The periodic table of chemistry embodies a
lot of knowledge of the field of chemistry. It took a long time for chemists to understand
the table’s patterns. In modern particle physics, the equivalent is the Standard Model,
which still has some unexplained features. And that tale is the basis of this
week’s episode of Subatomic Stories. The periodic table of chemistry was invented in
1869 by Russian chemist Dmitri Mendeleev. You probably learned about it in chemistry class. The
table consists of rows and columns of elements. Each column consists of elements with similar
chemical properties, and the mass of elements was ordered, with lightest ones at the
top and heaviest ones at the bottom. As an example, in the left-most column, we
have the alkalis, with hydrogen, lithium, sodium, potassium, cesium, and so on.
They are all highly-reactive substances. And over here on the right we have the noble
gasses, with helium, neon, argon, krypton, and xenon. None of these elements react much at all.
For about fifty years, scientists were aware of the patterns, but didn’t understand them.
It took the theories of atomic and nuclear physics to clear things up. We now know that
each column has similar reactive properties because the elements in each column has a
similar configuration of outermost electrons. For instance, the noble gasses all have a
filled orbital of electrons, with no space to comfortably accept another electron. That’s
why they don’t react much. In contrast, the alkalis all have an extra electron, which makes
them willing participants in chemical reactions. You probably learned about all
of this in high school chemistry. Elements at the top of columns are lighter,
because the bottom ones contain more protons and neutrons. Thus, the reactivity of
columns is governed by atomic physics, while the increase in the mass as one goes from
the top row to the bottom is explained by nuclear physics. This was all clear by the mid-1920s.
Now for particle physics, the situation is similar to what it was during the first years
of the periodic table. We learned about the smallest known building blocks of matter in
episodes 2, 3, and 4 of subatomic stories. They are the quarks and leptons.
As a reminder, there are six quarks, with the names up, down, charm, strange, top
and bottom. There are three charged leptons, named the electron, the muon, and the tau. And
then finally, there are the three neutrinos, the electron-type, muon-type, and tau-type.
We ordinarily arrange them looking like this. The first row of quarks all have electrical
charge of 2/3 that of the proton, or plus 2/3. The second row of quarks have a charge of
1/3 that of an electron, or minus 1/3. The row of charged leptons all have a charge of
minus 1. And the neutrinos are all neutral. As you go from left to right in a row,
the various quarks or leptons get heavier, except for maybe the neutrinos,
which have a complicated mass story. The columns tell their own story. The left most
column are the building blocks of ordinary atoms. The second and third columns are carbon copies
of column one, but the particles are unstable. Particles in those columns are
not ordinarily found in nature. Physicists call the columns “generations.” And,
the simple fact is that we don’t know why there are multiple generations. We don’t know why the
quarks and leptons have the charges that they do. But we can make an analogy to
the chemical periodic table and say that the rows of the modern periodic
table are kind of “chemically similar,” and that subsequent columns are heavier.
But we don’t know why. On the other hand, we can maybe look to
the chemical periodic table for hints. For instance, in both cases – the increasing mass
and chemical reactivity – are caused by each atom having an internal structure. Perhaps the same
thing is true in the quarks and leptons and the rules that govern them are due to unknown rules
that govern the particles contained within them. Scientists have a proposed name for the building
blocks of quarks and leptons and that name is preons. Now before I say anything more, I should
tell you that there is exactly zero experimental evidence for preons. We’ve looked. >>I’ve<<
looked. If quarks and leptons contain preons, then the quarks and leptons will have a size.
And using data from the most powerful particle accelerator on the planet, we’ve tried
to measure their size and found nothing. Finding nothing doesn’t mean that quarks and
leptons have no size. It means that they are smaller than the smallest thing your instrument
can see. And the best instruments available to modern science can see things about one ten
thousandth the size of a proton. So, if quarks and leptons have a size, they’re smaller than that.
The preon idea is not super popular in the scientific community, but if you’re interested
in learning more about it, I wrote an article about it in Scientific American quite a few
years ago. I put a link in the description. But even if preons are crazy, there’s still the
question of why there are three generations. And, before you ask, it appears to be only three. Back in the early 1990s, scientist showed
that there were only three different types of neutrinos and, since each generation
seems to have a neutrino, that’s why we think there might be only three generations.
It’s all very mysterious. Nobody understands why there are three generations.
I certainly don’t. I wish I did, because this particular mystery bugs me a lot.
For me, it’s a spectacular outstanding question. Speaking of spectacular questions, let’s see
what questions our viewers have for us today. Man, there were soooo many good questions
this week. I couldn’t get to all of them. But I did pick a bunch, so let’s jump right
into them. Mark Sakowski asks about the cosmic microwave background radiation. He thought
it was due to photons from the plasma of the early universe and not gamma rays emitted by the
matter/antimatter annihilation. Hi Mark. Good news. You’re right. But so am I. So, naturally,
that means that the answer is complicated. The matter/antimatter annihilation emitted gamma
rays which then heated up the remaining matter. That matter absorbed the gamma ray photons
and re-emitted them at different wavelengths. The process repeated itself over and over and
the situation was compounded by the fact that the universe was expanding. The CMB photons are
from the plasma that existed about 400,000 years after the Big Bang, but their origin was from the
original gamma rays. So that’s the connection. MKSense J points out that positrons are emitted in
certain forms of radioactive decay and positrons are antimatter. Hi MKSense. Yep. That’s right. And
antimatter is made in particle accelerators too. It’s not that antimatter doesn’t exist. After
all, we’ve discovered it and we know a great deal about its properties. It’s that the primordial
antimatter has disappeared. That’s the mystery. Brent Jacobs asks what it is that dark energy
is repelling – matter or the fabric of space. Hi Jacob. Actually, there’s no huge difference.
