The Concept of Mass – with Jim Baggott

The Concept of Mass – with Jim Baggott

September 17, 2019 76 By Stanley Isaacs


[MUSIC PLAYING] Now this is called mass. But I’m not taking confession. Well, if you really
insist, maybe later on, we can do something along
those lines over a pint of beer in the pub afterwards. I know it doesn’t
always seem like it. But trust me. An author really
does need a reason to sit down and write a book. So my reason for wanting to
write this particular book was to try to convey something,
some sense of what has been an understanding in
contemporary physics for already quite
a number of years, but which I felt
actually wasn’t really that wonderfully well understood
or commonly understood. And that is the way
that modern physics conceives the nature of
matter and, in particular, the property of mass. So what I want to try
and do is to give you a sense of the journey that
I went on myself researching and then writing this book to
give you some sense perhaps also of the sense
of astonishment or wonder at where
contemporary science feels that we’ve landed up. And I’m going to begin by
setting myself a hopefully not too difficult mission. So this is my mission, Jim,
should I choose to accept it. Here’s a cube of ice. And I’m going to ask
myself two really, hopefully, quite simple
questions about this stuff. I want to know
what is it made of, and I want to try to
answer the question where would I look
to find its mass. So we know ice. Don’t we? It’s what you put in
your gin and tonic or, increasingly, the
cosmopolitans among you, in your glass of Sauvignon
blanc, as the Italians do. We know it’s made of water. And we’re going to start the
story with the ancient Greeks because much of our common
understanding of the nature of material substance
actually comes from a handful of Greek
philosophers dating back about 450 years before year
0, before the Common Era– names like Leucippus,
if he really existed, Democritus, then later, 100
years later or so, Epicurus. And much of Epicurus’
work was actually translated into a grand poem by
the Roman poet and philosopher, Lucretius. And a great place to start
because Epicurus once said nothing comes into being
out of what is non-existent. That’s philosophers for you. I can guarantee you the
cryptic crossword puzzles were ace in ancient Greek times. We’re talking, of course, about
the famous Greek elements– earth, air, fire, and water. That’s good because we
know ice is made of water. And we’re interested in
exploring a little bit more about what ice– what
that water is made of. And we get the sense
that nothing comes out of stuff that doesn’t exist. A great star of what
Epicurus is really saying is it’s a common observation
that nothing magically appears out of nothing. Stuff just doesn’t appear. And there’s a corollary to that. If it’s a common experience that
stuff doesn’t magically appear, it is also a common experience
that stuff doesn’t magically disappear. Push that to its
logical conclusion, and you end up in
a situation where you have to accept that
nature resolves everything into its constituent atoms. And nothing, no substance
can be resolved completely into nothing. It’s a simple
logical consequence. If nothing can
come from nothing, and nothing can be
resolved into nothing, then by definition, when
I have something, it must resolve into some
indestructible indivisible bits of stuff, what the
Greeks called atoms. Even better, we know that
the ancient Greeks speculated that if you accept that
substance, like water, consists of atoms, then
by definition atoms must be moving in
something, which the Greeks called the void. Empty space is how
we think of it today. And there had to be a
reason for that motion. And so the Greeks had
no real difficulty in ascribing properties
to these atoms. As you can see from
the little diagram, they gave them different shapes. Some were spiky with hooks
that would cling to each other, giving them certain
characteristic properties that we can actually see
manifested in our experience of different substances. But if they were
perpetually in motion, then the argument went they must
have something called weight. They fall through the void
much like heavy rain drops will fall from the heavens on an
otherwise warm June, Sunday afternoon. OK, even further,
seawater, being fluid, OK, must consist of round atoms. So here we’re getting
quite a few answers to our opening questions. What is it made of? Well, ice is made of water. Water is made of hard,
round atoms that are indestructible and indivisible. And those atoms
in their turn must have the property of weight. OK, good start. Everyone happy? Everyone happy? Yes. Good. All right, then we have
to wait quite a long time. I don’t want to give you
the impression that there was nothing going on in
the 1,300 or 1,400 years between the ancient Greek
philosophers and the times, perhaps a little bit
before Isaac Newton, the times of Galileo,
Bacon, Robert Boyle, and others contemporary
with Newton. But I just thought I’d pull
this quote from an online encyclopaedia of philosophy
called the Stanford Online Encyclopaedia of Philosophy. And this, when I read it,
really struck a chord. Here is a recipe for producing
mediaeval philosophy, the stuff that was going on in the
1,300 years between the Greeks and Newton. “Combine classical pagan
philosophy, mainly Greek, but also in its
Roman versions, with the new Christian religion. Season with a variety of
flavouring from the Jewish and Islamic
intellectual heritages. Stir, and simmer for 1,300
years or more until done.” So there was a lot going on, but
most of the intellectuals, most of the minds of
thinkers in this period were devoted to
trying to reconcile the pagan philosophical
