Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Jon Francis on 25/08/2010 19:41:16
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If gravity is related to mass. Why does a black hole stop light from escaping when it was created from a star with the same mass that had been emitting light previously?
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Because the black hole is so very much smaller and its gravity is so much stronger. Our sun is around one million miles across. A black hole with the mass of the sun is around one mile across that is one million times smaller as the gravity goes up as the inverse square of the distance the gravity of the black hoe at the surface of the black hole is one trillion times as great as the sun. Note the gravitiational field of the black hole with the mass of the sun is exactly the same as the sun at the distance of the surface of the sun.
Now as light escapes from a gravitiational field it loses energy. This is not significant for the sun but it is observable for very dense stars like white dwarfs a black hole is defined as the point where light trying to escape has its energy reduced to zero and photons trying to escape from whatever is inside are turned back on themselves and fall back just like a ball thrown upwards on the earth falls back to the earth
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I understand that photons have no mass? and that gravity acts on mass. Why are photons affected by gravity?
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Because gravity curves space and time...
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I understand that photons have no mass? and that gravity acts on mass. Why are photons affected by gravity?
Gravity is caused by and acts on both mass and energy. Photons have energy.
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I understand mass and energy are interchangeable? and that the reason photons can travel at light speed is because they are massless? They do not require infinite energy to achieve light speed?
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Thinking about mass is natural, since mass is something you experience every day, but its not the best way to think about relativistic problems. What really matters in relativity is energy and momentum. Those are the things that determine how an object moves and also how it interacts with gravity. You can define mass (there are two commonly used definitions) in terms of energy and momentum, too.
An object that isn't moving only has energy, so that you can basically use mass and energy interchangeably. For objects that are moving slowly (compared to light speed), the momentum doesn't matter that much, so that mass and energy are still pretty much interchangeable. For faster objects, you have to be more careful about what you mean by mass, so its usually safer to define things in terms of energy and momentum.
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I am usually a bit clumsy at this and someone will add to help me express or extract my expressions.
Maybe I can shine some light on this matter in more physical layman's terms.
In a natural way, what creates light?
Categorically speaking, isn't it an atomic and/or chemical reaction, relative to a strong enough differential of electrical charge of matter, creating an energy release in some form of light?
Since gravity acts on natural matter naturally, and naturally the acceleration of gravity is
derived from the accumulation of matter, how much matter is needed to produce a mass to achieve an acceleration larger then 299,792,458.00 m/s2. It is said, the first derivative of acceleration at this value is 99.9% achievable.
When this is achieved what happens to matter as it comes near this effected region of gravity?
Will it be also attracted and achieve the same speed and add mass to the already growing mass?
Will this mass get larger and become denser, due to the pressure resultant of gravity exerted at the mass surface?
Can we say as the approach, getting closer to the center, there is a relief of the acceleration of gravity, but pressures are still growing?
We are just interested in the acceleration of gravity at the surface!
Can it be viewed as, it catastrophically growing in density and gaining in gravitational acceleration relatively?
Is this acceleration of gravity now faster than the speed of light?
No, it is the Universal Speed limit. Maybe 99.9% close?
At this speed, can any chemical reaction at the surface of this massively dense hunk of matter have any time to really react to produce and release any light energy?
Any atomic or chemical reactions below the surface where a significant relief of gravity permits a reaction, have the energy to overcome the acceleration of the surface gravity.
I think it is said that a photon created in the nuclear furnace of our sun takes approximately a billion years to reach the surface. I am not sure of the number of years but it is a Long time.
The gravity of the sun is a comprehensible value
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If gravity is related to mass. Why does a black hole stop light from escaping when it was created from a star with the same mass that had been emitting light previously?
There are two significant factors at play here.
The most important is that the 'strength' of gravity decreases as the distance from the mass that's causing the gravity increases, so that the closer you are to the mass, the stronger the gravitational field, and the second is that the gravitational field for any spherical (or near spherical) body is at its greatest on the surface of the body.
The second factor is probably easier to understand, for if you're 'down' beneath the surface of a spherical body then a proportion of that body's mass must be 'above' you, pulling you back 'up', so it's only when you're actually right on the surface that all of the body's mass is effectively below you, pulling you 'down', and none of it is 'above' you, pulling you back 'up'.
Now the first factor, where the strength of the gravitational field decreases as you move further away from the mass means that you should feel the strongest gravitational force when you are closest to the mass but then that's where the second factor comes into play, for this means that you can't really get any closer to the mass than its surface.
However, one of the most important things that happen when an ordinary body, such as a star, becomes a BH is that it becomes much, much smaller than it was (I don't know off-hand what the diameter of the Event Horizon for a solar mass BH is, but I do remember that if the Earth was turned into a BH the diameter of its EH would be a little under 9mm) and this means that you can get much closer to the mass once it has collapsed than you could before, and as a consequence the gravitational field you'd experience there would be correspondingly stronger.
If you were to stay at the same distance from the BH as you were before it collapsed you'd feel no difference in the gravity, so if our sun were to collapse into a BH, the Earth would quite happily continue along its current orbit because it was no closer than it was before.
It is then, because the size of a BH is so much smaller than the star from which it formed that the gravitational field can become strong enough to curve space-time back into itself, making it 'closed', and once it's reached that point nothing can escape from the space within the EH - it has effectively been sealed off from the rest of the universe.