
Does gravity CAUSE the bending of spacetime, or IS it the bending of spacetime? - rrauenza
https://physics.stackexchange.com/questions/413846/does-gravity-cause-the-bending-of-spacetime-or-is-gravity-the-bending-of-spacet
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6nf
So correct me if I'm wrong, but if gravity is merely a curvature in space-
time, does that mean there's no such thing as a graviton?

~~~
raattgift
General Relativity is a classical field theory, so there's no graviton in it.

However, there are various quantum gravity research programmes which have
tried to quantize the (classical) tensor fields of General Relativity in one
way or another, for instance by turning (classical) gravitational waves or
position and momentum variables into quantum particles. In these theories the
graviton is a spin-2 gauge boson and induce an attractive force on fermions
with like gravitational charge. This compares with the photon, a spin-1 gauge
boson that induces a repulsive force on fermions with like electric charge
(and an attractive force on fermions with opposite electric charge). The spin
also corresponds with the rank of the (classical) fields, where in General
Relativity we have rank-2 tensors and in electrodynamics we have vectors
(which are rank-1 tensors).

Unfortunately this approach has a problem: particles other than fermions also
have a gravitational charge, including gravitons themselves. (Photons do not
have an electric charge, but they _do_ have a gravitational charge). Quantum
field theories use a variety of tricks to remain linear theories, but General
Relativity's Einstein Field Equations are non-linear. That non-linearity
manifests in graviton-graviton interactions in this sort of quantized General
Relativity, and we do not know how to stabilize the resulting quantum field
theories.

In more concrete terms, let's "drop" an electron from higher potential to
lower potential. The electron is a fermion with electric and gravitational
charge. When we drop it from higher to lower electric potential, out pops a
photon. When we drop it from higher to lower gravitational potential, out pops
a graviton. But we can't move the photon around the electric field and get it
to produce another photon, because the photon is not self-interacting. A
graviton, on the other hand, can be dropped down a gravitational potential
causing it to emit another graviton. That in turn influences the potential,
making it easier for those two gravitons to each emit a further graviton,
which influenes the potential, making it easier for those ... and so forth to
infinity. Our original electron is in that potential too, so it in turn can
emit further gravitons as well. Fortunately this explosion of gravitons only
happens in strong gravity, and we only find that deep inside black holes (or
in the very early universe), rather than somewhere we can outright observe
(assuming the cosmic censorship principle is true), so we can still use this
type of theory to describe most of the universe.

Unfortunately, gravitational waves are hard enough to detect; when they are
quantized, they turn into lots and lots and lots of gravitons, each of which
has a tiny fraction of the detectability of a gravitational wave. Detecting an
individual graviton experimentally is beyond the ability of present human
technology, or anything reasonably likely to appear in the next few decades.
So if we ever figure out how to stabilize this type of approach to quantum
gravity, we will only be able to assess it against observations of many-
graviton systems, likely at extreme events like black hole/neutron star
mergers or asymmetrical hypernovae.

This is just one broad example of quantum gravity; there are several other
approaches, some with a "graviton" dynamics (e.g. string theory's gravitons
are a string state) and some without.

All these approaches share a common problem: General Relativity describes our
universe exceptionally well in practice, and its fundamental components are
among the best-tested of any scientific theory to date. Where there is weak
gravity, barring significant masses in superpositions of position, any type of
quantum gravity has to _exactly_ reproduce the results of General Relativity.

It is likely that we will be able to bring microgram masses into such
superpositions in the next few years, and we will be able to directly probe
one aspect of quantum field theory on (classical) curved spacetime (aka
semiclassical gravity). However, _if_ General Relativity fails in that
situation, it would certainly allow some useful comparisons between competing
theories (with and without gravitons) hoping to extend General Relativity as
it in turn extended Newtonian gravity. If only graviton-containing theories
match the experimental results, that would be evidence in favour of the
existence of a graviton, even without direct detection; conversely, perhaps
only non-graviton theories match, in which case the existence of a graviton
would seem pretty dubious.

Right now, though, we just don't know: in the absence of evidence for or
against, gravitons are purely theoretical.

~~~
mrkstu
Is there any implication with the gravity wave detection, that since gravity
has been verified to travel the speed of light that it is transmitted via
particle?

