Is it possible to explain how a black hole collapse can occur to a lay person with no phsyics background? It is hard for me to comprehend how something so dense could collapse, I can easily imagine an explosion.
A massive star is trying desperately to crunch down to as small a space as possible. This huge “pressure” forces light elements, like hydrogen and helium to fuse, releasing energy (“heat”)which causes the star to expand.
It’s this equilibrium between gravitational collapse and fusion driven expansion that lets stars live. Eventually though you reach a point where the elements fusing together effectively becomes consumers of energy. So now there is less “heat” pushing against gravity leading the star to begin to collapse. If it’s large enough it will collapse past the point where escaping the gravitation attraction of the body is greater than the speed of light.
I can’t believe you just explained that in a very short paragraph, I can’t thank you enough.
Note that, in the process, you also get a big explosion, which does in fact blow away a good portion of the star’s mass. Just, not all of it.
In white dwarf stars, there’s another form of pressure that pushes back against the pressure of gravitational collapse: degenerate pressure. This is a kind of pressure that comes from quantum mechanical effects. When the electrons in the core of the star are compressed to a high enough density, they are forced into higher and higher quantum energy states because the lower states are all filled. This, of course, requires energy; if the force of gravitational collapse can’t provide enough energy, the star can’t collapse further.
If the collapsed star is large enough, however, the gravity will be strong enough to force the electrons to combine with the protons in the atomic nuclei, which turns the protons into neutrons and creates a neutron star. The neutrons also have a degenerate pressure which prevents the star from collapsing further. If the star is large enough for gravity to overcome even that, then it either collapses into a black hole or possibly becomes a quark star.
You probably got it, but just to be clear: any body has an “escape velocity” that’s a function of the mass and the distance from the center of mass. On the Earth’s surface, that’s roughly 7 miles per second. If anything goes faster than that upwards, it will never return to Earth due to gravity.
The bigger the mass, the higher the escape velocity. Black holes happen when the mass gets big enough for the escape velocity (at some nonzero radius) to be greater than the speed of light.
During a collapse, this radius starts out very small, and grows rapidly once it starts sucking all the mass inside.
But as Grey so succinctly put, there’s more to it than mere mass; if there’s some force (and there are a number of different ones, different cases) that keeps the mass density low enough at the center to ever get to the point where the escape velocity reaches the speed of light, then a black hole doesn’t form.
In a neutron star, there are no atoms and few protons or electrons, they’ve collapsed into neutrons. The forces that keep neutrons from collapsing into each other keeps it from becoming a black hole, as long as the total mass isn’t enough to exceed that force. I think a neutron star is the densest thing that isn’t a black hole. The next less dense is a white dwarf. Not sure what’s after that.
For completeness: doesn’t have to be upwards. 7 miles per second in any direction that doesn’t run you into the ground will lead to escaping (assuming we’re considering only gravity and neglecting air resistance and the like).
That’s a fair statement. Other contenders are squarely in the hypothetical realm, with one being quark stars.
Thanks for the clarification.
The “escape speed greater than c” thing is a common explanation for black holes, and it gives the right numerical value for the event horizon radius, but it’s misleading. First of all, the calculation for escape speed isn’t a relativistic one as it should be: The correct calculation ends up with a couple of extra factors of 2 that cancel each other out. Second, you don’t actually have to go at escape speed to escape from a celestial object: That’s just the speed you need if you have one moment of thrust and coast from there, but if you’re able to thrust continually, you can escape at an arbitrarily low speed. But you can’t do that with a black hole.
A better explanation, I think, is that there simply isn’t any path from any point inside the event horizon to any point outside of it. Inside the hole, r is a timelike direction, not spacelike, with the outside in the past. So you can no more reach the outside of the hole than you can reach last Tuesday, no matter what speed you go at, or how you thrust.
Just curious and not being a smart ass but can your brain actually conceive this or is it just a matter of knowing how it works and accepting it?
I’m not entirely sure what distinction you’re making, there.
It’s possible to visualize it if you restrict yourself to two dimensions at once, make one of those dimensions time, and make sure you know how rotations work when you’re rotating into and out of the time direction. (You probably think of those kinds of rotations as “accelerations” and “decelerations”.)
The next step is to see gravity as creating acceleration which obeys the rules you learned about in the first link. This is a way to visualize how “falling due to gravity” is really “moving straight in a region of spacetime bent by mass”.
There’s a ton of resources out there for this if you search “Special Relativity” (the real geometry of spacetime) and “General Relativity” (how gravity comes about from the fact mass and energy deform spacetime in certain ways).
