Rethinking the origins of the universe
unc.edu
unc.edu
Whether black holes exist or not, there's something really dense at the center of our galaxy. It's a challenge to come up with a consistent non-black-hole explanation for the observed motion of stars at the galactic center.
http://www.galacticcenter.astro.ucla.edu/images/2011orbits_a...
More here: http://www.galacticcenter.astro.ucla.edu/journey/smbh.html
Not as conclusive as the evidence for black holes. Remember that the article's hypothesis requires proof that black holes do not exist -- in other words, proof of a negative. Proof of a negative is generally regarded as an impossible evidentiary burden, and calls into doubt the hypothesis that relies on it.
There's pretty good evidence for very large, very dense masses at the center of most galaxies including our own. The default conclusion based on the evidence is that they're black holes -- they have the right mass and density.
We should all remember that a mathematical theory isn't a scientific theory until observation bears it out (produces positive evidence), and to the exclusion of competing explanations.
This doesn't work in physics. If you say they can only exist if they've always existed, then you're obliged to say how they came into existence earlier -- the deep meaning of "always".
That sounds implausible. Lots of things that cannot possibly come into existence at the present time did exist at the moment of the Big Bang (and shortly afterward), because the conditions back then were so radically different from they are now.
Perhaps the only singularity that ever existed was the Universe at the moment of the Big Bang. After the One Singularity has blown itself apart, no other singularity can come into existence. I don't see any inconsistency in that scenario.
Yes, that's true, but if the paper's thesis is correct, then there aren't any black holes or singularities at all, at any time. If the thesis is false, then there can be black holes at the universe's beginning, and they can form today, given the right conditions.
> Lots of things that cannot possibly come into existence at the present time did exist at the moment of the Big Bang (and shortly afterward), because the conditions back then were so radically different from they are now.
Not so different, and from a theoretical standpoint, they lie on a continuum from the present to the past. The condition of the universe as a whole has changed over time, but from the present we can mimic earlier conditions using high energies.
It is often said that one goal of accelerator technology is to move backward in time toward the Big Bang. As technology advances, as accelerator energies increase, we create conditions more like the conditions in the early universe. The ultimate hope is to create conditions as much as possible like the first few seconds of the universe's existence, to be able to see the genesis of present conditions and refine physical theory.
> Perhaps the only singularity that ever existed was the Universe at the moment of the Big Bang. After the One Singularity has blown itself apart, no other singularity can come into existence. I don't see any inconsistency in that scenario.
A physicist would immediately see the inconsistency in that. If the conditions of the Big Bang theory can exist in reality, then given the same conditions of temperature and pressure, we should be able to imitate an earlier reality on a small scale. That's what an accelerator is designed to be -- an imitation of nature at an earlier time.
We have no evidence that the singularity that existed at the moment of the Big Bang was the result of a collapsing blob of matter. Therefore, we have no reason to think that it could not have occurred.
In fact, it might not even make sense to think of that initial singularity as the result of anything at all, since causality requires time and there was no time before the Big Bang. Time is asymmetric, and this is especially important when we're talking about t = 0. You cannot simply imitate it from the other direction.
If true, it calls into question the existence of the initial singularity thought to precede the Big Bang. The alternative -- that the working of physical theories depends on circumstances -- isn't a very good one.
People sometimes hypothesize that this universe arose as a fluctuation in some other universe. For this kind of speculation to have any credibility, we have to assume that any singularity can be explained by physical theory, not just present-day singularities.
My point is that a theory whose workings depend on the circumstances isn't a very powerful one.
> We have no evidence that the singularity that existed at the moment of the Big Bang was the result of a collapsing blob of matter.
Look at this logically. If there was a singularity at the moment of the Big Bang, then it arose from the same physical theory that creates them in the present, but on a bigger scale. The alternative is to argue that physical theory is inconsistent, depending on circumstances. That sort of condition undermines physical theory, makes it more like a soap opera than science.
> In fact, it might not even make sense to think of that initial singularity as the result of anything at all, since causality requires time and there was no time before the Big Bang.
Yes, true, but once time exists, then we can start describing phenomena as having causes and effects. So physical theory only applies after time zero, not before. But if, at time zero, there was a singularity, we can try to apply physical theory to it. I'm not saying we will be successful, but it would be a mistake not to try.
For example, we know that things tend to behave weirdly at quantum scales. It that because the usual laws of physics do not apply at quantum scales? No, our best theories should be able to explain why the scale makes such differences, and therefore accurately predict phenomena at every scale, both large and small.
Likewise, there would be nothing wrong with singularities occurring under some circumstances and not under others, as long as we can come up with a compelling theory to explain how the different circumstances affect the outcome.
That's the kind of theory we should try to come up with. Not hasty generalizations based only on limited observation of familiar circumstances. I'm not saying we will be successful, but it would be a mistake not to try.
The author's claim that the big bang theory is now invalid because of this calculation of energy loss of a collapsing star being too fast to result in a black hole seems incorrect to me. The singularity from which the universe expanded (according to the big bang) is sort of "untouchable" by physics at the moment. I would guess there are some of the string/brane theory models out there that postulate what caused the big bang but currently it is my understanding that whatever information was around about the cause of that singularity is not accessible to us. Again, who knows what we will discover in the future but as of now, making any bold assertion about the lack of existence of that initial singularity seems presumptuous.
