January 1, 2019 • 36 mins
Why is gravity so much weaker than the other forces?
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Speaker 1 (00:07):
I just think it's fascinating that it's such a fundamental
force in the universe, right Like, it's it's basically the
thing that builds galaxies and keeps planets moving right and
gets structured to the entire cosmos. That's right on the
largest scales, actually the most important force. It's the reason
why things look the way they do, the reason why
(00:29):
our planet is around, it's the reason why we're on
the planet. It's pretty important, and yet we don't know
a lot about it, right, Like, there's some really deep
and strange mysteries about it. On one hand, we have
a theory which works really really well. On the other hand,
we have questions about it, but to seem really really basic.
And not only that, it's it's very different than all
(00:50):
the other forces of nature. That's right, one of these
things is not like the other ones. Ye hi am more.
(01:15):
I'm a cartoonist and I'm Daniel, I'm a particle physicist.
And this is our podcast Daniel and Joe explained the universe,
in which our cartoonist and physicists try to figure out
how to make the universe understandable to anybody. Yeah, and
today on the podcast we are examining a very heavy topic,
(01:39):
gravity and specifically why is gravity so weak and strange. Gravity,
as we said earlier, is something which controls the structure
of the universe. I mean, the reason the Solar system
looks the way it does is because of gravity. The
reason the Earth is round is because of gravity. The
reason we have galaxies is because of gravity. The reason
(02:00):
we weighed so much, it's it's because of gravity. Right,
it's not my belt, No, that's because of late night
cake eating. Um. But it's such a fundamental force of nature. Rights, Like,
it's present in our everyday live We spend a lot
of time thinking about gravity, right, how not to fall down,
(02:20):
how not drop things, how to go up buildings, how
to go down buildings? Right, that's right. It seems like
one of the most important forces. I mean, if you
ask people, you know, to name a force or what
kind of forces the experience in their life, gravity is
the one that's present in their lives. Right, you're climbing upstairs,
you're fighting gravity. Trip, you fall down, you're feeling gravity.
You look around you. The shape of things is controlled
(02:41):
by gravity, and that's why it's particularly strange that gravity
is the weakest force of all the forces we've discovered,
it's by far the weakest. Yeah, it's really strange to
hear you say that, Like, how can gravity be weak?
Like you know, like it's it's keeping the whole Earth together,
it's making the entire higher planet swing around, going to
(03:02):
circle basically, right. Without gravity, we would just shoot off
into space. That's right. It's a really strange situation. And
there's other things about gravity we don't understand as well.
It's really strange. It doesn't play well with the other forces.
It's very, very weak. It's a total mystery to science,
except that we have a theory which works beautifully. Right.
We can calculate exactly how mercury orbits the Sun. We
(03:23):
can send things into outer space and note with two
millimeter precision exactly where they're going to land. We have
a working theory that we can use, right, but we
don't understand it on a conceptual level. We have these basic,
deep questions about about what gravity is and how the
universe works because of it. So it's a weird question,
and maybe one of the people hadn't thought about before.
So Daniel went out as usual and ask people on
(03:45):
the street, why do you think gravity is so weak.
Here's what a random selection of folks who are willing
to talk to me on a Tuesday morning had to
say about gravity. I don't know. I should don't know
about that, all right, I was. I was thought it
was a pretty strong force. So I don't know. But yeah, because, um,
it depends on the distance and it's long range one,
(04:06):
so that's why we feel it's very weak most of
the time. Cool. No, okay, I have no I'm sorry,
it's not very fruitful. I have no idea, but I'd
be interested in finding out why. All right, that wasn't
that was pretty good. Most people weren't surprised when you
said gravity is weak. I don't know. I feel like
(04:27):
of all the questions I've asked people, this is the
one that flams them the most. You know. People were like, what,
I have no idea, or um, they had crazy ideas
why gravity must must be weak. I feel like, usually
we get one person who knows what the answer is
or has a good clue about what what's going on.
This time, I feel like almost everybody was pretty clueless.
I mean, one person said I always thought gravity was
(04:48):
pretty strong, right, which kind of sums up the situation. Right.
Gravity is omnipresent in our lives. It dominates our experience,
and yet it's so weak compared to the other really
powerful forces we've discovered. Well of people, A couple of
answers were that had to do with distance, Like, gravity
gets really weak with distance, that's right. And the problem
there is that all the forces get weak with distance,
(05:09):
like electromagnetism also falls as a as a distance grows. Right,
So all of these forces followed this one over our
squared rule or are as your distance from the thing
that's that's giving you the force, right, maybe maybe right? Maybe? Yeah,
mostly we think and uh, and so that can't be
the answer, right, because all the other forces have that
(05:29):
same feature. So when you say it's the weak is
it's not that it changes over distances differently than the
other forces. That's right. So maybe we should talk about
what the forces are and compare them to each other.
So focus and get understanding of how crazy weak gravity is. Right. So, Daniel,
what are the forces of nature besides a bad movie
with Ben Affleck in center Bullock? Well, I think comedy.
(05:53):
Comedy is definitely force of nature. You know, it solves
big problems around the world. You know. The fundamental forces
are electromagnetism, right, that's the one that controls electricity and
magnetism obviously and is responsible for the cool things like
light and lightning and all that all that cool stuff. Um.
And then there's the weak nuclear force, which is a
(06:15):
force which is responsible for radioactive decay of a nuclei. Right.
And the cool thing about electricity and magnetism and the
weak nuclear force is that we actually have shown that
there are two sides of the same coin as a particle. Physicists,
we refer to them as one force. We call it
the electro week. So sort of magnetism lost out there
in the name merger, right, it should be electro magnetic week.
(06:36):
But nobody voted to keep magnetism in the the name
of the partners on the law firm. Nobody, nobody lobbied
for weak electro or magneto weak force. Yeah. Yeah, again,
we are suffering the fate of some anonymous committee of
scientists that get to name these things. Right, who are
these people? It's probably some grad student, right or some
(06:58):
you know, like this is really weird call it this. Yeah.
So we have electricity and magnetism, which is a single force.
We have the weak nuclear force, which is really should
be combined with electricity magnetism. And then there's the strong
nuclear force, and this is the one that holds the
nucleus together. You know, the nucleus is just a bunch
of positively charged protons and neutral neutrons, right, so it's
(07:19):
only positively charged particles in the nucleus. So you might think,
what it even holds the nucleus together, right, you have
all this positively charged stuff should be repelling themselves. Well,
it's the strong nuclear force, and it does so by
exchanging these crazy little particles we call gluons, and that
holds the nucleus together, and it's pretty strong. It's even
stronger than electromagnetism. Well, let's take a step back. So
(07:40):
in the universe there's stuff. There's like, yes, affirm that
there is stuff in without reservation, there is stuff. I'm
glad we saw that question. But I mean it's like
there's stuff that has substance to it, that has masked
to it, or you know that it sort of exists.
And then there's also besides the at how these things
(08:01):
interact with each other, like how they affect each other
that's right. There's the matter and then there's the forces. Right.
The forces affect how they interact with each other, and
that's pretty much the universe. That's that's like, it's matter
and forces. Yeah. One way to look at the universe
is that it's particles, right, or you would say matter
and their forces. In modern particle physics we think about
one level deeper, which if we think of quantum fields,
(08:23):
and quantum fields are responsible both for matter and for forces.
So we can talk about that maybe in another podcast.
What is a quantum field and how can I get one?
You know for leaser rent um? What can they do
for me? But yeah, I think it's it's fair still
to think about the universe in terms of particles and forces.
On that note, let's take a quick break. There are
(08:56):
only four kinds of forces. Yeah, there are four kinds
of forces. So after magnetism, weak nuclear force, strong nuclear force,
and then of course gravity, right, that's the fourth force
that we've discovered. The fascinating thing is that different particles
feel different forces, right, Like, some particles feel this set
of forces, some particles feel those set of forces. For example, right,
(09:17):
particles with electric charge feel electromagnetism. Right. The electron, for example,
is negatively charged, the proton is positively charged. You bring
them close together, they're gonna pull on each other. They're
gonna suck each other together, right, because they have opposite charges.
