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Duration:04:28
Uploaded:2014-02-14
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Hank explains the science behind recent reports that physics great Stephen Hawking said "there are no black holes." There are. They're just super complicated.
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I love you Steve- almost as much as Science.

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Sources:
http://arxiv.org/abs/1401.5761
http://arxiv.org/pdf/1401.5761v1.pdf
http://www.popularmechanics.com/science/space/deep/no-black-holes-heres-what-stephen-hawking-actually-said-16442205?src=spr_TWITTER&spr_id=1457_42616980
http://www.pbs.org/newshour/updates/hawking-meant-black-holes/

 Introduction (00:00)


We've taken from the comments on last weeks SciShow news that a lot of you really, really do not like, or at least don't care about american football. We totally get it, message received. So how 'bout this: (sings) are you ready for some physics?

 "There are no black holes" (00:23)


Every now and again, we gotta clarify some important concepts that come up in the news, that the news doesn't particularly cover very well. That happened last week, when none other than Stephen Hawking appeared on Google News homepage's and Tumblr dash's and everywhere under headlines announcing that Hawking had suddenly declared that black holes do not exist. The exact quote that you might have heard was Hawking having written in a recent paper "There are no black holes" that's right. Actually, it's not at all right, but it is what a lot of the headlines said. Naturally, the whole of the internet took the tl;dr route and didn't bother actually reading what Hawking wrote.

I understand that. That's what we're here for. Instead, a lot of people took it to mean that there was some kind of major breakthrough in astrophysics or maybe some major breakdown in astrophysics, or as one news site put it: "Maybe Hawking just changed his mind." To better understand what Hawking's actually talking about, let's dial it back like maybe like 40 years? That's when Hawking first introduced the idea that black holes aren't really what we thought. 

 Event Horizons (01:16)


The prevailing model of black holes, the one that you're probably familiar with, is that of an infinitely dense, powerful singularity, surrounded by a boundary known as an event horizon. And anything that's inside of this event horizon gets sucked into the black hole, anything that's outside of it can't see, cannot observe the stuff beyond the event horizon. 

But 1975 Hawking blew minds everywhere with his calculations that showed that black holes actually radiate energy. This had a whole bunch of ramifications for the universe as we know it, but one of them was the implication that black holes will in time eventually radiate out all of their energy until they simply vanish. So this, in turn, raised the question of what actually happens to all the stuff that's gotten sucked into the black hole before it disappears. And by stuff, I don't just mean matter or light, or anything else that you can actually detect, I'm also talking about the properties of that matter and light that define those things at a particular point in time. 

Physicists refer to this simply as information. And one of the many challenges posed by Hawking's evaporating black hole idea was the fact that the laws of physics, at least as we understand them, require that all of the information in the universe be conserved. Even if the information gets sucked into a black hole, it has to go somewhere. It can't just fade way. So to make room for this law, Hawking has revisited the idea of the event horizon. In his latest paper, given the somewhat lovely title of "Information Preservation and Weather Forecasting for Black Holes" Hawking suggests that maybe black holes don't have event horizons.

 What He Really Said (2:40)


Maybe instead they have what he calls apparent horizons. Boundaries that are essentially as temporary as the black holes themselves. So, when the black hole evaporates the apparent horizon goes with it, and, theoretically at least, all of that precious information that was hoovered into the black hole gets released back out in the universe, somewhere, sometime. So a lot of us heard Hawking saying "there are no black holes" but what he really said was: "The absence of event horizons means there are no black holes in the sense of regimes from which light can't escape to infinity. There are, however, apparent horizons which persist for a period of time." 

Sounds a little different when you hear the whole sentence. And no, it is not an issue of Hawking "changing his mind" although it would have been perfectly fine if he did. In fact, in Hawking's latest paper, he does the opposite, and reinforces the revolutionary ideas that he first introduced 40 years ago. Ideas that helped make him one of the most important physicists of our time.

 Conclusion (3:33)


This of course is the formative version of what Hawking said about black holes, we've included the link to his paper in the notes below for you to look at yourself, because this stuff is complicated. Just a little bit more complicated than football. 

Thanks for watching this episode of SciShow News, which is brought to you by audible, which is giving away a free audio book to SciShow viewers. Go to audiblepodcast.com/scishow, there's a link in the description, where you can download many of Stephen Hawking's works and listen to them in your ears and it will make you so much smarter. Including his master work: A Brief History of Time, also A Briefer History of Time, The Grand Design, The Universe in a Nutshell and The Theory of Everything. So go to audiblepodcast.com/scishow, sign up for your free book.

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