- 5 Levels
- Season 1
- Episode 6
Astronomer Explains One Concept in 5 Levels of Difficulty
WIRED has challenged NASA's Varoujan Gorjian (Research Astronomer, NASA Jet Propulsion Lab) to explain black holes to 5 different people; a child, teen, a college student, a grad student and an expert.
Released on 07/05/2018
Hi, I'm Varoujan Gorjian, I'm a Research Astronomer
at NASA's Jet Propulsion Laboratory.
I've been challenged today to talk
about once concept at five different levels
of increasing complexity.
Today we're going to be talking about black holes.
A basic definition of a black hole is
that it's a lot of mass crammed into a very tiny volume,
such that the escaped velocity is the speed of light.
So have you ever heard of something called a black hole?
What is a black hole?
Well, it has to do with, a lot with gravity,
do you know what gravity is? No, not at all.
It's what keeps us on the earth.
The reason we're not just flying off the earth is
because earth has gravity, so if we throw something up,
it comes back down, so that's why
when we're walking on the earth,
we don't fly off the earth because the earth has gravity,
and it keeps us down.
The main thing about black hole to remember is
that it's just, like I said, how the earth holds you down,
the black hole pulls you in, as well.
Now, try and take the ball from me, black hole--
Why do you hold it so tight?
I'm holding it tight to show you
then when you're trying to pull it,
a black hole will really hold onto it with its gravity.
I got it.
The main thing is that if something falls
into a black hole, it can never get out, it's--
What about the earth? What if it rolls into it--
Oh, if the earth rolls into it?
It would be bad, we wouldn't be able to get out.
So it could never happen?
It's never, not likely to happen, no.
Only in movies, right?
Well, yeah, definitely in movies, yeah.
Yeah, but it's not real because it's just pictures of it.
What if I went in there? The black hole will sort
of stretch you out as you're falling in.
It'll, like, stretch my body out?
So what do you think about black holes?
It's kind of dangerous.
So tell me, what do you know about black holes?
Well, I know they're created when stars,
once they start growing, it doesn't,
it can't expand anymore, so they collapse inward.
You got, pretty much, you got a very good idea about it.
Basically you have a star,
at the core its generating energy,
which is countering the pressure
of all that mass trying, being pulled in by gravity.
Once it can't generate that energy anymore,
the core collapses, once the core is collapsing,
it keeps collapsing, and collapsing, and collapsing,
and that's the critical thing that makes a black hole.
You have enough mass in the volume,
such that the escaped velocity becomes the speed of light,
once light can't escape, hence the black part. (laughs)
But that's the thing, is that just
because there's an intense amount
of gravity very close to a black hole,
things don't start behaving differently.
And my favorite analogy is a vacuum, everybody thinks, like,
if you point a vacuum cleaner at something,
it's just gonna suck everything in--
But no, the wind, the pull isn't,
if you're not close enough, it doesn't work.
Right, you will feel the gravitational pull,
and it'll redirect you in some way,
and if you're far enough way,
it's just like you're very far away
from the sun, or something else.
If you're close enough, it will divert your path more,
if you're close enough still, then you will get
what are called tidal forces, where the difference between,
for example, if you're standing next to a black hole,
the difference in gravity between your feet
and your head actually becomes significant.
Next time you see a science fiction movie,
if somebody says, Oh, my god, we've been caught
in the gravitation field of a black hole,
and we're gonna fall in, it's like,
no, no, just, if you're far enough away,
just thrust a little bit this way,
and then you'll slingshot around the black hole.
So are there any movies that actually do,
like, get space right?
I wouldn't call them educational,
but the movie Interstellar actually had,
as one of the people involved in both writing it,
and as a science advisor, Dr. Kip Thorne,
who's a Professor at Caltech, who was part of the team
that detected gravitational waves,
and just won a Nobel Prize for it.
So he made sure to try and get it as accurate as possible,
so Interstellar, I think, is one of the best examples
of getting black holes right.
