What I THINK I know: Supermassive celestial body collapses in on itself and becomes so dense light can't escape it.
What I decidedly do NOT know: what kind of mass is in there? is there any kind of molecular structure? Atomic structure even? Do the molecules absorb the photons, or does the gravitational force just prevent their ejection? Basically, help!
Posted Aug 3, 2011, 7:35 PM UTC
I think this is an open question in Physics. There is no universally accepted answer. Proposed answers to "what's in a black hole?":
"An entire solar system" http://news.discovery.com/space/black-hole-alien-life-110413.html
"Our entire universe" http://www.popsci.com/science/article/2010-07/we-might-be-living-black-hole-scientist-says
Of course the mainstream answer is "a singularity" but I think Physicists look at you funny if you ask what the physical manifestation of that mathematical singularity is.
Please explain these downvotes. This is the only post in this thread to link to peer-reviewed papers (two of them in a prestigious journal, no less), yet this is the only downvoted post.
edit: I want to remind people of the etiquette of this subreddit. From the AskScience rules: "Please do not downvote answers/comments you disagree with or are wrong. Downvote answers/comments that are off-topic or distracting from the conversation. Wrong answers can have great discussions and educational opportunities that expand on the OP's question, and it's best if everyone can see them. "
So even if you think what I'm linking to is wrong, and it very well may be, please tell me why you think so before simply downvoting. It would be preferable if you could provide contrary physics journal publications talking about the interior structure of a black hole (or lack thereof), which is what the OP asked about.
discovery.com and popsci.com are not prestigious journals. And that conclusion from the actual paper is... not highly regarded in the science community (the paper is actually about the possibility of stable orbits). Made better fodder in the science journalism community because it sounded cool. you can search site:reddit.com/r/askscience civilization inside black hole for threads where we've discussed this paper specifically.
discovery.com and popsci.com are not prestigious journals.
The links to the journals are right in those articles:
Radial motion into an Einstein–Rosen bridge. Nikodem J. Popławski. Physics Letters B Volume 687, Issues 2-3, 12 April 2010, Pages 110-113 Editor: S. Dodelson.
Cosmology with torsion: An alternative to cosmic inflation. Nikodem J. Poplawski Phys.Lett.B694:181-185,2010
I've search previous threads. They're dominated by RRC saying that this is an invalid question and that there can be nothing in a black hole. I'm trying to make an argument that this is not universally accepted by the physics community.
Mass, spin, and charge.
And lots of it.
None of these answers are right because the truth is we simply don't know whats inside. We are just starting to understand whats on the outside.
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Kinda, late, but I always wondered. Why do event horizons have non-zero Width? i mean if you truly got rid of all standard conventions of "Volume", then why isn't it a point? I mean, if the mass does really collapse on itself, shouldn't it collapse completely? Or is the construct of an event horizon the range at which anything inside is effected? Essentially inside there is absolutely nothing, save for a single point in the center with all the mass.
I love how I can just look at the question being asked and say to myself, "Well how is RobotRollCall going to answer this one?"
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This is wonderful, thanks man!
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Black holes have no insides, so there's nothing in them.
It's basically impossible to give a short, succinct description of black holes that is also in any way even vaguely correct. They are so completely different from anything we encounter in daily life that even metaphors fail.
So the best way to think of it, for the layperson just going about life wanting to be essentially educated as to how the universe works, is to imagine a very large, very old star. This star has used up all its fusion "fuel," if you will, and will soon collapse, exploding spectacularly in an apocalyptic cataclysm of radiation that will, briefly, outshine its whole galaxy.
Inside the very core of that star, there's, well, more star. The end hasn't come yet; the star is still being a star for the moment, so the interior is still star. But it's fantastically dense. In a minute, when the star explodes, it's going to become denser still. Because you see, the thing that explodes when a star goes supernova is the outside of the star. Imagine a bowling ball coated in cake icing … made of plastique explosive … and wired to a timer … okay this metaphor isn't very good. But the point is, it's the outer layer of the star that's actually going to do the exploding here in a minute.
So let's wait.
And wha-boom.
Okay, that was a supernova. Nice one, right? It happened kind of fast, so you might've missed it if you weren't watching carefully: The interior of the star reached the point where it no longer had sufficient pressure to hold the outer layers of the star up, so it essentially collapsed. The outer layer, meanwhile, began to drop like a rock, because all the pressure that had been supporting it suddenly vanished. This caused the star's outer layer to heat up unbelievably quickly, which caused lots of violently interesting things to happen. There was a stupendous outrushing of radiation, first, and matter shortly behind it — helium and lithium ions mostly, and some other stuff. But what you couldn't see was that that same explosion also went inward.
A spherically symmetric shockwave propagated inward, down toward the core of the star, compressing the already hellishly dense matter that was there until … well, the world came to an end.
There is a limit to how much stuff can occupy a given volume of space. This is called the Bekenstein limit, after the boffin who figured it out, and I won't elaborate on it here because maths. But suffice to say, there's a limit.
When that limit is reached — and in this case, due to the simply incomprehensible pressure exerted by that inward-focused shockwave, it was — the volume in question simply goes away. Poof. It ceases to exist. If you like, you can imagine God Almighty being offended by the ambitious matter and willing it out of existence in an instant. Just pop. Gone. Forever.
