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.
If you are liking the series, please like, subscribe, and share with all of your friends.
We want to get our numbers up. Higher numbers mean more people can enjoy the deep mysteries
of physics and that would be a good thing because, even at home, physics is everything.