texts of the Greeks with the demands
effectively of the Catholic Church and other orthodoxies. And eventually, things
started to free up. The first universities,
of course, were created out of monasteries
to all intents and purposes. So that kind of sense
of monastic scholarship translated itself into
academic scholarship. And it slowly, over a
long period of time, began possible to
start speculating along lines that were not
any longer theological. You could start to
speculate about the nature of the natural world
that didn’t necessarily have always to reference
back to some kind of religious orthodoxy. So to be fair though,
Newton, we tend to regard Newton
as one of the first among a generation
of scientists. But in truth, Newton was
a mechanical philosopher. His famous book,
published in 1687, it’s English title is the
Mathematical Principles of Natural Philosophy. So these folks
understood that they were doing natural philosophy,
but of a particular mechanical kind. It was the mechanical
investigation of nature. Newton, of course,
had a lot to say about things like
motion and gravitation, which we’ll talk about
in a little while. But these philosophers, these
mechanical philosophers, also held an understanding
that substance was ultimately composed of
indivisible, hard atoms. But their concept
of mechanical atoms was not really that
much more sophisticated than the conceptions
that the ancient Greeks had put forward hundreds or
thousands of years before. All right, Newton,
though, went further. Newton speculated
that not only were these little hard billiard
balls of substance moving in the void, they
might also actually have forces acting between them. That was something the
Greeks never latched onto. As far as they were
concerned, all of the motion was due to the
weight of the atoms. The idea that there might
exist different kinds of forces between atoms was new,
but very speculative. Newton had no
experimental grounds for making that
kind of statement. The other thing that we
would look to Newton for is really a good
understanding of things that are manifest in our
visual world of experience to do with the motion of
objects, things with mass, things with acceleration,
as a result of the acting of a force of some kind. And so here, at least if we
can’t get further insight into the nature of
atoms themselves, we can at least
get some insights into the nature of
this thing that we’re calling mass or weight,
which I’m not differentiating between in this talk. And indeed, there
is a definition of mass in Newton’s
Mathematical Principles of Natural Philosophy. And it reads
something like this. “The quantity of matter or
mass is a measure of the same, arising from its
density and bulk,” which we can interpret
as volume, “conjointly.” Do you see anything
wrong with that? It was Ernst Mach, coming a
couple of hundred years later, who actually pointed out,
“The formulation of Newton is unfortunate. As we can only define density
as the mass of a unit of volume, the circle is manifest.” And that, by the
way, in the corner there is a vicious circle. So Newton, who we would expect
to be the champion of clarity, his second law of
motion is force equals mass times acceleration. These are concepts
that are embedded deep in our common
understanding of what’s known now as classical physics. And I would say
that’s a physics that is just consistent with
our everyday observations. Watch a game of tennis. Watch Andy Murray lose in the
semifinal of the French Open to Stan Wawrinka, and you
get a sense for the way that the motion of the tennis
ball is affected by a force. Watch a game of
snooker on the TV. Get in your car and accelerate
at high speed along the M4, well, until you get to the
first set of traffic cones. And you get a sense for what
Newton’s classical physics is trying to tell you. But start to pick
at it, and you’ll find that some of the
fundamental concepts that we are so very familiar
with start to unravel a little bit because,
in truth, something as important and
fundamental as mass was never really defined
properly in the first place. Mach had a go at defining
mass, but only relative to other masses. There was no real
attempt to come up with a firm understanding,
a derivation almost, as to what mass is. All right, so we’ve
got some problems. We’re not any further along
with our understanding of the nature of atoms. And we’ve got this
bit of a wobble when it comes to
understanding what mass is. But let’s keep going. Those two questions where
I started at the beginning seem so fairly straightforward. We should surely be
able to get some light at the end of the tunnel if we
keep our heads down and keep going. OK, move on. Well, when you don’t get
clarity from the physicist, you can always rely on chemists. And some 100 years after
Newton, again, John Dalton wasn’t the only one here. I’m picking out and singling
out these heroes just really to encapsulate
what was essentially a movement, the responsibility
of many individuals involved in its development. But John Dalton
famously said that he’d come to some enlightened
understanding of the nature of chemical
substances by looking at their weights, an
understanding, in fact, that he could understand
chemical substances in terms of the nature of the
atoms that they contained. So this is the beginnings of
a burgeoning understanding of chemistry and, in
fact, if I’m honest, really the foundations,
along with the development of the science of
thermodynamics, the development of the
beginnings, the seeds, of the Industrial Revolution. Dalton was pretty
curmudgeonly when it came to an understanding
of the composition of water. As far as he was concerned,
it was one atom from hydrogen and one atom of oxygen.
