Hi there, physics fans.
In the last episode, I told you about astronomical
mysteries that have led some scientists to
postulate that there exists an unseen form
of matter that is five times more prevalent
than all of the stars and galaxies seen by
the best telescopes.
Seeing the unseen sounds like a perfect topic
for this week’s episode of Subatomic Stories.
From the 1930s, astronomers saw galaxies both
spun and moved through space more quickly
than expected.
They saw images of distant galaxies more distorted
than they could explain.
Truth be told, it’s a bit of mess.
Even today, we still don’t definitively
know the explanation.
The most accepted answer is that a substance
exists called dark matter.
But first let me kind of breeze through ideas
that were investigated and discarded.
In the last video, I mentioned a delightful
analogy that I believe originated with Stacy
McGaugh, professor at Case Western Reserve
University.
He treated the observed discrepancies as roots
of a tree, whose branches and leaves are a
metaphor for potential solutions.
If the issue is a disagreement between the
motion of stars and galaxies and their observed
gravity, then it stand to reason that scientists
either misunderstand how astronomical objects
move or the gravity that governs them.
Ideas on how researchers may not understand
motion fall under the umbrella name MOND,
short for Modified Newtonian Dynamics.
The first MOND theory was proposed in the
early 1980s, and it did a brilliant job explaining
away the discrepancy of having the outer edge
of galaxies rotating too fast.
The original theory didn’t do as good a
job on the other mysteries and the theory
was obviously incompatible with the theory
of relativity.
In recent years scientists have devised improved
versions of MOND, which do a better job and
are now relativistic, but there are two observations
that make me personally lean away from the
MOND explanation for the mysteries at the
root of the tree.
I’ll get to those in a moment.
If the answer isn’t misunderstanding motion,
then what remains is gravity.
Gravity could be wrong because we literally
don’t understand the theory of gravity or
because there is invisible matter.
That’s the next branch in the tree.
Both of them remain possible explanations.
If the issue is unobserved matter, then we
can ask what kind of matter it is.
In the 1980s, the most obvious and least adventurous
explanation was that perhaps there were invisible
astronomical bodies, like black holes, brown
dwarfs, and planets that have been ejected
from their solar system and now wander the
stars.
Researchers looked for these objects, called
Massive Compact Halo Objects, or MACHOs.
Some were observed, but it became clear that
there simply aren’t enough to solve the
mystery.
So, scientists turned their attention to a
form of dark matter that is essentially a
gas throughout space.
One thought was that the gas was quick moving
neutrinos, while another thought was that
maybe it was a new form of stable, slow moving,
and electrically neutral matter – something
like neutrons floating around.
When the universe was young, whether dark
matter was fast or slow would have strong
consequences on how galaxies formed and we
can use powerful telescopes to look at ancient
galaxies and we are quite confident that dark
matter doesn’t move quickly.
Because, in a gas, temperature and the speed
of molecules are related, we call this form
of dark matter, cold dark matter.
A decade or so ago, physicists thought that
the theory of supersymmetry would provide
for a cold dark matter particle called a WIMP,
short for Weakly Interacting Massive Particle,
but the theoretical enthusiasm for that has
waned of late.
WIMPs would be like stable neutrons with a
mass in the range of hundreds or thousands
of times of that of a neutron.
However, if we’re being honest now, we have
really no idea the mass of individual dark
matter particles.
They could be far lighter than an electron
and as heavy as an asteroid.
In fact, we’ve looked for cold dark matter
at all of these sizes and masses and have
found precisely zero.
Nothing.
Nada.
Bupkis.
And, when I say we looked, I mean we’ve
looked.
I could spend hours telling you all of the
ways.
But nothing.
So, we’re at a point where cold dark matter
and modified gravity are possibilities.
Is there any reason to pick one over the other?
Maybe.
Actually, there are two.
The first, is called the Bullet Cluster.
This is an astronomical phenomenon where two
clusters of galaxies passed through one another.
If dark matter is real, then a cluster of
galaxies consists of galaxies, hydrogen gas,
and dark matter.
When they pass through one another, the galaxies
miss and pass through, and the hydrogen gas
collides and heats up in the middle.
