New efforts!

[Arcana]

I read along the way dark matter is all bar confirmed as real - but what it is exactly isn't understood as yet...

Hmm.  I was actually only going to answer the question below, but I figured might as well start here for the benefit of anyone that doesn't understand where all of this comes from.

To put it very simply, we believe we understand how gravity works well enough that we can rely on its predictions.  Gravity is a product of mass, so when we see the effects of gravity in terms of how it affects the motions of objects we can see, we automatically presume that the mass necessary to produce that effect has to be there, whether we can see it or not.  Is this absolutely guaranteed to be true?  No, nothing in science is absolutely guaranteed to be true.  But we have to make reasonable assumptions about what we can rely on to function in science.  Hold this thought, I'll return shortly.

*If* we believe we understand gravity, then we can use the motions of objects in the universe to map where all the mass is.  The most famous of these studies was the one credited to Vera Rubin.  Rubin used special telescopes to measure galactic rotation: she took images of a lot of galaxies and used a spectroscope to measure the doppler shift in light emitted by those galaxies.  Imagine you're looking at a spinning vinyl record, or a CD if you prefer, kinda roughly edge-on.  Think one of those pretty pictures of Saturn.  Okay, the galaxy is rotating, so on one side of the galaxy the stuff is moving towards us and on the other side it is moving away from us.  We can measure that, and calculate the rotational velocity of the galaxy.  What's more, we can measure this across the width of the galaxy.

We expect that the stuff close to the center will be moving faster, and the stuff farther away will be moving slower.  Just like in our solar system, the issue is that most of the mass is centered on the Sun.  The close planets experience a stronger force of gravity, and orbit faster.  The far planets feel a weaker force of gravity because they are farther away, and thus orbit slower.  We expect that, and Rubin expected that.  But that's not what she observed.

What she actually observed was that orbital velocities seemed to be roughly constant across the width of the galaxies she measured, or even increasing.  That's not consistent with all the mass being concentrated in the center of the galaxy.  It is actually consistent with the mass density being roughly constant from the center of the galaxy to the edge, or even getting higher.  But when we look at the galaxy with telescopes, it looks like most of the bright stuff is concentrated near the center.  And when tiny "satellite" galaxies' motions were observed that were orbiting large galaxies, that basic measurement continued to hold.  In other words, even in completely dark areas outside of the visible galaxy, the motions of objects was telling us that nope, there is still a lot of mass there.

Gravity was telling us that while the galaxy looked like a bright pinwheel with a really large central bulge and a bunch of thinner cruft circling farther away, the actual mass of the galaxy was distributed more like a gigantic cloud ten times larger than the galaxy we could see, and was about equally dense everywhere.  The bright spots, the dim spots, and the spots that looked totally empty all contained roughly the same amount of mass.  Gravity was saying that most of the mass of the galaxy was stuff we couldn't see with telescopes.

It gets weirder.  Suppose that stuff was just some invisible gas, and not stars.  Well, if it was just a haze of gas surrounding the galaxy, it would slowly heat up as all the stars inside the galaxy sent light in all directions.  But that gas doesn't glow in infrared, or any other spectrum of radiation we can see.

Okay, maybe its some weird stuff that doesn't absorb radiation.  Special dust particles that somehow reflect all the light that hits them.  Well, now the problem is how did it remain a diffuse hazy cloud surrounding every galaxy we can see?  It has mass, so it should respond to gravity.  It surrounds galaxies, so gravity almost certainly gathered the stuff up in the first place.  So why hasn't gravity caused it to clump up near the center of the galaxy like all the rest of the normal matter?  It is as if the stuff doesn't interact with itself with any sort of "friction" that would cause it to clump up like all other matter we're aware of.

So it doesn't absorb radiation, doesn't heat up, doesn't emit radiation, doesn't interact with itself, doesn't clump up, but does have mass and does produce gravity and does cluster around other massive objects like galaxies.  So what is it?  We don't know.

