
'Dark fluid' with negative mass could dominate the universe - pavel_lishin
https://theconversation.com/bizarre-dark-fluid-with-negative-mass-could-dominate-the-universe-what-my-research-suggests-107922
======
galadran
Paper:
[https://arxiv.org/pdf/1712.07962.pdf](https://arxiv.org/pdf/1712.07962.pdf)

Not a physicist, but my summary of the author's point would be:

If you allow for matter with a negative mass and plug it into a n-body
simulation. You seem to get what looks like dark matter from first principles.
Namely, a halo of non interacting matter around galaxies in just the right
"non-cuspy" shape. Their conclusion is that this is worth considering, as
almost all mainstream cosmology makes the assumption that mass >= 0, despite
the fact mass <=0 doesn't seem to violate any physical invariants we know.

The idea is persuasively simple. Every other force besides gravity is
polarised, so why not also gravity? It seems interesting to me, but I'm
nowhere near knowledgeable enough to know whether it can be easily refuted.

~~~
andrepd
Here's an argument:

If you think about it, "mass" actually means two different things. One one
hand, there is mass as the "charge" of gravitational interaction, that is, the
gravitational field generated by a body is proportional to its mass. Let's
call this concept "gravitational mass". On the other hand, you have mass in
the sense of "intertial mass", a body's resistance to change of velocity.
Although we kinda take it for granted, it's weird that these two concepts
coincide, and not something that would be necessarily true _a priori_.

Asserting this equivalence as a principle is the starting point for Einstein's
theory of General Relativity, which is one of the most beautiful and
thoroughly tested theories in modern physics.

On the other hand, you can show that the inertial mass cannot be negative
(else the Lagrangian would not be bounded from below and therefore you could
not minimise it; the whole of physics then comes crumbling down). Therefore,
if that equivalence stands, the gravitational mass also cannot be negative.

~~~
thaumasiotes
> On the other hand, you can show that the inertial mass cannot be negative
> (else the Lagrangian would not be bounded from below and therefore you could
> not minimise it; the whole of physics then comes crumbling down).

Here's a purely thought-experiment argument for inertial mass not being
negative:

If you had an object with negative inertial mass, then pushing on that object
would cause it to accelerate into your hand. This is a positive feedback loop
that immediately results in the object and your hand exerting infinite amounts
of force on each other, because applying force just leads to applying more
force.

~~~
FabHK
Here's a thought experiment that shows that there can't be electrons:

"If you had an object with negative charge, then putting that object near a
positive charge would cause it to accelerate towards the positive charge."
Getting closer increases the force. "This is a positive feedback loop that
immediately results in the object and the positive charge exerting infinite
amounts of force on each other, because" getting closer "just leads to
applying more force."

Thus, there are no electrons.

So, I don't think the thought experiment rules out negative mass. In fact, the
article explicitly says "if a negative mass was pushed, it would accelerate
towards you rather than away from you.", and I assume that if that's an
obvious contradiction, the author would have caught it.

~~~
thaumasiotes
So, electrons are attracted to protons by the electromagnetic force, but they
don't make it all the way there because they are repelled by some other force.
You don't have a paradox because our model of the electromagnetic force
doesn't specify that it's the only force there is.

But if a negative mass is moving towards you and encounters a repulsive force,
that repulsive force will, by definition, move the negative mass further
toward you. It would need to encounter an _attractive_ force to reach
equilibrium.

Note that, unlike in the electron example, it doesn't matter what kind of
force is being applied; the object's negative mass tells us that the response
to any repulsive force is acceleration towards the force.

~~~
millstone
> they don't make it all the way there because they are repelled by some other
> force

A repulsive force is not the best way to think about it. The potential of the
nucleus is the usual -1/r, and goes to (negative) infinity at zero. A
repulsive force would be incorporated into the potential and appear as a bump
around the nucleus, and would mess up the electron orbital.

A hand-wavy explanation of why the electron doesn't fall in: if you try to
push the electrons into the nucleus, you necessarily localize the electron
into a smaller volume; this means its wavefunction must get "spikier" and
therefore it has more kinetic energy. This kinetic energy rises faster than
the potential energy drops, so the state of lowest energy is actually found at
an average radius > 0.

Note that it took quantum mechanics to rescue the atom. A classical electron
could fall into the nucleus.

I do not see any problem with a negative-mass particle accelerating towards
the force. The analogy with opposite charges seems right to me.

~~~
btilly
A different hand-wavy explanation. The places that an electron can be found
are described by a wave pattern. If the electron is staying in place around a
nucleus, that wave pattern has to be a standing wave that reinforces itself.
To reinforce itself it has to wrap around the nucleus an integer number of
times.

This explanation doesn't just explain why it doesn't fall in, it also explains
why there are discrete shells that it could be found in, corresponding to how
many times it wraps around the nucleus. (It doesn't explain why only a finite
number can fit in each shell though. Or why bigger shells can have more
electron orbitals. Or...well the actual theory has to be good for something!)

~~~
thaumasiotes
I'm not really interested (here) in the reality of the interaction between the
electron and the nucleus. I don't think the existence of an electromagnetic
field is a good argument against its own existence as argued by FabHK further
up.

I also don't think FabHK's argument works as an analogy to my problem with
negative mass. The electromagnetic field is not self-reinforcing in the same
way.

Imagine that you're holding a marble of negative glass in your fist. Negative
glass is indistinguishable from ordinary glass except that its mass is
negative rather than positive.

As we all know, the first step in solving any physics problem is to draw a
free-body diagram. ( [http://www.smbc-
comics.com/comics/20130616.png](http://www.smbc-
comics.com/comics/20130616.png) ) Let's draw one here. First, we'll do one for
an ordinary marble:

1\. The enormous mass of the earth attracts the marble downward
proportionately to the marble's mass.

2\. The marble cannot accelerate downward, because it's stuck in your fist.
Your fist experiences a downward force equal to the weight of the marble.

3\. By Newton's third law, your fist exerts an upward force on the marble
equal to the force exerted by the marble on your fist. This is exactly equal
to the weight of the marble, but in the opposite direction. The two forces
cancel, and the marble is at rest.

Now for the negative marble:

1\. The enormous mass of the earth attracts the marble downward
proportionately to the marble's mass. Because that mass is negative, the
marble attempts to accelerate upward.

2\. The marble can't accelerate upward, because it's stuck in your fist. Your
fist experiences an upward force equal to the weight of the marble.

3\. By Newton's third law, your fist exerts an downward force on the marble
equal to the force exerted by the marble on your fist. This is exactly equal
to the weight of the marble, and in the same direction, effectively doubling
the marble's weight. The marble is now trying twice as hard to accelerate
upward into your fist.

2\. (Again.) The marble can't accelerate upward, because it's stuck in your
fist. Your fist experiences an upward force equal to double the weight of the
marble. Nothing has moved; we're still just trying to work out the balance of
forces within the system at rest.

3\. (Again.) You can see where this is going.

What is the conceptual breakthrough that rescues negative mass from this trap?
(Note that saying the marble has negative inertial and gravitational mass, as
opposed to negative inertial mass and positive gravitational mass, doesn't
help: the marble will be trying to accelerate downward instead of upward, but
it will still be doing it with infinite force.)

~~~
petre
Maybe that's precisely why objects with negative mass do not exist on the
Earth? But what stops them to exist in the void of space, between regular
matter? And it doesn't need to be actual "objects", but isolated particles or
a particle gas. In relativistic physics mass is dependent on the body's
energy, so negative mass implies negative energy.

