
One night of telescope time rules out black hole/dark matter idea - Tomte
https://arstechnica.com/science/2019/04/one-night-of-telescope-time-rules-out-black-holedark-matter-idea/
======
mikorym
When I went to listen to a dark matter talk at the observatory in Cape Town,
my first response was: Why on earth do they keep insisting that the r^2 rule
_should_ hold?

Maybe a physicist can explain why, but as a mathematician my first quest would
be to try to form a mathematical system that follows what we are observing,
_without_ introducing physical objects that we aren't actually seeing.

~~~
GuB-42
1- 1/r^2 is the most natural in 3D space. Think of it as the surface of en
expanding sphere. A lot of things follow the inverse square law: electric
fields, light intensity, sound intensity,... It is not a strong argument:
there are other things that don't follow 1/r^2 and we are not even sure our
universe really is 3D (though there is strong evidence for it).

2- Theories that don't involve 1/r^2 (MOND) exist, but they are not sufficient
to match observations without involving dark matter. And following Occam's
razor, since MOND introduces another parameter (the behavior of gravity on
long scales), but doesn't simplify anything (dark matter is still there), it
is not preferred.

3- While we don't know the details about dark matter, we "see" it in a way. We
see it affect the movement of galaxies, and the gravitational lensing effect
it has. It is just that we can't observe it with our preferred methods, which
involve electromagnetism. I mean, we don't need to see a glass panel to know
that it's there. Sure, we'd like to know more, and evidence of dark matter is
weaker than our glass pane, but we simply don't have anything better right
now.

~~~
mikorym
A question on 1-: does introduction of intermediary objects stack well? What I
mean, for instance: If the moon is between us and the sun, the light gets
blocked out but gravity does not. Light intensity then does not stack well
(for visible light frequencies) when there are intermediate objects. Or does
the analogy here require a more formal transposition to GR and the distortion
of spacetime?

~~~
Steuard
I'm not sure that I understand your question. Certainly all of our
understanding of astronomical effects includes (and often depends on) the
effects of intervening clouds of gas and dust: that hasn't been overlooked or
omitted from any professional-level analysis. (My memory/guess is that in the
presence of an absorptive medium, influences like light intensity will fall
off as e^(-a*r)/r^2 rather than just 1/r^2. But the point is that we can model
that absorptive term reasonably well, and the underlying 1/r^2 still results
naturally from 3D space.)

(Meanwhile, to a first approximation, intervening dense objects like planets
or stars are rare enough to be negligible: even in a region like our solar
system where such objects are most likely to be found, the available volume is
overwhelmingly empty space.)

~~~
mikorym
My question is whether gravity works like light, or whether you can just
ignore the planets in between. When for example you calculate the effect of
the sun on the earth but venus or mercury happen to be in a line of sight,
does it influence the sun-earth calculation component? (Of course, the other
planets have their own gravitational influence in line with r^2.)

~~~
xenophonf
Are you indirectly asking whether planets can be dark matter candidates? If
so, those are called MACHOs (massive compact halo objects). They have been
studied via the gravitational microlensing effect. The presence of MACHOs
can't account for most dark matter.

[https://arxiv.org/pdf/astro-ph/0001272.pdf](https://arxiv.org/pdf/astro-
ph/0001272.pdf)

[https://arxiv.org/pdf/astro-ph/9904401.pdf](https://arxiv.org/pdf/astro-
ph/9904401.pdf)

When you see figures like "1E-7 to 1E-2 M⊙ mass range" (from the above paper's
introduction), note that a Jupiter mass is about 1E-3 M⊙---one thousandth or
0.1% the mass of the Sun. Earth is 3E-6 M⊙, the Moon is 3.7E-8 M⊙, and Pluto
is 6.6E-9 M⊙; or 0.00037%, 0.000004%, and 0.00000066% the mass of the Sun
respectively. Compared to stars, planets comprise a tiny fraction of the mass
of a galaxy.

TL;DR - No, that's not how gravity works, but you can still ignore the planets
in between because they're tiny.

~~~
ruds
I think the question is: given a 3-body system with objects A, B, and C, is
C's gravitational effect on A altered in any way by the position of B, or are
B's and C's effects on A independent?

The reason for the question, I think, is because of an intuition built up
around EM. In the system

A B C

B can block and otherwise interfere with C's EM radiation so that its effect
on A is different depending on B's location.

~~~
fhars
No, if you want to think of gravity in analogy to light, you can see every
mass as analog to a fully transparent light source. There is nothing in this
universe that can absorb gravity.