If dark matter is real, it should stay with
the galaxies and, if dark matter isn’t real,
then it should stay with the gas.
And the data says it stays with the galaxies.
So that’s respectable evidence that dark
matter is real.
Now there are some people who do not find
this to be persuasive.
So, let me tell you about another observation.
Most galaxies rotate more quickly than can
be explained by the stars and gas they contain,
but there are a couple of instances of galaxies
that rotate exactly the speed you’d expect
solely from the observed stars.
If the explanation for McGaugh’s roots is
either modified gravity or motion, it’s
hard to imagine that it somehow would be turned
off in a galaxy.
However, it’s not too hard to imagine a
galaxy that, by chance, simply doesn’t contain
dark matter.
Thus, in a moment of astronomical irony, galaxies
without dark matter seem to be excellent evidence
for the existence of dark matter.
Now, like I said, most astronomers believe
dark matter exists, but it has never been
directly observed.
And, until it does, we have to be cautious.
If you are a young person interested in physics,
it’s an important unsolved problem and the
person who solves it will be a scientific
hero.
Okay, let’s see what sorts of questions
we have this week.
There were tons of interesting questions this
week.
Let’s jump into them.
Balazs Adorjani asked if dark matter could
form a black hole.
Hi Balazs.
The answer to your question is both yes and
no.
Assuming dark matter is the particle form
currently favored by astronomers today, dark
matter exerts gravity just like ordinary matter.
If you were to clump enough of it together
in a small location, the result would be a
completely ordinary black hole.
That’s the yes part.
But that “clump together” thing is key.
Since dark matter isn’t affected by electromagnetism,
it doesn’t clump together.
And, even if dark matter experiences forces
that only affect dark matter, we already have
proved that dark matter isn’t in the form
of massive and compact objects.
So, since dark matter doesn’t make objects
like dark matter stars, we can be confident
that dark matter doesn’t make dark matter
black holes.
Good question.
Gerry Galweski acts if dark matter could be
ordinary matter in some sort of parallel universe.
Hi Gerry.
The answer to your question is no.
Mostly, at least.
First, ordinary matter clumps into compact
objects like stars or galaxies.
Dark matter is much more diffuse than that.
So even if there are ghost universes, we know
that they’re not ones in which matter acts
like matter in our universe.
But there’s a much trickier thing to think
about.
Gravity weakens as one over the distance squared.
This is a direct consequence of there being
three dimensions of space.
In your scenario, gravity exists in both our
universe and the parallel one.
That probably means that there are six dimensions,
which means that gravity should weaken as
the fifth power of the distance.
Now, obviously such statements depend critically
on the exact multiverse model you’re considering,
and how dimensions work and all that, but
hopefully you can see that your supposition
would require some disciplined effort to carefully
lay out a well developed theory.
Geetesh Lashkari asks what the path is for
becoming an astrophysicist.
Hi Geetesh…that’s simple.
Start out trying to be a particle physicist
and fail.
Your fallback plan would be to be an astrophysicist.
Okay- I’m kidding, of course.
I actually think astrophysics is totally awesome.
No, the answer your question is pretty much
the same for any field of physics.
Basically, you need to do well in high school
and learn science, mathematics, and English.
Then you go to a university and study physics.
Note I didn’t say study astrophysics.
For an undergraduate degree you should get
a firm grounding in basic physics.
It’s in graduate school that you begin to
specialize.
Now my advice for graduate school is to attend
the most prestigious graduate school that
will accept you.
That means getting good grades in your undergraduate
career.
This is mandatory.
One thing that is not as obvious is the need
to do research as an undergraduate student.
Ideally, this would be in astrophysics, but
that’s not super important.
The important thing is to get some experience
when you are young.
That will help you get into a good graduate
school.
I should probably point out that the details
vary from country to country, but what I’ve
said here is pretty universal.
Still, you should ask a physicist in your
country for advice.
It’s just safer.
Good luck.
Vivek Yadav asks if the laws of physics can
change over time and if dark matter might
convert into ordinary matter.
Hi Vivek.