We all know that, um. But you bring a neutral
particle nearby, it just totally ignores it. Right, It doesn't
(09:37):
feel it at all. Right, It's like it's like somebody's
walking through a crowd of people shouting, but they have
headphones on so they can't hear anything, and they're totally
oblivious to it. It's kind of like how we talked
about in a previous podcast. They're almost like languages or
like social media platforms. Like some people are on Twitter,
some people are on Facebook, but some people are not
on this. And so if somebody if you're not on
(09:58):
Twitter and so many sense to your tweet, you're not
going to get it. And so it's just different ways
the particles in right, that's right. Gravity is the Google
plus social media right because nobody uses the trendstor it's
ancient but powerless. Yeah, And so different particles feel different forces.
And for example, an electron, while it feels electromagnetism. Because
(10:20):
it has a negative charge, it doesn't feel a strong
force at all. It will pass right by a bunch
of particles that are really tugging on each other with
a strong force and not be affected at all. Whereas corks.
Corks feel all the forces. They feel a strong force,
which is how they get pulled together in the nucleus. Remember,
protons and neutrons are made of quarks. Quarks feel electromagnetism
(10:41):
because they have electric charge. They feel the weak force.
They also feel gravity, of course, because they have mass.
So quarks get their fingers in everything they feel. They
get the feels for everything. They feel everything's right, They
got the strong feels. Course, they're really deeply emotional part
of um. And on the other side of the spectrum,
you've got things like neutrinos. Patrinos don't have electric charge,
(11:04):
so they ignore all electricity magnetism, right, They don't interact
with light. They're invisible. They pass right through anything that that.
They ignore electro magnetic bonds, so they pass through most materials.
They don't feel the strong force. The only way they
interact is with the weak force, and the weak force
is pretty weak, which is why neutrinos can mostly just
pass through matter unaffected. So we have four fundamental forces, right,
(11:31):
and gravity is one of these forces. And so when
you say that antravity is weak, you actually mean it's
weak compared to these other three forces, that's right. And
so the ranking is the strong force is the strongest,
so that one is actually well named. Congratulations, you know,
anonymous group of scientists. Yeah, yeah, we should be called
the as currently known to be the strongest force force. Um.
(11:55):
Right after that comes electromagnetism, and you know, we know
that force is pretty powerful. You stick your finger in
a socket, you're going to feel the wrath of electromagnetism, right.
It's it's not an unfamiliar feeling, right, right to think
your finger in anything? You feel it, right, because it's
electromagnetism is the force that keeps you from basically passing
through the table or passing through your car. Right, that's right.
(12:17):
Because electromagnetism is the basis of chemical bonds, right. And
chemical bonds are really the thing that form the structure
of your body. Right. You think if your body is
like a bunch of particles, but it's held together by
all these forces, it's like a chain link fence binding
together these little particles and prevents you from passing through
something else. Yeah, so we got the strong force, and
then electromagnetism, and then actually the weak nuclear force. Right,
(12:40):
this is the force that like powers neutrinos and radioactive decay.
It's much weaker than electromagnetism um and much weaker than
the strong force. Even weaker than the weak is gravity.
That's right. If you make a list like strong force, electromagnetism,
the weak force, then you should leave like a hundred
blank pages and then you get to grab because when
(13:00):
we compare these forces, we put things like an equal
distance apart and compare the strength of the forces. Gravity
is ten to the thirty six times weaker than the
weak force. But that's ten with thirty six zeros in
front of it. But it isn't that sort of a
matter of units or scale, do you know what I mean? Like,
it's much weaker, but only if you compare apples to apples, right,
(13:25):
or orange to oranges. That's right. But put two protons
next to each other, right, Two protons have a certain
amount of mass and a certain amount of electric charge,
and the force of their charges is going to be
much much stronger than the force from their masses. So yeah,
if everything was much much more massive, then there would
be stronger gravity. But you can compare these things apples
to apples by comparing them, you know, the same distance
(13:46):
and the same basic unit of interaction. Right, But what
if you take an apple put it next to another apple? Well,
I think you can do that experiment. Nothing's going to
happen because gravity is so weak. Right. You don't see
two apples like pulling themselves together on the counter, right,
then they'll alden apple collider um. You know, the apples
are not drawn to each other. Gravity is a super
weak force, and you can see this yourself. Right, you
(14:07):
can do an experiment where you counter the entire gravitational
force of an enormous celestial body like the Earth. Right,
take a small kitchen magnet and use it to hold
up a nail and think about what's happening there. Right,
you have the nail is being pulled down by every
single rock in the Earth. It's pulling with all of
its gravity. But a tiny little kitchen magnet totally overcomes that.
(14:31):
It can lift the nail even though it's pulling. It's
being pulled down by the whole entire planet Earth, right exactly. Now,
imagine a magnet the size of the Earth, right, I
mean that would be that would be extraordinarily powerful. And
so you have basically like a gravitational blob the size
of the Earth. Still pretty ineffective compared to electromagnetism. So
(14:53):
so it's weak if you sort of compare it by object,
Like you said, if you take a proton and put
it next to proton, the the force are going to
feel from electro magnetism is so much bigger than the
force of gravity. They're going to feel towards each other
the same with like two electrons or two cords and things.
So in the scale of like the particles that we know,
(15:15):
it's a really weak force, that's right, exactly. And yet
and yet it seems to dominate. Right. That's a bit
of a puzzle. Like, on one hand, it's super duper weak,
and we're telling you that it hardly counts for anything.
On the other hand, it's responsible for the structure of
the Solar System, man for the galaxy, and it's the
reason the universe looks the way it is, right, And
so that can be confusing to people, Like how do
(15:35):
you reconcile those two things in your head? Yeah, Like
why doesn't the Earth feel an electromagnetic force with the Sun,
which it would be so much bigger than the force
of gravity. Yeah, what, it would be pretty shocking. And
and that's actually the reason is um gravity is different
from the other forces and that it can't be canceled out. Right,
if there was some huge electrostatic difference between the Sun
(15:56):
and the Earth, like a bunch of positive charges there
and a bunch of negative charges year, it would create
such an enormous force that it would be very quickly balanced.
Like that's what lightning is, right, When there's a charge
differential between clouds and the ground, it's it doesn't take
that much before those charges want to rearrange themselves to
a lower energy configuration. They rushed down to the ground,
(16:18):
or they rushed up to the clouds and they jump
to balance themselves up. Because you have two kinds of charges,
you have positive and you have negative, so you can
find an arrangement where basically everybody is happy. It's an equilibrium, right,
But that's not true for gravity, Okay, I get it.
So for example, if the Earth was every particle on
Earth had a positive electromagnetic charge and every particle in
(16:40):
the Sun had a negative electromagnetic charge, there would be
a humongous pool from electromagnetism pulling the Earth into the Sun. Yeah,
we'd be toasted pretty quick. Yeah, I would be huge.
Even the opposite if we were all positive and the
Sun was all positive, we would get shot out of
the solar stem very quickly, that's right. And that's why
(17:02):
you know, early days of the Solar system being formed,
you have these gases and the gas and dust coalescing,
and very rapidly things neutralize, right, because anything that feels
an electrostatic force to something else is going to find
the opposite charge and they're going to coalesce and they're
gonna make something neutral. Right. That's why most of the
things around you are neutral, Right, Most of the elements
(17:23):
are neutral, because any deviation from neutral results in a
powerful force to neutralize it. So, thankfully the Earth is
made out of both like equal amounts of positive and
negative particles, right, that's right. Thankfully we're sort of balanced electromagnetically,
and so even if the Sun was all positive, we
would look like neutral, like a neutral ball to to
(17:45):
the sun. Yeah, that's right, we're on large scales. The
Earth is neutral, right, I mean there might be some
residual positive or negative charge depending on the solar wind, etcetera.
But basically the Earth is neutral, and so the largest
force of the Earth feels is the gravity in the sun,
even though gravity is super duper weak. Right, it doesn't
take a lot to counteract gravity. But it's the only
(18:05):
player left because everybody else is sort of pair it
up and danced off for the night, and gravity is
just there left holding the bag. And gravity can't be balanced, right.
