Black Holes NOT Science?

[AdkinsJr]

I read an interesting article which asserts that the existence of black holes cannot be falsified, and therefore they do not qualify as science. Has anybody heard this argument before? Any comments?

[russ_watters]

No one answered the question, though....

Black holes are objects and they most certainly exist as they have been observed countless times. In addition to the factual existence of the object, there is also a theory (many theories) about what, exactly, they are. These theories are pretty good but not yet complete (and may well never be). One particular aspect - what, exactly goes on behind the event horizon may be unfalsifiable due to its unobservability.

[jobyts]

Is there an equation that calculate how much gravitational force is needed to bend the light, say, to form an arc of x radius?

EDIT: Never mind. Some google search pointed me to the following url.
http://www.mathpages.com/rr/s6-03/6-03.htm

I don't think I'm knowledgeable enough to understand these complex equations. It's more than my brain can handle. Wish they were simpler.

[matt.o]

Black holes are objects and they most certainly exist as they have been observed countless times. In addition to the factual existence of the object, there is also a theory (many theories) about what, exactly, they are.

I think you state this too strongly. We have much observational evidence for the existence of black holes, such as jets and accretion disks associated with supermassive black holes and other high energy phenomena, or the orbital motions of stars close to Sagittarius A. However, the defining feature, an event horizon, has never been observed.

[twofish-quant]

However, the defining feature, an event horizon, has never been observed.

But curiously enough you can observe something that would cause a "major problem." If you have something with a hard surface like a neutron star then stuff that falls on it will accumulate and if it is hydrogen eventually you get enough to cause a flare. With a black hole, there is no hard surface so stuff keeps falling in. The other thing is that to get a pulsar you have to have something getting emitted from the surface.

So if you have a very massive compact object which emits flares or is a pulsar, then you have some explaining to do. It so happens that we don't observe anything like that, and all of the accretion disks that do flare up and pulsars are objects which are below the black hole cutoff line.

Since you have observations that would falsify the existence of black holes (i.e. if you say an eight solar mass pulsar), it's science.

[russ_watters]

I think you state this too strongly....

However, the defining feature, an event horizon, has never been observed. I'm not sure I'd call the event horizon "the defining feature", but semantics aside, I don't see why that would matter anyway. By analysis of gravity alone, you can identify a region of space where a massive object must reside. That object emits no light. Therefore, that object is a black hole. Not being able to visualize the boundary of that object doesn't change the fact that we know for certain we are observing (via its gravitational pull) a massive object that doesn't emit light.

[matt.o]

I'm not sure I'd call the event horizon "the defining feature", but semantics aside, I don't see why that would matter anyway.


Well, observationally speaking, since the event horizon is the "last frontier" beyond which nothing more can be observed, I would say then that it would be the best that can be done as far as providing ultimate proof of the existence of a black hole.


By analysis of gravity alone, you can identify a region of space where a massive object must reside. That object emits no light. Therefore, that object is a black hole. Not being able to visualize the boundary of that object doesn't change the fact that we know for certain we are observing (via its gravitational pull) a massive object that doesn't emit light.

By your line of reasoning, Dark Matter can also be classified as a black hole since it too is massive and neither emits nor absorbs light. All we can say currently is that there exists a massive, dense object. I believe that recent Very Long Baseline interferometric observations have come close to detecting an even horizon, but that's as close as we've come.

[trini]

for the gravitational field at a point to be strong enough to justify itself as a black hole, there must be an extremely dense point of charge. because light itself is a wave function, one can probably imagine that a body of charge which is compacted to extremely dense states will not be able to vibrate internally. remember that in infinitely high gravitational fields, "time" slows to zero, which is to say, there is no motion of charged bodies relative to each other. because an em wave is caused by a perturbation in a static electric field, when there are no vibrations, there is no light.

I believe that energy, on the whole, behaves in such a way that it likes to be in motion. this is why we can't achieve 0 K to date, energy just doesn't like remaining still[perhaps due to the fact that we ourselves on earth are part of systems which change position relative to others, ie, our planet orbits in the soalr system, which in turn orbits the galactic bar, so on some tiny level, there is always likely to be some "universal shake" occurring due to our primary source of rotation]

Because energy must be very dense in a black hole, it becomes very hard to move. Since it wants to move, the black hole effect of sucking in whatever is around it may be the core energy bombarding itself with as much energy as possible to try and achieve vibration.

the event horizon itself may be a ball of vibration-less energy, that is to say, when things are sucked onto the event horizon, super huge gravity instantaneously stops any potential vibrations caused by the collision of absorbed matter with the core, including the vibrations in the absorbed particle itself, so the particle then becomes part of the event horizon, it goes "black".

black holes exist however, whether or not our understanding of them is correct, even if it turns out they weren't holes, the concept of the black hole is that of a region of high gravitational potential(ie bends light), coupled with no observable vibrations coming from within.

[russ_watters]

Well, observationally speaking, since the event horizon is the "last frontier" beyond which nothing more can be observed, I would say then that it would be the best that can be done as far as providing ultimate proof of the existence of a black hole. But your complaint above was that the event horizon is not observable, wasn't it? Is the event horizon observable even in theory? If the event horizon is not observable, why should it be a problem that we can't observe it?

Anyway, this still doesn't trouble me at all. If you close your eyes and walk around your room, you might bump into something that feels like a chair. Is it a chair or do you have to see it with your eyes to know for sure?By your line of reasoning, Dark Matter can also be classified as a black hole since it too is massive and neither emits nor absorbs light. All we can say currently is that there exists a massive, dense object. I believe that recent Very Long Baseline interferometric observations have come close to detecting an even horizon, but that's as close as we've come.
Is dark matter of a similar density to black holes?

