Stephen Hawking: 'There are no black holes'
nature.com
nature.com
How scientific.
there are no black holes with event horizons
If you take the mass of a sun and squeeze it into a few miles wide, you are definitely going to get inescapable gravity no matter what you want to call it.
Now, it may be fair to label this as sensationalism anyway. The source here is a brief, conceptual talk by Hawking at a conference (and an associated writeup): it's not as if he's proven a theorem (as he did years ago when he first demonstrated Hawking radiation, for instance). But honestly, it may be newsworthy that Hawking is even considering this sort of resolution to the current "firewall" debate. It would be one heck of a change in perspective!
the event horizon is frequently treated as "uncrossable" from inside to outside which is really different from "inescapable". The later means that whatever speed you have on or below the event horizon, you can't reach infinity. The former is just an impression by an outside observer because in his observation the time has stopped on the event horizon, while a stone thrown up from below the event horizon would cross the event horizon just fine on the way up and on the way down and would return back down successfully in its proper time (the point of "inescapability" is that the stone would always return). While above the horizon, the stone can interact with other stuff there and result of this interaction can be observed outside (doesn't mean on practice by us today or tomorrow :).
Many models seems to treat the event horizon as "uncrossable". For example quantum information disappearance - matter goes in, evaporates as Hawking. Yet, just for example, when Hawking radiation decreases the mass of the black hole it causes the shrink of the event horizon and thus whatever "stones"/photons on their way up were stuck (for external observer) in the stopped time of the horizon become free - doesn't mean though that we can observe them on practice as getting out of that gravitational well does take time (again in our time) and redshifts them into oblivion. Like proverbial "the check is in the mail". It is the reason why we can't really observe Hawking radiation until we develop technology to observe light with extremely long wavelength and i just don't have the numbers right now on whether 13B years is enough for the radiation originating right above the event horizon to get out of that well and reach the interstellar space.
This doesn't agree with my understanding of GR. A stone "thrown up" from within a black hole interior cannot cross the event horizon in any reference frame - it cannot even get closer to it.
Look at the future light cones within the black hole interior, e.g. in the illustration at http://en.wikipedia.org/wiki/Eddington–Finkelstein_coordinat.... The future light cone of every event within the EH is skewed so far that even light rays directed outwards are drawn closer to the singularity.
The Schwarzschild radius of the mass of the observable Universe is 10B light years. So, several billions years ago, when observable Universe had 10B radius, it would be a black hole (though i think that in less expanded space of the earlier Universe the constants like "c" had different values and thus that Schwarzschild radius was less). We can imagine it in another way - increase 125 times (the observable Universe has 40+B light years radius) the amount of matter, ie. galaxies, stars, etc... inside the 10B radius ball around us, and you'd get the black hole with 10B light years Schwarzschild radius (and i don't think we would ever notice the change - only with time the galaxies's movement will be affected). Obviously, the gravitational field on the surface of that imaginary 10B radius sphere and inside it would increase somewhat (like 125 times on the surface, 125 times 0 pretty much 0) - not even close though to any values to affect space curvature or to prevent anything from crossing it from inside. Of course, anything that would cross it from inside would return back eventually.
The Schwarzschild radius does have the property that, if you plug it into the classical equation for escape velocity, you get the speed of light. That doesn't mean arbitrary classical analogies (like the "thrown stone" example) hold.
In particular, consider that you can escape the earth while never achieving escape velocity. Just keep firing your rockets to counteract gravity. But if this were possible with black holes, then they would not be very interesting!
> The various local effects like time dilation, light path curving, etc... are defined by the value of gravitation force, not gravitational potential.
These are not local effects! Locally, there is no time dilation, and no curvature of light. I see my watch tick at the same rate, no matter where I am, because my watch and my eyes are in the same local reference frame.
In order to see effects like time dilation or curvature of light, you must compare events separated in spacetime. For example, we can look at the paths of light emitted by distant stars. And since the light had to get to our eyes, we have to account for the entirety of the path that it took. So the time dilation I observe for an event depends on the entirety of spacetime along the path from the event to me, not just the local curvature for the event.
In mathematical language, if you want to move a vector from one point to another on a curved manifold, you can't just specify the start and end point. The path you take affects the resulting vector.
