
Dark matter nightmare: What if we are just using the wrong equations? - lordnacho
http://backreaction.blogspot.com/2019/10/dark-matter-nightmare-what-if-we-just.html
======
superqd
I very much wish no one had used the terms "dark matter" and "dark energy", as
those imply far more knowledge than we have. It implies to those unfamiliar
that there is in fact some form of energy or matter that has been "sort of"
identified, but whose properties are unknown.

That isn't the case at all. We have equations we use to predict observations,
and in certain cases, those observations are not quite matching what we
predicted. There are a lot of reasons why this can happen, and using the
equations poorly is but one of them.

I really wish they'd just said we've found an _inconsistency_ , between theory
and observation, which is the actual case. It would, I think, better inform
the public about the actual process of science, rather than making it sound
like a new discovery has been made, which has not happened, clearly. Or
perhaps the real discovery _was_ the inconsistency, and that should be the
framing of the problem. Describing it using sort of known words like 'energy'
and 'matter' makes the general public think a new type of matter has been
found, rather than what has actually been found which is a hole in our
understanding.

~~~
nabla9
I disagree.

Dark matter is form of matter (matter is any substance that has rest mass) and
it's dark (implying we can't see it and don't know what it is made of). Most
hypotheses around the dark matter assume it's either new kinds of particles,
or macroscopic objects.

Dark energy is also good name for hypothesis for unknown form of energy.

~~~
simonh
"Dark matter is a form of matter..." is a definitive statement, but you can't
point to a single example of a direct detection of dark matter. It's quite
possible the hypothesis is wrong.

The article, and the OPs aren't saying we shouldn't explore these hypotheses.
They're just pointing out that they are, well, hypothetical. We should be
cautious of talking about these things definitively as though we know they
exist.

~~~
ailideex
> "Dark matter is a form of matter..." is a definitive statement, but you
> can't point to a single example of a direct detection of dark matter.

Are these not all cases of direct detection:
[https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...](https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence)

~~~
rpedela
Direct detection would be detecting/trapping the dark matter particle(s).
Those observations are indirect and it may turn out that there is another
explanation other than dark matter such as we are using GR equations
incorrectly as suggested in the post.

~~~
snowwrestler
What kind of detecting/trapping? We have to be specific. If we hypothesize a
kind of matter that does not interact electromagnetically (only
gravitationally), then it's kind of nonsensical to say that only direct
electromagnetic interactions with such matter will count to confirm its
existence.

~~~
rpedela
Well that is the crux isn't it? How do we directly detect gravitational
particles?

~~~
snowwrestler
Maybe we don't. The evidence for general relativity is all indirect--we can't
directly detect space-time. But plenty of people seem to think it's a solid
theory anyway.

~~~
simonh
There is a significant difference in that GR made numerous very testable
claims about all sorts of phenomena, which would not be at all expected
without GR to explain them.

It also depends what is consider direct detection, which I accept is a matter
of opinion. For example I'd consider comparing the times on atomic clocks
inside and outside a gravity well to be direct measurement of time dilation.

------
philwelch
Yeah, maybe. The thing is, there are a lot of independent observations that
match up really really well with the hypothesis, "there's a bunch of matter
around that only interacts with us via gravity". If our models are off, they
would have to be off in exactly the right way to affect our observations of
gravitational forces but not really anything else. Which may very well be the
case--or it may be the case that baryonic matter isn't really the end-all be-
all of the universe and we're just a bunch of baryonic chauvinists for
thinking so because that's what we're made of ;)

Obviously, nobody knows for sure yet. But I would caution most fellow laymen
not to rely too heavily on the intuition that says, "dark matter? pffft!" or
even, "but what about all of those observations that didn't match Newtonian
dynamics?". Physicists (including the author here!) are generally smart enough
to have thought of those objections themselves, which is why they've spent
decades trying really hard to disprove the notion of "dark matter" by
compiling all of these anomalous observations. Sometimes the universe is just
unintuitive, at least to dumb apes like us.