~~~
Steuard
LIGO was able to detect gravitational waves, which I'm pretty sure must
require non-zero transfer of energy in one form or another. (I'm hedging a
little only because I know LIGO was measuring relative changes in metric
distance, which might possibly not require absorbing energy... but just
intuitively it's hard for me to fathom any transfer of information without a
transfer of energy: it simply shouldn't be possible.) But 1) it's a remarkably
small loss of energy, since the gravitational coupling to matter is so weak,
and 2) LIGO is very specifically reacting to gravitational _waves_ rather than
to static (or quasi-static) gravitational fields, and the absorptive
properties there will certainly be different (just as they are for
electromagnetism).

~~~
fhars
General relativity will probably add some effects like that, but then quantum
electrodynamics adds light scattering off of light, so the analogy is still ok
as far as analogies go.

------
UweSchmidt
It seems like the number of telescopes are a limiting factor in astronomy, and
while expensive, new telescopes seem to be a straightforward way to get
interesting new results. It is unfortunate that researchers have to wait for
telescope time like computer scientists had to wait for computer time back in
the day.

~~~
petschge
The request for additional funding for extra telescopes is always met with the
question "but what use is that knowledge?" Politicians don't want Moon
missions, they want Teflon coated no-stick pans. They don't want Higgs bosons,
they want the WorldWideWeb. Problem is you never know what commercially useful
things you will accidentally find while you hunt pure knowledge.

------
skybrian
Given that they're staring at an entire galaxy for such a long time in quite
high resolution, I wonder that they didn't notice anything else that's
unusual?

~~~
Etheryte
Given the sheer amount of data (870 megapixels every 90 seconds for a night)
you will only find or not find things you're very specifically looking for.
The volume of data is simply too large for random discovery. As the article
covers, looking for a very specific type of interaction, the researchers were
still faced with 15,500 possible matches that they then needed to filter.

~~~
EnigmaticLion
7 hours of observation, one image every 90 seconds: that's 280 images. My
phone takes 8 megapixel photos and those are about 2 MB, based on that the 870
megapixel image would be about 200 MB. 200 MB * 280 = 56 GB. Doesn't sound
that much.

Did they publish the images? I'm sure many astronomers would be happy to
examine them.

~~~
blattimwind
This isn't a smartphone camera. The raw images are 16 bit monochrome, which is
about 2 GB. Additionally you'll make dark exposures etc. to compensate for
errors, which overall results in about ~13 GB per shot [1].

[1]
[https://hsc.mtk.nao.ac.jp/pipedoc_e/e_hsc/index.html](https://hsc.mtk.nao.ac.jp/pipedoc_e/e_hsc/index.html)

------
jniedrauer
When I write a test, I intentionally make the condition fail to validate that
the test works like I think it does.

The experiment described here uses new equipment that presumably has never
been used for this purpose before. But I don't see any mention of efforts to
verify that it can actually be used to detect black holes. Hopefully the
remaining 10 days of their experiment can confirm that it works...

~~~
maxander
The “Hyper Suprime-cam” instrument came online in 2012, so it’s not new, and
was presumably calibrated against well-understood targets; so its light-
detection capabilities have been well tested. The physics of gravitational
lensing is also well understood and verified, so they know what light they’re
looking for.

If you can test each part of a system independently, and the parts connect in
a reasonably trivial fashion, it’s about as good as testing the whole.

~~~
Chilinot
> If you can test each part of a system independently, and the parts connect
> in a reasonably trivial fashion, it’s about as good as testing the whole.

Not really, which is why integration tests are so important.

~~~
scriptkiddy
Depends. In functional programming, if all components of a pipeline are tested
independently, you can all but guarantee that the pipeline as a whole will
work as intended.

That said, software testing isn't really an apt analogy now that I give it
some thought. Testing real world equipment has to contend with a lot more edge
cases and environmental factors.

------
toufiqbarhamov
To be fair, this is after years of lending surveys already closed the loophole
for MACHO theory down to the pinhead closed by this last survey. Still a good
read.

------
karmakaze
Here's a different observation report[0] which alludes to smaller scales of
primordial black holes but wasn't explicit.

[0]
[https://news.ycombinator.com/item?id=19591020](https://news.ycombinator.com/item?id=19591020)

~~~
mirimir
It's the same report. Suprime camera on Suburu telescope, observing Andromeda.

------
joemag
Sixty Symbols, which is a great channel for these kinds f topics, did a great
video on primordial black holes a few years ago:
[https://youtu.be/gs3mtZPySeM](https://youtu.be/gs3mtZPySeM)

------
ngcc_hk
We have burnt before. We invent something to fit our idea eg all wave need a
medium. Hence light run in ether and ... in fact we even call Ethernet in
remembrance of this ether. But it does exist.

We have done it before. We believe there is one rule from smallest to the
largest. God does not play dice with us and definitely not in the dark. We
were wrong.

Do not know whether this dark material (and the total different case of dark
energy). Just to say eliminate one more theory like dark hole may not be the
end of non- or better the birth of Dark Matter. If it is just weak ... but
dark. Still need convincing most positively.

 __* search for why Ethernet is called Ethernet you should find this :

“In 1973, Metcalfe changed the name to “Ethernet.” He did this to make it
clear that the system he had created would support any computer, not just
Alto’s.

He chose the name based on the word “ether” as a way of describing an
essential feature of the system: the physical medium carrying bits to
stations.

He thought this was much like the old luminiferous ether was once thought to
propagate electromagnetic waves through space.”

------
PhantomGremlin
One of the best pithy comments on general relativity and dark matter was made
by someone on HN about a week ago. I think it summarizes the situation quite
well. But, inexplicably, people don't seem too concerned about it:

>"[dark matter] seems to make up about 95% of the galactic mass...crushingly
narrow passages between observation and tests of GR"

GR only matches observation because you added in 20x more stuff that is
undetectable other than as a deviation from the predictions of GR.

[https://news.ycombinator.com/item?id=19537192](https://news.ycombinator.com/item?id=19537192)