If it is presumptuous to present calculations that may invalidate a _theory_ based on assumptions, then it is also presumptuous to disregard such calculations and state that the _theory_ itself is automagically correct, just because the assumptions are untouchable.
A bold assertion indeed.
https://medium.com/starts-with-a-bang/yes-virginia-black-hol...
"There's been a story going around that it's been proven that black holes don't exist.
Yes, they do. <link>"
https://twitter.com/StartsWithABang/status/51488524891693875...
Edit: Re-reading my original comment I see my choice of words was poor.
In her blog post from January, she says:
"What Hawking is saying is essentially that he believes that a matter collapse only leads to a temporary apparent horizon but not to an eternal event horizon. That is an opinion which is shared by many of his colleagues (including me) and there is nothing new about this idea whatsoever."
But in her more recent paper, she says:
"More specifically, we find that collapsing stars slow down their collapse right outside their horizon, while substantially reducing their mass through Hawking radiation. ... The star never crosses its horizon, so neither unitarity nor causality are violated, thereby solving the longstanding information loss paradox. This investigation shows that universally collapsing stars bounce into an expand- ing phase and probably blow up, instead of collapsing to a black hole."
I am not a physicist, but when I read those two statements, they are in contradiction. The first statement says to me: it may not be an absolute horizon, but it is a horizon. The second statement says to me: actually, we never reach any horizon. So I think that the more recent statement means that she has changed her conclusions based on her recent work.
I am a literary critic, though, and I have to say that it's bittersweet news at best. Black holes are so much a part of the space-age imagination. The article mentions Hollywood, but black holes are woven deeply into our everyday metaphors and have been for decades. They have stood among the most wondrous things in the universe -- an awe-inspiring thing.
It reminds me a bit of when the press reported that the Catholic Church had "gotten rid of Limbo" (the actual case was a bit more complex, and involved some fairly weighty theology).
Then again, we all still talk of things "being in Limbo." Perhaps that next project will still be a black hole, even if it turns out that there's no such thing.
Again, inflation does not replace the Big Bang theory, it complements it. Big Bang and inflation are both parts of our current understanding of the early universe.
The Big Bang theory was more than just the idea that there was a "big bang" at the start of space-time. It was its own theory. And in the current theories, the 'big bang' at the start remains. But much else has changed.
Since we've carried the thread so long, I'll relate something marginally off topic that you might find kinda cool. At the time (I was in grad school then), it was a big deal to make the from Big Bang to Inflation, for political reasons. The "Big Bang" theory, in order to explain the smoothness of the observed universe, required a very special Creation event. Natural law wouldn't do it. This was a comfortable place because 80-90% of Americans believe in a Creator, and science was saying it was necessary. So Church leaders and science got along (except for the batshit folks).
When Inflation gained acceptance, this need for a Creator to explain the smoothness went away, and there was genuine concern in the field about the ability to continue to get funding from these Americans.
So, really, I suspect that the 'really dark dense thing' will remain, and be called a 'black hole,' even if our understanding of 'Black Holes' significantly changes.
This is false -- inflationary theory complements Big Bang theory, it does not replace it, as you claim. Big Bang and inflation are both parts of our current understanding of the early universe.
Really? Given that science doesn't turn on belief but evidence, still, the Big Bang remains the best explanation for existing evidence. It's the prevailing cosmological theory of the universe' beginnings. Like all scientific theories, it's subject to replacement as new evidence appears and as new theories are crafted, but it's a good match at the moment.
That depends entirely on what we mean when we say "science". If by "science" we mean as defined and as practiced when it's done right, then no, belief plays no part -- it can't. If it did, it wouldn't be science.
> Scientist aren't robots, mechanically and dispassionately ingesting evidence and spitting out theory.
Scientists aren't robots, but they are also not hothouse orchids.
> They're humans with imaginations, beliefs, agendas, and egos, guided (hopefully) by a methodology that leads to discovery.
You have left the topic of science. The reason science exists is precisely because people are the way you describe them. Science is meant to be a counterpoint to natural human instincts, beliefs, passions, and other logical failings.
> I think belief is what drives science, and it's not all that bad of a thing.
You're now confusing science with religion. In religion, you let sincere feelings guide you to a conclusion. In science, you let logic guide you to a conclusion.
In religion, something is true until evidence proves it false. In science, something is false until evidence proves it true -- exactly the opposite.
Much of scientific discipline is meant to guard against what we believe is true or want to be true. Experimental and control groups, the classic double-blinding precautions in human studies -- all are meant to minimize the corrosive, undermining influence of our beliefs.
The perfect religious follower is guided by belief, hoping only that his work reflects his passion. The perfect scientist doesn't care what conclusion his work comes to, hoping only that it reflects reality to the best of his ability.
Belief and science are in absolute opposition.
Or it depends on what we mean when we say believe. The weight lying on "science" does form a conceptual framework that completely ignores the notion of something beyond our reach, although it inarguably is the driving force behind science.
> In religion, something is true until evidence proves it false. In science, something is false until evidence proves it true -- exactly the opposite.
alright, but the word of god himself isn't evidence enough? j/k :)
> In religion, something is true until evidence proves it false. In science, something is false until evidence proves it true -- exactly the opposite.
