
No, negative masses have not revolutionized cosmology - benwr
http://backreaction.blogspot.com/2018/12/no-negative-masses-have-not.html
======
mbell
I'm not a physicist, just an interested bystander but some of these arguments
don't seem to hold water to me:

> There’s a more general point to be made here. The primary reason that we use
> dark matter and dark energy to explain cosmological observations is that
> they are simple.

> A creation term is basically a magic fix by which you can explain everything
> and anything.

Dark matter is an unknown 'substance' that interacts only through gravity
(weakly) and must have a very specific and complex distribution in the
universe to 'work'. That strikes me as neither 'simple' nor much different
than tossing a constant into an equation to make it work. Similarly Dark
Energy is an unknown form of energy that in uniformly distributed through the
universe, which to me is just a fancy way of saying "we added a constant to
make it work". In both cases I don't see how either are implicitly better than
adding a 'creation term'.

~~~
rwilson4
I believe the author's argument is: however simple or complicated dark matter
and energy are, negative (inertial) mass just makes things even more
complicated, without offering any new (testable) insights. The author herself
points out that she (and the GR community) may be wrong about a lot of this
stuff (e.g. that gravity is mediated by a spin-2 field), but absent a testable
hypothesis, negative masses don't actually move the field forward at all.

Seemingly "crazy" ideas sometimes turn out to be correct! But in science, we
demand radical ideas at least make testable hypotheses.

~~~
guitarbill
> "it’s highly problematic to introduce negative inertial masses because this
> means the vacuum becomes unstable. If you do this, you can produce particle
> pairs from a net energy of zero in infinitely large amounts. This fits badly
> with our observations."

Basically, if you mess the the equation, you have to be very sure you aren't
simulating something silly. Which is easy to do, unfortunately I've done it
often.

I still need to read the original paper in detail to confirm, but if the post
is correct, the N-body simulation might have some issues.

~~~
Izkata
Waaait a minute, isn't that describing something we've already observed -
spontaneous creation of virtual particle pairs in a vacuum?

~~~
aroberge
We have definitely not observed spontaneous creation of virtual pairs in a
vacuum. I do not blame you for thinking so as it is often described this way.

Almost no computation in Quantum Field Theory can be done exactly. What
physicists do is using perturbation theory which is very similar to doing
Taylor series in introductory calculus. Feynman, in a genius inspiration,
found a way to represent the various mathematical terms that occur in these
types of calculations as pictures which could be described in words. In this
pictorial language, one would say things like "this term correspond to the
creation of a virtual pair of particles", etc." The perturbation expansion is
a mathematical "trick" done so that we can do obtain approximate results. Each
individual term in that expansion has no physical meaning - in spite of the
pictorial language used.

------
benwr
This is a critical response to
[https://news.ycombinator.com/item?id=18609375](https://news.ycombinator.com/item?id=18609375)
; in the comments is a response by Jamie Farnes, the author of the paper, and
a rebuttal by the blog's author.

~~~
jcranberry
I have absolutely no idea who is right, but I don't see why Dr. Hossenfelder
is being so patronizing? In her rebuttal in the comments she says

>Trust me, I do not enjoy doing this, but I do not want false claims to spread
in the popular science literature.

The only way to stop popular science lit from misinterpreting/aggrandizing
scientific findings and publishing what eventually amounts to false claims
would be to just never publish anything provocative at all...and then they'd
still probably find some BS to publish. I don't really see why such reasoning
would prompt this response for the paper.

~~~
jessriedel
> I don't see why Dr. Hossenfelder is being so patronizing

This is her personality. I don't think it's productive to resolving scientific
disagreements and don't endorse it; she looks obnoxious in contrast to the
author's polite reply in the comments. But it's definitely her natural state,
and it seems to produce a writing style that readers enjoy.

