Maybe someone could help me understand why a simple explanation for dark matter isn't possible.
I get how you can measure the mass of stars, and how you could also maybe measure the mass of interstellar gasses. You add it all up and there is missing mass. But how could you estimate the mass of larger objects? Couldn't the interstellar medium be full of moon sized cold rocks? You'd have no way to observe them on a galactic scale since they're not illuminated, they have effectively no absorption spectrum b/c of their small surface area and yet I'd think they could make up the difference you need for this "dark matter"
The explanation seems too simplistic, so I'm clearly missing something :)
Larger objects, such as the rogue (not members of a solar system) rocks you mention, are classed as MACHOs (massive compact halo objects) in possible dark-matter explanations.
Recent studies [0][1] have pretty much ruled out MACHOs as an explanation, by looking for gravitational microlensing events by MACHOs between us and the Magellanic Clouds and Andromeda Galaxy. They found about 1/1000 of the events predicted by a quantity of MACHOs necessary to explain dark matter's effect on our galaxy's gravity. For very small MACHOs that wouldn't produce enough lensing to be detectable, then you need so many that they'd darken the light, like a gas/dust nebula. This is also not observed.
I don't work in the field, but I studied astrophysics, and I have one or two astronomer friends - it seems these results are regarded as a pretty definitive nail in the MACHO theory's coffin. Current research is mostly looking for WIMPs (weakly interacting massive particles), a novel class of fundamental (atomic/subatomic?) particle, expected to be present throughout all of space, which doesn't interact with the electromagnetic force, so doesn't interact with light, including virtual photons of attraction and repulsion, (such as those holding the particles of your feet together and also repulsing your feet from the ground right now,) that mediate the forces we interpret as "solidity" and "pressure", but has mass.
Thanks for the explanation. It raises other questions, like how they would know small MACHOs haven't darkened the sky (I'm guess either you'd see twinking or it'd shift the stars downward on the Hertzsprung–Russell diagram to some "unnatural" location) - but I get the gist of it. It's an interesting area of research. Really appreciate you illuminating it for me :)
Sorry to be a pendant, but the forces holding things together is just ordinary EM (well, QED probably better stated) while virtual photons are different (virtual since they only appear in higher order corrections in qft).
I love pedants, don't apologise! Happy to be corrected and keep learning. It's been quite a few years since I was in physics classes, so I went and looked this up, and I think my memory was right, as for example, descriptions of the Coulomb force (electrostatic repulsion and attraction as in my foot example) all seem to be phrased in terms of virtual boson exchange.
I agree the forces holding things together is "ordinary EM", but virtual particles as force carriers seems to be ordinary, not rare.
(1) The quantity of rocks that would be needed to explain such a gigantic gap in mass is bound to be observed - if they are dark, then at least we should be able to see them when they block light. You have to remember we need dark matter to be around 80% of total mass of the universe. So they either need to be super dense, like tiny black holes [1], or there needs to be so much of it that there is no way we won't see them.
(2) Dark matter is too spread out. If they were rocks or any of the known matter types, they should have coalesced on their own. So they have to be such that dark matter doesn't interact with itself.
I ran some numbers, for the Milky Way[1], the visible matter is roughly 1e11 solar masses, while the dark matter halo contains[2] about 1-2e12 solar masses (depends on your cut-off), I'll use 1e12.
The moon is about 3.7e-8 solar masses[3], so you' need about 2.7e19 of these "rocks" to make up the Milky Way dark matter halo. For comparison the Milky Way has about 250e9 stars[4].
So we're talking over 100 million "cold rocks" per star in our galaxy.
Nebulae are an argument in favor. They are observable b/c they're not dense. They have a very high surface area to mass ratio.
Light passes through them and they have absorption spectrum. They also emit a lot of black body radiation.
By contrast a moon size object has a very low surface area to mass ration. It has no absorption spectrum b/c light can't pass through - and any black body radiation would be negligible b/c of relatively small surface area
Seems like they observed distortions of space time, but not seeing any (dark)matter?
Just for fun, what if matter actually has coordinates (x,y,z,h1,...,hn). Where hn is some number of hyper dimensions. Visible matter is stuck in our 3 space (x,y,z,0,...,0) and gravity is the only force that permeates across hyperspace. We can’t see the hypermatter except by the gravitational imprint it leaves on 3 space material.
If we had the ability, it’d be neat to see if our visible matter can alter the configuration of dark matter, just as dark matter seems to affect our visible matter.
In an infinite universe with random local density of matter, the gravitational constant would vary by location. Anyone know what the consensus is among astonomers if the universe is infinite or not ? Is there even a way to theoretically know ?
Theory can say what shape the universe should have, for example inflation predicts a flat universe[2]. Of course you then have to check if the theory matches what we observe.