Antoine Lavoisier wasn’t sure. But Antoine Lavoisier didn’t
survive the French Revolution. I’m afraid he was
guillotined for his efforts, not for his
scientific efforts, I have to say, but for his
efforts as a tax collector. And it took a little while
after some confusion. Maybe the clarifying voice
was an Italian chemist called Stanislao Cannizzaro
who in this quote makes it quite clear what he
thinks the nature of chemical substance is all about. “The different quantities of
the same element contained in different molecules
are all whole multiples,” and that was the singular
thing that the chemists were observing, “all whole
multiples of one and the same, which, always being
entire, has the right to be called an atom.” I love that quote. And of course, coming
out of the work that was being done on understanding
the nature of the relationships between the constituents,
the atomic constituents, of different
molecular substances, we came to the
firm understanding that water is a molecule of H2O. OK, I won’t tell you the
amount of confusion created around even that simple
understanding because, of course, if you take
hydrogen as a gas, your instinct is to think
that it’s a monatomic gas. It’s one atom of hydrogen. If you take oxygen as a
gas, your initial instinct is to think of oxygen
as a monatomic gas, O. But when H and O were
combined to produce water, things didn’t work out. And it was only the realisation
that hydrogen is actually a molecular gas. It’s H2. And oxygen is a
molecular gas, O2. Combine those, and you
can then begin to work out how water can be H2O. All right, so we’ve
come quite some way. I actually think we’re
up for a mission update. OK, so we started
off this evening with two very simple questions
around this cube of ice. And as a result of endeavours
beginning with the ancient Greeks 2,500 years ago,
we’ve understood that ice, being water, is made of
round atoms with weight. We’ve got a lot more
sophisticated thanks to the efforts of the
mechanical philosophers in the 17th and 18th
centuries and in the chemists in the 18th and 19th centuries. And we now understand
that we can drill into ice as a substance. And what we’ll find is a
lattice of molecules of H2O. Here the red ball
represents an atom of oxygen. And the two little
white balls represent atoms of hydrogen. So ice is a regular
lattice of water molecules, which we write as H2O. Where would we then
look to find its mass? Well, we can find its
mass or its weight– I’m not differentiating– in the
mass or weight of it’s hydrogen and oxygen atoms. But we have to remember
the caveat, whatever that is, because we haven’t got
a good definition of mass yet. OK, everybody happy? We all knew this. Right? I’m sorry. I feel I might have wasted 20
minutes of your lives going through stuff you already know. But I think it’s important
you understand the nature of the journey that we’re on. OK, so that’s good. Let’s move forward
a little further. Ah now, see. Pesky physicists are
now back in the picture. You can’t trust them. Just at the time at the
beginning of the 20th century when we were starting
to get hold of evidence that atoms really existed– they weren’t just figments
of a fertile imagination– just at the time when
we were getting evidence that atoms really
existed, physicists were working out how
to split them apart. I don’t know, honestly,
not to be trusted. And again, this is a model
that should be very familiar, the kind of planetary
model of the atom. Rutherford famously
did some experiments bombarding thin gold foil
with something called alpha particles, effectively
the nuclei of helium atoms, and was astonished. Expecting that this is like
shooting 15-inch shells at a piece of tissue
paper, how astonishing was it then to find some
shells bouncing back at him? And what that meant,
simply, was that all of the mass of a
gold atom or any atom is actually concentrated,
firmly concentrated, in a small central nucleus. And in fact, we now
understand that oxygen atoms consist of nuclei surrounded
by orbiting electrons. And those nuclei contain a total
of 16 particles, eight protons, positively charged,
eight neutrons, neutral. Hydrogen is the periodic
table’s lightest element. It consists of just a single
proton in its nucleus. Oh, fantastic, so time for
another mission update. OK, so we’ve gone a bit further. OK, the physicists have meddled,
but we’ve gone a bit further. We now understand that, in
fact, our molecule of water can be imagined as hard central
nuclei, oxygen, two hydrogen atoms, in a structure
around which are wrapped orbiting electrons. And it’s the nature of the way
that the electrons wrap around these three atoms that create
the molecular properties of something like water. Fantastic, the
even better news is 99% of the mass of an atom is
to be found in its nucleus. We still don’t
know what mass is. But we know where