Why do those have to opposite, can't they be reconciled into one, using some form of ternary logic? Edit: I mean, some things in life are unknown until proven true or false and then theres all kinds of methods to counter the fact.
You reconcile them using Bayesian probability. There is no "proven to be true", there is no "proven to be false", there is only probability that flows between different models based on the evidence we discover.
"In science, something is false until evidence proves it true" actually means that any random hypothesis you come up with has an equivalent probability to be right to any other random hypothesis - which is infinitesimal, because possible hypothesis space is huge. Then, as evidence comes knocking, you update those probabilities, and some hypotheses become more probable (i.e. more likely a good models of reality), while other become too improbable to care about.
Yes, but an idea doesn't become a matter of interest to science until it comes within our reach, in the sense of being observable in a way that forces different, similarly equipped observers to the same conclusion (the scientific meaning of "objective").
> Why do those have to opposite, can't they be reconciled into one, using some form of ternary logic?
Yes, an interesting question, but not really about science. Science concerns itself only with things that can be reduced to empirical observation, not belief.
> I mean, some things in life are unknown until proven true or false ...
That's true, but the basic scientific precept is the null hypothesis, the idea that things without evidence are assumed to be false. This is a great time-saver compared to granting credence to ideas without evidence, or assuming that something might be true until proven false (the unscientific outlook).
Seems i was wrong calling it inarguable, in fact i like to argue back and forth a lot.
> observable in a way that forces different, similarly equipped observers to the same conclusion (the scientific meaning of "objective").
no, that's still subjectivity, you just shifted the goal post. That kind of subjectivity might be your threshold for accepting some cognition as close enough to your supposition of objectivity, but it still requires trust on an individual level.
> an interesting question, but not really about science. Science concerns itself only with things that can be reduced to empirical observation, not belief.
Specifically by including subjectivity through empiricism, you in fact beget belief. I.e. measurements have an inherit uncertainty and you do what you can to reduce it.
> but the basic scientific precept is the null hypothesis, the idea that things without evidence are assumed to be false
You seem to fall for the fallacy of negated implication. My best guess is, the Null Hypothesis really states that evidence implies reality.
Or as my teacher put it, just because it didn't rain, the street doesn't have to be dry. It's hard to explain and I'm tired, sorry. I often fall for it, too.
You're making the post-modern argument. Are you aware of this? Everything is subjective, there are no objective shared truths, it's all a matter of opinion. But you haven't thought this viewpoint through to its logical conclusion, which is that, if it's true, then it applies first to the argument itself, fatally undermining it.
Before we have advancement of theory, we have to have inspired hypothesis, and for that, we need imagination. If it weren't so, we wouldn't need great minds to propel us forward. While I agree that science promotes confrontation of bias, I don't think it serves any useful purpose to pretend that science actually happens in a clean room.
Wait, are we discussing science or the problems science is designed to solve? You could make your remarks just as well about law, arguing that, because laws are broken, therefore they shouldn't exist, or they don't really mean what they say. You could also say this about mathematics, which only rarely exactly agrees with experience, but is still extremely useful in making predictions about an imperfect world.
Science is meant to be an ideal, that's its purpose.
> If you want to rule out everything where there's a strong human, non-mechanical element from the world of science, then there's very little science going on to talk about.
How are you missing the point that science is supposed to represent an alternative to everyday human affairs? And how are you missing the fact that, no matter how emotionally attached to a particular outcome, no matter how driven, a person still has to toe the science line in order to accomplish anything useful?
> ... I don't think it serves any useful purpose to pretend that science actually happens in a clean room.
True -- only good science, memorable science, happens in a clean room.
I can't believe you haven't figured out that science exists precisely because people are the way they are. If this were not the case, if people could divorce themselves from passion and perceptual distortions when circumstances required it, there would be no science -- what we call science would be one variety of normal human behavior and no one would think about it.
No. The universe is too smooth for the Big Bang theory. It has been supplanted by the Inflationary theory.
As is typical when this happens, parts of the original theory remained. For example, the "big bang" event- the idea that everything started from an initial 'explosion' of space-time- remained. And the term is still used.
My understand is that this is also true with Darwin's Theory of Evolution- I am not a biologist however so I may be off-base here. The concept of evolution- specifically survival of the fittest and speciation- remain in the current theories. And people still speak of 'evolution.' But the accepted/currently debated theories differ in some fundamental aspects, notably the gradualness of the process.
So my point was that we will be speaking of 'black holes' for a long time even if the current 'Black Holes' as we think of them today are shown to not actually exist.
Another question: Something very massive and very small is at the center of our galaxy. At least, we sure think so. Does this make us question that? Or does this just make us question whether that massive thing is in fact a black hole?
With regard to the question "If not black holes, then what?", this is a problem. There is a strict upper limit on neutron stars of about 1.4 solar masses. Beyond this, gravity is stronger than the repulsive core of the strong nuclear force, and the star should collapse. If the process described in this paper is what actually occurs, the star will shed mass due to Hawking radiation while collapsing. The end-state of that process must be either a neutron star (which can't have more than 1.4 solar masses) or something else. There doesn't seem to be any "something else" in the offing, which makes that million-solar-mass thing in the centre of our galaxy deeply mysterious.
A new theory that makes an old "settled" phenomenon mysterious is not all that uncommon in the sciences, so it's reasonable to take a wait-and-see attitude toward this idea, but I'm not enormously hopeful that it'll pan out very well in the long run.
