What I find really interesting is that black holes don't really "require" infinite density; the mass density inside the event horizon never exceeds neutron star levels, and becomes in fact lower the heavier the black hole gets (=> event horizon enclosed volume grows faster than mass).
We don't know if a black hole's singularity actually has infinite density because the volume being 0 means the density is undefined as division by 0 is undefined.
The event horizon radius grows linearly, so it's possible for a black hole to have a lower density than water (around 7 billion solar masses). Another curiosity is that if you plug in the mass of the regular matter in the observable universe into the schwarzchild radius equation (R = 2GM/(c^2)) you get a black hole event horizon larger than the the observable universe.
> it's possible for a black hole to have a lower density than water
But that density is not homogeneous. It's much denser in the middle. As you cross the event horizon you wouldn't notice anything happening (as long as the BH is big enough that the tidal forces were small so you didnt get spaghettified). But things would feel very weird as you got closer to the middle.
Probably. Nobody has proved any of this but it seems likely.
Therefore... what? The universe is a black hole? (If so, what if anything does that change? Is there a singularity that we hit within a finite proper time?)
Or the universe is not a black hole? If that's the case, why isn't it?
Is not a well-defined number. The number you are calculating assumes that the horizon is an ordinary 2-sphere enclosing an ordinary 3-ball [edited, see grandchild post] in Euclidean space, and divides the hole's mass by the calculated volume of that 3-ball. But the spacetime of a black hole doesn't work that way. The "space" inside the hole's horizon is not like any kind of ordinary space that your intuitions will work for.
Also, the interior of the hole is vacuum, so its mass is not something that's distributed throughout any kind of volume; it's just a geometric property of the spacetime.
You can't build an ordinary 3-sphere in 3D Euclidean space; a 3-sphere is a hypersphere. Are you considering 4D spacetime as the container of the 3-sphere, or did you just mean the volume of the 2-sphere in 3-space (as opposed to its shell area)?
The event horizon isn't the black hole. It's just the point of no return. As far as we know the black hole is a singularity with infinite density. But that probably isn't true. It's just a model and the model isn't perfect
I can't think of a way of getting a singularity. As soon as the mass in a region of space becomes high enough to start a black hole, time would freeze and it would take several millions universe lifetimes for the mass to fully collapse into itself, no matter how close of far the observer is to the mass. The way I see it, things would go on as usual inside the black hole, but there's a universal curtain preventing anyone from accessing the interior.
That's not how black holes work. It takes an infinite amount of time for the light at the event horizon to escape, true. But you can think of it as if it's an "optical effect".
If you are a free-falling observer, _nothing_ changes for you locally at the event horizon. You will cross it and reach the singularity within a finite (and quite short) time.
For large blackholes of course. And this actually makes other interesting property. Our universe is in fact giant blackhole with our own event horizon outside it. This also support cyclic big bang theory.
Once entire universe will collapse to singularity, the smallest possible particles shatters, leaving pure energy. Pure energy doesnt have "static" mass so no gravity => kaboom, we have new iteration. Theory of multiverse (multiple black holes aka universes) is out of scope here because we will never be able to escape ours.
The observables in our sky do not match any interior metric for a black hole. In particular generically black hole metrics concentrate a central mass, and (from a Raychaudhuri equation perspective) the known sources of shears and pressures that work against this concentration are insufficiently strong to block ever greater collapse. Additionally, shears and pressures enter into the stress-energy tensor. That's why the centre of a BH can be a singularity.
Observations of distant galaxies and the cosmic microwave background do match the (expanding vacuum) Lemaître-Robertson-Walker metric with the distribution of matter-density described by the Friedmann equations. Distant galaxies fill the sky and are receding from us, from our perspective; they are not concentrating in any sort of middle.
There is insufficient evidence to conclude that a big crunch is in our future.
"Pure energy" is represented in the stress-energy tensor just fine. General relativity's Einstein Field Equations exactly relates the stress-energy tensor to the curvature of spacetime. It gravitates, your argument just falls down.
One of the most remarkable aspects of physics is that Newtonian gravity calculations are quite spot on for all the parts of black holes layfolk consider important.