But do we know dark matter exists?  We think we do, because we trust our knowledge of gravity and relativity.  But what if we're wrong?  It is possible.  Maybe general relativity is wrong.  Maybe our understanding of gravity is wrong.  Maybe the universe treats gravity differently on large scales than small ones we can directly measure because of some unknown process or property.  It is not impossible.  But lets say that's true.  How would we prove it?  How would we prove that one of the strongest and most carefully tested and confirmed physics theory of all time is wrong?  We would need very compelling evidence.  Like, say, we could try to prove that one of the predictions of relativity was wrong.  In particular, we could try to show that the predictions of relativity in predicting the mass distribution of the universe was wrong.  That dark matter didn't really exist.  And how would we do that?  We would do that by carefully considering, studying, and analyzing all possible ways dark matter could exist, try to find them, and demonstrate that none of them exist observationally.  If we did that, we'd prove relativity was wrong, or at least incomplete, and its predictions about mass in very large scales was unreliable.

In other words, getting back to the question of "are we sure what gravity is telling us is true" the answer is: we think it is, and because of that we assume dark matter exists, and the job of physicists is to try to find it.  But if relativity is false, the best way to prove that would be to conduct a search for every possible kind of dark matter and demonstrate none of them exist.  So if relativity is true, searching for dark matter would be discovering something new about the universe that relativity gave us a strong clue to find.  If relativity is false, searching for dark matter would ultimately provide the evidence that relativity is false, by virtue of always failing.

When it is practiced correctly, this is how Science is supposed to work.  Science is a tool to help us poor monkey-brained bags of goo make accurate predictions about the world, given what we can observe.  It is our observations of the world that cause us to formulate theories about how the world works.  Those theories make predictions.  When those predicts fail, that theory gets discarded, because they don't work.  When they succeed, we start to trust them.  At some point, we trust our theories enough that when our observations don't match our theories, we start to assume our observations are incomplete in some way, because monkey-brained bags of goo have historically been less than perfect themselves.  We do that until we either discover a gap in our observations or the theory fails enough to make us not trust it anymore.  Dark matter is essentially physicists saying they trust our understanding of gravity more than they trust our ability to find every single thing in the universe with mass.  We're pretty certain dark matter exists because our trust in gravity and relativity is extremely high, given how often that knowledge has proven to be correct again and again.  On the other hand, we don't yet assume that all of our observational astronomy is good enough to be absolutely certain that everything we can see is everything there is.  So if we have to place a bet, we're betting on Einstein, at least today.

Whew.

I'd like to know why anybody thinks it will turn out to be anything exotic or even useful at all though?

Well, as I mentioned previously, whatever it is, it is likely to be exotic because while we can't say what it is, we can say what it isn't.  It isn't a normal gas, or plasma.  It isn't likely to be rogue planets that are too small or dim to see because for there to be enough of them flying around they would create observable effects (microlensing events, for example).  It is unlikely to be something like neutrinos because extremely low mass particles like neutrinos generally move extremely fast - near the speed of light.  So they wouldn't stick around to hover around galaxies on long time scales.  Whatever it is, it doesn't seem to absorb or emit observable amounts of radiation.  It doesn't seem to interact with itself in ways that would cause it to clump up.  But it does interact weakly to enough of a degree to cluster around massive objects like galaxies.

The laundry list of what dark matter is not, and what it has to be able to do, is rather weird.  So many physicists assume its going to be something exotic.  Will it be "useful?"  Well, anything we know nothing about always has the opportunity to alter our understanding of how the universe works.  That almost always generates some interesting results.   Whether they will be "useful" results is a crap shoot.  But people love to speculate.

[LateNights]

Just realised I'd kinda answered my own question in regards to being exotic or useful with the atomic bomb example earlier... :o

Ok, so how are gravity and the Higgs - boson related, because they both have been stated as why things have mass...

And where do the "fields" you spoke about factor in?

Sorry to bother everybody, but if I don't ask I'll forever vaguely know what these things are but never be able to see them in context...

Edit : Just found this...

https://images.weserv.nl/?url=www.extremetech.com%2Fwp-content%2Fuploads%2F2015%2F06%2Fhiggs-4.jpg

Might be important then, lmao!!