You feel gravity if you have any mass, right, But
there's only positive masses and no such thing as a
negative mass to give anti gravity. Well, let's keep going,
but first let's take a quick break. Okay, So that's
(18:40):
how gravity is so much weaker than the other forces. Um,
so how's it different than the other three forces of nature?
There's like no end to to weys, the gravity is weird,
you know, there's no end to like the puzzles of
Gravity's fascinating bottomless pit. That's right, it's a black hole
of questions. Um. And one of my favorites is just
(19:01):
that we have no way to sort of fit gravity
in with the way the universe works according to everything else.
You know, we talked earlier about how we have particles
and we have forces or quantum fields equivalently, and that's
a really successful way to describe the universe. You know,
we have the Large Hadron Collider to explore these things,
really high energies, and we've understood all sorts of things
(19:22):
using this theory. But that theory is used as quantum mechanics.
So the way we describe interactions, you know, the way
we talk about two electrons um repelling each other, or
the way lightning is formed or anything involves passing quantum
particles back and forth. And that's just not true for gravity.
What does that mean? Passing particles back and forth? Like
(19:42):
when like if I have two magnets and they're attracted
to each other, they're not. They're not It's not like
an invisible telekinesis pulling on each other. They're actually swapping
particles and I can't see that. Is that kind of
what you mean? That's exactly what I mean. Um that
the way two things interact via some four is by
exchanging particles. And so for example, electromagnetism, right, is the
(20:04):
force behind a magnet, And the way electromagnetism works, we
think at a sort of microscopic particle level, is that
there's a particle that transmits that force, that sends sort
of the information back and forth between two things that
are feeling it. And in the case of electromagnetism, that
particle is the photon. Right. The particle is also a
packet of light. So each of the quantum forces that
(20:26):
we talked about before, electromagnetism, the weak force and the
strong force, each of them have a particle we associate
with it. And that's not just like some name tag
we put on and say, hey, you get this one,
you get this one. We think that that's the particle
that's responsible for making the force work. So when two
electrons come near each other, how do they repel each other?
How does that actually happen? But we think that they
(20:49):
send photons out right, The electric field of a moving electron, right,
and accelerating electron generates photons, and those photons um interact
with the other chron's and so basically the passing messages
back and forth using these quantum particles. So gravity is
weird because we don't know that there is an quantum
(21:10):
particle being exchanged when two things get attracted gravitationally. That's right.
So we have this great framework. We say, oh, maybe
all forces are quantum mechanical fields interacting with each other. Right,
Let's apply that to the electromagnetic field. Yeah it works.
Let's apply that to the weak force. Yeah it works.
Let's applies to the strong force. Oh cool, it works.
Maybe this is something deep about the way the universe works.
(21:31):
Let's apply to gravity. Oh it doesn't work, right, So
what does that mean? What does it mean when I
say it doesn't work? Well? For a theory to work,
it has to provide predictions for experiments. You have to
be able to say, okay, theory, what would happen in
this configuration if I shot a proton and another particle.
Predict what would happen, and then you can go off
(21:53):
and do the experiments and compare it. Right. Well, when
you do that for gravity, you try to form a
quantum theory of gravity, it doesn't work. You get nonsense answers.
You can answers like infinity right, or things disappear, or
it just it doesn't mathematically function like there's no way
to build a theory of gravity that we've discovered so
far that works, that actually explains the way these things happen.
(22:17):
There are a few candidates out there there pretty far
from being a functional theory of quantum gravity. There's a
loop quantum gravity or string theory. But the basic problem
is that quantum mechanics and general relativity, which is our
best theory of gravity, do not play well together. And
we have no functioning quantum theory of gravity. So does
(22:37):
that mean that we don't have the right theory or
is that gravity is just not quantum in nature? That's
exactly the question we don't know the answer to. Right
In a hundred years from now, somebody will know the
answer that I hope, and they'll look back and they'll wonder,
you know, why do those guys see the clues but
we don't know. It could be that there is a
quantum theory gravity, we're just not smart enough to think
it up yet, right, Like the right person hasn't been
(22:59):
born yet to put the math together, or maybe it
requires a different kind of math that we're using. Right,
there's some assumption we're making that's a that's a mistake
or maybe just giving it a wrong name, like maybe
it should be or gravitinos grabby toss exactly. That's definitely
(23:19):
the problem. That's step number one when we made a mistake,
and step number one when we could define the particle um.
The other option, of course, is that maybe gravity is
not a quantum force the way the other forces are. Right,
the other forces we call them quantum forces because they're
well described by quantum mechanics, but gravity is kind of different.
I mean, the current theory we have a gravity general relativity.
(23:40):
It doesn't like to describe gravity as a force, right,
describes gravity instead as a bending of space. It says
that when you have mass somewhere in space, space no
longer becomes straight, becomes bent, so the things moving curves
and circles. And it's not like an actual just a
mathematical nuance where a mathematical perspe active. What's really confirms
(24:01):
it is the idea that gravity can affect things that
don't have mass. Right, That's how we know it's more
than just a force between things that have mass. It
actually like affects space for things that don't have mass. Right,
that's exactly right. So if you shoot a photon through
space that has mass nearby. The photon will not moving
what we consider to be a straight line, right, it
(24:23):
will find a path through this bent space that involves
basically curving. And this is what Einstein predicted with his theory,
and they and they saw it, you know. And you
can see in space it's called gravitational lensing. You can
see photons get bent by heavy objects. And it's because,
as you say, the heavy objects are bending space itself. Right,
It's not like gravity is pulling the photon because the
(24:44):
photon doesn't have any mass, right, that's right. The photon
doesn't have any mass, you know. So that's so that's
how it's different. Like gravity seems to affect things that
don't have sort of its fundamental property, you know, like
electromagantic forces can affect something that does not have an
electric charge. That gravity can affect things everything else. Right. Yeah,
that's a pretty deep insight there. I'm not bad for
(25:07):
a cartoonist at all. Yeah. Um, that's a fascinating to
think about. I think that's totally correct. Um. Yeah, And
so if gravity is instead of being a force, if
it's a way we change the shape of space itself,
then maybe that's why we don't have a quantum theory
of it. Right, And that's amazing and it's fantastic and
it's exciting um. And another reason why we have a
(25:29):
hard time bringing these two things together is that quantum mechanics,
the theory we've developed only works so far in flat space.
That is, if there's really heavy stuff nearby, we don't
know how to do those quantum calculations. We can basically
only do quantum mechanics in places where there isn't really
strong gravity. So wa, it's a quantum physics doesn't work
(25:50):
in reality basically, is that what you're saying, like, it
doesn't work in the space that we actually live in, Well,
it works basically everywhere except for those to black holes. Right.
You need to basically a black hole have enough gravity
to break down quantum mechanics, because it's when when space
gets really distorted that you start to see the effects
(26:11):
of gravity on space and and then it becomes comparable
to the strength of other stuff, and that's when that's
when quantum mechanics breaks down. Yeah, quantum quantum field theory
works basically what we call flat space, whereas gravity bends space.
So earlier when we categorize gravity as part of these
(26:32):
four fundamental forces, maybe that's just the wrong approach. Maybe,
you know, do you know what I mean? Like, maybe, um,
we shouldn't be categorizing these four things as is one
category of quote forces. That's right. It could be that
it could be that there is no quantum theory of
gravity as a fundamental force because it isn't one. Yeah,
and it's just a feature of space, right, Absolutely, that's
(26:53):
one possible explanation. But then we still need a way
to make quantum mechanics work in bent space, right, And
we still need understand how to make our theory of
general relativity play well with quantum mechanics, because we think
quantum mechanics describes the universe, right, and general relativity is
not a quantized theory. It's it's continuous, right. It treats
(27:13):
space and everything as if it's infinitely divisible. Right, it's
not a quantum theory all the fact that it came
about before quantum mechanics was even invented. And so while
the basic tenants of it how it distorts space are
probably correct, I been verified to zillion degrees of accuracy. Right,
it doesn't feel like it can be a fundamental description
of nature because it's not quantum mechanical. So like we
(27:35):
want to call it a force because it seems to
move things like all the other forces, but it's it.
Maybe it's not a force. Maybe it's just kind of
like a some other weird property of space. Yeah, exactly.
