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| | |.---.-..----.| |--..-----..----. | | |.-----..--.--.--..-----.
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on Gopher (inofficial) |
| Visit Hacker News on the Web |
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COMMENT PAGE FOR: |
| James Webb Space Telescope finds evidence for alternate theory of gravity |
|
librasteve wrote 4 hours 6 min ago:
Here is what Sean Carroll has to say about MOND⦠[1] I find this
treatment more compelling.
|
| [1]: https://www.preposterousuniverse.com/blog/2011/02/26/dark-matt... |
|
Glyptodon wrote 9 hours 26 min ago:
Is there a quantized version of MOND where the increased acceleration
is because a quantized unit of gravity will exert force across
distances that would otherwise suggest that that the force would be
less than a "g quanta" or because maybe quantization "ceilings" more
than floors at very large distances? If gravity does have some kind of
particle or fundamental quantization like a photon, and basically still
exerts at huge or "infinite" distance, does it make sense that it's
more likely there's some kind quantization floor or maybe quantization
bands or something? Or is it thought that quantization of gravity
imposes a limit on distance for the exertion of gravitational
attraction? (Or is it thought that that with quantized gravity that
what's happening is a decreased rate of "gravitons" interacting between
the objects?)
Anyway, a bit clueless about this, just curious what gravitons are
supposed to mean for either theory (MOND, LCDM, etc.).
maronato wrote 12 hours 16 min ago:
For a dark matter researcherâs take on MOND, see this video:
|
| [1]: https://youtu.be/qS34oV-jv_A |
|
I_am_tiberius wrote 13 hours 34 min ago:
What I find implausible about MOND is the constant a0 (~1.2). Why stick
with a measurement based constant instead of exploring a parameter that
varies with distance?
pikseladam wrote 13 hours 39 min ago:
It shows early galaxies forming way faster and bigger than expected,
which kind of shakes up the whole dark matter idea. Seems like it
supports the MOND theoryâthat gravity might not work the way we
think. Pretty wild, but itâs still up for debate.
steve_adams_86 wrote 13 hours 38 min ago:
That would be a fun surprise to me as a lay person who doesnât
actually understand these things, because I see a lot of
disparagement towards the MOND theory.
docflabby wrote 14 hours 13 min ago:
Dark matter is just made up bs if you replace "magic" for dark whenever
its mentioned its the same difference - theres no tangable evidence it
exists at all.
akvadrako wrote 12 hours 56 min ago:
Dark energy is literally this - it just means something is different
than predicted by current leading theories.
There is plenty of evidence that either dark matter or an alternative
is needed and CDM is just the most popular take.
XCSme wrote 13 hours 55 min ago:
There is no evidence that anything exists...
verzali wrote 15 hours 27 min ago:
Why why why do people share articles with sensational headlines like
this? Its no wonder science journalism gets a bad rap. This kind of
thing really undermines all the people who are actually trying to
communicate science properly.
muglug wrote 15 hours 1 min ago:
Without this article and HN discussion Iâd never have known about
MOND, which is (at the very least) a fun theory.
verzali wrote 5 hours 48 min ago:
There are much better articles on MOND that don't make misleading
claims that the James Webb has proven it. This one, for example:
|
| [1]: https://physicsworld.com/a/cosmic-combat-delving-into-the-... |
|
prof-dr-ir wrote 12 hours 44 min ago:
The trouble is that MOND is just not worth your time. In fact, I
would even object to calling it a 'theory' in the first place.
MOND is just some wild idea, but a little thought should convince
every physicist of its uselessness. It has major issues both in
explaining experimental data and in its theoretical consistency. It
justifiably receives next to no attention from the vast majority of
(astro)physicists.
In popular science the idea however does not seem to want to die,
perhaps because it is so easily explained to a layperson. Of course
this is a little frustrating for the community, but perhaps we
should look at the upsides: more attention for science is probably
a good thing, and explaining to people why MOND is so useless
provides a good opportunity to discuss some proper physics.
lloeki wrote 5 hours 0 min ago:
> The trouble is that MOND is just not worth your time. [...]
MOND is just some wild idea
Sometimes you gotta be wrong before you get it right.
I mean, Newtonian mechanics are "wrong" but served us well at
some scales for a while, and that it observationally failed in
others led us to relativity. Even "relativity" took iterative
steps, from Poincaré's Lorentz invariant theory (or even earlier
with Galilean relativity) all the way to GR via
special/restricted relativity, the latter name having been
retconned because it's only valid in restricted special cases and
fails to unify generally. And we know GR fails to unify with
quantum mechanics, so one of them (or both) gotta give.
So even if something as MOND were "wrong" and known to be wrong
(definitely so), there's still value in experimenting with it to
get a better understanding of things. That's just how things
work.
prof-dr-ir wrote 2 hours 35 min ago:
> there's still value in experimenting with it
I disagree: some experiments are just not worth our time. I
wrote about such a situation three years ago: [1] My view is
that it applies here as well.
|
| [1]: https://news.ycombinator.com/item?id=26656206 |
|
ogogmad wrote 5 hours 14 min ago:
This is a weirdly arrogant comment given both TFA and the fact
that professional physicists have worked on MOND and disagree
with everything you've said. [1] Is this typical behaviour for
physicsts? Extremely strong opinions expressed in an abrasive
way, out of proportion to the available evidence?
|
| [1]: https://www.youtube.com/watch?v=n33aurhg788 |
|
prof-dr-ir wrote 2 hours 44 min ago:
I just want to convey the following point: for the vast, vast
majority of physicists the status of MOND is akin to what
doctors think of the anti-vaccine theories. The evidence in the
opposite direction is simply overwhelming.
You refer to a non-scientific article and to a youtube video,
but any vaccine sceptic can probably easily find exactly the
same kind of material to support their view. That would almost
certainly include a video by a "professional doctor".
You might call me abrasive, but I am really just trying to be
as clear as possible: this is the consensus in the field.
And before you continue this discussion it might be worth
pondering the following questions. How do you think doctors
should convince vaccine skeptics that vaccines work? And how
big a percentage of their weekend do you think they should
spend engaging on the details with anti-vaxxers? (And, in this
forum, how many downvotes from obvious non-experts should they
be willing to accept?)