> The potential defines the fact that anything originating at or below the horizon would never escape completely the gravitational field, i.e. never reach the infinity.
So objects originating outside the event horizon reach infinity in a finite time? Huh?
No, it can't; the "acceleration due to gravity", which is the acceleration required to "hover" at a constant altitude above the horizon, diverges as the horizon is approached.
What can be made as small as desired by making the hole's mass large enough is tidal gravity at the horizon.
several billions years ago, when observable Universe had 10B radius, it would be a black hole
No, it wouldn't. A black hole is a stationary spacetime. The spacetime of the universe is not stationary. The spacetime model that describes the universe is very different from the spacetime model that describes a black hole; the fact that you can plug the mass of the observable universe into the Schwarzschild radius formula and get a number out does not mean that number has any physical meaning for the universe.
exactly. There is no stationary spacetime in our Universe. Black hole is pretty artificial model where pure mathematical artifacts of singularity at the horizon is taken for the real thing.
> the fact that you can plug the mass of the observable universe into the Schwarzschild radius formula and get a number out does not mean that number has any physical meaning for the universe.
Taking a big chunk of space and calculating escape speed from its gravitational field - how is that not a physical meaning? At what specific size of the chunk you think it becomes not meaningful?
The universe as a whole is not stationary, nor even close to being so; but portions of the universe are very close to being stationary. The solar system, for example. Black holes do not have to be exactly stationary; if they are as close to being stationary as the solar system, that's plenty close enough.
Black hole is pretty artificial model
The exactly spherically symmetric solution is an idealization, yes; but there are plenty of numerical simulations that show that non-symmetric spacetimes still form event horizons.
where pure mathematical artifacts of singularity at the horizon is taken for the real thing.
There is no physical singularity at the horizon. Some coordinate charts have a coordinate singularity at the horizon, but that's easily fixed by just using a different coordinate chart. The only physical singularity is at r = 0.
Taking a big chunk of space and calculating escape speed from its gravitational field
Escape to where? You can't escape from the universe as a whole. The concept of "escape speed" has no meaning for the universe as a whole.
take a 1B light years radius ball, populate it with density of our Milky Way - that ball will have 1B Schwarzschild radius. Calculation of gravitational potential (escape speed) from it to the rest of the Universe makes sense, doesn't it?. And due to this gravitational potential it will be bona fide black hole from the point of view of the rest of the Universe.
Yes, but that's not the same as escaping from the universe as a whole.
And due to this gravitational potential it will be bona fide black hole from the point of view of the rest of the Universe.
So what? What does that have to do with assigning a gravitational potential to the universe as a whole?
Nor mine. You're correct, any object in the interior of the hole, even one that is moving radially outward at the speed of light, gets closer to the singularity (i.e., its r coordinate decreases) with time, which means it can't get closer to the horizon (that would require increasing r).
I always wondered if a black hole could become completely visible again if it collected enough mass to spread its surface area out so that gravity would become thin enough for light to escape again. Kind of like how only the surface of a pond sometimes freezes.
(Of course, singularities don't make any sense - which is part of what Hawking's talking about. But that's the classical model of a black hole)
One certainty is that the space inside a black hole is bigger than the size of the black hole observed from outside of it. So it's entirely possible that you could have a solar system or even an entire universe on the inside. According to GR the space inside a black hole should pretty much behave like the rest of the universe, as long as the black hole is growing. As for "not getting out", well it's impossible to approach the edge of the universe too, despite that border having a physical location. But it's expanding at light speed, so ... no way to get there.
So density of a black hole is an entirely different concept from the outside and the inside. Seen from the outside it is infinite. Seen from the inside it is probably not infinite.
No, it doesn't. There is no edge to the universe. Our best current model is that the universe is spatially finite, but even if it turns out to be spatially finite, it is unbounded--the spatially finite model is a 3-sphere; like the surface of the Earth but with one more dimension. The surface of the Earth is finite, but it has no edge; a spatially finite universe would be the same way but with one more dimension.
Seen from the outside it is infinite.
No, it isn't. Seen from the outside, the hole has a finite mass in a finite volume, so its density is finite. (The finite volume an outside observer would assign, based on the surface area of the hole's horizon, is not the "actual" volume of the hole; but it works well enough as an "effective volume", the volume that the hole occupies from the standpoint of the rest of the universe.) The singularity can be viewed as infinitely dense, but the singularity does not occupy the entire black hole; it only occupies the center at r = 0.