1. Both predict the same critical radius, now called the Schwarzschild radius.
2. Both predict a central singularity with formally infinite density in idealised collapse.
3. Both predict exactly the same finite time to fall into the singularity under equivalent idealised conditions.
4. Both give the same equation for radial free fall, using different definitions of time.
What GR added was meaning to these things (and of course precise calculations outside of very idealised scenarios and symmetric conditions).
One significant difference in interpretation that layfolks do consider important is that it's possible to exit from within the event horizon of a black hole in Newtonian gravity (without exceeding the speed of light) whereas in general relativity that is not possible.
In relativity the event horizon really is a point of no return, whereas in Newtonian physics it's the point at which escape velocity is equal to the speed of light. The escape velocity on the surface of the Earth is 11 km/s, but that doesn't mean that I as a human can't escape the surface of the Earth momentarily just by jumping and I certainly don't jump 11 km/s or even 1 km/s for that matter.
Similarly in Newtonian gravity it would be possible for an object to enter the event horizon of a black hole, and then "jump" back out at a much lower speed than the speed of light and then get sucked back beyond the event horizon, and you can in principle do this over and over again so long as you had some sufficiently powerful source of thrust.
In general relativity this is in principle not possible.
in what sense. the black hole is a result of the interaction of light with the curavture of space time. this is not modelled by newtonian gravity (or physics for that matter) at all. light is assumed to not interact with gravity in that framework in any way.
>light is assumed to not interact with gravity in that framework in any way.
Light absolutely interacts with gravity in Newtonian mechanics. One of the neat features of Newtonian gravity is that gravitational charge (which is mass) equals inertia so the two cancel each other out exactly in the equation of motion. No other force has this property.
This means gravitational acceleration is independent of inertia, even in the case where inertia/mass approaches zero.
>> Newtonian gravity calculations are quite spot on
Which is why the concept existed long long before any talk of relativity or time dilation, back before anyone knew the name Einstein. (He was like 4yo when this came out)
From American Physical Society:
"the true “father” of the black hole concept was a humble 18th century English rector named John Michell–a man so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than a century later."
Both have finite time falling in to the singularity. If you use a point test particle Newton and GR agree on that finite time.
That's proper time I.e. the infalling particles wristwatch time.
In GR, external observers would never see the infalling particle cross the event horizon but that's a coordinate artifact. If using our wrist watch time for that particle.
I thought I was the only one. But there is a really good video linked at the bottom of the page that has the two images and a summary of the work to build them.
I also loved the analogies of reading the date of a coin in LA from NYC or seeing a tangerine on the moon. But they kept talking about how surprisingly small black holes are. I wanted that described in relative terms. Maybe I just did not read carefully enough but are they the size of a pinhead? The solar system? I know I can look this up.
The EHT is a remarkable achievement in datagathering and collaboration. But I always sort of wonder, a bit of doubt if you will. seeing as the process(simplified nearly to the point of absurdity) is that of building a image generator based off what we think a black hole looks like then feeding it very vague spectral data and getting it to generate an image. isn't that a huge confirmation bias. That is. of course the black hole looks like simulated ones, that is how the generator was programmed to make them.
presumably the only way to get a good picture of a blackhole would be to get very lucky and catch a supernova going off next door, preferably at a slight oblique angle from one of the poles and be able to light the acretion disk and any dust
but I dont think we have the visible light gathering capacity to search at the distances required for even a slight chance of capturing the event, so the answer to the question is : you wait.
"So we had been perfecting these imaging algorithms on synthetic data. We’d fed these algorithms made-up data. So we would take a black hole image that we might be looking at. We’d corrupt it with noise, and we’d send it through these algorithms, and we’d refine these to the point where we were pretty confident in these algorithms."
Algorithms? For what? So they're not just taking a photo of something observed through a telescope.
They're not really explaining this either.
I'm trying to keep an open mind but I'm still not buying the whole black hole thing.
There needs to be something more intellectually compelling than "we know it's a black hole because we know it's a black hole".
Yes, you read that right. I am doubting this religion.
"That was a moment where there was a lot of crying and screaming."
another odd quote from this article, provided one is assuming this is supposed to be around the scientific method.
It's not taking a photo. It's computation, based on a lot of real data, with a lot of inference and synthetic data. The idea is that it's broadly accurate but cannot capture fine detail.