> The only way to stop popular science lit from misinterpreting/aggrandizing
> scientific findings and publishing what eventually amounts to false claims
> would be to just never publish anything provocative at all

Hossenfelder is claiming much more than that the work is provocative. She's
claiming it's highly disfavored for widely known reasons that the authors do
not sufficiently address. She's probably also implicitly suggesting the
authors are allowing their work to be marketed directly to the lay public, who
do not have the expertise to assess the work, for personal gain. Obviously you
can always give alternative explanations (the establishment is close-minded,
or whatever), but keep in mind that Hossenfelder is waaaay outside the
establishment.

------
duality
"[T]he gravitational interaction is exchanged by a spin-2 field, whereas the
electromagnetic force is exchanged by a spin-1 field. Note that for this to be
the case, you do not need to speak about the messenger particle that is
associated with the force if you quantize it (gravitons or photons). It’s
simply a statement about the type of interaction, not about the quantization.
Again, you don’t get to choose this behavior. Once you work with General
Relativity, you are stuck with the spin-2 field and you conclude: like charges
attract and unlike charges repel."

There was a bit of discussion about this on
[https://news.ycombinator.com/item?id=18609375](https://news.ycombinator.com/item?id=18609375)
as well. Working through the exercise of how spin-2 mediated forces differ
from spin-1 is a worthwhile exercise for those who are so inclined.

~~~
pfdietz
If negative mass experiences an attractive force (that is, one that adds
momentum toward the source of the force), doesn't that push it away? After
all, with negative mass, the momentum vector and the velocity vector point in
opposite directions.

------
khawkins
Occam's razor is a heuristic used in the investigative process, not a
principle. While it's fair to point out that a theory makes everything more
complicated and thus might make it less likely to be sound, it's not fair to
push back on the first bit of research into a theory because they haven't had
the time to fully develop it.

Indeed, a scientific revolution generally starts by treating as false an
assumption previously held as true. Not to say that this is a revolution, but
if you push back too hard it won't be, whether it's true or not.

~~~
sgt101
History : Dun Scotus (really) probably invented Ockham's razor - William was
his student and used it to avoid the thing that all medieval philosophers
needed to avoid, namely being tied up and put on a bonfire.

Why did Dun invent it (probably)… well here's the thing, Christianity has a
God with three faces, son, father and holy ghost thing. Why? The answer is -
don't multiply entities beyond necessity, so God has the number of faces
necessary to do the job, no more, no less.

I'm wittering on because this is where that heuristic came from, literally
it's angels on pins stuff. So don't invest in it, I'd bet a bit that if we got
Dun and Bill together with a few pints of mead they'd laugh themselves silly
to here that 21st Century physics pins any weight onto their measure.

The Greeks wouldn't have, the Chinese didn't, why do we?

~~~
c1ccccc1
I mean, we still _call_ it Occam's razor, but I don't think that it's really
even the same principle by this point, it just shares the same name. In
science today, we don't really care about the number of entities. We don't
reject the idea that the stars are suns like our own just because that would
imply that the number of atoms in the universe is drastically larger than the
number of atoms in the solar system. Instead of hypotheses with the least
number of entities, we favour hypotheses with the smallest Kolmogorov
complexity. [1] As a slogan for the modern version, I like the phrasing by
John Von-Neumann: "With four parameters I can fit an elephant, and with five I
can make him wiggle his trunk." [2]

[1]
[https://en.wikipedia.org/wiki/Kolmogorov_complexity](https://en.wikipedia.org/wiki/Kolmogorov_complexity)
[2] [https://www.johndcook.com/blog/2011/06/21/how-to-fit-an-
elep...](https://www.johndcook.com/blog/2011/06/21/how-to-fit-an-elephant/)

~~~
comex
Nitpick: The historical problem with stars being suns wasn't (just) the
universe being larger than the solar system, but that the telescopic
observations available at the time seemed to imply that every other visible
star would have to be much larger than the Sun, in fact larger than the
_orbit_ of Saturn. This was because early astronomers didn't understand
optical diffraction and thought the Airy disks visible around stars were the
stars themselves, making their angular radius in Earth's sky seem vastly
larger than the reality. [1] Both characterization of the Airy disk and
observation of stellar parallax didn't occur until the 19th century, by which
point religious objections wouldn't have had the same status as in Galileo's
day anyway (for example, Darwin's work was published only a few decades
later).