As far as I've gathered, the current measurements are consistent with a flat universe. Here's[1] a post about a recent, improved measurement.
Alternative title: "Group of scientists assuming that the current model of gravity is correct look at the discrepancies in the observations and calculate a map of where and how much matter would be necessary to fix this difference. Some of them believe this additional mater is real, and some of them doubt, but meanwhile to make the titles shorter they use the catchy name of dark matter."
It's clear you didn't even try to read the article.
They mapped the distortion effects on visible light thought to be caused by dark matter. They don't know what it is or how it works, but they know _something_ is causing apparent distortion to the light.
Won’t it be interesting when our model of physics is updated to explain these cosmological phenomena? Who knows what new technology will be discovered as a result!
There's a very old short story I've never been able to find again that beat you to it.
Every civilization that discovers FTL travel contributes to a more rapid expansion of the universe. The older civilizations beg them to stop, but inevitably they fail.
We have observations that match quite well what would be expected if there was additional matter in the universe. We cannot detect this matter though, so the conjecture is that there must be something that interacts gravitationally with the rest of the universe but not in other ways. We call it dark matter for now. We do not yet know what it consists of, there are even competing theories but mapping the effects of it? That we can do.
This approach is not at all unusual. If you want to know about another example from a different field, look no further than genetics.
Gregor Mendel observed rules for inheritance of certain traits in plants and was able to make predictions from that. He discovered genetics. In the 1850-60ies. This was a long time before Crick, Watson and Franklin discovered the structure of the underlying mechanism for inheritance (DNA).
What if Dark Matter is just that? Dark matter. If there were lots and lots of something like planets out there, which for one reason or another don't reflect light, that would be something dark right?
Or what if Dark Matter was lots of Black Holes? We (more or less) know what Black Holes are so why don't we assume there could be lots of them having a gravitational effect but no visibility?
In other words why do we need to assume that Dark Matter is made of some kind of exotic unknown matter?
Black holes are indeed[1] a dark matter contender!
Just because the best explanation[2] we have right now is cold dark matter (CDM), doesn't mean physicists are content. They're looking for new ways[3] to probe not just CDM but other dark matter models as well, like primordial black holes[4].
[2]: http://pirsa.org/20040087/ "Dark Matter: A Cosmological Perspective", nice colloquium talk giving overview of what we know about dark matter.
[3]: http://pirsa.org/20100052/ "Strong gravitational lensing as a cosmological probe: new ways of constraining dark", random technical talk about new way of probing CDM.
Dark Matter is just a placeholder name, don't read too much into the "Matter" part. There are no assumptions there and plenty of alternate explanations are proposed, like Modified Gravity (MOND)
They have actually looked for black holes of many sizes and have ruled out most of the range. It might still be possible, though improbable because they looked very hard for black holes.
From a societal perspective, PhDs and postdocs take part in the economy, they have a job, make money, spend money, produce research sometimes in partnership with the industry, sometimes with immediate applications, sometimes theoretical with applications that will only come decades down the line. There is no reason to set the line at Bachelors, as a society we've found it's best to have a subset of the population go into higher studies. We used to have children participate in the economy 10 to 15 years earlier in fields and factories and it wasn't so good.
From the perspective of the individual, getting a PhD isn't always much longer than getting a Masters, and teaches you skill you will not acquire anywhere else. It also opens the door to research positions that would not be accessible to you without it. And you can have greater expected monetary returns on the medium and long term. I've known a lot of people doing PhDs, a good amount doing it for prestige, because it was the expected next step after a masters. Many others were doing it out of passion, for the freedom to study things they were interested in an amazing environment. So on an individual level many people find it worth it whether their priority is money or intellectual pursuits.
Not sure who invented the academia-industry dichotomy, but he/she deserves a society-wide slap. Society loses, the individual loses. My career is just a big middle finger pointed at this dichotomy.
> My career is just a big middle finger pointed at this dichotomy.
I only saw your comment after the parent it replied to was flagged/removed. This section of your reply jumped out to me, care to elaborate?
My experience (undergrad, minimal research -> industry) has shown me that the gap is quite real, but both sides are important. They simply align to different goals. The most extreme trope of this in tech is industry engineers making fun of researchers who produce systems which "only work on paper" (paraphrased, some meaning is lost in that phrasing).
I find that criticism too harsh (and rude), but it does highlight the differing goals: industry frequently needs to build systems which 1) work, and 2) scale, among other things. Academia can afford to push the cutting edge by compromising on constraints real world systems have to obey. It's the classic exploration vs. exploitation problem. Industry thinks the exploration isn't important because they need to optimize the local maxima/minima now. Academia cares about the global maxima/minima, pushing the state of the art, exploring new solutions in the hope of advancing knowledge and understanding.