In other words, what could I do to convince you in a reasonable
amount of time?
trimethylpurine wrote 13 hours 48 min ago:
Personally, I think it would be better that way. Science works in
pursuit of truth, not towards the obfuscation of it for personal
and selfish financial gain. That should hopefully explain the
outage that scientists have towards articles like this one. In
place of relying on articles like this, you might try searching
scholarly articles or subscribing to them.
mgraczyk wrote 15 hours 53 min ago:
Are any of the MOND theories consistent with this new data also
consistent with recent gravitational wave observations? My
understanding is that gravitational wave detectors have recently ruled
out most plausible MOND theories. The linked paper doesn't seem to
discuss this.
RicoElectrico wrote 15 hours 55 min ago:
Waiting for Angela Collier to make a video on this, I'm sure many
people will forward her this article. MOND is actually a niche in
cosmology despite its PR.
astroH wrote 16 hours 13 min ago:
In my opinion, this article is misleading at best. "...scans of ancient
galaxies gathered by the JWST seem to contradict the commonly accepted
predictions of the most widely accepted Cold Dark Matter theory,
Lambda-CDM." --> LCDM doesn't predict what galaxies should look like,
it simply predicts how much mass is in collapsed structures and that
dark matter haloes grow hierarchically. In contrast, with JWST we see
light and need to infer what the underlying properties of the system
are. It was shown very early on that the theoretical upper limit (i.e.
taking all of the gas that is available in collapsed structures and
turning it into stars) predicts a luminosity function (i.e. number of
galaxies per unit luminosity) that is orders of above what JWST has
observed (e.g. [1] ). This means that there is plenty of space within
the context of LCDM to have bright and seemingly large and massive
galaxies early on. Based on current JWST data at these early epochs,
there are really no convincing arguments for or against LCDM because
it's highly sensitive to the galaxy formation model that's adopted.
|
| [1]: https://ui.adsabs.harvard.edu/abs/2023MNRAS.521..497M/abstract |
|
ajross wrote 14 hours 38 min ago:
> there are really no convincing arguments for or against LCDM
because it's highly sensitive to the galaxy formation model that's
adopted.
To be fair, that is absolutely not the way ÎCDM would have been
described to someone in the pre-Webb days. It was a well-regarded
theory and the hope was (a-la the Higgs detection) that new data
would just better constrain the edges and get us on to the next phase
of the problem.
But instead it's a wreck, and we didn't see what we were expecting at
all, and so now we're retreating to "Well, ÎCDM wasn't exactly
proven wrong, was it?!"
That doesn't mean it's wrong either, and it for sure doesn't mean
MOND is right. But equally for sure this is a Kuhnian paradigm shift
moment and I think it's important for the community to be willing to
step back and entertain broader ideas.
Davidzheng wrote 13 hours 49 min ago:
Absolutely not in the field, so if you are please completely
disregard. But from conversations with physicists (not
cosmologists) I always thought people thought a lot of evidence for
ÎCDM was dubious at best.
astroH wrote 14 hours 26 min ago:
Again, LCDM and galaxy formation are two different things. "...and
we didn't see what we were expecting at all..." It depends on who
you ask. There were many pre-JWST models that did well in this
regard. A particularly interesting one is this from 2018 ( [1] ).
That group even had to write another paper reminding everyone of
what they predicted ( [2] ). Another example is here ( [3] ) which
shows results from a simulation from ~2014. I can provide numerous
other examples of this. My point isn't which theory is or isn't
wrong, my point is that what is presented in this particular
article is not a constraint on any realistic theory of gravity as
the sensitivity of these particular observations to galaxy
formation modeling is so strong.
|
| [1]: https://ui.adsabs.harvard.edu/abs/2018MNRAS.474.2352C/abst... |
| [2]: https://ui.adsabs.harvard.edu/abs/2024arXiv240602672L/abst... |
| [3]: https://ui.adsabs.harvard.edu/abs/2023OJAp....6E..47M/abst... |
|
uoaei wrote 15 hours 7 min ago:
> with JWST we see light and need to infer what the underlying
properties of the system are
Every theory of dark matter is based exclusively on light-emitting
objects. There is no "contrast" between JWST's methods and those of
others. Casting aspersions on JWST because it can only see light is
like casting aspersions on Galileo because he could only build
telescopes. If we could teleport to the things we study and get more
information that way, it would be nice, but we live in reality and
must bend to its rules.
> highly sensitive to the galaxy formation model that's adopted
I should only need to remind the reader of the classic idiom "cart
before the horse" to remind them that this line of reasoning is
invalid.
MattPalmer1086 wrote 3 hours 41 min ago:
You are missing the point. JWST is not being singled out as
different here, and no aspersions are being cast.
It is the entirely general point that all we can observe is the
light, and we have to infer what that means. Maybe things are
bright because there's a lot of stars. Maybe there aren't but
there is not much dust. Maybe there aren't so many stars but they
are bigger and brighter. There is room to fit many different
models on the basis of the light that is observed.
astroH wrote 14 hours 34 min ago:
This is a misrepresentation of what I am saying. By no means am I
casting an aspersion on JWST. I am casting an aspersion on this
particular observation as a test of MOND and LCDM. Also I highly
disagree about your comments on my line of reasoning. The fact that
you can obtain a huge range of possible galaxy properties in the
context of LCDM indicates that in general, tests of LCDM and MOND
that rely on galaxy formation model are in usually not strong
tests. This is the key issue with using the abundance of high-z
galaxies (or even their masses -- despite the fact that these
aren't measured) as a test. In the context of LCDM, you need haloes
to form galaxies but it has been shown many times that there are
enough haloes to solve the problem (see the paper linked) by a huge
amount.
uoaei wrote 14 hours 30 min ago:
The skepticism you display in this comment is completely absent
when you reference lambda-CDM elsewhere. Consistency invites zero
criticism :)
astroH wrote 14 hours 25 min ago:
And so you have proved my point. The observations presented in
this article can be made consistent with both...as such one
should think about stronger tests of both LCDM and MOND.
uoaei wrote 14 hours 22 min ago:
Your point was orthogonal to the point of epistemology. This
isn't Reddit, we respect actual arguments here.