I would think that as inflation is not limited by the speed of light, gravity
as a purely geometric phenomenon, would possibly function at supra-lightspeed.

~~~
raattgift
> [does gravitational wave detection imply the waves are particles]

No, not really.

However, if we deliberately quantize the gravitational waves, replacing the
classical waves with large numbers of particles, the fact that the classical
waves propagate at the speed of light means that the particles must be
_massless_ , like photons.

There are a couple of families of alternatives to General Relativity that have
"massive gravity", which requires that gravitational waves (or the particles,
if we quantize gravitational waves) have a small nonzero mass. This means
light will propagate faster than gravitational waves, and with some care, this
can solve a couple of the handful of problems about the very early hot dense
universe, namely in how it became so thermalized across huge regions of space:
any sort of pressure density released lots of photons which carried the local
energy away faster than gravitation could "react" and contain the heat
locally. Unfortunately for such theories, the mass of gravitational waves has
to decay away well before the formation of the cosmic microwave background,
since massive gravity would leave unmistakeable markings in it that we do not
see.

Verifying that gravitational effects do not outrace optical effects
effectively kills off all but the most complicated (early-decaying, thus
dynamical) massive gravity theories, and those are rejected on parsimony
grounds (the complexity is not needed to describe observations).

You're right that matter embedded in an inflating universe is not limited by
the speed of light. As with galaxies now, the matter remains more-or-less at
the same comoving coordinates throughout the history of the universe, even
during inflation. We don't see shear effects in distant galaxies even given
the redshift (if all Doppler/motion related) implies ultrarelativistic
recession velocities. The galaxies far away look pretty normal, moving slowly
within clusters, with the clusters separating. However, lots of new space is
being created between each and every galaxy cluster, but the galaxies do not
"feel" acceleration. They continue merrily along their almost exclusively
timelike worldlines.

Inflation is essentially an extremization of this: lots of new space gets
created in between the hot dense low-entropy matter of the pre-inflationary
universe, and more new space gets created in less-dense/higher-entropy spots,
which makes them even less dense / higher-entropy, which lets yet more space
be created there, and so forth, for many "e-foldings". The pockets of gas do
not "feel" like they are moving relative to one another, although if they had
radars or the like, they could certianly see that during inflation they get
much further away from each other (lots of radar signals will simply never
return to the sender, or even be received at the target).

At some point in the inflationary epoch the production of new space "cools
off" and inflation ends, after which the accelerating expansion is driven by
the residual small positive cosmological constant. Structure formation then
collapses the relatively dense relatively low entropy matter into the first
stars and galaxies, while the space between these areas gets ever less dense
and ever higher entropy. Some time after inflation we switch to the modernized
version of the expanding Einstein-de Sitter universe (1932), which in the late
1980s/early 1990s was the starting point for the current standard cosmology
(\Lambda-CDM). \Lambda-CDM does a much better job describing observations
relating to the early universe (before the first galaxies) and recent-epoch
acceleration than the Einstein-de Sitter model.

(Note, this is a broad stereotyping of cosmic inflation models; and for
completeness, one can still get \Lambda-CDM from the hot dense phase (the "big
bang") without inflation. Evidence from e.g. the Planck satellite has killed
off some flavours of inflation and non-inflation proposals, and forthcoming
evidence from Planck and other platforms will surely kill off many more.)

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dsnuh
I try to be well read on physics, but a lot is admittedly beyond my grasp at
the moment, so forgive me if this is a silly question.

Is this saying that gravity would not be possible without the curvature of
space because if space-time paths, then no matter as we know it would be
possible because all particles would be on their own timeline orthogonal to
all others? And since no collisions could happen if this were the case, no
energy in the system would be converted to mass to bend the space-time and
create the geodesic curvature gravity emerges from? If I have this right, does
this mean all energy in this system would be massless (photons?)

~~~
raattgift
I'm not sure what you're asking, exactly. Please ask again, if this doesn't
help:

Photons have "gravitational charge", in that they both feel other bodies'
gravitational influence, and induce a gravitational influence on other bodies.
One sees the "feeling" part in observations of gravitational lensing even
around our own sun; the generating part is harder to demonstrate, but consider
[https://en.wikipedia.org/wiki/Kugelblitz_(astrophysics)](https://en.wikipedia.org/wiki/Kugelblitz_\(astrophysics\))

In General Relativity, we explain the gravitational charge in terms of
spacetime curvature; light both generates curvature (because it has momentum)
and responds to curvature (because of the universality of free fall).

An isolated photon (or even a non-extreme classical beamlike pulse of light)
generates so little curvature that we can safely ignore it in an otherwise
Minkowski (flat) spacetime; however, in both cases if it traverses a region of
spacetime that is not flat, the isolated photon and classical pulse will
respond accordingly.