And there is no(t much) doubt that very massive dark objects are out there, we can observe them indirectly. So even if the classical black hole creation process turns out to be not possible, there are still objects out there that a very much like classical black holes.
But it does, because one hypothesis about the Big Bang is that it commenced from conditions identical to a black hole's singularity. So our understanding of black holes is critical to at least one idea about the universe's beginnings.
> Or does this just make us question whether that massive thing is in fact a black hole?
The answer is that there is something like a black hole at the center of our galaxy, but people are free to theorize about what exactly it is.
Any theory about causes of that initial singularity are basically just conjecture at this point. My understand is that some of these high dimensional theories of quantum gravity postulate colliding 'membranes' and things but they are still very much in development and this calculation does nothing to further those theories along.
Another way I was thinking about it is that the initial singularity and creation of the universe is probably more a result of large scale, topological dynamics where this paper is concerned with more local, geometric dynamics. That's my own conjecture, there. :)
Not an issue, only that a singularity can exist, and therefore might come into being in the present.
If in a consistent physical theory we grant the reality of a singularity arising from very high matter density, then this narrows the range of possibilities for their nonexistence in the present.
So we have conventional densities easily explained by current theory, and we have the possibility of a singularity as an endpoint of increasing matter density, which narrows the discussion to the means whereby a mass might collapse past the critical density. I'm just saying this increases the burden on those who try to argue that the transition isn't possible. Especially given the absence of evidence in favor of, and plenty of evidence against, the stated thesis.
Er, what? I don't think this is true, or Hawking would have his Nobel prize by now. Maybe consistent in the sense that the predictions are that Hawking radiation is undetectably low for stellar mass black holes. Also, analogue systems don't count.
Hawking not getting a Nobel by now doesn't mean much. Einstein never got one for relativity (either Special or General), and when he did get his Nobel he was told not to mention "relativity" in his speech.
So... Hollywood? Star Trek wasn't filmed on location - Desilu and Paramount were both located in Hollywood.
Not that this, y'know, matters - just found it amusing.
So instead of the "black hole" being massive, it could be that time is running fast!
Now, if time ran faster, like if you made the measurements while accidentally having the video speed in fast forward. The stars would accelerate faster, and you would get a higher mass.
You can't create time distortions without gravity distortions, and you can't create gravity distortions without instantiating mass, so... big massive black holes have to be running slow; if they exist at all and aren't just cosmic bees sitting at the 2D boundary of the universe and projecting nothingness into our inflated space.
If you're speaking of time and space, no, they're different -- they're certainly all part of one spacetime, but they're distinct. This distinct:
t' = t √(1-v^2/c^2)
t = time
v = velocity
c = speed of light
t' = time at velocity v relative to velocity 0
Put into words, this special relativity relationship shows that an increase in velocity (movement in space) causes a reduction in time's rate of passing (movement in time). The general relativity version has more terms, and shows a relationship between mass and both space and time.
So space, time and mass aren't the same thing, but they're part of an interrelated system, one easily described mathematically.
> Time runs slower for us here inside the gravity well of Earth than it does for astronauts in zero gravity.
Yes, true, but astronauts in orbit aren't in zero gravity, they're in free-fall. The gravitational force at typical orbital heights is nearly as strong as it is at the surface.
I would have put it the other way around. Time passes more slowly near large masses relative to other locations.
In reality, it's the other way around. At the bottom of a gravity well, time passes more slowly. An astronaut on the moon observing a laser beam from earth's surface would see it as red-shifted compared to a local reference, reflecting the fact that time passes more slowly at earth's surface than it does on the moon. The other equivalent interpretation under GR is that the light loses energy climbing out of the gravitational field and is therefore red-shifted. One of the beauties of Einstein's theory is that, if you do the math using either assumption (slower time or lost energy), the result comes out the same.
The classic confirmation of GR conducted in 1919 during a solar eclipse showed that the paths taken by starlight near the temporarily blocked sun were curved toward the sun. This is consistent with the idea that time passes more slowly near the sun. Consider that a light beam a bit closer to the sun would have a longer transit time (because of slower time passage) than one farther away, which would have the effect of curving the stellar light wavefront toward the sun.
This article has some graphics and deeper explanations to assist in understanding these ideas:
If you're deep in a gravity well, you observe physical processes happening outside that well to go along more quickly, not more slowly. And the reverse is true if you are outside a gravity well, observing something in it.
Take some observer, Alice, far away from a massive body, who sees some process occurring near it take one second to complete. A second observer, Bob, nearer to the massive body, will see the same process take 1 - ε seconds to complete (and Alice will say Bob's clock is slow). In effect, Bob has passed through one second of (Alice) time in only 1 - ε seconds of (local, to Bob) time.
That's what I mean when I say "pass through time / move along the time axis more quickly". To think of it another way, ask who ages faster, Alice or Bob?
In your earlier post, you didn't clarify which observer was seeing which time pass more quickly, and where. In this post, you do -- sort of.
> Bob, nearer to the massive body, will see the same process take 1 - ε seconds to complete ...
If Bob and the process being observed are in the same frame of reference, Bob will see the process require a "normal" amount of time, i.e. a time consistent with classical physics. Alice will see Bob's process require more time on her clock, from her perspective. Bob, in the gravity well, will see Alice's time appear to be passing more quickly compared to his own.
