Hi there, physics fans old and new.
In case you haven’t noticed, this series
has turned its attention to gravity.
Like the microcosm, gravity is often misunderstood
by the public in many circumstances, perhaps
most notably when dealing with black holes.
Misunderstandings are opportunities, so black
holes are the topic of the next couple of
episodes of Subatomic Stories.
Black holes are places with gravity that is
so strong that they bend space and time and
even the mind.
The first two are bad enough, but bending
the mind leads to misconceptions.
So, let’s spend this episode learning about
black holes and some of the common misconceptions.
Black holes are locations in the universe
where so much mass is concentrated in such
a small space that gravity is so strong that
not even light can escape.
They are most commonly created by the death
of an ultra-massive star that used up all
of its fuel and then exploded, leaving a black
hole in its wake.
Astronomers are still working out the rules
that govern how big black holes can be if
they are made this way.
The smallest one found so far has a mass of
3.8 times the mass of the sun and the biggest
ones seem to be maybe thirty times the mass
of the sun or so.
There are another class of black holes called
supermassive black holes, but those are a
different thing and they’ll have to wait
for a future episode.
So, the first misconception about black holes
is that they are some sort of ravenous monsters
that reach out and grab surrounding matter.
That doesn’t happen.
At reasonable distances, black holes are no
different than stars of the same mass.
If it were possible to make a black hole with
the mass of our sun and we replaced the sun
with it, the planets wouldn’t notice.
Well, you know, except for that whole “it
got dark” thing.
No, black holes and stars are different simply
because you can get closer to them.
The force of gravity changes as a function
of distance from a massive object.
That’s true in general.
Since a hypothetical black hole with the mass
of the sun is about three and a half miles
or six kilometers wide, you can get very close
to it.
And it’s that strong gravity near the surface
that's hard to escape.
Speaking of size, that’s another thing that
is confusing to some people.
The most useful measurement of the size of
a black hole is what is called the Schwarzschild
radius, named after Karl Schwarzchild who
calculated the size of a black hole while
fighting on the Russian front in World War
I.
Between artillery barrages and bayonet charges,
he used Einstein’s equations to figure out
the size of a non-rotating black hole.
That’s dedication to physics.
The Schwarzschild radius is the distance from
the center of a non-rotating black hole where
gravity becomes so strong that not even light
can escape.
Sometimes people call it the event horizon.
For non-rotating black holes, they’re the
same thing.
The Schwarzchild radius for an object with
the mass of our sun is about three kilometers.
For a mass equivalent to the Earth, it’s
just nine millimeters.
And for a smallish adult size human, it’s
about a tenth of a trillionth the size of
a proton.
For me, it’s a bit bigger, but, hey- I’m
working on it.
There’s another size that is often mentioned
when people talk about black holes.
When a black hole is formed, the matter that
made up the star is crushed down to a tiny
size.
According to the equations of general relativity,
that tiny size is literally zero size.
It’s what’s called a singularity.
Now the idea of an object having zero size
seems unreasonable to most people.
And that intuition turns out to be sensible.
In spite of the frequently repeated claim
that a singularity lies at the center of a
black hole, no scientist takes it seriously.
A singularity just means that the equations
have been pushed beyond the realm of the physical.
Singularities are math things, not physics
things.
I don’t want you to come away with the idea
that the scientific community doesn’t know
what it’s talking about.
It’s certainly true that the matter that
makes up a black hole is crushed to very small
size.
But the claim that it’s zero size is a consequence
of general relativity, which we know fails
in the world of the super small.
For that, we need a theory of quantum gravity,
which we don’t have yet.
We talked about quantum gravity in the last
episode.
Because we don’t have a theory of quantum
gravity, we don’t know exactly what the
center of a black hole is, but we can be pretty
certain that it’s not a literal, mathematical,
singularity.
Of course, since we can’t escape a black
hole, it’s going to be hard to ever make
direct measurements of what the center of
a black hole really looks like.
You know, there’s an awful lot to say about
black holes.
I think I’ll stop here and continue talking
about black holes for the next episode or
two.
I expect that we’ll have some spectacular
questions and answers.
Speaking of questions, let’s see what questions
the episode on quantum gravity have inspired.
Learning about the one force not yet explained
at the quantum level has certainly generated
some very serious questions.
I hope I can answer with the appropriate gravity.
I crack myself up.
Okay, I’m getting serious now.
Let’s see what questions our viewers have
for me.
Michael Blacktree asks if gravitons exist
if they’re the force carrying particle for
space-time and if that means that space-time
is an energy field.
Hi Michael.
The answer to your question is hard to give
definitively.
What we know is that general relativity says
that the force of gravity can be explained
as the bending of space and time.
We also know that, if it exists, that the
graviton is the quantum carrier of gravity.
That’s simply a definition.
In order for the graviton to be, as you said
“the force-carrying particle of space-time,”
the connection between energy and matter,
space and time would have to persist at the
quantum level.
Without a theory of quantum gravity, I think
that’s a step too far.
It may be that quantum gravity shows that
the connection between the distribution of
energy and spacetime isn’t causal, but rather
arises from a deeper principle that governs
both.