What's left, in its place, is a wee tiny … not. An isn't. Part was, part isn't, part won't-ever-be, in the shape of a perfect sphere that doesn't exist.
The boundary between where that sphere isn't and where the rest of the universe still continues to be is called the event horizon. The event horizon is not a surface. It's not an anything. It's an isn't. But it behaves like a surface in most respects. A perfect, impervious, impenetrable surface. If you threw something at it, that something would shatter into its component bits — and I don't mean chunks, or even dust, or even atoms, or even protons and electrons. I mean individual discrete field quanta. And those field quanta would spray off into space in all directions like bits of strawberry out of a liquidizer that has been unwisely started with the lid off.
That's what happens to all the stuff that was in the centre of that star, as well. Eventually, it'll be sprayed out into the universe in the most fundamental form possible, as little individual quanta of energy and momentum and spin and charge.
Except due to a combination of relativity and thermodynamics, you will not actually witness that happening. Because the process takes a while. For a typical stellar black hole right now? The process will take on the order of a trillion years. So don't wait up, is what I'm saying here.
So black holes? They have no insides. They aren't. That's their defining characteristic, qualitatively speaking: They aren't. There's nothing in them, because there's no in, because they aren't. There's stuff which is, even right this very moment as we sit here talking about it, in the process of scattering off black holes. You can't see, observe, detect or interact with any of that stuff, but we know it's there, because it has to be. And we know eventually it'll spray out into the universe, first and for hundreds of billions of years as photons — a few a day — with such long wavelengths that they can barely be said to exist at all. Later, hundreds of millions of millennia after we, our species and our solar system have long since ceased to exist, black holes will start emitting radiation we'd recognize as radio waves. Then, in an accelerating process, all the way up through the electromagnetic spectrum until finally, in the last tiny fraction of a second before the black hole evaporates entirely, the potential energy available will be in the hundreds-of-electronvolts range, and we'll get the first electrons and antielectrons, then a few protons, and then a cataclysmic burst of short-lived exotic particles that lasts hardly longer than a single instant, then the black hole will have ceased to not exist.
This is possibly the best quote ever associated with physics.
Reminded me a lot of Douglas Adams.
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Same here. I miss that hoopy frood.
Sounds like a line from Doctor Who
Great line. Just finished Bill Bryson's Short History of Nearly Everything which is chock-full of awesome physics metaphors.
I second this! Must-read for laymen.
Runner up
By a mere whisker.
Reminds me of the Louis CK bit about answering questions from a 4-year old. "BECAUSE THEN NOTHING WOULDN'T BE!! YOU CAN'T HAVE NOTHING ISN'T!!"
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Thank you for a very eloquent post. I had to link this to my friend whom I incorrectly "explained" black holes to her kids. During that conversation we talked about black holes having really strong gravity, but in your post you don't really speak of gravity. The way I understand what you said, is that they have no mass, so then I infer they must not have gravity. Would you care to comment on this. I didn't see any mention of it on this entire post.
Gravity's really the least interesting aspect of black holes, to be honest. I mean yes, it's interesting from the perspective of finding solutions to the field equations that describe how black holes gravitate, but for the most part all that work has been done. There's not that much new to say on the subject, and hasn't been for many decades.
The short answer to your question is that mass is not the source of gravitation. In the Newtonian approximation, we assign a number to every body in a system. That number is proportional to inertia — it's the term in the equations that distinguishes between how different particles will accelerate under the same change in momentum — and we call it "mass." But don't let the technical-sounding name fool you. It basically just means "stuffness." A heavy thing, we say, has more "stuffness" than a light thing, and we put a term quantifying that into our equations because it works. It makes our equations describe reality.
In truth, the concept of "mass" is far more subtle than that. It's not a single, fundamental quantity, but rather a composite quantity made up of many different contributions. You know about the "mass defect," right? An atomic nucleus with (just making up some numbers here) twenty-six protons and thirty neutrons should have the same mass as 26 × the mass of the proton + 30 × the mass of the neutron … only it doesn't. Okay, no problem, we say. There's stuff holding the nucleus together — which makes sense, seeing as how it has net electric charge and really ought to fly apart — and that stuff is what makes the nucleus heavier than the sum of its parts.
Except that's wrong. Because a nucleus isn't heavier than the sum of its nucleons. It's lighter! There's less mass in an iron-56 nucleus than there is in twenty-six protons and thirty neutrons.
Why? Because if you wanted to take an iron-56 nucleus apart nucleon by nucleon, you'd have to put energy in. A stable nucleus is in a lower energy state than it would be if each of its nucleons were separated. Which means it has less "mass." Less stuffness. Even though it's the same amount of stuff.
A black hole is the extremal case of this. A black hole has no stuff at all. Yet it gravitates. Why? Because mass is not actually the source of gravitation. Mass doesn't gravitate. Energy gravitates. (Technically, what gravitates is energy density, energy flux, momentum density and momentum flux, plus the diagonal terms composed of those components — pressure — and the off-diagonal terms, sheer stress. But whatever.)
There are no fermions — no matter particles — associated with a black hole. You can't meaningfully say, "Oh, this black hole has so-and-so many fermions inside it," because black holes have no insides. So when it comes to that thing we call mass in casual conversation, black holes have none.
But they gravitate anyway, because mass isn't the source of gravitation.