The approach has been calibrated against known objects like NRAO 530, a quasar near Sgr A* that has some known properties, and it's been used on both Sgr A* and M87. And the team has admitted there's a lot of uncertainty about exactly how accurate it is.
Here [1] is a pretty good article about how it works.
There's overwhelming scientific consensus that Sgr A* and M87* are black holes. It's not really provable at this point, but I don't think anyone doubts it.
Due to the insane distortion of "space"[1], every spare molecule in the vicinity is going to be lighting up at the speed of $E = mc^2$ . Which tends to be real visible. Black holes power some of the brightest things in the universe - light so powerful that it sometimes has its own gravity.
A tame black hole is possibly the least fussy high-efficiency furnace imagineable. Of course, "tame black hole" possibly . . . isn't.
[1] Whatever that happens to be, either bouncy rubber sheet or emergent property from deeper Hilbert Spaces or something beyond our imaginations. Or something that's just an illusion, an artifact of our specific perspective from our little spot in existence, like a rainbow.
What I find really interesting is that black holes don't really "require" infinite density; the mass density inside the event horizon never exceeds neutron star levels, and becomes in fact lower the heavier the black hole gets (=> event horizon enclosed volume grows faster than mass).
We don't know if a black hole's singularity actually has infinite density because the volume being 0 means the density is undefined as division by 0 is undefined.
The event horizon radius grows linearly, so it's possible for a black hole to have a lower density than water (around 7 billion solar masses). Another curiosity is that if you plug in the mass of the regular matter in the observable universe into the schwarzchild radius equation (R = 2GM/(c^2)) you get a black hole event horizon larger than the the observable universe.
> it's possible for a black hole to have a lower density than water
But that density is not homogeneous. It's much denser in the middle. As you cross the event horizon you wouldn't notice anything happening (as long as the BH is big enough that the tidal forces were small so you didnt get spaghettified). But things would feel very weird as you got closer to the middle.
Probably. Nobody has proved any of this but it seems likely.
Therefore... what? The universe is a black hole? (If so, what if anything does that change? Is there a singularity that we hit within a finite proper time?)
Or the universe is not a black hole? If that's the case, why isn't it?
3 replies →
> the mass density inside the event horizon
Is not a well-defined number. The number you are calculating assumes that the horizon is an ordinary 2-sphere enclosing an ordinary 3-ball [edited, see grandchild post] in Euclidean space, and divides the hole's mass by the calculated volume of that 3-ball. But the spacetime of a black hole doesn't work that way. The "space" inside the hole's horizon is not like any kind of ordinary space that your intuitions will work for.
Also, the interior of the hole is vacuum, so its mass is not something that's distributed throughout any kind of volume; it's just a geometric property of the spacetime.
You can't build an ordinary 3-sphere in 3D Euclidean space; a 3-sphere is a hypersphere. Are you considering 4D spacetime as the container of the 3-sphere, or did you just mean the volume of the 2-sphere in 3-space (as opposed to its shell area)?
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The event horizon isn't the black hole. It's just the point of no return. As far as we know the black hole is a singularity with infinite density. But that probably isn't true. It's just a model and the model isn't perfect
I can't think of a way of getting a singularity. As soon as the mass in a region of space becomes high enough to start a black hole, time would freeze and it would take several millions universe lifetimes for the mass to fully collapse into itself, no matter how close of far the observer is to the mass. The way I see it, things would go on as usual inside the black hole, but there's a universal curtain preventing anyone from accessing the interior.
That's not how black holes work. It takes an infinite amount of time for the light at the event horizon to escape, true. But you can think of it as if it's an "optical effect".
If you are a free-falling observer, _nothing_ changes for you locally at the event horizon. You will cross it and reach the singularity within a finite (and quite short) time.
I never thought about this, but I guess it makes sense. What an insane thing to consider - one more reason for to love neutron stars.
Some of.the biggest ones woild float on water... which tells you its not very useful to compare density based on Schwarzschild radius
For large blackholes of course. And this actually makes other interesting property. Our universe is in fact giant blackhole with our own event horizon outside it. This also support cyclic big bang theory. Once entire universe will collapse to singularity, the smallest possible particles shatters, leaving pure energy. Pure energy doesnt have "static" mass so no gravity => kaboom, we have new iteration. Theory of multiverse (multiple black holes aka universes) is out of scope here because we will never be able to escape ours.