And then there are theories of gravity which
says that it emerges from quantum entanglement
encoded in a two dimensional holographic representation
of the universe.
And, yes, you should be forgiven if that just
sounds like a bunch of gobbledygook.
It isn’t.
It’s at least a semi-respectable theory.
The bottom line is that it’s probably premature
to answer your question.
The answer to your second question is easier.
According to general relativity, there is
a one to one correspondence between energy
and spacetime.
So, at least in that theory, yes.
Luis Lopes congratulates me for simply mentioning
the possibility that relativity can be modified.
Hi Luis.
This is a sticky point.
It boils down to the fact that there are a
lot of science enthusiasts who are completely
confident that they’ve found some fatal
flaw in either special or general relativity.
While I admire their enthusiasm, many of them-
indeed most of them- don’t understand relativity
well enough to have a substantive criticism.
The criticisms are often philosophical or,
when they include equations, they use special
case equations for general cases.
Or, and this is a common situation, they don’t
know all of the tests to which relativity
has been subjected and successfully passed.
In short, many of these critics simply haven’t
mastered the subject well enough.
But they don’t realize it and many become
quite upset if you point any of those things
out.
However, among the professional science community,
we always remain open to changes of accepted
theories.
That’s how science works.
There’s never a rigid insistence that any
theory cannot be questioned.
After all, I’ve said that scientists are
well aware that general relativity fails in
the quantum realm and there are a number of
modifications of relativity that are supposed
to solve the dark matter mystery.
But, with that said, relativity has passed
many very stringent tests.
We know where it works and where it doesn’t.
And, if anyone has a new theory that they
claim is better than either form of relativity,
they need to expect to be confronted with
a huge body of knowledge and a daunting gauntlet
of data that relativity has successfully survived.
Overthrowing relativity is possible- just
difficult.
Emmett O’Brian asks if there was a hope
that the LHC would produce evidence of a graviton.
Hi Emmett.
The answer to your question is yes and no.
There was zero chance that the LHC would produce
a classical graviton.
Zero.
However there was a possibility of finding
a graviton if there exist extra dimensions
of space that gravity can enter, but the other
forces can’t.
These extra dimensions would explain the relative
weakness of gravity.
These extra dimensions would be small- smaller
than a millionth of a meter.
Probably much smaller than that.
Furthermore, they are cyclical, like the surface
of a sphere.
And, if we use the LHC data to study objects
the size of these extra dimensions, the gravity
will suddenly become strong.
Furthermore, these gravitons would be massless,
but they could be trapped in these small dimensions,
which would mean they have energy, but no
momentum.
That means, according to us big beings, they
would appear to have mass.
I made a long form video on extra dimensions
which explains this all in more detail.
The link is in the description.
It’s a very cool idea.
Divyansh Vishwakarma says that we know that
the smallest possible length is the Planck
length and doesn’t that give some proof
that loop quantum gravity is real?
Hi Divyansh.
That’s a good question.
The answer is no, but it’s a reasonable
question to ask.
In fact, I think I’m going to issue an IOU
for this one, since the Planck length is frequently
badly misunderstood.
You’re going to have to wait for a couple
episodes while I deal with black holes.
But I’ll get back to it.
Bjarni Valur asks about the origins of the
Fermilab logo.
Well, there’s a long history and I put a
URL in the description that explains it all.
But basically, it’s an artistic combination
of the two most important magnets in a particle
accelerator, the dipole and the quadrupole.
Dipoles bend the path of particle beams and
quadrupoles focus them.
In an optical analogy, dipoles are prisms
and quadrupoles are lenses.
Take the two plates of a dipole and combine
it with the four bent plates of a quadrupole
and, voila, one Fermilab logo.
Beautiful, isn’t it?
Skorj Olafsen points out a difference between
particle physicists and cosmologists regarding
the observation of dark matter.
I suppose it’s true.
But even cosmologists admit that dark matter
isn’t a complete lock- well, honest ones,
anyway.
Now there are tons of lines of astronomical
evidence that dark matter is real.
Galaxies rotate too quickly.
Clusters of galaxies should tear themselves
apart.
The images of distant galaxies are distorted.
All of these are examples of observations
that clearly point that we’re missing something
and that something is often called dark matter.
But dark matter hasn’t been directly observed.
We haven’t directly detected a dark matter
particle “in the wild,” so to speak.
Hints of indirect observation of dark matter
in the center of galaxies appear, and then
are explained by more ordinary phenomena.
Particle accelerators try to make dark matter
and fail.
If we’re honest, all we know is that there
are astronomical mysteries that we need to
explain.
Dark matter probably is real.
I think it is probably real.
But it would be hasty say that the story is
over.
Guilherme Matos Passarini asks me where I
get my hair cuts.
Hi Guilhereme.
It’s easy.
Physicists are masters of matter and energy,
space and time.
You think that we let such mundane concerns
as haircuts get in our way?
My hair wouldn’t dare cross me.
Oh yeah, and clippers work wonders.
Can’t go wrong with clippers.
Okay, so that’s all the time we have for
questions.
You know the drill.
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Let’s spread the physics love far and wide,
because- well, you know- even at home, physics
is everything.