That is not a certainty at all. The entire premise of Hawking's paper is that the object we call a 'black hole' is just a really degenerate dense lump that slowly turns incoming matter/energy into background radiation with as much entropy as possible. And that's it. No fancy Time Lord bigger-on-the-inside science proposed here.
It's pretty neat, actually: if you had a black hole containing the mass of a whole galaxy (which isn't unthinkable, especially in the distant future), local space-time at the event horizon would seem completely ordinary (at least as far as we understand from classical relativity and semi-classical quantum corrections like Hawking radiation). You wouldn't have the foggiest idea that you had crossed that fateful line... until you caught on to the fact that you weren't actually making any progress when you tried to turn back.
I am not a point mass. If I can cross the event horizon and not know it, then there is a time when my feet are on one side and my head on the other. If signals cannot cross the event, how would I not notice my missing feet? If signals from my doomed feet can reach my not yet doomed head, then how is it the event horizon?
Now, if you were wearing some sort of rocket collar that fired powerfully just before your head crossed the horizon and carried it safely away from the black hole (or even just held it there), you're right: you wouldn't ever receive that signal from your feet. But I'm pretty sure that would be the least of your worries! (And you would notice your missing feet. And torso.) Roughly speaking, the only non-orbital trajectories close to an event horizon that don't fall straight in must be moving pretty close to light speed as seen by infalling observers.
It's wrong. Time dilation is not something you observe yourself to experience; it's something another observer observes you to be experiencing. "Time" is relative, so you can feel your own time to be ticking away perfectly normally while it appears to be ticking much more slowly to someone else.
As your feet approach the event horizon, the tingling will get slower and slower, because time passes much faster for your head than for your feet.
No, this is not correct. As you fall toward and through the horizon, signals travel between your head and your feet just fine, and you don't notice any difference in how fast they travel. But, as I posted upthread, if your foot emits a signal towards your head when it's below the horizon and your head is above it, then by the time the signal reaches your head, your head will have fallen below the horizon.
This is a little bit weird, I know!
So in your example, for a large black hole, you will experience no difference in the rate of tingling as you cross the event horizon. This is what the principle of relativity tells us: you are in free fall, so the laws of physics must appear the same to you.
If your feet are below the horizon and your head is above it, then you must be falling through the horizon; it's impossible to remain at a constant altitude at the horizon, even for an instant. So a signal emitted by your foot would still reach your head; but by the time it reaches your head, your head will have fallen below the horizon.
Basically, you can change your reference point and consider that the signal is not moving, but your head is still moving towards the signal. This way, you're still seeing your legs. (Note that I have no idea what I'm talking about)
BTW, it's not hard to convince yourself that masses and springs in a centrifuge behave exactly like those near a large body. (In Newtonian mechanics even.) How are you sure you know the difference?
But the horizon isn't a point; it is in fact a surface (a 3-surface in 4-dimensional spacetime). Whether you want to call that the "surface" of the black hole is a matter of terminology; the hole is not a solid object--it's not really an "object" at all in the ordinary sense of the term, so it doesn't have a "surface" in the ordinary sense of the term.
3.8 miles (1.9 mile Schwarzchild radius), to be a bit more precise.
'No black holes' is good enough for me. Now, Hawking's been chipping away at these ideas for a long time - remember his essay 'black holes ain't so black' - but this is indeed a radical departure. Cue fooljhardy adventurers dashing off in search of black holes and an outbreak of highly similar sci-fi plots. /opens Final Draft
Astronomical. Astrological would suggest it depends which star sign the masses are in.
Neutron stars are in a sense "failed" black holes because of not enough mass. I wonder if they could eat more mass and achieve that status though.
http://en.wikipedia.org/wiki/Tolman–Oppenheimer–Volkoff_limi...
https://news.ycombinator.com/item?id=7104056
Basically it's unknown if it is a two star system where the neutron consumes the partner, or a three star system where two neutron stars collide head on.
The article being referred to seems to be a short piece where he summarizes his points and puts forward a (new?) idea. Maybe it's obvious to researchers in the field, but I don't see much argument or reasoning supporting his claim. It seems like he's just introducing a new idea for people to consider (brainstorming with the community).
