Hi there, physics fans!
In the last few episodes, we’ve talked about
the particles and forces that make up the
subatomic realm.
But just what is like to experience the world
of the super tiny?
A few movies have represented it, like the
2018 Marvel movie Antman and the Wasp, but
just what is it like?
Sounds like a great topic for this episode
of Subatomic Stories.
We’ve all heard just how weird the quantum
world is.
We hear about such things as electrons simultaneously
being waves and particles, and objects not
existing until someone looks at them, and
that’s all true to varying degrees.
But I don’t want to talk about these familiar
features of the microcosm.
Those features are all soooo 1930s and arise
from when quantum mechanics was first invented.
Quantum mechanics explores the behavior of
individual matter particles.
Instead, I want to dive down to the very smallest
scales and explore new features that were
revealed in what is called the second quantum
revolution, which occurred in the late 1940s.
That’s the era when Richard Feynman and
others devised the mathematics that illustrates
that not only was matter quantized, but so
were force fields.
Instead of classical electromagnetism, which
incorporates electric and magnetic >>fields<<,
quantum electrodynamics, or QED, described
quantum forces as matter particles exchanging
individual force carrying particles.
I explained the basic idea of this in episode
five.
QED was the first theory that quantized forces,
but the idea could be generalized and it was
applied to the strong force, which now is
best described by a theory called quantum
chromodynamics or QCD and the weak force,
which doesn’t have a particular name.
I made long form videos about these and the
URLs are in the description.
The term that describes these class of theories
that quantize subatomic forces is quantum
field theories, or QFTs.
So, what consequences do quantum field theories
have?
Are they testable?
How do we know that this crazy sounding idea
is real?
There have been several testable predictions
of QFTs.
Let me tell you about two of them.
The first one involves predictions and measurements
of the magnetic properties of charged subatomic
particles with spin one half, like the charged
leptons and the quarks.
Precisely measuring the magnetic properties
of quarks is hard, as they are buried inside
protons and neutrons, but particles like electrons
and muons are a different thing entirely.
1930s quantum mechanics makes a specific prediction
of what is called the magnetic moment of a
charged, spin one half, particle.
You can sort of think of the magnetic moment
as the magnetic charge of the particle.
When the magnetic moment of the electron was
measured, it was discovered to be approximately
0.1 percent different from the prediction
of classical quantum mechanics.
Beginning in 1947 and for several years after
that, theoretical physicists applied the mathematics
of QED to the problem to get an accurate understanding
of what is going on near an electron or a
muon.
The current thinking is that an electron is
constantly emitting what are called virtual
photons.
Virtual photons are ones that do not need
to conserve energy and momentum.
This is basically the Heisenberg Uncertainty
Principle of classical quantum mechanics.
These photons are then reabsorbed by the electron.
Occasionally, the virtual photons can temporarily
convert into virtual electron/positron pairs
that then recoalesce into the virtual photons.
Looking even more closely, those virtual electrons
and positrons can emit their own virtual photons
and so on.
The result is a chaotic and complicated mess
of virtual photons, electrons, positrons,
and even other subatomic particles all flickering
in and out of existence in the vicinity of
the actual bare electron.
The bare electron is enshrouded in a cloud
of virtual particles flickering in and out
of existence.
Now this idea is kind of hard to believe,
but researchers have measured the magnetic
properties of electrons and muons to twelve
digits of accuracy and the theory can only
make correct predictions if this virtual cloud
is real.
So that’s strong evidence that the theory
is true.
Perhaps even more interesting is a measurement
of the magnetic properties of the muon, which
is also precise to twelve digits of accuracy,
but for which the theory and measurement disagree.
That could mean that a new theory must be
devised to account for the discrepancy.
If the discrepancy is real, it could require
a significant modification of the Standard
Model of particle physics.
A new measurement of the magnetic properties
of the muon is underway at Fermilab, called
the g-2 experiment.
It’s hard to make predictions, especially
about the future, but they could announce
the results of their measurement within a
year.
It’s one of the most anticipated measurements
in modern physics.
I made a long form video about g-2 and the
URL is in the description.
In quantum mechanics, electrons and photons
are both particles and waves, but my description
of the magnetic properties of electrons and
muons used particle language.
Have any experiments looked for the wave behavior
of virtual particles and is the idea of a
cloud of virtual particles broader than describing
the subatomic world close to charged particles?
Yes.
The cloud of virtual particles exists everywhere,
even in empty space, even inside you.
At any spot in space, virtual quarks and leptons
and photons and all manners of subatomic particles
are appearing and disappearing.
The microcosm is a chaotic mess.
There is a measurement that validates both
the cloud of virtual particles in space >>AND<<
their wave nature and it is called the Casimir
effect.
This experiment is conceptually simple.
