
Gravity Is Not Like the Other Forces - pseudolus
https://www.quantamagazine.org/why-gravity-is-not-like-the-other-forces-20200615/
======
phn
I know this is tangential, but as lay person regarding physics I am stuck on
the following thought experiment regarding gravity:

We know about gravitational potential energy. We harness it all the time with
dams and whatnot. Generally speaking, when we lift matter high above, we get
some of the used energy back when it falls back down.

But when we talk about sending say, a spaceship into space, it may fall on a
different planet, with a different gravity (a function of the planet's mass),
changing the amount of energy yielded by the fall.

The question is: Where is the energy stored in the meantime? Where does the
extra energy, if the ship falls into a heavier planet, come from?

It seems that the energy is being stored somewhere, and that matter itself
allows us to tap into that, but is not the place where it is stored. Is there
anything I can read to better understand what we know about this?

~~~
simonh
That turns out to be a really interesting question. In physics the
gravitational energy of a massive body is considered to be negative.

That takes a bit of getting used to. If you consider the universe after the
Big Bang, it consisted of extremely thinly dispersed clouds of hydrogen and
helium atoms. Over billions of years these collapsed into galaxies and stars.
During the process of that collapse these atoms accelerated towards each
other, impacted and heated up considerably due to the release of kinetic
energy. This kinetic energy has to come from somewhere, so we consider that
the energy of the gravitational field of a star or planet is negative energy.
It's an energy deficit.

As an side, it is estimated that if you add up the energy in star, including
the matter it's composed of, and deduct the negative energy of it's
gravitational field they cancel out. It seems likely that the net energy of
the universe may well be very close to zero.

[https://en.wikipedia.org/wiki/Zero-
energy_universe](https://en.wikipedia.org/wiki/Zero-energy_universe)

~~~
goldenkey
I am happy that you provided the only realistic answer here. Gravity is the
negative to light...radiation pressure pushes and gravity pulls. We live in a
yin yang universe. Like Alan Watts would say, the universe is in the middle of
cosmic dance, and eventually all positives and negatives will neutralize and
we will be back where we started. This bides well for cyclic theories of
cosmology.

[https://en.m.wikipedia.org/wiki/Radiation_pressure](https://en.m.wikipedia.org/wiki/Radiation_pressure)
Radiation pressure - Wikipedia

~~~
kleer001
I'd love to hear a refutation to this (however simple) rather than an
anonymous and empty downvote.

~~~
simonh
Gravity and light are unrelated phenomena, one being a curvature of space and
the other a fluctuation in electromagnetic fields. They are in no way the
opposite of each other. It’s like saying apples are the opposite of oranges.
It’s not even intelligibly refutable.

~~~
kleer001
Sorry, that leaves me even more confused. I didn't mean to start an argument
though.

Light pressure is a thing. And gravity certainly pulls. And pulling seems the
opposite of pushing.

> It’s not even intelligibly refutable.

So, your calling your refutation unintelligent? I don't think that's what you
meant.

~~~
undersuit
Just light, what about the rest of the electromagnetic range of emissions?
Gravity can push, ever tried to roll a car uphill? The weak and strong nuclear
forces are also from Bosons, why are light and gravity special? Did you know
you can send enough light into a region that you can cause that region to pass
into a blackhole, it's a called a Kugelblitz.

Rapid fire of some refutations.

~~~
goldenkey
When a physicist says 'light' they mean electromagnetic energy of all
frequencies - not just visible light. All the forces become unified at high
temperatures. The universe is thought to have 0 total charge, so even the
attractive coulomb force does not counter gravity -- it in sum total, does
nothing. Gravity doesn't push the car down the hill, the car pushes you,
because it itself is being pulled, silly semantics.

The gravitional force is a negative, it destroys separation and distance.
Light and the other forces create separation and distance.

In your black hole example, the light has a momentum, and that doesn't vanish
if a black hole forms, the black hole will continue moving.

Rapid fire of some refutations.

~~~
undersuit
>Gravity doesn't push the car down the hill, the car pushes you, because it
itself is being pulled, silly semantics.

That was my point. You're looking for some wonderful unifying human beauty. I
can make the same arguments.

>In your black hole example, the light has a momentum, and that doesn't vanish
if a black hole forms, the black hole will continue moving.

My mistake, it's a bit more complicated, what if you pointed two lasers at
each other and created a pulse from each of such magnitude the when the light
pulses pass by each other the Schwarzschild radius is passed? The blackhole is
formed from the photons and nothing can escape the interior of the blackhole.
Too much EM radiation can destroy separation and distance.

Also: [https://www.quora.com/Since-photons-are-always-moving-
would-...](https://www.quora.com/Since-photons-are-always-moving-would-a-
kugelblitz-be-travelling-at-speed-of-light)

~~~
goldenkey
Blackholes aren't the end - they evaporate.

The photons had a gravitational field before the black hole. Their
gravitational field is merely summed. Their total gravity is still conserved.

Nothing has really changed. The fate of the universe is still going to be a
balancing act..

------
tzs
Imagine some kind of alien life form that arose and evolved in intergalactic
space. They evolve intelligence and start developing physics. Their physics
knows about electromagnetism and maybe the strong and weak nuclear forces, but
they have never seen any sufficient concentrations of mass to notice gravity.

They would still know about mass but for them it would just be a property of
matter that determines how strongly it reacts to force.

Their model of how force works would be that for each kind of force there is
some kind of charge. How much of that kind of force you have between two
bodies is determined by how much of that kind of charge each has and how far
apart they are.

How each body responds to that force is determined by its mass.

Mass and charge are independent for all of their known forces. You can have
two bodies with the same mass but different charge for a given kind force, or
two bodies with the same charge for that force but different mass.

Eventually they would discover gravity and I think it would immediately stand
out as fundamentally different from the other forces because its charge is not
independent of mass. You double the gravitational charge on a body you also
double its mass. You double its mass you also double its gravitational charge.

