Hi everyone. Welcome again to my den.
If I were to ask you to name a dangerous substance,
one that could easily kill you if you mishandled
it, you might think of nitroglycerin. Nitroglycerin
is a liquid that is unstable and explosive
and not to be trifled with, as we see in this
video clip by Adam Smith, of MythBusters fame.
But while nitroglycerin is undoubtedly dangerous,
physicists have discovered a substance that
could level an entire city using only a single
gram. Do I have your attention? Let me tell
you more about it in this episode of Subatomic
Stories.
It was in 1928 that British physicist Paul
Dirac first tried to unify the laws of quantum
mechanics and Einstein’s theory of special
relativity. He was successful and the result
is a mathematical expression called the Dirac
equation.
Solving the Dirac equation even for a single
electron is challenging, but when the dust
settles, the solution looks something like
this, with both a positive and negative answer.
The positive answer described the electron,
but what about the negative one? Many people
thought it should be ignored, but Dirac was
confident that it meant something. He was
right. Dirac had predicted the existence of
antimatter.
It was only a few years later that a young
professor at Caltech by the name of Carl Anderson
discovered the antimatter electron. The antimatter
electron is called the positron. It has identical
properties to the electron – mass, spin,
size, you name it – except for electrical
charge. The electron has a charge of negative
one and the positron has a charge of positive
one. It took some time, but the antiproton
was discovered in 1955. It’s just like the
proton, but with the opposite charge.
The antineutron was discovered a year later.
It is also electrically neutral, just like
the neutron. Some people wonder if they have
the same charge, how they can be opposites
of one another? We now know it’s because
they have a different quark makeup. The neutron
is made of quarks and the antineutron is made
of antiquarks.
Antimatter versions of every particle have
been discovered. There are antiquarks, antileptons,
antineutrinos. More bizarrely, there are things
like antiphotons, but in that case, the photon
and antiphoton are the same particle, just
like in numbers plus zero and minus zero are
the same.
Let me tell you three interesting antimatter
facts.
First is that antimatter is uncommon in the
universe. Uncommon, as in basically non-existent
except in very special circumstances. But
we can make it in particle accelerators using
Einstein’s equation E equals m c squared.
That equation says that energy and matter
are equal, but the truth is a bit more subtle.
What really happens is that energy can be
converted into equal amounts of matter and
antimatter. This happens.
In physics, processes usually work both ways.
If you can convert energy into matter and
antimatter, then you can convert matter and
antimatter into energy. And then Einstein’s
equation teaches us something interesting.
That c squared thing is a really big number,
basically a one with seventeen zeros. That
means a little antimatter is a lot of energy.
In fact, if you combined a gram of antimatter
with a gram of matter, the energy release
is the same as the nuclear explosion at Hiroshima.
So, important safety tip. Don’t do this
at home. Antimatter is dangerous.
Luckily, antimatter is very hard to make.
The most powerful antimatter manufacturing
facility on the planet took a quarter century
to make enough antimatter to brew fifty medium
sized coffees. So, you don’t need to worry.
The second fact is that there is nothing all
that dangerous about antimatter if it’s
not near matter. You could take antiprotons,
antineutrons, and positrons and make antimatter
atoms, molecules, everything. There could
be an antimatter me, and nobody would be the
wiser. It’s only when matter and antimatter
are mixed that things get dicey.
The third fact is that the absence of antimatter
in the universe is a surprise. During the
Big Bang, there was a lot of energy, which
should have converted into equal amounts of
matter and antimatter. But we see no antimatter
anywhere in the universe. We don’t really
know why. I mean, we have some ideas, but
the truth is that nobody knows the answer.
But we’re working on it. It’s an unsatisfying
state of affairs to be sure, but that’s
life on the frontier of human knowledge. Welcome
to my world.
I’ve made a couple of longer videos that
talk about all of these things in more detail,
including one where I meet an antimatter version
of myself. It’s an explosive experience.So,
that’s a short introduction into antimatter.
Now, what do you say? How about we get to
viewer’s questions?
Hi guys, it’s that time of the episode where
I answer viewer’s questions. There are some
pretty good ones this time.
Jason_K asks if electromagnetism and the weak
force both originate from the electroweak
force, why do they have different force carrying
particles?Hi Jason. Hoo boy…you don’t
pick easy questions, do you? The answer to
that question is extremely complicated, but
I’ll try to give you the high points.
Suppose you’re outside and it’s a hot
and muggy day. You look around you and see
you’re surrounded by air. Nothing weird
about that. Then the temperature drops and
what happens is that it rains.
Thus, when the temperature is high, what you
call air is a mixture of oxygen and nitrogen
and stuff, but also with water vapor. The
two things look completely alike. But when
you change the temperature, you get a phase
change for the water, but not for oxygen and
nitrogen. Above some temperature the air and
water look the same. Below, they don’t.
