Hi there physics fans. Another week, another
video. We’ve been talking about black holes
for a while and it’s getting to be time
to move on. However, before we do that, I
should probably spend a little time talking
about how we know that black holes even exist.
Indeed, that’s a great topic for this week’s
episode of Subatomic Stories.
Let’s start with talking about the different
types of black holes. The most common one
and the one we understand the best are what
we call stellar mass black holes. These are
black holes with a mass several times the
mass of the sun. They are created when very
heavy stars – maybe twenty to a hundred
times the mass of the sun burn through their
hydrogen fuel.
They then burn through helium, and a series
of heavier and heavier elements until they
hit iron. At iron, fusion stops. You can’t
make energy fusing elements heavier than iron.
So, fusion stops, gravity takes over, the
outer layers of the star smash in, heating
everything up as it goes. The density and
gravity at the core get high enough to make
a black hole and the outer layers get hot
enough to get blown out into the universe
is a supernovae. The black hole has a mass
in the range of five to thirty times the mass
of the sun or thereabouts.
Then there are much bigger black holes. These
are monsters found at the center of nearly
every galaxy. These are called supermassive
black holes. The smallest one known is at
the center of a galaxy located about 340 million
lightyears away in a dwarf galaxy called RGG
118. That’s big, but it barely counts as
supermassive. Our own Milky Way hosts a black
hole at the center that is about four million
times heavier than the sun. And the largest
supermassive black hole known contains a whopping
40 billion solar masses. It’s located in
a galaxy called Holm 15A, located about 700
million light years away.
Supermassive black holes aren’t made by
the explosion of stars. Truth be known, we
don’t know how they are made. There are
several theories and all I can say is that
“Hey…we’re working on it.” But it
does seem that they were made very early in
the history of the universe and they have
a huge role in the evolution of galaxies.
So that’s the two known kinds of black holes.
Intermediate black holes don’t seem to exist.
That’s a clue of some sort, although we
don’t understand what it’s telling us.
OK, if those exist, how do we know? Well solar
mass black holes were the first observed,
indeed the first one astronomers saw is called
Cygnus X-1. Of course, you can’t directly
see black holes, but Cygnus X-1 is locked
in tight orbit with an ordinary star. The
black hole is siphoning off some of the mass
of the companion star and, as the mass spirals
down into the black hole, it heats up and
emits x-rays.
Black holes can be indirectly seen if they
are in a binary system with an ordinary star.
If that happens, the two objects – the star
and the black hole orbit a point between them.
Since we can’t see the black hole, what
it looks like is a star just wobbling in space.
And, of course, we talked in episode 16 about
the gravitational waves emitted by two merging
black holes, so that’s another way we can
see them. But it turns out that with supermassive
black holes, we have much more compelling
ways to know they’re real.
For the supermassive black hole at the center
of the Milky Way, we can simply point radio
telescopes in that direction and watch the
stars orbit a heavy and invisible thing much
heavier than any star. Astronomers use radio
telescopes to watch these stars because there’s
too much dust near the center of the galaxy
to use visible light.
Astronomers have been watching for many years
and they’ve even made movies of the stars
orbiting this supermassive black hole. It’s
very cool.
But the most compelling evidence for the existence
of a black hole came in April of 2019. Astronomers
used radio telescopes distributed across the
globe to transform the entire Earth into a
single huge radio telescope and turned it
to directly view a super massive black hole.
You’d think that they’d try to image the
one at the center of the Milky Way and they
did, but it’s not the one they saw first.
The first supermassive black hole that was
photographed is in a galaxy called M87, located
about 55 million light years away from Earth.
This particular black hole dwarfs the one
in the Milky Way. It has a mass of about 6.5
billion times the sun – over 1,500 times
bigger than the black hole at the center of
our galaxy. And here’s the what the actual
picture looks like. That’s just pretty cool.
Next to it is an artists rendition of what
astronomers think it would look like if they
had perfect telescopes.
You might wonder why astronomers first succeeded
with this distant black hole and not the much
closer one here in the Milky Way. Basically,
because it’s so much bigger, anything orbiting
it takes much longer to go around. Since these
orbiting things can mess up the picture, you
want to look at black holes that have things
that change more slowly.