Bengalilol wrote 16 hours 44 min ago:
« Stunning evidence »
⦠then later on:
« Instead, the readings _seem_ to support a basis for MOND, which
_would_ force astronomers and cosmologists to reconsider this
alternative and long-controversial theory of gravity. »
Whatâs conditional evidence? I may be missing the overall picture,
but I view such writing as non precise at its best.
yieldcrv wrote 16 hours 34 min ago:
There is no consensus yet, there is no repeatable metric
It is perfectly valid to say âhey look over there for further
reviewâ
bbor wrote 16 hours 37 min ago:
Well, itâs evidence that a) must be verified on a mathematical and
empirical level, and b) (arguably) fits better with a currently
unpopular theory than the dominant one. Thereâs so many unknowns in
physics that opponents can easily reply âwell your theory doesnât
explain XYZ yet, so we likely just need to tweak our theoryâ.
In other words, reasonable minds do disagree. AFAIU as an amateur.
MattPalmer1086 wrote 16 hours 40 min ago:
It's just typical pop sci journalism, with a click baity headline.
Read the paper instead.
joe_the_user wrote 16 hours 5 min ago:
Not entirely typical. MOND proponents seem to be trying more and
more sell their approach to the public.
It annoys me but I suppose every theory has to do that now, "the
mouse trap must go to market now" and all.
akvadrako wrote 12 hours 59 min ago:
Well you have to convince somebody to pay researchers for their
time, which ultimately means selling your idea to non-experts.
Bengalilol wrote 16 hours 35 min ago:
Thanks, I will.
|
| [1]: https://iopscience.iop.org/article/10.3847/1538-4357/ad834... |
|
samsartor wrote 17 hours 44 min ago:
My hangup with MOND is still general relativity. We know for a fact
that gravity is _not_ Newtonian, that the inverse square law does not
hold. Any model of gravity based on an inverse law is simply wrong.
Another comment linked to [1] , which is an excellent read. It makes
the case that GR has never been tested at low accelerations, that is
might be wrong. But we know for a fact MOND is wrong at high
accelerations. Unless your theory can cover both, I don't see how it
can be pitched as an improvement to GR.
Edit: this sounds a bit hostile. to be clear, I think modified gravity
is absolutely worth researching. but it isn't a silver bullet
|
| [1]: https://tritonstation.com/new-blog-page/ |
|
twothreeone wrote 13 hours 37 min ago:
GR says spacetime is curved by mass, right. So what's the basis for
explaining the curvature of space (which can be measured, e.g., LIGO)
in MOND?
oneshtein wrote 5 hours 22 min ago:
> GR says spacetime is curved by mass, right.
Wrong. GR says that gravitation can be modeled as acceleration.
mog_dev wrote 4 hours 33 min ago:
General Relativity states that mass-energy curves spacetime, and
objects follow the straightest possible paths (geodesics) through
this curved geometry. The equivalence principle relates gravity
and acceleration, but it's not the main description of gravity in
GR.
MathMonkeyMan wrote 9 hours 16 min ago:
MOND has nothing to say about the curvature of spacetime, since
MOND is Newtonian (MOdified Newtonian Dynamics). It goes back to
"F=ma and gravity is a force" and modifies the rules so that
gravity grows weaker faster at a certain scale.
The fact that MOND fits a lot of the data troubled cosmologists,
because they know that a General Relativistic theory is needed to
explain pretty much the rest of gravity.
TeVeS is an extension to General Relativity that reduces to MOND in
the non-relativistic limit. For comparison, General Relativity
reduces to Newtonian gravity in the non-relativistic limit. The
non-relativistic limit is when speeds and spacetime curvature are
small.
Gooblebrai wrote 3 hours 45 min ago:
How does MOND deal with the effects of time dilation and length
contraction? Do we have to go back to Newton's time where there's
a universal time?
throwawaymaths wrote 14 hours 30 min ago:
> My hangup with MOND is still general relativity.
Fwiw, we know for a fact also that for edge cases GR is wrong because
it doesn't agree with quantum mechanics (unless QM is wrong), so it's
maybe not right to take GR as gospel, especially for a theory that
only seems to also change GR in edge cases, and the only reason why
"it doesn't agree" might amount to "the math is hard and the
physicists haven't put enough work in yet"
To wit, accepting a mond-ified GR is probably not going to change how
GPS works so the claim that "GR has withstood the test of time and
engineering" is not a totally solid refutation of MOND
pdonis wrote 7 hours 21 min ago:
> for edge cases GR is wrong because it doesn't agree with quantum
mechanics
What "edge cases" are you talking about? I agree that GR is not a
quantum theory, but it's not established that that has to be a
problem, nor is it a matter of "edge cases".
scotty79 wrote 10 hours 23 min ago:
> because it doesn't agree with quantum mechanics
I don't think it doesn't agree. It's just that we never managed to
neither formulate quantum mechanics on 4 dimensional space time nor
quantize gravitational force. So we simply have no idea what
happens in small scale in significant gravitational fieldd.
mort96 wrote 14 hours 11 min ago:
Well this doesn't seem like such a conundrum. We know for sure that
ND is wrong because it predict things incorrectly which GR predicts
correctly. We know GR is wrong because it is incompatible with any
form of QM and we know some form of QM is more or less correct.
Essentially, GR and ND are both wrong, but ND is more wrong than
GR.
meindnoch wrote 16 hours 26 min ago:
>We know for a fact that gravity is _not_ Newtonian, that the inverse
square law does not hold
[citation needed]
The consensus is that gravity - outside of extreme mass/energy
environments - works just as Newton described it to many many decimal
places.
Emphasized part added because people in the replies thought that I
literally think that General Relativity is somehow wrong. Don't be
dense. All I'm saying is that gravity at galactic scales works as
Newton described it. General Relativity has extremely tiny effect at
those scales.
nimish wrote 14 hours 13 min ago:
>The consensus is that gravity - outside of extreme mass/energy
environments - works just as Newton described it to many many
decimal places.
It absolutely does not. Newtonian gravity occurs instantly. It has
no notion of information taking time to propagate. But we know
gravitational waves happen, so Newtonian gravity is wrong _at even
very large scales_. If the sun disappeared Newton tells us we'd
know immediately. In GR we'd know about 8 min later.