[matt.o]

But your complaint above was that the event horizon is not observable, wasn't it? Is the event horizon observable even in theory? If the event horizon is not observable, why should it be a problem that we can't observe it?


You should re-read my post.


Anyway, this still doesn't trouble me at all. If you close your eyes and walk around your room, you might bump into something that feels like a chair. Is it a chair or do you have to see it with your eyes to know for sure?

Well, if all scientists settled on logic like this, there would be nothing more to do, right?

Is dark matter of a similar density to black holes?

No, it is not, but your criteria was that to be defined as a black hole, an object just has to be massive and not emit light:


By analysis of gravity alone, you can identify a region of space where a massive object must reside. That object emits no light. Therefore, that object is a black hole.

[russ_watters]

You should re-read my post. Uh, ok.... You said: However, the defining feature, an event horizon, has never been observed....

...the event horizon is the "last frontier" beyond which nothing more can be observed... ......so what is your point? Well, if all scientists settled on logic like this, there would be nothing more to do, right? Huh? Nothing I have said suggests that science has nothing more to do. There is a lot of work to be done on black hole theory. No, it is not, but your criteria was that to be defined as a black hole, an object just has to be massive and not emit light: Is dark matter "an object"? In any case, perhaps I was slightly too simplistic. So what? I think you get the point - I have no idea why you are being so argumentative.

[matt.o]

I'm trying to show you that statements like:


Black holes are objects and they most certainly exist as they have been observed countless times. In addition to the factual existence of the object, there is also a theory (many theories) about what, exactly, they are.


Are simply not true.

[Chronos]

I'm pretty convinced of the existence of black holes, but, have not heard any claim of proof to date. We know there are some suspiciously dense objects in the universe, and can at least infer they are probably black holes. That invisible spot in sagittarius with stars zipping around it at ridiculous velocities looks fairly compelling. An interesting point, however, is it appears extremely massive stars blow off too much mass to allow formation of stellar mass black holes. Neutron stars with masses in excess of about 1.33 solar mass are virtually unobserved to date - far short of the ~ 3 solar masses necessary to form a black hole. The smallest black hole detected to date weighs in at nearly 4 solar masses [http://www.space.com/scienceastronomy/080401-smallestblackhole.html]. This is quite a mystery, imo. Where are all the 'tweeners'?

[russ_watters]

I'm trying to show you that statements like:

Are simply not true.
Why didn't you say that before? Before you just said "too strong". I don't find it useful to be argumentative like that: if something is wrong, say it is wrong and explain why.

So far your quibbles have just been with the particulars of my admittedly simplistic definition. But I don't see those quibbles as being substantive/useful. Ie, no cosmologist would say an identified black hole is also consistent with dark matter, would they? Would a cosmologist identify this photo as a photo showing the aftermath of the creation of dark matter? http://chandra.harvard.edu/press/08_releases/press_041608.html

The wording of the article seems pretty unequivocable to me. They aren't saying that SGR A 'appears to be' or 'is theorized to be' a black hole. It is a black hole.

[WhoWee]

I'm pretty convinced of the existence of black holes, but, have not heard any claim of proof to date. We know there are some suspiciously dense objects in the universe, and can at least infer they are probably black holes. That invisible spot in sagittarius with stars zipping around it at ridiculous velocities looks fairly compelling. An interesting point, however, is it appears extremely massive stars blow off too much mass to allow formation of stellar mass black holes. Neutron stars with masses in excess of about 1.33 solar mass are virtually unobserved to date - far short of the ~ 3 solar masses necessary to form a black hole. The smallest black hole detected to date weighs in at nearly 4 solar masses [http://www.space.com/scienceastronomy/080401-smallestblackhole.html]. This is quite a mystery, imo. Where are all the 'tweeners'?

Chronos, (as you've pointed out) "A fish cannot comprehend the existence of water. He is too deeply immersed in it."

[chroot]

Russ,

I think your conviction is understandable, but also too strong.

As Chronos said, we have observed curious things in the universe -- immense sources of energy, jets, accretion discs, large gravitational effects on other objects, even gravitational lensing -- which can only be understood as the consequences of extremely massive (and dense) objects.

Currently, the only theoretical candidate that we have to explain these observations is the black hole, as described by the general theory of relativity.

If you take the definition of "black hole" as "super dense body," then yes, there is observational proof that black holes exist. If you take the definition of "black hole" to mean the narrower "body as described by general relativity," then their existence is, at best, plausible. The actual nature of these super dense bodies could be radically different than anything we think we know today.

- Warren

[matt.o]

Why didn't you say that before? Before you just said "too strong". I don't find it useful to be argumentative like that: if something is wrong, say it is wrong and explain why.

Actually, I think I did explain why I thought that statement was too strong. I'll admit that perhaps I was too strong in saying your statement was not true, and would have been better to say it was misleading.


So far your quibbles have just been with the particulars of my admittedly simplistic definition. But I don't see those quibbles as being substantive/useful. Ie, no cosmologist would say an identified black hole is also consistent with dark matter, would they? Would a cosmologist identify this photo as a photo showing the aftermath of the creation of dark matter? http://chandra.harvard.edu/press/08_releases/press_041608.html

My quibbles are with your penchant for dogmatically stating things. This is unscientific and does not help someone's understanding.


The wording of the article seems pretty unequivocable to me. They aren't saying that SGR A 'appears to be' or 'is theorized to be' a black hole. It is a black hole.