~~~
gpderetta
It is not so different. To double the electric charge of an electron you also
have to double its mass.

~~~
MereInterest
True, but only if you stick to electrons. Switch to muons, and you have more
mass, but the same electric charge. Depending on what particles you use, the
mass and charge vary independently.

~~~
gpderetta
the point that I tried, and failed, to make is that you can't arbitrarily
change the charge of a body without changing its mass.

------
jscipione
Since reading Brian Greene’s books I have a question that I can’t seem to get
an answer to. If there really are extra curled up dimensions and the graviton
is not connected to the brain of the 3 extended dimensions in our Universe and
is therefore free to travel into and out of the curled up dimensions then why
does gravity obey the inverse square law? Shouldn’t we be able to detect the
extra dimensions by the effect of gravity diminishing faster than inverse
square as the gravitons go into and out of the extra dimensions? The inverse
square law of gravitational propagation indicates that there either are no
extra dimensions or that the graviton is also connected to a brain that
prevents it from entering and exiting these extra dimensions. Since the other
3 forces also obey the inverse square law the extra dimensions might as well
not exist making the conclusions of string theory invalid. What am I missing?

~~~
JimBlackwood
No, what you’re saying is correct. If the inverse square law was obeyed
perfectly, space would be restricted to 3-spatial dimensions.

The extra spatial dimensions are usually theorized to be extremely compact so
the effects of a higher power law (the inverse square law can be generalised
to 1/r^(n-1) with n as your amount of dimensions.) can be neglected.

Of course, to prove it you’d have to at some point measure these effects, and
there is some evidence that the inverse square law breaks down.
([https://arxiv.org/pdf/hep-ph/0307284.pdf](https://arxiv.org/pdf/hep-
ph/0307284.pdf))

~~~
jscipione
I would think that over cosmological distances even if the effect of extra
dimensions on the propagation of gravity was tiny, Less than half a human
hair, the net effect would be measurable especially if there are not just 1
but 8 extra curled dimensions as M-theory predicts. Well thanks for confirming
that what I was thinking was an accurate criticism and I hope that physicists
can devise an experiment to prove or disprove the extra dimensions some day
soon.

------
bollu
I put in the work to study GR from Susskind' theoretical minimum (fantastic
lecture series: [https://theoreticalminimum.com/courses/general-
relativity/20...](https://theoreticalminimum.com/courses/general-
relativity/2012/fall) ). I still don't understand GR.

Yes I can now say "oh gravity is '''just''' curvature on a lorentizian
manifold", but what in the world does it _mean_? My current picture is
something like this:

A Lorentizian manifold looks locally hyperbolic, so we need to imagine a light
cone. Now the curvature of this manifold tells us how we need to __distort
light cones __at different points to build up the full space.

How the fuck do I visualize this? Is there some way to embed this in low
dimension to play with it? For example, to understand regular curvature, you
can play around with a sphere and a cone to get a feel for curvature ~= angle
deficit ~= loopy deficit ~= holonomy.

I don't know how to visualize the curvature of space-time, and I'm completely
unconvinced I actually understand it beyond squiggles on paper. I'd love a way
to embed and visualize this. Please tell me if you know how!

As an aside: It is definitely possible to get a feel for _hyperbolic geometry_
by playing HyperRogue:
([https://roguetemple.com/z/hyper/](https://roguetemple.com/z/hyper/)) --- A
free roguelike set in hyperbolic space.

What I don't grok at all is the _curvature_ across hyperbolic spaces,
analogous to how a sphere has _curvature_ across flat spaces. I'd love to know
how to get a feel for this.

~~~
Ono-Sendai
Sure, I can explain how to visualise it (I think :) )

First off, a note: lots of people think they know how to visualise GR, but
actually don't. For example, explaining with marbles falling down curved
rubber sheets doesn't help, because you're using gravity (the marble being
pulled down) to explain gravity. And if you think you can just explain it with
geodesics along a curved rubber sheet with a heavy weight in the middle, then
think about how to explain why a stationary object falls towards the heavy
weight.

Anyway, here is how to visualise it:

Imagine you are driving a car along some surface. We require that each wheel
rotates at exactly the same speed with respect to the surface. Now imagine
there is a decreasing curvature gradient from left to right across the path of
your car - in other words the surface below the left wheel is curving faster
than under the right wheel. Seen from above, the left wheel has extra distance
to travel in the vertical direction than the right wheel, to achieve a given
horizontal distance. This causes the direction of the car to turn slowly to
the left - in other words the car is 'gravitationally attracted' to the left -
i.e. towards the region of greater curvature, i.e. towards the attracting
mass.

To explain why static objects are gravitationally attracted to a mass, you
just have to realise that according to Einstein, there are no such things as
stationary objects (in some sense). Rather, even a 'stationary' object is
travelling rapidly through spacetime. To go back to the car metaphor - the car
is not stationary, rather it is always driving forwards (through spacetime).

Some notes on this - think about what a bizarre, weirdly-shaped surface the
car must be driving over.

You can also think of the car front axle as being a wavefront of light.

And finally - if you think this explanation is too bizarre, don't blame me,
blame Einstein. Personally I don't actually like GR as a theory due to how
weird it is.