In the largest companies, the cost/benefit analysis shifts back toward exploration. Google, Microsoft, Xerox PARC, etc. And the natural extension of academics elsewhere pushing the state of the art is finding new optimums and bringing that back to industry (e.g., as a startup). So I feel there gap is real, with lots of symbiotic back and forth.
I don't 100% agree, but let's play with "academia is exploration, industry is exploitation".
How would you ensure you explore something useful? How would you ensure to transfer what you explored, so you can exploit that knowledge? What if you exploitation is stuck in a local minimum, how would you ensure you send feedback back to exploitation?
I feel a more useful model is Technological Readiness Level (TRLs). Academia deals with lower TRLs, while industry deals with higher TRLs. Somewhere in between there is -- there needs to be -- a huge overlap to ensure a proper continuum.
This can also be phrased in a risk perspective. Lower TRLs are extremely high risk, almost like throwing money out the window, but have extremely high societal reward. Think curing cancer or terraforming Mars. These are best funded via grants. High TRLs are low risk, low rewards; issue some bonds. Somewhere in the middle is VC territory.
But how? PhD students doing industrial internships; PIs doing industrial sabaticals; engineering managers becoming adjunct lecturers or giving guest lectures; academia and industry working on common projects; making patents could as much as papers on an academic CV.
Is anyone in the world either exploring or exploiting? Then why don't we create a system that allows people to quickly switch hats, as required to achieve larger goals?
I like your comparison to TRLs. I think we’re saying the same thing, actually! I don’t mean that one is exploitation and one is exploration exclusively. Re-reading my post, the phrase “gap” might be a misnomer — it’s more like a balancing act of priorities.
> How would you ensure you explore something useful? How would you ensure to transfer what you explored, so you can exploit that knowledge? What if you exploitation is stuck in a local minimum, how would you ensure you send feedback back to exploitation?
I’d argue you can’t necessarily be sure. Not all research works out. But we need to ask questions and get answers to move forward, even if what we discover may not be worth exploiting (getting tired of my own phrasing). Getting stuck in local min/maxima is part of why we need both angles. This is the flip side of what I said about industry needing to build systems that work — lower budget for risk tolerance, but also increased likelihood of getting stuck in a local optimum.
It sounds like I’m using those terms to describe the same spectrum you are with TRLs. At least, I wholeheartedly agree with everything you said — the “but how” portion sounds exactly like the symbiotic overlap/crossover I had in mind.
I guess to answer my own original question, it sounds like you’ve swung back and forth a few times?
> Then why don't we create a system that allows people to quickly switch hats, as required to achieve larger goals?
Preaching to the choir ;) and appreciate your perspective
> I guess to answer my own original question, it sounds like you’ve swung back and forth a few times?
Indeed, I couldn't decide if I want industry or academia. What I (think I) want is to experience the whole innovation pipeline, from TRL 1 to 9. I'm now working 80% in industry and 20% in academia, and it satisfies my intelectual needs, both to explore moonshots and to get sh*t done. :)
At least where I live, many start earning some salary from academic contribution of some minor sort during the bachelor's trajectory, typically by assisting in lessons to lower classes, most art to do so during master's, and Ph.D. students are employed by the institute that promotes them and earn a full salary for their research.
The real issue isn't that, the real issue to me is quite a few research fields are purely taking part in œconomy and entertainment, and essentially funded of sponsorships generate by spectacular news reports and nothing more.
A very large quantity of scientific research is purely infotainment and doesn't actually generate knowledge that is used for anything but merely interesting to read, which is what could cause the replication crisis to go unnoticed for so long among other things: no one found out, as no one was using it in any way that relied upon it's veracity, and many even made up data for decades and went unnoticed.
I recently read a paper that studied some social parts of the acquisition of language in Japanese, — quite interesting to read, and as usual it was quite spectacular because the findings went against the established ideas, but this knowledge merely exists to be “fascinating” and it won't ever be used for anything that assumes it's veracity, and as a consequence the data could have been pulled from the æther and no one would ever find out and replication will surely never occur,and even if it would, it would be too late as some years would have passed and the original auctor can always argue that these social matters changed within the interval.
I get how you can measure the mass of stars, and how you could also maybe measure the mass of interstellar gasses. You add it all up and there is missing mass. But how could you estimate the mass of larger objects? Couldn't the interstellar medium be full of moon sized cold rocks? You'd have no way to observe them on a galactic scale since they're not illuminated, they have effectively no absorption spectrum b/c of their small surface area and yet I'd think they could make up the difference you need for this "dark matter"
The explanation seems too simplistic, so I'm clearly missing something :)