So please don't blow this out of proportion.
This firewall issue is just complicated, and it has stubbornly resisted consensus from the physics community despite a year or so of intense interest. I think we're in the market for crazy new ideas at this point.
If you right down the equations for a simple black hole you immediately encounter a singularity at the event horizon, but this is swept under the rug by making a coordinate change. The trouble is that coordinate change breaks the warrantee on the space-time continuum because it will stretch a plank length out to something macroscopic. making quantum phenomena visible.
A simpler paradox which has never been taken seriously is that an outside observer sees that it takes forever for something falling into a black hole to hit the event horizon. However, knowing about Hawking radiation, you know the black hole doesn't last forever, so an outside observer never sees anything fall into a black hole.
Thus the classical black hole was always a pipe dream. In the 1970s the information paradox showed that a classical black hole (which makes information disappear because you can't tell the difference between a black hole that was made out of gold bars or feathers) makes information disappear, but information can't disappear in a quantum mechanical universe.
The quickest way to counter your claims of suppression, I think, is to do a Google Scholar search for a term like "black hole information paradox". No, we don't entirely understand what's going on there, but figuring it out has been one of the most active and fruitful areas of research in relativity and high energy physics for decades.
As for your "breaks the warranty on the space-time continuum" claim, I'd love to know why you're choosing the "spherical coordinates" coordinate system as the fundamental one (so that we should judge coordinate stretches as "too big" relative to its measure). Not that I like the idea of specifying one reference frame as "preferred" at all, but if I had to, the only natural choice would be the reference frame of a freely falling observer. And that's precisely the coordinate system we typically change to in order to show that there's no essential singularity at the event horizon. So please don't lob insults at the active physics community based on your own misunderstanding of the theories that they spend their lives striving to understand.
[Credentials: I'm a professional physicist: a professor specializing in string theory with a background in general relativity.]
> [Credentials: I'm a professional physicist: a professor specializing in string theory with a background in general relativity.]
That's my one of my biggest problems with physics (other is Copenhagen interpretation of quantum mechanics). As I understand it in our frame of reference, nothing ever (before present moment) crossed any event horizon because of gravitational time dilatation.
When I tried to solicit people on the internet to help me understand it they said that in general relativity theory you can draw line of simultaneity arbitrarily and that causes "before" and "after" to not make sense for two distant points.
I can imagine that's true (although I'm doubtful), after all in classical theory simultaneity is straight line orthogonal to time, in special relativity it's straight line but slanted dependent on speed of reference frame. I was imagining simultaneity line in general relativity theory as a well defined curve dependent on speed of reference frame and mass distribution but I can also imagine that it's not defined in unambiguous way at all. I am yet to read some actual scientific material about defining simultaneity in GRT and how it relates to gravitational time dilatation that seem pretty well defined to me.
If you could recommend some reading material about simultaneity in GRT (or some more general concept that gets reduced to simultaneity in universe without mass) I'd be most grateful.
It doesn't have to be too simple. I was International Physics Olympiad national level laureate and I'm pretty motivated to understand that as it costed me some sleepless nights over the last ten years or so.
I've got to concur with the other people you've heard from: simultaneity in classical (Galilean) physics is the same for every observer, simultaneity in Special Relativity is different for observers moving with different velocities, and simultaneity in General Relativity often isn't even globally well-defined for one observer at a time. That's not too surprising given other features of GR. You've no doubt heard about "gravitational lensing", where a large mass curves space so that "straight" light rays from the same distant object appear to come from two different directions. Now imagine the same thing happening to a "line of simultaneity", and you'll start to understand the issue.
I suspect that one concept that might satisfy a part of what you're looking for as a "generalization of simultaneity" is a Cauchy surface or Cauchy hypersurface: http://en.wikipedia.org/wiki/Cauchy_surface This idea, essentially, is an arbitrary (possibly curved but everywhere spacelike) plane that divides time into "future" and "past", and that can serve as an "initial value surface". Have a look at that article for more information, and see if it's something like what you're looking for.
[Also: I don't like the Copenhagen interpretation, either. I've become more of a "many worlds"/Everett person ever since a fantastic and highly mathematical quantum course in grad school, despite the somewhat uncomfortable philosophical issues with it.]
