashing things together. It's always fun to smash things together. Black holes are like fundamental subatomic particles, like an electron or the Higgs. But they require much much higher energies for their creation than we anticipate reaching in our accelerator experiments.

The problem with mini black holes is that they evaporate through Hawking radiation nearly instantaneously. The smaller the black hole, the more explosive the evaporation. There may be a way to stabilize them by fattening them up, but that would be dangerous since they'd be able to eat your laboratory.

Another question, thank you for your time and attention. If the event horizon of a black hole turns out to be a hologram, is it theoretically possible to recover the information encoded on it or is it lost forever in some lower-dimensional abyss?

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You mean like something sci-fi-y like a singularity power source or Zero-Point-Module power source? I wish. But, you never know.

Maybe some sort of.. "energy field manipulator" ..?

How would you hold it? Anything it touches would be absorbed?

Some sort of electromagnetic containment. Black holes can have an electric charge. And if they are spinning they should have a magnetic field too.

there went my phace!

imagine black hole powered flying vehicles!

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Black holes have magnetic fields, so that's a possibility.

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it would have to be big enuff so it didnt immediatly evaperate(hawking radiation) even if we were able to contain it we would still have to deal with the gravity maby use it as artificial gravity in space travel? i dont know if hawking radiation can be converted to useful energy but that might be a posibility.

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How does an actual work day look like for a person in your position ? is it mostly meetings and coordination ? or do you crunch numbers over an excel sheet ? do you calibrate and finetune your devices ?

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

An ordinary day involves a pen and paper and calculations, a blackboard and discussions with a collaborator, and noodling with a code to generate images, maybe finishing a technical article and considering more calculations.

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Have you seen the movie "interstellar?" How realistic is the halo of light around the black hole in the movie? Also, if "white holes" do exist, what is their relation to black hole and how would they affect spacetime, compared to a black hole?

Astrophysicist and Author | Columbia University
Original Poster28 points · 6 months ago

Yes. I am a big fan of Kip Thorne's, the brilliant astrophysicist who wrote the original treatment for Interstellar and who won the Nobel prize alongside Rai Weiss and Barry Barish for LIGO's success. Not a bad run. The black hole is very realistic. The animators actually used the general relativistic equations to simulate the event horizon. Essentially you see a bright accretion disk around the hole and you also see it above and below because the curved spacetime sends you light from the other side of the accretion disk on bent paths that reach you from around the north and south poles.

About white holes, all I can say is that they are beautiful conjectures. Possibilities not predictions. They are the opposite of a black hole in some sense. Stuff can only come out, never go back in.


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Not OP. They actually had a real physicist who helped work on the science of that film, named Kip Thorne. He later wrote a book detailing all the scientific accuracy of the movie, called “The Science of Interstellar”. Highly readable and enjoyable, I recommend it!

Not the op, but the halo of light surrounding a black hole is realistic. As materials heat up, they have a tendency to glow. If a black hole is spinning and there is material orbiting the black hole, just before reaching the event horizon, that material is going to be moving incredibly fast. This movement is going to generate a lot of heat. There is an immense amount of energy generated by black holes.

But didn't they not took doppler effect in account? If they did it would've been a bit dull than as shown in the movie.

3 points · 6 months ago · edited 6 months ago

I believe that is the case, one side would be bright and the other darker as it rotated away

Edit: not quite the site I originally read it on but found some information here

It's a real thing and it's called an accretion disk.

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46 points · 6 months ago · edited 6 months ago

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

Astrophysicist and Author | Columbia University
Original Poster27 points · 6 months ago

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?

Black holes are not "rips" in space time, and don't have anything to do with Planck length. Black holes are objects that are continually collapsing under their own weight.

If you get close enough to such an object (e.g. its Schwarzschild radius), you can reach a point of no return where nothing can escape. For such objects that are non-rotating, and electrically neutral, this radius effectively forms the surface of a sphere. We have no way of knowing what's beyond that surface, but as best we can tell, there is nothing extraordinary on one side of the surface vs. the other.

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And would this have anything to do with "white holes?"

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Hey there, thanks for the AMA!