Ugh. This is a press release, and I also have problems with the way things are stated "as fact" in these things. Read the wording in the following journal articles: Detection of the Intrinsic Size of Sagittarius A* Through Closure Amplitude Imaging (http://adsabs.harvard.edu/abs/2004Sci...304..704B), The Event Horizon of Sagittarius A* (http://adsabs.harvard.edu/abs/2009ApJ...701.1357B) or Title: Event-horizon-scale structure in the supermassive black hole candidate at the Galactic Centre (http://arxiv.org/abs/0809.2442) and note the use of phrases like "black hole candidate".

[matt.o]

Russ,

I think your conviction is understandable, but also too strong.

As Chronos said, we have observed curious things in the universe -- immense sources of energy, jets, accretion discs, large gravitational effects on other objects, even gravitational lensing -- which can only be understood as the consequences of extremely massive (and dense) objects.

Currently, the only theoretical candidate that we have to explain these observations is the black hole, as described by the general theory of relativity.

If you take the definition of "black hole" as "super dense body," then yes, there is observational proof that black holes exist. If you take the definition of "black hole" to mean the narrower "body as described by general relativity," then their existence is, at best, plausible. The actual nature of these super dense bodies could be radically different than anything we think we know today.

- Warren

This is very well put, and conveys my thoughts too.

[Chronos]

We do not understand how matter behaves at extreme density, and there is no known way to replicate it in a laboratory. As Chroot noted, we may be quite surprised by the answer. The gap between neutron star masses and imputed 'black holes' remains very puzzling to me.

[Phrak]

No one answered the question, though....

Black holes are objects and they most certainly exist as they have been observed countless times.

I don't think so. It takes an infinite amount of time for matter to cross an event horizon. I think this quantifies the question "how long does it take a black hole to form?". I've had no informed responses on the this, so I can only make an ill-educated guess. Forever is a long time.

I would like to see the argument that a proto-black hole is measurably different than a black hole, and that observed objects touted as black holes in the pop-science press are measurably disinguishable.

Edit: I've jumped in without reading all the post, so forgive me if im not in sync.

[Dmitry67]

Black holes are more than objects: they are timespace structures

When we ask 'Does X exist'? (existed, will exist) we assume some global 'NOW' in neutonian sense. It is not applicable to the BH.

To make it clear, formulate the question this way: Does the Blach Hole exist NOW?
Obviously, the question does not make any sense: SOME timeline trajectories from you WILL 'hit' the black hole (if you decide to fly there), others - will even never cross the lightcone from there.

[russ_watters]

Russ,

I think your conviction is understandable, but also too strong.

As Chronos said, we have observed curious things in the universe -- immense sources of energy, jets, accretion discs, large gravitational effects on other objects, even gravitational lensing -- which can only be understood as the consequences of extremely massive (and dense) objects.

Currently, the only theoretical candidate that we have to explain these observations is the black hole, as described by the general theory of relativity.

If you take the definition of "black hole" as "super dense body," then yes, there is observational proof that black holes exist. If you take the definition of "black hole" to mean the narrower "body as described by general relativity," then their existence is, at best, plausible. The actual nature of these super dense bodies could be radically different than anything we think we know today.

- Warren Warren, while you said my position is too strong, your description of the situation agrees exactly with what I said. I don't know quite what is going on here, but it seems like people are reading something in my posts that I didn't say.

Perhaps a more stark example of the same concept would help. The ancients observed many "planets" and gave them proper names like "Venus". They were woefully wrong about what, exactly they were viewing, nevertheless, they were viewing the same object we still call "Venus" and classify as a "planet" today.

So, fast forward 1000 years and assume turns out the exact body predicted by GR is woefully inadequate to describe what is being observed. All I am saying is that there is still an object SGR A - it's really there - and I even think that in 1000 years, they'll still call that object a "black hole". Whether they do or not, that doesn't change the fact that there really is an object there, just like regardless of what name and theory we use to describe Venus, it is still the same object the ancients were looking at.

[Jonathan Scott]

All I am saying is that there is still an object SGR A - it's really there - and I even think that in 1000 years, they'll still call that object a "black hole". Whether they do or not, that doesn't change the fact that there really is an object there, just like regardless of what name and theory we use to describe Venus, it is still the same object the ancients were looking at.

So far, even the blackness is theoretical. These objects typically emit large amounts of energy at some wavelengths (for quasars, this amount is almost impossibly huge), but it is usually assumed on theoretical grounds that this emission is coming from accretion disks around a black hole (as we cannot yet resolve the central component separately). If theory proves wrong, and significant energy is being emitted by the central object too, then I don't think that such objects would continue to be called "black".

[twofish-quant]

Neutron stars with masses in excess of about 1.33 solar mass are virtually unobserved to date - far short of the ~ 3 solar masses necessary to form a black hole.

The 3 solar mass limit depends crucial on the equation of state for neutron star material which is unknown. If you assume that neutron star material is soft (which it may be), it's not hard to get a limit of 1.33 solar mass as the cutoff.

The smallest black hole detected to date weighs in at nearly 4 solar masses. This is quite a mystery, imo. Where are all the 'tweeners'?

It's not that much of a mystery. A neutron star has a surface and once material falls onto it, there are ways of getting the material off. Once material falls into a black hole it's going to stay there.

[twofish-quant]

I don't think so. It takes an infinite amount of time for matter to cross an event horizon.

No it doesn't. Material falling into a black hole will take a finite (and quite small) time to pass through the event horizon to the singularity at which point who knows what happens.

