Is it good reading? Lisa Randall -Dark Matter and Dinosaurs

Recommend, review, and discuss science related books that you have read, movies/tv programs you've watched, or Podcasts you listen to.

Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Ormond,

Your questions are sensible. They have just been asked and answered by the scientific community. It's not as strong a case as the case for the spherical Earth and there are some dark matter Flat Earthers out there, but the debate has moved on.

Nobody will be 100% sure until we directly detect dark matter, but that was true of the Higgs boson as well.

For me, the clinching piece of evidence was the observation of the Bullet Cluster. Google it.

I don't mind questions like these, but I do mind ongoing and willful ignorance. So look up my reference and draw your own conclusion. But make it an informed one. Remember that there are idiotic antivaxxers out there and Flat Earthers and climate deniers and fake Apollo mission claimants and those who reject the connection between cigarettes and cancer.

Just because a video exists doesn't mean the debate is credible.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Hi Lincoln,

I did watch the video. Is that you? Whoever it is, they did a great job, very well done.

Upon reflection, I think what I really want to discuss is less about dark matter specifically than a cultural issue which perhaps dark matter brings in to better focus. So as to not derail the thread, I'll do that elsewhere.

Like you, I don't object to being challenged at all. Just as you have your reasonable objection to willful ignorance, my little nit pick thing is appeals to authority. I hope no one will be offended if I don't accept statements just because a scientist made them. I really mean no insult, I'm just trying to be a good "scientist wannabe", which necessarily seems to involve being as inconvenient as my skills will allow.

Ok, dark matter is a theory, a matter being investigated. That works for me.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

It is a theory with considerable thought behind it. And you shouldn't believe in dark matter because I said so. But you should believe that the community has evaluated all proposals to date and the dark matter is the only viable one. Even MOND theories require at least a little dark matter.

I will believe in dark matter only when we have direct evidence for it. But I would bet heavily on it at this point.

And, for the record, I was agnostic on dark matter until the Bullet Cluster observation.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Hi again Lincoln,

It is a theory with considerable thought behind it.

Well, if anyone is actually desiring a thread derail :-) I can say the same thing about the God theory.

But you should believe that the community has evaluated all proposals to date and the dark matter is the only viable one.

Well, to be inconvenient, I have a physics theory which works perfectly so long as I assume the existence of an infinitely large Easter Bunny that no one has ever seen, and which has none of the properties normally associated with existence.

Do you see the cultural issue I'm trying to get at? Why is it ok for scientists to propose invisible things which have none of the properties normally associated with existence, but if we in the public do that same thing, the science clergy rolls their eyes at us? Why is it ok for scientists to have faith, but not us?

To me, a counter explanation for the observed phenomena could be that physicists are operating at the edge of their current ability, and simply don't know what is going on. Doesn't that explain the issue just as credibly as a massive invisible substance?
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Ah...

Well, this is a sad direction for a thread to go.

I don't know your background and I am simply reading between the lines here. But, I hazard a guess that your scientific training isn't terribly advanced. And this is not a dismissive "don't bother the great man" thing. I don't bet on horses because I don't know diddly about the subject, in spite of having raised them as a kid. Mild familiarity does not a disciplined thinker make.

Regarding gods...you give me any specific model of a god, and I'll kill the model. Most models are easy to kill. A few are inherently unkillable. The standard ones, Christianity, Islam, Judaism, etc., are all quite easy to kill. They are either logically inconsistent or they are inconsistent with data. There are "watchmaker deity" types that aren't yet killable, but neither are they worth much...simply because of their inherent non-interactivity. The fact is that a model that is purely consistent with natural forces isn't much of a model.

Regarding dark matter, the real point of this thread, it's not a simple case of people saying "I believe in dark matter." In the video (and, even better, the paper I wrote for the Physics Teacher), I show the simple idea. There are only a handful of possible culprits. These could be incorrect kinematics, incorrect gravity, missing forces or missing matter. Now you can hypothesize new kinematics (e.g. MOND) and see what predictions it makes. This methodology fails testing with data. (And I'm not talking some airy-fairy test, I'm talking real curves with real data and real uncertainties and complete failure of the model. The only way MOND can survive is if one postulates MOND >>AND<< some dark matter.) Modified gravity is another possibility. But that also fails tests. That's one of the nice things about astronomy is that you can test a theory under a huge range of distance scales. Modified gravity also fails. (Meaning modifications can explain one or another discrepancy, but not all discrepancies.) Additional forces suffers from similar weaknesses.

