I am Janna Levin. Theoretical physicist, Big Bang and Black Hole author. Ask me anything about the universe.

What exactly is on the other side of a black hole? And can anything survive the trip through it?

Love black hole questions. We don't really know what's inside a black hole. If we trust Einstein's theory all the way through, then we argue space and time sort of switch places inside. This means you are forced toward the singularity at the center as surely as you are forced forward in time. And there you will be crushed to death.

Do you fell that there are any questions left in the death of Alan Turing?

Alan Turing bit from a cyanide laced apple to die as his favorite character snow white did (her sleeping death). But some people argue it may have just been a clumsy accident, that he was sloppy with his experiments and accidentally poisoned himself. Others think, he left this possibility open as an interpretation his mother could lean on. There are some things we'll never know for sure.

One of the things I find confusing that I was wondering if you'd could clarify for me is: the universe is expanding, yet at the same time there are galaxies that are colliding with each other. Now, are these galaxies colliding cause they are in each others' paths or is it gravity pulling these galaxies together? See, what bothers me, is if the universe is expanding then how is it possible that galaxies are colliding. How can some be pulling away from each other while others are being drawn to each other?

Yes, this is true. The universe is expanding so most galaxies are moving apart. Yet still some collide. The local gravitational pull of a neighboring galaxy is stronger than the overall expansion. So we fall towards Andromeda for this reason. Similarly, the entire galaxy says gravitationally bound together and doesn't rip apart with the expansion. One day the expansion may be strong enough to tear galaxies apart, but not yet.

Hawking and Krauss posit a quantum fluctuation as the source of the singularity in the big bang. From what I've read, there is a relationship between the amount of energy in a quantum fluctuation and how long it can persist for, so Hawking and Krauss use a gravitational field's potential negative energy to balance the net energy to zero.

My question is this: In the absence of space and mass to warp it, where is the gravitational field coming from?

PS: I'm a laymen, so if possible, avoid a mathematical answer :)

The original work from Hawking and others tried to ask how a universe can come from nothing. To do this, they imagine a space of possible universes -- of possible spacetimes -- and they ask the probability for one of them to pop out of the virtual quantum sea and become real. So there is a virtual spacetime. Super abstract and fun question to start us off.

Sounds very Leibniz-y best of all possible worlds? Am I understanding this concept correctly?

It may be more, the most energetically favorable of all worlds. But that is actually part of the debate. Which criterion is the right one to select the most probably universe?

Thank you for clarifying :) as for the criteria I've not a clue haha.

These days people talk more about a landscape. So instead of imagining that there was nothing before the big bang, we wonder if there is a landscape of spacetime. Occasionally on that landscape a patch gets caught in a high energy state and bubbles off into a big bang. This is multiverse view in which there are many universes and the conditions in each may be different.

Don't you find it absurd for anyone to believe that this "Earth" of ours would happen to be the sole source of intelligent life in the universe?

Yes. But on the other hand, the signals we've sent into space have only gone about 70 light years, a little more. I would be shocked (and excited) to discover there is intelligent life that close by, just in our backyard, and that the lifeforms have two eyes and a nose, and that their industrial revolutions coincides so closely with ours.

Can you explain the Baryon Number Problem and how(or if) it poses a problem for the Big Bang Model?

In the early universe, we expect that matter and anti-matter are made in equal proportions. But then there would be no galaxies and stars in the universe because all of the matter and anti-matter would annihilate. So the question is, why is there more matter made so that a residue is left over out of which to form everything we see? We don't have the answer to this question, but we're close and it informs out theories of everything.

What kinds of math do string theorists use and why, and how has string theory changed mathematics? Have you got any ideas of how we might test String theory, and do you think anyone has any plausible ideas(at this very moment)? Last, but not least: Do you think/speculate about what a testable prediction of String theory might do to the state of "things" in this world, and how we view the universe? (Like GM/SR followed by nuclear weapons, GPS..)

