The Naked Scientists

The Naked Scientists Forum

Author Topic: Black Holes and light  (Read 2558 times)

Offline RealEarthling

  • First timers
  • *
  • Posts: 6
    • View Profile
Black Holes and light
« on: 05/09/2009 11:54:39 »
I am an engineer, but not a scientist - so a simple question:  If gravitational pull depends on mass, and if matter cannot be created or destroyed, then it seems that a BH should have exactly the same gravitational pull after Super Nova as before Super Nova.  So then how did light escape from this gravitational pull before the BH became more dense.  Logically, density should not matter if the amount of mass is the same.  ????


 

lyner

  • Guest
Black Holes and light
« Reply #1 on: 05/09/2009 12:02:17 »
You are right in that the gravitational pull at a great distance will only depend upon the total mass - not the density. A BH wouldn't be expected to suddenly start sucking stuff into it from all over the place.
The black hole needs to have a high enough density so that enough mass is concentrated with the Schwarzschild radius. A bit further out than that, you can expect some GR effects, due to the high gravitational field (which could perhaps cause a decaying orbit) but, if we (Earth) were orbiting a BH, the same mass as the Sun, I think we would orbit in the same year and with the same year length.
« Last Edit: 05/09/2009 12:07:00 by sophiecentaur »
 

Offline syhprum

  • Neilep Level Member
  • ******
  • Posts: 3818
  • Thanked: 19 times
    • View Profile
Black Holes and light
« Reply #2 on: 05/09/2009 16:23:02 »
An intriguing thought replacing the Sun by a blackhole of similar mass although I cannot for the life of me think how such a BH could have been formed.
Firstly it would be rather chilly and the alien astronomers from another solar system who had established an observatory here would have to rely on geothermal heating.
I wonder what they would see when they pointed their telescopes in the direction of this BH what effect would it have on the view of the background stars would there be any radiation from matter spiralling into it ? .
It would of course be a very good location for IR astronomy as the detectors would need little additional cooling and there would be no problem from an atmosphere.
 

Offline Vern

  • Neilep Level Member
  • ******
  • Posts: 2072
    • View Profile
    • Photonics
Black Holes and light
« Reply #3 on: 05/09/2009 18:29:28 »
There should be a gravitational lensing effect. Maybe there could be a halo of one distant star showing up multiple times around the black hole.
 

Offline RealEarthling

  • First timers
  • *
  • Posts: 6
    • View Profile
Black Holes and light
« Reply #4 on: 05/09/2009 23:19:25 »
Thanks very much - I appreciate your thoughts, all of them.  Larry
 

Offline LeeE

  • Neilep Level Member
  • ******
  • Posts: 3382
    • View Profile
    • Spatial
Black Holes and light
« Reply #5 on: 06/09/2009 05:26:16 »
The reason that density is important is because it relates to size and because gravitational 'force' follows an inverse square rule.

Imagine two large planets that have the same mass but are different sizes, one ten units in diameter and one five units in diameter.

When you stand on the surface of the five unit diameter planet all of its mass is within five units distance of you, but when you stand on the ten units size planet more than half of its mass will be more than five units distant from you; because half of its mass is further away the gravitational force you feel will be lower.

Iirc, the Schwarzchild radius for the Earth is a little under 9mm.
 

The Naked Scientists Forum

Black Holes and light
« Reply #5 on: 06/09/2009 05:26:16 »

 

SMF 2.0.10 | SMF © 2015, Simple Machines
SMFAds for Free Forums
 
Login
Login with username, password and session length