So that leaves us with dark matter. Dark matter is >>NOT<< the same as some silly god model. Dark matter makes very clear predictions on the time evolution of the clumpiness of the universe since the beginning to now. (Success.) It makes predictions of the rotation curves of galaxies (success). It makes predictions of the internal motion of clusters of galaxies (success). It makes predictions about gravitational lensing (success). It successfully predicts the observations of the Bullet Cluster (success). It makes predictions about the number of the number of satellite galaxies around large galaxies like the Milky Way (terrible failure until two years ago when the DES experiment started finding an explosion of population of satellite galaxies that had been undiscovered before that.) Thus there was a prediction of DM that ended up being true, even though we though the initial data disfavored the model.

These are all numerical predictions and tests and they all pass with a single and internally consistent model (dark matter). The fact that you are posting as you do means you are simply unaware of the vast amount of highly precise predictions and tests of the theory.

Now...with all that said, we'd still like to see a direct observation of the phenomenon. We are working on that. We now know that dark matter cannot interact by the weak nuclear force unless more particles are postulated. That isn't some vague thing. It's a real prediction with real tests and inescapable conclusions. We know >>TONS<< about dark matter. That's not the same as knowing everything.

It remains possible that there is something not yet considered. Scientists concede this. But we scientists are in the business of showing that each other are foolish and that we are smarter than our colleagues. We have tried >>VERY HARD<< to kill the DM idea. There have been long and rancorous debates over decades on this. However, regardless of these valiant attempts to kill the theory, we have come to a point where we can't kill it and, in trying to kill it, have found that there are strong reasons to believe in it.

I am willing to debate the DM question, but you had better know your data and predictions and the tests that have been done. Because what you've stated here is just some sort of vague "I don't believe in authority" silliness that doesn't apply here. There are thousands of papers on the subject. You don't need to have read all of them, but you certainly have to have read some of the better survey papers. If you've done that and still think you have a case, fine. Otherwise, you aren't doing science.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Ormond » January 24th, 2016, 7:40 pm wrote:Well, to be inconvenient, I have a physics theory which works perfectly so long as I assume the existence of an infinitely large Easter Bunny that no one has ever seen, and which has none of the properties normally associated with existence.

That'd be awesome! There's nothing wrong with asserting an infinitely large Easter Bunny if your theory helps humans to make better predictions about the physical world.

Of course other people will improve your theory by simplifying it. For example, if this entity doesn't have to be called the Easter Bunny, people will just start calling it by whatever simplest description is adequate to maintain the predictive power of the theory.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Hi again Lincoln, you are a good sport, and that is noticed and appreciated here, thanks.

Well, this is a sad direction for a thread to go.

It depends. If members wish to stay safely inside a science clergy authority bubble, then ok, yea, I suppose this could get sad and we'll require rescue from Braininvat. If members wish to explore beyond the authority bubble, then this can be a lot fun. The key to success is probably for each of us to keep in mind that the discussion has nothing to do with any of us personally.

I don't know your background....

This thing we're doing now, engaging nerd topics on a net forum, I've been doing that every day the whole time you've been studying physics. I promise I can keep up, in my own way, if your time permits. I do recognize that you're not retired like me, and thus your time may not permit.

But, I hazard a guess that your scientific training isn't terribly advanced.

That is entirely true. And I'm guessing that your forum and religion training isn't terribly advanced, so it's a fair contest as I see it. What we have going for us is that we're coming at the same subjects from very different backgrounds, which can be promising ground for an interesting thread.

And this is not a dismissive "don't bother the great man" thing.

Um, yea it is, but that's ok. As you've seen, I don't write perfectly either. No problem.

Regarding gods...you give me any specific model of a god, and I'll kill the model. Most models are easy to kill.

See? This is science clergy syndrome. The assumption that because you are highly trained in one arena, you are therefore qualified to casually dismiss thousands of years of thought in another arena.

Again, by challenging science clergy syndrome I am attempting to do that which you would very reasonably expect me to do if I was chatting with a Catholic cardinal. I'm attempting to provide an even handed challenge to anybody making large assertions, no matter what ideological culture they come from.

They are either logically inconsistent or they are inconsistent with data.