From a distance, it appears that material will take an infinite amount of time to fall in, but that's something of an optical illusion.

[Ivan Seeking]

but that's something of an optical illusion.

Uh, no.

[Dmitry67]

'optical' is not the right word
But I would not call the infitite time dilation for the fallign object real

Here is a best explanation of the BH I have ever seen
http://www.valdostamuseum.org/hamsmith/DFblackIn.gif

[Bob_for_short]

'optical' is not the right word
But I would not call the infitite time dilation for the fallign object real

Here is a best explanation of the BH I have ever seen
http://www.valdostamuseum.org/hamsmith/DFblackIn.gif

No, the plot is wrong. A black hole is as a spherical black body, that's it.

[Phrak]

No it doesn't. Material falling into a black hole will take a finite (and quite small) time to pass through the event horizon to the singularity at which point who knows what happens.

hmm. Matter will take an infinite amount of time to cross an event horizon.

There is no event horizon that is crossed for this in-falling frame of reference.

There is no clock that will record a local and finite elapsed time to cross an event horizon. You have to confuzzle coordinate systems to get this to work using the distant observer's coordinate singularity (the horizon) and the in-falling observer's clock.

From a distance, it appears that material will take an infinite amount of time to fall in, but that's something of an optical illusion.

Without this optical illusion there is no Hawking radiation.

[twofish-quant]

hmm. Matter will take an infinite amount of time to cross an event horizon.

Incorrect. See

http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html

Also it references Misner, Thorne, and Wheeler.

[Dmitry67]

No, the plot is wrong. A black hole is as a spherical black body, that's it.

Look at the diagram. It is spherical.

[sweetsimple]

A black hole is defined as a dead star, whose escape velocity is greater than that of the speed of light.

[Phrak]

hmm. Matter will take an infinite amount of time to cross an event horizon.

Incorrect. See

http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html

Also it references Misner, Thorne, and Wheeler.

If you are going to throw things out like this you should provide a quote.

Matt talks about some "useful sense". I have no idea what is supposed to be useful about falling into a black hole. Apparently this doesn't mean a "physical sense".

[George Jones]

Matt talks about some "useful sense". I have no idea what is supposed to be useful about falling into a black hole. Apparently this doesn't mean a "physical sense".

What in the world do you mean? I agree with twofish-quant.
hmm. Matter will take an infinite amount of time to cross an event horizon.

This is wrong.
There is no event horizon that is crossed for this in-falling frame of reference.

And this is wrong.
There is no clock that will record a local and finite elapsed time to cross an event horizon. You have to confuzzle coordinate systems to get this to work using the distant observer's coordinate singularity (the horizon) and the in-falling observer's clock.

And this is wrong. Nothing has to be confuzzled. The in-falling observer's clock works just fine by itself.

[Phrak]

What in the world do you mean? I agree with twofish-quant.

This is wrong.

And this is wrong.

And this is wrong. Nothing has to be confuzzled. The in-falling observer's clock works just fine by itself.

I keep getting these one liners as if this solves everything.

Please provide a world line that Contains a coordinate singularity.

[twofish-quant]

As Chronos said, we have observed curious things in the universe -- immense sources of energy, jets, accretion discs, large gravitational effects on other objects, even gravitational lensing -- which can only be understood as the consequences of extremely massive (and dense) objects.

In the case of accretion disks they can only currently be understood in terms of massive and dense objects that seem to behave according to how black holes should behave under the rules of GR. For some of the accretion disks, it's not merely a massive and dense object, but a massive and dense object with no apparent surface.

The actual nature of these super dense bodies could be radically different than anything we think we know today.

Possible but not likely. The thing about accretion disks is that a lot of them have been studied enough to reduce the likelihood of "weird physics." If there is something radically different than what we think we know then we have to explain why all that radical stuff only seems to affect the core object and not anything else.

Personally, I think that the observational evidence for black holes is roughly equal to that of exoplanets, and a finding that "black holes don't exist" would be as shocking as "exoplanets don't exist."

[twofish-quant]

Please provide a world line that Contains a coordinate singularity.

Someone already put out a diagram that has one. The physics problem with singularities is that if you fall into a black hole, you'll hit it in finite time. If you never hit the singularity then there would be no point in worrying about what they are.

[Dmitry67]

http://www.valdostamuseum.org/hamsmith/DFblackIn.gif
It shows a worldline of infalling observer.
You can see his proper time over that line.

[twofish-quant]

One curious thing about black holes is that the thing that kills you (i.e. the tidal forces) really are large only for "ordinary" small black holes. If you have a large enough black hole, the tidal forces diecrease, and nothing particularly strange happens to you once you cross the event horizon.

[Dmitry67]

Yes. ANother interesting thing about the proper time of freely falling observer is that to have a longest lifespan you need to relax and enjoy the view. If you try to resist, pushing the pedal of your spaceship to the metal, then you decrease your proper time. But this a quite obvious property of pseudo-euclidean space.

[George Jones]

Please provide a world line that Contains a coordinate singularity.

I'm not sure what you mean. Relativity models spacetime as a semi-Riemannian differentiable manifold. What is the definition of "semi-Riemannian differentiable manifold?" Also, what is a "coordinate singularity?" I am a little unsure of the (general, not specific to this case) answer to the second question, but I do have something in mind.

[atyy]

I read an interesting article which asserts that the existence of black holes cannot be falsified, and therefore they do not qualify as science. Has anybody heard this argument before? Any comments?

http://arxiv.org/abs/gr-qc/9808035

"All the methods for finding black holes described above are indirect. They essentially
say that there is a lot of mass in a small volume. Direct proof that a candidate object is a black hole requires a demonstration that the object has the spacetime geometry predicted by Einstein’s theory. For example, we would like to have evidence for an event horizon, the one feature that is unique to a black hole."