So that’s the first analogy.
Now for the electroweak force, there is a
temperature above which the electromagnetic
and weak force seem to be the same. Under
these circumstances, mass doesn’t exist.
Even weirder, electrical charge doesn’t
exist. Instead the electroweak force has two
sets of massless force carrying particles.
A single particle called a singlet and which
we’ll call B and a trio of particles called
a triplet and which can be called W1, W2,
and W3. These are the particles of the electroweak
force.
In this high temperature environment, the
Higgs field is zero. Now, we lower the temperature.
Initially, the change in temperature doesn’t
matter. However, there is a specific temperature
at which the conditions change, like when
air can no longer hold water. At that temperature
a few things happen. First, the Higgs field
goes from being zero to being non-zero. Now
that the Higgs field isn’t zero, the B and
W particles can interact with it and some
of those particles get mass and some don’t.
The particles become the familiar massive
W and Z bosons and the massless photon.
I’ve skipped over some details, including
some complicated quantum stuff, but that’s
the gist. The massless electroweak particles
above the magic temperature become a mixture
of massless and massive force carrying particles
below it.
If you want to learn even more about this,
there is a very accessible explanation by
Flip Tanedo, a physics professor at the University
of California, Riverside. He was a blogger
for the website “Quantum Diaries,” and
he is a gifted explainer of complicated physics
topics. I’ve put several links in the video
description if you want to read his explanations
and I really recommend them. They’re very,
very, good.
Matt H asks how massless particles can have
no energy, especially if mass and energy are
the same.
Hi Matt…I’m glad to see you’re trying
to reconcile the words you hear because they
are indeed confusing. Not everybody digs so
deep.
The reason for this is simple. The rules of
relativity are much trickier than you generally
encounter in explanations for the public.
Take Einstein’s venerable equation E equals
m c squared. While it’s certainly right,
it’s simply a special case of the full equation.
The full equation is the more complicated
E squared equals m c squared, all squared,
plus pc, all squared. E is energy, m is mass,
c is the speed of light and p is momentum.
For a massless particle, we can set m equals
to zero and we get E equals pc. This is the
proper equation for a massless photon. We
can also see what happens if we set the motion
of the particle to zero, this means setting
the momentum or p to zero and we get the more
familiar E equals m c squared equation. Basically,
E equals m c squared only applies if an object
is motionless.
I’m sure someone is going to mention in
the comments the idea of relativistic mass,
which is the idea that mass changes as velocity
changes. That’s simply not true. Now before
you have a heart attack over that or think
I’m some sort of fringe relativity hater,
no way. Einstein is totally right. It’s
just that science popularizations cut corners
when explaining his ideas.
Saying that mass changes with velocity is
a bit dodgy, but it helps students intuitively
understand the consequences of relativity.
I’ve even taught it myself in classes. Sorry
about that if you were one of my students.
If you want to understand what is really going
on and why the claim that mass increases as
velocity increases is both right in one sense
and yet wrong if you dig deeper, I made a
long form video on that subject. The URL of
the video is in the description.
In any event, the answer to Matt’s question
is that the equation E equals m c squared
is a special case of the much more complicated
equation and this simplification is true only
for stationary massive particles. The simplification
for massless particles tells a very different
story.
Nick Lag asks if electromagnetism and the
weak force can combine to the electroweak
force, can the electroweak force combine with
the strong force to be some, single, super
force.
Hi Nick. The short answer is that nobody knows,
but most particle physicists think it will
happen. There are two reasons, one of which
is a bit dodgy, and the other is more persuasive.
The dodgy one is just because unification
of forces is what we’ve seen in the past.
Besides the electroweak thing, there was the
unification of electricity and magnetism into
electromagnetism back in the 1870s and Newton’s
unification of celestial and terrestrial gravity
in the 1670s. Unification seems to be the
way of things. That’s the dodgy motivation.
The more substantive reason is that we know
that the strength of the three subatomic forces
changes with energy. This is established fact.
And, if we take the trends we measure in current
experiments, we can project them out to higher
energies – energies we can’t currently
test directly. If we do that, we see that
the strength of the three subatomic forces
all become the same at a specific energy.
Now that energy is very high – a quadrillion
times higher than what we can currently study
and a lot of new physics probably awaits us
between the two energy scales. So the real
situation is probably more complicated.
But if we just go with what we’ve got, we
see that the strengths of the forces eventually
become the same and that is a serious hint
that the known subatomic forces eventually
become the same. And here’s a kicker…we
think that at even higher energies, gravity
joins the party and there is just one force.
Okay, so that’s all the time we have for
questions. There were some tough ones today.
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