to look for it. That’s good news. OK, we can worry about
what it is later. OK, so our attention
now turns to the nature of the protons and neutrons in
the nuclei of atoms themselves. That’s where we look to find. We now know what
water consists of. We know its atomic structure. We know its nuclear structure. We’re going an awful long way
to answering the first question. What is water made of? What is the ice cube made of? And we think, at
least, we’re getting some clarity in
where we think we need to look to find its mass. OK, keep going. Ah, now I can’t tell you
what kind of mess this makes. Again, just when you thought
things were starting to become clear, we hit this period of
scientific endeavour where we get nothing other than
madness and confusion. So we can credit Prince
Louis, 5th duc de Broglie, with the insight that the
discovery made by Einstein in 1905– what did
Einstein discover in 1905? He discovered that light waves
can be particles, what we now know as photons. Louis de Broglie
speculated that, as a result of some
further observations in experimental science over
the subsequent nearly 20 years, then maybe it’s
also a possibility that electrons can be waves. Now we’d always
cherish the notion, right from the beginnings
of the speculations of the ancient Greeks, that we
would be able to take material substance, accept
that, ultimately, we must hit a final kind
of indivisible stuff out of which everything is made. And now we’ve got
this French prince telling us, well,
actually, you know, what you thought were little,
hard billiard balls of material substance that happened
to be negatively charged electrons can also be waves. Why is that a problem? Well, let’s have a quick look. I just want to spend
a few minutes talking about what I call the essential
mystery of quantum mechanics. There’s a famous experiment. It may already be
familiar to you. It’s called the
two-slit experiment. And it’s easy to understand what
we see in the context of a wave theory of light. We take a light source. We take two narrow
slits or holes. And we shine the
light through these. Now there’s only one caveat. The distance, the
spacing, between the slits has to be of a
certain magnitude. And the slits
themselves have to be of the order of the
wavelength of light. And the chances are
you’re going to see what you need to see
only if that light itself is monochromatic. In other words, it has
a single wavelength. It’s not contaminated
with different colours. Do that, and what you see
projected on a far screen is what’s known as the
two-slit interference pattern. It’s very easy to understand. As the light waves
squeeze through the slits, they diffract. They spread out beyond. And in the space beyond where
a wave crest runs into a wave crest coming from
the other slit, you get what’s known as
constructive interference. The waves add up to
give a stronger wave. Where a trough meets a trough,
you get a deeper trough– constructive interference. But where a wave crest
meets a wave trough, you get a cancellation– destructive interference. And the result is a
pattern of alternating light and dark fringes. These were first discovered by
Thomas Young in about 1804– easy to understand with
a wave theory of light. But de Broglie is now telling
us electrons can be waves. So how about if we do that
experiment with electrons? And how about we
do that experiment in such an arrangement
so that only one electron goes through
these two slits at a time? Think about that for a second. An electron is an
indivisible bit of– it’s a
fundamental particle, an elementary particle. It has a negative
electrical charge, but it also has a
mass, whatever that is. We’d anticipate that
the electrons surely must go through one or
other of these two slits. And the one thing that
you don’t expect to get is an interference pattern
coming out of that. How can it possibly? But de Broglie was saying
electrons can also be waves. And a wave passes through
both slits simultaneously to interfere on the far side. So let’s do the experiment. Here’s what we see when
a few electrons have passed through these two slits. This is fine. What we see is,
for each electron, we see a definitive spot. It says an electron
hit here, struck here, and that seems entirely
consistent with the idea that a single electron
maintains its integrity, goes through one
or the other slits to be detected on the
screen on the far side. Then let’s slit in a few
more electrons and a few more electrons and a few more. Now, although the resolution
is a little bit fuzzy– this is not HDMI quality– we get the sense that even
though these electrons are passing through
this apparatus one at that time, what we’re seeing
is an interference pattern of light and dark fringes. What I love about
this experiment is, if indeed the electron is
passing through both slits simultaneously as a wave,
what happens to its mass while it does that? Now I don’t know
