November 30, 2019 28

Four fundamental forces | Scale of the universe | Cosmology & Astronomy | Khan Academy

Four fundamental forces | Scale of the universe | Cosmology & Astronomy | Khan Academy


What I want to do
in this video is give a very high-level overview
of the four fundamental forces of the universe. And I’m going to
start with gravity. And it might surprise some of
you that gravity is actually the weakest of the four
fundamental forces. And that’s surprising
because you say, wow, that’s what keeps us glued–
not glued– but it keeps us from jumping off the planet. It’s what keeps the Moon
in orbit around the Earth, the Earth in orbit around
the Sun, the Sun in orbit around the center of
the Milky Way galaxy. So if it’s a little
bit surprising that it’s actually the
weakest of the forces. And that starts to make
sense when you actually think about things on maybe
more of a human scale, or a molecular scale,
or even atomic scale. Even on a human scale, your
computer monitor and you, have some type of
gravitational attraction. But you don’t notice it. Or your cell phone and
your wallet, there’s gravitational attraction. But you don’t see them being
drawn to each other the way you might see two magnets
drawn to each other or repelled from each other. And if you go to
even a smaller scale, you’ll see the it
matters even less. We never even talk about
gravity in chemistry, although the gravity is there. But at those scales,
the other forces really, really, really
start to dominate. So gravity is our weakest. So if we move up a
little bit from that, we get– and this is maybe
the hardest force for us to visualize. Or it’s, at least, the least
intuitive force for me– is actually the weak
force, sometimes called the weak interaction. And it’s what’s responsible
for radioactive decay, in particular beta minus
and beta plus decay. And just to give you an example
of the actual weak interaction, if I had some cesium-137–
137 means it has 137 nucleons. A nucleon is either a
proton or a neutron. You add up the protons
and neutrons of cesium, you get 137. And it is cesium, because
it has exactly 55 protons. Now, the weak
interaction is what’s responsible for one of
the neutrons– essentially one of its quarks flipping
and turning into a proton. And I’m not going to go into
detail of what a quark is and all of that. And the math can
get pretty hairy. But I just want to
give you an example of what the weak
interaction does. So if one of these neutrons
turns into a proton, then we’re going to
have one extra proton. But we’re going to have the
same number of nucleons. Instead of an
extra neutron here, you now have an
extra proton here. And so now this is
a different atom. It is now barium. And in that flipping,
it will actually emit an electron and an
anti-electron neutrino. And I’m not going to go
into the details of what an anti-electron neutrino is. These are fundamental particles. But this is just what
the weak interaction is. It’s not something that’s
completely obvious to us. It’s not the kind of this
traditional things pulling or pushing away from
each other, like we associate with the other forces. Now, the next strongest
force– and just to give a sense of
how weak gravity is even relative to
the weak interaction, the weak interaction
is 10 to the 25th times the strength of gravity. And you might be saying,
if this is so strong, how come this does it
operate on planets or us relative to the Earth? Why doesn’t this apply to
intergalactic distances the way gravity does? And the reason is the weak
interaction really applies to very small distances,
very, very small distances. So it can be much
stronger than gravity, but only over very,
very– and it really only applies on the
subatomic scale. You go anything beyond
that, it kind of disappears as an actual force,
as an actual interaction. Now, the next force
up the hierarchy, which is one that we
are more familiar with, it’s what actually dominates
most of the chemistry that we deal with
and electromagnetism that we deal with, and that’s
the electromagnetic force. Let me write it in magenta,
electromagnetic force. And just to give a sense,
this is 10 to the 36 times the strength of gravity. So it kind of puts the
weak force in its place. It’s 10 to the 12th times
stronger than the weak force. So these are huge
numbers that we’re talking about, either
this relative to that or even this
relative to gravity. And so you might be
saying, well, you know the electromagnetic force,
that’s unbelievably strong. Why doesn’t that apply over
these kind of macro scales like gravity? Let me write it
there, macro scales. Why doesn’t it apply
to macro scales? And there’s nothing about the
electromagnetic force, why it can’t, or it actually does
apply over large distances. The reality though, is you don’t
have these huge concentrations of either Coulomb charges or
magnetism the way you do mass. So the mass that you have