[Eskreema]

If anything this exotic matter may function like a check for the universe: forcing it into certain constraints.  Take the rotation of a galaxy.  Dark matter was theorized because the outter region rotated at he same speed as the central regions.  This could not happen if gravity were the dominant force on the galactic scale.  If something was present to redistribute excess energy back into the system as a stabilizer it may explain why we see things the way we do, most of the universe is devoted to non-interacting matter that decentralizes.  It makes the geometry and gravity and matter and energy are the side-effects.

[umber]

What she actually observed was that orbital velocities seemed to be roughly constant across the width of the galaxies she measured, or even increasing.  That's not consistent with all the mass being concentrated in the center of the galaxy.  It is actually consistent with the mass density being roughly constant from the center of the galaxy to the edge, or even getting higher. 

I've have an lingering question concerning that bit that I've yet to stumble across a good answer for online but I bet Arcana has it readily available! 

"If galaxies aren't mass-centered at the core, why are they spinning around the core?"

From what I can gather, before Dark Matter reared its head we had a pretty decent notion of how galaxies formed, why they look the way they do.  Basically, stuff accumulates and spins around a central clump.  *Later* we notice the discrepancy in rotational speeds of stars the further out from the core and the answer to that question would be that the galaxy is within a sphere of dark matter but I've wondered since DM first started being discussed, do we have models showing why galaxies would form the way we see they've formed if, instead of a central mass being the defining trait, a fairly homogeneous distribution of mass in a giant galaxy-sphere defines the gravity landscape?  My nagging suspicion is that galaxies would look different, perhaps hugely different, if the Dark Matter galaxy-sphere theory is true.

I dunno, when I was in high school back in the late-80s one of the big scientific debates was the nature of Gamma Ray Bursters with a goodly number of scientists putting forth that whatever these events are, they "must" be localized to the Milky Way because the amount of energy they would represent if extra-galactic was too large to consider.  But the bursters were coming from all different directions in the sky, if they were localized they probably would have been greater numbers showing in the galactic plane.  Their answer to that was to hypothesize a bubble of... something around the Milky Way that was producing the bursts.  Increasing evidence for extra-galactic sources eventually won over and I've been a little leery of scientific "throw bubbles at the problem" solutions ever since, there is the faintest whiff of epicycles here, my gut is telling me that DM might turn out to be real but that the current ideas of how DM is operating are going to prove to be not quite on-target.

[Biz]

If anything this exotic matter may function like a check for the universe: forcing it into certain constraints.

For fans of The Office, this may help you understand.

https://www.youtube.com/watch?v=Viccnu603_I

[Arcana]

Ok, so how are gravity and the Higgs - boson related, because they both have been stated as why things have mass...

Technically, general relativity says that what we think of as "the force of gravity" is really bent spacetime, and it is *energy* that bends spacetime.  "Mass" is just a kind of energy: E=mc^2.  But general relativity is silent on why some stuff has mass.

There's actually two kinds of "mass" or to be more precise, when we say something has mass we generally mean one of two things.  One: it generates and responds to gravity (technically, bends spacetime, but even physicists say "generates gravity" because its easier to say than the GR version).  So here on the surface of the Earth, things with mass weigh something.  Gravity pulls them downward to the surface of the Earth with a force we can measure.  The other thing we mean is "it has inertia" which means when we try to apply a force on it, the thing accelerates at some rate: the more mass, the less acceleration.  In other words, its hard to move.

The Higgs field is what gives things with mass their inertia.  They resist acceleration.  You can kind of thing of the Higgs field as the thing that massive particles have to "push through" when they move, and the more they interact with that field, the slower they accelerate under force.  That's not really a very good analogy to what actually happens, but it is good enough for our purposes here.

[Arcana]

I've have an lingering question concerning that bit that I've yet to stumble across a good answer for online but I bet Arcana has it readily available! 

"If galaxies aren't mass-centered at the core, why are they spinning around the core?"

Basically, the math works out so that given any object in something like a galaxy - a roughly symmetric collection of mass - if you pick out any individual object in that galaxy the net force of gravity it "feels" coming from all directions from all of the mass around it is equal to the gravitational force it would feel if the only mass that existed was the mass closer to the center of gravity than it is.