~~~
vagab0nd
> For example, explaining with marbles falling down curved rubber sheets
> doesn't help, because you're using gravity (the marble being pulled down) to
> explain gravity.

Exactly, and it drives me nuts. There are many similar examples in SR and GR
that maybe help a layperson intuitively, but creates confusion and frustration
when you actually try to learn the theory. I can think of 2:

The "escape velocity" explanation of black hole event horizon. Yes, the number
comes out correct. No, it does not make sense and does not help me understand
how black hole works.

"Mass increases when object moves". No it doesn't. And no it doesn't explain
why massive particles can't reach speed of light, because in the frame of the
object, its speed is zero.

I can't tell you how much time I've spent searching for the answers, and
drowned by these answers that seemingly make sense.

~~~
ISL
For the latter concern, you may find this Physics Today article from Okun
helpful:

[http://www.hysafe.org/science/KareemChin/PhysicsToday_v42_p3...](http://www.hysafe.org/science/KareemChin/PhysicsToday_v42_p31to36.pdf)

------
hektorrr
I would describe myself as a hopeless philosopher in terms of physics. I like
to theorise about the nature of everything in universe. In no circumstances
I'd say I know what I'm talking about. I'm aware that I lack a lot of
knowledge in this field, which actually makes my journey pretty amazing
through the learning about the complexity of the universe.

These few words just to let you know that whatever you read here it might be a
complete nonsense and it's not even a pseudoscience but some fantasy/fiction
reflections.

I'm most amazed by the nature of tri-dimensional space itself. Back then,
there was popular belief in Aether - the substance that carries wave of
lights. To be honest I tend to believe such 'thing' could explain many of the
most weirdest questions that appears in my head.

I wonder if it's possible that the Aether is the space itself. And the space
is the Aether. Tiniest, basic 'particles' that are frame for everything else.
There would be no 3-dimensional space without at least 4 of such Aether
quants. Also for example photons don't just 'fly' through space, but jumps
from one quant of Aether to another one. That's how they 'fly' in straight
line. That's why when the space is curved they will follow that curvness.

And the gravity is not just a curved spacetime but something like an
atmosphere made of Aether quants that are repelled by the particles containing
Higgs boson. Which naturally makes the Higgs boson the only particle that can
interact with Aether.

I know that so far none experiment has confirmed the existence of Aether. But
I can't let go the feeling that we are missing something here.

~~~
at_a_remove
You are not the first to think of this, or even the hundredth. It's been
decades since I first saw it, but I did see someone propose kind of an
interesting approach, for amusement: why not take the aether thing as far as
it could go? Keep adding bizarre properties to it to explain experimental
results. Go wild!

If I recall, the end result was something very much like the properties of
spacetime. But with no particles. Just ... take the particles away. You're
attached to the particles.

The reason you cannot let go of that feeling is that you're trying to attach
your base human experience (which is largely Newtonian, low spacetime
curvature, quantum effects not immediately visible, a fairly confined region
of temperatures and pressures) to a domain where your base experience is
completely irrelevant. All of the waves you witnessed were the disturbance of
matter as a traveling phenomenon. It's what you grew up on. And so envisioning
a scenario wherein the wave isn't carried on some kind of matter, however
hidden, "feels" wrong.

Also, don't do physics without math. If you just want to keep reaching from
one analogy to another, you can build all kinds of castles in the sky, but you
won't have anything at the end of it. Remember, physics is a _quantitative_
prediction. Your concepts have no math or calculations, and so there is no
measurement to contradict them. Nor can you "farm the math out" to other
parties. Nobody capable of doing the math will do the math _for_ you. If you
desire to pursue this, you must begin to do the math, yourself.

~~~
saagarjha
I should note that it’s really not possible to know that aether doesn’t exist
or whatever, all of these are just models that experimentally match our
observations. And your answer describes why we don’t use it: it’s not that
aether is wrong, it’s just that if you try to extend the model to account for
observations, you get the same model as what we currently have but corrected
slightly because you’re assuming that aether exists. It turns out that it’s
just simpler to not assume that, which is why we have our current model.

------
Raro
For some context: the article only interviews quantum gravity researchers, so
it has that particular slant. They are concerned with coming up with different
strategies to mitigate the issues general relativity has with singularities,
and these strategies range from "sensible deviations from general relativity"
to "fun, but highly unlikely".

In terms of how the theory of gravitation is not like the other fundamental
forces, I think the most salient point was made by Sera Cremonini: gravitation
is not a renormalizable theory. At least in the sense of how we have
approached renormalization with the other forces. It is intrinsically non-
linear, because the source is the field (and vice-versa).

To get anywhere close to a grand unified theory that incorporates gravitation
requires a rethink of the normalization procedure (and likely requires an
entirely new geometric framework—this was a pre-requisite for both general
relativity and Newton's Universal Law).

~~~
codethief
> For some context: the article only interviews quantum gravity researchers,
> so it has that particular slant.

