Mass-Energy Equivalence
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05-05-2017, 05:18 AM
RE: Mass-Energy Equivalence
My understanding of a black hole is that it breaks down everything until only gravity remains.

It's a large gravity well that pulls everything to it.
If light enters, the energy of that light adds to the gravity.

My understanding of it is quite old, so I'm probably wrong about all of this and more than happy to be corrected.

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05-05-2017, 06:41 AM
RE: Mass-Energy Equivalence
(05-05-2017 05:18 AM)Rahn127 Wrote:  My understanding of a black hole is that it breaks down everything until only gravity remains.

It's a large gravity well that pulls everything to it.
If light enters, the energy of that light adds to the gravity.

My understanding of it is quite old, so I'm probably wrong about all of this and more than happy to be corrected.

If i understood you correctly, your view is very much..messed up...wrong.

I will try to clarifiy:

When we talk about a "black hole" colloquially, we usually mean the event horizon around it (around the sigularity). The event horizon marks the threshold in space beyond which escape velocity from the celestial object called "black" hole is greater than the speed of light. Ergo, no light can escpae, ergo we can gather no information about anything beyond the event horizon, because no radiation (of any frequency) can escape. However the space inide this event horizon is (most probably, we never can see it) "normal" space, just graviationally so distorted that there is not way out, even for light.

Light and gravity inside the black hole have nothing to do with each other. Light can just not escape from beyond the event horizon, and eventually falls into the singularity. The energy of the ligh t(radiation) should be negligible compared to the (gravitaitonal) energy added by mass falling into it (see: E=MC2 for mass, and E=hf for radiation, h= plancks constant= 10^-34J/s , f= frequency of radiation, max 10^18Hz for gamma rays, visible light has Tera-Hz, which is 10^15).

Black holes do not "pull everything into them" in general. Only things that are beyond the event horizon. On "this" side of the event horizon you can basically orbit a black hole (yet, you need insane speeds to overcome gravity).

The only thing that ads to the gravity of a black hole, is matter falling into it, making the black hole heavier and increasing the radius of the event horizon (
= making it "bigger").
The increasing radius of the black holes horizon has one counter intuitive effect: The speed of increase of gravitational force (first derivative) near the black hole is smaller for holes with bigger mass, and larger for holes with smaller mass. This means you have a lower risk of being spaghettified and being torn apart by tidal forces near a humongous, multibillion sun masses black monster hole with an event horizon bigger than our entire solar system, than orbiting a 1.4 solar masses (Chandrasekhar limit) minimal size black hole.

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05-05-2017, 10:49 AM
RE: Mass-Energy Equivalence
(05-05-2017 06:41 AM)Deesse23 Wrote:  On "this" side of the event horizon you can basically orbit a black hole (yet, you need insane speeds to overcome gravity).
You need insane speeds only if you orbit close to the center of the hole. You can orbit a black hole at the same kind of distance that you orbit an ordinary star, and the orbital speeds would be similar to ordinary orbits. The difference is that, with a black hole, you can orbit much closer to the center.

(05-05-2017 06:41 AM)Deesse23 Wrote:  The increasing radius of the black holes horizon has one counter intuitive effect: The speed of increase of gravitational force (first derivative) near the black hole is smaller for holes with bigger mass, and larger for holes with smaller mass. This means you have a lower risk of being spaghettified and being torn apart by tidal forces near a humongous, multibillion sun masses black monster hole with an event horizon bigger than our entire solar system, than orbiting a 1.4 solar masses (Chandrasekhar limit) minimal size black hole.

This is an interesting point that is often overlooked. For a "typical" black hole with the mass of a star, the tidal forces become huge long before you get near the event horizon or close enough to see any of the neat General Relativistic effects. There would be a force on the order of a billion times Earth gravity pulling your head and your feet away from each other. So almost every science fiction story or movie about people falling into or close to black holes is wrong. You can't get close enough to a stellar black hole to see any of the cool stuff.
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