You know, maybe we've been trying to put a round
peg into a square hole all these years, a gravity
peg and a quantum hole. That's right, that's right. And
(27:56):
there are other ways that people are trying to solve
this problem. Like one way is thinking that maybe gravity
is a fundamental force, but it just works a little
bit differently from the other forces. For example, people think
about how the universe might have additional spatial dimensions, you know,
like instead of just being able to move in three
spain directions, maybe there's like four or five six dimensions
that you can move in. And folks who are interested
(28:17):
in that should listen to our podcast on extra dimensions. No, yeah,
we did a whole episode on extra dimensions, but we
didn't start of get into this particular topic. Um, so
tell us how extra dimensions might explain why gravity is
so weak. Yeah. The idea is that maybe gravity isn't
so weak. Maybe gravity is just as strong as all
the other forces. But if there's a whole other set
(28:38):
of dimensions out there, there's ways directions that think can move.
It might be that gravity is the only thing that
feels those dimensions, right. It might be that those dimensions
are invisible to electromagnetism and to the weak force into
the strong force, but visible to gravity. And what that
means is that gravity might be basically leaking out into
those other dimensions. Know, we talked about how the farther
(29:01):
your way you get from something, the weaker the forces.
So like Mercury feels the force of the Sun's gravity
much more strongly than Pluto does. Right, irrelevant of whether
or not you call it a planet, it doesn't feel
gravity very strongly. And that's because it's further from the Sun, right.
I mean that goes like one over our squared or
r as the distance. It's one of our squared because
we have three dimensions. If we had six dimensions, it
(29:23):
would be one over our five, right, which falls much
more rapidly. So if there are additional dimensions out there, okay,
and only gravity feels them, think that might be the
reason why gravitational force fall so quickly. Maybe gravity is
actually just as strong as everything else when you get
really really close. But then these extra dimensions exist, and
(29:44):
most of gravity leaks out into those other dimensions, sort
of like between you and me, there's not just the
three dimensions between you and me. May there are other
secret hidden spaces kind of between you and me or
these other dimensions exactly, other ways for gravity to spread out. Right,
So gravity would be like just as strong as all
(30:04):
the other forces, but it's just flexing its muscles in
these other spaces that we can't see or feel exactly.
It's like, you know, if somebody's at the center of
a crowd and they let go a really stinky far right.
The people next to them they smell it strongly, and
the people further away they smell it much more weakly,
and people outside don't smell it at all. Right, now,
imagine really suddenly, but let's let's let's go with it. Hey,
(30:27):
I'm trying to make this successible. You know, this is
something everybody going to appreciate. Trying to make it. And
but if there was somewhere else for that fart to go,
you know, if it could move not just sideways, but
also could float up right. So you had a really
tall room in the floor, fart floated up, then people
wouldn't feel it as much because most of the fart
would dissipate into the upper corners of the room. And
(30:50):
so gravity might be the same way. It might be that,
you know, for the first millimeter, so the first centimeter,
so gravity gets very weak, very quickly, it falls off
really wrap ly, and that then you know, a normal
distances like a meter or ten ms or whatever, you
don't feel those other dimensions anymore because other other dimensions
only activated really really short distances. This is the theory
(31:10):
people came up with, and we don't know if it's real.
You know, we've tested it so far. It seems like
gravity works the same way um for galactic scales and
for Earth scales, and for microscopic scales. It seems to
always fall off at the same rate as a function
of distance. So nobody's ever seen any evidence of these
extra dimensions. But it's a fascinating theory and it's a
you know, it's one that would give a kind of
(31:31):
a natural explanation for why gravity would fall off so
quickly and why gravity is so weak it wouldn't explain
all these other things. But people sort of try to
use gravity to see if there are other dimensions, right, Yeah,
that's right. It would be a really cool clue, right if.
And and that's a fascinating way that science has done.
You know, you try to look at everything around you
and see if you can fit it all into one framework,
(31:52):
like can I use this one set of ideas to
describe everything canto one part of concepts? That's right, and
my far theory of you in the universe. Um, The
best possible way I think to unravel this is to
actually go visit a black hole, because a quantum mechanics
(32:14):
and general relativity tell you very different things about what's
happening inside a black hole. Right, As we said before,
general relativity tells you it's an infinitesimal dot of of
almost infinite density. Quanto mechanics says, you know, the universe
is quantized first of all, so you can't have infinitesimal
dots um. And also this sort of a minimum size
to stuff, right, and you can't have all that stuff
(32:35):
compressed in such a tiny little area. And so if
you could see inside a black hole, you would learn
a lot about gravity. So what would be the plan.
You would go into a black hole. You would observe
and discover how the universe works, and then and then
you'd be stuck there. That's right. They would have to
send your Nobel Prize into the black hole. Act, which is,
assume you'd figure it out, and the Nobel Prize into
(32:57):
space into the black hole. Congratulate for anybody who's listening.
Please do not go into a black hole. Please, please
do not go into a black hole. But you know,
we don't need to visit black holes. We could try
to create them here on Earth. That sounds like a
great idea. Yeah, doesn that sounds like a great idea.
I mean I'm excited make a quality. Yeah, let's create
a black hole and study here. Right, Um, if gravity
(33:19):
gets really really powerful when you get two really short
distances because of this extra dimension theory, then it might
be that if you shoot two protons together really really
hard and they get really really close to each other,
that you can create a super duper mini extra cute,
little fuzzy black hole. Right. I'm trying to make it
sound like a cozy thing. Yeah, you're trying to sell
it and so um, before we turned on the Large
(33:45):
he collided about ten years ago, people thought maybe by
smashing these protons together we could actually create black holes
and we could study them. We can reveal the deep
secrets of gravity, right um. So, then the idea would
be to try to make them at the Large Hattern
Collider and just kind of see what happens, like, does
it tell us something about gravity or quantum physics at
the same time exactly by seeing how often they're made
(34:08):
and how strong they are and what they turned into
when they decay, we can understand something about the way
black holes work, and that would have been really powerful.
But unfortunately or fortunately, depending on how you feel about
black holes, we haven't made any black holes at the
Large Hadron Collider that we've discovered that you but maybe
isn't it true that maybe you've made them but they evaporate. Yes,
(34:29):
these black holes would be very short lived, but you know,
everything we make at the Large Hadron Collider is really
short lived. These things last we like tend the negative
thirty seconds or tend the negative twenty three seconds. We're
pretty good at seeing short lived stuff because it usually
blows up into other things. And a black hole would
have a really unusual signature in our detective it would
be pretty clear to see if we had made them. Okay,
but short of going into a black hole or detecting
(34:52):
farts and extra dimensions, we may not know in the
near future what what makes gravity so different? That's right,
it's going to take some work. I mean. The other direction,
is theoretical, is to build up a theory of quantum
gravity sort of from the bottom up, like like, start
from the beauty of math and physics and then try
to build it up to our level exactly. And that's
(35:13):
that's a wonderful way to do, is to say, like,
maybe the universe works in this way, that's most basic
fundamental nature, and build it up from there and see
if you can describe the universe that we see around us.
All right, Well, that's um, it's pretty shocking to think
gravity it's such a place, such a big role in
our lives, and yet it's it's like the the weakling
(35:35):
in the universe, right, It's like, imagine, imagine if if
gravity was stronger, life would beat a lot more chaotic,
right and crazy? Yeah, exactly. We would be closer to
the Sun and everything would feel more intense. It's fascinating
to me that gravity has been a mystery to physics
for hundreds of years. I mean, it was the focus
of Isaac Newton's studies, you know, like hundreds of years
ago people working on gravity. And still today, even though
(35:57):
we've made so much progress in terms of gravity, we
still have so much, so many basic questions about it
that we don't we don't know the answers to, not
even really beginning of how to answer them to meet.
That's fascinating. Gravity is such a rich source of mystery
for physics and for everybody. Wow, alright, cool, I think
it's maybe time to push down this question. Thanks for
(36:17):
joining us. If you still have a question after listening
to all these explanations, please drop us a line. We'd
love to hear from you. You can find us at Facebook, Twitter,
and Instagram at Daniel and Jorge That's one word, or
(36:38):
email us at Feedback at Daniel and Jorge dot com.