There's a lot of differential geometry and topology because of the spacetime aspect of the theory. There might be clues from the Large Hadron Collider if dark matter is found or supersymmetry or extra dimensions are probed. If any of these aspects are revealed, then string theory will be brought closer within our observational reach.

I don't know how this would work mathematically as I just like thinking outside the box. However, is it possible that the reason we cannot find anti matter but so much matter exists within our world because the universe is flipping back and forth between expansion of matter and contraction of anti matter over very long periods of time that we haven't been able to track yet? Almost like a reversible reaction in chemistry.

Well, the matter and antimatter occupy the same space so we'd have to try to segregate them over billions of light years and that probably will amount to the same thing as trying to create an excess of one over the other.

Is there any theory that as of right now is deemed "controversial" but you tend to agree with, or at the very least are intrigued by?

Many theories are controversial, which is exciting. There are things we don't know and we have to try out ideas and be willing to change our minds as information comes in. We may live in a universe with extra dimensions. I love that. The extra dimensions might trap energy that poses as the dark energy. I love that. But then again, maybe not.

Do we have any idea how big the universe actually is?

We know how old the universe is, a bit less than 14 billion years old. Light can only travel a certain distance on an expanding space in this time, about 45 billion light-years (this is bigger than 14 billion light-years because space is also stretching). So our observable universe appears to be that big. Beyond that, we don't know. Maybe the universe is finite but we'll never see far enough to know.

Wouldn't Venus be a better planet to terraform instead of Mars?

Interesting. I think Venus has sulphuric acid rains, so that's not good.

With the massive size of the universe, I think you can agree that we are not the only intelligent species. But with even the closest galaxies being thousands of light years away, do you think humans will ever develop the technology to reach an alien civilization?

I am extremely excited to think we will discover life in the universe in my lifetime. Surely we'll find microbial life before anything else. Intelligent life may not be long-lived. Look at our subtly suicidal tendencies. How long will we survive compared to the dinosaurs? That reduces our odds to ever meet other industrially advanced species.

Stephen Hawking recently claimed that "Black holes don't exist", in the sense that the ways we've been thinking about them aren't necessarily correct.

After doing a bit of reading on it, I am still a little confused.

He differentiates between an "apparent horizon", and an "event horizon", and the difference between the two I don't understand.

Could you shed some light on this?

Hawking's comment doesn't really apply to astrophysical black holes, which he doesn't deny exist. The argument is really about the end of the black hole life when it evaporates due to Hawking radiation. None of our astrophysical black holes formed as the death state of heavy stars evaporate because they absorb more ambient radiation from the universe than they can emit through the Hawking process. Having said that, yes he may well be right that a true event horizon never forms, just a temporary convergence of light-rays so that light doesn't escape until the end. In some sense the idea is not that radical and we don't yet know how it pushes the conversation forward. But it's Hawking, so we should pay attention.

We feel time moves faster as we get older. But the universe is expanding at a faster rate all the time (right?). Are the 2 related?

I shouldn't think so. Our experience of time can be understood locally, meaning in our little neighborhood. But it is interesting to wonder if there will be any notion of time in our far future. If the universe continues to expand faster and faster than there will be nothing to see in the sky and no changes to perceive and our time will effectively stop ticking.

The Earth rotates around it's own axis, and orbits the Sun, the Sun orbits the center of the galaxy and the galaxy is also moving somewhere.

If we combine all those movements, how fast are we REALLY going?

We really need to think of motion as relative. So we always have to ask, how fast is something moving relative to what? If you imagine an astronaut floating in empty space, there is no meaning to her absolute motion. Someone else may believe she's cruising past them at the speed of light. But she will be convinced she's still and the other guy is moving past her.

How can something with no center and no edge expand?

Our intuition wants to imagine a lower-dimensional object expanding into a higher-dimensional object. Like the skin of a balloon expanding into a room. The skin expands the same everywhere without a center. Now all we have to do is challenge our imagination to accept the mathematical and physical possibility that a space can expand without expanding into another one.

