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.