I would go even further: Natalie Wolchover only interviewed string theorists
and quantum field theorists. They all assume that gravity in some way or
another will just be yet another quantum field theory.

There is not a single piece of evidence for this, though. Personally, I also
don't think it is a particularly promising approach. As Hawking so eloquently
put it:

> But I believe [gravity] is distinctively different, because it shapes the
> arena in which it acts, unlike other fields which act in a fixed spacetime
> background.

(Hawking in Hawking & Penrose: The Nature of Space and Time, chapter 1)

~~~
Raro
> I would go even further: Natalie Wolchover only interviewed string theorists
> and quantum field theorists. They all assume that gravity in some way or
> another will just be yet another quantum field theory.

I didn't want to push that point too much. I am personally uncomfortable with
the lack of emphasis on empirical evidence. However, that's not to say there
can be fruitful results.

In terms of re-thinking geometric approaches, I quite liked (what I understood
of) Nima Arkani-Hamed's suggestion for avoiding issues with localization.
Similarly for Penrose's twistor theory.

~~~
codethief
> I quite liked (what I understood of) Nima Arkani-Hamed's suggestion for
> avoiding issues with localization.

Which suggestion are you referring to? Do you happen to have a link?

~~~
Raro
The 'Amplituhedron':
[https://arxiv.org/pdf/1312.2007.pdf](https://arxiv.org/pdf/1312.2007.pdf)

The idea is to focus on the amplitude of scattering interactions from the
momentum-space twistor perspective, rather than perturbations about a point in
space-time.

For background: Nima made a name for himself by greatly simplifying complex
particle interactions, from tens of pages of Feynman diagrams to around a
page. I think of the above as an extension of that project. However, I don't
recall all the details, as it's been six years since I went to his talk on
this.

~~~
codethief
Oh yes, I've heard a bit about the Amplituhedron. Unfortunately, it only
serves to simplify the _perturbative_ calculations in Yang-Mills theories
order by order, i.e. the Feynman diagrams. It doesn't describe the full (non-
perturbative) theory, so it's not really a candidate for a replacement of
"classic" local QFT.

~~~
Raro
Thank you for pointing this out; this accords with my recollection, but you
clearly stated a fundamental issue with the approach in its current state.

I thought of the idea as more a sketch of how things might be looked at with a
perspective change, rather than a full-blown theory (at least in the form I
saw it in 2014). I guess I was holding out for the (highly unlikely)
possibility of an asymptotic limit or something similar. It just seemed to be
rooted more in reality than most of string theory/LQG. Of course, that could
be my bias towards novelty!

~~~
codethief
> I guess I was holding out for the (highly unlikely) possibility of an
> asymptotic limit or something similar. It just seemed to be rooted more in
> reality than most of string theory/LQG. Of course, that could be my bias
> towards novelty!

I totally understand! I was quite excited about the Amplituhedron, too, when I
first read about it!

------
rurban
They should have started with Newton, not Einstein. Heisenberg is also
missing.

Newton was so excited to detect this new supernatural attracting force over
great distances to hold the planets together, as clear proof that God exists.

Mechanics cannot explain contact-less attracting forces, in mechanics there
are only repelling forces, caused by direct contact. Newton never gave up and
disappeared into this gravity=god hole forever.

Quantum-mechanics had the same problem, as Heisenberg detected. The dual
nature of particles and waves don't hold for gravity, there cannot be a
gravity particle as counterpart to the attracting force. Einstein was the same
opinion, but still the Boor folks had their way. Even the Einstein-Podolsky-
Rosenstein Paradoxon, which disproved causality in the standard model, was
explained away.

Same for black holes, the mysteriously postulated dark matter and more.

~~~
ganzuul
If you are OK with reversing time then mechanics can explain attractive
forces. It then becomes a question of why our perspective is one-sided.

~~~
mnw21cam
Nope, still repulsive forces. Think about it - two balls head towards each
other, collide, and move away from each other, having experienced a repulsive
force. Reverse time, and it looks exactly the same.

Or more mathematically, force is measured in Newtons, which is kg * m / s / s.
Reverse the sign of the seconds part of that, and, oh, it's squared so it ends
up being exactly the same.

------
scotty79
I don't understand why people think that gravity should be describable by same
math other forces are. Why should there be a quantum gravity field or quantum
mediating particles for gravity?

As far as we know gravity is just acceleration due to non-flat shape of
space(-time). Why would that have anything similar with exchanging photons,
gluons or other bosons? I understand how you might think there's a quantum
math for a field. But how can there be quantum math for acceleration?

~~~
joeberon
Quantum physicist here. The reason is not that we _want_ to describe both
gravity and the standard model using the same formalism, it's that currently
general relativity and the standard model are actually fundamentally
incompatible and the most obvious way to come up with a model where there are
no incompatibilities is to try to describe both using the same mathematical
framework.

>Why would that have anything similar with exchanging photons, gluons or other
bosons?

You make it sound like it is obvious that the strong force would be mediated
by gluons or the electromagnetic force by photons, but it actually isn't at
all, and it is only relatively recently that we have started describing all
forces as being mediated by these so-called "force carriers". It is not
obvious at all that there are any photons at all involved in, for example, two
magnets repelling each other, or in electromagnetic induction of a current.

~~~
scotty79
Thank you for your response. What's the source of incompatibility between
standard model and general relativity?