Source:
[http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.612...](http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.612.5617&rep=rep1&type=pdf)

------
apo
Can't help but think of phlogiston:

 _Eventually, quantitative experiments revealed problems, including the fact
that some metals gained mass when they burned, even though they were supposed
to have lost phlogiston. Some[who?] phlogiston proponents explained this by
concluding that phlogiston had negative weight;_

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

Eventually, the mass paradox was resolved by the realization that combustion
is really something else altogether: the combination with a then-unknown
element, oxygen:

 _Phlogiston remained the dominant theory until the 1770s when Antoine-Laurent
de Lavoisier showed that combustion requires a gas that has mass
(specifically, oxygen) and could be measured by means of weighing closed
vessels._

~~~
JumpCrisscross
Side note: it’s pretty incredible how phlogiston theory correctly linked
burning and rusting, despite being unaware of oxidation. (Also, that plants
have a part to play in the oxidation-carbon cycle.)

I don’t know if oxygen theory would have developed without phlogiston theory
first linking these phenomena.

~~~
lisper
Almost certainly. Joseph Priestly, the discoverer of oxygen, was a confirmed
phlogistonist. He even called oxygen "de-phlogisticated air". The reason
oxygen theory wasn't developed sooner was technological limitations, not
intellectual ones: scientists simply didn't have any way of preparing pure
oxygen to experiment on before Priestly.

(FYI, I just happen to know all this because I'm in the middle of preparing a
series of lectures on the history of science, and I just finished the segment
on the atomic theory. It's a really fascinating story.)

------
onhn
It is fun to read these things, and of course discussion is a good thing, but
I do hope that blogs and comment sections do not end up serving any formal
role in the scientific literature.

There are several reasons for this: a blog's visibility is low compared to a
preprint server (in the scientific community at least), the contents of the
blog probably won't be as well-preserved, and there is a tendency to be more
casual with the arguments.

~~~
dpq
> I do hope that blogs and comment sections do not end up serving any formal
> role in the scientific literature.

Why not? It all depends on the qualification of the people writing blog posts
and comments. Remember that some 300 years ago a lot of scientific results
existed in epistolary form only, and that first journals that looked like
today's thing (like the Bulletin of the Royal Society) were basically
compilations of letters sent by astronomers, naturalists etc and reactions of
their peers sent as follow-up letters.

Naturally, this doesn't mean that anybody and their dog should have a voice in
discussing matters like cosmology if they actually have no clue about it.

~~~
onhn
The continuation of the system you mention still exists today, and is the main
channel of scientific discourse. I think it is a better channel to use when
challenging someone's work.

(I don't agree with your statement that it depends on qualifications -- I
don't think qualification is a measure of the importance of a contribution.)

------
diffeomorphism
> These equations tell you that like masses attract and unlike masses repel.
> We don’t normally talk about this because for all we know there are no
> negative gravitational masses, but you can see what happens in the Newtonian
> limit.

I feel like I am misunderstanding what the author wants to say here. It seems
to me that this would only be the case if you change the sign of the
gravitational mass (F[-m_1,-m_2]=F[m_1,m_2]), but not of the inertial mass?

In the Newtonian case you get that the force is proportional to m_1 _m_2, so
+_ +=+, + _-=- and -_ -=+, but then F=ma flips the direction of the
acceleration, right?

+ _\+ gives F >0 and a>0, so attraction.

-_\- gives F>0, but negative inertial mass yield a<0 and hence repulsion.

+*- gives F<0, so the positive inertial mass sees a<0 and is repelled, while
the negative inertial mass sees a>0 and is attracted.

What am I missing here?

------
eutropia
Tone of this blog post is extremely pedantic and comes across as if the author
were personally insulted by the paper.