I'm not saying you can't have a hunch that we're just doing the math wrong
somehow; I just wouldn't be overly confident that's the case just yet.

~~~
lonelappde
I think you misread. Sabine is arguing that maybe dark matter is just the
error term in our approximations of real matter, and pointing out that no one
yet has actually taken the time to disprove that Occam's Razor theory.

~~~
philwelch
I wouldn't presume to argue against her, because she's a physicist and I'm
not.

I'm mostly making the case to the peanut gallery here to maintain a state of
epistemic noncommittal. It's a very popular intuition to think that "dark
matter" is just a weird consequence of modeling things improperly, kind of
like how we used to think there was another planet between Mercury and the sun
because that was the only way to make Mercury's orbit consistent with
Newtonian dynamics. If we _are_ modeling things improperly, I suspect it would
have to be a pretty weird bug--and I think the one she's suggesting qualifies
as such.

------
QuesnayJr
It's amazing that every dark matter discussion goes the same way, with the
same amnalogies to aether or epicycles, completely unlinked to the actual
evidence. (Unlike the blog post, where Hossenfelder suggests a specific
methodological problem, and that GR may in fact be completely correct. This is
completely unlike anything people have suggested here in the comments.)

Dark energy is less well established, but based on the evidence we have,
Occam's razor points strongly towards dark matter. Dark matter is a specific
theory that makes predictions, and as far as we can tell these predictions are
borne out. There was a competing hypothesis -- that GR itself is wrong and
needs to be corrected. These were in competition, but the evidence has
decisively broken in favor of dark matter. If it was just a question of fixing
the equations, then you should be able to predict the apparent pattern of dark
matter from the distribution of ordinary matter. For a while it looked like
maybe you could, but more recent evidence has shown that you can't. (This is
the reason why the Bullet Cluster comes up as an example so much -- the Bullet
Cluster looks like what you would predict if galaxies have dark matter halos
and two galaxies collided.)

People also misunderstand the incentives here. If you could write down a
modified theory that could explain all of the evidence, you would be the
greatest physicist since Einstein, maybe the greatest since Newton. You would
achieve immortal glory as one of the central figures of the 21st century. So
people try to write down that theory, but it turns out to be really hard to
do.

~~~
akvadrako
Dark Matter is not a specific theory. Are you talking about WIMPs?

~~~
QuesnayJr
Some of the astronomical data is compatible with any kind of dark matter,
though other observations (such as the Bullet Cluster) points at WIMPs.

------
sparker72678
I’m imagining this thread if it were particle physicists and cosmologists
arguing about the difficulty of the traveling salesman.

I’m not convinced we hobbyist physicists have anything to add to this topic.
;)

~~~
mindcrime
_I’m not convinced we hobbyist physicists have anything to add to this topic.
;)_

On average, I'd say that's a safe premise to adopt. But I'd add two things to
that:

1\. The question "is that the point?" That is, cutting edge cosmological
research isn't (AFAIK) generally done on HN. The "real experts" probably
aren't coming here looking for new insights, and the rest of us idling around
chewing the cud on this isn't really hurting anything.

2\. That said, this "cud chewing" may be beneficial to the participants in the
conversation, or perhaps - in a "long game" sense - to science at large. If
somebody reading all this is inspired to take up physics as a career field, or
just learns something that they turn around and use as metaphorical
inspiration in some seemingly unrelated field, well... maybe that makes it all
worthwhile.

Anyway, at worst this is entertainment, and arguably of a higher quality than
watching the latest episode of the sit-com de jour.

~~~
sparker72678
For sure. If I didn’t totally agree with you I’d never read anything on HN.
Entertainment indeed!

------
Veedrac
I have no expertise at all in the matter, but my understanding is that the
evidence for dark matter is overwhelming. In particular, this Reddit comment
(which is mostly just a TL;DR of [2]) seems pretty convincing:

[https://www.reddit.com/r/Physics/comments/kucdk/dae_think_th...](https://www.reddit.com/r/Physics/comments/kucdk/dae_think_that_physicists_of_the_future_will_look/c2nb7nc/?context=2)

It seems to me a dark matter hypothesis is hardly extraordinary—that is, I
don't see why anyone would have priors that particularly disfavoured it—and,
if it were to exist, us not knowing much about it doesn't seem particularly
surprising either.