how many of you are familiar with the
work of Tom Stoppard. He wrote a play called Hapgood. It was put on stage in, I
think, the late 1980s, 1988 or thereabouts. He had a character, Kerner. It was a play about a double
agent in MI6, I think. But the double
agent was a metaphor for wave-particle duality. Stoppard is a clever guy. And Kerner said, “Every
time we don’t look, we get wave pattern.” Because, of course,
faced with that kind of puzzling
experiment, you might be tempted to say, OK,
well, I’m bloody well going to trace the path of an
electron through this thing. I’ll show you. But the minute you do
that, every time we look to see how we
get the wave pattern, we get the particle pattern. The act of observing
determines the reality. And that’s the
essential mystery. OK, Einstein and Bohr
had a famous debate. The problem with
this kind of thing is that, when we see a
single spot on the screen, there’s a phrase. It says that if the electron
is described as a wave, it’s kind of distributed. It could be anywhere
across that screen. It ends up being
in only one place. It’s detected there, but that
place cannot be predicted. It’s left to chance it seems. That’s the nature of
quantum probability. And Einstein didn’t like that. He said God does not play dice. Bohr, on the other
side of course, answered it is not
for us to tell God how he should around the world. All right, so this is the
mystery of quantum mechanics. We were doing so well. We’d started with
our cube of ice. We got molecules of water
in a regular lattice. We found the mass of
molecules of water in the protons and
neutrons in it’s nuclei. And now we run into
this sea of confusion called quantum mechanics. I’m going to press
on because, OK, the thing about
quantum mechanics is that it works really well. It is by far and away one of the
best theories of physics that have ever been designed,
even though it’s bizarre and nobody understands it. You think I’m joking. I’m not. There’s an extension
of quantum mechanics, perhaps less familiar
than some of these things, called quantum field theory. And one of the first successful
developments of quantum field theory was this guy here, the
charismatic American physicist, Richard Feynman. But there were others involved. Julian Schwinger,
Shin’ichiro Tomonaga, and an English physicist called
Freeman Dyson were responsible for putting it together. It’s called quantum
electrodynamics. And the subtlety
and sophistication of quantum electrodynamics
is a thing to behold. I think Feynman once said that
the predictions, the things you can calculate with
quantum electrodynamics, is like knowing the distance
from San Francisco to New York to within the width
of a human hair. It is so precise
that you can’t but accept that this version of
quantum field theory is– it’s got some
essential truth in it, despite the fact that
we don’t understand it. And that was fine. QED worked really well. But then when
physicists’ theories started to assemble about 20
years after this– something called the Second
World War intervened. 20 years afterwards,
when theorists started to try to
create a quantum field theory to describe protons
and neutrons, they hit a snag. In the meantime, quantum
waves, by the way, so we’ve not lost the idea of
wave-particle duality in this. We still have to deal
with this confusion. It’s just that those
wave ideas have been translated into a field. as still an extended,
distributed structure. We’ve still got the problem
of the collapse of the wave function. We still understand that
an electron field somehow interacts with the screen and
ends up producing a single dot over here in a way that
cannot be predicted. There was a problem. And that is that early
quantum field theories, they dealt with only
massless particles. Now the photon is a good
example of a massless particle. And so having got the clarity– even though I use clarity
probably in inverted commas– having got the clarity of
quantum mechanics and quantum field theory, we’re
now at a situation where things have gone
horribly wrong again. And we’ve lost sight of mass. We cannot get to the mass
of protons or neutrons, even though we know that
these things do have a mass. So what do we do? Well, actually, the
first thing to do is to actually understand what
a massless particle actually looks like. And for that, I’m
afraid I’m going to have to ask you to indulge
me a little bit of Einstein’s special theory of relativity. I promise it won’t
hurt too much. So here’s a particle,
very simply conceived. It’s a billiard ball type thing. It has a diameter. I’ve called it d0. You with me? OK, I’m going to push
that particle to travel. It’s travelling
with a velocity, v. And I’m going to push that
particle so that it moves at ever-increasing speeds
up to the speed of light, which is given the
special symbol, c. All right, now to understand