We’re pretty sure it can’t.
And this isn’t supposition.
It’s data.
For instance we can look at galaxies in deep
space.
Light from those galaxies took millions, billions,
or as much as 14 billion years to get here.
The laws of physics always seem to be the
same throughout the history of the universe.
So we can rule out your conjecture with some
certainty.
Banang points to my favorite tree metaphor
and asks if dark matter can be multiple leaves
on the tree.
Hi Banang.
You make a good point.
Cosmic neutrinos are a form of dark matter,
as are black holes, brown dwarfs, etc.
However we now know enough to know that neither
of those things can make up the majority of
dark matter.
Dark matter is five times more prevalent than
ordinary matter, and neither neutrinos, nor
black holes are anywhere near enough.
After decades of work, the favored explanation
is that dark matter is likely to be a single
form of undiscovered form of matter.
On the other hand, until we’ve solved the
dark matter puzzle, we need to not be close
minded and focus on a single answer.
It remains possible that dark matter is multiple
leaves on the tree.
However, we can definitively say that we’ve
ruled out many of the leaves as being the
main solution to the dark matter mystery.
Dam dam notes that dark matter doesn’t interact
with light and ask if we could just run into
dark matter in outer space.
Hi dam dam.
I think the answer is no.
After all, when we touch something, what is
going on is an electromagnetic reaction between
atoms.
Dark matter doesn’t experience electromagnetic
reactions, so it is likely it would pass through
you, just like neutrinos do.
Good question.
Giovanni P notes that Mercury orbits the sun
faster than the outer planets and is asking
how this applies to galactic motion.
Hi Giovanni.
What you said is true.
But there is a key difference.
Planets orbit the sun, which is a concentrated
form of mass.
Stars orbit galaxies, which are a distributed
form of mass.
And just as important, stars are actually
inside the galaxy itself.
The orbit of stars inside the galaxy are governed
only by the mass of other stars inside their
orbit.
The ones outside don’t matter.
It’s easy to show this by inventing an imaginary
galaxy that is a solid sphere.
According to Newton, the orbital velocity
of a star inside the sphere increases as a
function of distance from the center and drops
off outside the sphere.
You can see what I mean here.
And I put a link to a paper in the description
that shows the relevant math.
In the case of real galaxies, the drop off
in orbital velocity at large radii is because
the star is outside the galaxy and the mass
of the entire galaxy is governing the star’s
motion.
That’s like when planets orbit the sun.
Good question.
Artcamp7 asks if the phenomenon of light bending
around his or her finger when held up to a
bright light is the same as gravitational
lensing.
Hi Artcamp.
Actually, no.
It’s not.
Light bending around your finger is a phenomenon
called diffraction and it’s a behavior that
waves do.
All waves bend around corners.
The gravitational effect predicted by Einstein
is a different thing entirely.
And, finally, Michael Kingsford Gray claims
that dark matter is the modern form of luminiferous
aether.
Hi Michael.
There’s part of me that agrees with you
and part that doesn’t.
It’s certainly true that the term “dark
matter” is a place holder name for a solution
to the mysteries I described in the last episode.
We don’t know the true nature of dark matter,
or even if it exists at all.
However, there never was any evidence for
the existence of the aether.
It was simply a postulate because nobody could
imagine that electromagnetic waves like light
could travel without a medium.
After all, all other forms of waves need a
medium.
When Zwicky invented the term dark matter
back in the 1930s, it was just a placeholder.
But dark matter is a more advanced concept
these days.
Many, and I mean many, postulated explanations
have been falsified.
Killing ideas is not the same as discovering
ideas, even though they both are crucial steps
in the scientific process.
But an honest person must admit that we don’t
know the final solution of the mystery of
dark matter.
We need to be open to all possibilities that
have not yet been ruled out by measurement.
Okay- that’s all the time we have for questions
today.
Actually, we went a bit long…long enough
that I don’t have time to ask you to like,
subscribe, and share.
Oops.
I guess I just did.
But I couldn’t help talking so long.
Get me started talking about physics and I’ll
go all day.
And who could blame me?
After all, even at home, physics is everything.