So that’s the deal. We’ve photographed
a huge black hole in a distant galaxy and
astronomers are hoping to figure out ways
to directly image the one in the Milky Way.
They’re real. Science is amazing, isn’t
it? OK, let’s take a look at this week’s
stack of questions.
Questions. Questions are fun, but before we
get into them, I want to draw your attention
to an error in the numbers I reported for
my example supermassive black hole in episode
18. When I reported the amount of force felt
by a 1 kilogram mass 15,000 meters above the
event horizon, my spreadsheet pointed at the
wrong mass for the black hole. The correct
force felt by a 1 kilogram mass in that location
is about 400 >>tons<<, a number which is about
the same in metric or imperial units.
The number I quoted for the difference in
forces felt by that object and another one
kilogram object a meter farther away from
the event horizon was wrong too, and for the
same reason. The correct difference is about
a thousandth of an ounce or about 30 milligrams.
The overall message is unchanged. Basically,
the two objects feel a nearly identical force
and there is no spaghettification at that
location. And, if you want to reproduce the
numbers yourself, I used ordinary Newtonian
gravity equations to do the estimate.
I’d like to thank YouTube user thedeemon
for taking the time to try to reproduce the
calculation and pointing out the error. If
you recall, I am compiling a list of individuals
who have contributed many thoughtful and correct
answers to viewers questions. I can’t promise
that they’re always right, but they’ve
demonstrated some expertise. Thedeemon has
done this more than once, so I’d like to
welcome the newest member of the Hall of Heroes.
Moving on. A vast number of you tried to drag
me into the gif/jif wars and, as tempting
as it is, I’m going to take a pass on that
one. But it’s always instructive to see
what sorts of things people choose to comment
on more than science. Speaking of science,
how about we get to the science questions?
Captain Cruise notes that they learned that
temperature is a measurement of random kinetic
energy and wants to understand how it applies
to space. Hi Captain, that’s a good question
and it rests on a lot of physics shorthand
physicists use when discussing the subject.
Space itself doesn’t have a temperature
in the sense you mean it. So it’s not at
all surprising that you’re confused.
Instead, we need to talk about objects that
do have temperature in the sense you mean
it. If they do, they can emit radiation. The
range of wavelengths of radiation they emit
depends on the temperature. That’s why heated
steel glows red, but hotter steel glows blue.
When the universe was young, it was filled
with a hot plasma of free protons and electrons
that radiated light. The universe expanded
and cooled and when it hit a temperature of
about 3000 Kelvin, the temperature was low
enough for protons and electrons to combine
into neutral hydrogen, which made the universe
transparent. The light could move freely across
the universe. Scientists then kind of sloppily
say that the universe had a temperature of
3000 Kelvin because of the wavelength of light.
The universe became transparent nearly 14
billion years ago. Since that time, the universe
has expanded to be about 1,100 times bigger.
This expansion has also stretched the wavelength
of light that was emitted when the universe
was 3,000 Kelvin. That light has been stretched
into radio waves. In order for an object to
emit the radio waves we observe today, it
would have to have a temperature of about
2.7 Kelvin.
So space isn’t literally that temperature.
The 2.7 Kelvin is the temperature matter would
have to have to emit the radio waves we see
now. As the universe continues to expand,
the wavelengths will continue to lengthen.
In the distant future, astronomers would say
that the temperature of space would be one
Kelvin, but it would be the same light we
see now that was emitted in the early universe
when it was filled with a plasma with a temperature
of about 3,000 Kelvin. Good question.
Swapnil Kumar notes that this channel makes
him want to leave engineering and study theoretical
physics. Hi Swapnil. Makes sense to me, but,
then again, I’m partial. These are hard
decisions. There are more jobs in engineering,
but personally I like science better. My actual
advice is if you are really considering physics
to be open to experimental physics as well.
Most young people considering physics want
to go into theoretical physics. I did, for
example. There is a tendency to imagine experimentalists
as guys or gals who turn wrenches. And, while
I admit to having used a wrench or two in
my day, the reality is so much more exciting.
Theoretical physics has its charm, but it’s
important to remember that physics is an experimental
science. The experiment is king. Experiments
are what forces the universe to give up its
secrets.
Personally, I don’t regret changing my focus
to experimental work. It’s not for everyone.