what goes on, I need to recognise one of the effects
of Einstein’s special theory of relativity is that distances
contract, and time dilates. Don’t ask me to go
into that right now. But anyone who wants to buy
a beer afterwards for me, I will happily regale them with
the reason why that happens. So what we do is we
push our particle. Let’s push it to something
like 87% of the speed of light. This factor here, given
the Greek symbol gamma, is called the Lorentz factor. You don’t have to worry
where it comes from or what it represents. You just need to know that it
started off with a value of 1. And now it has a value of 2. And what it means, according
to that little equation up there it means
that the diameter of this particle in
the direction of travel has compressed to half
its original diameter. That’s special theory
of relativity for you. Push it a little bit further,
now 98% of the speed of light– by the way, we’re
getting now to the kinds of speeds at which protons are
hurled around the Large Hadron Collider at CERN. They get up to about 99%
of the speed of light. We see now that this
Lorentz factor, gamma, is moved to a value of about five. That means the diameter
of this particular is a fifth of its original
diameter in the direction in which it’s moving. I think you can
figure out what’s going to happen if
we push this all the way to the speed of light. We’re going to end up with the
thing going off the top there. And we end up with,
effectively, a dimensionless, a two-dimensional particle,
if that makes sense. Now in truth, we
can’t accelerate. We can’t move particles with
mass at the speed of light. Only massless particles
can travel at this speed. It’s a characteristic. And by the way, massless
particles only ever travel at the speed of light. OK, so what that means is a
massless particle travelling at the speed of light is
flat or two-dimensional. It’s kind of lost
the third dimension. It cannot possibly exist
in a third dimension. And in fact, for
those of you who know about light
polarisation, you’ll know that light
actually has only two states of polarisation, which
we can think of, perhaps, as vertical and horizontal. There’s no light polarisation
in this direction. If this is like travelling
towards you here, it’s either– I always say horizontal when
I do that and then vertical. Vertical or horizontal,
there’s no polarisation in this direction for
the very simple reason is that it has no third
dimension to travel in or to be polarised in. OK, so that’s a
bit of a problem. So in effect, to
fix this problem in quantum field theory in
the early 1950s, what you need is a trick. We need massless
particles going in. We need something
magical to happen. And we need to get particles
with mass coming out. You know what this is? It’s called the Higgs field. And the fundamental
particle of the Higgs field is this thing called
the Higgs boson. Now here’s a dirty little secret
about theoretical physics. If you’re a
theoretical physicist, sitting down, pondering
great thoughts about the nature of
material substance and elementary
particles, your mission is to get the maths
to work out correctly. That’s your first priority– get the maths to
work in a way that’s consistent with theoretical
structures that have gone before and, hopefully, in
such a way that might give you some insights as to
an experimental test you can do or give
you something to look for in a laboratory like CERN. But these theorists are
not overly concerned as to what it means. And it’s then left– when these things do turn out
to have a bit of life to them, it means that we’re
left scrambling to try to understand
what on earth this means. As far as they’re
concerned, they’ve got a mathematical trick. They invoke something
called a Higgs field, and, suddenly, mass is
switched on as a result. What is supposed to happen? Well, believe it
or not, politicians get puzzled by this too. And if you can cast your
mind back, those of you are old enough, to another
conservative government that actually in the end
became a minority conservative government under
John Major in the 1980s, John Major had a
science minister called William Waldegrave. And William Waldegrave
was facing a challenge of understanding as to whether
it was worthwhile for the UK to continue funding the
European Centre for Nuclear Research, CERN. I think we spent in those
days about 50 million pounds on CERN. And, of course,
the message he was getting from high-energy
physicists– oh, but we need to find
the Higgs boson. And William Waldegrave says tell
me what the hell this is then on one sheet of A4 paper. And I will give the best entry
a bottle of vintage champagne as a reward. And he got many entries. And in fact, he got
many good entries. But perhaps the
best one actually comes from a guy called
Professor David Miller, close by here at University College
London, who said, well, maybe think of it like this. Imagine a singularly