What is the best way for someone who just has a basic understanding of physics to get into astrophysics and understand the universe a bit more ? Can you recommend any series, youtube series or book series that explain things in a simple yet interesting and detailed way ? Could you recommend something for my 11 year old sister as well?

Astrophysicist and Author | Columbia University
Original Poster15 points · 6 months ago

There are many popular science books that range in style and level. I personally love very narrative books and think they provide a very human illumination of the passion of science. I'm not going to plug my own books (ha, ha just sorta did: Black Hole Blues). Kip Thorne's book, Black Holes and Time Warps is a great example of a pedagogical style. Take your sister to look through a telescope. It's transformative.

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Not Dr. Levin, but I warmly recommend the YouTube channel PBS Space Time: . Their explanations are at the perfect level for someone serious about but new to physics: it's not the watered-down hand-waving often fed to the laity, but it doesn't drown you in tensor calculus, and what math it does use isn't strictly necessary for understanding the concepts themselves. The content, which is conveniently sorted into playlists, ranges from general relativity to quantum field theory, the two heavyweights of tried-and-tested modern physics, and it's just a phenomenal source of information: clear pedagogy, helpful visuals, and appropriate level (no nightmarish renormalizations!).

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I second this!

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24 points · 6 months ago

Black Hole Apocalypse discussed a black hole that is 6,000 light years away. In our search for other black holes, do we have enough data to know that there are not any within close enough proximity to affect our sun and Earth in the next thousand or million years? My 11 year old son wants to know after watching your show.

Astrophysicist and Author | Columbia University
Original Poster14 points · 6 months ago

Sweet. I hope we have not provoked anxiety. If there were a black hole so close that it could cause our solar system harm, we would actually be able to detect it through gravitational lensing. The black hole could be seen causing a twinkling of the light from other stars as it moved across the sky. We're safe. At least from that. There may be other black holes closer than 6000 light years, but not so close that I'll lose any sleep over them.

3 points · 6 months ago

Your guess was correct that my son was a little anxious after watching the show. I just read your answer to him and he will sleep better tonight. He says thank you!

Thank you kindly for doing this! As a physics major, I deeply respect-admire the work that researchers like you do!

Question: how would you characterize the geometry of a black hole? Like, is it a hole that makes spacetime incomplete? Are they holes that each add 1 to spacetime's genus? GR says that BHs are singularities with infinite curvature, but do you believe that they're true singularities, or just imperceptibly small? (Nature doesn't seem to like singularities, let alone infinities, after all, not to mention the pursuit of quantization.) Is talking about them having size or shape even physically meaningful?

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

Thank you for tuning in. I hope you get this even though it's the next day.

The geometry of the black hole is fully described by the metric in general relativity. The Einstein equations teach us how to derive the metric given the energy and matter distribution. That's the problem Schwarzschild tackled. The singularity itself is the hole in that matter reaches the singularity in a finite time, according to its own clocks, and just catastrophically drops out of existence. That's pretty bad for the whole physics endeavor so the singularity is probably just telling us that relativity isn't working so well there any more.

Seems like an odd question, but what inspired you to become an astrophysicist?

Astrophysicist and Author | Columbia University
Original Poster28 points · 6 months ago

Not odd at all. I was in college studying philosophy because I loved the big questions. I felt astrophysics had better answers. And, unlike with Kant, no one was laboring over what Einstein meant. Once he laid out relativity it was a gift. Once the tool was acquired, anyone could own it and use it. Transcended everything.

That's so funny, I switched from astrophysics to philosophy :-) I felt like astrophysics was telling me a lot of what and how, but I wasn't getting much why.

Science doesn't really try to explain why, so that makes sense.

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9 points · 6 months ago(1 child)
2 points · 6 months ago(0 children)

What's the next step for data collection? Are you able tell us anything about the improvements being done to Advanced LIGO if there are any?

Astrophysicist and Author | Columbia University
Original Poster8 points · 6 months ago

LIGO is currently offline as upgrades are made. The team is analyzing noise that continues to haunt the high frequencies and fascinating advances are being made on the quantum aspects of the detector, involving squeezed quantum states of light. However, LIGO might not do more than a few times better than it is now. For that, we might need to go to space.

