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When You Fall into a Black Hole, How Long Have You Got?

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In chatting with colleagues after a talk this week, Joe Polchinski said he’d love to fall into a black hole. Most theoretical physicists would. It’s not because they have some peculiar death wish or because science funding prospects are so dark these days. They are just insanely curious about what would happen. Black holes are where the known laws of physics come into their most direct conflict. The worst trouble is the black hole information paradox that Stephen Hawking loosed upon the world in 1976. Polchinski and his colleagues have shown that the predicament is even worse than physicists used to think.

I first heard about their brainstorm while visiting the Kavli Institute for Theoretical Physics in Santa Barbara this spring, and the team—Polchinski and fellow Santa Barbarans Don Marolf, Ahmed Almheiri, and James Sully—wrote it up over the summer. Polchinski blogged about it a few months ago, and another theorist who helped to usher in the idea, John Preskill, did so last week. Polchinski’s talk to the New York University physics department drew a standing-room-only crowd, not a single person snuck out early, and he was still fending questions an hour after it ended.

Almost as much has been written about Hawking’s original paradox (including by me) as about the fiscal cliff, so I’ll jump straight to the new version. Step #1 of the argument is what Polchinski and his co-authors call the “no-drama” principle. According to current theories of physics, a black hole is mostly just empty space. Its perimeter or “event horizon” is not a material surface, but just a hypothetical location that marks the point of no return. Once inside, you are gripped too tightly by gravity ever to get back out. By then, falling at nearly the speed of light, you have a few seconds to look around before you reach the very center and get crushed into oblivion. But nothing noticeable should happen at the moment of crossing. One of Einstein’s great insights was that observers who are freely falling—whether into a black hole or toward the ground—don’t feel the force of gravity, since everything around them is falling, too. As they say, it’s not the fall that kills you; it’s the sudden stop at the end.

An outside observer knows you’re doomed, but likewise doesn’t think anything untoward happens upon passing through the event horizon. Indeed, this observer never sees anything actually cross over. Because of a kind of gravitational mirage, things seem to slow down and freeze in time. All the stuff piling up at the horizon forms a ghostly membrane, which obeys the usual laws of physics and has conventional properties such as viscosity and electrical conductivity.

Step #2 is to relate these two viewpoints. To the infalling observer, space looks like a vacuum, and in quantum theory, a vacuum is a very special state of affairs. It is a region of space that is empty of particles. It is not a region that is empty of everything. There’s no getting rid of the electromagnetic field and other fields. (If you could, the region would not merely be empty, but nonexistent.) A particle is nothing more or less than a vibration one of these fields, and what makes a vacuum a vacuum is that all the possible vibrations cancel one another precisely, leaving the fields becalmed. To maintain this finely balanced condition, the vibrations must be thoroughly quantum-entangled with one another.

To the outgoing observer, the horizon (or membrane) cleaves space in two, and the vibrations no longer appear to cancel out. It looks like there are particles flying off in every direction. This is perfectly compatible with the infalling observer’s viewpoint, since the fields are what is fundamental and the presence of particles is a matter of perspective. To put it differently, emptiness is a holistic property in quantum physics—true for a region of space in its entirety, but not for individual subregions.

For consistency between the two viewpoints, the outside observer infers that each particle he or she sees has a doppelgänger inside the horizon. The two are quantum-entangled, like those particles in laboratory experiments you read about. (Watch this lighthearted video that my colleagues made earlier this year to explain entanglement.) Individually, both particles behave completely randomly, but together they form a matched pair. See the diagram at left: the infalling observer sees vacuum state a, the outside observer sees entangled particles b and b′. Particle b is part of what physicists call the Hawking radiation.

Step #3 is to consider the long-term fate of the hole. Like everything else in this world, black holes must decay—quantum mechanics mandates it. In the process, a hole must gradually release everything that fell in. If Joe Polchinski jumps into a black hole, he will get scrambled with all the other theorists who have done the same, and the morbid gruel will emerge particle by particle in the Hawking radiation. Though mangled beyond recognition, each martyr to the cause of knowledge can still be separated out and pieced back together. To enable this reconstruction, the particles of the Hawking radiation must be thoroughly entangled with one another.

So, by step #2, each particle flying away from the hole must be thoroughly entangled with its doppelgänger inside the hole. By step #3, the particle must also be thoroughly entangled with other particles that are flying away from the hole. These two conclusions clash, because quantum mechanics says that particles are monogamous. They can’t be thoroughly entangled with more than one other partner at a time. They can be partially entangled, but that is not enough to ensure consistency between the observers’ view or to reconstruct the infalling physicists.

This formulation of the black-hole paradox vindicates Hawking’s original argument. For years physicists hoped that the devil lay in the details—that more precise calculations would reveal an escape route—only to be serially disappointed. Now they have officially given up hope. One of the basic premises must be wrong—which is to say, something deep about modern physics must be wrong. “You need huge changes, not just quantum-gravitational corrections, to invalidate Hawking’s argument,” Polchinski told the assembled multitudes at NYU.