Ok, here's a taste. If you want to continue, which I hope you will, Braininvat will probably ask us to "get a room" in the religion section.

You are assuming that the rules of human reason are binding on all of reality, and thus any proposed gods within. Within that assumption, it's correct to dismiss any proposals which fail the logic test. The problem here is that nobody can prove the fundamental assumption the entire atheist case rests upon, the notion that human reason is binding on everything everywhere. We don't even know what the phrase "everything everywhere" refers to. Thus, the atheist case is that no god exists in an arena that no one can define in even the most basic manner.

Atheism is faith, just like religion. The only real difference is that atheists typically don't know they are relying on faith, whereas theists typically do.

BTW, if it matters, I'm not religious, but a "Fundamentalist Agnostic". I assure you that I'm good at this thing we're doing now, which I say only in the hope that you might believe me and give me the opportunity to prove myself worth your time in further dialog.

Regarding dark matter, the real point of this thread, it's not a simple case of people saying "I believe in dark matter." In the video (and, even better, the paper I wrote for the Physics Teacher), I show the simple idea. There are only a handful of possible culprits.

None of which appear to be, physicists are working at the edge of their ability and simply don't know what is going on. To me, this seems a possibility that should be added to the mix. Not as the only possibility, not as a "one true way", just one of many possible answers, dark matter among them.

This is a reasonable challenge to present any time anybody proposes something invisible which has none of the properties normally associated with existence. I'm not being an uneducated hillbilly by presenting such a challenge, I'm just doing the very thing scientists reasonably ask we in the public to do, challenge everything.

So that leaves us with dark matter. Dark matter is >>NOT<< the same as some silly god model.

Well, the god model explains all unexplainables, not just one.

These are all numerical predictions and tests and they all pass with a single and internally consistent model (dark matter).

Ok, fair enough, good presentation. I'm not really challenging the existence of dark matter, but something else. A notion that we should believe in invisible things just because the science clergy announces them. I'm presenting the same challenge to dark matter as I would present to other invisible things such as the Holy Spirit. Where is it? Show me! etc.

The fact that you are posting as you do means you are simply unaware of the vast amount of highly precise predictions and tests of the theory.

I will again repeat that I am not a scientist. If scientists are the only people you are willing to learn from then, yea, I will be a disappointment. Which is ok, really it is. Surely no one is obligated to attend to my posts.

But we scientists are in the business of showing that each other are foolish and that we are smarter than our colleagues.

Yes, I like that about scientists! I'm like that too, though I try to be cheerful and friendly about it (and don't always succeed).

I am willing to debate the DM question, but you had better know your data and predictions and the tests that have been done.

If you are only willing to talk to scientists about science, which is of course your right, I have no role to play here and can safely be ignored. If your mind is interested in exploring a wider circle, maybe I'll be able to contribute in a manner members will find engaging.

Otherwise, you aren't doing science.

Correct, I'm not doing science, agreed. I'm not doing science both because I am not a scientist AND because science exists within a larger realm which I hope to address in a manner at least some readers will find engaging.

As a forum poster, my job is to understand that forums are primarily about entertainment (irregardless of any claims to the contrary) and my job is to make a good faith effort to put on a show which is at least somewhat entertaining. If you will allow me to be me, I might be able to contribute. If you demand that I be you, then I can't.

Thanks for reading this far, too many words I admit, apologies.
Last edited by Ormond on January 25th, 2016, 8:06 am, edited 1 time in total.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

My thoughts and question, too, Ormond. If you can't see it, how do you prove it exists? It's almost impossible to get an answer to that question. At least one this unknowing one understands.

That said, I do hope you two continue your conversation. There is more wealth in your back-and-forth responses than maybe even you realize.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Viv...

Don't be led astray. Not seeing is not the same thing as not detecting. You can't see radio waves, radiation, heat, sound, and the invisible curse that follows the Chicago Cubs, but they all exist and can be detected.

>>OF COURSE<< dark matter can be detected, first gravitationally (which is why it was hypothesized) and secondarily via either indirect annihilation or direct detection in underground detectors or by being created in particle accelerators. All of those experiments are underway, thus far without success. It is that "without success" that keeps the Nobel Prize committee from awarding a prize for the whole dark matter thing. This is because the standards of science are high.

