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The Sun, Black Holes and Light

Nikkolas

Nikkolas

Location
Texas, US
So fair warning, it's been over 10 years since I was in high school and even then I took a very low-level science class because I hated science. The result is I don't really understand basic concepts.



This video was a guy trolling pretending he was a Flat Earther and demanding evidence for a spherical Earth. The pro-science people said gravity effects all mass which is why a black hole can absorb even light. The false Flat Earther asked "The Sun has a lot of gravity, why doesn't it just draw in its own photons?" The response was photons have no mass or almost no mass and also maybe that blackholes absorb energy or bend space time and that's why light can't escape? It was very confused and jumbled.

I don't believe the Earth is flat, by the way. But it's sort of like how I know my computer will turn on if I push a button. I couldn't explain why or how it does this. Same for all the argument about mass and gravity. Can anybody help?

Sorry, I'd totally fail "Are You Smarter Than A Fifth Grader?"
 
Photons are pulled back by the sun's gravity, as they exchange some of their energy for gravitational potential energy, they redshift because that's what photons do when they lose energy.

You can do the same with a flashlight point the light straight up and look down at it and it will be redder as the photons that reach you have been struggling to escape Earth's gravity well... And if you hold the flashlight above you and shine down on yourself it will be bluer as they gain energy. Of course earth's gravity is so slight that the difference is nigh imperceptible even to fancy instruments, never mind mere eyes.

One way to look at an black holes event horizon is that it's the point at which a photon going radially outward gets redshifted to infinity... Which means a photon with no energy... Which is the same as saying it doesn't exist.
 
One of the things about gravity is that distance from the center of the object matters almost as much as the object's mass. So denser objects have stronger surface gravity.

Black holes are physics-breaking objects of infinite density. Stars are not. That's why photons can escape a star but not a black hole's event horizon.
 
The Sun's schwarzschild radius is about 3 kilometers. Ergo if I understand correctly light from that teeny tiny section does not in fact escape. Outside of that, it can still take a 100,000 years for light to get from the core of the Sun to the surface, even though the Sun has a radius of 2.3 light seconds, though more than just gravity plays a factor in that.

As for why it doesn't collapse, gravity is in a contest with the nuclear forces that maintain the shape of atoms, and losing because nuclear forces are stronk and gravity is weaksauce. Neutron stars are an object where the mass and gravity has scrunched things up to the point that you no longer have discrete atoms and just get a big mush of neutrons, and black holes when the schwarzschild radius exceeds the object radius and the whole thing scrunches up to an infinitely small point. Well actually naked singularities wouldn't but we aren't sure if those exist, or possibly if they even can exist.
 
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firefossil said:
The Sun's schwarzschild radius is about 3 kilometers. Ergo if I understand correctly light from that teeny tiny section does not in fact escape.
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At 3 kilometers from the center the force of gravity depends only on the mass inside that 3-kilometer radius, with the force of gravity for all mass above that radius cancelling out as per the shell theorem. The force of gravity at the exact center of the sun (or any other spherically symmetric massive object) is actually zero.
 
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firefossil said:
Outside of that, it can still take a 100,000 years for light to get from the core of the Sun to the surface, even though the Sun has a radius of 2.3 light seconds, though more than just gravity plays a factor in that.
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Gravity plays no factor is that. Light moves at light speed unless there is stuff to interact with, then the interaction slows it down, the gravitational field can curve the path light takes and red or blue shift it, but it can't affect it's speed. Only matter can do that, and even then you have to be careful about what's going on.
 
There's a bunch of ways to think of this. The most direct, but probably not the most intuitive, is that for a Schwarzschild black hole, spacetime is bent in such a way that there is no (future-directed) trajectories that lead outside. No matter which direction you turn, it's toward the singularity.

One alternative way to think of a Schwarzschild black hole is to ask how it looks not to stationary observers, but to freefalling ones, starting far away at rest. In that case, space is flat at every instant in time, but falling towards the centre at the escape velocity, analogously like water down a drain.
So to go farther way, one should 'swim against the current'. The event horizon happens when the inflow velocity in the speed of light, so once inside everything is inevitably carried inward.

This analogy is mathematically exact, and is allows for hydrodynamical simulation of black holes; there will be a sonic event horizon when the inflow velocity reaches the speed of sound, and no sound signal can propagate outside. The difference is that there is no genuine physical problem with particles going locally faster than the speed of sound. ... Isolated rotating black holes have a similar but more complicated inflow+twist picture, but general GTR situations do not.

Catty Nebulart said:
You can do the same with a flashlight point the light straight up and look down at it and it will be redder as the photons that reach you have been struggling to escape Earth's gravity well... And if you hold the flashlight above you and shine down on yourself it will be bluer as they gain energy. Of course earth's gravity is so slight that the difference is nigh imperceptible even to fancy instruments, never mind mere eyes.
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Though using a radioactive source rather than a flashlight, the Pound–Rebka experiment (ca. 1960) measured the gravitational redshift across a four-story building. The frequency shift was a few parts per 1E15.
 
The main thing is that the Sun's gravity isn't strong enough to keep light in. It does bend light a little, which can be measured during solar eclipses.

firefossil said:
As for why it doesn't collapse, gravity is in a contest with the nuclear forces that maintain the shape of atoms, and losing because nuclear forces are stronk and gravity is weaksauce.
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The strong nuclear force doesn't matter until neutron stars. Ordinary stars are held up by the pressure of hot gas, white dwarfs by electron degeneracy pressure.
 
Vorpal said:
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This analogy is mathematically exact, and is allows for hydrodynamical simulation of black holes; there will be a sonic event horizon when the inflow velocity reaches the speed of sound, and no sound signal can propagate outside. The difference is that there is no genuine physical problem with particles going locally faster than the speed of sound. ... Isolated rotating black holes have a similar but more complicated inflow+twist picture, but general GTR situations do not.
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Hydrodynamical simulation of black holes is very very cool:



I like the experimental practicality of it, compared to the massive accelerators or satellites that are usually used to impress the public with physics, it's so simple and clever.

Almost something you could do in your own bathtub.
 
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