[2]
[https://en.wikipedia.org/wiki/Dark_matter#Observational_evid...](https://en.wikipedia.org/wiki/Dark_matter#Observational_evidence)

~~~
mmusson
There is no direct evidence for dark matter. It is a placeholder for
observations that don’t match theory. There are many current experiments
trying to detect proposed particles but so far none has been detected.

~~~
scarejunba
Well, it's a placeholder, but a very specific kind of placeholder: it fits
exactly everywhere. It's not like it's special-cased to one of the problems.
It actually works in all of them.

~~~
denton-scratch
The reason it "fits exactly everywhere" is that the only definition we have is
that it's the exactly gap between observation and theory.

~~~
DiogenesKynikos
No, the definition is cold, gravitationally interacting matter. It just so
happens that adding that one extra ingredient reconciles a whole number of
different observations with theory. That didn't have to be the case. For
example, if the dark matter is hot, structure formation doesn't match
observations, or if dark matter is composed of massive, compact objects, we
would observe a lot more microlensing events. But putting in just the right
amount of cold dark matter makes everything we can test so far work. That
could be a giant coincidence, but most cosmologists think it's because cold
dark matter really exists.

------
Tharkun
SciShow Space recently did an episode on Brown Dwarfs. They suggest that there
might be so many brown dwarfs (which are hard to see), that they could account
for a large portion of the "missing" mass.

And there's a theory that says there are small "primordial" black holes all
over the place, which are also hard to detect individually, but together could
also account for a large part of the missing mass.

Perhaps invisible brown dwarfs and invisible black holes are an easier pill to
swallow than mysterious dark masses/energies?

~~~
DCKing
This is the MACHO hypothesis, which seems to have fallen somewhat out of favor
since we can measure this to some extent and we can't find anything close to
the required mass [1]. Although it is likely there is unaccounted regular
matter, I think it is considered unlikely that this will account for the 85%
of "missing matter" that we need, not in its entirety and likely not for a
significant part of that either. You will need a humongous amount of brown
dwarfs to compete with the regular stars and known (supermassive) black holes
that are already in the known 15%.

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

"A survey of gravitational lensing effects in the direction of the Small
Magellanic Cloud and Large Magellanic Cloud did not detect the number and type
of lensing events expected if brown dwarfs made up a significant fraction of
dark matter."

------
scotty79
I think recently there was observation of galaxy with very little of dark
matter. I think it makes the dark matter harder to explain as some sort of
theory glitch not the real object.

~~~
mkl
That interpretation seems to be still up in the air:
[http://www.astronomy.com/news/2019/09/astronomers-cant-
agree...](http://www.astronomy.com/news/2019/09/astronomers-cant-agree-on-
galaxies-without-dark-matter)

------
rurban
The classical multibody problem, Heisenberg already struggled with it in
quantum mechanics.

And Hossenfelder really means the we are using the equations wrong. Not that
are using the wrong equations.

~~~
mxcrossb
This is a really important point, a lot of comments in this thread seem to
think the article is related to proposals for new laws of physics.

~~~
rurban
And only CS can help with this problem. If you cannot average non-linear
functions trivially, you certainly can approximate it and minimize the
differences. Something like a simplex method. The problem is called "Nonlinear
averaging".

e.g.
[http://www.eebweb.arizona.edu/Faculty/chesson/Nonlinear_aver...](http://www.eebweb.arizona.edu/Faculty/chesson/Nonlinear_averaging.htm)
or
[http://faculty.cs.tamu.edu/schaefer/research/nonlinearSub.pd...](http://faculty.cs.tamu.edu/schaefer/research/nonlinearSub.pdf)
with nice graphs

------
DCKing
Can "using the wrong equations" explain gravitational lensing effects
appearing apparently out of the void? The Bullet Cluster is a famous example
referred to in this comment thread, but it's far from the only example.

