Are Black Holes Proven or Just Theoretical Constructs?

[stglyde]

Are Black Hole 100% proven already or are they just theoretical construct? If the latter, how come the news item in the following speaks as if Black Holes are 100% certainty?

http://www.bbc.co.uk/news/science-environment-16178112

 

[twofish-quant]

Are Black Hole 100% proven already or are they just theoretical construct? If the latter, how come the news item in the following speaks as if Black Holes are 100% certainty?

Nothing is 100%, but I'd put existence of black holes as 99%.

 

[e.bar.goum]

Well, we can observe really really massive things (in the center of the Milky way, for instance) that seem to behave as we would expect black holes to behave, as far as we can see. Whether or not one is willing to call them a black hole is a matter of taste. Most people do though.

 

[twofish-quant]

Well, we can observe really really massive things (in the center of the Milky way, for instance) that seem to behave as we would expect black holes to behave, as far as we can see. Whether or not one is willing to call them a black hole is a matter of taste. Most people do though.

It's one of those. It looks like a duck. It quacks like a duck. No one has come up with a better explanation for it than being a duck. How sure do you have to be that it is a duck before you call it a duck?

 

[e.bar.goum]

It's one of those. It looks like a duck. It quacks like a duck. No one has come up with a better explanation for it than being a duck. How sure do you have to be that it is a duck before you call it a duck?

Indeed! Black holes are an extreme example of this (rather common) phenomenon - we cannot ever actually observe a singularity directly, so, that which we call a black hole is one that looks and quacks like we expect one to.

 

[phinds]

... we cannot ever actually observe a singularity directly ...

Yes, but we MAY some day (not any time soon, though) be able to observe a black hole as it appears from the outside; that is NOT an impossibility. Observations will still be somewhat indirect but they will be direct enough to conclude that it IS what we call a black hole.

 

[e.bar.goum]

Yes, but we MAY some day (not any time soon, though) be able to observe a black hole as it appears from the outside; that is NOT an impossibility. Observations will still be somewhat indirect but they will be direct enough to conclude that it IS what we call a black hole.

Well yes, as it appears from the outside being the key phrase - you still can't see behind the event horizon, and say "yep, that's a singularity", as I interpreted the OP. Space-time outside the black hole could still be examined though, and that would be good enough.

 

[Chronos]

The only thing reasonably certain about a black hole is it has an event horizon. That does not necessarily require a singularity.

 

[phinds]

The only thing reasonably certain about a black hole is it has an event horizon. That does not necessarily require a singularity.

Just the point I was about to make in response. Thanks.

There have even been hypotheses that there IS no singularity in a BH but rather some sort of unbelieveably dense plasma. I don't "get" that, but it's what I've read.

 

[stglyde]

Just the point I was about to make in response. Thanks.

There have even been hypotheses that there IS no singularity in a BH but rather some sort of unbelieveably dense plasma. I don't "get" that, but it's what I've read.

In the Big Bang what is the minimum possible length of the original core (1 Mile)? How is it calculated? Whatever it is, the Black Hole core (singularity or whatever) can't be smaller than the BB original core.

Also does it mean the BH core of masses equal to billions of suns is the same size as a BH with few times the solar mass?

 

[e.bar.goum]

The only thing reasonably certain about a black hole is it has an event horizon. That does not necessarily require a singularity.

For sure, and kind of my point. I think we're actually in violent agreement here.

 

[twofish-quant]

There have even been hypotheses that there IS no singularity in a BH but rather some sort of unbelieveably dense plasma. I don't "get" that, but it's what I've read.

I do and those papers are non-sense papers written by people that don't understand general relativity. This issue was settled in the 1970's, but you have the occasion paper from people without clue.

 

[phinds]

I do and those papers are non-sense papers written by people that don't understand general relativity. This issue was settled in the 1970's, but you have the occasion paper from people without clue.

Thanks for that. I was not aware.

 

[phinds]

In the Big Bang what is the minimum possible length of the original core (1 Mile)?

There is no such thing. There was no center to the BB.