The observables in our sky do not match any interior metric for a black hole. In particular generically black hole metrics concentrate a central mass, and (from a Raychaudhuri equation perspective) the known sources of shears and pressures that work against this concentration are insufficiently strong to block ever greater collapse. Additionally, shears and pressures enter into the stress-energy tensor. That's why the centre of a BH can be a singularity.
Observations of distant galaxies and the cosmic microwave background do match the (expanding vacuum) Lemaître-Robertson-Walker metric with the distribution of matter-density described by the Friedmann equations. Distant galaxies fill the sky and are receding from us, from our perspective; they are not concentrating in any sort of middle.
There is insufficient evidence to conclude that a big crunch is in our future.
"Pure energy" is represented in the stress-energy tensor just fine. General relativity's Einstein Field Equations exactly relates the stress-energy tensor to the curvature of spacetime. It gravitates, your argument just falls down.
<https://en.wikipedia.org/wiki/Stress%E2%80%93energy_tensor#/...>
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It is my understanding that energy produces a gravitational field regardless of the form it takes.
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What is “pure” energy?
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Of course, they didn't actually take a picture of a black hole. They took a picture of the matter surrounding it...
One of the most remarkable aspects of physics is that Newtonian gravity calculations are quite spot on for all the parts of black holes layfolk consider important.
1. Both predict the same critical radius, now called the Schwarzschild radius.
2. Both predict a central singularity with formally infinite density in idealised collapse.
3. Both predict exactly the same finite time to fall into the singularity under equivalent idealised conditions.
4. Both give the same equation for radial free fall, using different definitions of time.
What GR added was meaning to these things (and of course precise calculations outside of very idealised scenarios and symmetric conditions).
One significant difference in interpretation that layfolks do consider important is that it's possible to exit from within the event horizon of a black hole in Newtonian gravity (without exceeding the speed of light) whereas in general relativity that is not possible.
In relativity the event horizon really is a point of no return, whereas in Newtonian physics it's the point at which escape velocity is equal to the speed of light. The escape velocity on the surface of the Earth is 11 km/s, but that doesn't mean that I as a human can't escape the surface of the Earth momentarily just by jumping and I certainly don't jump 11 km/s or even 1 km/s for that matter.
Similarly in Newtonian gravity it would be possible for an object to enter the event horizon of a black hole, and then "jump" back out at a much lower speed than the speed of light and then get sucked back beyond the event horizon, and you can in principle do this over and over again so long as you had some sufficiently powerful source of thrust.
In general relativity this is in principle not possible.
in what sense. the black hole is a result of the interaction of light with the curavture of space time. this is not modelled by newtonian gravity (or physics for that matter) at all. light is assumed to not interact with gravity in that framework in any way.
>light is assumed to not interact with gravity in that framework in any way.
Light absolutely interacts with gravity in Newtonian mechanics. One of the neat features of Newtonian gravity is that gravitational charge (which is mass) equals inertia so the two cancel each other out exactly in the equation of motion. No other force has this property.
This means gravitational acceleration is independent of inertia, even in the case where inertia/mass approaches zero.
>> Newtonian gravity calculations are quite spot on
Which is why the concept existed long long before any talk of relativity or time dilation, back before anyone knew the name Einstein. (He was like 4yo when this came out)
From American Physical Society: "the true “father” of the black hole concept was a humble 18th century English rector named John Michell–a man so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than a century later."
https://www.aps.org/apsnews/2009/11/1783-john-michell-black-...
Could you provide a bit more detail? I don't quite see how one would define the event horizon for instance.
The radius at which the escape velocity is the speed of light.
Newtonian gravity gives infinite time to fall into black hole?
I thought you need at least special relativity for time dilution.
Both have finite time falling in to the singularity. If you use a point test particle Newton and GR agree on that finite time.
That's proper time I.e. the infalling particles wristwatch time.
In GR, external observers would never see the infalling particle cross the event horizon but that's a coordinate artifact. If using our wrist watch time for that particle.
6 replies →
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Is it just me, or could they have added a, say, picture of a black hole?