As a non-physicist, observed dark matter gravitational lensing really points
to "there's more gravity than we expect" rather than "gravity works
differently than we thought" for me.

~~~
mmusson
The Bullet Cluster observations are commonly misunderstood. There is indeed a
missing baryonic mass problem for MOND but this is also a problem for ΛCDM
too.

------
andbberger
This seems very plausible as an explanation for dark matter but I'm confused
as to whether the author proposes it as an explanation for dark energy as
well.

Are there not galaxies on opposite sides of the universe, expanding away from
each other too fast for them to ever interact?

~~~
jfengel
The case is specifically being made about dark matter, and in particular about
the galactic rotation curves that are the primary motivation for the dark
matter hypothesis. Dark energy has a completely different basis that wouldn't
be subject to this particular alternative hypothesis -- though the very
general point of "maybe the approximations we're using are too simple" does
still apply. Though really they apply to pretty much any open question.

------
mschwaig
I don't understand what the author means. Maybe someone can help me
understand.

> Now, what we do when we want to explain what a galaxy does, or a galaxy
> cluster, or even the whole universe, is not to plug the matter and energy of
> every single planet and star into the equations. This would be
> computationally unfeasible. Instead, we use an average of matter and energy,
> and use that as the source for gravity.

Does this mean instead of individual objects we make our calculations with x
units of mass and energy in every given patch of space and this may be an
issue, or am I getting this wrong?

~~~
Cogito
> Does this mean instead of individual objects we make our calculations with x
> units of mass and energy in every given patch of space and this may be an
> issue, or am I getting this wrong?

Pretty much yes, as we can only take observations like that.

More importantly, the GR equations are non-linear, so taking these 'average'
observations and feeding them into our models may introduce significant error.

Or another way of saying that, it's possible that the model would give
accurate predictions if you were able to feed it precise and complete
information (the details of every single point mass) but not give accurate
predictions if you feed it the sampled data we have (observations about whole
galaxies, for example). People have discussed this elsewhere in this thread as
the N-body problem.

It's also saying that even if we did have that level of detail, our numerical
simulations aren't currently capable of calculating at that fine a level of
detail.

------
ars
I'm one of those students who questioned it.

Never got a satisfactory answer.

For example the gravity between two objects depends on the relative speed
between them (since gravity is proportional to energy, not rest mass). Are we
calculating that for each and every sun orbiting in a galaxy?

It's a lot of interactions, each sun interacting with all the rest.

It gets worse - gravity is also proportional to _potential energy_. So you
need to include how much energy you would have if every sun fell into every
other sun, not just their rotation.

I just can't shake the feeling that dark matter is just this unaccounted for
energy in a galaxy.

~~~
philwelch
That sounds somewhat plausible to me because it fulfills the criterion of
something that would mess up our observations of gravitational force but
nothing else. Is the n-body problem really so intractable we can't even
handwave our way to an approximate answer that accounts for this stuff?

~~~
dodobirdlord
Everyone has thought "maybe it's just a bad gravity equation?" at some point.
It's a fun idea, but it's contrary to observation.

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

~~~
andbberger
Oof, a gravitational lensing based result at 8 sigma... so much for this fun
little theory

~~~
heavenlyblue
>> gravitational lensing based result

What's wrong with basing your results on gravitational lensing? Is it related
somehow to the complexity of inferring on how much the images are wrapped by
gravitational lensing?

~~~
andbberger
nothing, I'm talking about OP's theory. A gravitational lensing based result
is particularly strong due to it being model independent. OP's theory cannot
account for lensing inferred CoM differing from the baryonic CoM in the bullet
cluster.

------
ymolodtsov
I’m so tired of this blog and its author that just tells we’re wrong without
really leading anythere.

~~~
mmusson
The post was intended to educate the general public. Physicists already know
about this problem.

Most pop science, even the ones featuring physicists, present speculative
ideas as if they were well established facts. (Hello string theory)

Finally, criticism of a theory does not require presenting an alternate
theory. She presents an argument. If you don’t agree with her conclusion then
you need to rebut her argument.

~~~
ymolodtsov
I understand, but she has the same point about essentially every modern idea
in physics.

