APOD: When Black Holes Collide (2015 Oct 20)

[geckzilla]

If the recent realizations of the existence of dark matter and dark energy and the acceleration of the universe are anything to go by,
then a full understanding of HOW gravity operates is only very much in its infancy ?!

By this same logic I could say that because dark matter and dark energy were discovered recently, you yourself must be an infant. It doesn't make that much sense, though.

[Markus Schwarz]

If we go to the science bookstore and ask for a book on the finer details of precisely how gravity operates,
what choice of books do we have ? What science papers or journals spell it out in precise detail. ??

You can start with this. Even though it only has 1200+ pages, it covers many of the finer points. If you want to read more on the "cutting-edge" of gravity research, I recommend you look here, which has a dozen new scientific articles per day.

[geckzilla]

Only 1200+ pages, he says... Only.

[Chris Peterson]

If the recent realizations of the existence of dark matter and dark energy and the acceleration of the universe are anything to go by, then a full understanding of HOW gravity operates is only very much in its infancy ?!

Not at all. Dark matter has no impact on gravitational theory. Indeed, it is our understanding of gravity that allows us to detect dark matter. Dark energy could potentially impact gravitational theory, although most physicists doubt it will. It is a minority view that dark energy is a manifestation of gravity operating over cosmological distances. However, even if that turns out to be the case, we'd just be looking a small adjustment to GR, not replacing it with something completely different.

If we asked a conference of physicists and astronomers if gravity was a push or a pull, what split of the audience would we see. ?

We'd see close to 100% stating that gravity is an attractive force.

A Mr Einstein gave some very advanced (for the time) explanations of the 'what', but the 'how' is as yet still totally unexplained.

"How" is a meaningless question as you express it. "What" and "how" are usually the same thing. It bothers many non-scientists that they can't express scientific ideas in ways that are physically intuitive, but that doesn't point to any problem with the theories.

[suicidejunkie]

What I found rather interesting were the stages that the smaller black hole went through in it's death spiral. At a couple points it seemed to temporarily split into two separate black holes. Trying to mind experiment this but I am falling short of the manifestation of a third gravity well.

I believe that is an image of the larger black hole coming from the light being bent around the smaller hole.

You can see a similar effect on the larger hole, showing an image of the smaller hole. I'm guessing that it isn't 180 degrees away due to the extremely rapid motion and the fact that it takes the light significantly longer to travel a curve around the back side than it does to travel the shortest path towards the camera. You can see the lag on the second image of the small hole increasing as the merger happens, from a few degrees to about 90 at the 13 second mark as the merger approaches.

[MarkBour]

Among the galaxies observed to have a large central mass, and hence thought to have a supermassive central black hole, what would be the largest mass and the largest radius for an event horizon? What would be the radius of its Einstein ring?
...
Have we never seen a black hole transiting a star?

A black hole is only a few kilometers across. That's way too small for a transit to be detected photometrically with our current technology.

Thanks, Chris. I took some time to look this up and study the equations a bit. A stellar mass black hole would have a Schwarzchild radius (= event horizon) as you describe, perhaps a few km. So, as you noted, we'd not expect to be able to catch one passing in front of a star, even if there were quite a few roaming around the Milky Way. I wonder how common they are (noting that it's possible that they don't actually exist, current astronomical theory aside).

On the other hand, a supermassive black hole could be much larger. The one that might lurk in NGC 4889 has been estimated at 62 billion km for its Schwarzchild radius. While that's much larger, all of these type would be at galactic distances, and would pretty much always be obscured by a great deal of galactic material, so those will also be very hard to ever observe directly, in terms of visible light effects.

What I found rather interesting were the stages that the smaller black hole went through in it's death spiral. At a couple points it seemed to temporarily split into two separate black holes. Trying to mind experiment this but I am falling short of the manifestation of a third gravity well.

The simulation attempts to envision what one would see at a point near the danse macabre. Just because the image of the smaller black hole broke up, it does not imply that the black hole itself did so. There are many other items in the image that are moving around in the observer's view and showing multiple images, which are not actually undergoing those changes themselves. It is just that the light from those objects is being strongly manipulated by the two dance partners.

I suspect that this simulation did not take everything into account. Clearly, the two holes had a net angular momentum, but I don't think the effects of that angular momentum were necessarily preserved in the simulation after the merger.