Let me start over. When a gas cloud contracts under gravity to become a star, it heats up from the release of potential energy. At a certain point, assuming it is about 80 Jupiter masses or more, the pressure at the center becomes hot enough to start a fusion reaction. This reaction fuses H to He at first. This increases the heat and pressure. When all the H at the center is fused into He and, assuming the star is sufficiently massive, the He begins to fuse into C. Some stars stop there (I believe our sun is one), grow into red giants and then gradually cool off. For the sun, this is estimated to take about 10 gY (billion years). Larger stars will have a hotter center that continues fusion until finally the center is producing Fe. But each of these reactions produces less and less energy. But producing all elements of higher atomic number requires energy and cannot ordinarily happen. But once the center stops producing energy, the core will collapse under gravity. If the star is sufficiently massive (the required mass is several solar masses, although how many varies with the different accounts I have read) even the degenacy pressure cannot contain it and in about a second, the star collapses totally (whatever that means) producing so much energy that all the further elements up to U (and beyond, apparently, although the short lived radioactives were never found on earth) and then the star explodes, spewing most of the star into the sky. What’s left is either a neutron star (just one extremely large nucleus, essentially) or a black hole, depending on mass. That explosion is called a supernova.
All the elements found on earth except for H and He come from the ashes of supernovas. The more massive a star is the shorter is its lifetime, so there was time for a lot of supernova ash when the sun formed about 8 gY after the big bang.
It’s a little more complex than this.
Remember the core of the star does not mix with the star outside the core. Which is to say the core does not get new hydrogen from outside the core. It has whatever it started with and that is it.
When the core has used up all its hydrogen fusion ceases. With no outward force from fusion balancing against the inward pull of gravity the core starts to shrink. As it shrinks it gets hotter and more dense. Eventually it will be sufficient to start fusing helium.
Here’s the thing, a new shell outside of the core will form where it is sufficiently dense and hot to fuse the hydrogen that was outside the core.
In time the core runs out of helium, starts to collapse some more till it gets dense enough and hot enough to fuse carbon (I think that is next). Outside of the core there will then be a ring of fusing helium and around that a ring of fusing hydrogen.
And so it goes till the core is filled with iron. At this point the star will be like an onion with different layers fusing different things. Once iron is all that is left in the core the star is as good as dead. It take more energy to fuse iron than iron gives off. The core will now start a collapse that will only be stopped by degeneracy pressure and explode as a supernova. In these last moments (it all happens very, very fast at this point) the star fuses all the other elements (such as uranium and gold). If the core is massive enough (1.7 times the mass of our sun) nothing will stop its collapse and it will shrink into a black hole.
FWIW our star is too small for such a climatic ending and will instead eventually settle down to be a white dwarf star.
By the same token, once past the event horizon, regardless of how powerful your rocket engines are, you can no more avoid falling into the singularity than you can avoid tomorrow. I’ve long been fascinated by that consequence of the math describing the spacetime geometry of a black hole, but is it not really rather speculative? That is, the way the signs change places between t and r beyond the event horizon suggest this intriguing possibility, but how can we ever know that this math is a valid model of the inside of a black hole? I suppose it’s the best conceptualization we have.
Great care should always be taking when interpreting coordinate systems physically: the switching of the ‘r’ and ‘t’ Schwarzschild coordinates from spacelike to timelike and vice versa when going from outside to inside the event horizon is due to the bad behaviour of the coordinates at the event horizon when extending them through the EH. A different choice of coordinates can eliminate this bad behaviour (though possibly replace it with some other bad behaviour). It’s also worth noting that a similar switching happens in Minkowski spacetime (i.e. flat spacetime) when extending Rindler coordinates through the Rindler horizon. In other words the switching is a property of the coordinate system, though that’s not to say physical implications can’t be drawn from the switching in the case of the Schwarzschild black hole.
What lies beyond the event horizon is an interesting question, but the least would should assume is that the basic physics that predict the existence of the interior region are the same as to assume otherwise would be paradoxical. In a Schwarzschild black hole anything inside the event horizon will meet the singularity, though that needn’t be the case in a general black hole. For example the interior region of a rotating and/or charged black hole has certain (rather odd) features that don’t occur in a Schwarzschild black hole. However as the interior regions of such black holes are inherently unstable, the interior region of a realistic black hole is thought to be similar to that of a Schwarzschild black hole.
But no matter what coordinate system you use, it remains true that any path from inside the horizon to outside goes backwards in time, and any path forward in time eventually reaches the singularity (assuming a Schwarzschild hole or that the infalling particle has nonzero mass).
As a hijack, if I’m inside the event horizon what would I see looking outwards?
What do you mean, “looking outwards”? Are you trying to point your face towards the outside? Because you can’t do that, any more than you can point your face towards yesterday.
Away from the singularity or along the path I took to enter the event horizon.