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This is about 1 kilogram of mass. Then again, so is this. These two objects are different sizes, different shapes, made of different materials, and have
different purposes. So, when I say something like this is 1 kilogram,
what am I saying about it? What the heck is mass?! In old school physics, mass shows up in two
different places. Kicking it old school with my home-boy Newton.
He’s no fool and that’s totally proven. He’s got a rule that’s meant for computing,
a major uncool of a mass executing. What. Whatever. According to Newton’s second law, mass measures inertia, an object’s ability to resist changes in motion. More mass means less acceleration. Anyone who’s ever tried to push a broken
down car knows exactly what I mean. The second place mass appears is gravity. Newton’s universal law of gravity, which is in no way universal, says two masses exert gravity on
each other over some distance. More mass in either object means
more gravity between them both. Both laws are aspects of something we call
classical or Newtonian mechanics, for obvious reasons.
And it works really well most of the time. We know from a lot of videos on this channel
that Newtonian mechanics doesn’t explain everything. It’s not fundamentally correct. However, to be clear, Newton’s laws are unbelievably accurate
for 99% of things in 99% of the universe. Most of the time, you’re good with Newton. But his laws don’t really explain what mass is, just what effects it has, which isn’t very satisfying. The easiest definition I’ve heard is that
mass is the amount of stuff something has inside it, but that’s kind
of vague. What stuff?! Yeah, exactly. This is a really deep question and the deeper
questions we ask, the more that other 1% of 1% is going to become important. Let’s focus on that for a few minutes and, maybe, we can make this definition a little more specific. The other 1% of 1% of stuff is governed by one of two types of physics: Relativity and Quantum Mechanics. We’ll start with relativity because,
well, it’s easier. In relativity, Newton’s second law of motion
and his law of gravity have been corrected to more closely match
the universe. But there’s a consequence to this correction. Like a lot of other things in relativity, mass is relative to the observer. That means it depends on who or what is measuring it. Yes, angry commenters. Relativistic mass is a real thing. It isn’t always useful and sometimes it’s unnecessarily confusing, but it’s still a valid way of looking at the world. No matter what a particle physicist might tell you. To understand this, we need to take a look
at the most famous equation of all time. E equals m c squared. This mass here is relativistic mass. Whether you’re trying to accelerate an object
or you’re weighing it on some kind of scale, you’re measuring relativistic mass. In other words, you’re measuring its complete energy content. That energy could: elastic, kinetic, thermal, whatever. If the object has more energy, it’s harder to accelerate, it’s affected more by gravity, and it generates more of its own gravity,
no matter what type of energy that is. But there’s one type of energy that usually
dominates. Rest energy! The part of mass associated with rest energy is often called rest mass, so the distribution usually looks something like this. It’s almost all rest mass, so we don’t have to distinguish most of the time. Why do we call it rest mass? It’s the mass we’d measure if it were at rest
relative to us. Say an object starts to speed up. Because its kinetic energy increases,
so does its total mass, otherwise known as relativistic mass,
but its rest mass stays the same. While rest mass usually dominates, it doesn’t
always. If something is small enough or moving fast
enough, kinetic energy can noticeably contribute to
overall mass. There are even things that don’t have any
rest mass at all. In order for something to have rest mass,
it must have a rest frame. There must be a frame of reference where it
is at rest or stationary. Light doesn’t have one of those. Light is always traveling at the same speed
according to all observers everywhere. It has relativistic mass, but not rest mass.
All of its energy is kinetic energy. In fact, anything without rest mass is going to have
that same speed-weirdness as light. Gluons also have no rest mass,
so they also travel at the speed of light for all observers. Speaking of gluons, let’s take a look at quantum mechanics. Because we still haven’t quite explained everything. How is this rest energy stuff actually energy?
What is it?! It can’t just be existence energy.
Photons and gluons exist and they don’t have it. It comes down to what type of stuff
objects are made of. You mean steel? No! Particles! Steel is mostly iron and carbon atoms in a
crystal solid. So far, there’s nothing super enlightening.
Steel has mass because its atoms do. Taking a closer look at one of the atoms,
we have the same problem. Atoms have mass because their subatomic particles do and most of that mass is from the protons
and neutrons. Inside one of the protons though, something
weird happens. Protons are made of three quarks,
but the mass of those quarks only represents 1% of the total mass of the proton.
The other 99% comes from the kinetic energy of those quarks
and, more importantly, the binding energy of the gluon field holding those quarks together
as a proton. On a fundamental level, 99% of the mass of
you or me, what we would call our rest mass,
is actually just other types of energy. So the break down actually looks more like
this. Almost the entire chart is made up of different
types of energy. But what about this remaining 1%?
That’s just the rest mass of all of these particles over here. According to quantum field theory, those particles
have mass due to an interaction with something called the Higgs field.
See, most quantum fields hover around zero energy. The Higgs field hovers a little higher than the others. Its equilibrium is a non-zero energy that fills all space. The gruesome details of this we'll save for another video in the distant future. The point I’m making here is that it’s still about energy. The Higgs field is an energy field. All quantum fields are. So, what the heck is mass? When you measure the mass of something, anything, you’re just measuring its energy content.
That energy can be made up of a wide range of different types,
but, in the end, it’s all just energy. And, just so I make this abundantly clear,
less than 1% of that is from the Higgs field. The other 99% or more is from other energy
that has nothing to do with Higgs. Whatever part of that you happen to be calling
mass just depends on who are and who you’re talking to. But since this famous equation says that mass is really just energy, maybe it’s time we stop discussing mass all together. So, what’s the most massive thing you’ve
ever seen? Please share in the comments.
Thanks for liking this video. Don’t forget to subscribe if you’d like
to keep up with us. And until next time, remember, it’s OK to
be a little crazy. The featured comment come from Gustavo who
asked: What exactly did he mean by detect?
A quantum detection, observation, measurement, whatever you want to call it,
is any interaction where one particle carries away information about the other.
In our case, the electron’s location. Anyway, crazies. Thanks for watching!