[Markus Schwarz]

I suspect that this simulation did not take everything into account. Clearly, the two holes had a net angular momentum, but I don't think the effects of that angular momentum were necessarily preserved in the simulation after the merger.

If I remember correctly, angular momentum is not conserved in these situations (two massive objects rotating around each other and emitting gravitational waves). This is because the Einstein equations, which govern this behaviour and which the simulation solves, are non-linear.

[RocketRon]

Post by Chris Peterson » Wed Oct 21, 2015 2:21 pm

"How" is a meaningless question as you express it. "What" and "how" are usually the same thing. It bothers many non-scientists that they can't express scientific ideas in ways that are physically intuitive, but that doesn't point to any problem with the theories.
Chris

Observing something and explaining precisly how something operates are 2 VERY different things.
Including fabricating quite complex laws of observed and predicted behaviour, without understanding the 'how' whatsover.

Phlogiston theory was wildly popular and widely understood at one point in time,
and quite solidly evidence and observation based....

"With his theory of relativity, Albert Einstein explained how gravity is more than just a force: it is a curvature in the space-time continuum."
Where does the 'attraction' part fit into this ??
It would seem to be infinitely more complex than a mere simple attraction ?

And, why is there so much discussion out there in the ether about whether gravity is a push or a pull.
And higgs bosons seem to appear quite repeatedly in these discussions too
??

[RocketRon]

RocketRon wrote:
If we go to the science bookstore and ask for a book on the finer details of precisely how gravity operates,
what choice of books do we have ? What science papers or journals spell it out in precise detail. ??

Post by Markus Schwarz » Wed Oct 21, 2015 7:33 am
You can start with this. Even though it only has 1200+ pages, it covers many of the finer points. If you want to read more on the "cutting-edge" of gravity research, I recommend you look here, which has a dozen new scientific articles per day.

Can we find a precis of these 1200 pages,
rather than a lot of hedging about the bush....

E=MC squared was a very nice summary of a heck of a lot of theory,
does anything similar exist for gravity yet.

And will the Nobel Prize go to the man/woman who comes up with it - in our lifetimes maybe even - or not.

[geckzilla]

Phlogiston theory was wildly popular and widely understood at one point in time,
and quite solidly evidence and observation based....

If we were using the phlogiston equivalent to gravity, a bunch of things wouldn't work. Satellites, Apollo Program, GPS, predicting where a planet is located without having even observed that planet... there's a whole laundry list of things that work even with Newtonian gravity. If you want to say that our current understanding of gravity is likely to be overturned or only in its infancy, you are going to have to show us the holes in it rather than showing us that other things are not yet understood. Attack the theory of gravity itself, not unrelated things.

[daddyo]

Observing something and explaining precisly how something operates are 2 VERY different things.

Amen!

[Chris Peterson]

E=MC squared was a very nice summary of a heck of a lot of theory,
does anything similar exist for gravity yet.

Yes. It's called General Relativity.

And will the Nobel Prize go to the man/woman who comes up with it - in our lifetimes maybe even - or not.

No. Because it was developed by Einstein, and he's dead. So he can't win a Nobel Prize for this particular work.

[Chris Peterson]

Observing something and explaining precisly how something operates are 2 VERY different things.

Not necessarily. Gravity is precisely explained by General Relativity. It isn't an observation, but an explanation.

[RocketRon]

Some folks would like a more comprehensive explanation of precisely HOW gravity operates than Einstein provided.
Or is that where we leave exploring how this mystery operates....

Gravitons have been, and gone, how does that fit in here ???
Dark matter was 'discovered' (invented ?) because of anomalies in our current understanding of gravity,
ONLY when it was noted that the Universe was accelerating ??

Some might call this an ad hoc approach, rather than a comprehensive complete and thorough understanding of this subject ....

[RocketRon]

Not necessarily. Gravity is precisely explained by General Relativity. It isn't an observation, but an explanation.

So General Relativity perfectly explains Black Holes ?
And covers gravitons, gravity waves, gravitational radiation, dark matter, and dark energy.

Earlier you said that black holes were a few km across.(or something along those lines)
How do we know that ??
If someone suggested they were only a few mm across, could we debunk that ?

[RocketRon]

geckzilla wrote
If we were using the phlogiston equivalent to gravity,

Phlogiston actually explained things pretty well, for quite some time, until observations demonstrated a better explanation was required.
Thats how science works, after all.