I just think it's fascinating that it's such a fundamental
force in the universe, right Like, it's it's basically the
thing that builds galaxies and keeps planets moving right and
gets structured to the entire cosmos. That's right on the
largest scales, actually the most important force. It's the reason
why things look the way they do, the reason why
(00:29):
our planet is around, it's the reason why we're on
the planet. It's pretty important, and yet we don't know
a lot about it, right, Like, there's some really deep
and strange mysteries about it. On one hand, we have
a theory which works really really well. On the other hand,
we have questions about it, but to seem really really basic.
And not only that, it's it's very different than all
(00:50):
the other forces of nature. That's right, one of these
things is not like the other ones. Ye hi am more.
(01:15):
I'm a cartoonist and I'm Daniel, I'm a particle physicist.
And this is our podcast Daniel and Joe explained the universe,
in which our cartoonist and physicists try to figure out
how to make the universe understandable to anybody. Yeah, and
today on the podcast we are examining a very heavy topic,
(01:39):
gravity and specifically why is gravity so weak and strange. Gravity,
as we said earlier, is something which controls the structure
of the universe. I mean, the reason the Solar system
looks the way it does is because of gravity. The
reason the Earth is round is because of gravity. The
reason we have galaxies is because of gravity. The reason
(02:00):
we weighed so much, it's it's because of gravity. Right,
it's not my belt, No, that's because of late night
cake eating. Um. But it's such a fundamental force of nature. Rights, Like,
it's present in our everyday live We spend a lot
of time thinking about gravity, right, how not to fall down,
(02:20):
how not drop things, how to go up buildings, how
to go down buildings? Right, that's right. It seems like
one of the most important forces. I mean, if you
ask people, you know, to name a force or what
kind of forces the experience in their life, gravity is
the one that's present in their lives. Right, you're climbing upstairs,
you're fighting gravity. Trip, you fall down, you're feeling gravity.
You look around you. The shape of things is controlled
(02:41):
by gravity, and that's why it's particularly strange that gravity
is the weakest force of all the forces we've discovered,
it's by far the weakest. Yeah, it's really strange to
hear you say that, Like, how can gravity be weak?
Like you know, like it's it's keeping the whole Earth together,
it's making the entire higher planet swing around, going to
(03:02):
circle basically, right. Without gravity, we would just shoot off
into space. That's right. It's a really strange situation. And
there's other things about gravity we don't understand as well.
It's really strange. It doesn't play well with the other forces.
It's very, very weak. It's a total mystery to science,
except that we have a theory which works beautifully. Right.
We can calculate exactly how mercury orbits the Sun. We
(03:23):
can send things into outer space and note with two
millimeter precision exactly where they're going to land. We have
a working theory that we can use, right, but we
don't understand it on a conceptual level. We have these basic,
deep questions about about what gravity is and how the
universe works because of it. So it's a weird question,
and maybe one of the people hadn't thought about before.
So Daniel went out as usual and ask people on
(03:45):
the street, why do you think gravity is so weak.
Here's what a random selection of folks who are willing
to talk to me on a Tuesday morning had to
say about gravity. I don't know. I should don't know
about that, all right, I was. I was thought it
was a pretty strong force. So I don't know. But yeah, because, um,
it depends on the distance and it's long range one,
(04:06):
so that's why we feel it's very weak most of
the time. Cool. No, okay, I have no I'm sorry,
it's not very fruitful. I have no idea, but I'd
be interested in finding out why. All right, that wasn't
that was pretty good. Most people weren't surprised when you
said gravity is weak. I don't know. I feel like
(04:27):
of all the questions I've asked people, this is the
one that flams them the most. You know. People were like, what,
I have no idea, or um, they had crazy ideas
why gravity must must be weak. I feel like, usually
we get one person who knows what the answer is
or has a good clue about what what's going on.
This time, I feel like almost everybody was pretty clueless.
I mean, one person said I always thought gravity was
(04:48):
pretty strong, right, which kind of sums up the situation. Right.
Gravity is omnipresent in our lives. It dominates our experience,
and yet it's so weak compared to the other really
powerful forces we've discovered. Well of people, A couple of
answers were that had to do with distance, Like, gravity
gets really weak with distance, that's right. And the problem
there is that all the forces get weak with distance,
(05:09):
like electromagnetism also falls as a as a distance grows. Right,
So all of these forces followed this one over our
squared rule or are as your distance from the thing
that's that's giving you the force, right, maybe maybe right? Maybe? Yeah,
mostly we think and uh, and so that can't be
the answer, right, because all the other forces have that
(05:29):
same feature. So when you say it's the weak is
it's not that it changes over distances differently than the
other forces. That's right. So maybe we should talk about
what the forces are and compare them to each other.
So focus and get understanding of how crazy weak gravity is. Right. So, Daniel,
what are the forces of nature besides a bad movie
with Ben Affleck in center Bullock? Well, I think comedy.
(05:53):
Comedy is definitely force of nature. You know, it solves
big problems around the world. You know. The fundamental forces
are electromagnetism, right, that's the one that controls electricity and
magnetism obviously and is responsible for the cool things like
light and lightning and all that all that cool stuff. Um.
And then there's the weak nuclear force, which is a
(06:15):
force which is responsible for radioactive decay of a nuclei. Right.
And the cool thing about electricity and magnetism and the
weak nuclear force is that we actually have shown that
there are two sides of the same coin as a particle. Physicists,
we refer to them as one force. We call it
the electro week. So sort of magnetism lost out there
in the name merger, right, it should be electro magnetic week.
(06:36):
But nobody voted to keep magnetism in the the name
of the partners on the law firm. Nobody, nobody lobbied
for weak electro or magneto weak force. Yeah. Yeah, again,
we are suffering the fate of some anonymous committee of
scientists that get to name these things. Right, who are
these people? It's probably some grad student, right or some
(06:58):
you know, like this is really weird call it this. Yeah.
So we have electricity and magnetism, which is a single force.
We have the weak nuclear force, which is really should
be combined with electricity magnetism. And then there's the strong
nuclear force, and this is the one that holds the
nucleus together. You know, the nucleus is just a bunch
of positively charged protons and neutral neutrons, right, so it's
(07:19):
only positively charged particles in the nucleus. So you might think,
what it even holds the nucleus together, right, you have
all this positively charged stuff should be repelling themselves. Well,
it's the strong nuclear force, and it does so by
exchanging these crazy little particles we call gluons, and that
holds the nucleus together, and it's pretty strong. It's even
stronger than electromagnetism. Well, let's take a step back. So
(07:40):
in the universe there's stuff. There's like, yes, affirm that
there is stuff in without reservation, there is stuff. I'm
glad we saw that question. But I mean it's like
there's stuff that has substance to it, that has masked
to it, or you know that it sort of exists.
And then there's also besides the at how these things
(08:01):
interact with each other, like how they affect each other
that's right. There's the matter and then there's the forces. Right.
The forces affect how they interact with each other, and
that's pretty much the universe. That's that's like, it's matter
and forces. Yeah. One way to look at the universe
is that it's particles, right, or you would say matter
and their forces. In modern particle physics we think about
one level deeper, which if we think of quantum fields,
(08:23):
and quantum fields are responsible both for matter and for forces.
So we can talk about that maybe in another podcast.
What is a quantum field and how can I get one?
You know for leaser rent um? What can they do
for me? But yeah, I think it's it's fair still
to think about the universe in terms of particles and forces.
On that note, let's take a quick break. There are
(08:56):
only four kinds of forces. Yeah, there are four kinds
of forces. So after magnetism, weak nuclear force, strong nuclear force,
and then of course gravity, right, that's the fourth force
that we've discovered. The fascinating thing is that different particles
feel different forces, right, Like, some particles feel this set
of forces, some particles feel those set of forces. For example, right,
(09:17):
particles with electric charge feel electromagnetism. Right. The electron, for example,
is negatively charged, the proton is positively charged. You bring
them close together, they're gonna pull on each other. They're
gonna suck each other together, right, because they have opposite charges.
