Hi there, physics fans.
When I started this series, I told you that
I’d tell you what I know, what I don’t,
and unsolved problems of fundamental physics.
I’ve talked about the Standard Model and
the gravitational world of general relativity,
but now it’s time to move on into the unknown,
into things we don’t know the answer two.
Take a walk with me on the mysterious side
of Subatomic Stories.
Since 1933 or so, astronomers have known something
is awry in the universe.
Since the days of Isaac Newton we’ve known
about his laws of motion and gravity.
In principle, you can combine those two ideas
and understand in detail how everything in
the cosmos moves.
A Dutch astronomer by the name of Jan Oort
tried to figure out how fast the Milky Way
galaxy was rotating.
He found that it seemed to be rotating faster
than he predicted.
Then there’s the story of Swiss astronomer
Fritz Zwicky who was measuring the motion
of galaxies clustered together in what is
called the Coma cluster.
Clusters of galaxies are held together by
the gravitational attraction between individual
galaxies.
As long as the galaxies aren’t moving too
fast, gravity should hold the cluster together
much like the planets in the solar system
stay in the vicinity of the sun.
Zwicky’s measurements showed that the galaxies
were moving way too fast to be held together
by the gravitational force of the visible
matter in the Coma cluster.
The Coma cluster should be torn apart.
Zwicky hypothesized a form of matter that
was invisible to telescopes, but exerted the
gravity, was what was needed to keep the Coma
cluster together.
He called it dark matter.
He never figured out what dark matter was.
Fast forward to the late 1960s.
Vera Rubin was a bright female astronomer
whose talent and drive was obvious.
Due to the era, she encountered obstacles
because of her gender, including being denied
admission to some universities.
She found ways around these obstacles and
eventually received a PhD from Georgetown
University.
Because she was married with children, she
had to do much of her work from home.
Because of her situation, she decided to avoid
some of the contentious astronomical issues
of the time and study how galaxies rotate,
beginning with our neighbor galaxy Andromeda.
Through a combination of talent and some luck,
she both confirmed Zwicky’s observation
four decades prior >>AND<< overturned the
wisdom of the astronomical community of the
1970s.
Rubin used Newtonian physics to predict how
fast galaxies would rotate.
Now galaxies aren’t rigid bodies like a
frisbee.
Newtonian physics predicted that stars near
the center of galaxies would orbit slowly.
Stars towards the edge of the galaxy would
orbit very quickly.
And stars and gas at the very outskirts of
the galaxy, where stars were few and far between,
would orbit slowly.
That’s what Newton’s laws predicts.
What Rubin found was that in the center and
mid-part of the galaxy that Newtonian predictions
were correct.
However in the very outskirts of the galaxy,
they were totally wrong.
Stars far from the center of the galaxy moved
far faster than predicted.
I’m going to wait until the next video to
talk about the possible causes of Rubin’s
observations, but Zwicky’s dark matter is
an obvious possible explanation.
We know that Newton’s laws are incomplete.
After all, general relativity was invented
partially for just that reason.
The fact that Newton’s laws didn’t accurately
describe the motion of individual galaxies
or clusters of galaxies could be just another
general relativity thing, but that turns out
to not be true.
Remember when I described in episode 13 how
Sir Arthur Eddington showed that general relativity
was right?
He did this in 1919 by looking at stars near
the sun during an eclipse.
The stars appeared in the wrong place due
to the strength of the Sun’s gravitational
field and how it affected the passage of light.
Well, in modern times, astronomers can do
a similar thing using not a single star, but
rather entire clusters of galaxies, with masses
trillions or even hundreds of trillions of
times heavier than the sun.
The universe is big, with some galaxies near
and some far.
Astronomers can look at distant galaxies and
see how they appear distorted by the gravity
of nearer clusters of galaxies.
They then can add up how much matter is in
the stars they see in the nearer cluster and
see if the mass of the nearby cluster and
the distortion they see of the more distant
galaxies makes sense.
And it doesn’t.
Astronomers see more distortion than they
can explain.
So, this means that something is going on
with gravity and it’s not just that Newton’s
laws are wrong.
General relativity also makes incorrect predictions.
There is definitely a cosmic mystery to solve.
I will talk in the next episode about possible
solutions, but I want to draw your attention
to a metaphor that I like very much.
I can’t take credit for it.
I stole it from Stacy McGaugh, an astronomer
at Case Western Reserve University.
He likened the mysteries that I’ve sketched
here as the roots of a large tree.
The possible explanations of the mysteries
are embodied in the trunk, boughs, branches,
twigs and leaves.
The most accepted explanation of these mysteries
is that there exists a substance, called dark
matter, that is invisible to light and other
electromagnetic radiation, but participates
in gravity.
Dark matter, if it’s real, is a single leaf
on the tree.
But, before you can pick out the right leaf,
you need to have ruled out all the rest.
This follows Sherlock Holmes statement from
“The Hounds of the Baskervilles.”“Once
you've ruled out the impossible, whatever
remains, however improbable, must be true.”
In the next episode, I’ll rule out the impossible.
But, in this episode, it’s time for questions.
Let’s see what you’ve got for me this
week.
It’s question time.
Let’s jump right into it.
Adithya Ramanujam asks if my video on the
Planck length means that our physics theories
work for length scales larger than the Planck
scale.
Hi Adithya.
So the answer to your question is no.
What I said is that even if our current theories
were perfect, they would break for lengths
smaller than the Planck length.
But that doesn’t mean we know everything
at larger length scales.