------
dragonbonheur
Looks like the rest of the world is about to catch up with Jean-Pierre Petit's
theories about negative masses surrounding positive mass objects like galaxies
and holding them together... Even the proposal for the modification of
Einstein's theories... Times are a'changin.

It took almost ten years (2007-2017) from JPP being shunned by the scientific
community (and the highly politicised French Wikipedia) to others
"discovering" the same things he had been saying all along.
[https://arxiv.org/abs/0712.0067](https://arxiv.org/abs/0712.0067)

[http://jp-petit.org/papers/cosmo/2018-AstrophysSpaceSci.pdf](http://jp-
petit.org/papers/cosmo/2018-AstrophysSpaceSci.pdf)

You may give yourselves the smallest pat on the back in the universes.

~~~
DoctorOetker
figure 1 of that paper is virtually identical to the galaxy rotation curve of
the current paper! both were N body simulations

thanks for pointing out this earlier work

------
scott_s
I found the author's Twitter summary of the paper useful as well:
[https://twitter.com/Astro_Jamie/status/1070302359325151233](https://twitter.com/Astro_Jamie/status/1070302359325151233)

~~~
jobigoud
Threadified:
[https://threadreaderapp.com/thread/1070302359325151233.html](https://threadreaderapp.com/thread/1070302359325151233.html)

------
FabHK
I've personally verified this part of the paper:

> Observations clearly indicate that the Universe is not empty.

EDIT to add: Another nice, slightly Douglas-Adams-y quote:

> This implies that our Universe is just one of those things that happen on
> occasion, and we can simply think of its existence as being illustrated by a
> 1 billion-σ statistical event.

------
lawlessone
>The gravity from the positive mass galaxy attracts negative masses from all
directions, and as the negative mass fluid comes nearer to the galaxy it in
turn exerts a stronger repulsive force onto the galaxy that allows it to spin
at higher speeds without flying apart.

I don't get this, seems contradictory.

~~~
sp332
Maybe it was modeled as a curvature of space? So positive masses curve space
so that everything moves toward them - including light, massless particles,
and negative mass. And negative mass repels everything, including other
negative mass, by shaping space differently.

[Edit: jbay808 has a better explanation below, and negative mass _attracts_
negative mass.]

That behavior is described here
[https://en.wikipedia.org/wiki/Negative_mass#Runaway_motion](https://en.wikipedia.org/wiki/Negative_mass#Runaway_motion)
but it's not what I was expecting either.

~~~
astrocat
Yeah this helped me create a mental picture that kind of makes sense. It would
clarify why negative mass is also concentrated in areas of high mass, and
could also provide the mechanism for constant universe expansion, as the space
between masses is constantly being pushed away by the diffuse negative mass.
Heh, armchair physics is fun :)

------
radarsat1
> Air bubbles in water can be modelled as having a negative mass.

Forgive the naive take on this, but it's a weird point to me... if air bubbles
in water can be _modelled_ as having negative mass, but clearly do _not_ (they
are made of particles with positive mass), then they are only "negative" in
relation to the material surrounding them. So even if they can be modelled
this way successfully, clearly the model doesn't imply for certain that they
do have negative mass. So by analogy, just because a negative-mass model can
explain the observations of dark matter, doesn't it still leave the question
of whether anything "real" is implied by it? Perhaps it's just a reflection of
a negative relative to some unknown bias (field), like the Higgs field, but in
"absolute" terms is not actually negative.

(Not sure "absolute" has any meaning here..)

~~~
archibaldJ
The concept of whether something is "real" is a metaphysical amusement. And it
is never meant to be answered in modern physics. Physics has come a long way
into what it is today from natural philosophy. I believe all physicists in
this day and age subscribe to the notion of model-dependent realism in one way
or another. [https://en.m.wikipedia.org/wiki/Model-
dependent_realism](https://en.m.wikipedia.org/wiki/Model-dependent_realism)

Perhaps someday in the distant future we will discover that many assumptions
in physics are simply wrong since we really do indeed live in a computer
simulation and it is one where the Von Neumann architecture holds so it is
always possible to alter "physics laws" like the Pauli exclusion principle and
"physics constants" like the speed of light. And these "physics laws and
constants" do get altered under certain if-statements or else there will be
bugs and the simulation will crash, rebooting itself as a result.

Or perhaps this simulation does crash and reboot itself very often. It's like
elixir. We simply are not aware of it. Maybe that could be why we have things
like the Heisenberg uncertainty principle and quantum tunnelling.

~~~
TangoTrotFox
Or not so distant future. There has been a relative standstill in physics in
spite of perhaps more 'known unknowns' than ever before. The problem with
models independent of 'reality' is a lack of falsifiability. This in turn
means the model itself becomes indefinitely self sustaining even when it's
completely wrong as each wrong prediction is then simply massaged back into
the model to artificially produce a correct answer.

For instance a recent conversation I had was discussing the big bang and the
cosmic microwave background radiation. Somebody thought that the CMB was
evidence of the big bang. Of course when the CMB was first observed/measured
it completely falsified our idea of a big bang due to the horizon problem --
regions of space that should not be causally connected are somehow
homogeneous, which should be impossible. But instead of this observation !=
prediction indicating some fundamental problem we instead add in a magical
hyper-inflation period to the model of the big bang so that the model can now
"predict" what we see. And now you have people that believe that the CMB
itself is evidence of the model's accuracy, when in reality the CMB refuted
our models and we were forced to massage it back into the model in a rather
arbitrary fashion. There is absolutely no physical reason to accept hyper-
inflation other than 'if we add this magic event, then we can keep using this
model.'

For a less contemporary example consider geocentricism - the idea that
everything rotated around the Earth. Prior to Newtonian mechanics and other
'real' systems of interaction, we simply used models for orbital mechanics.
And these models ended up being quite rediculous. You had planets doing
magical swirlies, stopping and going backwards, and all other sorts of things.
But because you can't falsify models detached from reality, everybody just
shrugged their shoulders and accepted it for something on the order of
centuries. And as time passes these models become even more difficult to
replace because it ends up meaning you'd basically have to toss something on
the order of decades, if not centuries, of past work. In this particular case
astrology, for instance, was in the past a scholarly and academic field
analogous in many ways to psychology today. Refuting geocentricism involved
completely scrapping this entire field, and centuries of work within it, due
to the fact it took things such as Mercury going backwards as key components
of its analysis of human psychology.

The point of this all is that as you accept models, it can be that you're
searching for answers at the entire wrong level of a problem. For instance
astronomers in times of a geocentric universe may have been trying to explain
why Mercury went backwards during its orbit, yet it's extremely difficult to
answer something when you start with a wrong assumption -- in that case that
the planet does indeed go backwards at some point, planets have these orbital
swirlies, and so on. People like to rewrite history to blame geocentricism on
the church, but inertia is not limited to divine belief.

------
mirceal
If anyone is interested in dark matter and dark energy I recommend this book:
[https://www.amazon.com/Our-Mathematical-Universe-Ultimate-
Re...](https://www.amazon.com/Our-Mathematical-Universe-Ultimate-
Reality/dp/0307599809)

Written by this guy
[http://web.mit.edu/physics/people/faculty/tegmark_max.html](http://web.mit.edu/physics/people/faculty/tegmark_max.html)
is does a fantastic job of going from really simple concepts/experiments to
blowing your mind.