> In effect, Bob has passed through one second of (Alice) time in only 1 - ε seconds of (local, to Bob) time.
This way of describing it is confusing. Here you are saying that Bob's experience of time is equal to Alice's time minus ε, when it's the reverse -- Bob sees Alice's time passing at 1 - ε, while his own time passes at a "normal" rate. It's a matter of how one describes it, because I suspect you understand how this works, the only problem is the prose.
> That's what I mean when I say "pass through time / move along the time axis more quickly".
Again, it's a matter of how one chooses to describe it, and it only proves the advantage of mathematics as a language. If we were discussing SR instead of GR, I would want to say, about the time experienced by (A)lice and (B)ob if (B)ob is moving at velocity v:
A = B / √(1-v^2/c^2)
B = A √(1-v^2/c^2)
In other words, Bob's time is slowed relative to Alice's time, as observed from Alice's frame of reference. The above two equations are only valid as written if Alice's time is compared to Bob's time after Bob completes his journey and brings his clock into Alice's frame of reference.
You will not understand the full implications of it (well, unless you are an astrophysicist), and I didn't bother with the meat of the paper (because I am also not an astrophysicist). But the abstract, introduction and conclusion are rather understandable, and far less hyperbolic than the press release. And even though I can't understand all of it, I'd much rather get a misunderstanding from the original source material, then get a compounded misunderstanding from a press-release.
https://news.ycombinator.com/item?id=7191324
A quick google shows his response to this:
https://twitter.com/abhasmitra/status/514804101024997376
His response:
"Hawking radiation is unproven. And particle physicists are not aware that even without any Quantum Mechanics, classical gravitation dictates that all gravitational collapse must be accompanied by radiation. :
http://link.aps.org/doi/10.1103/PhysRevD.74.024010
(http://arxiv.org/pdf/gr-qc/0605066v3.pdf)
And then grav collapse should naturally lead to radiation supported ECO: http://www.sciencedirect.com/science/article/pii/S1384107606...
http://mnras.oxfordjournals.org/content/369/1/492
http://mnrasl.oxfordjournals.org/content/404/1/L50
And as far as non -formation of ``Event Horizon'' is concerned, in contrast to the conjectures of the present yet unpublished paper I gave EXACT proof:
http://www.ias.ac.in/pramana/v73/p615/fulltext.pdf
But phys.org never highlighted my research which is infinitely more accurate than the present paper whose authors are not even aware that Question of Hawking Radiation Would Arise Only If There Would Already Be a Black Hole With an Event Horizon. Therefore This paper is not self-consistent. But that does not matter:
The lead author of this paper has CAMBRIDGE AFFILIATION.
So phys.org is glad to highlight an inconsistent and yet unpublished paper by ignoring my series of my exact and original papers on the same topic."
I don't have any background in astrophysics, can anyone comment on Dr. Mitra's response and the relation of his work to the above research?
The best discussion I can find on Dr. Mitra's work is on quora: https://www.quora.com/Is-it-true-that-Abhas-Mitra-resolved-t...
>The lead author of this paper has CAMBRIDGE AFFILIATION.
>So phys.org is glad to highlight an inconsistent and yet unpublished paper by ignoring my series of my exact and original papers on the same topic."
welcome to modern science. It is a professional guild like a guild of shoemakers of Middle Age Europe cities with about the same rules. Or like another my friend (former second-rate professor from a first rate university :) put it - it is a mafia, at every level. Researchers of the highest tier universities publish in the highest tier journals, researchers of the next tier - in the next tier journals, ... A good physicist should be able to understand the laws of Nature behind such formation :)
"Laura Mersini-Houghton has mathematically proven that quantum effects are strong enough to stop the formation of black holes..."
If I'm not mistaken, Mersini-Houghton's work has just hit the scene. It still has to be vetted by the larger community before we could say that anything's been "proven."
"The paper, which was recently submitted to ArXiv, an online repository of physics papers that is not peer-reviewed..."
I'd wait until something like this is peer-reviewed too.
As to "mathematically proven", it's an unfortunate juxtaposition of math (where things really can be conclusively proven) and physics (where they cannot be).
The result doesn't say that black hole is impossible. It says that a star's collapse loses mass faster than acceptable for the collapse to end in black hole.
Some misconceptions in the web-post:
>They are the ultimate unknown – the blackest and most dense objects in the universe that do not even let light escape.
a black hole isn't necessary most dense. After all a black hole with a mass of visible Universe would be only 125 times denser than current average density of the visible Universe (i.e. 45B light years radius of visible Universe have 10B light years Schwarzschild radius)
> So the story went, an invisible membrane known as the event horizon surrounds the singularity and crossing this horizon means that you could never cross back. It’s the point where a black hole’s gravitational pull is so strong that nothing can escape it.
"membrane" is very bad illustration. It is constantly changing solution to the gravitational equations. One moment it is here, another moment - it has moved because gravitation of black hole changes and a lot of oscillations/perturbations happen. One moment you're inside, another - outside.
"gravitational pull is so strong that nothing can escape it" - that's bad wording too. It is actually "gravitational well is so deep that nothing can escape it". Feel the difference :) A huge black hole may have pretty weak gravitation at its horizon.