Now, I explained before one example of how energy can look and act like mass — like stuff. So what's the point of distinguishing between mass and energy? There is none. And in fact, in modern physics we really don't. We describe the inertia of matter particles in terms of energy units, and we talk about the mass of fields which aren't associated with matter at all. "Mass," to a physicist these days, is just a particular type of energy that behaves according to certain rules, and down at the smallest scales even those rules become indistinct to the point of irrelevance. So we often talk about the mass of a black hole. Just like we often talk about the mass of a scalar field that fills all of space. Even though neither are associated with matter.
But to the everyday public, "mass" and "matter" are intrinsically linked concepts. Mass is a property of matter, matter has mass, things which aren't made of matter have no mass.
So in contexts like this one I try to go out of my way to talk about the effective mass of a black hole, rather than just being lazy and talking about the mass of a black hole. It's an effort not to confuse people who believe — and not unjustifiably so — that mass means matter and matter means mass.
Maybe it backfires. Because confusion frequently arises, only in the opposite direction. "Black holes aren't made of matter, which means they have no mass, which means they can't gravitate, right?" And then we're having the discussion anyway even though I tried to avoid creating a need for it.
I really don't know. All these years, and I'm still really quite rubbish at teaching.
No, no you are not rubbish at teaching... I find you to be quite the opposite, actually. Thank you for your contributions they have cleared up any confusions I have had on this subject.
This is the post that made it all click for me--really liked the explanation of a Fe nucleus having less mass than the summed neutrons/protons. Thanks for the post!
And then it gets really fun when the atom is heavier than iron, 'cause then it has more mass than the sum of its parts and that's what makes plutonium bombs go bang.
And then it gets really fun when the atom is heavier than iron, 'cause then it has more mass than the sum of its parts and that's what makes plutonium bombs go bang.
The bit about a nucleus having slightly less mass than its components would on their own really helped.
I think this means that black holes are a whole lot of energy in an infinitesimal point?
Can you help me understand why people keep coming back to the "infinitesimal point" thing? It's wrong, but I don't know how to address it because I'm not sure where it's coming from.
But what is energy? Isn't energy more of an accounting method to describe the interaction between two physical objects? Can one have a 'ball of energy' such as in a black hole?
Meh. Questions like "what is energy" bore me; I'm not philosophically minded. Energy is energy. It's that thing you put into various equations to make predictions about how systems are going to evolve over time.
This was all making a ton of sense to me and then I "remembered" the term "supermassive black hole" and how there's supposed to be one at the center of our galaxy and I went and checked out this wikipedia page and now I'm greatly confused, because they're talking about black holes with varying mass and you're saying that black holes have no mass. I'm wondering if this is something you could elaborate on. Is this page wrong, or is my interpretation of it wrong?
Pardon me, but the question you just asked was answered in excruciating detail in the comment you replied to with the question you just asked. What part of it did you have trouble with?
Uhhh, mass IS energy. If you add energy to an object (i.e. speeding it up), you will increase its mass.
You even proved this wrong with your example about iron-56. You did not change the matter, but you changed the energy, thus changing the mass. Mass is the amount of energy, not a property of matter. A photon has 0 mass at rest, while it does have mass while moving.
This is a 21 day old comment on a 5 month old post, but what the heck, here goes.
Photons do not have mass. They move at the speed of light. They cannot be at rest.
This is excellent, but not quite complete I think. It's more correct to say that the theories we have about matter and space, theories which have been verified to greater precision than any other theories in history, tell us that there's no meaningful model for what happens inside the event horizon of a black hole. However, that's not necessarily the end of it. We have yet to reconcile quantum mechanics and general relativity and perhaps a theory of quantum gravity will yield more interesting answers as to what lies on the other side of the event horizon.
Nevertheless, the exercise is somewhat academic as even if there were the most exiting circus hijinx occurring within a black hole it would have no impact on our Universe whatsoever.
I think you're working from old data. That's exactly what one would have said about twenty years ago. We have new models, and they tell us exactly what goes on "inside the event horizon." More specifically, they tell us nothing goes on, because there is no inside. With a few convolutions and excursions, that basically follows from Noether's theorem.
I am unfamiliar with these new models. Which models are they?
There are a few. There's Hawking's anti-de-Sitter model, there's 't Hooft's S-matrix model, and there's Susskind's string-theory maths formalism. The consensus is that these three models are all just different ways of expressing the same essential truth, but of course the work of sorting them out continues apace.
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Are you a Dr. Who fan? I ask because that sounds very much like something the Doctor would say.
Are you the Doctor?
No, no, no!!!!
There is no particular reason to believe that space doesn't exist beyond the event horizon. The fact that the Schwarzschild metric has a singularity there doesn't mean that spacetime has a singularity there, any more than the fact that spherical coordinates have a singularity at polar angles and pi means that there is a geometric singularity at the north and south pole of the Earth.
The black hole singularity is at the center of the black hole, not at the event horizon, and GR predicts a finite amount of time passing for an observer in freefall to fall from the event horizon to the singularity (for example, an object of very small mass compared with the black hole, whose initial conditions were v/c<<<1, r/r_s>>1).
See: http://en.wikipedia.org/wiki/Schwarzschild_metric#Singularities_and_black_holes
As for what happens at the singularity, that's beyond present understanding; it involves gravitational quantum field theory, which is presently not understood, seeing as field theory as currently understood is formulated using an approximation that space is flat – i.e., it assumes flat Minkowski spacetime. It breaks down when spacetime is curved (e.g., at high energy densities; see: renormalization energy scale cutoff), but spacetime is not necessarily very highly curved at the event horizon, especially for large black holes, such as those in galactic nuclei.
