Existence of super-massive black holes

sri sharan

Imagine that you are an astronaut standing very far from a black hole.Now you throw a luminous body (a bulb may be) directly towards it.Now as it gets nearer the black hole,the light from the bulb as you observe it becomes more red-shifted.Eventually from your frame(consider it is an inertial one) you just observe the body getting nearer and nearer to the black hole but never quite crossing it.The light just keeps on getting red-shifted.

Doesn't this imply that super-massive black holes cannot exist, or if they exist at least the way we think they are formed(stellar black holes which gobbled up a large no of stars and other matter) is wrong?

marcus

It sounds like you are saying that because we cannot see something it doesn't happen.
I guess I don't understand your chain of reasoning.

DaveC426913

As marcus points out, it is our point of view that is limited. The light bulb really does fall into the BH. If you threw yourself into it instead, you would pass the event horizon without noticing anything and would reach the centre quite quickly.

sri sharan

no,I am saying that with respect to the external frame of reference the the body will never cross the event horizon.It will just keep getting nearer and nearer to it.

DaveC426913

But then you go on to say "Doesn't this imply that super-massive black holes cannot exist,"


No. It doesn't.

sri sharan

Ya,that's because the frame of reference changes.No, you are no longer the observer and you are subject to the gravity of black hole.

Ok,lets think of it this way.If a body crosses the event horizon I must no longer be able to see it right.But,this luminous body that is thrown in,the astronaut will always be able to see it,it will never cross the event horizon from his frame of reference(it does cross with respect to the body's own reference frame)

DaveC426913

The image you are seeing of the object is not the object; it is the light of the object, on its long journey to climb back out of the BH's well. Lots of things can happen to that light after it has left the infalling object.

sri sharan

Yes,but doesn't the fact that I always see it,no matter how much time i stay there imply that the body will not cross the event horizon.If it does cross the horizon eventually I should stop getting light from it
Can you please elaborate?I am a little confused

DaveC426913

What's confusing you?

The object falls into the BH, emitting light all the way. That light heads out of the BH back toward the observer. The last photons that leave the object before it crosses the event horizon has a tough time climbing out, and gets red-shifted. The very last photons in the last nanoseconds before it crosses the event horizon take the rest of eternity to climb out of the well, which is what the observer sees. But the object is looooooong gone.

sri sharan

Ohh,yes i now understand but.hmm,does that mean at least some black holes(which have devoured some stars may be) should shine in some highly red shifted(radio wave) frequencies.Can we perhaps use this as a way of detecting possible black hole candidates??Looking for frequencies that are so low that they cannot be emmited by normal means?

Rick88

Black holes won't shine. They are black. They emit no light.
The only way to "see" a black hole is to take a picture of the same patch of sky at two different times.
If in the second picture something seems to have "disappeared", it's very likely that a black hole is hidden there.

R.

sri sharan

By light I dont mean the shining of black holes here .I meant the 'residual' light from any gobbled up stars,as i said in my previous post