Re: New rbr Science Officer

On May 2, 4:50 am, "Kurgan. presented by Gringioni."
<kgringi...@xxxxxxxxxxx> wrote:
On May 2, 2:52 am, "b...@xxxxxxxxxxxxxxxxx" <bjwei...@xxxxxxxxx>
wrote:



They _may_, but black holes decay, and the smaller, the faster.

What does "They may" refer to?  Ultra-high energy cosmic rays
entering the atmosphere may create black holes?  This has never
been observed.  Black holes have never been observed to decay,
either.

<snip>

Dumbass -

Am I correct in assuming we can't observe "Hawking Radiation" because
we lack the technology?

Sort of. From the astronomical black holes that we know
to exist, it's probably utterly hopeless to detect, regardless
of technology. I mean, there is a difference between
phenomena that are undetectable with present
technology, but we might be able to do it in 10 or 100
years; and phenomena that have such negligible effect
that we can't even really think about how to address
the problem. It's like the difference between detecting the
presence of an Earth-mass planet around a nearby
Sun-like star (which will probably happen in the next 10 years,
although we won't get a direct image of it), and figuring
out how to exile Kunich there.

Back to black holes. A larger mass black hole also has a
larger Schwarzschild radius, the radius of the event horizon,
which is where action such as Hawking radiation occurs.
The math works out that for a larger mass black hole, the
curvature of space at the event horizon, the local gravitational
acceleration, and the gravitational tides, are weaker than
for a smaller mass black hole. (Basically, the Schwarzschild
radius is R = 2GM/c^2, and the local gravitational
acceleration, or surface gravity, is: a = GM/R^2 - that's a
Newtonian formula, but approximately useful here. This
means that the local acceleration a ~ 1/M.)

If we dropped Astronaut Brian Lafferty into a black hole of one
solar mass (Schwarzschild radius 3 km) he would likely be
torn to pieces by the tidal stress as he approached the event
horizon. But because the acceleration and tides are weaker
in a larger black hole, if we dropped Lafferty into the
million-solar-mass black hole at the center of the galaxy
(S.radius ~ 3 million km), he would feel minimal tidal force
as he crossed the horizon, and might not even notice the
moment of crossing, although from the outside we would see
him getting dimmer and dimmer due to gravitational redshift,
emitting a last few messages (about chess?) before
disappearing entirely.

Hawking radiation is a quantum-mechanical process that
depends on the curvature of space (or local acceleration,
equivalently) at the horizon. It turns out, and here I have
to trust the experts, that the Hawking radiation is a black
body with temperature T ~ 1/(8pi M). So a large-mass
black hole emits weaker, cooler Hawking radiation.

We know that black holes exist on astronomical mass
scales - a few solar masses, the remnants of massive
stars, and a few million solar masses, supermassive BHs
at the centers of galaxies. The Hawking radiation from
a solar mass BH is predicted to have temperature about
60 nano-Kelvin and total power about 10^-30 watts, even
weaker than Henry after too many donuts.

For comparison, the BH is sitting in a bath of cosmic
microwave background radiation at 2.7 Kelvin, plus a roughly
similar energy density from Galactic starlight. Plus, black holes
generally emit radiation from the accretion disk of matter falling
into them. So the prospects of detecting Hawking radiation from
an astronomically-known black hole are essentially zero.

As BHs emit radiation, they should lose mass and
eventually evaporate, and in their final moments, the
temperature would increase to the point where we might
hope to detect it as a chirp of increasing gamma rays.
The time to evaporation of a solar mass BH is about
2 x 10^67 years (http://en.wikipedia.org/wiki/Hawking_radiation),
and the poor old universe is only 1.4x10^10 years old,
so again we'll never see a solar mass BH disappear.

The only possibility is if very small black holes, much smaller
than an Earth mass, or even down to subatomic-particle
energies, both exist and are plentiful enough to be close
enough for us to detect their evaporation screams. There's
no way to create such a small BH from an astronomical
object like a star. There are speculative possibilities that
there are small black holes either created as primordial
BHs in the hot, dense early universe, or created by
interactions of high-energy cosmic rays with other matter.
I don't think any of the mechanisms suggested for
creating these small BHs are more than some theorist's
half-worked-out pet.

NASA's recently launched Fermi gamma-ray satellite is
capable of observing gamma-ray squeals from evaporating
BHs if they happen often enough and close enough, and
can be distinguished from other types of gamma-ray
emitters. If Fermi finds something that looks like BH
evaporation, then maybe we have to take mini-black holes
seriously, but until then they are a curiosity.

Ben
P.S. Strangely, the song that popped onto the stereo as
I typed the last sentence of this post was Beck's
"Gamma Ray." I don't think modern science can
explain this phenomenon. Perhaps it is evidence for
intelligent design.



.

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