The bigger problem is not that the quantitative effect is large,
but that the _qualitative_ difference of going from the
instantaneous effect to one that needs to propagate is enormous.
It's the whole point of relativity as a concept.
Even going to GEM as a true, non-singular linear approximation of
GR would be a step up from Newton's laws, at least there we can
have gravitational waves and causal flow of information.
the__alchemist wrote 1 hour 4 min ago:
Thanks for bringing this up; this is the central reason why I'm
skeptical of Newtonian models that predict dark matter, and why I
don't think the term MOND makes sense as the simplest
alternative.
wbl wrote 15 hours 55 min ago:
We can see gravitational redshift on Harvard's campus thanks to
gamma ray Mossbauer spectroscopy.
EPWN3D wrote 16 hours 0 min ago:
You're simply wrong. There's no other way to put it. The GPS system
would have been simply impossible to deploy without the general
theory of relativity. There's no extreme energy or mass involved,
just precision requirements that are influenced by the minuscule
differences in time experienced by the surface of the earth and
orbiting satellites.
Also Newton's laws famously could not account for Mercury's orbit.
Mercury is just an ordinary planet orbiting an ordinary star.
Nothing extreme is involved. He knew his laws were incomplete. But
they were so dead-on in basically every other scenario that could
be physically observed at the time that he figured there was some
small tweak missing (or maybe another planetary body that hadn't
been spotted yet).
tzs wrote 11 hours 53 min ago:
Compared to the gravitational fields galaxies orbiting other
galaxies deal with Mercury orbiting the Sun is extreme. So are
GPS satellites orbiting Earth.
Mass of Sun: Ms = 1.99e30 kg
Distance to Mercury from Sun: Rm = 5.83e10 m
Mass of Milky Way galaxy: Mg = 6e42 kg
Q: At what distance R from the Milky Way would something have to
be to experience the same gravitational field strength from the
Milky Way that Mercury feels from the Sun?
A: We want R such that Ms/Rm^2 = Mg/R^2 or R = Rm sqrt(Mg/Ms) =
1.0e17 m.
Let's convert that to lightyears. There are 9.46e15 m/ly. The
final result is 10.75 ly. Note that everyplace that close to the
center of mass of the Milky Way is inside the galaxy. Anything
actually outside the galaxy would be at least 5000 ly away and
feel a gravity field at most 1/200000th as strong as what Mercury
feels.
For Earth use the same calculation from above but replace Mg with
the mass of the Earth, 5.97e24 kg. That gives that the distance
from Earth where something would feel the same field strength
from Earth that Mercury feels from the Sun is 1.0e9 m. That's a
little over 4x the radius of the orbits of GPS satellites, so GPS
satellites are feeling a little under 16x the field strength from
Earth that Mercury feels from the Sun.
ahazred8ta wrote 13 hours 59 min ago:
We know that spacetime is einsteinian, not euclidean, yes. But
that's not what's being discussed here. The issue is whether the
force of gravity deviates from the expected 1/r^2 value.
Experiments, measurements and observations within the solar
system have not revealed any deviation. The precession of mercury
is not due to a deviation from 1/r^2; it is due to space near the
sun being bent instead of flat. Ditto GPS; we have to adjust for
time dilation and curved space, but not for any deviation from
1/r^2. MOND theories predict that MOND gravity is
indistinguishable from normal at short ranges less than several
light years; the MOND effects only show up at distances of many
light years.
meindnoch wrote 15 hours 21 min ago:
Easy there champ. Noone is shitting on general relativity.
All I'm saying is that the effect of general relativity at
galactic scales is so minuscule, that galactic dynamics is - for
all intents and purposes - governed by the Newtonian limit of
gravity.
If you propose that gravity doesn't behave like the Newtonian
limit at those scales, then you're contradicting general
relativity as well, since the far-field limit of the
Schwartzschild metric is literally Newton's inverse square law.
In layman terms, modified Newtonian gravity, that the article
talks about, is an attempt to explain why galaxies don't rotate
the way they should according to Newton (and Einstein, because at
those distances the two are the same!!!).
fpoling wrote 5 hours 44 min ago:
We already know that one must not use Newtonian gravity on the
galaxy scale. For example, properly accounting for GR effects
is enough to explain the observed rotational curve for our
Galaxy without the need for any dark matter hypothesis.
Similarly there are papers that tries to explain the effects
attributed to dark matter on the scale of tenths and hundreds
megaparsecs using just proper accounting of GR effects. They
are rather speculative, but still they show that even on very
huge distances Newtonian approximation may not be valid.
magicalhippo wrote 3 hours 37 min ago:
> For example, properly accounting for GR effects is enough
to explain the observed rotational curve for our Galaxy
without the need for any dark matter hypothesis.
Do you have some references handy for this? Or are you
talking about the work of Deur?
jfengel wrote 14 hours 36 min ago:
I had the impression that "shitting on general relativity" was
exactly what MOND was about. That is, it starts from the
position that Einstein is wrong, and searches for ways to
support that.
throwawaymaths wrote 14 hours 21 min ago:
Can you explain how MOND shits on GR? My understanding is
it's more like. "GR is mostly right but...". As for MOND
being exclusively Newtonian, yeah. In terms of solving the
math, you gotta crawl, walk, run. Let's not kid ourselves,
GR invokes way harder math than algebra and simple integral
calculus. TeVeS Is a first attempt at "walk", let's say, but
even it might not be correct even if adjusting gravity may be
correct.
If someone emerges with a proof that the two systems are
irreconcilable then yeah you have an argument that it's
"shitting on GR"
at_a_remove wrote 5 hours 31 min ago:
Hi! Physics BS, but they let me take some grad courses,
including a Spacetime and Relativity class. I can help.
The word "mass" is used in physics in three different
general contexts. First, we have mass in mass-energy, as
in "how much energy can I get for trading in this mass?"
Mass-energy is the coin paid as the price of existence. If
it exists, it has mass-energy. Mostly mass for us.
Mostly. We can skip that one for now.
The second context of mass is inertial. Mass has the
property of inertia, of resisting a change in its direction
or speed. It resists stopping if it is motion, and if it
is stopped, it resists moving. The degree of the
resistance is also called mass. Put a pin in this one.