In short, don't use your and Ormond's incomplete knowledge of the matter as being a useful indictment against the dark matter idea. Scientists know >>TONS<< about dark matter. I can spend all day telling you what it isn't and another day what it must be, if it exists. I can spend a third day telling you about the various detection methodologies currently employed. And I can spend years telling you about the analytic models that have been developed and the statistical methodologies that will be employed when a valid signal arises.

In short, Ormond's questions do not arise from a place of wisdom and knowledge, but rather a place of ignorance. I'm not one of those scientists who dismisses the public simply because of a lack of a degree. However a lack of knowledge is simply unforgiveable in these discussions. And, perhaps sadly, that puts the onus on you to catch up on the known subject matter. (Mind you, if we were discussing something that you have mastered that I haven't...say knitting, boy bands from the 90s, juggling or neurosurgery, well then the onus would be on me.)

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Viv...

Don't be led astray. Not seeing is not the same thing as not detecting. You can't see radio waves, radiation, heat, sound, and the invisible curse that follows the Chicago Cubs, but they all exist and can be detected.

>>OF COURSE<< dark matter can be detected, first gravitationally (which is why it was hypothesized) and secondarily via either indirect annihilation or direct detection in underground detectors or by being created in particle accelerators. All of those experiments are underway, thus far without success. It is that "without success" that keeps the Nobel Prize committee from awarding a prize for the whole dark matter thing. This is because the standards of science are high.

In short, don't use your and Ormond's incomplete knowledge of the matter as being a useful indictment against the dark matter idea. Scientists know >>TONS<< about dark matter. I can spend all day telling you what it isn't and another day what it must be, if it exists. I can spend a third day telling you about the various detection methodologies currently employed. And I can spend years telling you about the analytic models that have been developed and the statistical methodologies that will be employed when a valid signal arises.

In short, Ormond's questions do not arise from a place of wisdom and knowledge, but rather a place of ignorance. I'm not one of those scientists who dismisses the public simply because of a lack of a degree. However a lack of knowledge is simply unforgiveable in these discussions. And, perhaps sadly, that puts the onus on you to catch up on the known subject matter. (Mind you, if we were discussing something that you have mastered that I haven't...say knitting, boy bands from the 90s, juggling or neurosurgery, well then the onus would be on me.)

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Ormund,

Interestingly enough, my religion and philosophy training is reasonably advanced, inasmuch as I have minor degrees in them. And my forum training isn't so bad either, inasmuch as I have been on this forum for over a decade and have many more posts than you too. (I presume you post elsewhere as well, but then maybe so do I.)

I have been away from SCF for quite a while, as the pressures of research, writing, and other science communication efforts have proven more valuable than this particular forum. And I will disappear from SCF soon for exactly those reasons. So sadly, the conversation will likely not advance as far as you might hope. Sorry about that, but the graduate students don't beat themselves.

It is funny that you return to this phrasing of science clergy, which is true in some cases but not in most. The real problem is that you simply don't know enough about the science to say anything valuable. Sorry...but it's just not worth the time of someone who has spent decades mastering the subject to have a conversation that basically boils down to "I don't believe in what you're saying because I don't know much about it but could you spend valuable time teaching me stuff that I could well learn on my own." And the answer to that is "Go and learn and come back when you have even a basic understanding." Yes, some call that academic arrogance, but I wouldn't argue with a sociologist about the effects of first contact of isolated Amazonian tribes without learning something about it first. To participate at a reasonable level requires real effort.

Briefly touching on the religion thing, I'm sorry, but you're simply wrong. Most ideas of gods are easy to kill. Once a person makes a statement about the world around us that is tied to a particular deity idea, we can test that hypothesis. And the standard Abrahamic religions are particularly easy to kill. There was no great flood, for instance. And if you kill just one prediction of a hypothesis, the hypothesis is wrong and must be either abandoned or modified.

There are god theories that have been modified sufficiently to be consistent with existing data. Those god theories are either indistinguishable from science or they revolve around the "brain in the vat" idea, in which no data you have been fed can be trusted. While I can't disprove those ideas, I can constantly refine the "indistinguishable from science" god hypothesis until there is a difference between the hypothesis and the science and prove or disprove the hypothesis. The brain in the vat hypothesis is simply vapid posturing and does not necessarily lead to religion. After all, you could exist and I could be poking you with an electronic probe. No god.