------
Pxtl
Would this be only gravitational mass that's negative or would it be both
gravitational mass and inertial mass? Because negative inertial mass is zany
and wierds me out. Negative gravitational mass behaves in a way that makes
sense to me.

~~~
walrus1066
Has to be gravitational only. Otherwise positive mass is repelled from
negative mass, but the negative mass accelerates towards it. So both masses
accelerate in the same direction, until they have infinite kinetic energy!

Conservation of energy & momentum would still hold, because negative mass
would have negative energy & momentum, cancelling out that of the positive
mass.

But the above surely would be an 'unphysical' situation.

~~~
jon_richards
>Negative masses are a hypothetical form of matter that would have a type of
negative gravity – repelling all other material around them. Unlike familiar
positive mass matter, if a negative mass was pushed, it would accelerate
towards you rather than away from you.

Pretty sure the author means both.

A continuously accelerating system seems "unphysical", but we have an example
already: the accelerating expansion of the universe.

------
mklauber1
I'm curious about the idea of "allow[ing] negative masses to not only exist,
but to be created continuously." Given the law (in the scientific sense) that
matter/energy cannot be created or destroyed, on a scale of 1 to FTL travel,
how crazy is this idea?

~~~
pavel_lishin
I thought it was less a law and more of an observation.

~~~
Retra
Scientific laws are just quantified observations.

------
russdill
Sounds a lot like the concept of the Dirac sea to me.

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

Rather than have dark matter particles, have negative mass dark matter
particles in every other location instead.

~~~
sixo
This is explicitly very different from that. There's no claim of "negative
mass dark matter particles" in _ever_ location, rather, they have a specific
distribution.

~~~
tylerjwilk00
That distribution being all the places that aren't normal matter particles. So
primarily outside of galaxies.

~~~
jon_richards
No. The negative mass matter clusters around galaxies because it accelerates
towards positive mass and away from other negative mass. The area between
galaxies becomes largely empty in the simulations.

------
agapon
I recall reading article in a popular science magazine about hypothetical
negative mass with exactly these properties. It was about 25 years ago and the
magazine was Russian language. Unfortunately, I can't recall its name now.

~~~
agapon
Oh, thanks to the power of the internet search, I found a scan of that
article: [http://epizodsspace.airbase.ru/bibl/tm/1990/10/otrits-
massa....](http://epizodsspace.airbase.ru/bibl/tm/1990/10/otrits-massa.html)
The journal is "Техника-молодежи" (a Soviet style name that I am incapable of
translating to English) and the article is from 1990.

And that article refers to a publication by Robert Forward in a journal with a
name like Aerospace Technology or Aerospace Engineering. But I found an
article in on the same topic in NewScientist:
[https://www.newscientist.com/article/mg12517084-200-the-
powe...](https://www.newscientist.com/article/mg12517084-200-the-power-of-
negative-matter/)

Wikipedia, of course, has an article on Negative mass as well.

~~~
FiatLuxDave
Thank you! The first thing I did when coming to this thread was search for
posts containing "Robert Forward", as any discussion of negative inertial mass
is incomplete without mentioning his work. So, kudos.

Anyone interested in the implications of negative mass should probably check
out Timemaster, a sci-fi novel by Forward. Negative mass can hypothetically do
some interesting things. Things such as allowing lightspeed travel by matching
negative mass to positive mass in equal amounts, leaving a non-photon object
with rest mass = 0, or producing energy by accelerating negative mass to high
velocities.

Unfortunately, Forward is much better as an idea guy than he is as an author.
The scene where the hero travels back in time and has a threesome with himself
and his wife is a bit awkward...

------
hirundo
What's the relationship between negative mass and negative gravity and anti-
gravity? Dark fluid is repulsive, so is it theoretically possible to collect a
ball of it, put it between you and another positive mass, and fly apart?

~~~
RootKitBeerCat
Yes, that is essentially my understanding

------
sulam
Sounds a lot like phlogiston, especially since it apparently must be
continuously created to matched observable behavior.

~~~
therein

      Beware the Physical Chem, my son!
      The laws that arn’t, the constants that vary.
      Beware the phlogiston, my little one
      And of the molality be wary.
    
      He took his entropy in hand
      Long time the adiabatic foe he sought
      So rested he by the delta T
      And stood awhile in thought.
    
      And as in Newtonian Thought he stood
      The Physical Chem with Beer’s Law Plot,
      Came Bohring through the orbital wood
      And took quantum leaps when hot!
      PV! RT! And with fugacity,
      The entropy discharged his wrath.
      It reached a degenerate state, and being late,
      He returned by the mean free path
    

I wonder if anyone will recognize this. :)

~~~
deathanatos
If by recognize you mean that it's a variation/remix of "Jabberwocky"[1], then
yes.

The second corresponding verse (so this corresponds to "He took his entropy in
hand"), so others can see the resemblance (I chose the second since the first
is a bit harder to see):

    
    
      He took his vorpal sword in hand:
      Long time the manxome foe he sought—
      So rested he by the Tumtum tree,
      And stood awhile in thought.
    

[1]:
[https://en.wikipedia.org/wiki/Jabberwocky](https://en.wikipedia.org/wiki/Jabberwocky)

~~~
therein
Very cool. Today I learned something about a AXE (Alpha Chi Epsilon chemistry
fraternity).

------
hirundo
Here's an article describing the theory that sound waves have negative mass.
It also describes it in terms of the behavior of fluids.

[https://www.realclearscience.com/articles/2018/08/11/could_p...](https://www.realclearscience.com/articles/2018/08/11/could_phonons_have_a_negative_mass_110719.html)

Maybe these phenomena are linked, and it's more immediate than cosmology.

via: [https://twitter.com/robinhanson](https://twitter.com/robinhanson)

------
noetic_techy
This then leads to the inevitable questions, is an Alcubierre Warp Drive
possible, since one of its pre-requisites was particles of negative mass?:

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

>>a spacecraft could achieve apparent faster-than-light travel if a
configurable energy-density field lower than that of vacuum (that is, negative
mass) could be created

If this matter is popping into space continuously as the author describes, it
could be possible to harvest it. The question is, how much of it is popping
into existence in lets say a square km of space (something we could feasibly
cover with our current tech), and how do we detect and capture it.

The author does readily admit that his theory may be wrong, but useful as a
mathematical tool. It's like saying "My theory may be the Newtonian Mechanical
model of dark matter + dark energy, but not the quantum (read: real) theory."
I sure as hell hope he is correct and its a real particle we could capture. It
could open up technology we have yet to realize and get us off this rock.

------
anentropic
Two passages from the article:

> Negative masses are a hypothetical form of matter that would have a type of
> negative gravity – repelling all other material around them. Unlike familiar
> positive mass matter, if a negative mass was pushed, it would accelerate
> towards you rather than away from you.

> The gravity from the positive mass galaxy attracts negative masses from all
> directions

...why aren't these contradictory?

intuitively... wouldn't a negative mass be _repelled_ by positive gravity?

(I am not a physicist of any kind, obviously)

In the author's Twitter thread
[https://threadreaderapp.com/thread/1070302359325151233.html](https://threadreaderapp.com/thread/1070302359325151233.html)

there is this image:
[https://pbs.twimg.com/media/Dtp7YjWW4AEIvSp.jpg](https://pbs.twimg.com/media/Dtp7YjWW4AEIvSp.jpg)

..which seems to answer my question

If I understood the picture:

\- positive mass gravities attract each other and result in acceleration
towards each other

\- negative mass gravities attract each other, but the negative mass means
this attractive force results in an opposite acceleration (they behave as if
repelled)

\- a positive mass and a negative mass have gravities which repel each other
so the positive mass accelerates away from the negative, but the negative mass
inverts its gravitation repulsion into an acceleration _towards_ the positive
(I find it hard to imagine how this interaction actually plays out, it's a bit
counter-intuitive.. I guess they must cancel out rather than chasing each
other faster and faster?)

------
planck01
Interesting rebuttal from someone in the field (who is actually quoted in the
paper): [http://backreaction.blogspot.com/2018/12/no-negative-
masses-...](http://backreaction.blogspot.com/2018/12/no-negative-masses-have-
not.html)

------
jxcole
One of the most interesting properties of matter with negative mass is that if
something collides with it it accelerates in the opposite direction of the
incoming object.

[https://en.m.wikipedia.org/wiki/Exotic_matter#Negative_mass](https://en.m.wikipedia.org/wiki/Exotic_matter#Negative_mass)

------
hliyan
Titles that describe poorly understood scientific phenomena as "bizarre",
"mysterious" etc., hurts science, IMO. I would expect this from a journalist
(though not excuse it), but an academic?

Why not something like "Negative mass dark 'fluid' halos around galaxies as an
alternative to dark matter"?

------
debatem1
Stupid non-physicist question here: we know the universe is expanding, but
where does it expand at? Are there physical areas which we can observe
growing? And if so, shouldn't that mean that we observe distant objects as
lighter than their interactions with closer objects would imply?

