
Supermassive black holes may be lurking everywhere in the universe - dnetesn
http://phys.org/news/2016-04-supermassive-black-holes-lurking-universe.html
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MichaelApproved
Here's a classic video that helps grasp (if that's even possible) the size of
a supermassive black hole
[https://www.youtube.com/watch?v=QgNDao7m41M](https://www.youtube.com/watch?v=QgNDao7m41M)

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Fiahil
Thanks, it was short, but informative!

If I may, I would like to point to another kind of supermassive black hole:
[https://www.youtube.com/watch?v=Xsp3_a-
PMTw](https://www.youtube.com/watch?v=Xsp3_a-PMTw) :)

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SagelyGuru
Could someone please explain to me in plain English, how is it that the black
holes "emit radiation at many frequencies" and, at the same time, "the light
is unable to leave their pull"??

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danbruc
There are several effects.

Accelerating black holes, or actually any accelerated matter, emits
gravitational waves. This are disturbances of space-time itself and unlike
electromagnetic waves mediated by photons moving through space-time. At least
in a classical picture of gravity, not sure how that would work out within
quantum gravity when gravitational waves are mediated by gravitons. But I
guess you can just think of the gravitational waves being emitted at the event
horizon.

Then there is electromagnetic radiation emitted from matter falling towards
the black hole but still outside of the event horizon. The gravitational
forces of the black hole heat up this matter to the point that it emits
electromagnetic radiation in a wide spectrum.

Finally there is Hawking radiation which is a suspected electromagnetic
radiation due to quantum effects near the event horizon. The associated black-
body temperature is tiny, two billionth of a Kelvin for a 30 solar mass black
hole and decreasing with increasing mass. The temperature of the cosmic
microwave background with 2.7 Kelvin is in comparison gigantic and therefore
black holes absorb much more cosmic microwave background radiation than they
emit Hawking radiation. The universe has to become a lot cooler before Hawking
radiation can start to evaporate black holes.

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raattgift
Gravitational waves (GWs) propagate a _change_ in the gravitational field.
Consequently, GWs are only shed by accelerating matter where the change in
motion is not rotationally or spherically symmetric.

Spherically symmetric gravitational collapses (of dust or gas, for example)
are certainly systems of accelerated matter, but do not shed gravitational
waves. Rotating spherical bodies also do not shed GWs. To first order, this
means the rotation of stars and planets do not shed GWs, although a planet
revolving around the star in an essentially Kepler orbit will shed GWs, as
will a pair of stars in close orbit around each other.

GWs solve the apparent removal of momentum-energy from a system where there is
no removal of mass-energy by non-gravitational radiation. For most systems the
energy loss is negligible. That's why the first GW detection involved a late
stage inspiral of two compact massive objects with no way to ditch their
angular momentum electromagnetically or via neutrino (or other dark matter)
emission.

GWs themselves have spin 2 symmetry; in theories with gravitons, those are
spin-2 gauge bosons, typically massless (because they are long range), and you
get large numbers of them forming the classical GWs, just as you will find
large numbers of photons forming a bright flicker of light, although you can
detect tiny numbers of photons because electromagnetism is relatively strong
-- tiny numbers of gravitons are not going to be detectable by humanity any
time soon.

It's not curvature (and certainly not gravitational forces) heating up
accretion disks. The matter in the disks move on intersecting geodesics and
occasionally collide. Such collisions tend to ditch the particles' momentum
into emitted photons.

Hawking radiation is an effect of accelerated observers seeing more (and more
energetic) particles than non-accelerated ones, and gravitational collapses
producing an acceleration between past and future observers. Where a past
observer sees a tiny number of low-frequency particles in the "vacuum" outside
the collapsing object, a future observer (after the horizon has formed) will
see a larger number of higher frequency ones. The acceleration is curvature
dependent, and curvature just outside the horizon depends on black hole mass
-- more massive black holes have less curvature just outside the horizon
(which is further away from the gravitational singularity or whatever is at
the centre of a black hole). These particles near but outside the horizon can
escape, which takes mass-energy away from the dynamical spacetime just outside
the horizon. That in turn causes the horizon to retract and that exposes
stronger curvature. That in turn produces a stronger acceleration for further
future observers, and the process continues -- that's black hole evaporation.