The third context of mass is gravitational. Two masses,
attracting one another because a force between them, a
force which is not based on charge or the relatively nearby
exchange of some more exotic bosons, no, just attraction
based on how much mass is present. Nothing more special.
Now, curiously, values of each one of these seem to agree!
Einstein's absolute core concept in general relativity, the
idea from which all else is built, is that inertial mass is
identical to gravitational mass, not merely in number, but
so fundamentally intertwined that there is no real
difference between them, other than being two faces of the
same coin. Now, that does not sound like much, but it
gives birth to experiments such as an elevator which is
falling toward versus an elevator floating far from
gravitational sources, and that they are, from the inside
of the elevator, impossible to differentiate.
Einstein then constructs general relativity from this, that
the "m" in "F = ma" is identical to the first m in "F = -G
m1 * m2 / r^2"
In MOND, the two ms are not identical, they only appear
close most places, and so you cannot construct general
relativity atop it. You will get most correct
approximations but you're missing out in some cases.
meindnoch wrote 14 hours 26 min ago:
The Wikipedia article on MOND literally starts with galaxy
rotation curves: [1] There's zero mention of MOND being a
rejection of general relativity.
OF COURSE, any tweaking of Newton's formula at galactic
scales will necessarily invalidate general relativity, since
general relativity predicts Newton's formula at those scales!
But MOND tries to work backwards: they propose a modification
of the far-field Newtonian formula, and the belief is that it
can eventually be worked out to be a limiting case of a
"modified general relativity", for lack of a better name.
Just how Newtonian gravity was eventually worked out to be a
limiting case of a theory called general relativity.
|
| [1]: https://en.m.wikipedia.org/wiki/Modified_Newtonian_d... |
|
auntienomen wrote 16 hours 1 min ago:
Citation needed? That's ridiculous. The empirical evidence is
well over century old at this point. Start with the anomalous
precession of Mercury's perihelion. That already can't be
accounted for by Newtonian gravity.
ahazred8ta wrote 13 hours 45 min ago:
Samsartor seems to think that the inverse square law does not
hold at short distances (e.g. between the sun and mercury).
Meindnoch agrees with mainstream physics that the inverse square
law does indeed hold at short distances. You're confusing
newtonian physics (busted) with the inverse square strength of
gravity (still strongly supported); those are two different
things. GR says gravity should be strictly 1/r^2, and this is
what we observe in the solar system.
bobmcnamara wrote 14 hours 37 min ago:
I don't think they're saying the relativistic effects don't
exist, just that they're still largely unimportant compared to
Newtonian effects.
For precession of perihelion of Mercury we mostly noticed because
any error is cumulative over time and we could integrate over an
arbitrarily wide timebase. The relativistic effects are <10^-8 of
the total, around 1/10th of the change imparted by Newtonian
gravity of planets much, much further away. The BepiColombo
orbiter should allow us to correct for the relativistic effects
of other planets' pull on Mercury, but it's expected to be a
change of <10^-12.
So I guess "many, many decimal places" is in the ballpark of
6-12.
superjan wrote 16 hours 1 min ago:
These extremes exist, and GR predictions are better than Newtonâs
in those cases. Closest to home is mercuryâs perihelion drift. We
have observed black hole mergers, gravitational lensing, and GR is
also an essential component in understanding the universeâs
expansion(that we know from redshift and the CMB). Likely MOND will
address these, but Newtonian mechanics will not get you there.
samsartor wrote 16 hours 4 min ago:
[1] is the example I learned in school. You don't need to be
around a black hole for GR to suddenly switch on.
Newtonian gravity is an approximation. A perfectly acceptable one
in many contexts, but still measurably incorrect.
|
| [1]: https://en.wikipedia.org/wiki/Tests_of_general_relativity#... |
|
meindnoch wrote 15 hours 50 min ago:
Nobody said that general relativity is "switched on" around black
holes.
But ok, let me put it this way: outside of extreme energy/mass
environments, gravity is described by Newton's law of gravitation
with very high precision. If you look very hard, you may notice
differences on the order of 10e-MANY. But for all intents and
purposes, gravity is Newtonian in 99.99999% of the universe.
bobmcnamara wrote 15 hours 8 min ago:
My first thought was that we only know Cavendish's constant to
a little over 4 significant figures, so how could this be
right? The relativistic effects at Earth's surface would change
this by only ~10^-8, so I think the challenge in refining the
Cavendish gravitic constant lie elsewhere.
radishingr wrote 15 hours 18 min ago:
So spacetime (interactions between mass, space, and time) are
required for any sort of precision explanation. If "extreme"
means planet size masses, I guess, but I generally consider our
solar system pretty normal. However we cannot explain the
planetary motion of mercury without relativity, so define your
extreme.
But sure, newton is good enough to handle most ground based
scenarios where we only care about forces at low precision.
DiogenesKynikos wrote 15 hours 39 min ago:
The inaccuracy of the Newtonian theory of gravity is large
enough that it was already noticed by astronomers in the
mid-1800s.
samsartor wrote 15 hours 40 min ago:
Not for all intents and purposes.
If we are asking whether MOND is useful, then the answer is
probably yes. You might use it for simulations of galaxy
formation where Newtonian gravity is considered a reasonable
approximation today. But MOND is not a correct model of the
universe. There is no place in the universe that Newtonian
gravity applies, only places where the error is an acceptable
trade-off for simpler calculation.
meindnoch wrote 15 hours 33 min ago:
By the same logic, there's no place in the universe that
general relativity applies either, since it breaks down at
the quantum level. There's no place in the universe where any
theory other than the one true grand unified theory applies,
because everything else is just an approximation. At which
point we're just arguing about semantics, and I don't see a
reason for continuing it on my part.
radishingr wrote 15 hours 8 min ago:
There are vastly different scales where the approximation
is correct for newton vs general relativity. Perhaps you
can define the scales that you are calling relevant so we
understand what you mean.