Gravity is about acceleration and flow of time. What's the problem with
plugging general relativity in place of classical dynamics whenever you care
about flow of time and acceleration in quantum physics? As if those gravity
and other forces and particle had nothing to do with each other.

Why does general relativity is more incompatible with quantum mechanics than
with fluid dynamics for example?

My understanding is that general relativity says that gravity is not even a
real force. It's just a function of acceleration caused by misshapen space-
time (deformed by mass/energy).

Some other things I don't understand... How gravity is considered to be a
fundamental force, while Pauli exclusion principle is not, despite the fact
that it can keep neutron stars from collapsing. Is it because there's no hope
for finding mediating particle in the math that describes the principle
because the math is too simple?

~~~
joeberon
>Thank you for your response. What's the source of incompatibility between
standard model and general relativity? Gravity is about acceleration and flow
of time. What's the problem with plugging general relativity in place of
classical dynamics whenever you care about flow of time and acceleration in
quantum physics? As if those gravity and other forces and particle had nothing
to do with each other.

The difficulty is that the standard model uses "quantum field theory" (QFT) as
the physical model in the background to describe it. However QFT is actually
quite hacky in a lot of ways, the main one being that actually it predicts
that every point of space should have an infinite free energy (see here for
example
[https://en.wikipedia.org/wiki/Vacuum_energy](https://en.wikipedia.org/wiki/Vacuum_energy)).
The way we get around this is by subtracting off this infinite energy, which
is one of many similar techniques called "Renormalization"
([https://en.wikipedia.org/wiki/Renormalization](https://en.wikipedia.org/wiki/Renormalization)).

Well it turns out that if you produce a quantum field theory for gravity, it
is not actually possible (or at least in any way easy) to "renormalize" it
(i.e. it's really hard to subtract off these infinities)

The issue is more that QFT is really weird, rather than that general
relativity is

>Why does general relativity is more incompatible with quantum mechanics than
with fluid dynamics for example?

Well general relativity (GR) and QFT both describe fundamental interactions,
whereas fluid dynamics describes an emergent property of electromagnetic (EM)
interactions between many particles. Gravity is also usually very irrelevant
in fluid dynamics unless we are looking at very large objects like the flow of
matter in stars for example, so in most cases it is just EM interaction which
is fully described by QFT. In other cases we have relatively weak gravity +
QFT, for example in those stars, in which cases we can actually already
describe the dynamics fully using approximate versions of gravity and not have
any incompatibility. In cases where gravity is very strong though we do have
these issues as we just don't know what happens due to lacking a proven theory
of quantum gravity. One example is how the matter behaves in the super-dense
core of a neutron star, known as the neutron star "equation of state", which
is currently unknown, as in these cases both quantum and gravitational effects
are very strong.

>My understanding is that general relativity says that gravity is not even a
real force. It's just a function of acceleration caused by misshapen space-
time (deformed by mass/energy).

Yes that is completely correct to our understanding, but we could maybe model
it as a "real force" by writing a quantum field theory for it

>Some other things I don't understand... How gravity is considered to be a
fundamental force, while Pauli exclusion principle is not, despite the fact
that it can keep neutron stars from collapsing. Is it because there's no hope
for finding mediating particle in the math that describes the principle
because the math is too simple?

Honestly the fact that the Pauli exclusion principle isn't a force has always
been confusing to me too as it behaves like one in so many situations, but it
is similar to gravity really which as you said isn't really a force, but more
like an influence. In fact GR says that we follow the geodesic (roughly
translated to the "shortest path") along the curved space-time, i.e. it is by
minimising the so-called "action". This isn't really a force, more like a
fundamental law of motion. I think the Pauli exclusion principle is similar in
this way, but I don't think it is a satisfactory explanation

~~~
scotty79
> Well it turns out that if you produce a quantum field theory for gravity, it
> is not actually possible ...

I heard about that. But why would you want to have quantum field theory of
gravity? Why can't you just consider gravity to be only about the shape of
space-time in which everything including quantum field theories of everything
else happen?

> Well general relativity (GR) and QFT both describe fundamental interactions
> ..

Does GR really describe fundamental interactions? For me it seem that it only
describes shape of space-time where all the fundamental interactions take
place.

> In cases where gravity is very strong though we do have these issues as we
> just don't know what happens due to lacking a proven theory of quantum
> gravity.

Isn't it just because we lack data from systems where gravity is extremely
strong and systems are small/isolated enough to show any quantum effects? How
can we be sure that we need quantum gravity? Why it's not enough to calculate
QFT in curved spacetime instead of flat spacetime?

> One example is how the matter behaves in the super-dense core of a neutron
> star, known as the neutron star "equation of state", which is currently
> unknown, as in these cases both quantum and gravitational effects are very
> strong.

I see.