Regarding logic, well there are logics that transcend all of space and time. A and .NOT. A can not simultaneously be true without some sort of secondary conditional that distinguishes between the conditions under which A or .NOT. A applies.

Finally (I hope), your equating dark matter and Holy Spirit really demonstrates a deep, deep, deep misunderstanding of the nature of science and (especially) dark matter. There can be no discussion on anything you've raised here until you up your game in the whole "understanding of science" thing. And there is no clergy business going on here. This understanding is yours for the taking if you wish to pursue it. There are no roadblocks and you would be welcome into the circle of informed thinkers. If you choose to stay outside, commenting on something that you clearly don't understand, well that is your prerogative. But I personally find intentional and willful ignorance to be very sad and I don't understand why you would elect to do that. The "wider circle" that you refer to has no discipline and is not at all valuable. Sorry.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Seeing vs detecting. :-) Right. But I am not confining my comment to dark matter - or even including dark matter in my "how do you know" question.. My comment covers many things that science says exist but the question "how do you know" gets pretty much a blank stare - a "you wouldn't understand" reaction. Your "detection" is definitely the first step toward answering. If I ask, for example, how you know quarks exist? and you say "by detection", the next question becomes "how did you detect them?"

And, of course, it is a fact that I might not understand your answer - especially if it includes a long list of equations - but you would have at least tried. One reason I so much appreciate this forum is that we generally get answers. Sometimes I understand them and sometimes not but I always get a hint of what to start looking for.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

If you really want to know the answer to the quark question, then I recommend you read "Understanding the Universe: From Quarks to the Cosmos." I understand that the author is quite expert on such matters.

http://www.amazon.com/gp/product/9814374458/

There was a long thread in this forum on the 2004 version of this book, but you should read the 2012 version.

viewtopic.php?f=14&t=700

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Lincoln » January 25th, 2016, 8:50 am wrote:If you really want to know the answer to the quark question, then I recommend you read "Understanding the Universe: From Quarks to the Cosmos." I understand that the author is quite expert on such matters.

http://www.amazon.com/gp/product/9814374458/

There was a long thread in this forum on the 2004 version of this book, but you should read the 2012 version.

viewtopic.php?f=14&t=700

Thank you. I'll look for it at my next trip to B&N. I'm always looking for new books.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

You're better off with Amazon. The book was released 3 years ago and it is unlikely you'll find it on the shelves of a bookstore unless it's in a college town.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

vivian maxine » January 25th, 2016, 8:32 am wrote:Seeing vs detecting. :-) Right. But I am not confining my comment to dark matter - or even including dark matter in my "how do you know" question.. My comment covers many things that science says exist but the question "how do you know" gets pretty much a blank stare - a "you wouldn't understand" reaction. Your "detection" is definitely the first step toward answering. If I ask, for example, how you know quarks exist? and you say "by detection", the next question becomes "how did you detect them?"

It's all about providing falsifiable predictions. To test for the existence of anything - be it an object, abstract concept, or anything else - you make predictions that depend upon the assumption of its existence, then do those experiments.

Hypothetically, say that we found a solar system like ours,
,
except there's no star like our sun in the middle. Our current theories of gravity say that that shouldn't be possible, so what do you make of it?

There're two pretty reasonable first thoughts:
1. Our understanding of gravity is wrong, and you don't actually need a big star in the middle of a solar system.
2. Our understanding of gravity is right, and there's something invisible instead of a normal star.
Lincoln's already talked about both of these ideas. The first idea is modified Newtonian dynamics (MOND), and the second idea is dark matter.

In reality, we didn't find a solar system like that (which could be explained by something like a black hole in the center), but rather the galaxy rotation problem: that galaxies don't rotate like our theories predicted. In an attempt to explain this problem, physicists proposed hypotheses like MOND and dark matter.

As Lincoln said, the discovery of the Bullet Cluster was important because it provided important evidence:
1. The most popular versions of MOND fail to explain it (Wikipedia).
2. Dark matter could explain it reasonably well.
The Bullet Cluster alone isn't enough to "prove" dark matter, but it helped to discredit MOND while providing more circumstantial evidence for dark matter.

Currently astronomers are looking for more systems that aren't well-explained to see if a single, consistent theory of dark matter predicts their behavior. If we get enough evidence that scientists feel safe assuming that dark matter's there in their models, then they'll declare it to be proven.