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As space time curvature increases, time slows. At a black hole, time slows to 0. Is this at the singularity or the event horizon? Does time stopping mean velocity stops, so the fall into the singularity takes infinite time from an external point of view?

Astrophysicist and Author | Columbia University
Original Poster11 points · 6 months ago

The infinite time dilation you are describing happens at the event horizon. The time dilation is relative to the time of an observer far from the black hole. From the perspective of the space traveler crossing the event horizon, her own time is completely normal. Her clocks tick normally. She ages normally. It's the rest of the universe that's sped up.

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21 points · 6 months ago

What are your thoughts on death and consciousness?

Astrophysicist and Author | Columbia University
Original Poster22 points · 6 months ago

We had an event on Consciousness recently at Pioneer Works in Brooklyn, where I'm also director of sciences. My guests were Christoph Koch and David Chalmers. We were in conversation about the unsolved problems. Fascinating, truly. I suspect consciousness is an evolutionarily adapted tool to manage too much information than our processors could manage and still navigate the world. We have to simplify and declare coarse level comprehension quickly. Like identify "baby" nearly instantly. A computer can't do that. But a computer can process the glut of input data staggeringly more effectively than we can. So AI might not need consciousness. We had the conversation in a series at Pioneer Works called Scientific Controversies. We'll put the recordings out in the next few moths.

What's your favorite star?

Astrophysicist and Author | Columbia University
Original Poster15 points · 6 months ago

Ooooh. I'll have to think about this. But you gotta love Alpha Centauri for being so close.

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This is the kind of AMA where I learn just as much reading the questions as I do reading the guest’s answers, but I also love personal questions like this, I’m excited for her answer.

Not OP.

Sol. I couldn't live without it.

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Where do you think the matter pulled in to a black hole goes? Surely all the matter from millions of years can't just keep infinitely compressing? What do you think their inner..structure? Might be like? Will Matthew
McConaughey be ok?

For some reason, magnetars are extremely interesting to me, they're a fascinating level of fuckery. LIGO spotted two neutron stars smashing together recently, what would be the difference? Would the the fuckery be off the charts? Thanks!

Astrophysicist and Author | Columbia University
Original Poster16 points · 6 months ago

Agreed, agreed. That's in no small part why the interior of black holes continues to attract the attention of theoretical physicists. There are many suggestions out there including vibrating quantum remnants. But they all have noteworthy problems. The latest ideas involve the suggestion that the black hole is a hologram with no actual interior. According to this idea, all the quantum information is smeared around the event horizon in the form of oscillation strings. There is much mathematical evidence that there can be no more information packed into the interior than can be smeared on the exterior surface of the event horizon. That smells like a hologram.

If I understand correctly, the "information" of matter is just across the event horizon? Would that mean that there's a finite amount of matter that could be collected?

If the size of the event horizon was a constant then yes, there would be a limit to the amount of matter that can be "consumed". However, the event horizon has a relationship with mass that means the horizon expands the more massive it becomes. The most common portrayal of this direct relationship is the concept of the Schwartzchild Radius

PhD|Computer Science
5 points · 6 months ago · edited 6 months ago

Hi there, I'm a scientist but am not up on the latest research on black holes, so I hope you can help me with this. There's a 2010 documentary called "How the universe works" that claims there is no direct evidence for black holes. Has that changed? Do we now have observations that support its existence? I hate to see pseudoscience go unchallenged and would like to hear the latest known answers from the horse's mouth. Thanks.

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

Well, that depends on what you mean by "direct" evidence. There is no question that we can see the orbits of stars around an object and very simply deduce the mass and size of the object. At the center of our galaxy that leads to 4 million times the mass of the sun in a space a few times the width of the sun. And it's dark. That's what we mean by a black hole. Will it be exactly as predicted in general relativity? Maybe not. But it's dark and has curved spacetime enough to keep those stars on those orbits. LIGO also detected completely dark black holes through gravitational waves. But maybe someone wants a picture of the event horizon as more direct evidence and that will be forthcoming.

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Are we living in a black hole that formed in another universe?

