Download PHYSICS 015

Are Black Holes Inevitable?
Perplexing!
- but are we sure they exist?
Some Considerations
1.
2.
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4.
Neutron stars of a few solar masses are already very close to the
Schwarzschild radius, so it wouldn’t take much to tip the balance.
For the most massive stars, the Schwarzschild radius is already
too big. For example, if you wanted to allow a 10-solar-mass star
to settle down as a neutron star, about 10 km in diameter, it
already inside its Schwarzschild radius and is doomed to collapse!
Stars can’t ‘know’ that they should shed a lot of their dangerous
mass, to prevent this kind of collapse. And there are lots of stars
of 10, 20, or even 50 solar masses!
No mysterious new physics can save you.
Why No New Physics?
- a Special Circumstance
Imagine a huge
cluster of a billion
(109) stars, perhaps
near the center of
a galaxy.
(That’s about 1%
of the stars in the
Milky Way.)
Now Let Gravity Work
Allow gravity to draw these stars together, to within a
radius of 3x109 km – about the size of the Solar System.
(That’s a lot of stars closely clustered together, but not
unimaginable. There would be room!)
Overall, at this stage the average density is less than that
of water. (It is higher in the centres of individual stars,
of course, but not dramatically so.)
But this huge distribution of material is already within the
Schwarzschild radius, and now there is no escaping the
inevitable collapse! Even light is trapped.
No One Rings a Bell
As the stars draw together, no special conditions are
encountered, no exotic or extraordinary circumstances.
No one tells the material that a special new behaviour
has to rise up to save the day.
….but it’s already too late; gravity
will win.
SuperMassive Black Holes!
This reasoning tells us that SMBHs (Super-Massive Black
Holes) seem to be particularly inevitable, even if we
could find some way to avoid black holes that have the
mass of a single star.
Most obviously, we’d expect to find SMBHs in the cores of
galaxies, where there are many stars that can clump
together.
This almost certainly explains quasars, for example
[more on this later!]
Let’s Try This Out!
Meet the Milky Way
What We See
Zooming in on
the Center
In the Very Core:
A Super-Massive Black Hole
http://www.physics.queensu.ca/~hanes/Movies/MW-SMBH.mp4
Keep This in Perspective, However!
The Super-Massive Black Hole (SMBH) contains only about
1/1000 of one percent of the mass of the Milky Way –
it’s big, yes, but hardly dominant. (Moreover, there are
globular star clusters of comparable mass, located here
and there in the galaxy.)
So our SMBH doesn’t ‘control’ the Milky Way the way the
Sun ‘controls’ the planets!
There are galaxies in which we find evidence for billionsolar-mass black holes!