In other words, imagine you are moving through a tunnel that goes through the center of the Earth.  At any point in your journey you would feel a different force of gravity than when you were on the surface.  But how much?  You can do this calculation the long way, or you can make a simplification.  Suppose you are 100 miles underground.  The force of gravity you would feel is exactly the same as if you removed the top 100 miles of the Earth, and you were standing on a hypothetical surface of the stuff that's left.

It takes a bit of calculus to figure this out.  Which, by the way, is why Newton invented calculus.

So basically, for every object in a galaxy, they orbit as if all the stuff closer to the center of the galaxy was all there is, and all the stuff farther away cancels itself out.  So all that mega dark matter hovering far away from the galaxy doesn't affect the stuff orbiting closer to the center.  It does affect the stuff orbiting farther away.

From what I can gather, before Dark Matter reared its head we had a pretty decent notion of how galaxies formed, why they look the way they do.  Basically, stuff accumulates and spins around a central clump.  *Later* we notice the discrepancy in rotational speeds of stars the further out from the core and the answer to that question would be that the galaxy is within a sphere of dark matter but I've wondered since DM first started being discussed, do we have models showing why galaxies would form the way we see they've formed if, instead of a central mass being the defining trait, a fairly homogeneous distribution of mass in a giant galaxy-sphere defines the gravity landscape?  My nagging suspicion is that galaxies would look different, perhaps hugely different, if the Dark Matter galaxy-sphere theory is true.

Actually, dark matter helps explain galactic formation fairly well.  One of the problems with the theories surrounding galactic evolution is how normal matter collapsed densely enough to form galaxies in the first place.  It may sound counterintuitive at first, but matter doesn't actually want to collapse under gravity.  The problem is that a huge volume of stuff, say gas, has pressure, it has momentum, it has temperature.  When gravity tries to shrink that volume of stuff, that stuff gets concentrated.  Pressure, temperature, angular momentum all go up, like squeezing a ball makes it harder and hotter.  Those forces act to push back outward.  Gravity wants to shrink stuff, but gasses want to expand under their own energy.  That makes it difficult for them to collapse.  You need more gravity to overcome those forces.  Dark matter can help.  If there is already a dense halo of dark matter floating around in the universe, the tendency is that its gravity will attract and collect gas.  That gas will "feel" its own gravity as it shrinks, and also the gravity of the dark matter halo.  But the dark matter halo doesn't contribute any extra pressure or temperature, and it can carry away angular momentum by interacting gravitationally with the gas.  Dark matter can, in effect, "suck" some of the pressure out of the gas while also adding more "squeeze."  That can help kick start galactic formation, and computer simulations suggest this process works better than ones without dark matter to start with - it reproduces observations better.  Another potentially good sign for whether dark matter actually exists.

I dunno, when I was in high school back in the late-80s one of the big scientific debates was the nature of Gamma Ray Bursters with a goodly number of scientists putting forth that whatever these events are, they "must" be localized to the Milky Way because the amount of energy they would represent if extra-galactic was too large to consider.  But the bursters were coming from all different directions in the sky, if they were localized they probably would have been greater numbers showing in the galactic plane.  Their answer to that was to hypothesize a bubble of... something around the Milky Way that was producing the bursts.  Increasing evidence for extra-galactic sources eventually won over and I've been a little leery of scientific "throw bubbles at the problem" solutions ever since, there is the faintest whiff of epicycles here, my gut is telling me that DM might turn out to be real but that the current ideas of how DM is operating are going to prove to be not quite on-target.

There were always two theories behind gamma ray burst events: extra galactic and intra galactic.  The problem was that if they were inside the galaxy they were not distributed consistently with the rest of the matter of the galaxy.  But if they were extra galactic there was a separate problem: to be coming from randomly outside the galaxy they had to be enormously bright (the farther away, the brighter they had to intrinsically be).  Based on calculations on how large they could be and how bright they had to be to be extragalactic sources the conclusion was that the GRBs were putting out more energy than could possibly be contained within that space, even if the GRB somehow converted all of its mass directly into energy.

The solution turned out to be evidence that GRBs were "directional" - instead of exploding, there's a mechanism whereby they radiate almost all of their energy into two narrow jets of energy going in opposite directions.  We only detect GRBs if they just so happen to be pointed in our general direction.  Otherwise, we never see them.  Because they focus their energy into jets, the actual amount of energy they need to appear as bright as they do is a lot less than was first assumed.  As soon as this mechanism for GRBs being jets was established, the notion that they had to be nearby halos of objects was discarded.