More technically, light contributes to the matter tensor, whose components
encode the flux of momentum from one direction to another. Although light has
no rest mass, it does have energy-momentum (in special relativity, E = pc;
additionally, the Planck-Einstein relation for photon energy E = h\nu, where h
is Planck's constant and \nu is the frequency of the photon). So a higher
frequency photon contributes more to the stress-energy tensor than a lower
frequency photon, and even the lowest-frequency imaginable contributes more
than vacuum. (Aside: General Relativity runs into localization problems with
extremely low frequency photons if one piles up a lot of them in a region of
space comparable in diameter to their wavelengths; they should mutually
gravitate, and possibly collapse, but exactly how is so hard to show that
probably nobody's done so successfully. How do gravitational interactions
among the photons change their wavelengths? (This is an aspect of quantum
gravity; there are related practical problems with Hawking radiation,
especially near final evaporation, for instance)).

> particles would be on their own timeline orthogonal to all others

You can contrive a "dimpled" mostly-flat spacetime in which small masses are
so widely separated that the curvature they induce is not felt by their
neighbours or vice-versa. The contrivance is not especially awful, because we
do something similar with "swiss cheese" models of cosmology, where the holes
in the cheese are galaxy clusters generating a non-expanding or even
collapsing metric (like Schwarzschild or Lemaître-Tolman-Bondi) and the body
of the cheese is an expanding Robertson-Walker metric. Already separated holes
"float" in the cheese, and the expansion just separates them with ever more
cheese, without interfering in the evolution of the galaxies etc. in the
holes. Holes which are too close together coalesce into a single larger hole.
The spacetime curvature of an expanding "swiss cheese" is extremely strong,
but we can turn off the expansion and it is similar to the "dimpled" mostly-
flat one; in fact you could model the "dimples" as tiny patches of
Schwarzschild space stitched into Minkowski space [1]. The "holes" that are
sufficiently separated by the non-growing cheese would stay separated forever.
In all these cases, the worldlines of the centres of masses of the holes are
essentially entirely timelike.

\- --

[1] This kind of "stitching" using junction conditions is straightforward if
extremely complicated and subject to a number of conditions/assumptions.
Misner, Thorne & Wheeler's _Gravitation_ has a chapter on Israel junction
conditions, and various authors notably Darmois, O'Brien, Synge, Bonnor and
Raju have catalogued stitchings of different types of metrics into one
another. Most of these would likely require a trip to sci-hub, as they were
written largely in the 1970s to 1990s. Unfortunately, wikipedia seems very
sparse on this topic (see for example the Inhomogenous cosmology entry, which
is desperate for some love [<\- possible note to self]).

------
iamjdg
I think a mass bends/distorts spacetime the effect of this bent/distorted
spacetime on other masses is what we describe as gravity.

~~~
vardump
IANAPhysicist.

In other words, most likely the following is totally wrong armchair physicist
speculation. :) If there are any experts (=physicists) around, I'd love to
know why it's wrong.

What if mass doesn't even distort spacetime, but only "slows" down time
locally.

Maybe there's a some sort of "quantum progression" limit per volume of space.

You'd get gravity out of local time slowdown, because of gradient effects. The
object under influence of gravity would experience time slightly slower on the
side that is closer to a gravity well. This should cause acceleration towards
the larger mass.

Gravity's inverse square law could be a result of this local effect depending
somehow on wavelength. Longer wavelengths would affect objects further away.

Perhaps you could derive theory of relativity out of a local quantum
progression limit.

~~~
raattgift
The problem here is that you can't represent tidal deformations purely through
time dilation, since they bring (parts of) objects closer together. The tl;dr
is: "how does your idea explain that Earth is oblate, with objects at the two
poles slightly closer together than objects on the equator but on opposite
sides of the planet?" General Relativity explains this as tidal forces acting
on the planet, which is in approximate hydrostatic equilibrium, but rotating.
The same features explain why a measurement of the local gravity (the field
strength "g") at the poles gives a higher number than at the equator. It also
explains the shape of the moon in its synchronous orbit.

In General Relativity we can decompose the Riemann curvature tensor into the
Weyl tensor, the Ricci tensor, and the metric tensor. The Weyl tensor encodes
the squash-stretch deformation of moving objects experiencing shear, for
example, a rotating spherical body deforming into an oblate spheroid, or a
spherical body moving close to (including synchronously orbiting) a large mass
deforming into a scalene ellipsoid. The Ricci tensor encodes the volume
deformation of a body experiencing tidal forces; if we drop a ball of dust
(think very loosely packed coffee grounds) onto a planet, the Ricci tensor
describes the tendency of the individual dust particles to converge into a
smaller volume as they each fall "straight down" on individual lines
converging at the centre of the planet.