important personality in Conservative Party politics. Thatcher had gone, but
let me tell you now she was still a force
to be reckoned with. And imagine that we have a room
here full of Conservative Party workers. This is the Higgs field. Now Thatcher, being
two-dimensional and massless, comes into this
room of Higgs field. And immediately,
the field starts to cluster around her
because we all want to hear what she has to say. We’re waiting for her to
pronounce on, you know, the big political
decisions of the day. And as a consequence
of this grouping or this clustering of the field
around a massless particle, its motion is impeded. It can’t get through the room
in quite the speed of light that it was travelling before. And as a consequence,
it has acquired mass. Now it’s an imperfect analogy. But William Waldegrave
kind of liked it. Well, OK, so that’s how
the Higgs field gives elementary particles mass. What about the
Higgs boson itself? Well, the Higgs boson is
like a softly spoken rumour. Of course, this is clearly
something that’s contentious. We don’t want everyone
to be hearing this. So as the rumour
goes around the room, the party workers cluster
to hear what it says. And that motion, that
clustering of the field itself, is the Higgs boson, all
absolutely clear now. Good, all right, so
actually, you know the story. There was a search
for the Higgs boson. It was discovered
or found in 2012. I, rather incredibly, had a book
about this discovery in stores only six weeks after the
discovery was announced. I had an agreement
with my publisher. I will write a book that is 95%
finished, which you then print, and then we wait. And I actually listened to
the live webcast from CERN on the morning of
the 4th of July, 2012 and finished the chapter. And the book was then, as I say,
in the store six weeks later. I thought it was quite good. But so finding the Higgs boson
completes the standard model. Now this is, effectively, the
particle physicist’s equivalent of the chemist’s periodic table. These are the ingredients
that we need, finally, to get to our current
contemporary understanding of the nature of matter. We don’t need all this though. That’s the good news. We can shrink this down
to just a few bits. What we need is two things
called up and down quarks. These combine in
triplets, in threes, to form protons and neutrons. So protons and neutrons
are not in themselves elementary particles any longer. We need these things
called gluons. These gluons literally glue. Physicists have limited
creativity, really, at the end of the day. When they come up
with these names, they are normally
pretty obvious what they’re kind of getting at. So gluons glue the
quarks together inside protons and neutrons. We need electrons, obviously. Electrons are still
the thing that accounts for most chemistry
and most molecular biology at the end of the day. So we need them, and
they form patterns around the outside
of the atomic nuclei. The force that holds the
electrons and the nuclei together is the
electromagnetic force. And that is a force
that’s carried by photons, familiar
particles of light. We also need this thing
called the Higgs boson because the Higgs boson
is about the Higgs field. And the Higgs field
Higgs field is necessary in the standard
model of particle physics to give particles mass. Right, mission
update, are we ready? So we learned that a
cube of ice consists of a lattice of
water molecules, H2O. We learned that
an oxygen atom has a central nucleus with eight
protons, eight neutrons. Hydrogen atoms have a central
nucleus each of one proton. We drilled into
the proton itself. [ALARM] [LAUGHTER] I’m glad you didn’t all run
screaming from the room. Now we have a real problem
because you kind of would expect that, if we can trace the
mass of a substance like a cube of ice to its molecules, to its
atoms, to its atomic nuclei, to its protons and
neutrons, and we learn that protons and neutrons
are themselves composed of quarks, you might expect– now, OK, those quark masses
are coming from interactions with the Higgs field. But let’s not get
too detained by that. They have a mass. We know what that is. But when we do the sums, we
find that a mass of a proton– only 1% of the
mass of a proton is accounted for by adding up the
masses of its two up quarks and one down quark. Something seems to have
gone horribly wrong. Fortunately, there was
this guy called Einstein. And he wrote a paper in 1905. And what’s Einstein’s
most famous equation? E equals mc squared. Everybody knows that equation. Right? Maybe you’d be a
little bit disappointed to learn that in
his singular paper in 1905 about this aspect
of special relativity that equation doesn’t
appear at all. What Einstein discovered, his
big insight, is actually not E equals mc squared. It’s this– m equals
E over c squared. Mass is the measure of the