So it's, I'm assuming it's hard to,
like, detect a black hole, besides if it's,
if things are close enough that they're being pulled in,
so it's basically a theory?
It's, there're multiple theories,
there's observational evidence,
more than some other theories that are really feeding
into this because now we have the instrumentation,
both in X-rays and infrared, in particular,
because we don't have a direct optical line
of sight to the center of our galaxy
because there's just a lot of dust in the way.
But the infrared can penetrate the dust,
the X-rays can penetrate the dust,
the radio can penetrate all of that dust,
so by combining all of these different wavelengths,
people are really coming to a point
of okay, how is this happening,
by looking at different wavelengths of light,
we can get a better sense, but they're still working
on the theory, it's not all done.
So far, what do you know about black holes?
I never knew beforehand how hard it was
to get actual data of the black holes itself,
first of all, they're dark, and, like,
they're so far away, it's almost impossible just
to get a good image of them.
They were discussing a project in
which multiple radio telescopes of some sort, like, are,
like, pinpointed all across, from Greenland
to South America, and, like, and they're trying to
get an image of
the black hole in the center
of our galaxy because, as opposed
to just recording its impact
on the surrounding stars and planets.
So we've been, we've had, now,
effectively two different ways
of getting more direct measurements, one is the LIGO, which
is the Laser Interferometer Gravitational Wave Observatory,
which is where, getting the ripples in space time,
coming off of the merging of black holes.
The other one that you're mentioning is actually called
the Event Horizon Telescope, where they're using radio waves
to actually image the event horizon,
that region where light cannot escape
from the black hole at the center of our galaxy,
which I know they're working on it right now.
It's an amazing thing,
but that'll be the most direct imaging of a black hole.
LIGO is a direct detection of the consequence
of the merging of black holes.
The critical part has been, like,
for the super massive black hole at the center
of our galaxy, we've seen the stars orbiting it,
and we've measured the mass, so that way,
so if you look at a spinning black hole,
it actually fundamentally alters the emission
that's coming off the stuff that's falling into it.
These are discovered as what are called X-ray binaries,
that is, you know, there's an X-ray member
of the binary that is emitting in the X-rays,
and it's really not very bright
in the optical (mumbles) at all, so there's always,
people are looking at these X-ray binaries.
What sort of technology and, like,
I guess tools have you been using
in your studies, or, like, just in general,
in the study of black holes?
For my studies, I actually, when I started at UCLA
in graduate school, I worked
with a professor named Matt Malkin who was,
gotten a lot of data observations
from the Hubble Space Telescope, so that was one
of my very first projects to work on, so any,
space-based observatories have been a really big advantage,
and then I've moved on now to the Spitzer Space Telescope.
In addition to that, then there's other people
who've used a lot of X-ray telescopes,
NuSTAR, Chandra have used data from that.
It's been a combination of both ground-based observatories,
as well as space-based ones, and going everywhere
from X-ray observations, not done by me,
but certainly ultraviolet, and then optical,
and infrared, particularly, those are the ones
that I've been most involved with.
What got you interested in studying black holes?
The way I got really interested
in this field is I actually first came
to Caltech as a summer student, and I started working
in this research group called the NuSTAR Group.
Right now I'm doing my PhD in the field
of active galactic nuclei,
which are the most luminous compact objects in the universe,
and it's because of the extreme accretion
that we're seeing onto these super massive black holes.
We don't have such a simple picture,
that this central black hole is surrounded
by this donut shaped torus of material,
and that all these different classes of AGN simply arise
from a viewing angle effect of this torus,
as a very oversimplified geometry, and this--
Which is, by the way, what, when I was just starting out
in graduate school, that was the hot new thing, so.
That was, it was like, oh, wow, this might be it,
and then, but very early on, it was, as I just started my,
it was basically my second year of graduate school,
it was like, uh, this is not that simple.
You know, everybody's been just coming at it
from different wavelengths at the optical infrared,
and, but definitely the X-rays has been one
of those things where it's like, oh, finally.