> Yes that is completely correct to our understanding, but we could maybe
> model it as a "real force" by writing a quantum field theory for it

Apparently we can't due to problems with renormalization. And I argue we
shouldn't be able to because I believe it's completely different thing from a
real force and if same math worked for two dissimilar things I'd be worried.
:-)

I'm happy to hear that Pauli principle is unsettling not only to grumpy weirdo
like me. :-)

~~~
joeberon
>I heard about that. But why would you want to have quantum field theory of
gravity? Why can't you just consider gravity to be only about the shape of
space-time in which everything including quantum field theories of everything
else happen?

No, because it turns out that curved space-time is actually incompatible with
quantum field theory in certain extreme parameter regimes, in a way that it
actually makes it impossible to make predictions on scales where both
gravitational and quantum effects are very strong (e.g. neutron stars). We
want a quantum field theory of gravity as then it will "play nice" with other
quantum field theories.

>Does GR really describe fundamental interactions? For me it seem that it only
describes shape of space-time where all the fundamental interactions take
place.

I think it is quite fundamental that energy curves space-time and that it
affects the paths of objects (even light), but yes maybe I wouldn't call it a
fundamental interaction in the same sense as quantum field theories,
particularly because it isn't quantised so it isn't really a single "thing"
being exchanged (like a gauge boson). Most physicists do call it an
interaction or a force though. I mean that it describes nature on a
fundamental and indivisible level (at least so far as our current/most popular
understanding)

>Isn't it just because we lack data from systems where gravity is extremely
strong and systems are small/isolated enough to show any quantum effects? How
can we be sure that we need quantum gravity? Why it's not enough to calculate
QFT in curved spacetime instead of flat spacetime?

Of course we lack this data, which is why measurements of, for example, the
aforementioned neutron star equation of state will give us more understanding.
We can't be sure that we need a quantum theory of gravity, it could be that it
is actually just impossible to describe both gravity and QFT at the same time.
QFT in a curved spacetime is only a first approximation of general relativity
and doesn't capture all of the effects of GR (see here for example
[https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...](https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved_spacetime)).
I don't really know much about quantum gravity, but I don't believe that QFT
in curved space-time is sufficient, at the very least it doesn't solve the
cosmological constant problem
([https://en.wikipedia.org/wiki/Cosmological_constant_problem](https://en.wikipedia.org/wiki/Cosmological_constant_problem))

> And I argue we shouldn't be able to because I believe it's completely
> different thing from a real force

It's worth considering that what we call a force in quantum field theory is
very different from what Newton would have considered a force! Probably in
fact the gravitational "force" due to curved space-time would seem more like a
force to him, certainly QFT is much weirder than GR after all...

~~~
scotty79
> I think it is quite fundamental that energy curves space-time ... Most
> physicists do call it an interaction or a force though.

Isn't it sort of 'legacy' way of thinking and naming things?

> QFT in a curved spacetime is only a first approximation of general
> relativity and doesn't capture all of the effects of GR (see here for
> example
> [https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...](https://en.wikipedia.org/wiki/Quantum_field_theory_in_curved...)).

Oh. Thank you. This looks way more promising. But I see that it has few very
hard to accept results like lack of objective vacuum or particles loosing
meaning (which I'm ok with :-)). And I see how it still fails to include
influence of particles generated by gravity. But it makes interesting
predictions already.

I really think that's the way forward.

------
sudhirj
Isn't it conclusive already that gravity _insn 't_ a force? It works nothing
like what we consider forces, and we already have models that show it's
curvature in space-time, not force. A black hole doesn't exert infinite force
on things, for example – one could say it doesn't exert any force at all, just
curves all paths you could possibly take back in on itself.

------
Ono-Sendai
I think the big reasons gravity is different are as follows:

* very weak (many orders of magnitude less strong than the other forces)

* no positive/negative mass 'charges', the gravitational force is attractive only.

* works via curvature of spacetime (according to Einstein), instead of the exchange of force particles (e.g. instead of by jiggling fields)

~~~
klank
You last point is not necessarily a description of gravity, but a description
of the models we use to reason about it.

------
TheMightyLlama
I would like to join in with many of the statements in this thread describing
a joint lack of understanding of the physical universe as well as a deep
fascination with it. I certainly count myself among those ranks.

In watching way too many YouTube videos on quantum physics and gravity I began
to create a model of how the universe works at the very small scale and after
a while something occurred to me.

We know that the path of a photon can be bent if it passes near a black hole.
The key being that spacetime is being somehow consumed by a black hole and the
photon is being dragged towards it. Thus, its path appears bent. I thought
that similar effects should happen with volumes of a lighter mass. Perhaps
with the same mechanism.

Neutron stars would have a smaller effect on photons, and a more measurable
effect on heavier particles and molecules. Neutron stars would also somehow
'consume' spacetime. Just at a lower rate. Thus heavier particles would
escape.

So too with the earth. Only, in this case, the mass of the earth is small
enough that molecules can escape all on their own.

So, what is gravity? Gravity in this model is the effect of spacetime being
consumed. By analogy the spacetime is like standing under a shower. We are
glued to the earth because we cannot gain enough purchase against the torrent
of spacetime being consumed by the mass of the earth.

Which brings me to the second question. Well, this is where actually having
some physics would be useful. But after many drunken bouts of thought, I'm
convinced that this spacetime consumption is down to [color
confinement]([https://en.wikipedia.org/wiki/Color_confinement#QCD_string](https://en.wikipedia.org/wiki/Color_confinement#QCD_string)).

Anyway, enjoy. If nothing else, this should at least be useful for some sci-fi
anti-gravity mechanism.

~~~
Balgair
Great questions/ideas!

To maybe nit-pick a bit to help out:

Space time is not 'consumed', it's just curved in 4-D. The Einstein Field
Equations are a very good model of what we think is occurring.

