Comment by myrmidon
1 day ago
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).
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?
The universe is not a black hole because the "Schwarzschild radius equation" only applies in a very specific/narrow circumstance, namely a perfectly spherical, non-rotating mass surrounded by vacuum in flat spacetime.
Applying that equation to the observable mass of the universe violates basically every single requirement needed to apply the equation.
Often it's taken out of context by pop sci channels to propose the universe "could be" or even "is" in a black hole. But from my understanding, as a non-physicist, it's not surprising or interesting to most physicists that the math works out like this, and there's nothing else to suggest that we're in a black hole.
I don't think it means anything, it's probably just a curiosity. Simple equations like that probably don't work correctly when applied universe-scale.
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 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)?
Sorry, I meant to say 3-ball bounded by a 2-sphere in Euclidean space. The edit window of my GP post is still open so I've fixed it.
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#/...>
Thank you for your comment. I've heard the parent argument earlier as well. I wasn't aware of these points.
Science mapped out our solar system and developed base12 to account for rotation and orbit.
What I haven't found is an accounting of our solar system's patterns in our galaxy.
We know about axial precession but we don't know the official cause and some suspect it's related to a pull from another galaxy or a swayish wobble from our solar system's rotation in the galaxy.
Same for a supposed magnetic flip in the earths core. They might be related but we seemingly only know of the precession measured as 1 degree per 72 years but the magnetic north pole doesn't align with any noted patterns nor cause.
It is my understanding that energy produces a gravitational field regardless of the form it takes.
That's correct, in fact almost nothing Borg3 has said is remotely accurate.
One cool thought expressed by Penrose is the possibility that all matter will eventually decay or annihilate and all that's left will be massless radiation such as photons. If this happens then the concept of space and time itself will cease since from within the reference frame of a photon space and time are undefined.
In this scenario the distinction between an infinitely expanded, cold, and dilute universe and an extremely small, hot, and dense universe becomes undifferentiable. Without any physical scale by which to distinguish the two, the far distant future of our universe where all matter has disappeared can be equivalently viewed as the big bang of a new universe.
This is what Penrose calls conformal cyclic cosmology [1]. There was a time when physicists debated whether the universe would continue expanding forever or whether gravity would eventually halt its expansion and cause it to collapse back in on itself in a Big Crunch, potentially giving rise to another Big Bang. Conformal cyclic cosmology allows for an endless repeating cycle of universes even in the case where the universe expands forever, by identifying the infinitely distant future of one universe with the Big Bang of the next.
This is highly speculative and almost certainly false, but it's at least grounded in actual physics and is kind of poetic.
[1] https://en.wikipedia.org/wiki/Conformal_cyclic_cosmology
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What is “pure” energy?
It is the subtitle of a #3 synth-pop hit by Information Society, which debuted only 16 months before the release of the unrelated Belgian techno anthem, "Pump Up the Jam".
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Photons? They cannot be stopped, because they have no "static" mass? As far as they move, they react to gravity, not sure if they can provide its own gravity field. Might be a case as far as they move. Not expert here :)
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