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.