But that's not to say that the people who theorized those halos were wrong to do so.  They were exploring one possibility among many.  The only reason we are as confident as we are about our understanding of GRBs is not just that we found a good solution (jets), but also because we know a lot about all other alternatives, and why they are not as good.  The work done on galactic GRB halos was useful, because it told us what the limits of that idea were.  That kind of research is just as valuable as the kind that happens to stumble on the right answer.  It is not enough to like the theory we have, we also want to know that all other possible theories are not as good.

It is perfectly fine to be skeptical about the precise way dark matter exists and operates of course.  But along side that skepticism it is also a good idea to understand what the supporting evidence for dark matter is.  It isn't just one thing, like Vera Rubin's galactic rotation observations.  It is also cosmological gravitational lens effects which predict dark matter, cosmological matter distribution which imply distributions of dark matter, and many other observations all of which independently and separately suggest that our accounting for matter in the universe is incomplete.  What's more, all of them tend to predict the same kind of stuff being dark matter - all these separate independent studies of different things all say there's more stuff we can see, it acts like normal matter when it comes to gravity but it doesn't act like normal matter when it comes to almost anything else, and the amount of it that's out there is about five times the matter we can normally detect directly.  All that suggests that dark matter is real.  It would otherwise be a remarkable coincidence that each of these studies was flawed in just the right way to coincidentally suggest the exact same thing.  Possible, but increasingly unlikely.

[princezilla]

.... And when we beat space in the final boss fight then we get to revive CoX with all of our characters but shiny new graphics and a devoted dev team who puts out a new issue every other month?

[Arcana]

.... And when we beat space in the final boss fight then we get to revive CoX with all of our characters but shiny new graphics and a devoted dev team who puts out a new issue every other month?

I'm afraid you only get space shards for that.  But if you do it seventeen times you can trade those space shards for one spacial manifold.  Three of those get you a space geometry.  Combining one spacial geometry and one temporal geometry will unlock the spacetime slot.  You will need to slot your spacetime slot with a cosmology to use it.  I believe it currently takes thirty million trillion trillion trillion trillion inf to craft a cosmology. 

You also get a badge for that.

[umber]

Thanks Arcana, that really was helpful!

Don't get me wrong, this casual-observer enthusiast of science well knows how limited is his grasp of math skills  :'(

[Ohioknight]

Basically, Alcubierre sat down one day and said "since general relativity can be used to predict the geometry of spacetime for any given arrangement of matter and energy, why can't I do that in reverse: make up a geometry of spacetime I like, and run the equations in reverse to figure out what arrangement of matter and energy create what I want?"...

OhioKnight mentions that other people have taken Alcubierre's inspiration and tried to make the amount of negative energy required smaller.  There are ways to do that, but it is very difficult.  ...

In other words, interesting for the min/max community in a theoretical sort of way, but not really quite practical knowledge for anyone else, yet.

Yeah, exactly.  One thing worth noting is that Alcubierre was very directly trying to make Gene Roddenberry's warp drive concept from Star Trek work in General Relativity terms.  He was rather surprised to find an actual solution for it. 

It isn't possible without something that works like Negative matter and there's no actual indication that Negative matter/energy is meaningful in reality...  with the VERY MINOR exception that SOMETHING is making the Universe speed up it's expansion and we've named that SOMETHING (which we have absolutely no ideas about in any real sense) "Dark Energy".   (Some very simple analysis shows this stuff must make up 70-some percent of the total mass of the universe).  So when we figure out what "Dark Energy" might be, the discussion of Alcubierre warp drives MIGHT (or might not) be connected to some further theoretical possibility.

In the meantime, it's a fun numbers game.  The energy reduction solutions folks have come up with work as well as Alcuierre's solutions, so the Alcubierre brute force simple "mass of Jupiter" solutions aren't really relevant any more.  But yeah, it's all make believe with rules until our technology is vastly more capable of manipulating forces other than the electro-magnetic than it is now. 