such huge concentrations, it can operate over
huge, huge distances, even though it’s
way, way, way weaker than the electromagnetic force. The electromagnetic
force, what happens is because it’s both
attractive and repulsive, it tends to kind
of sort itself out. So you don’t have these huge,
huge, huge concentrations of charge. Now, the other thing you
might be wondering about is, why is it called the
electromagnetic force? In our everyday life, there’s
things like the Coulomb force or the electrostatic force,
which we’re familiar with. Positive charges or
like charges want to repel– if both of
these were negative, the same thing
would be happening– and different charges
like to attract. We’ve seen this multiple times. This is the Coulomb force
or the electrostatic force. And then on the other
side of the word, I guess, you have the magnetic part. And magnets, you’ve played
with magnets on your fridge. If they’re the same
side of the magnet, they’re going to
repel each other. If they’re the opposite
sides, opposite poles, they’re going to
attract each other. So why is it called one force? And it’s called one
force– and once again, I’m not going to go into
detail here– it’s called one force
because it turns out, that the Coulomb force,
the electrostatic force and magnetic force are
actually the same thing viewed in different frames
of references. So I won’t go into
a lot of detail. But just keep that in the
back of your mind, that they are connected. And in a future
video, I’ll go more into the intuition of
how they are connected. And it’s more apparent
when the charges are moving at relativistic frames
and you have– well, I won’t go into a
lot of detail there. But just keep in
mind that they really are the same force, just
viewed from different frames of reference. Now, the strongest of
the force is probably the best named of them all. And that’s the strong force. That is the strong force. And although you probably
haven’t seen this yet in chemistry
classes, it actually applies very strongly
in chemistry. Because from the get-go, when
you first learn about atoms– let me draw a helium atom. A helium atom has two
protons in its nucleus and it has two neutrons. And then it also has two
electrons circulating around. So it has an electron. And I could draw the
electron as much smaller. Well, I won’t try to do
anything in relative size. But it has two electrons
floating around. And one question that may or may
not have jumped into your mind when you first saw
this model of an atom is like, well, I see
why the electrons are attracted to the nucleus. It has a negative
Coulomb charge. The nucleus has a net
positive Coulomb charge. But what’s not so
obvious and what tends not to sometimes be
explained in chemistry class is these two
positive charges are sitting right next
to each other. If the electromagnetic force
was the only force in play, if the Coulomb force was
the only thing happening, these guys would just
run away from each other. They could repel each other. And so the only
reason why they’re able to stick to each
other is that there’s an even stronger force than
the electromagnetic force operating at these very,
very, very small distances. So if you get two of these
protons close enough together, and the strong force only
applies over very, very, very small distances,
subatomic or I should even say subnucleic distances,
then the strong interaction comes into play. So then you have the strong
interaction actually keeping these charges together. And once again, just to keep
it in mind relative to gravity, it is 10 to the 38th times
the strength of gravity. Or it’s about 100 times stronger
than the electromagnetic force. So once again,
the reason why you don’t see the
strong force, which is the strongest
of all the forces, or the weak interaction,
applying over huge scales is that their strength
dies off super, super fast. Even when you start going
to a larger radius nucleuses of atoms, the strength
starts to die off, especially for the strong force. The reason why you don’t see the
electromagnetic force operating over large distances, even
though in theory it can, like gravity, is
that you don’t see the type of charge
concentrations the way you see mass concentrations
in the universe. Because the charge
concentrations tend to sort them out. They start to equalize. If I have a huge
positive charge there and a huge negative
charge there, they will attract each
other and then become essentially a big lump
of neutral charge. And once they’re a big
lump of neutral charge, they won’t interact
with anything else. And gravity, if you have
one mass and another mass, and they attract
each other, then you have another mass
that’s even better to attracting at other masses. And so it’ll keep
attracting things to it. So it kind of
snowballs the process. And that’s why gravity operates
on these really, really large, large objects
in our universe and on the universe as a whole.