This was discussed elsewhere on the page, just so you know. I'm sorry to say your information is a couple decades out of date.
You mentioned several theoretical models. However, AFAIK none of them has ever produced a single experimentally verifiable prediction – at least, not verifiable with any experiment we are capable of conducting at present or in the foreseeable future. So until that happens, I'll take exact solutions of GR at face value.
Okay. You're certainly welcome to do that. Except of course for the fact that by themselves, the exact solutions from general relativity say thermodynamics is wrong. That's rather why we need more than just general relativity for this.
So the core of the star just ceases to exist? Your explanations are excellent, but I just can't reconcile this small point. Or is it that due to exceeding the Bekenstein limit, the matter jumps into a state of scattering that just happens to take a really long time? If that's the case, how does all that matter suddenly drop to 10^-7 K?
It just goes away. Poof. There's no reconciliation involved; it just happens.
...but... stuff doesn't blink out of existence. I'm confused :S
Okay, that's fair, we can clarify this.
Instead of stuff let's say conserved quantities. Okay? What are conserved quantities? Well, the technical answer is that any Noether charge is a conserved quantity, but broadly speaking we're talking about things like electric charge, angular momentum and (because we're talking about a local scale) total energy.
No conserved quantities are lost in this process. They all do exactly what I described above: They scatter off. It's just that this process takes a trillion years, so you won't see it finish.
But if instead, we focus not on conserved quantities but on the individual fermions involved, then yes, they just go away. Poof. Forever.
This shouldn't be surprising! Fermions go away poof forever all the time. Whenever an electron and an antielectron annihilate, they just go away poof forever. Their conserved quantities remain — mass energy becomes momentum, charge cancels out leaving no net change, and so on. But the fermions themselves? Gone wave bye-bye.
Wow. That's hard to fathom. It just flies in the face of the conservation laws.
Why? No conservation laws are violated. All the conserved quantities are still conserved.
The mass is converted to energy and is still there it is just isolated from the universe outside of the event horizon excepting the effects of that energy has on it's surrounding space (gravitation, hawking radiation, etc). Which makes it, from the outside perspective, not exist.
(warning, layman's physics)
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Thats not exactly true. In fact the core of the star, without the constant pressure from the fusion, condenses through gravity into an incredibly small point called a singularity but the matter doesn't disappear of the face of the universe, it is just compressed to incredibly small proportions but not destroyed.
You're attempting to correct an expert in the field with the "layman's physics" explanation. Before reading this I thought exactly the same thing you just described happened, but I have read enough of RobotRollCall's responses to know he or she knows what (s)he is talking about.
However, even in the simplified model you'd learn in school, anything on the inside of an event horizon would be 100% unable to interact with anything outside it ever again - so it would certainly disappear off the face of the universe by virtually any definition. Whether or not it still exists in some form doesn't change the fact that it is now no longer a part of the universe outside the event horizon.
I heard a theory a while ago, about when everything in the universe is consumed by black holes, and they only radiate photons, the universe could be considered empty, except for the radiation floating around. And as we know, radiation all travels at the speed of light. So relatively speaking, time is stopped and nothing exists, as everything is traveling at the speed of light. This could be the "what came before the universe", then in their last moments of existence, or non existence, the black hole explodes, maybe one massive black hole explodes, after billions of them have merged, creating all these exotic particles, creating a "new" universe, after trillions and trillions of years. Like a rebirth. I kind of like the idea that nothing really ends. Ideas?
Poetic, but no. None of that is compatible with, well, anything really. Sorry.
Should we be looking for the pre-WMAP photons randomly roaming the universe?
Thats an idea. But if it existed, wouldnt it be outside the "edge" of the universe.
What kind of energy output are we talking about for a late stage decaying black hole? The power output seems to grow exponentially, but how gradual is that slope?
For a few million years, will the decaying black hole go through a "star like" phase where it outputs energy of similar magnitude to a white dwarf, main sun-like star, etc? Or is the exponential increase so fast it goes from "power output of a wristwatch" to "power output of the present-day observable universe" in a fraction of a second?
Basically I'm thinking can we imagine some fanciful scenarios where some meager form of intelligent life is clinging to the dim glow of decaying singularities? Or would it behave more like a supernova, incredibly energetic, but uselessly short.
Energy is exactly conserved across the scattering. So every erg that goes in to the interaction comes back out. Eventually.
At the end, incredibly steep. It is in fact exponential, but in an interesting way.
The temperature of a black hole is an inverse function of its effective mass, right? So a large black hole — and for purposes of establishing a scale, we're going to call a stellar black hole "large"; galactic black holes are "oh my god I've never seen one that big before" — has a low temperature. On the order of ten-millionths of a degree absolute.
With a temperature that low, a black hole can only radiate very low-energy photons. There's just not enough energy in the system to radiate anything bigger than that. So while such a black hole does radiate, it does so incredibly slowly, in terms of watts per square meter of surface area. In fact, it gains effective mass, on balance, because it's colder than the universe. It gains more energy from background radiation alone than it radiates. Black holes, in other words, are being warmed by the Big Bang itself.
