# Balck hole within 20 light years?

## Main Question or Discussion Point

Can you get a thing like that within the sol's closest space? I mean, if there was one somewhere between us and say alpha centauri or barnard's star, is it possible that you cannot feel it's gravity force?

## Answers and Replies

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Black Holes's gravitational field is nothing special.If a black hole has an event horizon radius same as that of sun , its gravitational field would be same as that of sun , I doubt that Gravitational field of a BH at such a distance would make a major impact on field around us , and may go undetected unless BH is itself massive.

BJ

Labguy
Science Advisor
Dr.Brain said:
Black Holes's gravitational field is nothing special.If a black hole has an event horizon radius same as that of sun , its gravitational field would be same as that of sun
Not so. I forgot the "size" that our sun would have for an EH if compressed to a black hole, but it is very small. For example, for a star with a mass of 19.9 x 10^30Kg (A mass of about 10 times that of our Sun) would have a Schwarzschild radius of about 30 kilometers. That's only about 18.6 miles!

Dr.Brain said:
I doubt that Gravitational field of a BH at such a distance would make a major impact on field around us , and may go undetected unless BH is itself massive.
This is true, it is just another source of gravity that we may, or may not, detect.

EDIT: Found it (too lazy to calculate)
If the Sun were somehow compressed enough to become a black hole, it would be less than 6 kilometers (well under 4 miles) across.

Last edited:
George Jones
Staff Emeritus
Science Advisor
Gold Member
Labguy said:
Not so. I forgot the "size" that our sun would have for an EH if compressed to a black hole, but it is very small. For example, for a star with a mass of 19.9 x 10^30Kg (A mass of about 10 times that of our Sun) would have a Schwarzschild radius of about 30 kilometers. That's only about 18.6 miles!.
I think you and Dr. Brain are saying the same thing. The Schwarzschild radius for any (spherically symmetric) massive object can be calculated. If the the Schwarzschild radius lies within the object (i.e., the object has mass outside), then the object is not a black hole; if all the mass lies inside its Schwarzschild radius, then the object is a black hole. By Birkhoff's theorem, a distant observer cannot use gravity to tell the difference between equal mass (spherically symmetric) objects, be they stars, black holes, or even stars collapsing (spherically symmetrically) to form black holes.

Regards,
George

Labguy
Science Advisor
George Jones said:
I think you and Dr. Brain are saying the same thing. The Schwarzschild radius for any (spherically symmetric) massive object can be calculated. If the the Schwarzschild radius lies within the object (i.e., the object has mass outside), then the object is not a black hole; if all the mass lies inside its Schwarzschild radius, then the object is a black hole. By Birkhoff's theorem, a distant observer cannot use gravity to tell the difference between equal mass (spherically symmetric) objects, be they stars, black holes, or even stars collapsing (spherically symmetrically) to form black holes.

Regards,
George
Your description is correct, but I took his statement to mean an EH radius equal to the sun's radius, which would mean a massive BH. Not sure that's what he meant.

Labguy said:
Not so. I forgot the "size" that our sun would have for an EH if compressed to a black hole, but it is very small. For example, for a star with a mass of 19.9 x 10^30Kg (A mass of about 10 times that of our Sun) would have a Schwarzschild radius of about 30 kilometers. That's only about 18.6 miles!

This is true, it is just another source of gravity that we may, or may not, detect.

EDIT: Found it (too lazy to calculate)

I was trying to imply that whenever you are outside a black hole having a size of some star A, its gravitational field would be no different from that star . I mean that if the poster above had asked me if a star's gravitational field between us and alpha centauri could be detected , the answer would have been the same.

Concluding that as long as you are outside a black hole , it can be treated as just another body carrying massive mass just like a star.

BJ

Labguy
Science Advisor
Dr.Brain said:
I was trying to imply that whenever you are outside a black hole having a size of some star A, its gravitational field would be no different from that star . I mean that if the poster above had asked me if a star's gravitational field between us and alpha centauri could be detected , the answer would have been the same.

Concluding that as long as you are outside a black hole , it can be treated as just another body carrying massive mass just like a star.

BJ
I agree, but you said size instead of mass. Mass would be a more accurate description as size implies a measure of distance and mass is "a quantity of matter", regardless of size.

Chronos
Science Advisor
Gold Member
I think it is improbable there is any black hole near us. The gravitational lensing should have been obvious by now.

How close to Earth is the closest black hole we know of today? Hundreds of light years? Thousands?
Anyone know?

hellfire
Science Advisor
Chronos, I dont think the gravitational lensing experiments did cover sufficient sections of the sky to conclude such thing. (I might be wrong).