But her argument is basically "You didn't understand the math, and you
misunderstood the work that you cited".

I feel like the burden is a little higher -- i.e. put in some effort to at
least show the readers the math she's talking about and the counterfactual
conclusions they arrive it if worked out.

I would be much more convinced if she took the original paper's claims at
their strongest and most convincing and formulating a simple proof or
mathematical argument why the paper is wrong -- instead it feels like she's
knocking down a straw man and saying "you're too stupid to be doing this kind
of work"...

~~~
ajkjk
The argument presupposes that a person either believes (if they're not a
physicist) or knows (if they are) that a spin-2 field must obey those rules,
and that gravity is a spin-2 field, which makes the conclusion follow
naturally.

The math for this would not make any sense to a layperson, but is widely
accepted. Proposing a new theory of negative mass means proposing much more
significant alterations to the underlying theory of gravity; the theoretical
machinery supporting this current understanding is huge.

~~~
eutropia
True. I'm not a physicist, so help me understand -- we've only ever observed
fermions with spin 1/2 and bosons with spin 1 (and recently, Higgs at 0).
We've postulated that gravitons would have to be spin-2 because of how the
math works out (I don't understand the math but wikipedia suggests that its
because gravity is defined by the use of 2nd order tensors) but we've not
confirmed the existence of gravitons. Hopefully I'm not talking past you by
speaking of particles, correct me if I'm wrong but particles/fields are
interchangeable as a matter of quantization, right?

I definitely trust the general relativity math -- gravitational lensing /GPS
atomic clock corrections are perhaps the easiest bits to wrap my head around
as evidence.

Anyways, all that is to ask the question -- Is this negative mass model in
conflict with observations or is it in conflict with other models of those
observations?

~~~
ajkjk
I am not an expert on this either, though I guess I hope to be someday. But
when Hossenfelder says that Spin-2 necessitates like charges attracting, I
believe her. I've also heard that result elsewhere.

My understanding is that the spin of a field or particle is more of a result
of the equation (specifically, the Lagrangian) which governs its dynamics.
This is irrespective of whether you consider it as a field or as a quantized
particle of that field; either way the Lagrangian has certain symmetries. The
Fermion Lagrangian has symmetry on 4pi rotation, but not 2pi, which
(confusingly) we call spin-1/2\. I suppose that the GR Lagrangian has symmetry
under pi rotations, which corresponds to spin-2.

(that would make sense if the stress-energy tensor is contracted with two
vectors; it would essentially boil down to the fact that (-x^T) M (-x) = x^T M
x if you wrote everything as matrices. But while I have studied GR I haven't
studied it as a field theory so I'm not sure it's this simple.)

So the Spin-2 thing is not too questionable. I don't anything about how to
turn that into a statement about gravitational charges, though.

------
8bitsrule
"highly problematic to introduce negative inertial masses .... This fits badly
with our observations."

Science news is not scarce. Click-bait titles aren't either. There's no
shortage of catchy on-paper theories that 1)are not backed by evidence, 2)
make no predictions.

Cosmology has a big problem, and thrashing around in desperation is not
becoming.

------
hirundo
Negative matter is created spontaneously everywhere, according the article in
theconversation.com. Why then can't we collect it, isolate it, and poke and
prod it? Because the particles are too small and interact too weakly?

~~~
Florin_Andrei
> _the particles are too small_

That's not a good way to talk about these things. Usually it's the really
small stuff that is very strongly interacting - see quarks.

> _and interact too weakly_

That's better.

------
btilly
Obligatory XKCD: [https://xkcd.com/955/](https://xkcd.com/955/)

When you read breathless science journalism about the latest revolutionary
paper, remember how it usually works out. Enjoy it as entertainment if you
wish, but don't get your hopes up.

~~~
vymague
Reminds me of the SUSY bets.

------
ykonstant
Well, the negative consensus is disappointing, but the criticisms are
absolutely massive.