I guess my struggle now is about how to reconcile the fact that simultaneity in General Relativity often isn't even globally well-defined for one observer at a time with the pretty well defined gravitational time dilatation. How can one say that time flows slower in higher absolute gravitational potential then somewhere else without being able to unambiguously say what's before and what's after at those two points.
Cauchy surface at least from wiki description doesn't sound much like a simultaneity line/surface/subspace. Is it dependent on the speed of the observer? I'm looking for some concept that in universe without mass reduces to SRT simultaneity. It might be twisted, branching, looping as in you lensing example but smoothly transitioning towards SRT simultaneity as mass goes to zero. Also time distance at two remote points between such surfaces should be reflect value of gravitational time dilatation between those two points. Is there something like that?
This comment does nothing but discredit you and make you look like the Internet crank you seem to want to be. I already think you don't have the slightest clue what you're talking about.
Quantum entanglement in summary is that two or more particles are such that their quantum state relates them to each other, correct? In the big bang, all matter existed at a single point in "time", but we are confused about why there is more matter than anti-matter, correct?
Would it be at all possible that perhaps at the big bang or prior, the universe was more balanced in it's symmetry or perhaps even anti-matter existed in greater quantities than matter, but at some point there was (again, forgive my lack of physics knowledge) some sort of a collapse or antimatter blackhole which generated the big bang, and as antimatter collapsed in on itself and was spewed out the other side it became matter, with a side effect of quantum entanglement (since the matter (previously anti-matter) had existed at the same point in time (or some groups of matter had at least)). Now here is the key part, what if anti-matter isn't the opposite of matter, but it is indeed the preferred state of matter (eg, matter seeks to become anti-matter) but the only way that it can do so is to mass together until it becomes a black hole and collapses back into the anti-matter universe? (which would explain the lack of anti-matter where masses of matter exist)
Crazy lunatic layperson hypotheses: 1) Matter can become anti-matter and vice versa. 2) There is a natural tendency for balance in the universe. 3) Black holes are where matter goes to become antimatter. 4) White holes are anti-matter becoming matter 5) Gravity and many other observable effects are side effects of this process.
I was always wondering if the fact, that a lot of neutrinos (not anti-neutrinos) travel constantly through earth and all the experimental equipment, was properly ruled out as a possible cause of observed matter anti-matter asymmetry in those experiments.
To make sure I understand, would it be correct to say that relative to us, the time dilation of any massive collapsing star means that it would never even collapse to a black hole in the first place because it would take infinitely long, relative to us(?)
Disclaimer: I am just a physics enthusiastic, take that with a pinch of salt.
This reminds me of a competing theory that states black holes (as currently understood) can't exist: Instead they are Magnetospheric eternally collapsing objects http://en.wikipedia.org/wiki/Magnetospheric_eternally_collap...
That's far more interesting to me than the idea that the "event horizon" is gone.
Say that your leg is inside the black hole, but the rest of you isn't. How can the atoms just outside the horizon detect the quantum forces from the atoms just inside? Electromagnetic forces are transmitted by photons, which are forbidden to cross this barrier.
Thus, if you pulled back away from the horizon, you would have an event-horizon-curvature-shaped hole where your lower leg had just been.
Similarly, for any matter traveling through the black hole, unless two atoms and all their electrons passed through the horizon simultaneously, no chemical bond could survive (since from the point of view of one atom, the other "blinks" out of existence.)
Thus, anything that crosses an event horizon must be fundamental particle mush, no? Assuming gluons were also forbidden to cross back, even protons and neutrons would be torn apart.
[1] I mean one around a very massive black hole - so that spaghettification is yet to occur.
P.S. - This argument takes a very literal interpretation of fundamental particles; indeed, it would be interesting to test the quantum-ness of matter to see how this worked - i.e. whether "stuff" really is particles or waves - since waves could, potentially, exist on both sides of the barrier at once.
You wouldn't have to travel directly into the signularity, if that helps clarify things. You could orbit around it for a while and your orbit would decay in a spiral pattern.
This excerpt from Wikipedia might be helpful: https://mediacru.sh/rd-9YJEtJRGo
I would expect that you'd be torn apart by tidal forces before you reached the event horizon anyway.