But eventually, after many more e-foldings, the universe will cool to the point where black holes are in thermodynamic equilibrium, and then cool further to the point where black holes start to lose effective mass. This will take many hundreds of billions of years.
When that happens, though, the black holes will still only be slightly hotter than the universe. Which means they won't radiate much. They'll just continue kicking out a few very-long-wavelength photons — photons with wavelengths on the order of the size of the solar system — per day.
But each one will reduce the black hole's effective mass by a little bit, which will increase the black hole's temperature by a little bit. Which means the black hole will radiate more.
But in order for the black hole to radiate a particle, there has to be enough energy available. The lightest fermion is the electron, at about 500 eV. But you have to make a pair in order to conserve charge. So you need about one MeV for the black hole to start radiating fermions. One MeV is 10^10 degrees absolute, which means the black hole has to go from 10^–7 degrees absolute to 10^10 degrees absolute by radiating photons alone. That process takes a long time.
But once it does, things change rapidly. Electrons carry away a lot more energy than photons can, so the temperature of the black hole climbs faster and faster. Eventually you get muons, pions, all the way up to protons, and then even heavier baryons. The rate of black-hole decay is highly nonlinear at that point, since each, say, Δ emitted carries away a thousand GeV, or 10^16 degrees absolute, all by itself! So very quickly, the black hole simply vanishes, having radiated away all of its effective mass in one big burst of particles lasting just a tiny fraction of a second.
Ok. So it is incredibly quick. It goes from "too cold to emit a single photon" to "supernova impersonation" in a fraction of a second.
Hmm...I have an idea. Perhaps I should write a paper and submit it to a journal. :P Would not such an explosion make the most conceivably precise, but also most conceivably useless standard candle imaginable?
Won't every black hole "explode" at exactly the same mass? This seems a lot like a Type Ia supernova - an event that happens at very nearly exact same energy each time. I would imagine black hole explosions would be far, far more identical than Ia supernova.
So therefore, the new method to determine cosmic distances! It's the most conceivably accurate system imaginable. If you want to know how far away something is, just observe it until the black holes in that region of space start detonating!
Of course, you'll have to wait...awhile. But as with all things, patience rewards the diligent. :P
Not exactly. It takes something on the order of a trillion years to go from emitting very weak photons, to emitting photons with higher energies, to being almost able to emit electrons.
After all those eons of slow buildup, once electrons can be emitted, that's when it suddenly ramps up. At that point the process can build on itself much more quickly and culminate in the near-instantaneous kersplosion.
So if I'm interpreting this correctly, we should be able to predict the mass at which a black hole is in equilibrium based on our measurements of the cosmic background radiation? Do you happen to know what this mass might be, assuming that some sort of estimates have been made?
"Mass." It's not mass in the sense you're probably thinking, but rather just total energy. And yeah, you can work it out. It's on the order 10^22 kilograms. About half the mass of the moon. For scale reference, a typical stellar black hole is around five times the mass of the sun.
This is something that occurs to me (A lowly former physics major with a B.S): If the ability of the blackhole to emit certain fermions is dependent on the temp, and the temp change is different for the things emitted, does that mean that the function of Power Emission V. Time is non-continuous? It seems to me that when it goes from just emitting low-wavelength radiation to emitting fermions there would be a noticeable jump in the amount of energy emitted/second. Or am I just making things up?
In the blackbody approximation, energy per time per area at temperature, it's continuous. At the fine scale, it's as continuous as any quantum process. That is to say, not at all.
Does the matter/energy that was just occupying that volume still exist? I know you said it's scattered out over time, but before it gets scattered, where is it? Is it outside the event horizon? Directly on the event horizon? If not directly on, how far away from the horizon is it?
Also, I know this was all looking at the black hole from the perspective of a far-away observer, but can it be said that the black hole does have an inside if you look at it from the point of view of someone who's just fallen past the event horizon? Or are they still technically not inside the horizon, even according to them?
Eh. I don't want to get into a big discussion of what "exists" means. All the conserved quantities are conserved; I'd just leave it at that.
If you want to imagine that sitting at the exact centre of a star about to go supernova and make a black hole is, I dunno, a chair or something, and then ask whether it's ever possible in any way to recover that chair, the answer's no. It's gone forever the instant the black hole forms.
And the thing about "someone who's just fallen past the event horizon" is that that never actually happens unless you happen to be the person who's doing it. In which case you won't have anything to say about it.
Theoretically if we could build an atom with pure energy and then use those atoms to build a person couldn't we throw a person in a black hole, wait for the black hole to dissipate its energy and capture the energy that the person added to the black hole to then reconstruct them and let them tell us about their experience.
This assuming we live until a black hole dissipates, reconstruct atoms and with relative simplicity to that reconstruct a human with those atoms.
No offense, but I think you and I have very different ideas about what the word "theoretically" means.
Energy is the same regardless of its source. You could just reconstruct the person from sunlight if you had enough of it.
The problem would be that reconstructing the person requires you to have blueprints of that person's exact structure. That would include information about the structure of their brain, which I'm pretty sure would include all the memories, etc encoded in those neurons.
Other problems aside, black holes dissipate incredibly slowly. A black hole of 1 solar mass (I know too small, but a not terrible reference figure) would take 2e+67 years to dissipate from initial formation.
I have read all of your comments in this thread, and my understanding of black holes has been completely revolutionized.