Thus no-escape is valid only in the sense that escaping object just would never reach "a point at infinity" from the black hole and the light would get red-shifted into full oblivion.
I mean, what if could construct a collapse to your desire, e.g. smashing dense cold chunks, would those necessarily radiate enough energy to prevent collapse too?
the shown impossibility of star collapse to BH is somewhat questionable for massive stars because - as i cited wikipedia in another comment - the Hawking radiation carries away less mass/energy than cosmic background brings in - the net is positive for the collapsing star.
>smashing dense cold chunks, would those necessarily radiate enough energy to prevent collapse too?
we associate "collapse/smashing" with BH i think mostly because of collapsing stars. One can scale this process out to a lot of small chunks of cold matter as you suggest flying simultaneously toward common center. Imagine the sphere with radius 1000 times radius of Sun filled with material of density of water - this is already a black hole as its Schwarzschild radius is a bit larger than those 1000 radii of Sun. Now lets say we scale the sphere radius additional 10 times - the mass scales 1000 times and the Schwarzschild radius increase 1000 times too - thus all this water may be separated by a lot of space and still be a black hole. Now imagine that all this water is outside the Schwarzschild radius and spread on some imaginary sphere and simultaneously released to fly toward the center - once all these drops cross the schwarzschild radius, even without touching each other, and with each drop feeling only pretty small gravitation from the rest of the water, they nevertheless become a BH. What kind of miracles would happen in the moment these drops cross the imaginary sphere of Schwarzschild radius? Nothing. The only change here is that any light radiated by the drops close to crossing would reach far observers as a very redshifted and any light radiated at the moment of crossing and after would never be seen by the far observers. The article suggests that the loss of mass due to Hawking radiation would be fast enough to shrink the Schwarzschild radius faster than the drops can fall into. All this cold drops flying in empty cold space would supposedly cause very intense radiation close to the schwarzschild radius border? Nope. As mentioned above in the wikipedia citation Hawking radiation is pretty cold, low intense, for that size of black hole, and in particular it is much colder than CMB, ie. the losses are less than what is brought in by CMB. Though may be in the absence of CMB if the math in the article is right the losses (given the time slowing) would be enough to shrink the Schwarzschild radius fast enough. Yet not in our real Universe with CMB.
Not really. The amount of spacetime curvature at the event horizon is fixed in the theory -- it's always the same. It's how the event horizon is defined. Near the event horizon, light orbits endlessly (in principle), and (again in principle) if you were located at an event horizon and there was sufficient illumination, anywhere you turned you would see the back of your own head.
> Thus no-escape is valid only in the sense that escaping object just would never reach "a point at infinity" from the black hole and the light would get red-shifted into full oblivion.
From a more distant frame of reference, yes, but not at the event horizon itself.
No, its greater the smaller the horizon is (so, equivalently, less the bigger the horizon is).
> Near the event horizon, light orbits endlessly (in principle)
Sure, at the event horizon, light orbits endlessly. But the amount of curvature needed to do that is less the further across the horizon is.
> No, its greater the smaller the horizon is (so, equivalently, less the bigger the horizon is).
We're using different meanings of "curvature". For all black holes regardless of their properties, the event horizon has the same spacetime curvature -- that required to produce orbiting photons that cannot escape. Outside the event horizon, photons can escape. Inside, they cannot (and those photons don't orbit either, but cross the horizon). All the same.
> But the amount of curvature needed to do that is less the further across the horizon is.
No, it's the same. Same curvature, different large-scale geometry. For a sufficiently small zone near the event horizon, the conditions are identical.
Again, this is about the meaning of "curvature". The circumference of the horizon is greater for a large mass than a small one, but the spacetime curvature at the horizon is the same -- it's the specific value that causes photons to orbit perpetually, and interestingly from a local perspective, the entire horizon surface appears as a plane of infinite extent, sort of like two facing mirrors but with more dimensions.
From the perspective of a hypothetical observer at the horizon, he wouldn't be able to judge the size of the black hole using local observations -- there would be a plane of infinite extent for any black hole regardless of size.
If anyone else wondered how the radiation of massless particles could result in the reduction of mass, I've found a resource with simple explanations for a layperson like myself. https://www.physicsforums.com/threads/mass-lost-by-the-sun-b...
[1]http://astro.uchicago.edu/cosmus/projects/UCLA_GCG/ [2]http://en.wikipedia.org/wiki/Accretion_disc
http://en.wikipedia.org/wiki/Black_hole#Evaporation
"A stellar black hole of one solar mass has a Hawking temperature of about 100 nanokelvins. This is far less than the 2.7 K temperature of the cosmic microwave background radiation. Stellar-mass or larger black holes receive more mass from the cosmic microwave background than they emit through Hawking radiation and thus will grow instead of shrink. "
Because if that's true (and my feeble understanding is that it is true), then light can't enter a black hole or the reverse path would allow light to escape. What am I missing here?
So what happens if you direct a beam of light radially into a black hole? A distant observer will calculate that as the light beam descends into the black hole it will be (according to a distant observer) progressively blueshifted until it reaches the event horizon, at which point it will have blueshifted to infinite energy. In fact, according to the calculations of a distant observer, the light beam will never cross the event horizon, so the statement that the light beam is reversible remains true. The reason that the light beam (according to the distant observer) doesn't cross the horizon is that if you look at the metric, it states that ds^2 = (1 - R_Schw / r) c^2 dt^2 - 1 / (1 - R_Schw / r) dr^2, where R_Schw is the radius of the event horizon. Notice that at the event horizon r = R_Schw and the dr^2 term diverges. The distant observer therefore concludes that there is a singularity at the event horizon. This analysis lead physicists to initially conclude that black holes were unphysical.