Einstein’s theory of relativity says that matter and energy are tightly linked to space and time.
Change one and you change the other. However, I would say that dark energy is simply stretching
space, which moves matter. Dark energy isn’t like a force field that pushes matter apart in
the sense that would be natural to imagine it. The EyesofNye asks about experiments looking
for dark matter and future plans. Hi Eyes. There have been tons of experiments, mostly buried
deep underground to shield from radiation and also chilled to slow the motions of atoms in
the detector. The idea is that dark matter would pass through the detector and occasionally
crash into an atom in the detector. Researchers would look for the atoms to move. That’s how it
works. And there have been dozens of efforts. Nothing has worked so far, but there are some high
precision dark matter experiments on the horizon, like LUX-Zeppelin, to be conducted in South
Dakota, Super CDMS, to be conducted in Canada, XENON 1T in Italy, and PANDA-X II in China. Dark
matter can run, but it can’t hide. We’ll crack this nut one day in the not >>too<< far future.
Squirrel ASMR notes that I am charming, funny and smart. Mom…is that you?
Will Pittenger asks if inhabitants of an antimatter universe would call the stuff
they’re made from as “matter.” Hi Will. Yes, they would. Or, more likely, they’d call it
snorf-fluff or something, because…you know…they probably wouldn’t speak English. But which is
called matter, and which is called antimatter is simply a matter of convention. Except, of
course, we call the dominant form “Matter.” Betaneptune notes an incongruity between people
who say the universe is infinite in extent and that it was a singularity in the past.
Hi Beta. Yep. It’s true. And you’re in luck, as I’m going to talk about that in more detail
in the next video. But the short answer is that people are sloppy. It’s the >>visible<<
universe that was small or a singularity. That is true, even if the universe as a whole is
infinite. Wait until next week for more insights. Blackmark 52 asks if a question in the last
video was handled a bit too cavalierly, with an amusing reference to Sith Lord
emitting lightning from his fingertips. Hi Blackmark. Yes and no. If dark energy exists,
it is a potential energy source, but there’s a caveat. Uniform energy isn’t so useful. What
>>is<< useful is a difference in energy. Water behind a deep damn doesn’t generate electricity.
That’s because the water is the same everywhere. What generates electricity is when energy moves
from a high energy location to a low energy one, like when water behind the damn flows onto the
other side. If dark energy is the same everywhere in the universe, then there is no impetus for
the energy to flow from one location to another. So, there is no real prospect for dark energy
to be an exploitable power source for humanity. Michael Blacktree asks about whether my statement
that Einstein was responsible for the theory of the Big Bang was right, or if the credit
should belong to George Lemaitre. Hi Michael, well, the answer is complex. History
is rarely as simple as it is taught. Here’s the story. Lemaitre did propose the
theory of an expanding universe in 1927, two years before Hubble published his paper. Lemaitre also imagined the expansion might have
arisen from a much smaller universe that he called a “primeval atom.” So that’s true. But Lemaitre
published his paper in 1927 and his calculations relied on Einstein’s theory of general relativity,
which was published in 1915. And, of course, Alexander Friedmann published ideas similar to
Lemaitre’s in 1922, five years before Lemaitre. It’s true that Lemaitre was the first person
to mentally run Einstein’s equations back to a singularity. And he hadn’t seen Friedmann’s
work before he published his paper – after all, Friedmann was stuck inside the Soviet Union. But
Einstein knew of Friedmann’s calculations long before the Lemaitre paper. And none of it would be
possible without the theory of general relativity. So, you see, it’s complicated, but I
think Einstein gets the ultimate nod. Zack says that he’s a high school student
who hopes to work at Fermilab and wants to know how to do that. Hi Zack. That’s a great
question and I think it’s worth answering properly. Let me tell you what. I’ll think
about whether a video or a written essay is the better way to do it, and I’ll let you know
in the question section of an upcoming video. Hah…now you have to watch them all.
Daniel Jacobovitz says that he really appreciates the new video format. Hi Daniel. Thank
you. People seem to be split between the Subatomic Story format and the long form videos. The long
form videos reached a larger audience and I hope to transition back to them soon. However, I’d like
to hear from the viewership about what they like about either format and maybe the new version
will be a blend of what people like about both. Let me know in the comments.
And finally, Edward Siegel notes that it is often said that antimatter is matter
going backward in time, and maybe that means that antimatter went back in time and got
pushed back to before the universe began. Hi Edward. Yes, I’ve heard that too. And I think
it’s terribly misleading. It’s a case of someone loving their math just a little bit too much.
Let me explain, but I should warn you, there’s some math involved. Here is an equation
representing an antimatter electron moving through time and space. And here is the equation for
a matter electron moving backward through time and space. And you see that the equations are
the same. So, from a theoretical point of view, positrons moving through time are equivalent
to electrons moving backward through time. But that’s not necessarily physical. After all,
when we make an electron and antimatter electron, they are made at the same time and move
forward through time. To use the other mindset, an electron in the future came
backward in time to disappear at a time when a regular electron was created. It’s a bit
of a stretch in my opinion to say that this is a physical phenomenon. Some will disagree, but
I’ll leave that conversation to the philosophers. They have time on their hands.
OK, that’s all the time we have for questions today. There were a lot. That’s not
surprising, because our viewers are a curious lot. Each week, after they like, subscribe, and share,
they sit down and think about the physics we talk about here. And, how could they not? After
all, even at home, physics is everything.