Naively, we could say that there is a clear relationship between the Ricci
tensor and gravitational time dilation, since time is running slower in the
direction where the individual fall paths ("geodesics") converge. However, we
can make the Ricci tensor go to zero by dropping the dustball onto a planet
with enormous radius, such that it's effectively flat. Similarly, ultra
massive black holes have essentially no Ricci curvature just outside the
horizon, while low-mass black holes have significant Ricci curvature. However,
gravitational time dilation is still very strong just outside the horizon of
such a massive black hole. In this way we can distinguish between spatial
curvature (or if you like, length contraction) and curved time (or if you
like, time dilation) in the region around massive bodies. Gravitational time
dilation is not a tidal effect, but rather depends on the gravitational
potential, which in General Relativity is encoded in the metric tensor.

If we orbit our dust ball very rapidly around a heavy planet, it will have
substantial Weyl curvature (squashed along its axis of motion, stretched on
the other two spatial axes). If the planet has a big enough radius, the Ricci
tensor vanishes, leaving us with just the Weyl curvature. In that case, if we
measure periodic microscopic processes ("clocks") closest to the planet and
furthest from the planet, we will see that the latter are running faster than
the former, and the difference is precisely that of gravitational time
dilation. Again, this lets us distinguish between changes in length and
changes in duration in the region around massive bodies.

Gravitational effects are the result of curved _spacetime_ , although in
special circumstances practically all of the curvature can be in the timelike
axis, and in other special circumstances practically all of the curvature can
be in the three spacelike axes.

I don't understand what you mean by "quantum progression".

~~~
vardump
Thanks for your reply, I'll try to digest all that. :)

I don't yet understand why "local" slowing of time wouldn't generate all of
those tensor components.

> I don't understand what you mean by "quantum progression".

Just hypothetical local limitation on quantum state changes (=progression).

~~~
mikhailfranco
I think any computer scientist naturally and naively thinks about your
'quantum progression', since it relates to a transactional rate on
interactions experienced in the local proper time of a particle, as it
interacts with other real/virtual particles/fields.

For example, a particle in deep space experiences faster relative time,
compared to one on Earth, because it has 'fewer transactions slowing it down'.
The discrete 'tick' of proper time is then one iteration of the spinning
polling loop the particle executes while waiting for something to interact
with.

So if existence is (or requires) computation, then ....

~~~
raattgift
> a particle in deep space experiences faster relative time, compared to one
> on Earth, because it has 'fewer transactions slowing it down'.

Transactions with what? Other matter?

A precise weather and waterproof clock on the ground (so in air) on the
surface at the north pole will tick more slowly than an identical clock
immersed in seawater several metres below the sea level at the equator, thanks
to the oblation of the Earth (and the rotation that causes the oblation). The
equatorial water is much denser and warmer than the arctic air, so surely
there are more interactions between the water and the clock?

See the Early observations subsection of the Gravity measurement section of,
starting with 1672 :

[https://en.wikipedia.org/wiki/Pendulum?oldformat=true#Early_...](https://en.wikipedia.org/wiki/Pendulum?oldformat=true#Early_observations)

------
quandarium
If you reduce all energy exchange to particles in a vacuum, most of the
particles transfering energy are photons. Energy transfer, in general is
mostly reducible to photons at a fundamental level. In other words, across the
vacuum of space, in an idealized model, where there is nothing but void, any
exchange or transfer is via particles traversing the emptiness, and most
often, those particles are photons.

If two bodies at absolute zero are suspended in a void, and the limits of that
void are walls of matter frozen at absolute zero, light years away, and
nothing in the scene is emitting photons, what reason would drive the to
objects to be attracted toward one another?

This is to say, if the arrow of time is a deterministic effect of entropy, and
the irreversible aspect of time is a result of the chaotic interactions of
many discrete subatomic entities, why would spacetime still be curved by two
dark, massive, cold bodies, such that, even in the absence of photon exchange
between the two, there are forces acting upon them at a distance, leading them
to cross a void, travel down some unseen curvature or gradient and collide?

If empty space truly is nothingness and void, then entropy, and by corollary,
the arrow of time, only exists in the quantum state of each material body.

This would seem to herald a fact that the bodies each move themselves toward
eachother, as an incidental phenomenon of being condensed from raw unbound
energy to systems of materialized standing waves, (subatomic particles trapped
in fixed discrete systems of balanced energy, which does not readily decay
into other particles without action from external entities).

In total darkness, without the exchange of photons, why would two massive,
cold, inert objects move inexorably toward one another?

How would each massive object find the other's location and accelerate toward
the other, if there is nothing but empty space between?