28 Replies to “Four fundamental forces | Scale of the universe | Cosmology & Astronomy | Khan Academy”

  • cutiepie316289 says:

    "I'll wright it in magenta" That made me giggle :3

  • eurasianchica says:

    We <3 you khanacademy!

    I've always had a really hard time with math and I think I've learned more from these videos than in class. Teachers usually go so fast I stop understanding what they're talking about… my eyes glaze over and I hear but don't listen. It's so nice to be able to be able to pause and rewind the video as often as necessary to understand what you're talking about.

  • Yaron Zaret says:

    I ❤ khan academy!

  • elroyjenkins100 says:

    Studders too much.

  • Charlie Ulivarri says:

    So according to the Newtonian physics, gravity is a force of attraction.  However, according to the theory of general relativity, gravity is a geometric feature of space-time and not a force.  So why do we insist on talking about gravity as a force still?

  • Gautam Puri says:

    if you want more info (in more detail), check out Scishow's 5 videos on the 4 fundamental forces (yes, there's 5 videos).  way better.

  • Don Pedro says:

    so if i punch somebody in the face, which kind of force am i exerting? something doesnt add up here

  • Mr. Singer says:

    What fundamental forces are used in friction and digestion?

  • Tye says:

    This isn't the sort of lesson that I need as someone trying to get an understanding of physics. It's poorly organised, but well presented. This feels like more of a mixture of pop-science and a lesson – a mixture between a viral science video and a classroom lecture. 

  • JayPixx77 says:

    Wow. Really great video mate! You could use some music though. But still.. interesting and well explained 🙂

  • Dr. Drake Ramoray says:

    pretty awesome video! explained very nicely, without taking on the stuff people dont really need

  • YManCyberDude says:

    That was pretty darned nifty . . .

  • fabulous says:

    i cant seem to find the next video? pls help

  • John Payne says:

    Gravity is actually a theory and has not been proven by science.

  • Leandro Cruz says:

    I teach English. I would like to know which graphical interface you are using for this video. It is a tablet with a stylus of some sort. What are you using? I would like to use it in my classroom. Please, Thanks.

  • hey hey monkey says:

    this vid is great…made me feel relaxed and comfortable.

  • God may exist says:

    it's the magnetic force that's the strongest fundermental force in the universe for example gravity keeps us from floating off but it's the magnetic field force that repells the planets from colliding and when the earth'sagnetic force field weakens a collision will be iminate for example if you had 2 repelling magnetic force field balls and a trampoline and pull down the fabric in the middle of it now let go loose of the magnetic balls that repell eachother and watch them spin around without colliding into eachother.

  • Roxell says:

    the source of electromagnetic force is electric charge, what's the source of strong force?

  • Emperor Siddiantus says:

    what if they try strong force fission
    and create an artificial nucleus

  • nadoy AA says:

    thank you

  • Make Me Scientific says:

    good explanation thanks. #makemescientific

  • Jason L. N. Williams says:

    Not bad, these videos though need to be done in a more exciting way– such as Crash Course. This can get a bit boring.

  • Truth Truth says:

    Do we have devices that can detect and measure each force?

  • burjalmadre says:

    Explained better than my astronomy course in college.

  • Netional says:

    You always immediately address the questions arising from these topics, like why the weak force does not apply to planets. A lot of other explainers have a tendency to leave these questions hanging in the air.

  • L P Muzza says:

    He sounds like dj vlad (vladtv)

  • George Richards says:

    Does weak interaction happen by the weak necleur force or just weak force or are they the same thing. Also does strong interaction happen because of strong necleur force. I always see weak force, but strong necleur force.

  • Beatrix Mamí says:

    All these comments from years ago👀
    Finna come back n look for mine lol
    🚶🚶✨

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