Photons are massless, hence why they can travel at light-speed. Their energy
is the thing to worry about when doing GR.

~~~
TheMightyLlama
The crucial question I guess is: Why do objects fall down a gravity well if
its just a curvature?

The 'consumption' to me is more intuitive, but then again I'm just not a
mathematician. Is it the field equations specifically thatI should look at or
are there others?

And also what type of math structures should I be studying to understand the
field equations?

~~~
pa7x1
They don't fall a gravity well, they follow straight lines in a curved
spacetime. Not subject to the other forces everything follows the straightest
line (geodesics) it possibly can in a curved spacetime.

The field equations define the spacetime geometry but they are very hard to
solve, you will gain little insight from them. Instead you should try to
understand the concept of a geodesic, how they are extremes of a variational
problem, how they are calculated and the physical/geometrical intuition behind
them.

------
rudolph9
> The problem with a quantum version of general relativity is that the
> calculations that would describe interactions of very energetic gravitons —
> the quantized units of gravity — would have infinitely many infinite terms.
> You would need to add infinitely many counterterms in a never-ending
> process. Renormalization would fail. Because of this, a quantum version of
> Einstein’s general relativity is not a good description of gravity at very
> high energies. It must be missing some of gravity’s key features and
> ingredients.

Is there a proof backing up the claim that “ Renormalization would fail”? I
can see why people would jump to that conclusion but that doesn’t mean it’s
true.

------
macspoofing
>Gravity Breeds Singularities

Gravity or GR? Is there even any reason to take singularities seriously?
Aren't singularities simply places where GR as a theory falls flat on its face
and therefore singularities don't actually describe any physical phenomena?

~~~
codethief
The short answer to your question is: We don't know.

------
emsal
Theoretical physics is sporadically coming back into my mind again after a lot
of time learning CS and Stats.

I'm wondering now, is the goal of theoretical physics sort of like the goal of
topological data analysis[0]? Reading about things like how Max Planck came up
with quantum theory to explain the measurements of black body radiation
reminded me a lot of the ML task trying to fit a model to data.

My bias is definitely showing here but is this a useful framework to be able
to think about these things?

[0]:
[https://en.wikipedia.org/wiki/Topological_data_analysis](https://en.wikipedia.org/wiki/Topological_data_analysis)

------
markkat
I’ve long thought gravity is more likely to be the product of space between
masses, not something that alters it. My thinking is the “alteration” of we
perceive as gravity is lopsided entropy due to a field with a gradient of
space.

I wrote this thought experiment years ago, which was the start of this way of
thinking: [http://corpusdord.blogspot.com/2006/02/matter-space-and-
time...](http://corpusdord.blogspot.com/2006/02/matter-space-and-
time.html?m=1)

~~~
jcims
Maybe gravity is just diffusion of energy as it tries to smooth the tremendous
discontinuities between its condensed form (mass) and free (?) form (vacuum).

------
m3kw9
Can someone explain Why there is gravity without drawing on math?

~~~
Extigy
Imagine that you are standing on the equator of Earth, facing North. Now
imagine a friend is also standing somewhere on the equator -- not too far
away, but a fair distance -- also facing North. Now, if you both start walking
you'll find that you begin to move closer together. The distance between you
shrinks as you walk, until eventually you meet at the North Pole. The "force"
that is pulling you together is like gravity in that it is caused by the
curvature of the Earth's surface as you travel across it.

With gravity the curvature is intrinsic to space-time itself and is affected
by mass. Generally more mass means more curvature. So if you sit two objects
in space, at rest with respect to one-another (but still travelling forwards
in time), you will find they are pulled together and eventually meet in the
same way you and your friend were pulled together and met at the North Pole.

You might ask why mass affects the curvature of space-time? That I don't know
the answer to.

~~~
machiaweliczny
Im fine with curvature analogy but why things "move"?

------
logicallee
When thinking about gravity versus other forces, consider how weak it is.
Under you you have Earth weighing 6.6 sextillion tons (5.97 x 10^24 kg). It is
260 billion cubic miles (1 trillion cubic km). All that generates 1g of
gravity.

A basic fridge magnet (not even neodymium) easily generates a magnetic force
that can lift several times its own weight. It overpowers the sextillion tons
3:1, no contest.

------
exabrial
I always find it interesting we have the photon for light, electron for
electricity, but no particle for gravity.

~~~
brummm
Technically, the photon is also the particle for electricity, not the
electron.

~~~
jansan
What? Give me a link so I can learn.

~~~
pjc50
Trick question (or trick answer): depends what you mean by "electricity",
which is one of those questions that's simple to ask but collapses into a pile
of details on closer inspection.

The electric field and magnetic field are tied together in the manner
described by Maxwell's equations. This results in "electromagnetic waves",
which are indeed carried by photons in free space.

It is often helpful to look at the fields as being the real "electricity", or
at least use the lumped approximation we call "current", and this brings
various charge carriers along with it. Electrons are the common charge
carrier, but ions can carry charge (e.g. in batteries), and in semiconductors
we use "holes", which are the absence of an electron. Not an anti-electron,
just the space where it's supposed to be.

------
humanfromearth
Maybe gravity is not a force in a quantum sense, but just geometry of
spacetime in the presence of mass? Maybe there is no graviton and no quantum
theory is needed to explain gravity.

Is there any strong evidence that gravity is following quantum mechanics?
Maybe a real theoretical physicist can explain.