"Black holes connect to a wide variety of fields of physics. They are invoked to explain high-energy phenomena in astrophysics, they are the subject of analytic and numerical inquiry in classical general relativity, and they may provide key insights into quantum gravity. We also seem to be on the verge of verifying that these objects actually exist in nature with the spacetime properties given by Einstein’s theory. Finding absolutely incontrovertible evidence for a black hole would be the capstone of one of the most remarkable discoveries in the history of science."

http://arxiv.org/abs/astro-ph/0701512

"Probing the properties of Massive Dark Objects: black holes?
spinning?"

"Observers cannot yet definitively confirm the form of the metric in the strong-gravity region, in order to prove that BHs are indeed described by the Kerr metric."

"Gravitational waves from an EMRI can be used to map the spacetime of the central massive dark object. The resulting ’map’ can tell us if the standard picture for the central massive object, a Kerr BH described by general relativity, holds."

[Phrak]

Someone already put out a diagram that has one. The physics problem with singularities is that if you fall into a black hole, you'll hit it in finite time. If you never hit the singularity then there would be no point in worrying about what they are.

Perhaps the problem is in semantics. That was the diagram of the Schwarzschild metric. We need a diagram or metric for an in falling observer. Without conflagrating coordinate systems, we can use the observer's Riemann normal coordinates, good throughout the entire in falling trajectory (and excluding the central singularity, it seems).

There will be finite time elapsed on the in falling observer's clock, and a singularity free metric, up to and not including the central singularity.

[piareround]

Perhaps the problem is in semantics. That was the diagram of the Schwarzschild metric. We need a diagram or metric for an in falling observer. Without conflagrating coordinate systems, we can use the observer's Riemann normal coordinates, good throughout the entire in falling trajectory (and excluding the central singularity, it seems).

There will be finite time elapsed on the in falling observer's clock, and a singularity free metric, up to and not including the central singularity.

While there are something things I disagree about some of your points, I do agree this debate is gradually deviating from the original question:

Whether or not black holes can be falsified.

I read an interesting article which asserts that the existence of black holes cannot be falsified, and therefore they do not qualify as science. Has anybody heard this argument before? Any comments?


Many people have mention observed objects that one group scientists thinks they are black holes, other scientists thinks they are are not black holes, and sadly another group of scientists who refuse to answer. We call these things black hole candidates (http://en.wikipedia.org/wiki/Stellar_black_hole#Candidates) (here I am simply pointing to the list rather than the definition). Regardless of you stance on black holes, these black hole candidates (BHC's) have at least two primary properties of:

being themselves in a section of observable space or being in a section of observable space that does not emit/re-emit and/or reflect/refract light
have stuff orbiting around them that does emit/re-emit and/or reflect/refract light


1st Answer) So to give one answer to "can black hole be falsified", I would like to point out that that BHC's themselves can be falsified as not being black holes by these checking these two properties. However, I do not know of any examples of things we have confirmed are not BHC's. So I propose this as a question to the Relativistic Physics researchers out there:

Does anyone know of a historical example of a specific former BHC that has confirmed as a not being a BHC?

Now if really take a look at experimental detection of black holes you will find that there is a parallel with experimentation of gravity waves. This parallel is no coincidence because, historically, the gravitational waves have been tied to astrophysics and to large massive objects including black holes.

2nd Answer) So it would seem to me that if we are to talk about the validity of detecting black holes, we should talk about the validity of Gravitational Waves and thus the accuracy Gravitational-Wave Detectors (http://en.wikipedia.org/wiki/Gravitational_wave_detector). However I do not know enough about Gravitational Wave detectors to even begin talking about whether or not gravity waves are detectable. Yet, there are some particular questions about these detector's precision, that I think should be answered such as:

How does one calibrate something like a LIGO? How do you calibrate something to detect gravity waves before you start looking for them?
How do you calibrate it so that you know that the gravity waves you do detect come from a particular source?
How far out and to what frequency ranges does theory say these detectors can detect to?


I firmly believe that answers the above questions, will lead toward an answer of the validity of experimental detection of black holes from other massively large objects better than any debate of semantics of black hole theory can.

I also hope they will show that a single black hole can indeed be falsifiable, but it is, like most modern research, expensive.

[twofish-quant]

Many people have mention observed objects that one group scientists thinks they are black holes, other scientists thinks they are are not black holes, and sadly another group of scientists who refuse to answer.

I think this misstates the opinion and consensus in the field. All of the candidates for black holes are objects which pretty much everyone in the field think are black holes. The question is "how sure" are people that these objects are black holes. but that's a different question.

[Chronos]

The 'sure' thing has been an issue in this thread. No need to deal with that right now. Nothing is 'sure' in science. I am agreeable with 'highly probable' for the time being.

[Phrak]

My uninformed argument is fairly simple; that there are no black holes to be observed because it would require an infinite time for an event horizon to form, where time is being measured by Earthly observers.

My argument might be fairly easy to falsify: Locating any article in the literature that would validly support the formation of an event horizon of a collapsing mass in an outside observer's proper time. Any given distribution of charge, mass, or momentum, or displacement in distance or state of motion of the original mass would suffice.

Barring this, I doubt that a black hole can be measurably distinguished from a proto black hole. By proto black hole, I mean a collapsing mass, where there is (according to the proper time of a distant observer) mass remaining above the projected event horizon, however closely it resides.