And we've come a long way by seeing a wider range
of the spectrum, we can elucidate more
about the circumnuclear geometry,
and there's just been so much progress made
with all these new spectral models
that we use to fit AGN spectra,
and the different types of classes of AGN,
like type one and type two are believed
to be just a viewing angle effect
of seeing this torus at different angles.
It does to be that it may not even,
in parts, it may not be even connected
to this tiny, little torus at all
because part of the work that I did,
and others have done, is that type twos preferentially live
in different kinds of galaxies than type ones,
which should not in any way have to do
with something such a small scale.
They tend to be in smaller bulged,
SB and SC type spiral galaxies.
So there's something also having to do
with environment that gets you to be a type two,
and you can still maybe, timing wise,
but there's something else that's going on
on a larger scale because the type
of the AGN should not really correspond
to the host galaxy, but it seems to.
And that was one of the things
that we were finding out, and that was one
of the early little ideas that the,
individually, just as the torus model,
the unified model, can't explain everything
that we were observing at the time.
But it is one of those things that it's,
they're super luminous, they're all over the place,
and we don't have a really good picture of it,
which makes it exciting to study.
Yeah, and I think, you know,
pushing towards the future, that, like,
this whole multi-messenger era, and like,
you know, utilizing all the different wavelength telescopes
that we can is really the way to go.
We cannot just build up a picture purely
from X-rays alone, or purely from infrared,
and, you know, I think there should be increased effort
to try and have more coordinated observations
with the different telescopes, like NuSTAR--
Oh, that makes sense, but it's always
so difficult to get that.
It is, and it's difficult to even just coordinate,
you know, soft X-ray and hard X-ray telescopes together,
like, getting, you know, time for both simultaneous,
you know, say, Chandra observations,
and NuSTAR observations, or (mumbles) and NuSTAR.
It's a difficult thing, but, you know,
I think we really need to get a clear picture,
to be looking at wavelengths, of course.
So how do you do your observations
in optical and infrared?
So fortunately there's, I'm also doing it
from space with the Spitzer Space Telescope, so particularly
in the infrared, and my main interest has been to try and
study the environment around the super massive black holes,
not as close as where the X-rays are coming from,
but clearly there's something from the X-ray corona
that illuminates the rest of the accretion disk,
and the dust that's further out.
And so fundamentally, that's one of the key things
that I'm trying to use, is trying to see how long,
once you've got this sort of pulse
that's generated close to the black hole,
it propagates out, and so you can use optical wavelengths
to see that the accretion disk lights up
in the optical a little bit as it gets heated up
from the X-ray, and then later on,
the infrared dust, the dust absorbs it,
and emits it in the infrared.
And so that, I love that, the ability
to exchange time for resolution,
because these structures are so far away
that we're never gonna get a telescope big enough
where that has the resolution to see the accretion disk,
or the dust distribution around--
So do you get dimensions of the disk out of that?
Yeah, again, we don't know exactly where X, Y, Z,
zero is, we're assuming that it's something,
you know, the X-rays that are coming out are very close
to the event horizon of the black hole,
but this is still, you know, your realm of X-rays,
to really figure out those kinds of things.
But once the X-rays, once the photons hit the corona,
and are re-scattered, and up, energized,
and then they start illuminating the accretion disk,
it heats it up, and so just by the light travel time,
when the optical, if it gets, you know,
brighter and fainter, and then the infrared gets brighter
and fainter, two weeks later,
then the dust is two light weeks away from that.
So it's a one dimensional one, so we're averaging,
so we don't get the two dimensional,
or even three dimensional one.
And then we've done it now, of course,
we have better telescopes, there was a project
where you could do it with the Hubble Space Telescope
and the ultraviolet, you used the Swift Observatory,
which had optical and ultraviolet,
and then from ground base, we did optical,
and then from space we did it
with Spitzer and the infrared.
So you could actually see this bright flash go off
in a nearby AGN called NGC5548,
and then you see it propagate as it warms up the disk,
as all that light is falling onto it,
and then eventually you hit the,
the, further away, where the dust is,
and the dust tends to radiate in an infrared.