I thought I was the only one. But there is a really good video linked at the bottom of the page that has the two images and a summary of the work to build them.
I also loved the analogies of reading the date of a coin in LA from NYC or seeing a tangerine on the moon. But they kept talking about how surprisingly small black holes are. I wanted that described in relative terms. Maybe I just did not read carefully enough but are they the size of a pinhead? The solar system? I know I can look this up.
"bottom of the page that has the two images and a summary of the work to build them."
A picture + explanation below it would carry more weight than this entire article tenfold.
It's the one in your head that you picture, as not many have been taken:
https://www.si.edu/newsdesk/releases/astronomers-reveal-firs...
The EHT is a remarkable achievement in datagathering and collaboration. But I always sort of wonder, a bit of doubt if you will. seeing as the process(simplified nearly to the point of absurdity) is that of building a image generator based off what we think a black hole looks like then feeding it very vague spectral data and getting it to generate an image. isn't that a huge confirmation bias. That is. of course the black hole looks like simulated ones, that is how the generator was programmed to make them.
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Why does it look like it's moving to me?
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I remember times when the best available photo of Pluto looked like this.
This is a transcript of a 60-minute podcast. Would you like that TIFF in Zmodem format? Or they could read it out in base64?
Well, for starters you need a reasonably long exposure time.
there's a serious group of astrophysicists that actually disagree the photo was assembled properly at all
long paper made short I think their carefully demonstrated argument is it's a machine-learning hallucination to see what we want
Dr. Becky has excellent coverage of it, will link it in a moment when I find it
here it is, well worth watching:
* https://www.youtube.com/watch?v=9U6bvR6SzMo
Miyoshi, Kito & Makino (2024; claim the Sag A* image is wrong)
* https://academic.oup.com/mnras/article/534/4/3237/7660988
presumably the only way to get a good picture of a blackhole would be to get very lucky and catch a supernova going off next door, preferably at a slight oblique angle from one of the poles and be able to light the acretion disk and any dust but I dont think we have the visible light gathering capacity to search at the distances required for even a slight chance of capturing the event, so the answer to the question is : you wait.
Nothing beats a close-up.
First you'll need a camera that can detect mathematical equations.
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They got an image of something, yeah.
Or, did they?
Algorithms? For what? So they're not just taking a photo of something observed through a telescope.
They're not really explaining this either.
I'm trying to keep an open mind but I'm still not buying the whole black hole thing.
There needs to be something more intellectually compelling than "we know it's a black hole because we know it's a black hole".
Yes, you read that right. I am doubting this religion.
another odd quote from this article, provided one is assuming this is supposed to be around the scientific method.
It's not taking a photo. It's computation, based on a lot of real data, with a lot of inference and synthetic data. The idea is that it's broadly accurate but cannot capture fine detail.
The approach has been calibrated against known objects like NRAO 530, a quasar near Sgr A* that has some known properties, and it's been used on both Sgr A* and M87. And the team has admitted there's a lot of uncertainty about exactly how accurate it is.
Here [1] is a pretty good article about how it works.
There's overwhelming scientific consensus that Sgr A* and M87* are black holes. It's not really provable at this point, but I don't think anyone doubts it.
[1] https://www.discovermagazine.com/the-event-horizon-telescope...
I'd use a camera, personally
make sure you set the exposure to a negative value
f/0 should work ? :)
Easy. Space isn't empty.
Due to the insane distortion of "space"[1], every spare molecule in the vicinity is going to be lighting up at the speed of $E = mc^2$ . Which tends to be real visible. Black holes power some of the brightest things in the universe - light so powerful that it sometimes has its own gravity.
A tame black hole is possibly the least fussy high-efficiency furnace imagineable. Of course, "tame black hole" possibly . . . isn't.
[1] Whatever that happens to be, either bouncy rubber sheet or emergent property from deeper Hilbert Spaces or something beyond our imaginations. Or something that's just an illusion, an artifact of our specific perspective from our little spot in existence, like a rainbow.
Genuine question: why did you out $ signs next to the formula? I normally only see those put next to variable names.
LaTeX uses those to render math formulas. E.g. https://www.openmath.net/latex_reference.html
Some people are so used to it, that they can read in latex
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