~~~
nightcracker
Considering that gravity is produced by particles who's behavior is quantized,
there is essentially no other option than to assume gravity must be as well.

~~~
humanfromearth
Do we know for sure? I mean I don’t think the graviton has been observed in
action yet. Let me ask another question: is space quantized?

------
pier25
I could be totally wrong, but as a lay person I had the impression that
gravity wasn't actually force.

So the apple did not fall because it was attracted to the Earth but rather it
fell because of the curvature of spacetime.

------
tempodox
... and then there are the neutron stars, exquisite perversions of physics
where atom cores are packed so densely that the electrons have to form a cloud
layer around the star.

~~~
saagarjha
Not around but _inside_ the star: collapse to degeneracy doesn’t mean that the
whole star behaves like a giant atom with electron cloud around it. Most of
the electrons are captured by inverse beta decay, anyhow.

------
badrabbit
What does gravity have to do with energy levels? I thought it was a side
effect of motion in curved spacetime (einstein theory)?

~~~
klank
Curvature of spacetime is an effect of energy. Check out the Einstein field
equations, specifically the stress-energy tensor.

------
tia4tia
Well, some people say so, others say different
[https://en.m.wikipedia.org/wiki/Entropic_gravity](https://en.m.wikipedia.org/wiki/Entropic_gravity)

------
weeboid
Ahh, a primary favorite. "Gravity is the grand leviathan of forces". It is my
personal belief that, as light and energy were shown equivalent, the third leg
of the stool will emerge; that gravity and mass are equivalent. Not as we
understand them now; but as mc^2 understood light/energy equivalence.

Final thought exercise: is it possible, that gravity does not exhibit the same
behavior everywhere in the universe? That gravity may have states.

~~~
vecter
Man ... I remember when HN wasn't filled with crackpots. Sigh.

~~~
weeboid
Ha, yeah.

------
bArray
Disclaimer: Zero formal background in Physics, but I want to learn.

It has always felt like gravity is the result of some other "thing" going on.
There are two reasons for why I believe this:

1\. Gravitational waves are weird, the idea that the resulting force of
gravity can travel away from where it was created. I can't think of any other
examples where something similar happens with another force without something
"carrying" it.

2\. At lot of the issues we have with our current models come from gravity. If
it be on the quantum or Universe scale, our picture of what we believe gravity
to be seems to be limited to our localized experience of it.

I sometimes try to run through a thought experiment where I try to imagine how
gravity, space and time may behave at the very edge of the Universe, because
things there surely must seem weird. If space and time cannot be created or
destroyed, what is being "traded" to expand the Universe?

An alternate idea I've had is that the Universe is not expanding outwards, but
expanding inwards, where galaxies roughly stay where there are and the
properties of space-time between them changes. We always imagine the Universe
in the three dimensional space we experience it in when thinking about
gravity, but the quantum scale seems to suggest there must exist other spatial
dimensions.

Just some brain farts anyway, any reading material people can point me towards
I would be highly appreciative :)

~~~
macspoofing
>Gravitational waves are weird

Are they though? I know there is a lot of math behind how exactly
gravitational waves behave making specific predictions that stem from GR, but
even in the Newtonian sense, something like gravitational waves would
necessarily exist. That is, even under Newtonian laws, if a body emits a force
like gravity which drops in strength with distance, and if that body moves
back and forth in relation to you, you would necessarily experience waves of
force.

~~~
codethief
> That is, even under Newtonian laws, if a body emits a force like gravity
> which drops in strength with distance, and if that body moves back and forth
> in relation to you, you would necessarily experience waves of force.

Not if you take the term "wave" literally. Any action in Newtonian gravity is
assumed to occur _instantaneously_ , so while an observer far away from that
body would certainly see the gravitational pull varying in time, this would
not be the same as a wave traveling through space.

------
zackees
This guy who wrote a book on Magnetism says that Einstein has it wrong and
that only by starting over at Tesla's model and moving forward can the physics
of the universe be predicted well.

According to the Tesla inspired model of the ELECTRIC universe, gravity is not
a fundament field itself, but an emergent characteristic of "incoherent dia-
electric acceleration". The guy who said this is Ken Wheeler.

Whats also interesting is that this opinion is shared by pioneer of "Low-
Energy Nuclear Reaction" Randal Mills. I've been told by the people in this
space that the Mills paper provides a comprehensive model that is MUCH better
at predicting certain binding energy states of nuclei. In comparison to the
standard models all-over the place. It also solves problem of all the dark
matter in the universe, which is simply an abundance of tiny hydrogens called
hydrino's which contain an electron that has lost momentum and is zipping
closer to the nucleus, thus making a more elctro-neutral particle that weakly
interacts with other particles (WIMPS).

If anyone has doubts on the standard model, I suggest checking out "Ken
Wheeler" and Randal Mills. Fascinating and deep look at a minority consensus
of the universe that appears to be MUCH simpler.

~~~
beervirus
Simpler, but not supported by any evidence. Nobody's ever seen a hydrino.

~~~
macintux
Thank you for sending me down a rabbit hole. Quite interesting to read the two
incomprehensible (to me) and incompatible worlds represented by the believers
and the critics on Reddit.

[https://www.reddit.com/r/BrilliantLightPower/](https://www.reddit.com/r/BrilliantLightPower/)