meindnoch wrote 14 hours 47 min ago:
The scale of galaxies? Which the original article is
about? I feel like I need to spell out everything, but
ok:
The article is about modified Newtonian dynamics (MOND),
which is a theory that modifies Newtonian gravitation to
fix some observed differences in galaxies' motion,
without invoking dark matter. The original commenter then
proclaims "haha, MOND cannot be right, because we know
that Newtonian gravity is incorrect". Yeah, no shit
Sherlock; it is "incorrect" because it is just a limiting
case of general relativity. But that's completely besides
the whole point of MOND, which tries to "fix" gravity at
galactic scales, which is a Newtonian regime even with
general relativity. MOND is trying to tweak the Newtonian
formula at those extreme distances, and if it works, then
maybe it can be worked out to be a limiting case of a
"modified general relativity", just as Newtonian gravity
is a limiting case of GR. Got it?
exe34 wrote 15 hours 44 min ago:
that's like saying the visible mass of the universe is 99%
hydrogen and helium, so we don't need to learn about chemistry.
meindnoch wrote 15 hours 38 min ago:
So you're saying we should model galaxies down to the level
of individual protons? Lol.
Galactic dynamics is governed by gravity, which is Newtonian
at those scales.
exe34 wrote 14 hours 39 min ago:
No I did not say that.
meindnoch wrote 14 hours 22 min ago:
Ok, then how does your chemistry comment have anything to
do with the motion of galaxies? Reminder: you're
commenting on an article about MOND, which is a theory
that stems from trying to explain the motion of galaxies.
exe34 wrote 2 hours 54 min ago:
> outside of extreme energy/mass environments, gravity
is described by Newton's law of gravitation with very
high precision. If you look very hard, you may notice
differences on the order of 10e-MANY. But for all
intents and purposes, gravity is Newtonian in 99.99999%
of the universe.
I meant it in the sense that "most of the cosmos runs
on Newtonian gravity, therefore we can ignore GR" is
similar to "most of the visible matter in the cosmos is
hydrogen/helium, so we can ignore chemistry".
The interesting part is in the 0.0000001% that isn't
like the others.
hobs wrote 16 hours 17 min ago:
When you say "outside of" - that's the thing where it doesn't hold.
It's interesting and not even wrong to say "these rules work in
these contexts" but as far as I can tell we're looking for the
scenario invariant rules.
ajross wrote 17 hours 20 min ago:
To be fair, there are relativistic generalizations of MOND, in the
sense of relativistic theories that simplify to MOND dynamics in the
low energy case. My understanding (this not being my field) is that
they're sort of kludgey and non-calculable and that no one takes them
very seriously. All the "real work" on MOND is just done using the
classical stuff.
And yeah, that seems like pretty terrible cheating. It's one thing
to hang a big theory on a single conjecture, but you still need to be
trying to prove the conjecture.
MattPalmer1086 wrote 17 hours 37 min ago:
MOND isn't pitched as an improvement to GR. It was always a
Newtonian theory - it's in its name!
There are relativistic versions of MOND, for example, TeVeS [1], but
they all still have some problems.
|
| [1]: https://en.m.wikipedia.org/wiki/Tensor%E2%80%93vector%E2%80%... |
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samsartor wrote 17 hours 28 min ago:
TeVeS is definitely interesting, but it still has problems like you
said. AFAICT gravitational wave observations are particularly bad
for TeVeS theories. TeVeS isn't dead, but if dark matter theories
are criticized for being patched up post-hoc, that standard should
also apply to modified gravity.
gliptic wrote 17 hours 11 min ago:
The weirdest thing about TeVeS IMO is that it adds additional
fields that warp spacetime, so how is it not a dark matter
theory?
MathMonkeyMan wrote 16 hours 24 min ago:
For the fields to be considered particles, they have to be
freely propagating in space. TeVeS adds a vector field, a
scalar field, and some lagrange fields that are part of their
coupling. The degrees of freedom aren't consistent with one or
more particles.
uoaei wrote 18 hours 15 min ago:
I follow the lead author, Stacy McGaugh, via his blog where he posts
discussions and musings about the latest research into the dark matter
vs MOND debate: [1] His arguments are very convincing and relatively
clear. I am not an astrophysicist but I have two degrees in physics and
have always found the dark matter theory to be lacking -- in absence of
any evidence of causation whatsoever, dark matter can only be described
trivially as "where we would put matter if we could to make our theory
of gravity make sense," which is totally backwards from a basic
scientific perspective.
Predictions based on modern MOND postulates are shown to be more and
more accurate as our observational instruments continue to improve in
sensitivity.
|
| [1]: https://tritonstation.com/new-blog-page/ |
|
halgir wrote 17 hours 31 min ago:
I usually understand "dark matter" to be shorthand for the
discrepancy between theory and observation. The explanation might
indeed be matter that is dark, or it might be solved by entirely
unexpected observations and/or changes to theory.
mr_mitm wrote 17 hours 16 min ago:
Not really. You might think this after watching Angela Coulliers
video, but when you read something like "25% of the universe's
energy content is made of dark matter", they do not mean changes to
some theory. They literally mean non-baryonic matter.
zeroonetwothree wrote 15 hours 53 min ago:
Energy content not only comes from matter but also from fields.
OutOfHere wrote 17 hours 6 min ago:
Nope. It can mean change to some theory, without a need for
matter. It is the difference between relativistic gravity and the
corresponding observed mass.
antognini wrote 17 hours 37 min ago:
> where we would put matter if we could to make our theory of gravity
make sense
Dark matter behaves in a fundamentally different way from baryonic
matter. We can constrain the total amount of matter in the universe
(both dark and baryonic) from the observed abundances of
baryogenesis. But dark matter has a different effect on the relative
amplitudes of peaks in the CMB.
As far as I can tell, MOND has never really had any success outside
of modeling galaxy rotation curves.
The skepticism I've seen towards dark matter vs. MOND has always been
strange to me. Dark matter doesn't really require much in the way of
new physics --- there's just a new particle to add to the standard
model. But most MOND theories violate Lorentz invariance which is a
vastly more radical departure from standard physics. (And in my
mind, the more sophisticated MOND theories that maintain Lorentz
invariance like TeVeS are really a theory of dark matter dressed up
in the language of MOND.)
russdill wrote 16 hours 2 min ago:
The mond theories that add a factor that behaves like dark matter
do a rather good job of matching observational data.