~~~
danbruc
Space expands everywhere, also right in front of you, it is just a very small
effect over short distances. Two points one meter apart expand away from each
other with a speed of 0.07 nanometers per year. For short distances
electromagnetic and strong forces just pull everything immediately back
together as things want do move apart. Over larger distances, think the scale
of galaxies, gravitational forces keep the stars, the gas, and all the other
stuff together and prevent galaxies from expanding.

But once you move beyond the scale of galaxies and galaxy clusters there are
no longer any forces strong enough to counter the expansion of all that space
between galaxies and so in general the distances between galaxies are
increasing. There are still exceptions, in clusters of nearby galaxies
gravitational forces can still bind those galaxies together but with
increasing distances between galaxies the expansion finally overpowers
gravitational attraction.

It is important to remember that not only are all forces becoming weaker as
distances increase but also that points are expanding faster away from each
other as their distance increases. As said, two points one meter apart move
away from each other with 0.07 nanometers per year but two points one
kilometer apart move away with 70 nanometers per year because there are a
thousand meters in between them, all of them simultaneously expanding at 0.07
nanometers per year.

~~~
raattgift
> Space expands everywhere, also right in front of you, it is just a very
> small effect over short distances ... .07 nanometers per year but two points
> one kilometer apart move away with 70 nanometers per year because there are
> a thousand meters in between them

No, what you wrote is contradicted by evidence from within the inner solar
system (MESSENGER's observations of solar mass loss were highly sensitive; we
have excellent VLBI too), from other star systems, especially eclipsing
binaries and systems with occluding planetary systems) and from galactic
dynamics (LSR tests, peculiar motions, and lots of DM/dynamics evidence for
galaxies at different redshifts). It is not how the standard model of
cosmology works either. Grossly, 0.07 nanometers per metre per year expansion
everywhere would be impossible to hide from solid state physics, and even
several everyday polymers and ceramics; an expansion term would have to appear
in accurate descriptions.

We cannot say there is _no_ expansion at these scales, but the expansion is
highly constrained and evidence requires that it effectively vanishes. A
fifth-force mechanism like quintessence requires a shutdown mechanism inside
galaxies that contain star systems like ours or TRAPPIST-1 or PSR~J0337+1715.
Additionally it likely would need a shutdown in small structures (like
asteroids) ejected from galaxy clusters by violent events, although we will
not spot those in practice any time soon. Where there's a shutdown mechanism
there's also a wake-up mechanism that has to be considered too, and high-
redshift observations constrain the wakeup of quintessence action to
relatively late times. What suppressed quintessence for a bit more than three
billion years after the formation of the cosmic microwave background?

These type of modellers run into the hard problem that as they work out the
parameters of their theory, they (so far) find they all are consistent with
the standard cosmology. (That shouldn't be too surprising; LambdaCDM, the
standard cosmology, was carefully built to concord with evidence "forward-
compatibly".)

One way to put it is in the name: cosmic expansion, rather than universal
expansion, distinguishing between effects apparent at the largest scales, and
effects apparent everywhere in the universe, at all scales.

LambdaCDM is a model of an universe well-described by an expanding Robertson-
Walker metric and matter in the large obeying the Friedmann equations. The
expanding metric includes a term for the cosmological constant. The matter is
essentially a space-filling fluid at rest, isotropic, homogeneous, and being
diluted away by the expansion of the background Robertson-Walker spacetime.
(It is in a special frame of reference in this model in which Dark Energy
arises as a non-diluting homogeneous fluid imposing constant isotropic tension
on the matter fluids. In general frames Dark Energy is just the cosmological
constant.)

On the LambdaCDM model we can overlay the 'swiss cheese' model; this is
standard too, but we are departing from cosmology and heading toward
astrophysics. The motivation of 'swiss cheese' is simple: at the largest
scales, the Friedmann Robertson-Walker model sketched above describes all
observations between well and _extremely_ well. However, it does not describe
gravitationally-bound systems like galaxy clusters, galaxies, or star systems.
Those are much much better described with conventional clumping matter (and
dark matter) inside a collapsing Tolman spacetime. Notably, none of these
systems are homogeneous (they contain lumpy bits like planets and stars, and
sparse bits like the interstellar medium) and all of them contain radiating
orbiting material; that they are radiating implies gravitational collapse
(there's also plenty of other evidence for that). The Friedmann-Lemaître-
Robertson-Walker (FLRW) cosmological model does not describe these systems.

In 'swiss-cheese', the FLRW cosmological model is the cheese. If we treat the
cosmological-scale matter fluid as a dust that is only homogeneous at the
largest scales, the dust can be lumpier in some places and sparser in others.
The lumpiest bits represent galaxy clusters that remain gravitationally bound
over cosmological times. We then "swiss" this lumpy cheese procedurally: we
cut out a region of the Friedmann matter on the Robertson-Walker background
and replace it with a region of galaxy-cluster matter on a Tolman background,
and stitch the two together using junction conditions on the boundary.
(Typically we do this to the densest lumps, the galaxy clusters, but we could
do the same procedure in deep intergalactic space instead. A spherical region
of that will not be exactly void, since it will contain at least photons and
neutrinos produced in the early universe, likely a bit of baryonic matter from
big bang and supernova nuclear synthesis, and possibly a small amount of dark
matter too: however all this stuff is so sparse that it would not generate a
collapsing spacetime metric -- calculated out, it would resemble a slightly
perturbed expanding Robertson-Walker metric. So we don't put "holes" there;
our "holes" contain galaxies.).

In a swiss-cheese model there is simply _no_ expansion in the "holes", because
the Tolman backgrounds that work do not contain a cosmological constant term.
(The typical problem is that something that _does_ contain expansion, like a
Kottler vacuole, evolves away from a mass-compensating comoving void, e.g.,
the hole grows much too fast or in strange ways that do not match any of the
tens of millions of galaxy clusters in our sky).

Since Tolman/FRW swiss cheese models [a] are tractable in practice and [b]
match observations extremely well, it is perfectly reasonable to take the hint
that nature doesn't expand _at all_ inside the holes. That is, the non-
expansion of the hole background carries down hierarchically; you don't need
to introduce an expansion term into a metric describing an individual star or
planet.

It would be very exciting to find _any_ metric expansion within the solar
system, or in another solar system under close scrutiny, or at any scale
smaller than that of the swiss-cheese "holes".

Since real matter systems like stars and planets can be very well described as
a "hole" generating a metric like Schwarzschild out to some boundary, there is
no motivation to add in a metric expansion term (it would just vanish within
the boundary).

Finally, inhomogeneous cosmologies generally import these results, just like
they import the local and global measurements of H_0. The viable scale of
fluctuations in the Hubble parameter in such approaches is large compared to
solar systems. So there's no relief in a "well, your model is wrong"
(incidentally, I'd agree, there are some real problems with swiss-cheese).

In summary, the evidence is against your claim, and other strands of evidence
support theoretical frameworks in which there is _no_ metric expansion around
Earth/Earth-Moon. That includes frameworks in which one has explicit terms for
such expansion: those terms must vanish around here.

\- --

See also: the much less wordy way of taking the same position at
[http://curious.astro.cornell.edu/about-us/97-the-
universe/ga...](http://curious.astro.cornell.edu/about-us/97-the-
universe/galaxies/cosmology/538-as-the-universe-expands-why-don-t-galaxies-
get-stretched-out-intermediate)

Compare: [https://medium.com/starts-with-a-bang/ask-ethan-if-the-
unive...](https://medium.com/starts-with-a-bang/ask-ethan-if-the-universe-is-
expanding-why-arent-we-71b46b5e9974)