I'm an undergraduate; I've studied some basic topology and I knew that general relativity makes statements about the curvature of space, but for some reason I never made the connection to see black holes as things of topology.
Thank you.
Edit: If you have time for another question or two, can you tell me if my intuition here is more-or-less accurate? http://www.reddit.com/r/askscience/comments/j81b2/whats_in_a_black_hole/c2a1v85
Also, how do you feel about fuzzballs? Are they a plausible alternative explanation? Do they contradict the idea that the inside of a black hole doesn't spatially exist?
Well, general relativity is the beginning of understanding black holes, really. We can say that just as Newtonian gravity is an appropriate weak-field limit of general relativity, in a sense — I'm not being mathematically rigorous here, just making an analogy — general relativity is a weak-field limit when it comes to black holes.
General relativity, for example, tells you what the geometry — not topology, but geometry — of spacetime is like around a black hole. It tells you that if you consider a notional point we'll call the centre of the black hole, there must exist an event horizon that lies in a sphere at distance 2M from that point, where M is the effective mass of the black hole in units of length.
It can even tell you some basic things about what that event horizon really is. For instance, it can tell you that to an observer at infinity, any interval of proper time exactly at the horizon will be dilated to infinity, and any light emitted exactly from the horizon will be redshifted to infinity. Which is interesting enough, in a sense.
But those conclusions beg the questions of whether there can ever be a clock exactly at the event horizon, or whether there can ever be a charged particle to emit light exact at the event horizon. In order to understand those questions, you have to start contemplating how matter and fields behave in the curved spacetime around black holes.
Meanwhile, general relativity, naively applied, tells you something about black holes that actually isn't true. If you just examine the problem through the lens of general relativity, you find that black holes violate information conservation. Say you take a lump of iron — which has entropy — and drop it into a black hole. The lump of iron vanishes from observability before it gets to the event horizon — infinite time dilation and infinite redshift in the flat-space frame, remember — but we know from general relativity that the lump of iron doesn't just, like, stop. It eventually, after infinite time in the flat-space frame, reaches the event horizon, passes through it as if it weren't there, and then … well, yes. And so on. Bottom line, it never gets to come back out.
Which means you took some entropy, dropped it into a black hole, and decreased the total entropy of the universe. Which is, as one might say, a bit of a bloody problem!
Resolving this problem means you have to go beyond general relativity. General relativity tells us about geometry, which is fine. General relativity has nothing to say, however, about what matter and fields actually do. General relativity, by itself, can't tell us that black holes must have entropy, or that they must have a temperature, or that they must radiate. Nor can it tell us anything about the mechanism through which that radiation occurs. For that, we need quantum field theory, and more's the point we need to apply quantum field theory to curved spacetime.
It's only the synthesis of these two different ways of looking at the universe — general relativity is geometry, quantum field theory is stuff — that can fully describe black holes, their dynamics and their time evolution.
As for fuzzballs, superstring theory isn't my area of interest. But last I heard, that model is basically equivalent to all the other quantum models of black holes. They all yield up the same basic predictions — yes, black holes radiate; yes, that radiation has a thermal profile with corrections for information conservation; and so on — but differ only in details. It's entirely possible that someday, probably long after I'm dead and gone, we'll have the One True Model of black holes. But it's far more likely, if you asked me, that we'll instead have a variety of models which are fundamentally different mathematically, and make different predictions at the trans-Planckian scale, but that all reduce to the same predictions at observable scales. Which will be fine.
And this is why I love you.
Also, I read this whole post in the voice of Dave Tennant.
A very interesting read. One question:
Just to clarify. When you say they have no insides, are you referring to the single point right at the centre of the action. Or do you mean something else? I would imagine that if you fell into the event horizon of a black hole, there would still be some space for you to fall through before... whatever happens next.
So here's a sphere, right? I'm just talking about a volume of space, bounded by some arbitrary boundary we just made up. It has a centre, and we can define it mathematically as the set of all points for which r ≤ R, where r is the distance from that point to the centre, and R is the distance from that point to the boundary.
From a great distance — technically, from infinity, but that's just a mathematical tool we use in our models — a black hole looks like what I just described.
But it isn't. There's no sphere, no points inside it, none of that. It's an isn't. It's not.
Woah. I've read all about black holes and thought I had a 'firm enough' grasp on what's happening "in" them, but after reading your responses, this one in particular, my understanding has shifted.
I've always envisioned it as a very massive clump of matter compressed to a singularity which then causes spacetime to curve infinitely around it...creating a 'pit' so to speak where clump of matter sits at the bottom collecting additional mass as things fall into it.
But what you're saying is that the birth of the black hole actually converts the source matter into radiation and what's left is basically an energy phenomenon. If additional matter approaches it, it's shredded into radiation which then adds to the net energy of the 'hole', but nothing actually "falls in" because there is no hole...there's no pit. There's nothing there at all.
fascinating.
Well yes, but that's hardly surprising, is it? That same basic thing happens constantly. Energetic photons (which aren't matter) decay into electron-antielectron pairs (which are) which then annihilate releasing photons again (which aren't). Hit a pion hard enough, and the quark and antiquark will separate until there's enough potential energy in the gluon string between them to condense another quark-antiquark pair out of the vacuum. Matter appears and disappears from the universe all the time, all over the place.