 

[Pengwuino]

I do and those papers are non-sense papers written by people that don't understand general relativity. This issue was settled in the 1970's, but you have the occasion paper from people without clue.

In what sense would it not have a singularity according to these people? I mean, I'm under the impression the true nature of the singularity when you reach the quantum scale still need a theory of QG to really be talked about. Or are they saying that there's no singularity even at the macroscopic scale of GR?

 

[stglyde]

There is no such thing. There was no center to the BB.

I mean, not the center.. but the original core.. do you actually believe the entire universe with billions and billions of galaxies was once the size of an atom.. if not.. what's the size in miles before Inflation.

 

[Chronos]

I think the Planck density is a plausible limit on density. But, there is nothing to assure us quantum mechanics was not an emergent property of the universe. So, it is certainly conceivable the big bang originated from a true singularity.

 

[Pengwuino]

I mean, not the center.. but the original core.. do you actually believe the entire universe with billions and billions of galaxies was once the size of an atom.. if not.. what's the size in miles before Inflation.

There would be nothing preventing a sufficiently massive object from continuing to collapse into a singularity. So, Sun, Galaxy, or entire Universe, there's no reason (yet) to believe there's a point where a gravitational collapse would stop at some very small radius. In terms of the Big Bang, there is no reason to believe (yet) that there was a finite radius that the universe had to start out of. I mean to say that there is no minimum radius that must exist no matter how massive something is, not that the Big Bang was the result of a collapse.

(Edited for much clarity)

 

[Chronos]

There is a flaw in that logic. What collapsed to form the BB singularity?

 

[Pengwuino]

There is a flaw in that logic. What collapsed to form the BB singularity?

Ewww what horrible wording I chose.

 

[stglyde]

There would be nothing preventing the gravitational collapse, so our current understanding is yes, everything did collapse to a real singularity with no radius (or this is as much as we can say until we know the correct quantum theory of gravity).

So a 100 billion sun mass black hole has the same singularity or core size as a black hole a few times the sun mass and what differs is the mass.. just like the same wire can emit different magnetic field (or mass in analogy) resulting in different event horizon sizes for both (a few kilometers to a few light years)?

 

[Chronos]

The Planck density is quite high, so the radius of universe mass black hole would be virtually indistinguishable from the radius of a solar mass black hole.

 

[stglyde]

Black hole event horizon is said to be the point of no return.. that the cliche goes.. "not even light can escape..." but for quantum entangled pair.. inside and outside the black hole. They are still correlated right?

 

[stglyde]

I think the Planck density is a plausible limit on density. But, there is nothing to assure us quantum mechanics was not an emergent property of the universe. So, it is certainly conceivable the big bang originated from a true singularity.

I know the Big Bang created or expanded space and time but when it is just 1 mile in length. There is an edge.. the edge of the 1 mile spacetime ball. But it is said the universe has no edge.. how do you explain that?

Another thing. When we heard the phase how many angels can dance in a pin head. We laugh how ridiculous it sounds. But in actual life, many men on the street are not aware that the question "how many galaxies can fit in a pin head" has an answer and it is "All of the known galaxies". Anyway. How do you calculate again for this Planck density? It's so fantastic that all the universe as pure energy can be contracted to a Planck size region and can fit there. Is the calculation definite or just theoretical?

 

[D H]

I know the Big Bang created or expanded space and time but when it is just 1 mile in length. There is an edge.. the edge of the 1 mile spacetime ball. But it is said the universe has no edge.. how do you explain that?

The universe is not and never was a black hole.

The only thing that "Big Bang singularity" and "black hole singularity" have in common is that word "singularity." The natures of these singularities are quite different, and most physicists think that these singularities aren't quite real.

Ask astrophysicists what is at the center of a black hole and they will answer that general relativity says that the center of a black hole is a gravitational singularity. They will also be quick to point out that most physicists also think that this indicates that there is some unknown flaw in general relativity. Do black holes have an event horizon? Yes. Is all mass/energy truly concentrated at a single point? Almost certainly not.