~~~
cbzbc
I don't think this is correct:

"Grossly, 0.07 nanometers per metre per year expansion everywhere would be
impossible to hide from solid state physics, and even several everyday
polymers and ceramics; an expansion term would have to appear in accurate
descriptions."

On a small scale, the effects of the expansion are overcome by other forces --
this doesn't mean that the expansion doesn't occur - merely that its effect is
locally overcome and so it doesn't pull apart arrangements of atoms and
molecules which are bound together by other forces.

This is even described in your second link.

~~~
danbruc
My model was the following. Take the classical rubber sheet, place two marbles
on it, then stretch the rubber sheet. The marbles are unbound and therefore
expand away from each other with the expanding rubber sheet. Now connect the
two marbles with a spring and again stretch the rubber sheet. The rubber sheet
expands everywhere just as before but now the spring prevents the marbles from
being pulled apart and makes them slip across the rubber sheet.

I can see where this model might be misleading or go wrong, it requires some
friction between the marbles and the rubber sheet because all massive things
are bound together at least by a tiny force and without friction the weakest
imaginable spring would still prevent the marbles from separating. I have to
think about this more carefully and try to figure out how badly this affects
the model, maybe it's actually a really bad model.

But as far as I can remember, whenever raattgift said I am wrong, I was wrong.
So I will reserve some time this evening for trying to work through his
answer. I also skimmed the Ask Ethan article and at least at first it sound
like what I intended to say, especially the following paragraph. On the other
hand I did not notice any of raattgift's points in this article on a first
quick pass.

 _The reason for this is subtle, and is related to the fact that the expansion
itself isn’t a force, but rather a rate. Space is really still expanding on
all scales, but the expansion only affects things cumulatively. There’s a
certain speed that space will expand at between any two points, but if that
speed is less than the escape velocity between those two objects — if there’s
a force binding them — there’s no increase in the distance between them. And
if there’s no increase in distance, that impetus to expand has no effect. At
any instant, it’s more than counteracted, and so it never gets the additive
effect that shows up between the unbound objects. As a result, stable, bound
objects can survive unchanged for eternity in an expanding Universe._

~~~
raattgift
Your italicized paragraph is a good clue.

I started a reply which was getting too long and technical, with the aim of
looking at three prongs. Post-Newtonian elements are at work, and it is easy
to fool oneself thinking of the underlying one: metric gravitation and how it
manifests within extended objects. (It _does_ ; we can see that from the
figure of the Earth and its internal structure, and from other roundish
celestial bodies.) Where self-fooling is easiest is in thinking (1) in terms
of forces, (2) using too large a set of local coordinates or inappropriate
coordinates, and (3) studying an object that extends beyond the boundaries of
"local".

I have some things to do but will return to your comment and its parent in a
few hours with a more detailed comment that hopefully won't stray off into the
weeds of technicalities. :D

~~~
danbruc
After reading the Karen Masters link and your first response a few times, I
think I am starting to understand what you want to point out. If the universe
were empty and expanding, then some light non-interacting test dust would
indeed get pulled apart everywhere and at all length scales. But when we add
matter and let it collapse, the collapsing matter kind of pulls spacetime with
it as it collapses and may eventually reach a kind of equilibrium where the
effects of the expansion and the collapsing matter cancel out locally leading
to a non-expanding region. If this is kind of correct, I have a very wrong
model of the effect of mass on spacetime and have to rewatch Susskind's
lectures on general relativity.

~~~
raattgift
Yes, good, you've saved me some typing along these lines. (Well, in
retrospect, maybe not that much :D )

In typical vacuum solutions of the Einstein Field Equations the metric is what
determines the available geodesics[0]. Putting an isolated object into a
vacuum solution causes it to "select" an appropriate geodesic from the ones
available, and if left alone, that object will at every time be somewhere on
that geodesic. A "test object" is pointlike and massless, so adding it to the
vacuum does not change the background metric at all. But if we make it
heavier, or bigger and rotating or oscillating, then the spacetime is no
longer exactly modelled by the vacuum solution. At that point one might use
perturbation theory, and consider the vacuum metric as a background and the
metric the not-quite-a-test-object itself generates as a perturbation field
overlaid on that.

The algebra usually looks like g_{\mu\nu} = \eta_{\mu\nu} + h_{\mu\nu} where
the greek subscripts are the usual indices running 0,1,2,3 in 4-dimensional
Lorentzian spacetimes (like ours, and most that you'll ever run into), \eta is
the chosen background field, h is the perturbation field, and g is the "true"
metric. One can also add perturbation fields or play around with higher-order
contributions from self-interactions (say if the not-a-test-object is a black
hole binary, or a solid fragment of a supernova (an Earth-massed blob of hot
metal?) moving relativistically).

If we start with an expanding Robertson-Walker spacetime _vacuum_ (no matter,
not even dark matter), and add a single galaxy cluster, we need to figure out
the metric to see how it evolves (or to see what geodesics are available for
test particles we throw in as probes).

We can do this in a couple of ways: either using perturbative methods like
above, e.g. g = {RW} + {galaxy}, or start with a single g for the whole
spacetime from a known set of appropriate solutions like Schwarzschild-de
Sitter or Kottler, or by engaging in "inside"/"outside" stitching together of
metrics like I described earlier (swiss-cheese). (There are other approaches
too!)

Each has advantages and drawbacks.

The middle option has the problem that a galaxy cluster will only look like a
Schwarzschild source from so far away that it shrinks to a point. Closer
observers will see (optically, even!) that it's lumpy, and if they have
sufficiently sensitive gravimeters or a probe like Synge's "five-point
curvature detector" they will see that Schwarzschild is not quite accurate. In
particular, the geodesics the components of the gravimeter or probe "find" are
not the ones that would be generated in the model metric.