------
ryanthedev
The hubble constant is where the equations breakdown.

Remember why he even added it in the first place.

It was to create a model of the universe that is frozen. It's creating a
reference frame.

Depending on which reference frame we use, we get different numbers.

CMB vs Candles

~~~
ymolodtsov
In the most recent development it’s been debunked. The problem was with the
Hubble telescope measurements of the candles. Using another satellite and
another class of candles they got to the measurement that was very close to
CMB.

~~~
ryanthedev
idk the plank data is still very iffy. Some even say it still requires new
physics. Do you have some sauce where it's been "debunked"

------
gameswithgo
Can computational horsepower be thrown at this problem? To put some bounds on
how wrong the equations are? We have enormous capacity these days with racks
of GPUs and 64 core EPYCs and so on.

~~~
petschge
Cosmological simulations already take up month of run time on worlds largest
supercomputers. (astro-)Physicists were the first users of large computations,
in many cases inventing the computers along side with the physical theory the
were trying to compute. Computer people like you who think that physicists are
too stupid to make use of GPUs are as bad as physicists that think they are
qualified to comment on biology or chemistry.

------
tus88
Physics isn't about equations. It's about models of the universe that suggest
a certain underlying "working" of the system. If our models are not predicting
observations....it just means we still haven't figured out how things really
work.

Not surprising given we still can't even explain the double slit experiment
wwithout resorting to paradox and contradiction.

For all we know telepathy, magic, reincarnation and wormholes all might be
true, we just haven't advanced our understanding of the universe and reality
to that point yet.

------
YeGoblynQueenne
>> Dark matter filaments. Computer simulation.

To be honest, that looks just like a very old (ca. 1990's) noise filter from
3d Studio Max (where it would be used to make smoke, fog, but also very
realistically looking veins, etc).

("Filter" is not the right term. Sorry, I haven't touched Max for ages.)

------
alanbernstein
Doesn't that describe almost any unsolved problem in physics?

~~~
Cogito
The specific point is that we are using linear averages (over large
collections of objects, such as galaxies) to model a non-linear effects
(General Relativity interactions between individual objects).

It may be accurate to use those averages, or it may not be if for example the
chaotic nature of N-body problems causes reality to diverge significantly from
the simpler model; we don't know how large the error actually is and currently
don't have a way to measure it.

------
bronlund
It's the aether, stupid ;) If anything they should look into Maxwells original
20 equations.

------
Mizza
But surely, we already _know_ that we'll never be able to model the universe
without a law of intelligence.

For instance, if you were an alien scientist on the other side of the galaxy
measuring the atmospheric composition of Earth, there's no way that you could
make a prediction to match the result without also modelling emerging
properties of intelligence. We know that this is true, just as we know that
intelligence and computation are physical properties of the universe - so why
isn't there a serious branch of physics which tries to take this into account?

------
denton-scratch
So I'm no expert. My instinct is that these two dark stuffs are (as the OP
said) "placeholders" \- stuffs that fall out of the equations, but which we
cannot find, or even describe. If you want me to believe in stuff, then you
need to be able to point at it (at least in some kind of metaphorical sense).

I have a similar opinion about "cosmic inflation", as it happens; it appears
to be a solution to a problem with the equations.

In both cases, it looks to me very much as if someone has described the
problem, and then announced that their problem-description is actually a
solution. Nice work, if you can get it.

------
limeman
You know harry potter also deals with the same question in this chapter

~~~
dboreham
Dark arts?

~~~
alanbernstein
Possibly referring to [http://www.hpmor.com/](http://www.hpmor.com/)

------
ggambetta
To my entirely untrained eye, dark matter and dark energy always looked wrong.
Like a hacky way to patch our incorrect models. Not at all unlike Aether [0]
later rendered unnecessary by Special Relativity, or the Cosmological Constant
[1] later rendered unnecessary by, precisely, dark energy.

I find it very arrogant to believe that our current model is _the_ model. New
physics are discovered when the model is found to be broken at the edges, and
a new model emerges whose explaining power is a superset of that of the
previous model.

