
The Greatest Unsolved Problem in Theoretical Physics: Why Gravity Is So Weak - signa11
http://www.forbes.com/sites/startswithabang/2015/12/11/the-greatest-unsolved-problem-in-theoretical-physics-why-gravity-is-so-weak/
======
bd
I remember reading a comment somewhere on the web that made it "click" for me:

 _" Gravity is so weak that force of a tiny fridge magnet is able to counter
gravity pull of the whole planet"._

~~~
DennisP
Yes but on average, the mass of the planet is 4000 miles away. Put the magnet
4000 miles away and see how much pull you get.

~~~
light_hue_1
That's a confusing way to look at things. Lets make the two scenarios more
similar.

Say "earth" is a sphere with radius 1 meter and you're 1 meter away. And that
puny magnet defeats its gravitational pull in the same way it defeats earth.
How heavy would our sphere have to be in order to have the same pull as earth
if it has a radius of 1 meter and our magnet is 1 meter away?

If you remember Newton's law of gravitation the force an object will feel
looks like: F=Gm/r^2. G is small, 10^-11 Nm^2/kg^2, m is the mass of earth
10^24 kg, r is normally 10^6 m (the radius of the earth). Plug in these
numbers you get 10 m/s^2. The real answer is closer to 9.8, but we're looking
for the order of magnitude here. We're still remarkably close for how much
rounding we did.

How lets say that we're now 1 meter away. How heavy would the mass have to be
to still pull with 10m/s^2, which we know our magnet can defeat. (10^6)^2 ~>
10^12 so we have to be 10^24/10^12 ~> 10^12 kg.

So to defeat a tiny magnet 1 meter away you need 10^12kg. Lets pretend our
mass is a cube whose volume is 1m^3. That's 10^12kg/m^3. The center of the sun
has a density of 10^5kg/m^3. This is 10^7 times denser! We're lucky that our
sphere is far too light by many orders of magnitude to collapse into a black
hole (maybe not so lucky because it's going to explode immediately!) but it's
even denser than a white dwarf. This is on the order of the mass of Mt.
Everest (this is a very rough and unprincipled comparison and when you unpack
it can mean many different things, but it's something easy to visualize).

So you can see. Gravity is indeed insanely weak. A magnet can defeat Mt.
Everest.

~~~
jsprogrammer
This is much more confusing than the previous, one-line view.

~~~
Dylan16807
That the comment as a whole is confusing doesn't matter.

The point is that you should use something human-scale to show off the
strength of gravity.

So if you want a fridge-sized block that can hold things with gravity like a
normal fridge holds a magnet, how heavy does it have to be? As heavy as an
entire mountain.

------
wodenokoto
I don't understand why this is a problem.

It might sound silly, but isn't it like asking why blue is blue?

If Gravity is a fundamental force, isn't it like a mathematical axiom? We
can't really prove or explain it, it's just the basis for all the other rules.

~~~
chriswarbo
Electricity and magnetism used to be considered separate phenomena, but then
they were found to be two aspects of a single electromagnetic force.

Electromagnetism and the weak force used to be considered separate phenomena,
but then they were found to be two aspects of a single electroweak force.

Progress can't be made if we just accept everything as-is and don't
investigate more deeply.

~~~
noir_lord
Agreed, and the interesting fundamental thing we discover this year sometimes
has practical applications 30 years down the line.

The semi-conductor wouldn't have existed except for such fundamental work in
prior decades.

I dislike the "well, that's just the way it is" response to our not
understanding something (as do fortunately the smarter people than me
scientists!).

------
adekok
A related issue is "If electricity and magnetism are so strong, why do we feel
the effects of gravity more?"

To see why this is the case, let's look at magnetism first. Every magnet is
composed of a north pole and a south pole. If you split a magnet in two, you
get two magnets, each with a north pole and a south pole. You _never_ get a
magnet with more "north" charges than "south".

This means that the "north" and "south" portions of the field right are always
next to each other. Which also means that when you go very far from a magnet,
the _net_ effect of the magnet is zero. Because the two fields cancel each
other out.

So we should expect that magnetism doesn't have many practical impacts on our
daily life. If you're close to a magnet, you might feel it. But if you're far
away, you won't be able to feel a magnetic field.

Electricity is a bit different. The positive and negative charges can be
separated from one another, unlike magnetism.

This means that when electrical charges are separated, there is a force
pulling them together for differing charges, or separating them for similar
charges. The net effect over time is that most objects should be electrically
neutral. Because the charges will naturally come together.

So we should expect that electricity has an impact on our daily life. We
should be able to tell when the charges are separated. This occurs only for a
short time, until the charges move and cancel each other out.