MattPalmer1086 wrote 17 hours 31 min ago:
There are more successful predictions than just rotation curves.
For example, see:
|
| [1]: http://astroweb.case.edu/ssm/mond/LCDMmondtesttable.html |
|
antognini wrote 17 hours 23 min ago:
These successful predictions are all generally variants on
modeling galactic dynamics, though. The trouble is that galaxies
and galaxy clusters are very messy places, so it's hard to make
sure you've incorporated all the relevant physics.
By contrast something like baryon acoustic oscillations are very
simple to model, so you can be quite confident that you've
incorporated all the relevant processes. And in that regime LCDM
performs beautifully and MOND completely fails. So it's
reasonable to suspect that in more complicated environments the
problem is that we're not modeling the systems correctly rather
than that there's new physics going on.
gus_massa wrote 16 hours 8 min ago:
Very interesting. Do you know an article that ELI25 this?
antognini wrote 15 hours 34 min ago:
For a more non-technical overview, Sean Carroll had a nice
episode on his podcast where he talked about the evidence for
dark matter among other things: [1] For something more
technical, this article just came out as an overview of the
evidence for dark matter:
|
| [1]: https://www.preposterousuniverse.com/podcast/2023/07... |
| [2]: https://arxiv.org/abs/2411.05062 |
|
MattPalmer1086 wrote 17 hours 4 min ago:
There are other predictions MOND makes. For example, it
predicts higher collision velocities than LCDM, for example,
see: [1] And, of course, it predicted that the early universe
would have bigger and more structured galaxies (which is what
the posted article is about).
Dark matter has a slew of problems of its own; it's not the
case that LCDM is problem free, despite good success in some
areas.
|
| [1]: https://ieeexplore.ieee.org/document/8193356 |
|
kelseyfrog wrote 16 hours 12 min ago:
MOND doesn't cover the existence of CBM, distribution of
galaxies, non-metallic abundance - things all covered by
LCDM.
What MOND has going for it is that galactic rotation curves
are readily consumed by popsci readers and the story of the
"little guy" vs the scientific establishment is an easily
available frame story popsci authors can sell clicks for.
The proportion of lay people who think MOND could be true
greatly outnumbers the proportion of MOND researchers and
doesn't reflect the veracity of the theory.
MattPalmer1086 wrote 15 hours 14 min ago:
MOND is not a cosmological theory unlike LCDM, and it isn't
relativistic. So we should not expect it to cover the
range of things that LCDM tries to.
It's just a tweak to Newtonian gravity, which surprisingly
matches observation very well, and has accurately
predicted quite a few things in the regime it operates in,
before they were observed.
The fact it works so well in the areas it does apply to is
the reason that science hasn't given up on it yet
(regardless of what pop science or lay people think).
griffzhowl wrote 17 hours 45 min ago:
> which is totally backwards from a basic scientific perspective
This is not right, because if we have a situation where our theories
and observations don't cohere, it's not given whether the theory
requires modification or we're missing something in our observations
(or both). A classical illustration is the orbit of Uranus being
observed in the nineteenth century to be contrary to the predictions
of Newtonian theory. Calculations were made assuming the truth of the
Newtonian theory and that we were missing something in our
observations - the position of Neptune was predicted and it was
subsequently discovered.
On the other hand, the orbit of Mercury diverged from the prediction
of Newton's theory. Again, a previously unobserved planet closer to
the sun was postulated as being responsible, but in this case it
really did require a modification to the theory of gravity: general
relativity, which accurately predicted the 43 arcseconds per century
of perihelion precession by which Mercury's orbit diverges from
Newtonian predicitions.
GR has obviously made many other predictions, such as the
gravitational bending of light, black holes, and gravitational waves,
which have been vindicated.
So there's obviously a problem of the theory and observations not
cohering, but whether the solution is a modification of the theory or
a new form of matter is not clear in advance, and the latter is not
unreasonable and certainly it's not unscientific to make as a
hypothesis, to see where it leads.
The difficulty is in coming up with a theoretical framework that
retains all the successful predictions of GR while also accounting
for the galactic rotation curves.
njtransit wrote 17 hours 18 min ago:
One difference between dark matter and Neptune is that the
existence of Neptune is falsifiable. The formulation of dark matter
inherently is not. Falsifiable hypotheses is the cornerstone of
science.
griffzhowl wrote 14 hours 39 min ago:
I'm not sure it's inherently unfalsifiable. There are some
specific proposals for dark matter that could be ruled out by
experiments, such as right-handed neutrinos: [1] Maybe if you're
being very broad in definitions then some class of proposals
describable as "dark matter" might be unfalsifiable, but to be
taken seriously as a scientific proposal I think it should be
specific, concrete, and indeed testable, and there are a few of
these within the "dark matter" class.
Again, we're in the perhaps unsatisfying position of having
observations which don't cohere with our current theoretical
understanding. What's the solution? It's not easy...
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| [1]: https://en.wikipedia.org/wiki/Sterile_neutrino#Sterile_n... |
|
uoaei wrote 14 hours 20 min ago:
Have you ever encountered the phrase "grasping at straws"? The
pursuit of explaining dark matter has gone through many waves
of "we just need to invent detectors for this particle that has
never been observed" and is littered with the wreckage.
pixl97 wrote 16 hours 43 min ago:
I mean, dark matter may be discoverable, we just don't know how
if it exists. There was time between the irregularities that were
noticed in the orbit and the discovery of a new planet.
uoaei wrote 8 hours 35 min ago:
By that extremely simplistic logic, so is literally any other
theory of gravity. This is not an argument, this is a flailing
and empty justification.
LegionMammal978 wrote 16 hours 51 min ago:
Is the existence of a planet so easily falsifiable? It hasn't
been so long since the Planet Nine hypothesis started going
around, and while we've observationally ruled out a big chunk of
the original parameter space, there's still lots of room for a
big dark dwarf planet to be floating around out there. It doesn't
seem so different from how we've gradually been ruling out the
parameter space for dark-matter observations.
uoaei wrote 11 hours 20 min ago:
Planets that reflect light are easy to detect.
renewiltord wrote 16 hours 57 min ago:
Surely the idea of it being a new kind of matter that interacts
gravitationally but not electromagnetically yields some testable
result? Does it actually yield nothing testable with todayâs
experimental methods?