We all know that, um. But you bring a neutral
particle nearby, it just totally ignores it. Right, It doesn't
(09:37):
feel it at all. Right, It's like it's like somebody's
walking through a crowd of people shouting, but they have
headphones on so they can't hear anything, and they're totally
oblivious to it. It's kind of like how we talked
about in a previous podcast. They're almost like languages or
like social media platforms. Like some people are on Twitter,
some people are on Facebook, but some people are not
on this. And so if somebody if you're not on
(09:58):
Twitter and so many sense to your tweet, you're not
going to get it. And so it's just different ways
the particles in right, that's right. Gravity is the Google
plus social media right because nobody uses the trendstor it's
ancient but powerless. Yeah, And so different particles feel different forces.
And for example, an electron, while it feels electromagnetism. Because
(10:20):
it has a negative charge, it doesn't feel a strong
force at all. It will pass right by a bunch
of particles that are really tugging on each other with
a strong force and not be affected at all. Whereas corks.
Corks feel all the forces. They feel a strong force,
which is how they get pulled together in the nucleus. Remember,
protons and neutrons are made of quarks. Quarks feel electromagnetism
(10:41):
because they have electric charge. They feel the weak force.
They also feel gravity, of course, because they have mass.
So quarks get their fingers in everything they feel. They
get the feels for everything. They feel everything's right, They
got the strong feels. Course, they're really deeply emotional part
of um. And on the other side of the spectrum,
you've got things like neutrinos. Patrinos don't have electric charge,
(11:04):
so they ignore all electricity magnetism, right, They don't interact
with light. They're invisible. They pass right through anything that that.
They ignore electro magnetic bonds, so they pass through most materials.
They don't feel the strong force. The only way they
interact is with the weak force, and the weak force
is pretty weak, which is why neutrinos can mostly just
pass through matter unaffected. So we have four fundamental forces, right,
(11:31):
and gravity is one of these forces. And so when
you say that antravity is weak, you actually mean it's
weak compared to these other three forces, that's right. And
so the ranking is the strong force is the strongest,
so that one is actually well named. Congratulations, you know,
anonymous group of scientists. Yeah, yeah, we should be called
the as currently known to be the strongest force force. Um.
(11:55):
Right after that comes electromagnetism, and you know, we know
that force is pretty powerful. You stick your finger in
a socket, you're going to feel the wrath of electromagnetism, right.
It's it's not an unfamiliar feeling, right, right to think
your finger in anything? You feel it, right, because it's
electromagnetism is the force that keeps you from basically passing
through the table or passing through your car. Right, that's right.
(12:17):
Because electromagnetism is the basis of chemical bonds, right. And
chemical bonds are really the thing that form the structure
of your body. Right. You think if your body is
like a bunch of particles, but it's held together by
all these forces, it's like a chain link fence binding
together these little particles and prevents you from passing through
something else. Yeah, so we got the strong force, and
then electromagnetism, and then actually the weak nuclear force. Right,
(12:40):
this is the force that like powers neutrinos and radioactive decay.
It's much weaker than electromagnetism um and much weaker than
the strong force. Even weaker than the weak is gravity.
That's right. If you make a list like strong force, electromagnetism,
the weak force, then you should leave like a hundred
blank pages and then you get to grab because when
(13:00):
we compare these forces, we put things like an equal
distance apart and compare the strength of the forces. Gravity
is ten to the thirty six times weaker than the
weak force. But that's ten with thirty six zeros in
front of it. But it isn't that sort of a
matter of units or scale, do you know what I mean? Like,
it's much weaker, but only if you compare apples to apples, right,
(13:25):
or orange to oranges. That's right. But put two protons
next to each other, right, Two protons have a certain
amount of mass and a certain amount of electric charge,
and the force of their charges is going to be
much much stronger than the force from their masses. So yeah,
if everything was much much more massive, then there would
be stronger gravity. But you can compare these things apples
to apples by comparing them, you know, the same distance
(13:46):
and the same basic unit of interaction. Right, But what
if you take an apple put it next to another apple? Well,
I think you can do that experiment. Nothing's going to
happen because gravity is so weak. Right. You don't see
two apples like pulling themselves together on the counter, right,
then they'll alden apple collider um. You know, the apples
are not drawn to each other. Gravity is a super
weak force, and you can see this yourself. Right, you
(14:07):
can do an experiment where you counter the entire gravitational
force of an enormous celestial body like the Earth. Right,
take a small kitchen magnet and use it to hold
up a nail and think about what's happening there. Right,
you have the nail is being pulled down by every
single rock in the Earth. It's pulling with all of
its gravity. But a tiny little kitchen magnet totally overcomes that.
(14:31):
It can lift the nail even though it's pulling. It's
being pulled down by the whole entire planet Earth, right exactly. Now,
imagine a magnet the size of the Earth, right, I
mean that would be that would be extraordinarily powerful. And
so you have basically like a gravitational blob the size
of the Earth. Still pretty ineffective compared to electromagnetism. So
(14:53):
so it's weak if you sort of compare it by object,
Like you said, if you take a proton and put
it next to proton, the the force are going to
feel from electro magnetism is so much bigger than the
force of gravity. They're going to feel towards each other
the same with like two electrons or two cords and things.
So in the scale of like the particles that we know,
(15:15):
it's a really weak force, that's right, exactly. And yet
and yet it seems to dominate. Right. That's a bit
of a puzzle. Like, on one hand, it's super duper weak,
and we're telling you that it hardly counts for anything.
On the other hand, it's responsible for the structure of
the Solar System, man for the galaxy, and it's the
reason the universe looks the way it is, right, And
so that can be confusing to people, Like how do
(15:35):
you reconcile those two things in your head? Yeah, Like
why doesn't the Earth feel an electromagnetic force with the Sun,
which it would be so much bigger than the force
of gravity. Yeah, what, it would be pretty shocking. And
and that's actually the reason is um gravity is different
from the other forces and that it can't be canceled out. Right,
if there was some huge electrostatic difference between the Sun
(15:56):
and the Earth, like a bunch of positive charges there
and a bunch of negative charges year, it would create
such an enormous force that it would be very quickly balanced.
Like that's what lightning is, right, When there's a charge
differential between clouds and the ground, it's it doesn't take
that much before those charges want to rearrange themselves to
a lower energy configuration. They rushed down to the ground,
(16:18):
or they rushed up to the clouds and they jump
to balance themselves up. Because you have two kinds of charges,
you have positive and you have negative, so you can
find an arrangement where basically everybody is happy. It's an equilibrium, right,
But that's not true for gravity, Okay, I get it.
So for example, if the Earth was every particle on
Earth had a positive electromagnetic charge and every particle in
(16:40):
the Sun had a negative electromagnetic charge, there would be
a humongous pool from electromagnetism pulling the Earth into the Sun. Yeah,
we'd be toasted pretty quick. Yeah, I would be huge.
Even the opposite if we were all positive and the
Sun was all positive, we would get shot out of
the solar stem very quickly, that's right. And that's why
(17:02):
you know, early days of the Solar system being formed,
you have these gases and the gas and dust coalescing,
and very rapidly things neutralize, right, because anything that feels
an electrostatic force to something else is going to find
the opposite charge and they're going to coalesce and they're
gonna make something neutral. Right. That's why most of the
things around you are neutral, Right, Most of the elements
(17:23):
are neutral, because any deviation from neutral results in a
powerful force to neutralize it. So, thankfully the Earth is
made out of both like equal amounts of positive and
negative particles, right, that's right. Thankfully we're sort of balanced electromagnetically,
and so even if the Sun was all positive, we
would look like neutral, like a neutral ball to to
(17:45):
the sun. Yeah, that's right, we're on large scales. The
Earth is neutral, right, I mean there might be some
residual positive or negative charge depending on the solar wind, etcetera.
But basically the Earth is neutral, and so the largest
force of the Earth feels is the gravity in the sun,
even though gravity is super duper weak. Right, it doesn't
take a lot to counteract gravity. But it's the only
(18:05):
player left because everybody else is sort of pair it
up and danced off for the night, and gravity is
just there left holding the bag. And gravity can't be balanced, right.
You feel gravity if you have any mass, right, But
there's only positive masses and no such thing as a
negative mass to give anti gravity. Well, let's keep going,
but first let's take a quick break. Okay, So that's
(18:40):
how gravity is so much weaker than the other forces. Um,
so how's it different than the other three forces of nature?
