NARRATOR: Einstein showed
us that matter, mass, and the flow of time are
intrinsically connected, but opened the question,
are they even real? Let's find out. In a previous episode, we talked
about the speed of light-- the fastest speed there is. And we talked about how
this speed limit is really the speed of causality. It's the maximum speed at
which two neighboring bits of the universe can
talk to each other. Anything without mass
has to travel this speed. But what is it about mass
that prevents something from reaching the
ultimate speed? The answer to this will take
us to a much deeper question-- what is the origin
of matter and time? However, it's going to take
us a couple of episodes to get there. So today, we're going to look
at the true nature of matter and mass a little more closely. We've already covered
Einstein's famous equation, E equals Mc squared, and showed
that most of the mass of atoms comes from the kinetic
and binding energy of the quarks that make
up protons and neutrons. But saying that mass is energy
doesn't really get us very far. It just begs the
question, what is energy? That's a huge topic
that we'll build up to. For now, let's
look at this energy in terms of what's actually
happening in an object when it exerts this
property we call mass. Let's ignore the gravitational
effect of mass for the moment, and just consider mass as
the degree to which an object resists being accelerated. We call this inertial mass. A good place to start is
with a thought experiment that we'll call a photon box. Imagine a massless box with
mirrored walls-- impossible, I know, but it's an analogy for
something real, as we'll see. Now fill it with
photons, also massless, that bounce around inside
the box in all directions. All the walls of the box
will feel the same pressure, so there's no overall
force on the box. But let's give the box a little
nudge-- increase its velocity. Now the back wall
of the box moves into the incoming photons. It feels a little more pressure
from their impact than before. In the meantime, the
front of the box, moving away from the incoming
photons, feels less pressure. There's a net backward force
that feels like a resistance to the change in speed. The photons exert
a force on the box, the box also exerts a force
on the photons-- Newton's Third Law, which gives us
the conservation of momentum. Momentum lost by the box is
transferred to the photons. Now, if the box
stops accelerating, then everything jiggles
around and momentum gets shared out evenly between
the box and the photons again. But as long as
acceleration continues, the pressure
differential persists. Acceleration is
resisted in a way that feels exactly like mass. In fact, it's indistinguishable
from mass, because it is mass. The photon box is massive, even
though none of its components-- not the photons, not the
walls-- have any mass. Somehow, mass arises
in the ensemble where it doesn't exist in the parts. How much mass does the box have? It's the energy of
the photons divided by the square of the
speed of those photons. And you can derive the famous
E equals Mc squared just by looking at how
momentum transfers between the photons in the
box under acceleration. But E equals Mc
squared describes the universal
relationship between mass and confined energy, not
just confined photons. So let's look at another
example of confined energy. A compressed spring holds more
energy than a relaxed spring. It holds potential energy. So is a compressed spring more
massive than a relaxed one? You bet it is. Again, we can
describe this in terms of a straightforward
physical effect. An already compressed
spring is harder to compress further compared to
a relaxed spring. But that's exactly what you have
to do when you try to move it. Push the spring, and it doesn't
all start moving instantly. First, the rear
compresses a bit. And then a pressure wave
communications the force to the front until the
whole spring is moving. That initial push is harder
for the compressed spring than for the relaxed spring. It feels like it's more
massive, because it is. These seemingly very different
physical effects-- the box of photons and the
compressed spring-- both give the same translation
between mass and energy, E equals Mc squared, because the
underlying cause is the same-- the confinement of
interactions that themselves travel at the speed of light. Photons in the
photon box, but even in the spring, the
density wave is ultimately communicated by electromagnetic
interactions between the atoms. That itself is a speed
of light interaction, even if the resulting
density wave isn't. OK, so how does this stuff
translate to something like a proton? 99% of the mass of the proton
is in the vibrational energy of the quarks plus the binding
energy of the gluon field. The actual intrinsic mass of the
quarks is a tiny contribution. So the proton is a lot like
a combination of our photon box and our compressed
spring-- quarks, bouncing off the walls in the
binding gluon field, which itself acts like a
compressed spring, holding potential energy. And as we saw recently, even
those quarks, as well as electrons, gain
their tiny masses from a type of confinement
via the Higgs field. Take away the Higgs field,
and they are massless speed of light particles. It looks like
everything with mass is composed of a combination
of intrinsically massless, light-speed particles that are
prevented from streaming freely through the universe, as
well as the fields that confine those particles. So is mass really not
a fundamental property? Is it just the
result of massless particles and fields bumping and
sloshing around inside things resisting acceleration? Yeah, it kind of is. This acceleration resisting