Do you read science fiction at all? If so, do you tend to read the more realistic kind or more silly fluff scifi?

I love science in books, but I've been slow to read sci fi, except the early stuff: brave new world, 1984, the classics. I love Cormac McCarthy's the Road, Ishiguro's Never Let Me Go. Caryl Churchill's play A Number. These aren't sci fi in a traditional sense.

Oh, and one other light-hearted question. Are you a fan of The Big Bang Theory TV show and does the math they put up on whiteboards and discuss on it actually hold up?

I don't watch it enough but it definitely makes me laugh. And I think they do very well with real physics speak.

Is it a bit of PR saying that Voyager is in interstellar space? Won't it take the Voyager probes a very long time just to get past the Oort Cloud? Isn't interstellar really past the Oort Cloud?

It's important that Voyager has broken out of the sun's magnetic influence. Now some galactic Cosmic Ray work can be done. It's exciting that something we built has gone so far. Still, it's hardly gone anywhere in the bigger scheme of things. Some light hours away.

Light can't escape from a black hole. So you would assume the speed of light can be influenced by gravity. Is that right?

To any observer, locally, the speed of light is always c = 3 hundred thousand km per second. But it can take light longer or shorter than we expect to travel distances because of the warping of space.

Are there any forces or circumstances which might reduce the density of a black hole? Could anything reverse the density to the point that it is forced to expel matter which it once consumed?

Could our Big Bang be that expulsion of some other black hole?

There are some fun ideas that inside black holes are other universes, baby universes. If that's true, then a big bang bursts out of the interior of a black hole. The black hole could be infinitely big inside, even if it's only a few km's across outside.

Thank you for doing this AMA, I do have a question that's branches a little bit Into the unknown, but had its place in metaphysics. So really I'm looking for your opinion on this: the immovable mover. As an atheist my rationale has led me to believe that the only compatible answer with my belief is that time (existence if you will) always was, always is, and always will be. Otherwise there seems to be a need for the immovable mover, something that cataly the beginning of all we know (whether it be the big bang or whatever if anything exists outside our known universe). So I guess my question is, what do you feel about this? Can time beginning be compatible without the belief in some supernatural being?

Yes, the beginning of time can be comprehended without a belief in a supernatural being. These questions are profound and interesting, but when we rely on supernatural answers, we lose the opportunity to make scientific discoveries. And that would be a shame.

I whole heartedly agree, I don't like to take the easy way out either :) thank you'd for your answer, would you mid elaborating on what that compatibility could be?

Scientifically, it's nice to imagine we have the fewest assumptions possible and the greatest number of predictions from those sparse assumptions. Anything unnecessary should go.

Can you explain how under the "Holographic Universe" theory our universe can contain black holes, if the current universe is merely information encoded on the surface of an event horizon? Again, laymans terms here but it seems to suggest a potential Russian doll scenario of never ending universes encoded inside each other to infinity. (Apologies if I have my concepts wrong here, it's an idea I'm still trying to fully comprehend here)

Holography is so interesting but in a cosmological setting the idea hasn't been formalized as well as it has for black holes. The black hole can encode all of the information on its surface. What do we use for the universe as THE surface? The argument is that any surface will do, but again this is a work in progress and under debate. Some cosmologies have actual event horizons. And in those, we can more formally argue that holography seems to tell the right story.

What is your favourite Sci-Fi movie, and why?

2001 Requires further explanation?

My last remark is that, if you have not, I'd ask you to please consider viewing the film "Primer", as it is a most excellent science-fiction film to view...over and over again...!

Oh I remember Primer. Yes, great. Will watch again.

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People were worried that the LHC might make black holes. The scientists on the experiment didn't argue that black holes could never be formed. It was a possibility taken seriously, that black holes might be made in the high energy collisions. However, they would be so small that they would evaporate through Hawking radiation before they could absorb the Earth. The smaller the black hole, the hotter and faster the evaporation process.