By now, if you are paying attention, you know that the reason we don’t fall to the center
of the Earth, where spacetime flow is directing us to go, has to do with support forces, pushing us up. Drop a feather in air, the air pushes up on it, stopping it from accelerating with spacetime. Drop a rock and when it hits the ground, the ground pushes up on it, keeping it from falling more. Drop that same rock in a lake and, even before the bottom of the lake stops its descent, the water slows its travel with spacetime. How fast would these objects travel near Earth’s surface if there was nothing to slow them? They would increase their speed by 9.8m/s (about 32ft/s) for each second of fall. This rate is dictated by the specific amount that Earth’s mass curves spacetime, and is different for planets, moons, and stars of different masses and how close you are to them. For a refresher on how this works, see my With or Without You (Newton) article, which explains it in more detail.
To understand why planets are spherical, and not weird, potato shapes, we can use Einstein’s General Theory of Relativity. It is the amount of spacetime curvature which dictates whether there is enough “gravity” to form a sphere. Where there is more mass, there is more curvature. The more spacetime curvature, the harder it is for the mass to not follow the flow of spacetime to the center of the mass (which crushes it into a spherical shape). Once there is enough mass to cause a moon or planet to become spherical, in larger objects, what force can stop it from continuing to collapse, from crushing itself out of existence?
Here, Einstein does not have the answer. We need electromagnetism and quantum theory to understand. When electrons get close to each other, they repel. Not only that, but according to the Pauli Exclusion Principle, no two electrons can be squished into a space, occupying the same quantum state (spin, energy level, etc.). This gives them the ability to exert outward pressure when squished. We call this electron degeneracy pressure.
Stars exert thermal pressure to keep themselves from collapsing under the forces caused by the curved spacetime. Nuclear fusion in their cores provides the energy which pushes outward against the powerful gravitational flow of spacetime generated by the star’s huge mass and energy. Larger stars fuse hotter and faster than low mass stars, who live longer (stars are people too). When a low to intermediate-mass star finishes fusing elements, the outward pressure drops and the material left over condenses, becoming a white dwarf. It condenses because it now lacks the thermal pressure from fusion, but it stops collapsing when the Exclusion Principle comes into play, electrons pushing out (degeneracy pressure), supporting the new mass, in a very hot, dense environment.
When a larger star stops fusing elements, it explodes, and if its leftover mass is from 1.4-2.9 solar masses, spacetime is so curved that the crushing force produced is able to smash the electrons into the nuclear protons, leaving only neutrons. Thus, a neutron star is born. The matter that makes up this star is incredibly dense. One teaspoonful of it would weigh a billion tons (Go Mjollnir)! Just like the electrons, neutrons have their own degeneracy pressure they can produce when squished in this extreme way (Pauli’s Exclusion Principle again). This alone keeps the star from collapsing into a black hole.*
If you recall that neutrons are made of quarks, you might predict, as scientists do, that there is yet another level of crushing that could happen before a black hole is formed. With more mass, it is theorized that even the neutron degeneracy pressure isn’t enough to support the extreme spacetime curvature and the neutrons are crushed into their constituent quarks, creating a quark star held up by, you guessed it, quark degeneracy pressure. The existence of quark stars has yet to be verified, but strangely enough, the weirdest objects that exist, black holes, have been.
With more than three solar masses left over after the collapse and explosion of a large star, the matter that is left warps spacetime so drastically that nothing can resist the flow of its river. Not even light can get out, and thus the name “black hole” really applies. Without support, the density of the matter in a black hole approaches infinity when the volume of all the mass inside shrinks to zero. When infinities begin appearing in physics equations, physicists assume something is broken, that the theory is incorrect, and deep inside the black hole, they still believe the equations fail to tell us the complete truth. However, the idea that spacetime is curved so much that even light cannot escape has been verified by direct visualization of two supermassive black holes, and indirectly by many more.
Now, I know what you’re thinking, “How can you directly visualize something that by definition cannot be seen?” Good question. The center of a black hole will just appear black, but the edge is what really tells us we’re looking at a black hole. Just as Einstein’s equations predict, at the edge, light is curving with the fabric of spacetime, accelerating any particles that happen to get close in these curved paths at nearly the speed of light, which release massive amounts of radiation, which is what we see. That these bizarre extremes of physical reality actually exist, would have surprised even Einstein himself. In fact, Einstein wrote a paper in 1939 in an attempt to disprove the possibility of black holes. Since then, many other famous scientists used his Relativity equations to show that in this case, Einstein was wrong. Black holes can and do exist.
We all need support. Without it, we couldn’t walk around and eat pizzas. Without support, stars would not shine. They would collapse into black holes. Even Earth would do this with no support. I don’t know about you, but I am really grateful for all the support I am given on a daily basis. I don’t want to live in a black hole.
*Please watch this excellent ScienceClic Video to understand the concepts at a deeper level.
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