[Dmitry67]

My uninformed argument is fairly simple; that there are no black holes to be observed because it would require an infinite time for an event horizon to form, where time is being measured by Earthly observers

So, your argument is, if we express it more formally, the lightcone from the horizon will never ever reach an observer on Earth?

Then Cosmology is not science either; because when we say something like 'universe is flat' or 'spacetime is open' we talk about the regions behind the cosmological horizons. They have the same property.

Looks like you insist on the very strong version of falsibiability; we have no choice but to relax these requirements. We have no choice but
1) to develop the theory that fits observable data
2) apply the same theory to the regions we can (never) observe or experiments we can never perform (like the inside regions of black holes, Big Bang, Planks energies, behind the cosmological horizon).
3) We assume that if theory is valid in 1) then it is valid in 2)
4) If there are other alternative theories we chose the simplest/the most beautiful.

[Phrak]

So, your argument is, if we express it more formally, the lightcone from the horizon will never ever reach an observer on Earth?

No, not at all. Outward propagating signals would be a matter for observational evidence. I am referring to the time that would be required for matter moving in the other direction--the collapse of infalling matter to form an event horizon, as defined in an external observers spacetime coordinates, as measured in the proper time of the external observer.

:confused: I’ve been trying to make this as broad as possible.

How long, per an outside observer, does it take collapsing matter to develope an event horizon per the outside observer?

[Dmitry67]

Now you are saying nonsense.

Imagine 2 roads. When they are parralel, 1 mile on one of then is equal to 1 mile on the other.

If they are not parralel, then there is a 'dilation' - say, 1.5 on one of then is equivalent to 1 mile on another

Now there is a fork and one raod turns at 90 degrees and hides in a tunnel. Now it does not make any sense to compare the distance!

The same in BH: even in SR there is no absolute meamaing of simultaneously of events in different location, and no absolute meaning of age unless objects can return sooner or later to the same point.

If one observer falls into the horizon and another stays on Earth, they will never meet So your claims about 'as defined in an external observers spacetime coordinates' does not make any physical sense.

It is exactly like asking 'but on what of 2 moving ships time is moving slower'? Yes, in some coordinate system events freeze at the horizon. But the coordinate system you are talking about is impared because it can not cover events inside the BH. In another coordinates system your claim is false. So what?

The following pictures use coordinate systems where there is nothing special at the horizon, time inside the BH becomes spacelike, and proper time (line length) is limited.

http://www.valdostamuseum.org/hamsmith/DFblackIn.gif
http://nrumiano.free.fr/Images/lightcones_E.gif
http://www.pitt.edu/~jdnorton/teaching/HPS_0410/chapters/black_holes/bh_lightcones_st.gif
http://physics.syr.edu/courses/modules/LIGHTCONE/pics/bh2.gif

[Dmitry67]

How long, per an outside observer, does it take collapsing matter to develope an event horizon per the outside observer?

I repeat, your question IS NOT PHYSICAL

You can ask 'when I receive the signal from infalling observer' - but we both agree that these signals are infinitely redshifted

There is no 'when' in GR in curved spacetime for the events in different locations.

'How long' can be defined as 'proper time' for the same observer.

'How long per outside observer' can be defined as:
* proper time of some of the observers when thy finally meet - it is not the case here;
* time calculated based on the arrival of the signals
* time calculated based on some coordinate system. But it depends on the coordinate system and dos not have physical meaning (example: in twin paradox, when one spaceships stops and starts to go back, it has to 'switch' its coordinate system which leads to assymetry and resolves the twin paradox)

[Jonathan Scott]

I repeat, your question IS NOT PHYSICAL

You can ask 'when I receive the signal from infalling observer' - but we both agree that these signals are infinitely redshifted

There is no 'when' in GR in curved spacetime for the events in different locations.

'How long' can be defined as 'proper time' for the same observer.

'How long per outside observer' can be defined as:
* proper time of some of the observers when thy finally meet - it is not the case here;
* time calculated based on the arrival of the signals
* time calculated based on some coordinate system. But it depends on the coordinate system and dos not have physical meaning (example: in twin paradox, when one spaceships stops and starts to go back, it has to 'switch' its coordinate system which leads to assymetry and resolves the twin paradox)

I understand your point, but I reckon that's cheating. For example, time rates near a massive object vary with potential, but from the point of view of an observer using a specific background coordinate system (say isotropic) we can use that as a valid model of what "really" happens.

According to almost any coordinate system EXCEPT that of a free-falling observer falling with the material into the forming black hole, such events take an infinite time and therefore do not complete within the lifetime of the universe. From the point of view of a falling observer, things happen in a finite proper time, but any attempt to map that time back to external space-time gets very mixed up, as time and space have swapped roles.

We know from experience with ordinary levels of gravity that this "slowing" of proper time is a physical effect; if you go very close to a black hole and manage to get away again, a lot of extra time will have elapsed. On those grounds, it seems that the infinite delay should be considered an equally physical effect.

So what happens from the point of view of a falling observer and how can this be consistent with the external view? I don't know, and I've not seen any explanation I can believe.

[Dmitry67]

1
using a specific background coordinate system (say isotropic) we can use that as a valid model of what "really" happens.

2
According to almost any coordinate system EXCEPT that of a free-falling observer falling with the material into the forming black hole, such events take an infinite time and therefore do not complete within the lifetime of the universe. From the point of view of a falling observer, things happen in a finite proper time, but any attempt to map that time back to external space-time gets very mixed up, as time and space have swapped roles.