Are you telling me that the effects of the singularity, such as the event horizon shouldn't be considered as something real. I hope so, otherwise I'm hopelessly lost.
I don't know how to answer that question. "Real" doesn't have a single, objective meaning in theoretical physics. The process we're talking about happens. Matter and energy interact with a black hole, there's a scattering process, energy and eventually matter come out of that scattering process. That happens; that's real. Whether anything else is "real" depends entirely on how you're defining the word.
If I understand correctly, what you are saying is that a black hole has no inside. But at the same time, you seem to imply that a black hole has a volume that's defined by an event horizon, which should occupy an spherical space.
We end up with an area of space that has nothing in it but with infinite density. It's fascinating, but many people would find that answer unsatisfying.
Let's revisit what "a black hole has no inside" means. It doesn't mean "there's nothing inside a black hole." It doesn't mean "a black hole is empty." It means a black hole has no inside. From a distance, it looks like a spherical volume, but in fact it isn't. It's just a surface.
Given the current extent of physics and science if there is an 'inside' to a black hole it is entirely unknowable? Or that space-time simply doesn't exist in the volume one assumes would exist inside a spherical surface?
Nooooo … why would you think that's the case?
Like if you cut a hole in a sheet of graph paper?
Yes, exactly. It's completely unlike that in every way.
So in terms of space-time the black hole is a "volume" of space that has been completely removed?
Also; where does this leave the notion of the singularity being this pinpoint center in which all the matter/energy is absorbed into? Has this theory gone by the wayside?
Second also (if I am interpreting this all correctly); how does this relate to the entropy of a black hole? If the black hole itself does not absorb any energy and in fact scatters it how does the "volume" and overall entropy of the system grow?
Remember when I said metaphors fail? No, it's not a volume that's been removed. It is what it is.
As for the entropy of a black hole, it's exactly what you'd think it is. It's the entropy of the system. In fact, nature is so delightfully constructed that the entropy of a black hole is exactly equal to its surface area. Modulo some multiplicative constant, which I think is four if I remember off the top of my head.
To make it even more explicit, I believe RobotRollCall is saying that there actually isn't any space on the "inside" of a black hole.
I haven't studied general relativity yet, but from my understanding, GR is all about how matter/energy/stress-tensor-whatever causes space to curve. Here, the space curves so much that it... becomes disconnected.
Edit: Changed "matter" to "matter/energy/stress-tensor-whatever" :P Also adding: http://www.greatdreams.com/solar/magnetic-north-pole.gif If you made a coordinate system around a black hole, it'd look like the lines on that map, with the longitude-like lines just ending at the event horizon. But there'd be a bazillion more latitude-like lines getting asymptotically closer to the event horizon.
Beautiful and illuminating explanation. Thank you.
another neat one ;)
That is the best explanation I've ever received... of anything.
I wish I could give you more up votes for the explanation. Thanks!
This can't be right. For a supermassive black hole the tidal force at the event horizon is not strong enough to kill a human, much less disintegrate one into fundamental field excitations.
Although a far off observer will never see the plunge through the event horizon, it still takes place.
Even with the idea of black hole evaporation taken into account, an object will still plunge into the hole in a finite amount of time. The far away observer will "see" the object fall in at the same time the hole evaporates.
It's not tidal stress. It's technically the thermal properties of the stretched horizon. But that gets into a level of detail that I don't think is appropriate in an Internet forum.
Ok, looking up stretched horizon I am guessing that you are talking about Susskind's membrane model or something similar.
However, when I read Susskind's work, he does not say that an infalling observer of a supermassive black hole is blasted apart, he says quite the opposite:
http://arxiv.org/abs/hep-th/9306069
This meshes better with what I understand from GR. Granted, I am not an expert in black hole thermodynamics, but I cannot find anything to confirm your description of the event horizion.
If you don't think that this is appropriate for an internet forum, can you at least provide a reasonable review article?
I'm sure you won't have trouble finding what you're looking for in the literature.
Not sure if it's a very bad analogy, or what, but the everything certainly does not vanish upon collapsing into a black hole. And the event horizon is certainly not the edge of this sphere, not even remotely close. It's the point at which you would need to exceed c to escape the gravitational pull .
How black holes work
Is this pessimistic thinking? There's no way of escaping this fate, even with unforeseeable future advances in science/technology?
I'm not sure you're getting the scales involved here. We're talking hundreds of billions of years before anything interesting starts to happen at all. Our entire solar system will be long gone by then.
Well then what's the fucking point in dreaming of the future? How am I supposed to live knowing that nothing matters in the grand scheme of things? If energy can't be created or destroyed, can't it somehow be recycled in the future?
I have a pet. Obviously.
Welcome to the human condition. Enjoy your horribly and unfairly brief stay.
The meaning of life is purely and utterly subjective. Why can't you be content with the fact that you have been blessed with this amazingly unlikely ability to exist at all, especially in a universe that is so mind bogglingly vast and beautiful? I find the whole reality that we are tiny, insignificant specks of dust who's importance converges to zero in the grand scheme of the universe poetic, humbling, and reassuring because I know that, no matter how badly I fuck up, or how badly my government fucks up, or how many people suffer and die (including myself), or how many regrets I have, in the end, nothing really matters.
We're born, we live, we die, and in the end, we all experience a non-existence of peace and tranquility that is neither eternal nor instantaneous, but, at least, we are released from the torment that is the human condition.
