## black holes, gravity, and light

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### black holes, gravity, and light

Light must have mass because once it gets past the event horizon, the gravitational pull form the black hole pulls everything in, even light, so it must have mass so that the gravitational pull from the black hole can actually pull it in.
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papernuke
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Actually, this is a misunderstanding of how gravity works. Gravity distorts space-time itself, so even massless entities are influenced by it.
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The_Science_Guy
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Gravitational acceleration doesn't depend on mass. You can see this when you look at Newton's gravity equation.

PWrong
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What is it?

Im using my dad's computer, and its really slow.
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papernuke
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F = GMm / r^2
where:
F is the force
G is the gravitational constant
M is the mass of the larger object
m is the mass of the smaller object
r is the distance between them

Newton's first law says that F = m a, where a is acceleration.

So,
ma = GMm / r^2
Now divide by m on both sides:

a = GM/r^2

So the equation for acceleration doesn't have m in it. I hope that made sense.

PWrong
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A question popped into my mind after reading this... well actually it relates to something I have been thinking about anyway. The faster you travel the slower time becomes, locally speaking... which I think interesting since locality is all but nigh of being a false axiom.... but anyhow... time slows down, could you suppose that is because you are catching up with the velocity of time, not as opposed to, but rather than light.

And if time has velocity, which your own velocity can be relative to, just continuing with the assumption, is it faster than light? could it skim past an event horizon?

If we go with Einstein, light is the fastest thing on two legs, but I wonder if time, perhaps constrained to a different field than light, might outrun it in it's own way. What do you think?
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I think the difficulty you may have would be quantifying time. To say that time has a velocity would require you to be able to "follow" an "instant" of time, whatever that may be like we can with light and other such things.
houserichichi
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PWrong wrote:ma = GMm / r^2
Now divide by m on both sides:

But thats only possible for m!=0 and thatswhy massless particles arent influenced by Newtonian gravity.
bo198214
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What the hell is bo198214 talking about?

But thats only possible for m!=0 and thatswhy massless particles arent influenced by Newtonian gravity.

It doesn't matter what m equals because it is on both sides of the equation. It cancels! If m equaled zero the equation would end up as 0=0!
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3dftw
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Good point about the Big Bang. We can't really assume thats how it worked. That's just what the majority of people assume.
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3dftw wrote:What the hell is bo198214 talking about?

But thats only possible for m!=0 and thatswhy massless particles arent influenced by Newtonian gravity.

It doesn't matter what m equals because it is on both sides of the equation. It cancels! If m equaled zero the equation would end up as 0=0!

You're wrong there, actually. You can't divide by zero. This is a mistake we are constantly told to avoid making in our math class

Here's the right way to do it:
ma = GMmr<sup>-2</sup>

Take every term containing an m to one side (in this case, everything):
ma - GMmr<sup>-2</sup> = 0

Factorize out the m:
m(a - GMr<sup>-2</sup>) = 0

For this to occur, either or both of the products must be zero. Therefore, either a = GMr<sup>-2</sup>, or m = 0, or both. Since the second case does not include a formula for acceleration, acceleration is undefined for massless particles.

Keiji