I guess we're coming to the end of the "normal physics" stage of Kuhn's cycle
of scientific progress [2]. I wonder whether the current incentive structure
for science will let us progress to the next phase, or trap us for
unnecessarily long in the current one. I hope it's the former.

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

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

[2]
[https://en.wikipedia.org/wiki/The_Structure_of_Scientific_Re...](https://en.wikipedia.org/wiki/The_Structure_of_Scientific_Revolutions#Phases)

~~~
Cogito
> I find it very arrogant to believe that our current model is _the_ model.

I think it would be arrogant, but I've never met anyone who studies physics
and believes this (though I wouldn't be surprised if some exist). It's not
arrogant, however, to be dogmatic when faced with ideas that are incompatible
with established models. It's unreasonable to claim someone must be using a
broken model, because dark matter and dark energy looks wrong.

Scientific models are good when they are predictive, and powerful when they
are simple.

It's incredibly rare for a model-in-use to be _wrong_ \- we would never use
something that has no predictive power - but every day scientific models are
tested, refined, expanded, and simplified in order to make them either more
predictive (better accuracy) or more powerful (describe more things).

Sometimes this does manifest as being 'broken at the edges' but not always.
Sometimes a model is just fuzzy, and improvements add resolution to every
prediction.

~~~
ggambetta
I generally agree with all of your points.

> Scientific models are good when they are predictive, and powerful when they
> are simple. [...] every day scientific models are tested, refined, expanded,
> and simplified [...]

It seems to me that dark matter goes directly against this. The current model
of gravity can't explain certain observations, so we come up with a form of
matter and energy that we can't observe in any way, and doesn't interact with
anything, except with precisely the one thing we need for our observations to
match the theory. A theory that hasn't even been reconciled with quantum
mechanics yet!

Smells super fishy to me. But as I say, my knowledge of physics and
astrophysics is super basic, and I wouldn't call myself "someone who studies
physics" in any meaningful way. I understand your feeling that it's
unreasonable for me to criticise the status quo because dark energy looks
wrong, as I have no idea about the underlying math.

~~~
Cogito
> so we come up with a form of matter and energy

The current top comment by superqd[0] probably deals with this best. _Maybe_
there is a literal dark energy and dark matter, but the thing that is
manifested is a 'correction' term in the way we use these equations, and it so
happens that using those correction terms makes these equations more accurate.

I'd almost go so far as to say the uncorrected equations are a model that is
broken at the edges, and dark energy and dark matter are simply a
quantification of how broken they are (with the added benefit that adding them
to the equations makes the model better).

[0]
[https://news.ycombinator.com/item?id=21279200](https://news.ycombinator.com/item?id=21279200)

~~~
ggambetta
That makes a lot of sense, thanks!

------
briantakita
The Plasma Physics & Electric Universe models considers "dark matter" to be
plasma in dark mode & utilizes equations used in Electrical Engineering &
other Electrodynamic/Plasma models.

[https://www.amazon.com/Physics-Plasma-Universe-Anthony-
Perat...](https://www.amazon.com/Physics-Plasma-Universe-Anthony-
Peratt/dp/1461478189)

Relativity (time dilation) & > 3 physical dimensional mathematics is rejected
in favor of physical experimentation & interpreting physical phenomena through
the lens of plasma behavior (dark & glow & arc modes, discharge, z-pinch,
Birkeland Currents, etc). The EU model has successfully predicted surprising
phenomona of asteroids/comets & astrogeology and makes bold interstellar
predictions. Luminaries, deviating from the standard model, include Hannes
Alfvén, Anthony Peratt, Ralph Jurgens, Immanuel Velikovsky, Kristian
Birkeland, Nikoli Tesla, David Talbott, Donald Scott.

Recent physical experimental success include the SAFIRE project. "The SAFIRE
PROJECT reactor generates energy densities analogous to the Sun's ...in a
laboratory on Earth".

[https://www.youtube.com/watch?v=DTaXfbvGf8E](https://www.youtube.com/watch?v=DTaXfbvGf8E)