MattPalmer1086 wrote 16 hours 26 min ago:
There is a lot of indirect evidence for dark matter. All the
direct tests for dark matter particles we have performed have
found nothing so far - but since we have no idea what it might
be, there's a lot of possibilities to test.
uoaei wrote 11 hours 19 min ago:
"Evidence" in heavy scare quotes, considering, again, the
tautological nature of the claims around the existence of
dark matter. "Something must be here that we are missing" is,
frankly, a bullshit hypothesis that need not be entertained
unless researchers can actually prove there is some
worthiness to the claim. Anything stronger than "maybe our
theory is wrong" would suffice!
mannykannot wrote 9 hours 27 min ago:
It is tendentious to point out only the difficulties in
finding affirmative evidence for dark matter when MOND is
doing no better in that regard. If, by that standard, dark
matter is bullshit, then, mutatis mutandis, so is every
other hypothesis that has been presented so far - but the
observations that prompted them in the first place are not
going away. It is inconsistent to call just one of them
bullshit, and pointless to call them all that.
bbor wrote 17 hours 30 min ago:
Well put, thanks for sharing! Never saw it phrased in such a clear
narrative. As a novice, it seems like there's one big difference
between those anecdotes and the current situation, though: sample
size. Sure, if we were observing Andromeda spinning too slowly I'd
be open to our instruments not capturing some massive
objects/clouds, but we're actively observing, what, ~1E5-6
galaxies? In the case of a missing planet there were accidents of
history/solar system makeup that led to our otherwise solid
frameworks missing a key piece of information. But that clearly
couldn't happen millions of times; whatever explains the
inconsistencies we're seeing has to be a fundamental
misunderstanding.
Once we've arrived at this point, we can compare the two
theoretical re-workings on their own terms: one is that we're
glossing over some important detail of how gravitational relations
in spacetime work, and the other is that we're failing to observe
some new class of matter. I mean, right? There's no way this
conundrum will be solved by "whoops turns out there was more plain
ol' dust than we thought" at this point, right?
In those terms, I feel parsimony clearly favors one possibility
over the other. Every hypothesis is worth exploring (I mean, QM and
GR are dumb as hell, yet nonetheless turned out to be correct), but
when funding is on the line it's also not out of line to favor one
explanation explicitly. That's already happening anyway, just in
the other direction.
But also I'm just some kid who's awed and grateful to be living in
times of such profound mystery and discovery. Could be totally off
base -- I barely passed physics I!
griffzhowl wrote 14 hours 28 min ago:
Thanks. I'm also no expert - I'm just learning general relativity
- but that's also my rough understanding: either there needs to
be a modification of the theory, or there's a new form of matter.
It might seem more parsimonious to modify the theory, but then
how do you do that in a way that retains all the successful
predictions of GR while explaining the recalcitrant observations?
That's the hard part.
It seems at the moment that the minimal and most elegant
adjustment to the worldview required is to postulate the new form
of matter. But I think it's safe to say it's a genuine problem in
our knowledge: we don't know how to solve it
necovek wrote 17 hours 20 min ago:
> ...turned out to be correct
What we have learned so far is that our theories and models are
only correct up to our ability to precisely observe and measure.
In that sense, Newtonian physics is still very much correct under
a very wide set of circumstances, and as such amazingly useful.
GR improves on that (adds precision) on what would be extreme
cases for NP, but it is likely as correct as Newtonian laws are:
up to a point.
All this to say that "correct" is not the right term to use: many
of the theories are simultaneously "correct" with sufficient
constraints and a particular error range. What matters more is if
they are useful in predicting behaviour, and that's where I like
using "correct" instead (as above).
simonh wrote 17 hours 58 min ago:
I donât think thatâs quite fair. That approach is exactly how we
find planets. Hereâs an unexpected variance in the motion of a
planet or star. It could be explained by a planet over there. Oh
look, thereâs a planet over there.
russdill wrote 16 hours 1 min ago:
It's actually a better example than you think. This exact theory
led to long and protracted searches for the planet Vulcan, which
would explain Mercury's strange behavior.
solid_fuel wrote 17 hours 49 min ago:
Hypothesizing that a planet might be over there is a testable
hypothesis.
Have we found a way to verify the presence of dark matter yet? Or
is it still an untestable hypothesis sprinkled around distant
galaxies so their acceleration curves look right?
mr_mitm wrote 17 hours 11 min ago:
Dark matter predicted lensing effect which were successfully
tested. Same for the baryonic acoustic oscillations in the CMB.
MattPalmer1086 wrote 16 hours 50 min ago:
That's not quite true. General relativity predicts
gravitational lensing, not dark matter. Lensing has been used
as an experimental probe for the presence of dark matter.
elashri wrote 16 hours 44 min ago:
MOND is an alternative theory of gravity competing with GR.
People usually forget that while MOND started to present a
different explanation for Dark Matter, it is a theory of
gravity. Dark Matter is not a theory of gravity and is
compatible with GR.
zeroonetwothree wrote 15 hours 55 min ago:
Dark matter isnât much of a theory in the first place.
User23 wrote 17 hours 39 min ago:
Iâm particularly amused by the hypothesis that spacetime can be
bent without the presence of matter. We canât detect dark
matter because thereâs no such thing, itâs just a brute
topological fact.
MarkusQ wrote 17 hours 51 min ago:
Right, which is why it quickly led to the detection of dark
matter...hmm.
I think a better analogy would be "that approach is exactly how we
explain failing to find planets like Vulcan; we hypothesize that
they are made of as-yet-unknown stuff that you can't see, touch,
hear, smell, or in fact detect at all. But we know they're there
because our calculations say they are."
TheOtherHobbes wrote 17 hours 52 min ago:
Planets are visible when you look for them.
Dark matter - so far - isn't.
drdeca wrote 15 hours 17 min ago:
What do you mean by âvisible when you look for themâ? Like,
with light?
Does gravitational lensing count as âvisibleâ to you?
jchanimal wrote 19 hours 32 min ago:
Whatâs MOND really mean? Hereâs the Wikipedia entry
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| [1]: https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics |
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