1
And by changing that 'specific' coordinate system we can change what 'really' happens :) It is well known that just by walking in different directions in your room you can change what "now" "really" happens in the Adromeda galaxy by several years :)

2
Then you are violating the very central idea of the General Relativity - that ALL coordinate systems are equal in rights. Observers point of view on Earth is not more 'valid' in any sense than the point of view of a free falling observer.

Finally, it is not only "except the free falling observer" but a wide class of falling systems. Not only freely falling: if you resist falling into a black hole, but your engines are not powerful enough, then you are not freely falling but still you reach the singularity in finite proper time.

[Phrak]

I repeat, your question IS NOT PHYSICAL

Yes, well, I'll be sure to let Misner, Thorne and Wheeler know your assessment of their NONSENSICAL use of the language.

[Dmitry67]

Any arguments except the names?

[Jonathan Scott]

1
And by changing that 'specific' coordinate system we can change what 'really' happens :) It is well known that just by walking in different directions in your room you can change what "now" "really" happens in the Adromeda galaxy by several years :)

I disagree; I was talking about sequences of events which may or may not happen. These are the same in all coordinate systems. They can be described consistently in any one of them. Time, space and directions can of course vary.


2
Then you are violating the very central idea of the General Relativity - that ALL coordinate systems are equal in rights. Observers point of view on Earth is not more 'valid' in any sense than the point of view of a free falling observer.

There's no difference in validity within the range in which the coordinate system is valid, but if part of it corresponds to a non-existent region of the universe, that doesn't necessarily count as valid.


Finally, it is not only "except the free falling observer" but a wide class of falling systems. Not only freely falling: if you resist falling into a black hole, but your engines are not powerful enough, then you are not freely falling but still you reach the singularity in finite proper time.

True, which is why I said "almost".

[Dmitry67]

I disagree; I was talking about sequences of events which may or may not happen. These are the same in all coordinate systems. They can be described consistently in any one of them. Time, space and directions can of course vary.


No, because some observers cross the horizon

Let me explain my point of view in a different way.

"you-now" is a point in spacetime. There are infinitely many spacetime trajectories crossing that line. Each line represents some observer.

Some spacelines never hit the BH: for example, "you-remaining-on-EARTH". Some crosses the horizon: "you-decided-to-go-into-the-BH"

in GR all coordinate systems are equally vaid. So in some coordinate systems BH forms in finite time. In the others we lose communication with the inner parts of BH.

But if at least in some coordinate systems crossing "you-now" BH is formed, how can you say "it will never form"?

[Jonathan Scott]

No, because some observers cross the horizon

Let me explain my point of view in a different way.

"you-now" is a point in spacetime. There are infinitely many spacetime trajectories crossing that line. Each line represents some observer.

Some spacelines never hit the BH: for example, "you-remaining-on-EARTH". Some crosses the horizon: "you-decided-to-go-into-the-BH"

in GR all coordinate systems are equally vaid. So in some coordinate systems BH forms in finite time. In the others we lose communication with the inner parts of BH.

But if at least in some coordinate systems crossing "you-now" BH is formed, how can you say "it will never form"?

The region within the falling observer coordinate system which corresponds to falling past the horizon has no equivalent in the universe, as the time coordinate within the more conventional coordinate systems of its junction with the universe is "at infinity" (and the time coordinate on the other side is space-like).

Basically, as the observer falls, we see his clocks slowing to a halt, although he doesn't notice. This is just like the "stasis chamber" of science fiction. From the point of view of the observer, he doesn't know that he's been frozen in time, and if he is rescued and brought back to life, no time has elapsed. Even though from his point of view everything was normal up to the point where he was frozen, that doesn't necessarily mean that one can extrapolate to the assumption that he will be unfrozen again!

Some people insist that this is "not a problem", and insist that mathematical arguments or numeric simulations have shown that event horizons can form in finite time and things can fall inside them. I am personally yet to be convinced. Of course, in standard black hole theory, once collapse has started, it is inevitable, so the effect is the same anyway as a black hole.

[Dmitry67]

There is nothing to be convinced or not. Just look at the diagrams with the lighcones (I provided 4 links). What happens inside and around the BH is very simple. An attempt to think it terms of 'time dilations; leads to many misconceptions.

[Jonathan Scott]

There is nothing to be convinced or not. Just look at the diagrams with the lighcones (I provided 4 links). What happens inside and around the BH is very simple. An attempt to think it terms of 'time dilations; leads to many misconceptions.

I'm quite familiar with various different coordinate systems. If you take the falling observer's point of view there does not appear to be any problem with passing the event horizon (and hitting the singularity) in finite time. However, as I explained with the "stasis" example, that does not necessarily mean it's actually possible.

If you consider paths which fall very close to the horizon but then turn round and return, you see that the closer they go, the longer they take. This means that in the limit, as observed externally, the falling observer takes infinite time to cross the horizon, and could still in theory be rescued at any time during the entire future of the universe! So when do they actually fall in?

I find myself somewhat frustrated with books which show that in the falling observer's coordinate system, the horizon coordinate is passed in finite proper time and therefore conclude that the fact that this takes an infinite time as seen by an external observer is somehow just an irrelevant illusion. This isn't just a matter of different a point of view; it's a contradiction, and requires more than hand-waving to explain it.

[Dmitry67]

This isn't just a matter of different a point of view; it's a contradiction, and requires more than hand-waving to explain it.

I dont see any contradictions. The problem is that many people are trying to 'map' falling observer time to 'external' time, thinking in terms of

t' = x * t

where x is some variable. Obviously, you get into a problem when x becomes 0 or infinite. But who garanteed you that there is ONE 'river' of time and all times can be 'mapped' into each other?