energy content of a body. Now I’ve got to tell you– I mean who remembers The
Quatermass Experiment on BBC television all those years ago? I see a smile in the audience,
yes, a kindred spirit. Who remembers video footage
of atomic explosions in the 1960s, 1970s,
frighten you to death, as these bombs got ever bigger? You take the fission
of a uranium nucleus, uranium-235 nucleus, and a
fifth of the mass of one proton is converted into the energy
of an atomic explosion. You kind of have that almost
cultural understanding that E equals mc
squared represents the vast reservoir
of energy that is somehow locked up in mass. And when you convert
mass into energy, as was done towards the end
of The Quatermass Experiment, you get this enormous release. But that wasn’t
Einstein’s insight, despite the fact that E
equals mc squared became the most well-known equation in
the whole history of physics. So here’s what’s
really going on. It’s mass, Jim, but
not as we know it. The mass, about 1% of
the mass of a proton, let’s say, comes from
interactions between otherwise massless quarks and the Higgs
field, which is all around us, by the way. If it didn’t exist, if it were
somehow magically switched off, we’d all explode. Well, not in an
spectacular explosion, but all our particles
would become massless because there’d be no
mechanism to give them mass. So you hope that Higgs
field stays switched on. OK, so it comes from the
energy of these particles’ interactions in the Higgs field. But it’s only 1%
of the total sum. Where’s the rest of it? The bulk of the
proton mass comes from the energy
of the gluons that are dancing back and
forth between the quarks, holding them together. The gluons are
massless particles, but they possess very,
very, very high energy. And once they’re all
locked up in the confines of a proton or a
neutron, that energy translates into what we
understand and perceive as mass. Frank Wilczek– who is one of
the architects of the standard model, worked on something
called quantum chromodynamics, which is a theory that
describes quarks and gluons– put it this way. “If the body is a human body,
whose mass overwhelming it arises from the protons
and neutrons it contains, the answer is no
clear and decisive. The mass of that body,
with 95% accuracy, is its energy content.” I would quite like
to do something about the energy content of
a certain part of my body. But so far, I haven’t come up
with a diet that will actually disconnect the Higgs field
just in this specific region. But who knows? I’m hopeful. And what this is, it’s
mass without mass. In scrambling to try and
find a way to articulate this in the book, I said look. Mass is not a property. Ever since the
ancient Greeks, we’ve always understood that atoms
would have weight, weight or mass being an intrinsic
or primary property of these indivisible,
indestructible bits of substance. But now we learn that mass
is actually not a property. It’s not something
that matter has. It is rather a behaviour. It’s something that
quantum fields do. Now this isn’t the end. The standard model
of particle physics has lots of explanatory holes. The one thing that
it doesn’t do is it doesn’t explain,
for example, gravity. And at the moment, there’s
a lot of endeavour. There’s a lot of work going
on, both in the string theory community and in another
area called loop quantum gravity, to try to devise a
quantum theory of gravity. There may yet be more to learn. However, I’m pretty confident
that our understanding of matter and the nature of
mass is not going to change as a result of these endeavours. So get used to it. When you climb on the
scales in the morning, you’re weighing
the energy content of the gluons locked
up inside the protons and neutrons of your body. I don’t know whether that
will make a difference to what the scales will say. But sometimes, a bit of
enlightenment is a good thing. Now I want to thank you. I’ve gone on a little bit
longer than I’d intended. I want to thank Carlo Rovelli. He’s an Italian theorist
who very kindly agreed to read the manuscript
over my shoulder and make sure I didn’t
commit any howlers. Latha, Jenny, Phil,
who’s in the audience, are folks at Oxford
University Press who helped turn my
ramblings into a, hopefully, readable book. My mother, well, we should
all thank our mother. Right? But my mother, who’s 80 this
year, I’ve got to tell you. She has an endless curiosity. In her 70s, she
decided that she would study for a degree in history
at University of Warwick, which she did part time. Bless her. And she agreed to
read the manuscript, coming back saying,
Jim, why do you have to use all these big words. Can’t you make it just
a little bit simpler, which I did try to do. Martin Davis who
introduced me, thank you very much for asking me
to come along this evening, and of course you for
being so very patient. Thank you very much. [APPLAUSE]