Astrophysicist and Author | Columbia University
Original Poster11 points · 6 months ago

Even if a black holes interior leads to a big bang -- or even a series of big bangs for every little bit that falls in -- that new universe doesn't live inside the event horizon quite in the way I suspect you're visualizing. Imagine a long trumpet horn. The narrow part near the mouth piece is far from the big wide open horn and doesn't feel inside it. I admit, the analogy is imperfect. Plus the big bang will be in the past. Remember it's spacetime. Not just space.

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Suppose there was a known identical Earth with people on it within our galaxy. Do we have the ability to send signals that could be understood between each other, or at least tell that it must come from some intelligent life?

If not, what would it take to communicate with them? How much power would the signal require as a fraction of the brightness of the sun and the distance between us?

I've been thinking about the Fermi paradox. Could it be the galaxy is filled with intelligent life trying to discover and communicate with each other but their ability too weak and the distances too far? Stars are some of the most powerful things in nature and their light seems so faint to us. Any signal we would try to send out would be a near insignificant fraction compared to the brightness of our sun.

3 points · 6 months ago · edited 6 months ago

Not OP. Link to cached scientific paper on probabilities of detecting alien communication

Intentional, beamed to Earth signals: zero.

Unintentional (listening to Alien Public Radio): "the probability of detecting aliencivilizations by SETI projects can be either very small or very high. Minute variations in the equation’s parameters have a great influence on the final probability." (ie: not zero. Shrug)

Another quote: Quoting from Tarter (2001): "At current levels of sensitivity, targeted microwave searches could detect the equivalent power of strong TV transmitters at a distance of 1 light year (within which there are no other stars)...".

Meaning we couldn't detect ourselves at the nearest star just listening to chatter (TV, Radio stations).

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Is it theoretically possibly to look out in space and see Earth from long time in the past? If the light that left our planet was bent at exactly the right amount around another large object to have it return back to Earth at some point?

Astrophysicist and Author | Columbia University
Original Poster3 points · 6 months ago

Yes! My first book, How the Universe Got Its Spots, talks about exactly this possibility in a finite universe.

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What exactly IS the answer to life, the universe, and everything? Since you offered.

4 points · 6 months ago · edited 6 months ago

Not OP.

Nietzsche says there is no meaning, that the need for meaning comes from fear and seeking comfort. (Freud thought it was childhood fears.)

Freeing yourself from the need for meaning allows you to form yourself anew.

"We have left the land and have embarked. We have burned our bridges behind us—indeed, we have gone farther and destroyed the land behind us. Now, little ship, look out! Beside you is the ocean: to be sure, it does not always roar, and at times it lies spread out like silk and gold and reveries of graciousness. But hours will come when you will realize that it is infinite and that there is nothing more awesome than infinity. Oh, the poor bird that felt free and now strikes the walls of this cage! Woe, when you feel homesick for the land as if it had offered more freedom—and there is no longer any “land”.

  • Friedrich Nietzsche’s Die fröhliche Wissenschaft (The Joyful Science) (1882/1887); translation by Walter Kaufmann.

Camus, in The Myth of Sisyphus, takes this same course.

Life is meaningless, so make the best of it. It's your boulder to push fruitlessly up the hill. Enjoy the view and the effort.

Or were you hoping for a grade-school, "42", from a black hole researcher? Here you go.

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Also yeah, not OP.

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You say you study the cosmology of extra dimensions. What exactly does that entail? I've tried reading about the multiverse theory, and other dimensions, and for a hobbyist it was very confusing because there are so many different ideas floating around. What do YOU think about the possibility of a multiverse, and what exactly do you mean by the study of extra dimensions?

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No one has asked yet. So I'm going for it. Do you believe in extra dimensions? What do you think perception of these dimensions would be like? Do you believe we are some dort of soul attached to a living being. Or apart of some kind of life force?

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If black holes didn't exist, what major changes would be noticed/be different on both a cosmological scale (the observable universe) and planetary scale (our day to day lives)?

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

The supermassive black holes may have sculpted the universe on the largest scales, seeding galaxies and regulating their size and morphology. Our whole Milky Way might be different without them. Plus, life would generally be less cool.

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If a grad student wanted to study black holes further, which avenues would be the best approach? ie theory, LIGO type experiments, GR?