If we substitute for a machine? Could you, perhaps, build a strong enough spaceship to dip in close enough to dangle something passed the horizon, and then accelerate away again? I don't suppose you'd be able to measure anything from the resulting object, even if you could get it back...
Even if you couldn't, I still find it implausible that even large objects would travel across the even horizon effectively instantaneously.
Large objects in free fall wouldn't travel across the event horizon instantaneously. It's just that by the time a signal from the front of the object reached the back, the back of the object would have already crossed the event horizon.
Additionally, you still haven't addressed the issue of atoms, covalently bonded, either side of the horizon (and not in free fall downwards, but being pulled from a rocket above so as to be slower than that.)
The same would go for your legs. Event horizon is an abstract limit at which not even light can escape. __not even light__. I guess that the point of no escape for your rocket, no matter how powerful it was, was far, far more.
What happens if a contemporary earth rocket would go into orbit around Jupiter? At some level it couldn't get away with its engines. That would be the point of no return. Nothing inherently bad happens, except there is no way to go but down.
(PS: I think that advanced material you're referring to is also known as Unobtainium :)
In your frame of reference, you never pass through the event horizon. As you get closer, the event horizon seems to recede. A false event horizon seems to arise around and behind you. You will eventually reach the center, in a finite time - in your frame of reference.
Of course, in my frame of reference, something very different happens. As you approach what I think is the event horizon, you slow down more, and more, and more, until you get frozen forever in molasses of space-time.
This can only be true for a very short time; if your leg is inside the horizon, you must be falling through it (if you're not, your body is being torn apart). So your leg can send a signal to your head just fine; by the time it reaches your head, your head will have fallen through the horizon. Similar remarks apply to atoms, chemical bonds, etc.
No, it doesn't. The proper time (time by the infalling observer's clock) that it takes to fall to the horizon is finite.
their speed will approach the speed of light
This isn't correct either. The infaller will see the horizon pass him at the speed of light, but that's because the horizon is a lightlike surface--it is moving outward at the speed of light.
EDIT: OK, maybe it is, but from which observation point? An external observer may never see someone cross the horizon, but their local time would keep on ticking. Time always seems normal to you, remember, just other peoples' goes crazy in highly relativistic situations.
>Einstein denied several times that black holes could form. In 1939 he published a paper that argues that a star collapsing would spin faster and faster, spinning at the speed of light with infinite energy well before the point where it is about to collapse into a black hole.
http://en.wikipedia.org/wiki/Einstein's_unsuccessful_investi...
I never understand how Einstein's concept of relativity and his discovery of an arbitrary maximum speed of the universe can coexist. I mean, how can everything be relative when there's an absolute maximum velocity? The event horizon and Hawking radiation/the firewall naturally come out of the speed of light.
Physics confuses me ever so much.
In fact, the missile could launch a smaller missile forward at 0.5c, and that missile would still be moving under the speed of light relative to the observer. If you just added speed up classically, it'd be going 1.5c.
Really it always works like this, but at speeds we consider normal the error from simplifying to v1+v2=v3 is incomprehensibly small.
ow, my head.
More importantly, there is no privileged viewpoint. As far as a spaceship is concerned it's everything else that's moving and it's standing still and the same physics predictions work just fine it's only the measurements that seem different.
Pre-Einstein we assumed "Galilean relativity" almost by default: the idea that if you're moving at a constant velocity, the results of any experiment you can perform are the same as if you were standing still.
Maxwell's electrodynamics is a wonderful, beautiful thing and well worth learning, but the relevant part here is that it implies that light always propagates at a fixed speed. And we can perform experiments that verify this, and we can - more or less - perform those experiments while moving at speed (or we can find clever ways to test the theory using the motion of the Earth, as with Michaelson-Morely), and we observe that they still hold.
So then just think about objects moving at constant velocities, and how to reconcile their observations with each other - for simplicity, you probably want to start with 1D space. Plug the observations in, write some numbers, and special relativity more-or-less drops out as the only way things could possibly work. It really is a lot less confusing than it sounds.