mass, inertial mass, seems to be an emergent
property of the ensemble. But we can't talk about mass
without talking about gravity. Massive objects
exert and respond to the force of gravity. They have what we call
gravitational mass. But how does the inertial
mass of our photon box end up translating to
gravitational mass? Once we accept Einstein's
description of space-time as described by
general relativity, it's not so surprising
that the photon box feels the pull of gravity. The equivalence
principle tells us that the feeling of being
accelerated out in space is fundamentally the same
thing as the feeling of weight in a gravitational field. Holding up our photon box
against Earth's surface gravity has to be just as hard as
trying to accelerate it at 1 g in empty space. The photon box feels heavy. Same with the
compressed spring-- it's harder to accelerate
than a relaxed one, and it also feels heavier
in a gravitational field. In fact, the
equivalence principle tells us that the
gravitational mass of an object and the inertial
mass are the same thing. But mass doesn't just respond
to a gravitational field. It generates one. Mass curves the fabric of space. Actually, it turns out that it's
not just mass that bends space. The presence in the flow
of energy and momentum as well as pressure all have
their quite different effects on the curvature of space-time. Individual photons
affect space-time. And when you trap them in a box,
the curvature that they produce looks just like gravity. So confined massless
particles generate a very real gravitational field. OK, so mass is an emergent
property of the interactions of massless particles. What about time? A single photon
experiences no time, nor does any massless particle. Their clocks are frozen. But our photon box has mass,
so it must experience time. When and where does
this time arise? The individual
photons don't have it when they travel from one
side of the box to the other. Do they get time when
they bounce off the wall? Does the ensemble of
photons somehow feel time that individual photons do not? We'll explore these questions
when we delve deeper into the mystery
of matter and time in the next episode
of "Space Time." In the last episode
of "Space Time," we talked about how
the Higgs field gives elementary particles mass. And as always, you guys
had some great questions. Caleb Limb asks, does this
mean the Higgs field makes a little friction in space? Well, this is a bit of annoying
pop-sci misinformation. The Higgs field
isn't like molasses or like a crowd
full of physicists. It doesn't act like friction,
because friction slows down particles. The Higgs field doesn't
slow particles down. It gives them inertia-- a
resistance to acceleration. It makes them harder to
speed up or slow down. And importantly,
it prevents them from traveling at
the speed of light. Felix Feist points
out that given that the right-handed electron
doesn't have weak hypercharge, shouldn't it be massless? Well, yes and no. The right-hand electron can
interact with the Higgs field by picking up some
weak hypercharge. In fact, this flipping back
and forth between handedness is probably more
accurately thought of as the electron being
both right- and left-handed at the same time, because
the interchange happens on time scales shorter
than the Planck time. There's a quantum blur
surrounding the current state of the electron. It's really the composite
particle that has mass. The naked left- or right-handed
electron is massless. Death by PowerPoint
wonders whether there could be a point
in space somewhere where the Higgs field
takes on the value of zero, and what the
ramifications would be. Well, actually, yes--
or at least there was. At extremely high
temperatures, the Higgs field takes on a value
of 0 everywhere. And it's believed that this
was the case in the fraction of a second after the big bang. Then, without an infinite source
and sink of weak hypercharge, the weak nuclear force and
the electromagnetic force were all the same force. Only when the universe cooled
down did the Higgs field gain a nonzero value in a phenomenon
called spontaneous symmetry breaking. Then the weak force
carriers gained mass and became differentiated from
the electromagnetic carrier-- the photon. The ramifications--
we wouldn't have atoms without a nonzero Higgs field. [MUSIC PLAYING]
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This is by far my favorite physics channel, I binged all of their videos over a week
I have a question now. I always thought the circle you draw for an atom was just to represent it's component, but there isn't actually anything there. And the same would be true for all "physics balls" like electrons and protons and the like. Electrons were really some type of energy wave and the ball is just easier to draw. But the way they're talking about inertial mass in this video makes it sound like a proton gets its mass from containing the quarks. If the quarks were paired together, but there's no physical wall or field blocking them, would that change anything? I really didn't think those balls actually existed.
Best physics channel on the internet
I didn't know this channel. Thank you for posting!
i'm having a hard time going with the spring example. seems like all he's saying is that it's more stiff. if I put a compressed spring on a scale would it indicate any more weight after it settles?
Question: wouldn't the force difference also exist at uniform velocity? How does the force difference only show up for acceleration?
I'm just going to jump on the hype train and be the fiftieth person to say this is an amazing channel.