Hello Spacelab. I'm Gabe Perez Giz,
and I'm a research scientist at the Centre for Cosmology and Particle Physics
at New York University.
I'm here today to answer your questions.
The first question is,
"If the supermassive black hole in the center of our galaxy
"were to suddenly become a powerful quasar, what would it mean to life here on Earth?"
So, a quasar is not so much a thing,
but a phenomenon, that happens in the centers of galaxies
that are very, very distant, and are very young,
and that's why they're very far away from us.
It results in the emission of a lot of electromagnetic radiation,
a lot of light of various frequencies and wavelengths.
How bright are these things?
I mean, they have to be pretty bright for us to be able to observe them
from almost the edge of the universe, and in fact, they are.
Intrinsically, they emit about a hundred trillion times,
that's 10 to the 14, 1 followed by 14 zeroes,
a hundred trillion times the energy that our sun emits typically,
that's about an average, per second.
So now, the question is if all of a sudden
the center of our galaxy were to become a quasar,
and just to be clear, that would never happen,
but let's just say - suppose it did.
All of a sudden we throw a whole bunch of gas in there,
and set everything off into a very luminous quasar,
what would happen to Earth?
I mean, that's a hundred trillion times brighter than the sun,
with more energy per second coming off than the sun,
so you might think that we're in trouble.
But luckily, the center of the galaxy is very far away from our solar system.
How far away?
Well, the center of the galaxy is between 5 and 10 kiloparsecs away,
5 and 10 thousand parsecs.
One parsec is about 200 000 times the distance from the Earth to the sun,
an astronomical unit, ballpark.
So say we just give the quasar the benefit of the doubt,
put it as close as possible, say that it's 5 kiloparsecs away.
That's about a billion times further away than the sun is.
So, when you have an object
- this is now just basic physics - and it's emitting light,
and you want to know not how much energy is being emitted per second,
but how intense is that light at a given distance,
how bright does it seem?
You have to take the distance to that object, and divide by that squared.
So this object that's emitting a hundred trillion times
the energy the sun emits every second,
the intensity that we feel over here is going to be suppressed
by a factor of 10 to the 18, 1 followed by 18 zeroes.
The net result is that you would see an intensity
that's about one hundredth of a percent of the intensity of local sunlight.
Which means that regardless
of whether that quasar is emitting very harmful gamma rays,
or innocuous optical light, the relative intensity
compared to just the sun, over here would be undetectable.
The second question is,
"If nothing comes out of black holes, how come gamma rays shoot from the poles?"
This question comes from Dr.Seuss.
So, the short answer to the question is that,
nothing is coming out of the black hole per se,
nothing is coming from behind the horizon, not gamma rays or anything.
I think the question is addressing a phenomenon, or referring to a phenomenon
that astrophysicists call
"relativistic jets," or "relativistic beaming."
Whenever you have accretion onto compact objects,
like for example, a black hole,
there are complicated mechanisms in there,
say a lot of gas falling onto a black hole and all the particles in that gas
then start streaming around and churning at accelerated to very high velocities
because they're falling toward the black hole.
It often happens, in fact it seems to happen quite typically in phenomena
even like the quasars that we discussed in the last question,
that a lot of the matter ends up getting funneled
towards two sort of jets, as they're called.
So when all that matter ends up getting funneled up there,
and it's also moving extremely fast,
the fast moving electrons can collide with regular optical photons
and shift their frequencies up into the gamma ray part of the spectrum
and you can have a very quick, very large, sometimes sustained, emission of gamma rays.
This process is called a gamma ray burst,
we think it's accompanied with the formation of a black hole,
And that's why things seem to be shooting out from the holes,
but they're not per se coming out from the black hole.
But there's an interesting connection to the prior question,
unlike quasars, which would never form in our galaxy,
because the center of our galaxy just isn't that kind of place,
a gamma ray burst could, in principle, happen here in our galaxy.
Even though the event rate is very low,
maybe once per every million years for a typical galaxy like the Milky Way,
it could happen.
And if it happens to be the case that the Earth is sort of along the line of sight
of one of these poles of gamma rays,
that could have a significant adverse effect on life on Earth.
The third question is,
"How does Hawking radiation make black holes smaller to the point of disappearing?"
So the math describing Hawking radiation
is sometimes translated in popular treatments,
and this is, you know, maybe accurate,
into a picture of where you have particle production coming out of a vacuum.
Matter and antimatter appear or other kinds of particles being produced,
out of the vacuum in the region near the horizon of the black hole.
And if maybe one of those particles will fall in
and the other one gets ejected out
By the way, none of this has ever been observed,
this is just a part of the theoretical model,
the formula was developed by Stephen Hawking in the 1970's.
Well, if something is still falling into the black hole,
and that thing still has mass,
then how does that process of this particle falling into the black hole,
even if another one is being ejected,
how does that decrease the mass of a black hole?
It would seem, naively, that that should increase the mass of the black hole.
And the way out of this paradox is to remember
that in general relativity it is not just the raw mass of an object
that gravitates and that contributes
to what is effectively seen as the mass of a body like a black hole,
or some entity in space like a black hole,
it's all the energy associated with that object.
So now here's what's interesting.
There happen to be trajectories that a particle can be launched on to
as it crosses the horizon of a black hole.
That, as measured by an observer infinity, has negative total energy.
In other words, if you take not just the mass of that object,
but also all it's kinetic energy and all it's gravitational potential energy,
to the extent that you want to talk about it like that,
and if an observer infinity is saying,
what's the energy of that particle flying into the black hole?
This person at infinity will say, it's negative,
and that the energy of the particle coming out is positive.
There's nothing accreting onto the black hole,
and that process is taking, if you let it run for a long enough time,
eventually the mass of the black hole will get so small
that it will just, in a final blaze of glory, just go "pfft."
Hiccup, emit a few little bits of radiation and be gone.
Thanks for your great questions.
If you have questions for the next expert,
please leave them in the comments below.