Your confusion about how a "maximum speed" can coexist with "every reference frame is equally valid" (i.e. relativity) is completely understandable: it's really weird. One way of framing the basic issue is that the equations for how relative velocity works aren't as simple as we intuitively think they should be. "If I'm going 55mph and someone passes me 15mph faster than that, they must be going 70mph" is an awfully good approximation at low speeds, but it just turns out that the universe doesn't quite work that way when the speeds get really high. It's tough to wrap your brain around it, absolutely. (I like using "space-time diagrams" to visualize this. Here's a handout from one of my classes a few years back that probably isn't self-contained enough to understand from scratch but might convey a bit of the gist of it: http://www.slimy.com/~steuard/teaching/classes/spacetime.pdf )
The observations leading to relativity theory put this question differently: Apparently the speed of light is constant, no matter how fast it's origin moves [1] : so we have to interpret our measurements relative to the speed of the observer. I still find Einstein's explanation the most readable [2]. He stays close to the geometrical description of reference frames and avoids the pictorial freedom known from monad tutorials...
[1] http://en.wikipedia.org/wiki/Michelson%E2%80 %93Morley_experiment
This is because it is relative to the CoM and spin of the universe itself (although things get a bit wonky when we look at this even closer).
Yesterday black holes were there. Everybody knew about Schwarzschild radius, about Cygnus X-1, about black holes theorized to be occupying the centers of galaxies.
And now everyone here is commenting like it was an obvious fact that black holes were never real and no one considered them seriously.
What happend? What did I miss?
This is one of the dangers of popular science reporting, and here Nature is very much to blame. They've taken a theory which largely preserves the model of black holes and spun it into exactly the opposite.
Black holes are a mistake of general relativity (GR), specifically their absolute event horizon is incompatible with GR's equivalence principle postulate, as the blog (which is not mine) shows. In software terms, black holes are a bug in the theory.
Let's remember that Einstein disbelieved in black holes and for many years tried to prove they couldn't exist. Also there is no definitive evidence that they actually exist in nature. By their definition, such evidence can't be detected.
Other articles on that blog claim to solve other major problems in physics, in every case giving an answer that disagrees with (what I understand to be) the consensus of working physicists.
So I could trust the author of the blog, Just Because (which doesn't sound like a great strategy). Or I could trust my own opinion (which is that on the one point I looked at it looks as if there's a substantial conceptual mistake). Or I could trust the current mainstream consensus.
I'm having trouble seeing reasons why I should believe what the author of that blog says. Do you happen to know of any?
> but "seeing whether the small region can be extended to a larger region" is not a test you can perform within the small region
That law K doesn't apply in a frame falling through a horizon (a logical test within that frame, which is defined to be an inertial frame) indicates that physical tests can also show that the laws of physics differ between that frame and other inertial frames. The picture in the blog is an example of that. It's impossible to conduct a test where the cloud described there isn't splitting in two, unlike in some other inertial frame.
The "larger region" you need to look at to determine whether K applies is not small enough for the defining characteristic of the small region -- i.e., that spacetime curvature is negligible there -- to hold. The equivalence principle stops applying once you start considering regions large enough for spacetime curvature to be non-negligible within them.
Frame X, in which law K is tested, is defined to be small enough that the spacetime curvature is negligible there. So it's definitely small enough. The test of law K is conducted wholly within frame X.
That shows that you have good intuition. It is wrong; it relies on incorrect assumptions about how a local inertial frame straddling the horizon works. See previous discussion on HN in these threads:
The editorialized headline here is wholly unwarranted and does a disservice to the process of scientific advancement, Nature magazine should be ashamed. This is barely one step up from "Hawking says black holes don't exist, the explanation will shock you."
http://arxiv.org/abs/1401.5761
¹ You should question to what extent they're journalists, when their employer is a for-profit entity existentially threatened by the subject they're reporting on.
I really don't think that this is true. The useful bit of peer review is changes made to the original paper as part of the peer review process (both for accuracy and clarity of exposition). No feedback from arXiv readers has yet been incorporated into this paper; perhaps in a few months there will be an updated version that includes such feedback. Until then, people will have to search the net to get a feeling for how the wider field views his ideas.
In terms of feedback from conferences, plenty of results are presented at conferences prior to publication in Nature and other journals, so I doubt there is any difference in the review level here.
I imagine it has actually been reviewed by some of Hawking's peers before he published to the internet. I know that's not the generally accepted usage of the phrase peer review but I think it is probably becoming more normal practice.
The new terms do not sound as cool as "event horizon", "black hole" and "singularity".