The first option is hard to do and usually leads one into numerical
relativity. That's not such a bad thing these days, thanks to modern codes and
supercomputers. However, it's often hard to extract intuitions from numerical
solutions, and even harder to boil down into an explanation for others ("just
run this code and you'll see" is not very satisfying).

The last option has been around for decades and is a bit half-way. Its
advantage is that you generally can say intelligent things about what's going
on far from the thin shell boundary, and can develop a good map between the
evolution of the configurations of matter within the "inside" metric and that
outside. Its disadvantage is that it is a painful amount of work that
computers aren't very good at helping with yet.

Which one chooses will depend on some combination of the problem being
studied, personal preference, and trade-offs between manual work, accuracy,
and comprehensibility. One can combine the perturbation approach with the
inside/outside approach, so that one has (dropping the indices) g_{outside} =
\eta_{outside} + h_{outside} and g_{inside} = \eta_{inside} + h_{inside} and
an Israel-Darmois junction between g_{inside} and g_{outside} which is at
least in principle something one can think about functionally (a test object
crossing the junction will have its momentum altered; if it just drifts
across, the junction is something like a "kink" linking two otherwise smooth
geodesics).

Now, back to the standard cosmology, using the "inside"/"outside" approach. In
the "outside", the Robertson-Walker vacuum does just what you say:

> [as the] universe [is] empty and expanding, then some light non-interacting
> test dust would indeed get pulled apart everywhere and at all length scales

and then

> But when we add matter and let it collapse

... we can no longer describe the whole spacetime as a Robertson-Walker vacuum
that we're simply probing. Our matter perturbs the RW metric, so the true
metric g must be the combination of RW and whatever metric the introduced mass
generates. Since the introduced mass is collapsing, it's probably
approximately described some sort of Tolman dust. So we select e.g. the
Lemaître-Tolman-Bondi metric and perturb that in turn, so g = RW + LTB + O(h)
+ O(h^2) + ... where h is the deviation of the matter we introduced from exact
LTB, and h^n is higher-order perturbations.

This g, like any other metric, will generate a set of geodesics throughout the
spacetime. If our collapsing matter was a dust, then each of the individual
particles will select its own metric. Flashes of light will find their own
null geodesics determined by the metric, and so forth. The results are
different from Robertson-Walker, at least close to the matter that we
introduced. At even fairly short distances the higher order terms in h fall
into irrelevance, and at large distances h itself becomes negligible.

I want to write a bit about extended objects, but that'll have to wait until a
bit later. The key feature there is that the individual components of such
objects are stuck together, so they don't find their own individual geodesics.
The behaviour of extended objects in curved spacetime is fun!

> may eventually reach a kind of equilibrium where the effects of the
> expansion and the collapsing matter cancel out locally leading to a non-
> expanding region

You can move the thin shell of the junction around to explore that kind of
thing. Or taking a purely perturbational approach you can look for regions of
spacetime where geodesics have the characteristics you want (drop down test
objects near point in spacetime near a suspected "cancelling-out" region and
see if the inertially-moving test objects eventually collide or separate).

How you represent what's happening there is up to you. You could use your
colloquial expression, a line from the Karen Masters link or something like
it, or talk about the mathematical structures. The theory underdetermines the
precise mathematical expression (there are lots of exactly equivalent ways of
writing it down, and lots of so close it doesn't matter approximations); the
English has it even worse. :/

However, the key thing is that exact vacuum solutions stop being exact when
you add matter. Matter rarely cooperates with exact non-vacuum solutions (they
typically are maximally symmetric: spherical arrangements of matter of uniform
density, with no multipole moments from internal motions, etc. which is far
from what we see measuring Earth's or Moon's gravitational fields with e.g.
GOCE and GRAIL, and solar prominences big and obvious, galaxies can be spiral,
a tiny black hole orbiting a large one will induce tides on the large one, and
so on.)

\- --

[0] I'm deliberately going to avoid talking about non-geodesic trajectories
through spacetime for now. May come back to that when discussing extended
objects in a followup.

\- --

ETA: I should add that the _metric_ expansion is not really an expansion of
space, but rather an extra term tacked onto the metric g. So following the
perturbation notation above we could start with flat spacetime \eta and say g
= \eta + {expansion} and end up with g = {expanding Robertson-Walker}.

This is quite common: the "true" background is Minkowski flat-space, and then
the vacuum eternal spherically-symmetric black hole is a perturbation on that,
and then there may be other perturbations. (Instead of starting with the
"true" background of Schwarzschild.) This kind of thing leads to some insights
about asymptotic flatness.

So for an expanding cosmos, we can start with a perfect flat non-expanding one
and perturb that, expanding the _metric_.

(This is being really loose with terminology, but I think gets the point
across reasonably. It's the metric expansion of space because there is a time
parameter in the line-element of the metric; _spatial_ distances are
determined by _when_ (coordinate-time) you are. g = \eta + f(t), where f is a
function taking a time coordinate and generating a perturbation field; maybe
better to write g(t) = \eta(t) + f(t) where g(t) is the metric's _spatial_
distance functions for all events on a slice of space that where everything is
at the same time coordinate t; but this dives into 3+1 spacetime splittings
which is another big can of worms...).

\- --

ETA2: the Overview section of this puts it well in a different way. (No claims
about the other sections, I haven't read them yet.)
[https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/m/Metric...](https://www.cs.mcgill.ca/~rwest/wikispeedia/wpcd/wp/m/Metric_expansion_of_space.htm)

~~~
raattgift
A couple little things I won't turn into literal ETA3, just in case you've
already been reading the long post above. :)

(2) below is also mostly for me, I think.

1\. Thanks, Michael Weiss. This is essentially the same argument I'm making,
only terser.
[http://math.ucr.edu/home/baez/physics/Relativity/GR/expandin...](http://math.ucr.edu/home/baez/physics/Relativity/GR/expanding_universe.html)

2\. One important thing about the expansion of the universe is that people
hear "expansion of space" and reasonably think space has physical properties.
[https://arxiv.org/abs/0707.0380](https://arxiv.org/abs/0707.0380) is a good
rant about that, and
[https://arxiv.org/abs/0809.4573](https://arxiv.org/abs/0809.4573) is another
interesting take. There's a sort of Betteridge effect going on, in that none
of the authors think that the concept of expanding space is awful, but they're
instead capturing the "gotchas" that befall even working relativists stray
into what philosophers call manifold substantivalism. (I hope that doesn't
attract house philosophers, no offence. I don't think my text is all honey!)

~~~
delta-v
Could I interpret it this way?

In the EFE, if we move the cosmological constant factor to the right handside,
where the stress–energy tensor is, then stress–energy tensor and the
cosmological constant factor have opposite signs and stress–energy tensor can
overcome the cosmological constant factor.

When we say EM-forces/Strong/Weak forces "cancel out" the expansion of the
universe, it isn't because of those forces are holding matters together. But
they contribute to the stress–energy tensor and "offset-ed" the effect of the
cosmological constant factor.

Even without those forces, as long as the stress–energy tensor is greater than
the cosmological constant factor, the solution of EFE will be non expanding.