There are people who really do prefer the
world of theoretical calculations, but you
should be open to experimental work. There
are also the practical points that there are
more experimental jobs within academia and
a greater diversity of jobs outside academia
for a person with an experimental background.
The experimental path leads to many successful
career outcomes. Good luck.
Constellation Pegasus asks what made me want
to study particle physics? Hi Constellation.
Well, it wasn’t a guaranteed outcome. I’ve
always been interested in what one might call
“big questions,” like “how did the universe
come to be” and “does the universe have
to be the way it is?” And I investigated
many classical approaches to answering those
questions, including philosophy, religion,
and theology. Indeed, I have minor degrees
in all of those.
But in science, I found attractive the ability
to ask concrete questions, yielding concrete
answers, leading to demonstrable and reproducible
progress forward in human knowledge. When
the strength of science became clear to me,
I put aside those other disciplines and never
looked back.
Regarding particle physics, it’s simple.
For a person interested in these sorts of
questions, your choices are particle physics
or cosmology. When I was a student, cosmology
was more of theoretical and introspective
discipline. Definitive experiments were few
and far between. That’s changed over the
decades. Observational cosmology has really
come of age. If I were a student now, I would
have a hard time deciding between the two.
Macherla Komaraiah asks if black holes can
clear the mystery of gravitons and complete
the standard model. Hi Macherla. No. They
can’t. The theory of black holes includes
neither gravitons, nor the standard model.
In order to solve those unsolved questions,
researchers will have to devise a theory of
quantum gravity. That’s a hard nut to crack.
It’s going to take an Einstein or some fascinating
and unexpected experimental observation before
we’ll make real progress.
Tarendeep Singh asks if there is any experimental
proof of Hawking radiation. Hi Tarendeep.
No. None. But from what we know about quantum
mechanics and relativity, it would be hard
to imagine that it’s not a real thing. Still,
experimental confirmation is important. The
best I can offer you are some laboratory measurements
that study sonic or light analogues of Hawking
radiation. You can google them if you’re
interested.
Physics never dies asks in a roundabout way
if there is any possibility that antimatter
falls up. Hi Physics. Is there a possibility?
Sure. Is it likely? Probably not. But, in
physics, experiment is king, and people are
trying to measure this. At CERN, Fermilab’s
sister laboratory in Europe, scientists are
making antimatter neutral hydrogen atoms and
watching them to see if they fall up or down.
The experiments go by the name of Alpha-g
and GBAR and a few others.
I visited the Alpha experiment at CERN about
a year ago and spoke to the spokesperson.
They were confident that an initial measurement,
simply answering “does antimatter fall up
or down?” would arise in the near future.
Of course, the CERN accelerator complex has
been on hiatus recently, but when they resume
operations, I imagine we’ll know the definitive
answer not too long after. Precisely measuring
the force of gravity antimatter experiences
will take a little longer.
Muriel Bras-Jorge asks how can the first observed
black hole merger emit more energy than all
the stars in the universe? Hi Muriel. Good
question. The answer is simple. The gravitational
wave event took 0.2 seconds. You simply add
up all of the electromagnetic radiation emitted
by all of the stars in the universe in that
0.2 seconds and compare it to the energy emitted
by the merging of the two black holes. It’s
two different sources of energy and, briefly,
the merging was greater.
J D asks about why supermassive black holes
don’t cause spaghettification. Hi JD. First,
there was the mistake I made that I mentioned
earlier, but that doesn’t change the conclusion.
Spaghettification occurs because gravity is
changing rapidly with distance and, outside
a supermassive black hole, the force of gravity
changes slowly. In big picture terms, it’s
not so different than gravity here on Earth,
which changes with altitude, but not noticeably.
Near smaller black holes, the gravity just
changes with distance more rapidly. Now I’d
like to correct a misconception I saw in a
lot of questions. Spaghettification does occur
for supermassive black holes, it just occurs
inside the event horizon. So you still die
a painful death, but nobody will be able to
see it. So it’s both unpleasant >>and<<
lonely.
OK, so that’s all the time we have for questions
today. But, don’t be sad, there’s always
next week. In the meantime, please like, subscribe
and share. Next week, I’m going to talk
about a different kind of physics, which is
fantastic! Because physics is interesting!
Physics is awesome! And, well, of course,
even at home, physics is everything.