Hawking radiation is always emitted, for SMBHs that only grow by incidental
interception of CMB radiation, and even for stellar mass black holes.
Infalling matter simply replaces (and then some) the outgoing "fossil"
infrared radiation from the space just outside the black hole as the horizon
was forming.

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kowdermeister
Related reading, URL says it all [https://aeon.co/essays/is-the-black-hole-at-
our-galaxy-s-cen...](https://aeon.co/essays/is-the-black-hole-at-our-galaxy-s-
centre-a-quantum-computer)

Imagine a civilization building black hole computers in their solar system.
That's pretty badass :)

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swatkat
Off-topic query: The above linked aeon.co article has these opening lines:

 _After you die, your body’s atoms will disperse and find new venues, making
their way into oceans, trees and other bodies. But according to the laws of
quantum mechanics, all of the information about your body’s build and function
will prevail. The relations between the atoms, the uncountable particulars
that made you you, will remain forever preserved, albeit in unrecognisably
scrambled form – lost in practice, but immortal in principle._

Can anyone point me to the article or wiki of this theory/phenomenon? I would
like to read up more about it.

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wodenokoto
If you think of the world as being deterministic then the way your atoms will
react with other atoms (their place in time and space) will be a direct
consequence of where they were before, which again is a consequence of their
previous state and we can track this all the way back to your current life.

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deathanatos
That assumes that the universe is both deterministic _and_ reversible. SHA2 is
deterministic, but good luck reversing it. Essentially, to work backwards,
time, when going forwards from state A would need to move to some unique state
B.

Consider two hills with a valley between them. In one universe, I roll a ball
down one hill. In the other, I roll it down the opposite hill. In both, the
ball ends up in the valley, and the information about which hill the ball
rolled down is lost.

You could perhaps argue that other effects (me disturbing the air on the hill
I walked up) might still imply which hill the ball rolled down, but the
question really is if such a scenario could exist, probably in a more simple
form.

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tfgg
Right, and as far as we know, quantum mechanics evolves the wave function in a
time-reversible fashion.

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MichaelApproved
A couple of great YouTube channel for anyone interested in this stuff. The
videos range from surface explanations to actual math equations.

[https://www.youtube.com/pbsspacetime](https://www.youtube.com/pbsspacetime)
and
[https://www.youtube.com/user/sixtysymbols](https://www.youtube.com/user/sixtysymbols)

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ck2
Would our brains even be able to detect the moments the earth entered one?

We'd probably get a little warning from amateur astronomers as they freaked
out months before it happens.

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huuu
What if we entered one and came out on the other side without even noticing
something happened?

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gonzo41
there is no other side. its not a physical hole. its a gravity hole.

Think of a moon that drags you onto it. You can't ever have enough power to
reverse moving towards it. Light isn't quick enough to leave the pull. thus it
is dark.

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nonbel
>"there is no other side. its not a physical hole."

Isn't a black hole more like an edge of the universe? My understanding is that
you end up with a Zeno's paradox situation where no matter how close you get
you can never actually reach it.

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xtacy
Not quite -- in your reference frame, you cross the blackhole's event horizon
and go inside, but an observer (who is outside the event horizon) will only
see you reach the event horizon and slowly "fade away."

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ars
Which is why black holes makes no sense.

Nothing can ever fall into them, so they can't ever form, or grow. (Obviously
speaking from the POV of Earth.)

Since they can't form, and can't grow, how can you see a "supermassive one"?

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lisivka
Black hole has zero gravity at it center. ;-) Moreover, gravity can stop light
and other electromagnetic forces, but cannot stop gravity, so time is not
stopped.

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lisivka
WTF? I have a follower-downvoter? Every body has zero gravity forces at their
center of mass. We also can clearly see that mass continue to generate gravity
forces even after fall to black hole, so time is not stopped, at least for
gravity.

PS.

Every black hole has second event horizon in the center of black hole, and I
am very curios is it filled with mass or not. :-/