A more careful analysis reveals that this isn't the case, however. We have been using this metric which describes the shape of spacetime according to a distant observer. But what we're really concerned with is not what a distant observer calculates, but how a local observer perceives spacetime. If we now switch to a coordinate system in which we are free falling into the black hole along with the beam of light, we find that nothing remarkable happens when we cross the event horizon. The light is blueshifted relative to its initial energy, but it doesn't have an infinite amount of energy. Thus there is no true singularity at the event horizon. This singularity that the distant observer found must therefore have been an artifact of the particular coordinate system that the distant observer was using. The free falling observer can then hold up a mirror and send off a light beam and have it return back to him even after they have crossed the event horizon.
An analogous effect occurs for an observer stationed at rest, hovering above the event horizon. Such an observer will observe the photons from afar to be blueshifed in agreement with the distant observer. Moreover, as this stationary observer gets closer and closer to the event horizon, the photons will be blueshifted to infinitely large energies.
Yes, it would, assuming you have a way to reflect a beam back the way it came, like the kind of retro-reflectors the Apollo astronauts left on the moon. If you were an observer and saw a laser beam emanating from some distant place, and you wanted to assure that your laser beam would be most likely to be seen there, you would point it at the exact opposite heading -- assuming neither the source nor destination were in motion.
> Because if that's true (and my feeble understanding is that it is true), then light can't enter a black hole or the reverse path would allow light to escape.
That's a different question with a different answer. If you point a laser beam at a surface that absorbs most or all the radiation that falls on it, it's possible that none of the initial radiation will be returned to the source. But -- very important -- all the energy is accounted for. If the initial light beam has energy E, then the energy absorbed and reflected by the target will also equal E, not necessarily the same wavelengths but the same total energy. This applies to black holes as well -- they conserve energy.
In the case of black holes, the escape velocity of the black hole is simply too great to allow light to escape at it's velocity if it originates from within the event horizon.
It would be like if you were rolling tennis balls to a friend across a trampoline. If you place a bowling ball in the middle of the trampoline, suddenly rolling the balls straight to your friend won't work anymore, because you need to roll them angled to account for the bowling ball in the middle of the trampoline causing the trampoline to be bent a bit so the balls will curve around the bowling ball and a person on the other side of the trampoline can then grab them and roll them back to you with the exact same speed and angle that you rolled them to that person.
However, if you tried to roll a tennis ball instead from the MIDDLE of the trampoline (where the bowling ball is) to your friend you would find that no matter what angle you chose you could not get the tennis ball out if you used the same speed to roll it as before. On the other hand, your friend will of course have no problem rolling tennis balls from the side of the trampoline inward to you.
This is the situation you find light in. Because the absolute maximum velocity that ANYTHING including light can have in a vacuum is equal to c if the black hole is bending the trampoline of space time so much that that velocity doesn't surpass the curvature then it doesn't matter what angle the light faces, it's just going to end up heading right back towards the black hole.
The same thing happens in spacetime. As you get passed the event horizon, there is no way to move through spacetime so as to emerge back through it.
The reality is, though, that people working in a given field are much more likely to use the arxiv version as the basis for further work, simply because it is available so much earlier. It is not uncommon to reach the point of publication and then run around to try and find out where all the arxiv submissions you used were published, which can sometimes be challenging.
Some people do: http://en.wikipedia.org/wiki/Grigori_Perelman
What is the name of this law?
but from what I read in the article the EQ 13, is the weakness of their argumentation.
AA
astrophysics: the branch of astronomy concerned with the physical nature of stars and other celestial bodies, and the application of the laws and theories of physics to the interpretation of astronomical observations.
Much of the nature of the cosmos is indeed a mystery, but that doesn't diminish the importance of the multitude of things we've learned about our universe from various physical observations and measurements.
Astrophysics appears volatile simply because there is so much we do not know, but that is also what makes it fascinating, and of critical importance.
In the hypothesis Einstein's field equations obviously break down, and instead of acknowledging that any explanation will be enormously difficult to test we seem to have bought into this completely untested idea that matter can continue bending space-time around into a singularity and all that. Can someone with a better understanding point out what logic is (probably) missing here?
The other thing that sets off my hand-wave-o-meter is the pretending that we are somehow already observing these things ("new telescope detects hints of black holes") or have concluded that they exist without any positive evidence. Instead of "the mass at the center of the galaxy which is so large that we currently have no good way of modeling it without a black hole" we hear "the supermassive black hole at the center of the galaxy" and so on.
Even if this paper is correct, I don't think it's warranted to be so smug about people thinking they found black holes. We had a theory, and we had positive evidence for that theory. That's reasonable. If this paper is correct, then it turns out those things are not actually "black holes", but objects "very slightly outside" of being black holes. That's not too bad.
Disclaimer: I am not a physicist.
Also is it at all plausible that the center of the galaxy is evaporating slightly outside its event horizon in this way?
Anyway I'm not sure that the two of us are going to answer any of these questions without some help.