Do something you like doing, enjoy it. Not much else matters.
I don't see it as pessimistic thinking. Absolutely everything has a beginning and an end. Even if the human species manages to go as far as somehow make it to another galaxy after turning ours into an oxygen-breathing, carbon based life form's paradise, odds are we won't be around when the universe starts winding down. Its just the nature of things.
And regarding your existential crisis - I find that when you start regarding the time after your death in the same way you have always regarded time before you were born, it doesn't get so depressing - if anything you start appreciating life even more.
You live in a time that we as a species can look back to the dawn of the universe and even now have a discussion about the inevitable events in a hundred billion years time.
New York Professor Joseph Campbell in 1961 talking about the history of the various theories of the Universe presented by Religion, vs. what was now discovered by Science:
TRACK 7: Science: the Continual Quest
But now when one says truth as a scientist one is being sentimental, because really the wonderful thing and the great challenge of the scientific revelation is that science itself does not pretend to be true. It does not pretend to be final.
It is simply an organization of working hypotheses—hypotheses that seem to take account of the facts, as we now know them. But is there any intention to stay with these facts? No. There is a continuous quest for more, as though one were eager to grow as though the life of man, and of society here, were to be based on new things, on change, on transformation, rather than on petrifaction and rigidity. And so we don’t know anything. And even science itself is not the truth; it is only, so to say, an eagerness for the truth no matter where it may lead.
And so here again we have a still greater revelation than that of anything the old texts have to say; the old texts comfort us with horizons.
They tell us that a loving, a kind, a just father is out there. According to the scientific view nobody knows what is out there, or if there is any out there at all. There is just a display of things that our senses bring to us, and we are dealing with those. But what lies beyond is an ultimate mystery, and it is a mystery that is so great that it is going to be inexhaustible in its revelations. And man has to be great enough to receive it.
There is no thou shalt anymore. There is nothing you have to believe, nothing you have to do.
And if you want to play the game of the Middle Ages, go ahead, but don’t tell anybody else that that’s the only game there is. Or if you want to play the game of the Chinese Mandate of Heaven, go ahead—those are all lovely games. And the scientific game, after all, may not be any truer, but it is vaster, and it takes in a bigger range of facts and experience.
So it is this terrific moment that we face; it is a moment that has been maturing so to say since the days of the Greeks. And since the days of the Second World War, it has gone through all the planet.
Get in your flying car and go to another solar system.
Well, if the proton isn't stable, then no, even if we could survive the timescales RRC is talking about, we wouldn't be able to survive the eventual death of the universe.
Unless we somehow become beings of pure somethingness that isn't connected in any way shape or form to the structure of the universe. But that's a physical impossibility.
It's not pessimistic at all. It's just the reality of the situation.
Any time something does anything, it does so by converting potential energy into some form of kinetic energy. It does that by transferring energy from a high energy density state to a low energy density state. This can take any number of form. This can mean using the energy difference between a positively charged wire and a negatively charged wire. This can be generated by using the energy difference between high pressure, high temperature steam and low temperature, low pressure atmosphere. This can be generated by using the energy difference between low energy carbon dioxide and high energy coal and oxygen. This can be generated by using the difference between low temperature leaves and high energy sunlight. This can be generated by using the difference between low energy helium and high energy hydrogen.
In the future, we'll have burned all the coal. We'll have burned all the uranium and thorium. We'll have burned all of the hydrogen. We'll have burned all the helium, all the carbon, all the neon, all the everything until there's nothing left but iron-56. There will be no areas of high energy, nor any areas of low energy. There will be no energy gradients to sustain life, or any other activity of any kind.
'Unforseeable [sic] future advances in science/technology' to escape this issue would require breaking the first or second laws of thermodynamics. It's not just unforeseeable, it's theoretically impossible.
Further reading.
When did the transition from the theory that black holes had a singularity and event horizons that could be passed by matter into this theory of non-existance? Or is it two interpretations of the same mathematics? I just seem to remember a lot of reputible sources trying to model a descent into a black hole and it's anatomy as a single point source with very high gravitational fields inside the event horizon. I even think I remember you describing it as such in the past of r/askscience. Just seems strange to have such a shift.
Okay, first of all let's please not say "this theory of non-existence." That's way too misleading. And besides, I did not tell you about any theories. I just told you a story that's fairly reminiscent of the truth. If you want theories, go do the maths.
And second, the answer is, depending on who you ask, anywhere from about thirty years ago to any-minute-now.
Well, that gets into complementarity, which I decline to get into here. It confuses a lot of people who aren't immersed in the subject. Ultimately it boils down not to all the fiddly little details, but rather to "What does an essentially educated person need to know about black holes?"
Can you reconsider talking about complementarity? The wikipedia article is unhelpful, and I believe it's still true that, from the falling-astronaut FoR, nothing too exciting happens around the event horizon. If you don't set me straight I'm going to assume the astronaut goes infinity into the future and time starts on the other side of the event horizon and infinity + 1
What a gorgeous piece of literature. RobotRollCall, you are a rare, exceptional human being, a scientist who knows how language works.
I apologize for the late question, but I've always thought about black holes as if you cut a circle out of some fabric and sowed the edge together at one point. Is this a fairly good mental analogy?
Blew my goddamn mind.
Blew my goddamn load. Best nerdgasm I ever had.
You beat me to it....
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There, fixed it for you.