There's like no end to to weys, the gravity is weird,
you know, there's no end to like the puzzles of
Gravity's fascinating bottomless pit. That's right, it's a black hole
of questions. Um. And one of my favorites is just
(19:01):
that we have no way to sort of fit gravity
in with the way the universe works according to everything else.
You know, we talked earlier about how we have particles
and we have forces or quantum fields equivalently, and that's
a really successful way to describe the universe. You know,
we have the Large Hadron Collider to explore these things,
really high energies, and we've understood all sorts of things
(19:22):
using this theory. But that theory is used as quantum mechanics.
So the way we describe interactions, you know, the way
we talk about two electrons um repelling each other, or
the way lightning is formed or anything involves passing quantum
particles back and forth. And that's just not true for gravity.
What does that mean? Passing particles back and forth? Like
(19:42):
when like if I have two magnets and they're attracted
to each other, they're not. They're not It's not like
an invisible telekinesis pulling on each other. They're actually swapping
particles and I can't see that. Is that kind of
what you mean? That's exactly what I mean. Um that
the way two things interact via some four is by
exchanging particles. And so for example, electromagnetism, right, is the
(20:04):
force behind a magnet, And the way electromagnetism works, we
think at a sort of microscopic particle level, is that
there's a particle that transmits that force, that sends sort
of the information back and forth between two things that
are feeling it. And in the case of electromagnetism, that
particle is the photon. Right. The particle is also a
packet of light. So each of the quantum forces that
(20:26):
we talked about before, electromagnetism, the weak force and the
strong force, each of them have a particle we associate
with it. And that's not just like some name tag
we put on and say, hey, you get this one,
you get this one. We think that that's the particle
that's responsible for making the force work. So when two
electrons come near each other, how do they repel each other?
How does that actually happen? But we think that they
(20:49):
send photons out right, The electric field of a moving electron, right,
and accelerating electron generates photons, and those photons um interact
with the other chron's and so basically the passing messages
back and forth using these quantum particles. So gravity is
weird because we don't know that there is an quantum
(21:10):
particle being exchanged when two things get attracted gravitationally. That's right.
So we have this great framework. We say, oh, maybe
all forces are quantum mechanical fields interacting with each other. Right,
Let's apply that to the electromagnetic field. Yeah it works.
Let's apply that to the weak force. Yeah it works.
Let's applies to the strong force. Oh cool, it works.
Maybe this is something deep about the way the universe works.
(21:31):
Let's apply to gravity. Oh it doesn't work, right, So
what does that mean? What does it mean when I
say it doesn't work? Well? For a theory to work,
it has to provide predictions for experiments. You have to
be able to say, okay, theory, what would happen in
this configuration if I shot a proton and another particle.
Predict what would happen, and then you can go off
(21:53):
and do the experiments and compare it. Right. Well, when
you do that for gravity, you try to form a
quantum theory of gravity, it doesn't work. You get nonsense answers.
You can answers like infinity right, or things disappear, or
it just it doesn't mathematically function like there's no way
to build a theory of gravity that we've discovered so
far that works, that actually explains the way these things happen.
(22:17):
There are a few candidates out there there pretty far
from being a functional theory of quantum gravity. There's a
loop quantum gravity or string theory. But the basic problem
is that quantum mechanics and general relativity, which is our
best theory of gravity, do not play well together. And
we have no functioning quantum theory of gravity. So does
(22:37):
that mean that we don't have the right theory or
is that gravity is just not quantum in nature? That's
exactly the question we don't know the answer to. Right
In a hundred years from now, somebody will know the
answer that I hope, and they'll look back and they'll wonder,
you know, why do those guys see the clues but
we don't know. It could be that there is a
quantum theory gravity, we're just not smart enough to think
it up yet, right, Like the right person hasn't been
(22:59):
born yet to put the math together, or maybe it
requires a different kind of math that we're using. Right,
there's some assumption we're making that's a that's a mistake
or maybe just giving it a wrong name, like maybe
it should be or gravitinos grabby toss exactly. That's definitely
(23:19):
the problem. That's step number one when we made a mistake,
and step number one when we could define the particle um.
The other option, of course, is that maybe gravity is
not a quantum force the way the other forces are. Right,
the other forces we call them quantum forces because they're
well described by quantum mechanics, but gravity is kind of different.
I mean, the current theory we have a gravity general relativity.
(23:40):
It doesn't like to describe gravity as a force, right,
describes gravity instead as a bending of space. It says
that when you have mass somewhere in space, space no
longer becomes straight, becomes bent, so the things moving curves
and circles. And it's not like an actual just a
mathematical nuance where a mathematical perspe active. What's really confirms
(24:01):
it is the idea that gravity can affect things that
don't have mass. Right, That's how we know it's more
than just a force between things that have mass. It
actually like affects space for things that don't have mass. Right,
that's exactly right. So if you shoot a photon through
space that has mass nearby. The photon will not moving
what we consider to be a straight line, right, it
(24:23):
will find a path through this bent space that involves
basically curving. And this is what Einstein predicted with his theory,
and they and they saw it, you know. And you
can see in space it's called gravitational lensing. You can
see photons get bent by heavy objects. And it's because,
as you say, the heavy objects are bending space itself. Right,
It's not like gravity is pulling the photon because the
(24:44):
photon doesn't have any mass, right, that's right. The photon
doesn't have any mass, you know. So that's so that's
how it's different. Like gravity seems to affect things that
don't have sort of its fundamental property, you know, like
electromagantic forces can affect something that does not have an
electric charge. That gravity can affect things everything else. Right. Yeah,
that's a pretty deep insight there. I'm not bad for
(25:07):
a cartoonist at all. Yeah. Um, that's a fascinating to
think about. I think that's totally correct. Um. Yeah, And
so if gravity is instead of being a force, if
it's a way we change the shape of space itself,
then maybe that's why we don't have a quantum theory
of it. Right, And that's amazing and it's fantastic and
it's exciting um. And another reason why we have a
(25:29):
hard time bringing these two things together is that quantum mechanics,
the theory we've developed only works so far in flat space.
That is, if there's really heavy stuff nearby, we don't
know how to do those quantum calculations. We can basically
only do quantum mechanics in places where there isn't really
strong gravity. So wa, it's a quantum physics doesn't work
(25:50):
in reality basically, is that what you're saying, like, it
doesn't work in the space that we actually live in, Well,
it works basically everywhere except for those to black holes. Right.
You need to basically a black hole have enough gravity
to break down quantum mechanics, because it's when when space
gets really distorted that you start to see the effects
(26:11):
of gravity on space and and then it becomes comparable
to the strength of other stuff, and that's when that's
when quantum mechanics breaks down. Yeah, quantum quantum field theory
works basically what we call flat space, whereas gravity bends space.
So earlier when we categorize gravity as part of these
(26:32):
four fundamental forces, maybe that's just the wrong approach. Maybe,
you know, do you know what I mean? Like, maybe, um,
we shouldn't be categorizing these four things as is one
category of quote forces. That's right. It could be that
it could be that there is no quantum theory of
gravity as a fundamental force because it isn't one. Yeah,
and it's just a feature of space, right, Absolutely, that's
(26:53):
one possible explanation. But then we still need a way
to make quantum mechanics work in bent space, right, And
we still need understand how to make our theory of
general relativity play well with quantum mechanics, because we think
quantum mechanics describes the universe, right, and general relativity is
not a quantized theory. It's it's continuous, right. It treats
(27:13):
space and everything as if it's infinitely divisible. Right, it's
not a quantum theory all the fact that it came
about before quantum mechanics was even invented. And so while
the basic tenants of it how it distorts space are
probably correct, I been verified to zillion degrees of accuracy. Right,
it doesn't feel like it can be a fundamental description
of nature because it's not quantum mechanical. So like we
(27:35):
want to call it a force because it seems to
move things like all the other forces, but it's it.
Maybe it's not a force. Maybe it's just kind of
like a some other weird property of space. Yeah, exactly.