More surprisingly, Polchinski and his co-authors have shown that a popular approach known as black-hole complementarity, championed by Leonard Susskind of Stanford University, isn’t up to the task, either. Susskind reasoned that, although infalling and outside observers might see different and mutually incompatible events, no single observer can be both infalling and outside, so no single observer is ever faced with a direct contradiction. In that case, the paradox is only ever conceptual—suggesting it is somehow illusory, the product of thinking about the situation in the wrong way. But Polchinski and colleagues showed that a single observer can catch a particle in the act of polygamy by first lingering outside the hole and then jumping in.

The least radical conclusion is that the no-drama principle is false. Someone falling into a black hole doesn’t pass uneventfully through the horizon, but hits a wall of fire and is instantly incinerated. “I think it’s crazy,” Polchinski admitted. But in order for a black hole to decay and its contents to spill out, as quantum mechanics demands, the infalling observer can’t see just a vacuum. The firewall idea strikes me as similar to past speculation that black holes are somehow material objects—so-called black stars or dark matter stars—rather than merely blank space.

“I spent 20 years confused by this,” Polchinski said, “and now I’m as confused as ever.” It would be nice to answer the question, if only so that no one ever has to undertake the journey to answer the question.

Diagrams courtesy of Joseph Polchinski

George Musser About the Author: is a contributing editor at Scientific American. He focuses on space science and fundamental physics, ranging from particles to planets to parallel universes. He is the author of The Complete Idiot's Guide to String Theory. Musser has won numerous awards in his career, including the 2011 American Institute of Physics's Science Writing Award. Follow on Twitter @gmusser.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. Dr.Boris 3:21 pm 12/14/2012

    Hmmm… when falling past the event horizon, you SHOULD feel that you’re falling because it’s not uniform velocity, but acceleration with the tidal force at 1/r^3.

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  2. 2. gesimsek 5:44 pm 12/14/2012

    As far as I know prof.weiler once explained the decay phenomena by the universal principle that no information is lost. Each informaton contained in falling physicians’ quantum units (their massless vibration positions) comes outside at the end, but not their time and space coordinates, ie., the knowledge of assembling them back into original.

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  3. 3. rikacomet 6:57 pm 12/15/2012

    My question is the supposedly if the black hole is indeed something that pulls apart matter at its heart, then going by the newton’s law that matter is neither created or destroyed, what happens to the pulled apart matter? If its thrown outwards, then isn’t it a paradox in itself ? because a) if the two points of reverse magnitude gravity exist inside and outside the blackhole, then matter would be flying away at incredible speeds, which is not apparently so from observation.

    b) if the gravity is as strong as to pull the light in, then how is a reflection emitted at someone observing it from even nearby of a place ?

    Is it possible that black holes, convert matter into antimatter thus we do not see any mass continously going in and out?

    suppose the black hole is a machine crusher working in a opposite polarity, that instead of pushing in the matter together, pulling it out instead. then once any object is incinerated by it, what happens to its remains? if they are sent out, why are they sent out at (if not) at equal force as they were picked apart? and why are they not re-attracted towards the black hole?

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  4. 4. kebil 10:12 pm 12/17/2012

    I have read Susskind’s book on the matter, as well as seen many of his lectures, and fail to be impressed by this retake of the information paradox. First, why are there two particles, and second, even if you fall in after a particle has “split in two”, you would never be able to observe the second particle as it has fallen into the black hole before you, and no light can venture back to you once it has fallen past the horizon, even if you are also falling in.

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  5. 5. George Musser in reply to George Musser 10:47 am 12/18/2012

    @kebil You might enjoy reading Polchinski et al.’s paper, which I link to. Susskind himself says he finds the new formulation helpful.

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  6. 6. George Musser in reply to George Musser 10:54 am 12/18/2012

    @aurizon @Dr.Boris Yes, there are tidal forces, but for a large hole, these are fairly weak at the horizon. Infalling matter feels them no more strongly than does an astronaut in Earth orbit or a freely falling skydiver.

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  7. 7. George Musser in reply to George Musser 10:56 am 12/18/2012

    @rikacomet I’m not sure I follow.… What do you mean by “pulls apart matter at its heart”?

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  8. 8. genevehicle 3:54 pm 12/18/2012

    Cool Article
    @kebil I too read Susskinds’ book. The two particles are a particle/anti-particle pair, created from quantum field fluctuations. Normally, such pairs immediately destroy one another, unless they just happen to appear right on the event horizon of a black hole and are headed in the right direction. Then one can fall into the black hole and one flies off into space. The one that flies off into space is Hawking radiation.