That we have observed something is not in dispute, as I understand it. What's in dispute is what, exactly, these things are.
Are you asking what observational evidence exists for black holes? There are very massive, very dense objects at the center of most galaxies, objects that by their mass and density fall well inside the theoretical limits for black holes. At the center of our galaxy, there is a very massive, compact object, around which many stars are orbiting, in orbits that reveal the object's mass and approximate size:
http://astro.uchicago.edu/cosmus/projects/UCLA_GCG/
This is pretty good evidence -- if the object weren't within the mass/density realm that allows for black holes, the orbiting stars couldn't approach it as closely as they do without colliding.
Again, this doesn't prove anything, it only supports the idea that black holes are possible, and that our observations agree with that idea.
The observations are very good and offer little latitude for interpretation -- there is a very massive, very dense object at the focus of multiple stellar orbits near the center of our galaxy, and both the mass and the density of the object are easily and unambiguously derived from the orbits.
It's the same with other galaxies -- a massive, dense central object dominates the orbital dynamics of the galaxy near its center. We're obviously not free to say that means black holes exist, it's just another piece of evidence. But it's a way to exclude certain alternatives.
Actually, the evidence is pretty good. We have orbital velocities around the mass at the center of our galaxy that cannot exist unless there is a very massive, very dense object at one focus of the ellipses:
http://astro.uchicago.edu/cosmus/projects/UCLA_GCG/
This doesn't prove anything by itself, it only reduces the number of possible explanations.
We also have general relativity, which basically says that, once you exceed a certain spacetime curvature, you enter into the realm where black holes (or something like them) are inevitable.
The observations are very good, and the theory has stood the test of time, producing very reliable and consistent results. None of this is conclusive, there are difficulties with general relativity at the smallest scales, but the black hole idea is a reasonable conclusion to draw based on both theory and evidence.
> In the hypothesis Einstein's field equations obviously break down ...
General relativity doesn't break down for a black hole per se, only the realm inside it. If we observe phenomena from a great distance up to the event horizon, GR reliably predicts the outcomes. Within the black hole, i.e. between the event horizon and the hypothetical singularity at the center, we have a the region for which claims like "all physics breaks down" are appropriate.
But the fact that general relativity cannot explain everything isn't by itself a reason to doubt what it can explain. But it is a reason to look for a more complete theory, one for which GR is a subset. The same can be said about quantum theory -- it also has domains of excellent agreement with observation, and other places where it's less useful.
> ... we seem to have bought into this completely untested idea that matter can continue bending space-time around into a singularity and all that.
But that idea isn't hypothetical, it's pretty easy to see that it falls out of the mathematics. I won't present the general relativity treatment, but here's one from special relativity that's easier to understand. This equation tells us the relationship between space and time in SR:
t' = t √(1-v^2/c^2)
t = time
v = space velocity
c = speed of light
t' = time rate of passage at velocity v relative to velocity 0
Now try increasing v so it equals c (try traveling at the speed of light). See? Time stops, which in SR is roughly equivalent to an infinite curvature in GR.
Photons travel at the speed of light. This means in their frame of reference v = c, and therefore t' = 0. Does this mean that photons don't experience time? That's exactly what it means. In a photon's frame of reference, it's created in an atomic interaction, then it's taking part in another atomic interaction somewhere else, with no intermediate time having passed.
The above example is pretty remarkable when you think about it, but it doesn't mean the laws of physics have broken down, any more than they do at a black hole's even horizon.
Can't SR only model photons at c because some term disappears with zero mass which prevents infinite energies? Isn't it a whole different question whether GR is still reliable in the analogous case with massive objects?
[1] http://www.phdcomics.com/comics/archive.php?comicid=1174
That's the difference between science and journalism. In journalism, some things are proven true, while others are cast into doubt. In science, some things are proven false, while others are less doubtful than they once were, but never become true. The tl;dr: in science, things can only ever be proven false, never true.
As philosopher David Hume famously put it, "No amount of observations of white swans can allow the inference that all swans are white, but the observation of a single black swan is sufficient to refute that conclusion."
> Can't photons only travel at c because some term disappears with zero mass which prevents infinite energies?
I'm not sure I've successfully decoded your question, but only massless particles can travel at c. This is why, when it was established that solar neutrinos were changing their identities while traveling from the sun to our detectors, that meant they were experiencing time, which meant they had mass. All confirmed in later experiments.
> Isn't it a whole different question whether GR is still reliable at c when massive objects are involved?
From a mathematical standpoint GR is perfectly reliable from v = 0 to v = c, including places where large masses are present. Any velocity past c (or a sufficiently large amount of spacetime curvature) and GR is no longer able to provide reliable predictions. The reason is that the mathematical results include imaginary terms. As has been said by many, the relationship between mathematics and reality is much closer than we once imagined.
Mind-boggling stuff, thanks again for writing out these explanations.
Yes. At the event horizon, general relativity still predicts the outcome. Below it, no more conventional physics.
> I've heard that from a frame of reference outside the event horizon the time dilates and light would seemingly never get there, hence the whole idea of a black hole.
I've read that too, but in fact, because of the energies of accretion disks that are vacuuming up light and matter from the neighborhood, and the fact that some of the photons tend to orbit the horizon endlessly, it's actually a very hot place in most cases.
> ... thanks again for writing out these explanations.
You're most welcome.