Astrophysicist and Author | Columbia University
Original Poster7 points · 6 months ago

So sorry, have hit the end of the hour. I'd love to come back later and make it through the remaining questions. But I would say this. It has to be all of the above. I'm not an experimentalist. But you must take all the classes, study GR, cosmology, astrophysics, particle physics, and at keep an eye on the experiments if you are a theorists. Best of luck.

Can gravity exist without mass and act on mass outside its center?

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Excellent program. Long time NOVA watcher.

When LIGO detected the neutron star collision, you said that they were able to direct telescopes to look for the collision, and that they were successful in locating it.

How long did it take to communicate this detection to the community and get telescopes directed at the location of the collision?

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3 points · 6 months ago

What do you think about the interpretation of a black hole in Interstellar?

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

I may have hit this question a couple of times above, but I know they used very accurate models and you can tell. Kudos to Kip Thorne.

What are you thoughts about “tachyons” or any faster than light particles?

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What camp do you belong in for the information loss issue?

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

The camp that needs to spend more time trying to figure out the resolution. I'm fascinated by Lenny Susskind's ever wilder suggestions. He's a very visual, very intuitive thinker. Latest involves wormholes that create the illusion that there's an interior to the black hole, but it's really just the outside of the black hole.

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2 points · 6 months ago

Experimental observations show quite conclusively that matter dominates antimatter in the observable universe, at least to the CMB.

It is my understanding that gravitational waves could, with the appropriate instruments, allow us to detect events from further back in time and further away than the CMB. (If this is wrong, my question will not make sense).

Are there any hypothetical experiments that could be performed, using gravitational wave inferometry, to 'see' if this matter>antimatter dominance continues beyond the CMB?

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2 points · 6 months ago · edited 6 months ago

Do you believe in the Big Bang? Or is such a theoretical point in time a useful fiction governing a horizon we can’t see beyond (or around)?

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No question- I just wanted to let you know I loved "A Madman Dreams of Turing Machines". It's one of the most unique and beautiful books I've ever read.

Astrophysicist and Author | Columbia University
Original Poster5 points · 6 months ago

I cannot thank you enough for saying so. That means more to me than you can know. Honestly, I'm out here to hear from readers as much as anything. Thank you.

Thank you for your time, Janna.

I was particularly intrigued by the segment about how supermassive black holes form, likely in tandem with their host galaxies, such that the formation of one is a direct byproduct of the other. The vortex theory seems particularly interesting.

My question is threefold. I know this is long, so any answer you can give to any part would be appreciated.

First, what would prevent the infalling gas at the core of a proto-galaxy from igniting into one (or more) massive stars, whose solar winds would repel further accretion of material? In other words, what makes the vortex at the core of a proto-galaxy different from the process that leads to star birth as we observe it in the universe today? How can so much material accumulate so quickly, presumably getting extremely hot and dense, yet without undergoing fusion, somehow creating a supermassive black hole?

Second, is there a lower or upper bound on the mass of the black hole (and, relatedly, the mass of the soon-to-be galaxy)? Most galaxies have supermassive black holes, but not all do. The Triangulum Galaxy, our second-closest neighbor, is significantly smaller than the Milky Way, and, as I understand it, is not thought to harbor a supermassive black hole in its core, but rather a black hole of "only" 3,000 solar masses. What implications does the vortex theory have for galaxies like that? Or even for dwarf galaxies that orbit "parent" galaxies as satellites?

Third, if the vortex theory is correct, what implications does that have for galactic structural development? My intuition tells me that the process that creates a vortex also likely creates a flat disk, i.e., a spiral galaxy. But there are so many types of galaxies--spirals (including barred and grand design spirals, lenticulars, ellipticals, ring galaxies, irregulars, etc). Does this mean that spiral galaxies are the universe's "default" galaxy type?

Thanks again - I hope to see you on other science programs soon!

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Thank you for answering questions. Not sure if this is one you can answer, but I've been wondering on a universal level, how far a person travels through space in a single second. I know the earth is moving fast, and the solar system is rotating, the galaxy is moving, the milky way is headed somewhere, etc. So if I were to drop a beacon in space where I was one second ago, how far away would it be, not just relative to the earth's rotation, but in the grandest scheme of things?