    The thing is, Hawkings’ “quantum information paradox” shows a clear violation of the “universal principal” that no information is lost. That’s the problem. Either there’s some way to circumvent Hawkings paradox (and this new paper helps support the validity of Hawkings conclusions), or we have to take another look at some basic assumptions we’ve been making, namely, assumptions we’ve been making about entropy, information, and the second law. This is what Hawking was saying from the get-go. In short…”Houston…we have a problem….”
    Of course, like the article said, if we just allow in-falling matter to smash into, and disperse across, the surface of the event horizon, then information isn’t lost and our concepts about how we equate entropy and information are safe. But, in that case, we would be forced to re-evaluate our notions of event horizons an some other stuff.

    ……….? First Law, “energy” can’t be created or destroyed. I don’t think black holes work like you think they do. The gravity doesn’t reverse direction at the event horizon (if that’s what you were saying) and black holes don’t normally spit things out once they pass the event horizon. If they did, we’d have another set of paradoxes to solve.

    @George Musser

    Awesome article, thanks for the link.

    Personally, I think it’s about time we reexamine how we equate entropy and information. Why shouldn’t we lose some information every now and then? If black holes are, in fact, entropy (information) sinks, then so be it. The universe is no longer a closed system and it’s leaking….’cause it’s full of holes….black ones. After all, universes have to get their start somewhere.

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  9. 9. George Musser in reply to George Musser 4:56 pm 12/18/2012

    @kebil @genevehicle I purposely avoided the pair-creation picture in this blog post, instead using the observer-dependence of the number of particles in a quantum field, which I think captures the essence more faithfully.

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  10. 10. bucketofsquid 12:33 pm 12/21/2012

    I’ve never understood black holes before and I still don’t now, but I think that maybe I’m just a little less confused than I was.

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  11. 11. christinaak 12:09 pm 12/27/2012

    Tidal forces would kill one long before one crossed the event horizon. Even if one were to survive long enough to reach the event horizon, one’s body would not survive the penetration of it. It is most likely that the constituents of the hapless victim of such an experience would break down into some more basic form of matter, as the familiar fundamental forces (other than gravity) cease to operate upon penetration of the event horizon. If black holes possess a structure consistent with a quantization of space-time into discrete units (because singularities can not exist,) then matter as we know it (and the aforementioned fundamental forces must cease to operate) must decay into a more basic form upon penetration of the event horizon. During this ‘decay’ process the material that does not enter the black hole is released in the relativistic jets generated by black holes. christina knight

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  12. 12. christinaak 12:11 pm 12/27/2012

    Tidal forces would kill one long before one crossed the event horizon. Even if one were to survive long enough to reach the event horizon, one’s body would not survive the penetration of it. It is most likely that the constituents of the hapless victim of such an experience would break down into some more basic form of matter, as the familiar fundamental forces (other than gravity) cease to operate upon penetration of the event horizon. If black holes possess a structure consistent with a quantization of space-time into discrete units (because singularities can not exist,) then matter as we know it (and the aforementioned fundamental forces must cease to operate) must decay into a more basic form upon penetration of the event horizon. During this ‘decay’ process the material that does not enter the black hole is released in the relativistic jets generated by black holes. christina knight

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  13. 13. Zephir_AWT 12:00 am 12/30/2012

    How the fall into black hole looks like in AWT. I presume, it covers both complementarity, both firewall concept.

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  14. 14. neutrinoman 5:44 pm 01/2/2013

    It isn’t “two particles,” but a particle-antiparticle pair. What’s conserved locally is charge and mass-energy. The former is conserved by creating a pair of particles of opposite charge; the latter is not conserved locally, because there’s a “force doing work” on the vacuum, to put it in Newtonian language. That work results in particle-antiparticle pairs.

    Any object of finite size will be “spaghettified” by the tidal force, but not until it gets closer and closer to the singularity at the center of the black hole (not the event horizon).

    There’s no problem if you stop assuming that black hole entropy is the entropy of all the microstates held hostage by the hole — the chunk of quantum Hilbert space that the hole ate. Instead, Hawking’s argument requires that the BH entropy be no more than the entanglement entropy, the entropy associated with the quantum entanglement between particle and antiparticle.

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  15. 15. skeam 12:10 pm 01/20/2013

    Isn’t there a problem with the “gravitational mirage” at the event horizon mentioned in the article, where the outside observer sees the infalling physicist seemingly slow down and freeze? Is this really just a mirage…?

    It seems to me that the only consistent account of what happens, if Hawking radiation really makes a black hole to eventually evaporate, would be that the infalling physicist would see himself start disintegrating due to his particles evaporating, before he fully enters the black hole. (Or rather his particles being annihilated by other incoming particles, I would guess = the infalling part of the Hawking radiation.)

    Even though he may expect to hit the center of the black hole very soon, in his perspective, he doesn’t have time to do so because the black hole evaporates even sooner, in his perspective, although this corresponds to a very very long time for an outside observer. And the same would be true for simpler particles than a physicist.

    Or am I missing something in my understanding of black holes?

    (Do you mean the infalling physicist actually enters the black hole in a finite time even as measured by the outside observer, although they just can’t see it? Because if he doesn’t, I can’t see the paradox…)

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