Take two metal plates and put them parallel
to one another and separated by a very small
gap.
The virtual cloud exists both between the
plates and outside.
Now those virtual particles have all sorts
of different wavelengths, big wavelengths,
little wavelengths, medium wavelengths, the
whole gamut.
The key feature of the Casimir effect is that
only the short wavelengths fit between the
plates, while outside of them, all of the
waves fit.
That means that there are more virtual particles
outside than inside and the net effect is
those outside particles overpower the virtual
particles in the gap and the plates get pushed
inward.
And that’s >>EXACTLY<< what happens when
you do the measurement.
And, of course, I have a longer form video
that gives more details and the URL is in
the description.The idea that empty space
isn’t empty, but is rather a chaotic and
everchanging mess of virtual quarks, leptons,
photons, and all the rest briefly winking
in and out of existence is a real mind blower,
but it’s what the data says.
It gives you a very different mental picture
of the nature of space at the quantum level.
You’re going to be scratching your head
over this one for days.
OK, so that’s all the time we have for today’s
episode.
Let’s see what kinds of questions we have
today.
Nine episodes and nine sets of questions.
Let’s see what you have for me today…
Many viewers noticed a problem in the graphic
depiction of a water molecule in the last
video.
Of course, H2O should have two hydrogens and
one oxygen and not the other way around.
Sorry about that.
I wanted to make it up to you by telling you
a chemistry joke, but every time I tell a
chemistry joke, there is no reaction.
Tony Stark Iron Man asks if dark matter and
dark energy interact with the Higgs field.
Hi Tony…the most honest answer is we don’t
know.
Dark matter is a hypothesis that is very,
very, likely, but still technically unproven.
It has mass, which suggests an interaction
with the Higgs field.
So that’s a definite maybe.
But the Higgs field is tied up with the electroweak
force and we know pretty definitively that
dark matter doesn’t interact via the weak
force.
That’s surprising, but it seems to be true.
Given that dark matter seems to not interact
via the strong, weak, or electromagnetic forces,
it means we don’t have a handle on how to
make it and study its properties.With all
of that said, I think that it’s likely that
dark matter interacts with the Higgs field
and I’m pretty sure that dark energy doesn’t.
I’d bet money on both of those statements,
but not a lot.
The bottom line is until we understand both
of them better, we can’t answer your question.
Muhammed Hussein asks why electromagnetism
doesn’t bend spacetime like gravity does.
Hi Muhammed.
Actually, it does.
All energy bends spacetime to a degree.
It’s just that mass is a super-concentrated
form of energy, so it bends spacetime a lot.
But all energy has at least some effect, even
the heat in your morning cup of tea.
The Kwiatek asks if the “Higgs Charge”
must be quantized.
The answer is yes.
All electrons have the same mass.
Ditto up quarks, down quarks, etc.
However, nobody knows why the various particles
have the masses that they do.
They simply can’t be predicted.
At the moment, we just measure them and put
them into the theory.
Hopefully a deeper theory will answer that
question.
But, for now, the rough reason is basically
“just because.”
Anders Kallberg asks if the Higgs boson is
just a vibration of the Higgs field, why don’t
we see other Higgs bosons, from the vibrations
at other frequencies?
Hi Anders, the short answer is we don’t
know.
Actually, there are theories that predict
additional Higgs bosons for just that reason.
We’ve even looked for them.
In fact, back in 2015, the LHC experiments
thought that they might be seeing another
Higgs boson, but it turned out to be a statistical
fluctuation in the data.
Very sad, but, you know, that’s research
for you.
The bottom line is additional Higgs bosons
are possible, but for now, we’ve found only
one.
Laserkid7 asks if the mass of W bosons are
so large, how does this explain beta decay,
which is when a neutron decays into a proton
and that decay involves such a tiny energy.
Hi Laserkid.
Cool name.
As it happens, I made an entire long video
to answer just this question and the URL is
in the description.
The short answer is that short-lived subatomic
particles like the W boson don’t have a
unique mass.
While the average mass of the W boson is 80.4
billion electron volts, that’s just the
average.
The usual range quoted is 2 billion electron
volts, which means a W boson with a mass of
78 to 82 billion electron volts or thereabouts
is entirely common.
Masses farther from 80.4 are increasingly
rare, but even down with masses as little
as 0.002 billion electron volts that are required
for beta decay, you can occasionally find
a W boson.
It’s crazy rare, but it happens.
Furthermore, that rarity explains why the
weak force is so weak.
OK, so that’s all the time we have for questions
today.
I hope you’re enjoying these series of videos.
If you do, please like, subscribe, and share.
This video covered some really nonintuitive
physics – physics that is everywhere and
that you don’t usually see.
But knowing that physics is everywhere shouldn’t
surprise you because, as you are well aware,
even at home, physics is everything.