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

Fun. I can't answer quantitatively off hand, but yes, we're on a complex trajectory, first around the sun. Then the sun is orbiting the galaxy. The galaxy is moving toward Andromeda and the entire universe is expanding. We're not where we were a second ago.

Grad Student | Climate Science
2 points · 6 months ago · edited 6 months ago

Hello Janna, thanks for helping educate the population, both layperson and scientist alike. Your books, podcasts and TV appearances are entertaining to watch/listen/read/learn...even for the family. What are the odds you'll be able to answer even 1% of these comments??? Astronomical. (Edit: with about 600 comments, 1% would be easy...10% would be very tough in an hour!) ;)

If by some chance you read my post, two quick questions:

  1. Any tips on how to balance educational or speaking appearances vs conducting your research? Both take A LOT of time...I'm a climate scientist and battle with that a much smaller scale than you, I'm sure.

  2. The Nova special would be a great supplement to Black Hole Blues. Good for people to see Kip, Rai, and the chirp while reading. I know you decided by graphics for that book, but why? Diagrams of the Weber Bar or of a LIGO facility would have been great .

I can't for the life of me ask the question in a nice way because i don't really know the right terminology to ask the question. But here's a stab.

I see pictures of black holes consuming (adding to their mass) solar systems and other large entities, like stars and such. But there are 'projections or images' that indicate that some matter appears to skim off the event horizon. Or some matter doesn't get consummed by the black hole.

Why does this happen? I understood it to be, once your in, your in.

Also black holes sit at an extreme. Is there an opposing extreme?

While waiting for the op to respond, here is an excellent video from PBS Spacetime that discusses this:

If you have not already subscribed, I highly recommend this series!

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Hi Dr Levin! I’m curious.. we often phrase the question, “What happened before the Big Bang,” but this presupposes that there was a time before the creation of the universe. If the Big Bang was the birth of space as well as time, does it even make sense to talk about a time or event prior to there being any time at all? And what would that be like, having no time? How did things change or events like the Big Bang happen if there is nothing discerning one moment from the next?

Huge fan by the way! I love it every time you get to host StarTalk All Stars. Keep bringing the science of the universe down to Earth! Thanks!

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

Thank you! Great to hear.

Yes, it's fair to say that the question isn't meaningful. Sometimes people pose the analogy that it's like asking what's north of the north pole. But it might be that the big bang is more like a plume off a larger spacetime that caught fire in some sense, burst out and created our bubble, which is the only part of the universe we can see.

Sometimes I wonder if black hole formation and growth might be having an impact on vacuum pressure (aka dark energy) in surrounding spacetime. Relatedly I ponder the effects of dark energy within black hole event horizons. Are you aware of any papers, speculative or otherwise, that have explored these questions?

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From an aspiring astrophysicist; What was it that made you choose to become an astrophysicist? Did you ever think about going down a different path in physics, and if so, what? Thanks for doing this AMA:)

Astrophysicist and Author | Columbia University
Original Poster2 points · 6 months ago

Oh for sure. I started college as a philosophy major interested in art. I never finished high school so I never had physics or even calculus in high school. I secretly always wanted to be a writer too. And I struggled in graduate school to cut out all other interests but physics. I'm relieved and grateful that now I can integrate writing and culture into my life as an astrophysicist. I'm also director of sciences at an art center in Brooklyn, Pioneer Works.

I've always wondered about the properties of the Ergosphere on a rotating black hole, and the opportunities it might provide with regards to experiments relating to FtL travel.

From my understanding, the Ergosphere is the region just outside the event horizon where -thanks to the mass of the rotating black hole - spacetime itself gets "dragged along", which results in objects in the ergosphere appearing to go faster than light for outside observers.

It seems to me then, that if one were to slingshot an object past a black hole, in and out of the ergosphere, the object would appear to have gone faster than light, and it seems like it wouldn't be much of a stretch to set up an experiment this way in which we violate casualty, or appear to violate casualty for a specific (fast moving) observer.

So is my understanding wrong? Am i missing some key point that prevents us from violating causality? Do we have any idea what would happen if we were to set up such an experiment?

Nb; probably unnecessary info, but i'm a recently graduated math student who followed an introductory course on special relativity. So feel free to get a bit technical in your answer.

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