Ask cosmologists about what happened at t=0 and they will answer that the standard cosmological model says that at the instant of the big bang universe was a singularity. They will be even quicker to point out that this supposed singularity was nothing like a black hole. Moreover, nobody believes that the universe truly was a singularity. Singularities in a physical theory are an indication that the theory isn't quite right. A better answer is that standard cosmological models currently can't answer questions about what happened in the first 10^-30 seconds of the universe.

 

[stglyde]

The universe is not and never was a black hole.

The only thing that "Big Bang singularity" and "black hole singularity" have in common is that word "singularity." The natures of these singularities are quite different, and most physicists think that these singularities aren't quite real.

Ask an astrophysicist what is at the center of a black hole and they will answer that general relativity says that the center of a black hole is a gravitational singularity. They will also be quick to point out that most physicists also think that this indicates that there is some unknown flaw in general relativity. Do black holes have an event horizon? Yes. Is all mass/energy truly concentrated at a single point? Almost certainly not.

Ask a cosmologist about what happened at t=0 and they will answer that the standard cosmological model says that at the instant of the big bang universe was a singularity. They will be even quicker to point out that this supposed singularity was nothing like a black hole. Moreover, nobody believes that the universe truly was a singularity. Singularities in a physical theory are an indication that the theory isn't quite right. A better answer is that standard cosmological models currently can't answer questions about what happened in the first 10^-30 seconds of the universe.

When someone mentions about "first 10^-30 seconds of the universe", are you talking of real time like in my watch? Or some kind of cosmic time? This is because spacetime is just being giving birth so can the time duration be the same as it is now?

 

[Calimero]

I know the Big Bang created or expanded space and time but when it is just 1 mile in length. There is an edge.. the edge of the 1 mile spacetime ball. But it is said the universe has no edge.. how do you explain that?

Well, surface of the Earth doesn't have an edge. You can walk in any direction you like, as long as you like, and never reach an edge. That would be the simplest way to grasp edgeless universe. Furthermore, we don't know how much space is out there, so we talk about the rate of expansion mostly. For example, during inflation which lasted from 10^-36 seconds after the BB until 10^-32 seconds, universe expanded 10⁷⁸ fold. Or another common example, since the emission of CMB, some 380 000 years after the BB, until now - 13,7 Billion years after the BB, universe expanded 1100 fold.

 

[stglyde]

The universe is not and never was a black hole.

The only thing that "Big Bang singularity" and "black hole singularity" have in common is that word "singularity." The natures of these singularities are quite different, and most physicists think that these singularities aren't quite real.

I have never thought that the universe was a black hole. So I guess black hole can only occur when there is an existing spacetime. What would happen if for sake of illustration the dark energy were removed and the universe suffered a big crunch where all things would compact into a singularity (or near Planck scale). All the know spacetime would compatify so there would no longer be any space outside the singularity of the black hole. This means no event horizon can form?

Also we don't know what happens in a singularity where anything can occur. What's the refutation that we were not really inside a singularity now?

 

[stglyde]

Well, surface of the Earth doesn't have an edge. You can walk in any direction you like, as long as you like, and never reach an edge. That would be the simplest way to grasp edgeless universe. Furthermore, we don't know how much space is out there, so we talk about the rate of expansion mostly. For example, during inflation which lasted from 10^-36 seconds after the BB until 10^-32 seconds, universe expanded 10⁷⁸ fold. Or another common example, since the emission of CMB, some 380 000 years after the BB, until now - 13,7 Billion years after the BB, universe expanded 1100 fold.

I guess it would help by imagining the Big Bang as giving rise to 4D space and time, not just space isn't it. In order words, the Big Bang expands the differential manifold and not space as we know it. Right?

 

[Calimero]

When someone mentions about "first 10^-30 seconds of the universe", are you talking of real time like in my watch? Or some kind of cosmic time? This is because spacetime is just being giving birth so can the time duration be the same as it is now?

Both. Cosmic or cosmological time is exact term. For observers moving only with Hubble flow, being at rest with respect to expansion, it is in the sense of your question same as time on your watch.