Talking about the handwaving, what is NOT explained by the spacetime diagrams I provided? Lets talk about the physical things (what is observed, when signals arrive, etc) and avoid non-physical questions (when I am here on Earth, has black hole already formed? etc)

[Dmitry67]

If you consider paths which fall very close to the horizon but then turn round and return, you see that the closer they go, the longer they take. This means that in the limit, as observed externally, the falling observer takes infinite time to cross the horizon, and could still in theory be rescued at any time during the entire future of the universe! So when do they actually fall in?


I can ask you a similar question - without black holes.

Spaceship flies toward the Andromeda at very high speed. Then it turns back and returns back to Earth.

The trip took only few years measured by the clock on the spaceship, while on Earth it took millions. So while the austranaut on the spaceship aged only few years, his twin brother on Earth had died a long time ago.

The question you are asking "when actually (on the spaceship clock) the brother on Earth died?" Do you agree that this question is not physical?

[chronon]

But who garanteed you that there is ONE 'river' of time and all times can be 'mapped' into each other?Unfortunately most people tend to think that the different times can be mapped into each other. For instance in the webpage mentioned earlier in the thread http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html#evaporate it says I won't experience that cataclysm myself, though; I'll be through the horizon, leaving only my light behind. implying that the light he leaves behind is seen after he has crossed the horizon

[Jonathan Scott]

Unfortunately most people tend to think that the different times can be mapped into each other. For instance in the webpage mentioned earlier in the thread http://antwrp.gsfc.nasa.gov/htmltest/gifcity/bh_pub_faq.html#evaporate it says implying that the light he leaves behind is seen after he has crossed the horizon

Do you agree that the external time coordinate shows that he could in theory be rescued at any time in the future of the universe (although of course nothing we know is strong or fast enough to do so)?

I reckon that makes it reasonable to say that he hasn't yet fallen in.

[chronon]

Do you agree that the external time coordinate shows that he could in theory be rescued at any time in the future of the universe .

No, after a certain amount of time has passed it will be impossible for light to catch up with him before he crosses the event horizon, and so it would definitely be impossible to send a rescue mission (which would travel slower than light). On the other hand, if he had powerful enough rockets, then he could at decide to turn round at any time before he has crossed the horizon.

[Dmitry67]

oops, chronon was faster....

Do you agree that the external time coordinate shows that he could in theory be rescued at any time in the future of the universe (although of course nothing we know is strong or fast enough to do so)?

I reckon that makes it reasonable to say that he hasn't yet fallen in.

No, I dont agree.

for me, the point of no return is at the event horizon.

for you, when I am too close to the horizon it is too late to decide to flight to me to save me: when you approach the BH trying to 'save' me you see how I 'unfreeze' and sink deeper and deeper BEFORE you approach

To simplify, lets say that a simple signal from you can save me: if you send a signal 'please return, I forgive you :)' and I receive it I turn around and return. But if I am too deep inside the black hole then it would take a while for the light signal to cover the distance to the black hole, and it would be too late!


Will you see the universe end?
If an external observer sees me slow down asymptotically as I fall, it might seem reasonable that I'd see the universe speed up asymptotically-- that I'd see the universe end in a spectacular flash as I went through the horizon. This isn't the case, though. What an external observer sees depends on what light does after I emit it. What I see, however, depends on what light does before it gets to me. And there's no way that light from future events far away can get to me. Faraway events in the arbitrarily distant future never end up on my "past light-cone," the surface made of light rays that get to me at a given time.

observer falling into the BH will not see how the Universe ends, and even won't see any signals sent too late!

[Jonathan Scott]

No, after a certain amount of time has passed it will be impossible for light to catch up with him before he crosses the event horizon, and so it would definitely be impossible to send a rescue mission (which would travel slower than light). On the other hand, if he had powerful enough rockets, then he could at decide to turn round at any time before he has crossed the horizon.

I don't remember anything about a time limit from when I studied this area (which was admittedly long ago); I thought that it was simply necessary to get closer and closer to the speed of light to catch up the later you started. Can you quote a specific reference, please?

[Dmitry67]

Just take one of the diagrams and draw the worldlines there.

[Jonathan Scott]

Just take one of the diagrams and draw the worldlines there.

That's an example of what I mean by handwaving.

What I'd like to see is a bit of maths showing that there's a limit incoming light cone beyond which signals cannot reach the falling observer's geodesic. I don't remember having seen that calculation, but I should be able to work that out myself; it would just be easier if someone could point me to it.

[Ich]

What I'd like to see is a bit of maths showing that there's a limit incoming light cone beyond which signals cannot reach the falling observer's geodesic.
Look here (http://www.physicsforums.com/showthread.php?p=2417483#post2417483). There's a finite redshift at the EH, disproving the "he sees the future of the universe" thing. This could also be a starting point for your calculations.

[Chronos]

An observer inside an event horizon would suffer from reverse redshift. He/she would 'see' the universe age at infinite speed according to GR. Draw the light cone.

[Dmitry67]

observer inside the horizon reaches signularity in finite time. You mean, he will see everything before he hits singularity? this is not true and I am ready to draw a lightcone.

[Chronos]

My error, you are correct. Time will gradually speed up as observer approaches the singularity.

[Phrak]

in GR all coordinate systems are equally vaid. So in some coordinate systems BH forms in finite time. In the others we lose communication with the inner parts of BH.

Would you then argue just as animatedly against claims the existence of black holes?