Gravity is different again. There is no "positive" and "negative" charges with
gravity. It's _all positive_.

This means that when two gravitational bodies are separated, there will be a
force pulling them together. The net effect is that all objects with
"gravitational charge" will tend to clump together.

So we should expect that gravity has a _huge_ impact on our lives. We should
always be able to tell that gravity exists.

Note that these conclusions are _independent of the relative strengths of the
forces_. Magnetism and electricity will generally have net zero impact.
Gravity will have net positive impact.

If gravity was _strong_ , like the other forces, then gravitational objects
would group together. A _lot_. Which means that the universe would end up as
one big clump of matter.

It's only because gravity is _weak_ that we have gravitational objects which
are separate from each other for any extended period of time.

------
scotty79
I'm not sure why physicists think gravity is a force and Pauling exclusion
principle isn't.

Gravity is fake "inertial" force caused by shape of the space time. Why do
they insist that it should have gravitons when everything so far show it
doesn't fit quantum field models?

~~~
contravariant
>I'm not sure why physicists think gravity is a force and Pauling exclusion
principle isn't.

>Gravity is fake "inertial" force caused by shape of the space time.

Because all forces are like that? Electromagnetism is just a force created by
the shape of the electromagnetic gauge field.

In mathematical terms: it is the curvature of the gauge connection. In exactly
the same way that gravity is a force created by the curvature of the metric
connection.

~~~
hasenj
I'm not sure if this comparison holds.

Space-time is the environment that matter moves through. Curvatures in
spacetime affects how matter moves.

What is the electromagnetic gauge field? Why does it only affect charged
particles? Why does it affect particles differently depending on the type of
charge they hold (positive/negative)?

It doesn't seem to be the same thing as spacetime.

~~~
contravariant
Given the nature of this subject not all your questions have definitive
answers, but I'll try to give it my best shot. You should probably consider
these answers a subjective interpretation, rather than objective facts.

The most consistent interpretation of QFT I've seen so far views gauge fields
as principal bundles. This implies that matter not only propagate through
space-time but also through the gauge field, and the way it is 'changed' as it
passes through the gauge field is related to the charge.

Now technically the electromagnetic gauge field (and others) also affects
uncharged particles, mainly because it couples to gravity, but perhaps also in
more subtle ways (think of the way light can be affected by simply passing
through a piece of matter).

Admittedly there are quite a few differences (since the metric has a somewhat
special position) but there are also quite a few similarities. Unfortunately
nobody has yet found a (testable) theory that either unifies the forces or can
treat the separately. I'm placing my bets on the 'unification' side though.

------
rurban
Being so weak is not the only disturbing attribute of gravity. It's also being
the only far-reaching attracting force. Far reaching, not connected to
mechanical forces is only explainable with the wave model, not the mechanical
counterpart, and attractive is only similar to electro-magnetism, with a
completely different explanation.

That's what made Newton think of being a supernatural force created by god to
hold all the planets and the universe together.

And that's what made Heisenberg say: "Are you silly? An attracting force
having the mechanical interpretation, the Higgs boson, which jumps on each
other nucleons back to create an attraction?" That cannot be it.

------
castratikron
One of my favorite possible explanations is the Large Number Hypothesis (LNH)
by Dirac

[https://www.youtube.com/watch?v=-o8mUyq_Wwg](https://www.youtube.com/watch?v=-o8mUyq_Wwg)

------
javajosh
But physics doesn't answer this kind of question. We don't know _why_
magnetism, for example, works the way it does - we just measure it's effects
and model it with mathematics.

OTOH I think it's interesting to imagine universes with stronger or weaker
gravity, and to realize that in most of them it's hard to imagine life being
possible at all. (Life would not arise without gravity (since you need to
smush matter together to get life) but it also wouldn't arise with super
strong gravity (since life needs to be able to move around).)

------
spodek
> science will never progress unless we try, and that’s what these ideas and
> searches are: our attempt to move our knowledge of the Universe forward. And
> as always, as the LHC’s Run II has already begun, I can’t wait to see what —
> beyond the already-discovered Higgs boson — just might turn up!

If they want data from colliders, I would say it's a problem for experimental
physics, or physics in general.

------
xyzzy4
The reason is because of the anthropic principle.

------
rrggrr
I like to think of Gravity as a result of mass deforming space in the same way
a brown turd deforms toilet water as it spins inevitably toward the outlet
during flushing. Standing atop the turd you are affixed to its mass, but the
entire system is affixed to the spinning, evacuating forces of the great
flush. The Bono/Kurick South Park episode was instrumental in this theory's
formulation.