As we’ve seen in the first part of this
video, our current theories can’t explain
data that leads scientists to hypothesize
dark matter.
And this isn’t the only example.
There are tons of things that our current
theories can’t explain, and we’ll learn
about them over the next several episodes.
Matt Hunter claims that dark matter and dark
energy are just placeholder names for unexplained
phenomena.
Hi Matt.
That’s true, kind of, but it’s a little
unfair.
It’s true that dark matter, when it was
originally postulated back in the 1930s, was
an idea that wasn’t very well supported,
but that was nearly a century ago.
As I demonstrate in this episode and the next,
a lot of ideas have been tested and discarded.
Admittedly, we don’t have the final answer,
but we know a lot of things that dark matter
isn’t.
In our next episode, I’ll talk a little
about why researchers are favoring the dark
matter idea more and more.
Rhadian panji oki says that the energy needed
to start fusion is higher than the energy
to start fission and wonders why a star can’t
continue energy release by fission.
Hi Rhadian, there’s so much to say about
your question and I can’t cover it all.
First, it is true that the energy released
in fusion is much higher than in fission.
That’s very clearly shown in this graph,
which shows a very big difference in energy
between light elements.
But that’s only a small piece of the story.
Another piece of the story is that the universe
is 75% hydrogen, 23% helium, 1% oxygen, with
other light elements making up the remaining
1%.
So there are no heavy elements out there.
Then there’s the fact that for heavy element
fission to produce energy, the star has to
have made the heavy elements first via fusion,
so they can release energy when they split.
And that didn’t happen.
Finally, and this is very cool, if there were
a concentrated amount of, for example, uranium
in the universe, it could release some energy
via spontaneous fission.
That even happened about two billion years
ago in the African country of Gabon.
Look it up.
I’ll put a link in the description.
But, as interesting as that is, it doesn’t
help as a source of stellar power.
Fusion can make up to iron, then a supernova
or colliding neutron stars does the rest.
That’s just how it works.
MusicalRaichu asks if supernova explosions
can create heavy elements like Lawrencium.
Hi musical.
The short answer is, well.
we don’t know.
Obviously, we know that there is an astrophysical
process that makes uranium and even plutonium,
since we see it in nature, either on Earth
or in the spectrum of stars.
We’re not sure if Lawrencium is made, because,
if it is, it decays super fast.
There is a very interesting debate going on
in the astrophysical community about where
the very heaviest elements come from.
It could be from supernovae or it could be
from the collision of two neutron stars.
It seems that neutron stars make more heavy
elements per collision, but these collisions
are rare.
And so the conversation continues.
If you’re interested, I put a link in the
video description that is a pretty easy read.
Teotite asks why dark matter doesn’t form
large planet-like structures.
Hi teotite.
The reason is simple.
Forming big structures requires a strong interaction,
like electromagnetism, to hold matter together.
That’s how ordinary planets formed.
Dark matter doesn’t experience electromagnetism,
so that means no clumping.
There is one caveat.
There is an idea called complex dark matter,
which suggests that maybe dark matter experiences
an electromagnetic-like interaction that only
attracts other dark matter.
There is no evidence that this is true, but
it’s a cool idea.
I made a long form video on the subject matter
and also wrote a Scientific American article
on the subject with my colleague Bogdan Dobrescu.
I put links to both in the video description.
Daniel Fajardo asks what theoretical development
and experimental result would I like to see
in my lifetime.
Well, I think I’d like to see a theory of
everything developed and to see the sun grow
to be a red giant.
I don’t know that we’ll ever devise a
theory of everything, but the red giant thing
won’t happen for some five billion years
or so.
In case it wasn’t obvious, to see that happen,
I’d have to live for five billion years.
Hopefully I’d still have my youthful good
looks.
On a more practical note, I’d like to see
dark matter solved, both experimentally and
theoretically.
I’d like to see if quarks and leptons have
smaller building blocks.
Both of those could actually happen.
Going crazy, I’d like to see some sort of
faster than light propulsion be invented.
Dreaming super big, I wouldn’t complain
if someone invented tasty and calorie free
ice cream.
Cookie dough, preferably.
And, finally, Durin S. Bane asks if I could
give out a Nobel or two, to whom would I give
it.
Hi Durin.
Love the icon.
Cat people rule.
Well, obviously, I’d give one to me.
But, in all honesty, I haven’t earned one.
So I’ll just dream.
No, if I had to give out two Nobel’s, I’d
give one to Vera Rubin and the other to Chien-Shiung
Wu.
Rubin did more than anyone to put dark matter
on solid observational footing.
No matter what dark matter turns out to be,
it’s going to revolutionize physics.
And Wu conducted an experiment in 1956 that
was (a) insanely hard and (b) proved that
the weak force treated matter and antimatter
differently.
She did that by manipulating the spin of cobalt
nuclei and then watching the direction of
electrons the nuclei emitted when they decayed.
Rubin probably didn’t get the Nobel because
we still don’t know the answer to dark matter.
Wu didn’t get the Nobel, but two guys did.
And all they did was say “Hey, maybe you
should do this hard experiment and make that
measurement.”
But she’s the one who actually did it.
I’m still peeved about this.
Sadly, both women are no longer alive, and
the Nobel cannot be awarded posthumously.
They both deserved to receive the prize.
OK, that’s all the time we have for questions
today.
Please like, subscribe, and share.
In the next episode, I’ll talk about why
researchers believe dark matter is likely.
We follow the evidence, of course, and the
evidence points to new physics.
And that’s fantastic, because, as you know,
even at home, physics is everything.