(in theory, you can pump enough energy into a small enough space to make a tiny black hole, which you could then charge with electrical charge and control it with magnetic fields to make it generate gravity waves which might be the basis of another force-based technology like magnets and wires are for electricity, but that's pretty insanely theoretical for the foreseeable future).

[Victoria Victrix]

That thread is a bit long for me to read.  Would you mind summarizing all of the content in that thread and posting it here?  I would prefer if there was some narrative flair to the summary that would make it interesting to read.  Actually, if you have some contacts in the voice narration industry it would be useful if an audible version was posted that I could listen to in the car.  I would be willing to credit all of your efforts in this regard.

;D

100000 upvotes for snark!

[LateNights]

Yeah, exactly.  One thing worth noting is that Alcubierre was very directly trying to make Gene Roddenberry's warp drive concept from Star Trek work in General Relativity terms.  He was rather surprised to find an actual solution for it. 

Meanwhile, I'm just happy I managed to guess correctly that there is actually some study being done in the two fields I thought might be related somehow - so there was a "eureka" moment - but it wasn't quite the one people want.

;D

[pinballdave]

Yeah, exactly.  One thing worth noting is that Alcubierre was very directly trying to make Gene Roddenberry's warp drive concept from Star Trek work in General Relativity terms.  He was rather surprised to find an actual solution for it. 

It isn't possible without something that works like Negative matter and there's no actual indication that Negative matter/energy is meaningful in reality...  with the VERY MINOR exception that SOMETHING is making the Universe speed up it's expansion and we've named that SOMETHING (which we have absolutely no ideas about in any real sense) "Dark Energy".   (Some very simple analysis shows this stuff must make up 70-some percent of the total mass of the universe).  So when we figure out what "Dark Energy" might be, the discussion of Alcubierre warp drives MIGHT (or might not) be connected to some further theoretical possibility.

In the meantime, it's a fun numbers game.  The energy reduction solutions folks have come up with work as well as Alcuierre's solutions, so the Alcubierre brute force simple "mass of Jupiter" solutions aren't really relevant any more.  But yeah, it's all make believe with rules until our technology is vastly more capable of manipulating forces other than the electro-magnetic than it is now. 

(in theory, you can pump enough energy into a small enough space to make a tiny black hole, which you could then charge with electrical charge and control it with magnetic fields to make it generate gravity waves which might be the basis of another force-based technology like magnets and wires are for electricity, but that's pretty insanely theoretical for the foreseeable future).

Didn't the Romulans use some sort of singularity in their version of warp drive according to the STNG story line?

[Stealth Dart]

https://images.weserv.nl/?url=theinfininte.wikidot.com%2Flocal--files%2Fromulan-d-deridex-class-warbird%2Fstti_dderidex.jpg

    In the Star Trek Universe, the Romulan D'deridex class uses an artificial quantum singularity as power source/propulsion.

[LateNights]

So...how about that "new" planet?

700 years to orbit the Sun!!

[bugzee]

It will complete orbit before IP negotiations are completed.

[Arcana]

Yeah, exactly.  One thing worth noting is that Alcubierre was very directly trying to make Gene Roddenberry's warp drive concept from Star Trek work in General Relativity terms.  He was rather surprised to find an actual solution for it. 

It isn't possible without something that works like Negative matter and there's no actual indication that Negative matter/energy is meaningful in reality...  with the VERY MINOR exception that SOMETHING is making the Universe speed up it's expansion and we've named that SOMETHING (which we have absolutely no ideas about in any real sense) "Dark Energy".   (Some very simple analysis shows this stuff must make up 70-some percent of the total mass of the universe).  So when we figure out what "Dark Energy" might be, the discussion of Alcubierre warp drives MIGHT (or might not) be connected to some further theoretical possibility.

Just to clarify/correct a bit, as far as I'm aware to have the properties general relativity ascribes to it dark energy has to be positive energy and negative pressure, not negative energy.

[Arcana]

It will complete orbit before IP negotiations are completed.

On the plus side, if you live there you could be getting the game back by next year.

[Arcana]

100000 upvotes for snark!

I can't find the upvotes anywhere.  I must have posted a boojum by mistake.