You know, maybe we've been trying to put a round
peg into a square hole all these years, a gravity
peg and a quantum hole. That's right, that's right. And
(27:56):
there are other ways that people are trying to solve
this problem. Like one way is thinking that maybe gravity
is a fundamental force, but it just works a little
bit differently from the other forces. For example, people think
about how the universe might have additional spatial dimensions, you know,
like instead of just being able to move in three
spain directions, maybe there's like four or five six dimensions
that you can move in. And folks who are interested
(28:17):
in that should listen to our podcast on extra dimensions. No, yeah,
we did a whole episode on extra dimensions, but we
didn't start of get into this particular topic. Um, so
tell us how extra dimensions might explain why gravity is
so weak. Yeah. The idea is that maybe gravity isn't
so weak. Maybe gravity is just as strong as all
the other forces. But if there's a whole other set
(28:38):
of dimensions out there, there's ways directions that think can move.
It might be that gravity is the only thing that
feels those dimensions, right. It might be that those dimensions
are invisible to electromagnetism and to the weak force into
the strong force, but visible to gravity. And what that
means is that gravity might be basically leaking out into
those other dimensions. Know, we talked about how the farther
(29:01):
your way you get from something, the weaker the forces.
So like Mercury feels the force of the Sun's gravity
much more strongly than Pluto does. Right, irrelevant of whether
or not you call it a planet, it doesn't feel
gravity very strongly. And that's because it's further from the Sun, right.
I mean that goes like one over our squared or
r as the distance. It's one of our squared because
we have three dimensions. If we had six dimensions, it
(29:23):
would be one over our five, right, which falls much
more rapidly. So if there are additional dimensions out there, okay,
and only gravity feels them, think that might be the
reason why gravitational force fall so quickly. Maybe gravity is
actually just as strong as everything else when you get
really really close. But then these extra dimensions exist, and
(29:44):
most of gravity leaks out into those other dimensions, sort
of like between you and me, there's not just the
three dimensions between you and me. May there are other
secret hidden spaces kind of between you and me or
these other dimensions exactly, other ways for gravity to spread out. Right,
So gravity would be like just as strong as all
(30:04):
the other forces, but it's just flexing its muscles in
these other spaces that we can't see or feel exactly.
It's like, you know, if somebody's at the center of
a crowd and they let go a really stinky far right.
The people next to them they smell it strongly, and
the people further away they smell it much more weakly,
and people outside don't smell it at all. Right, now,
imagine really suddenly, but let's let's let's go with it. Hey,
(30:27):
I'm trying to make this successible. You know, this is
something everybody going to appreciate. Trying to make it. And
but if there was somewhere else for that fart to go,
you know, if it could move not just sideways, but
also could float up right. So you had a really
tall room in the floor, fart floated up, then people
wouldn't feel it as much because most of the fart
would dissipate into the upper corners of the room. And
(30:50):
so gravity might be the same way. It might be that,
you know, for the first millimeter, so the first centimeter,
so gravity gets very weak, very quickly, it falls off
really wrap ly, and that then you know, a normal
distances like a meter or ten ms or whatever, you
don't feel those other dimensions anymore because other other dimensions
only activated really really short distances. This is the theory
(31:10):
people came up with, and we don't know if it's real.
You know, we've tested it so far. It seems like
gravity works the same way um for galactic scales and
for Earth scales, and for microscopic scales. It seems to
always fall off at the same rate as a function
of distance. So nobody's ever seen any evidence of these
extra dimensions. But it's a fascinating theory and it's a
you know, it's one that would give a kind of
(31:31):
a natural explanation for why gravity would fall off so
quickly and why gravity is so weak it wouldn't explain
all these other things. But people sort of try to
use gravity to see if there are other dimensions, right, Yeah,
that's right. It would be a really cool clue, right if.
And and that's a fascinating way that science has done.
You know, you try to look at everything around you
and see if you can fit it all into one framework,
(31:52):
like can I use this one set of ideas to
describe everything canto one part of concepts? That's right, and
my far theory of you in the universe. Um, The
best possible way I think to unravel this is to
actually go visit a black hole, because a quantum mechanics
(32:14):
and general relativity tell you very different things about what's
happening inside a black hole. Right, As we said before,
general relativity tells you it's an infinitesimal dot of of
almost infinite density. Quanto mechanics says, you know, the universe
is quantized first of all, so you can't have infinitesimal
dots um. And also this sort of a minimum size
to stuff, right, and you can't have all that stuff
(32:35):
compressed in such a tiny little area. And so if
you could see inside a black hole, you would learn
a lot about gravity. So what would be the plan.
You would go into a black hole. You would observe
and discover how the universe works, and then and then
you'd be stuck there. That's right. They would have to
send your Nobel Prize into the black hole. Act, which is,
assume you'd figure it out, and the Nobel Prize into
(32:57):
space into the black hole. Congratulate for anybody who's listening.
Please do not go into a black hole. Please, please
do not go into a black hole. But you know,
we don't need to visit black holes. We could try
to create them here on Earth. That sounds like a
great idea. Yeah, doesn that sounds like a great idea.
I mean I'm excited make a quality. Yeah, let's create
a black hole and study here. Right, Um, if gravity
(33:19):
gets really really powerful when you get two really short
distances because of this extra dimension theory, then it might
be that if you shoot two protons together really really
hard and they get really really close to each other,
that you can create a super duper mini extra cute,
little fuzzy black hole. Right. I'm trying to make it
sound like a cozy thing. Yeah, you're trying to sell
it and so um, before we turned on the Large
(33:45):
he collided about ten years ago, people thought maybe by
smashing these protons together we could actually create black holes
and we could study them. We can reveal the deep
secrets of gravity, right um. So, then the idea would
be to try to make them at the Large Hattern
Collider and just kind of see what happens, like, does
it tell us something about gravity or quantum physics at
the same time exactly by seeing how often they're made
(34:08):
and how strong they are and what they turned into
when they decay, we can understand something about the way
black holes work, and that would have been really powerful.
But unfortunately or fortunately, depending on how you feel about
black holes, we haven't made any black holes at the
Large Hadron Collider that we've discovered that you but maybe
isn't it true that maybe you've made them but they evaporate. Yes,
(34:29):
these black holes would be very short lived, but you know,
everything we make at the Large Hadron Collider is really
short lived. These things last we like tend the negative
thirty seconds or tend the negative twenty three seconds. We're
pretty good at seeing short lived stuff because it usually
blows up into other things. And a black hole would
have a really unusual signature in our detective it would
be pretty clear to see if we had made them. Okay,
but short of going into a black hole or detecting
(34:52):
farts and extra dimensions, we may not know in the
near future what what makes gravity so different? That's right,
it's going to take some work. I mean. The other direction,
is theoretical, is to build up a theory of quantum
gravity sort of from the bottom up, like like, start
from the beauty of math and physics and then try
to build it up to our level exactly. And that's
(35:13):
that's a wonderful way to do, is to say, like,
maybe the universe works in this way, that's most basic
fundamental nature, and build it up from there and see
if you can describe the universe that we see around us.
All right, Well, that's um, it's pretty shocking to think
gravity it's such a place, such a big role in
our lives, and yet it's it's like the the weakling
(35:35):
in the universe, right, It's like, imagine, imagine if if
gravity was stronger, life would beat a lot more chaotic,
right and crazy? Yeah, exactly. We would be closer to
the Sun and everything would feel more intense. It's fascinating
to me that gravity has been a mystery to physics
for hundreds of years. I mean, it was the focus
of Isaac Newton's studies, you know, like hundreds of years
ago people working on gravity. And still today, even though
(35:57):
we've made so much progress in terms of gravity, we
still have so much, so many basic questions about it
that we don't we don't know the answers to, not
even really beginning of how to answer them to meet.
That's fascinating. Gravity is such a rich source of mystery
for physics and for everybody. Wow, alright, cool, I think
it's maybe time to push down this question. Thanks for
(36:17):
joining us. If you still have a question after listening
to all these explanations, please drop us a line. We'd
love to hear from you. You can find us at Facebook, Twitter,
and Instagram at Daniel and Jorge That's one word, or
(36:38):
email us at Feedback at Daniel and Jorge dot com.
Hosts And Creators
Daniel Whiteson
Kelly Weinersmith
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