Does this sound right to you? Thanks.

~~~
raattgift
Well, so you write down an initial values surface with that approach and let
the matter evolve: either you get something physically plausible or you don't.
We have some guides as to what is physically plausible, and of course evidence
that any of them is wrong would be very interesting.

More theoretically, the LHS doesn't care about how you arrange the RHS at all.
More finely, the metric does not care how you compose the metric. If you do it
perturbatively, the leading term could be Minkowski or Gödel for all it cares,
as long as the sub-leading terms then present a useful background for small
perturbations that remain small (or better, vanish) rather than explode into
large perturbations. Inside the solar system, Schwarzschild gets that right,
and nobody has shown you need to correct for the global behaviour of the
cosmic expansion.

> ... if we move the cosmological constant factor to the right handside ..

Before the 1920s (!) there was a discussion about where to put an everywhere
isotropic tension in the EFEs.
[https://arxiv.org/abs/1211.6338](https://arxiv.org/abs/1211.6338)

In section 3, the author quotes Einstein's response about how to determine the
density. The part after, "Later, he finishes with..." seems especially
germane, and cf. section 4 especially its last sentence (wherein "dark energy"
in that context means a source field with a potential and dynamics and a
coupling to the other source fields and a set of initial values).

[Heh, typo in the second sentence of the last paragraph of §3, "vale" instead
of "value".]

> (matter forces) contribute to the stress-energy tensor

Ok, but if you hold that the CC acts isotropically everywhere you can think
about decomposing the curvature tensor into its Ricci (and Weyl) components.
Weyl tensor encodes the stretch-squash astigmatism: a spherical volume in the
presence of a nonvanishing Weyl start to look more like a U.S. football. Ricci
scalar encodes the isotropic change in volume. For a volume, nonvanishing Weyl
with a vanishing Ricci is volume-preserving; a vanishing Weyl and a
nonvanishing Ricci is a contraction or expansion.

The cosmological expansion at cosmological scales clearly is Ricci scalar
rather than Weyl, since matter in the large remains isotropic and homogeneous.

Shuffling \Lambda over to the source side does not change that observation: we
don't see broken cosmological-scale isotropy, so we must still have a
vanishing Weyl and a nonvanishing Ricci scalar on the LHS. How does that
happen? Presumably in the metric, since it's the obvious choice of dynamic
field (although of course you could insist on making the RHS as complicated as
you like, and there have been cases where having the metric maximally static
with gravitational dynamics shuffled into "fake" matter is useful -- the
original Hawking Radiation paper did just that, for instance; and of course
you might want a quintessence-style field instead of a CC, and bite the bullet
on recovering observations through that field's behaviour).

Ultimately what we care about is that we have the correct EOMs, and for test
particles the EOMs in our labs and in our experimental platforms elsewhere in
the solar system, and observed for astrophysical objects throughout our galaxy
and the cluster it's in, are nowhere near the EOMs of FLRW. They are _much_
nearer to Schwarzschild.

------
kjeetgill
My favorite bit from the author on Twitter:
[https://threadreaderapp.com/thread/1070302359325151233.html](https://threadreaderapp.com/thread/1070302359325151233.html)

> The next-generation radio telescope - the Square Kilometre Array
> @SKA_telescope will be able to test this theory, and directly confirm or
> invalidate its predictions. 13/17 It is rather surprising that the model
> predicts the properties of a LambdaCDM universe.

As a casual observer this is is what gets me excited! We'll get our answers
one way or another.

------
starbeast
Presumably you could describe this as a theory of levity. For happy
spacetimes.

------
chicob
The possibilities for Star Wars puns are endless.

There are alternative explanations that theorized that the 2nd law is not
linear for extremely weak forces.[1] Thus, two masses that were sufficiently
far apart would experience an attraction larger than that predicted by an
inverse square law.

I think some particular applications of these theories were proved wrong some
years ago, but I can't find those results.

[1]
[https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics)

~~~
abruzzi
Actually, Star Trek had an episode where a giant space amoeba pulls the
Enterprise into a "zone of darkness" where thrust towards the amoeba pushes
the enterprise away from the amoeba. Sounds similar. Of course they destroyed
the creature with some antimatter.

[https://en.wikipedia.org/wiki/The_Immunity_Syndrome_(Star_Tr...](https://en.wikipedia.org/wiki/The_Immunity_Syndrome_\(Star_Trek:_The_Original_Series\))

------
matte_black
Negative mass... holy shit with this we could break the speed of light

~~~
point78
How?

~~~
matte_black
By offsetting the mass of a system, to produce a zero mass vehicle.

~~~
EthanHeilman
Take a net negative mass vehicle, and accelerate it by having it collide with
its propellant. Would this be an anti-rocket?

~~~
matte_black
How does it come to a stop, the vehicle gains too much mass?

~~~
EthanHeilman
The mass part of the vehicle could either detach from the negative mass
component. Or the vehicle could eject mass to reduce velocity.

------
mrhappyunhappy
“Negative masses are not a new idea in cosmology. Just like normal matter,
negative mass particles would become more spread out as the universe expands –
meaning that their repulsive force would become weaker over time. However,
studies have shown that the force driving the accelerating expansion of the
universe is relentlessly constant. ”

Is it though? How can you measure a universe-size force that takes billions of
years to “move” on a human lifetime scale?

------
SubiculumCode
Not a physicist. If negative mass exists, is it featureless, or is it composed
of units which can combine to form analogous negative mass atoms, negative
mass molecules, etc?

~~~
OscarCunningham
It would presumably be composed of units which could well join together into
larger structures. But these would have to be small scale (the size of atoms
or molecules) because gravity causes negative masses to move away from each
other. So it wouldn't clump together into planets and galaxies.

------
garmaine
Would this also explain galactic voids as regions of large negative mass? It
would seem that deviations in the Hubble constant bear a void would confirm
this theory.

------
ars
> Negative masses are a hypothetical form of matter that would have a type of
> negative gravity – repelling all other material around them. Unlike familiar
> positive mass matter, if a negative mass was pushed, it would accelerate
> towards you rather than away from you.

That assumes that inertial mass = gravitational mass, which is assumed to be
true, not not proven.

~~~
privong
> That assumes that inertial mass = gravitational mass, which is assumed to be
> true, not not proven.

The assertion has been tested experimentally. Inertial and gravitational mass
are seen to be equivalent within the bounds of experimental accuracy. The
wikipedia article on the "Equivalence Principle" has lists of tests for
various versions of the principle:
[https://en.wikipedia.org/wiki/Equivalence_principle#Modern_u...](https://en.wikipedia.org/wiki/Equivalence_principle#Modern_usage)

~~~
ars
And the negative mass postulated here? We don't know if it's still equivalent
there because we have never tested it.

It would be a lot simpler if inertial mass was negative gravitational mass in
this theory.

Then you have repulsive gravity with normal behavior under force.

~~~
privong
I see I misunderstood that you were specifically questioning the equivalence
for this postulated negative mass. I thought your statement was a more general
one.

------
sadness2
I love the choice of the word "could" for this title. Makes it sound like the
plot for a science fiction book X-D

------
NedIsakoff
Here comes the Alcubierre Warp Drive!