Could be that 'gravity waves', 'gravitons', 'dark matter' and 'gravitational radiation' may fall into the same category.
When that new Nobel Prize winner steps up and explains all.... (?)
Or someone produces a commercial anti-gravity machine - that throws all that theory out the window (!!).

[geckzilla]

geckzilla wrote
If we were using the phlogiston equivalent to gravity,

Phlogiston actually explained things pretty well, for quite some time, until observations demonstrated a better explanation was required.
Thats how science works, after all.

Yes, until tested, and then it fell apart. General relativity has been tested and proven itself over and over again.

[Chris Peterson]

Not necessarily. Gravity is precisely explained by General Relativity. It isn't an observation, but an explanation.

So General Relativity perfectly explains Black Holes ?

Perfectly? Hard to know what that even means. But precisely? Pretty much, yes. We have a rich understanding of black holes, with only a few missing pieces. And that understanding is due to GR. And when we put together the last unknowns, it's highly unlikely that the role of GR in our understanding will change much.

And covers gravitons, gravity waves, gravitational radiation, dark matter, and dark energy.

Gravitons are hypothetical, and may not exist. If they do, they're more related to quantum mechanics than to gravitation. GR provides a complete explanation for gravity waves and gravitational radiation. Dark matter has nothing to do with gravitational theory (although GR provides a complete understanding of the observations which lead to the conclusion that dark matter exists). Dark energy remains poorly understood, but isn't generally recognized as a part of gravitation.

Earlier you said that black holes were a few km across.(or something along those lines)
How do we know that ??

Because they are described by theory that is extremely well supported by a wide range of independent observations and accurate predictions. If by "know" you mean we have 100% certainty, we don't. But that's not the standard for our understanding of nature. We "know" with a very high degree of confidence, and there are no viable alternate possibilities.

If someone suggested they were only a few mm across, could we debunk that ?

There would be no need unless they provided evidence for their assertion. We would either agree (if the evidence was solid), disagree with an explanation (if the evidence was faulty or didn't support the conclusion), or ignore them completely (if the assertion was made without supporting evidence at all). That's how science works.

[Chris Peterson]

Phlogiston actually explained things pretty well, for quite some time, until observations demonstrated a better explanation was required.
Thats how science works, after all.

Actually, phlogiston never explained anything well. That's because it was proposed before we had a well developed scientific method, and it was never well tested after it was proposed. At best it was a hypothesis, not a theory. Once people began to understand how to test ideas, it completely fell apart. GR, on the other hand, has been extensively tested over the last 100 years with the specific intent of showing if to be false or incomplete. That's actually how science works. It is a logical fallacy to compare prescientific "theories" to modern thinking.

(BTW, a consequence of our high quality scientific thinking is that we rarely see major theories fail anymore. That's because our knowledge has become very accurate. Science these days is much more likely to result in adjustments to existing theory than the complete replacement of ideas. We no longer appear to have major gaps or errors in our understanding of nature. It's more like we've put together most of a jigsaw puzzle, and are now engaged in placing lots of loose pieces in small gaps. But we can see the overall picture accurately.)

[neufer]

[b][color=#0000FF]Non-Schwartzchild RING singularity for REAL rotating Kerr black holes. Credit: N. Rumiano [/color][/b]

---

Click to play embedded YouTube video.

<<A ring singularity is the gravitational singularity of a rotating black hole, or a Kerr black hole that is shaped like a ring. Since a point cannot support rotation or angular momentum in classical physics, the minimal shape of the singularity that can support these properties is instead a ring with zero thickness but non-zero radius, and this is referred to as a ring singularity or Kerr singularity.

Due to a rotating hole's rotational frame-dragging effects, spacetime in the vicinity of the ring will undergo curvature in the direction of the ring's motion. Effectively this means that different observers placed around a Kerr black hole who are asked to point to the hole's apparent center of gravity may point to different points on the ring. Falling objects will begin to acquire angular momentum from the ring before they actually strike it, and the path taken by a perpendicular light ray (initially traveling toward the ring's center) will curve in the direction of ring motion before intersecting with the ring.

An observer falling into a Kerr black hole may be able to avoid the central singularity by making clever use of the inner event horizon associated with this class of black hole. This makes it possible for the Kerr black hole to act as a sort of wormhole, possibly even a traversable wormhole.