~~~
wyattpeak
People aren't worried about it because it doesn't seem like a problem. We know
that weakly-interacting matter can exist (as demonstrated by neutrinos), and
there's nothing odd about the behaviour of dark matter: it seems to behave
exactly as we'd expect weakly-interacting matter to behave. That's certainly
not case closed __* , but it 's plenty to make the explanation perfectly
reasonable as a working theory.

On a more philosophical level, though, if experts aren't concerned about
something, and you as an amateur are, you're almost certainly the one who is
wrong. Assume you are, and look for why, rather than questioning their
conclusions. It's like the old adage about compiler bugs: they exist, but if
you think you've found one, you haven't.

 __* And indeed scientists don 't treat it as though it is - there are several
other theories, though it's the strongest contender.

~~~
thatoneuser
Id take what you say about amateurs worrying what professionals don't to be
true for the most part, but it's also not always true. Even in physics.

Iirc about a hundred years ago when we were trying to measure electron
diameter, several iterations of scientists bsed there results assuming the
last guy was accurate. Dark matter has a lot of similar mysticism and I would
not be at all surprised to see a metric shit ton of scientists just being
sheep and going along with incorrect models. I'd also not be surprised if they
were all spot on too.

For reference in grad school I worked in a neutrino lab right by a dark matter
lab underground. So I'm not an expert but do have decent exposure.

~~~
wyattpeak
I didn't say it was always true, I said it was almost certainly true. I stand
by that.

Untrained amateurs are right from time to time, but given that they have
almost no basis for their assumptions except gut feelings, it basically comes
down to chance.

I'm not arguing that scientists can't be wrong, certainly not. I'm arguing
that believing they are, or even believing you have a valid criticism, without
understanding on what they base their arguments, because "it sounds
reasonable" or "scientists are sheep" is deeply intellectually dishonest.

~~~
thatoneuser
Right, I didn't mean to sound like I disagree. More like I was making an
addendum.

~~~
wyattpeak
Fair, sorry, probably just a bit tired.

------
virgakwolfw
I've been reading Ars for many years now and I just wanted to say that the
clarity this specific article displays in explaining astronomical concepts to
a layperson is really impressive and appreciated. Sometimes I get lost in
these types of write-ups, but you gave readers the state of the field and why
this study - which simply provides evidence that helps likely nix one
potential explanation - truly was important in that context. It was all just
so digestible and satisfying.

~~~
cmroanirgo
Totally agree. I loved the implication that one simple 7 hour viewing yielded
15500 events to be filtered and that they could observe a binary star system's
change in that time frame. To me, that's awesome!

------
didibus
The article doesn't seem to say it rules out black holes or dark matter, at
least as I understand it. It just rules out primordial black hole.

Am I reading it wrong? Or why is the title so misleadingly?

~~~
SargeZT
The thing is that there's no conceivable mechanism we know of for a non-
primordial black hole to form and be as small and common as would be required
to explain away dark matter.

~~~
didibus
Yes, I understand that. So primordial black holes don't explain dark matter,
but dark matter as a hypothesis came before it no? To explain something else
no? The article wasn't very clear about all the links.

Also, there's zero indication in the article that this invalidates the
existence of black holes.

And about dark matter, the last sentence says: "But the study does slim down
the already thin chances that they are the source of dark matter's effects."

Insinuating that primordial black hole were a hypothesis for the effects of
dark matter. Not that dark matter theory is proven inconsistent by the lack of
evidence for primordial black holes.

I'm looking for someone who knows better I guess, to clarify. I'm only
interpreting this using my reading skills from the article itself.

~~~
SargeZT
I'm not positive, but I feel there might be a misunderstanding here. This
article isn't trying to say dark matter and/or black holes don't exist, just
that they aren't the same thing.

~~~
didibus
Okay, so that's what I'm saying as well.

But the title of the article insinuates so: "One night of telescope time rules
out black hole/dark matter".

So I wasn't sure if I had misinterpreted the article, or if the title was just
confusing.

Edit: Maybe I wasn't clear. From the title, I got the impression that the
observations ruled out both black holes and dark matter. But from the article,
I only got the impression it ruled out the hypothesis that black holes were
the force behind dark matter.