~~~
perlgeek
To quote the author
[https://twitter.com/Astro_Jamie/status/1070343971338153984](https://twitter.com/Astro_Jamie/status/1070343971338153984)

> It is quite likely that the negative masses in this study are a mathematical
> tool, rather than real physical matter. This rules out a lot of sci-fi,
> which is disappointing of course!

~~~
noetic_techy
To be clear, the author IS proposing that there MAY be negative mass
particles, not that this is all mathematical. That statement was simply a tip
of the hat to the fact that none have yet been detected and its possible his
theory is just a "accurate way to model" dark matter/+dark energy. It's just a
nice way saying "My thoery might be the Newtonian Mechanics of Dark
Matter/Energy, close but possibly wrong."

Why crap on the fact that this may lead to a warp drive.

------
jasonhansel
Reminds me of this (very old) theory of gravity:
[https://en.m.wikipedia.org/wiki/Le_Sage's_theory_of_gravitat...](https://en.m.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation)

~~~
_xgw
Non mobile link:
[https://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitatio...](https://en.wikipedia.org/wiki/Le_Sage's_theory_of_gravitation)

------
pmontra
No problem with negative masses, but where does this continuously generated
fluid come from?

------
RobertoG
I'm confused.

From the article:

"This is explained by the Hubble-Lemaître Law, the observation that more
distant galaxies are moving away at a faster rate."

If we observe an accelerating universe how can be that galaxies in the past
move away at a faster rate?

~~~
tzakrajs
Because more and more negative mass fills in the space left behind by the
expansion creating further expansion?

~~~
mike741
why would negative mass fill in the space rather than just "zero mass?"

~~~
tzakrajs
Negative mass has greater pressure to displace vacuum in a similar way that
mass does? That was my assumption. Perhaps not!

------
domparise
If we can prove that dark matter exists with negative mass, this may pave the
way for fundamental research on other phenomena with negative mass, like anti-
gravity effects.

------
scottmsul
If you put a negative mass and positive mass side by side and release from
rest, they will accelerate indefinitely and break conservation of energy. Of
course science should be open to any possibility, no matter how strange. But I
feel the author understates its strangeness. And requiring negative mass to be
continually created makes this theory not all that much "simpler" than the
standard dark matter+dark energy.

~~~
YaxelPerez
They would accelerate indefinitely, but it wouldn't break conservation of
energy. Gravitational force is inversely proportional to distance^2, so
potential energy (integral from 0 to infinity of Force dx) still converges to
a finite number.

Same with normal gravity, but backwards.

~~~
walrus1066
I think the weirdness is, the distance between masses wouldn't change, they'd
just accelerate together, in the same direction, indefinitely.

Both mass would experience negative repulsive force:

F=Gm(-m)/r^2

=-Gm^2/r^2

The positive mass accelerates away from the negative mass, because F=ma

=> ma=-Gm^2/r^2

=> a=-Gm/r^2

Now here's the kicker. If inertial mass (the m in F=ma), is the same thing as
gravitational mass. Then, _the negative mass accelerates towards the positive
mass_. Because

F=(-m)a

=> F=-ma

=> -ma=-Gm^2/r^2

=> a=Gm/r^2

So accelerates in same direction as positive mass.

Technically, conservation of energy & momentum is maintained, because the
negative kinetic energy of negative mass would cancel out positive energy of
positive mass.

But I think the above can't be natural, because the magnitude of kinetic
energy would grow to infinity, leading to even more impossibilities.

So you'd need to 'hack' the theory of gravity and make inertial mass different
to gravitational mass. So the particle has negative gravitational mass, but
positive inertial mass, to make laws of physics stay sane :)

~~~
FabHK
From what I gather, the negative mass repels the positive mass (which is thus
accelerated _away_ ), and the positive mass attracts the negative mass, which
(because it has negative mass) is thus also accelerated _away_.

Now, the kinetic energy of the positive mass grows, but the kinetic energy of
the negative mass is negative and falls (because its mass is negative...), so
energy is conserved.

EDIT to add: I think I gathered wrong. Apparently, there is runaway motion
(same direction), energy and momentum is still conserved (since one mass is
negative), and since total mass is zero, the two particles reach speed of
light. Intriguing. See paper.

------
DonHopkins
Could this repulsive 'Dark Fluid' be what they use on the Enterprise to
generate artificial gravity? They could pipe the stuff through the ceiling to
press you down against the floor. But that would make the gravity point the
wrong way in the decks above and below. But if they alternated the direction
of "up" on each deck, maybe they would cancel each other out.

------
RootKitBeerCat
Completely a layperson who’s only read science for the masses type physics and
astrophysics, but now imagine that there are all sorts of undiscovered
phenomena that are actually just the inverse of existing properties of nature
we know. Does that make sense, or resonate with more rigorous scientists,
Reade archers, students?

------
exabrial
Negative mass is required to produce warp drive... We might not be stuck here
forever

------
bmh100
Does his theory explain the Bullet Cluster and baryon acoustic oscillations?

------
3chelon
Dark fluid, negative mass?

Rubbish, it's turtles all the way down!

------
tigerlily
Anyone read up on David Wiltshire's timescape model?

After reading it, I became more skeptical of dark matter and dark energy
theories as the cause of the Universe's apparent accelerating expansion.

------
swamp40
Could be made into a nice Landspeeder!

------
time-domain0
Neither an experimental physicist nor cosmologist, but another possibility is
that our universe shares very slight space-time interactions with other
universe(s) in a multiverse where its mass-energy casts negative mass
"shadows" in ours. Blackholes in one creates areas of repulsion and expansion
in the other. Maybe this is where the missing mass-energy is? It's there, but
it's not in our universe.

------
throwaway487550
How abstract concepts, introduced to establish artificial symmetry, could be
used to describe aspects of reality which does not rely on any abstract
notions whatsoever?

Some processes are symmetrical to an outside observer, which does not imply
that anything must be or should be symmetrical. It is an observer's preference
to see symmetry everywhere.

There is no such fucking things as "dark" liquids or negative "masses" merely
because a model superimposed on reality have nothing to do with reality
itself.

The impossibility is the same as the principal impossibility of revealing the
CPU's actual wiring from the level of code it runs. There is not a single
fucking way to do it in principle, no mater what fancy guesswork one would use
or what kind of a wrongly simulated model one will superimpose on what one is
able to "see" and examine.

This is not even science, but some sort of contest of abstract theology,
crafted to please fellow reviewers and sectarians.

------
vtesucks
Alright question for all you genius physicists- what makes you visit this
forum which is primarily a discussion board for news and startups
(primarily)in the field of Information Technology? I'm curious because I don't
know you're your domain intersects with this forum's.

------
bitwize
Eezo?

------
DiabloD3
A "dark fluid" with negative mass COULD dominate the Universe, sure.

But so could galactic-scale electric currents and magnetic fields, and is more
likely.

No one ever talks about that.

~~~
XorNot
Both these are real things we measure with radiotelescopes.

They are not strong enough to produce the necessary effects, they are however
interesting phenomena -
[https://arxiv.org/pdf/1302.5663.pdf](https://arxiv.org/pdf/1302.5663.pdf).

~~~
jerf
The biggest tell that the Electric Universe theory is bunk is that anybody who
ever talks about it can't hardly get two sentences in before dropping in the
"but nobody ever talks about magnetic fields or electric currents at this
scale"... which is easily observed to be utterly false. Astrophysicists talk
about it a lot, and you can find even more discussion if you add "plasma" to
the search terms.

If Electric Universe advocates are so wrong about something so easy to check,
it doesn't give me much reason to check out anything else they say. (The fact
that I have done so nevertheless and I've never managed to find them making an
actual prediction is another. It seems to be one of those recursive handwave
theories that never quite lands on anything concrete no matter where you
search, but it sure does know that the standard theory is wrong.)

------
3327
Then we are definitely in a simulation. That is the fabric that holds the
universe together.

~~~
rjplatte
That's a complete non-sequitur.