It is generally expected that since the usual collapse to a point singularity under general relativity involves arbitrarily dense conditions, quantum effects may become significant and prevent the singularity forming ("quantum fuzz"). Without quantum gravitational effects, there is good reason to suspect that the interior geometry of a rotating black hole is not the Kerr geometry. The inner event horizon of the Kerr geometry is probably not stable, due to the infinite blue-shifting of infalling radiation. This observation was supported by the investigation of charged black holes which exhibited similar "infinite blueshifting" behavior. While much work has been done, the realistic gravitational collapse of objects into rotating black holes, and the resultant geometry, continues to be an active research topic.>>

[Markus Schwarz]

Can we find a precis of these 1200 pages,
rather than a lot of hedging about the bush....

E=MC squared was a very nice summary of a heck of a lot of theory,
does anything similar exist for gravity yet.

Since you asked for it: R-R g/2+Lambda g=8 pi G T/c^4. You can condense it even further to G+Lambda g=8 pi T, if you want to. And no, its meaning not immediately clear. There is a reason why it took Einstein and others about 10 years to figure out, and still takes one dedicated course in university to explain. Today, there are many books for laymen available, too. You can criticise the above equation for not being complete, which is true in the sense that e.g. quantum mechanics does not fit into this picture completely, yet. But calling our knowledge about gravity "at its infancy" just shows your ignorance, IMO.

There were, and still are, alternatives to general relativity. But they either were proven inconsistent with observation, or require even more (untested) assumptions.

Concerning, the whole "push or pull", keep in mind that this was a problem for Newton and his contemporaries. They had problems with the whole concept of "action at a distance". This is because in their time all they had for "moving things around" where ropes, springs, levers, and such, all of which require a contact. Nowadays we understand "action at a distance" by (force) fields. See Wikipedia for more.

[RocketRon]

Chris Peterson wote

Actually, phlogiston never explained anything well.

Could we dare suggest that 'dark matter' may well be in the same category ?
If the present theory/rationalization of gravity doesn't explain things too well, just invent something invisible and immeasurable to make up the difference. If 75 or 80% of the universe is made of it, thats a BIG discrepancy and invention .... !

If we look at theories in astronomy and astrophysics in say another 100, 1000 and 10000 years, what will be the standout features.
A better understanding of how gravity actually works.
Or an entirely new version thereof.
This subject is begging for another Einstein-like vision ?

[RocketRon]

Markus Schwarz wrote

While we are here, take a look at this Universe Today article.

http://www.universetoday.com/75705/wher ... come-from/

Spot the flaws and fallacies ?
And the general trend towards gravity being less than fully explained.
Not recent, but what has changed...

[Chris Peterson]

Chris Peterson wote

Actually, phlogiston never explained anything well.

Could we dare suggest that 'dark matter' may well be in the same category ?

If by "we" you mean yourself, you may suggest whatever you like. Personally, I follow the evidence, and the evidence strongly supports the idea that dark matter consists of nothing more than particles with non-zero rest mass which don't interact (or interact very weakly) with the electromagnetic force. That is consistent with observation, with other theories (our standard model as well as our cosmology model), and we already know of particles like this (the neutrino).

If the present theory/rationalization of gravity doesn't explain things too well, just invent something invisible and immeasurable to make up the difference. If 75 or 80% of the universe is made of it, thats a BIG discrepancy and invention .... !

Dark matter makes up about 20% of the Universe. And it is something we observe- it is not invisible.

If we look at theories in astronomy and astrophysics in say another 100, 1000 and 10000 years, what will be the standout features.
A better understanding of how gravity actually works.

Better, yes. Substantially different than current theory, probably not.

Or an entirely new version thereof.

Possible, but very unlikely.

[Markus Schwarz]

While we are here, take a look at this Universe Today article.

http://www.universetoday.com/75705/wher ... come-from/

Spot the flaws and fallacies ?

No, I don't! Please tell me what's wrong and why!

And the general trend towards gravity being less than fully explained.

From your cited article: "[...] we’ve been building and building a more comprehensive view of gravity. But we’re still not complete". I agree with that, what's bothering you? And you will have a hard time finding anything that is "fully explained". There is a whole branch of philosophy that asks how can we know anything in science.