So we got basically a structure,
and you just, you see this flashbulb go off,
and then it illuminates, effectively, the structure.
So you can map out the dust, where do you see it?
So you do see it, basically the dust sublimation radius,
and you see it at, and it tells you,
depending on what kind of dust it is,
and it's actually one of the problems
for me studying, whenever we try and do X-ray studies
of low luminosity active galactic nuclei in your,
by galaxies, because there're all these X-ray binaries
that are also emitting X-rays,
which make our lives difficult.
But they're also in black holes,
when it's this really interesting sort of both,
it's great, but ugh, it's also a source
of noise for those of us who are trying
to do X-ray observations of nearby galaxies.
We have the same problem,
we can't see the actual black hole
underneath all these very bright X-ray binaries.
It's a weird thing to be sitting in your own galaxy,
but not be able to separate all these,
four million solar mass black hole creating,
next to, how massive, it's like two
to three solar masses for the X-ray binaries?
The X-ray binaries, yeah, so they're,
no, they're typically like 10 solar masses, so from three,
you know, that's the smallest one you can have is
from three solar masses, and then all the way up.
So that's where we have LIGO, and LIGO has now directly,
I mean, this was all theory before,
that we knew that this was gonna happen,
and never seen it before, so LIGO is now the first time
that we have been able to completely verify this theory,
that you can have black holes
and neutron stars merging together.
And so what happens in the case
of two neutron stars, when they merge together,
now suddenly they become heavier,
they become heavy enough to turn into a black hole.
And so the first of these events happened in August,
and what happened here is you had these two neutron stars
that spun around each other, and then they merged,
and then for a very short time, we are talking
about 100 milliseconds, or tens of 100 milliseconds,
it actually remained a neutron star, probably,
it was a hyper massive neutron star because it was spinning
so fast, it didn't collapse under its own weight.
But then, you know,
the angular momentum gets dissipated away from the object,
and then, at that point, it can't maintain its own weight,
and then it collapses, and turn into a black hole.
All this theory that we knew about is now finally being,
Which is great, although it still doesn't help us
in the AGN community, because we don't know how the millions
to billions solar mass black holes came to be.
But it's, at least we're building up,
or hopefully that at some point,
and by understanding these lower mass,
how these lower mass black holes came to be,
then we can see where there are a large scale number
of mergers can potentially give us this,
or you really need something, other,
another corridor to fundamentally get us something
that's a million solar masses, you know,
on the minimum side, but definitely,
you know, we've gotten those which are billion.
So we know we can merge the million solar mass black holes
to get the bigger ones, but how do you get to those
in the beginning, particularly so early in the universe,
when you get quasars at really high red shifts,
so they're really early on.
Yeah, it is odd, it is very odd.
I mean, the other thing that's a little odd,
now we're going back to stellar mass black holes is,
so we look at a lot of supernova remnants,
and we, so we see them, we can only really see them
in our own galaxy, and so we have a lot
of supernova remnants, and so how we see them is,
you see the expelled mass from the star as it died,
so that creates an extended source, and then you look
for the compact object that was left behind.
And what's interesting is that you see,
you quite often see the neutron stars
because they pulse, so they're easy to see,
but so far, we've not found a single black hole
at the center of a supernova remnant.
And so, which is interesting, so you say,
you should see them, you know, you ought
to see some fallback, you know, you need some matter,
you need something, but no, never, not yet been detected.
And this goes to the idea
that you don't have a core balance,
that a supernova with a black hole as its final result,
actually never has any kind of expulsion,
that it all just goes. (imitates sucking)
Yeah, that might be, yeah.
Which was one idea, but again, it's,
I'll leave this to your theoreticians,
I think there are problems with that, as well.
I call it job security. That's right.
We have a lot of things about black holes,
both in terms of their formation,
or even how they exist as they are,
and how they're interacting with their environment
that we're still not understanding.
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