It's important to note that you can never really "know" anything for sure; for all we know, this could be a dream, or alien mind control, or something like that. Just, after there seems to be enough evidence for something, we call it good enough. We'll consider dark matter proven when there's enough evidence that experts no longer care to spend time doubting it.

PS:
Just to add, a big problem in looking for dark matter is that we can't see the universe very well. For example, we can estimate how far away particular stars are based on how their light looks, but it's not like we have a tape measure that shows us where everything is. Our estimates on star locations, sizes, movements, etc. are based on guesswork - some pretty reliable while some's quite speculative.

The Bullet Cluster is significant because we could see it well enough to rule out some popular MOND theories to a high degree of certainty ($8{\sigma}$, according to a paper cited by Wikipedia) while it's still consistent with dark matter.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Ormond » January 25th, 2016, 7:03 am wrote:
So that leaves us with dark matter. Dark matter is >>NOT<< the same as some silly god model.

Well, the god model explains all unexplainables, not just one.

In science, "explain" literally means "describe in terms of a predictive framework". God models aren't explanations because they don't provide a basis for prediction. This is, "God moves stars" doesn't actually tell us anything; it's an empty statement.

By contrast, models like MOND and dark matter do make predictions. So, we test them to see if those predictions are consistently correct, and if they keep working, then we consider them working theories and use them to make predictions about the universe. If they don't work, then we forget them and move on. That's Science.

For a historical example, scientists trying to explain heat proposed caloric theory. Caloric was kinda like a dark-matter version of heat; this is, we couldn't see caloric, but it interacted with other matter to provide temperature. But, after doing lots of experiments, we found that caloric theory didn't work very well as a basis for prediction, so we abandoned it in favor a MOND-like theory of heat. The same thing can happen to the theory of dark matter if it doesn't form a good basis for predicting what we see in the universe.
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Re: Checking the Extremes

Sometimes it's productive to check the extremes to find examples of a phenomenon which, while atypical for the species being sought, can nonetheless inform us in some ways of what we're hunting for.

For example, it is increasingly common to find neutrinos characterized as dark matter (DM), as I do. True, their masses are too low and velocities too high to explain the galactic gravitational phenomena which drew us to DM in the first place but we might gather from them the possibility that more massive DM particles may exhibit uniform helicity, may be Majorana particles (their own antiparticles), and may freely oscillate between three flavors. This could be a big time saver in solving future detection deficits for WIMPs, as occurred with the original the solar neutrino mystery.

At the other extreme, one might consider black holes as DM. From this we may consider the possibility that DM particles, instead of being fundamentally neutral, could be fundamentally both positive and negative to equal degree. Unlike neutrons, which have tiny internal dipole moments attributable to non-zero separation of their constituent quarks, DM may issue opposed (thus, self negating) EM fields from a single point origin. This could be important in explaining participation of DM in weak interaction by preserving electroweak theory. Otherwise, fundamentally neutral WIMPs would seem to require adjustment of electroweak theory to allow for purely weak interaction. The same consideration applies to neutrinos.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Thank you all. Dark matter may be at the center because there has to be something there? Or does there? The planets in revolution cannot be working against each other forcing each to stay in its own position? Am I attributing to planets the ability to sustain gravity on their own?

Did I just describe the discredited MOND? Have I given gravity a power that it doesn't have, so far as we know? There seems to be a lot of questioning about gravity right now. I feel new ideas are coming forward about it. Where it comes from; how it works; whether it has all the power we thought it had; all kinds of questions. Scientists with new ideas to be tested.

At any rate, thank you for that NCE. I now understand why we think dark matter exists. My idea about dark matter: It holds the entire universe together. Maybe that's where all the "gravity" comes from. How fast can that idea be shot down?

How far away a star is - I was forgetting about color shift.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Brief aside to Ormond: I am one of several moderators here, and my duties are not specific to any one forum. And I'm actually more digressive than some of our mods, so you shouldn't cite me as a higher authority (or enforcer) on thread relevance. And I'm planning to step down from mod status pretty soon, due to both time constraints and, after 2 years, feeling I've "served." Carry on.

I live just a few miles from SURF, the Sanford Undeground Research Facility, located deep in the old Homestake Gold Mine in Lead, South Dakota. The search for DM is part of SURF's research program - I will try to stay on top of their experiments and post anything worth noting here.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

vivian maxine » January 25th, 2016, 11:49 am wrote:Dark matter may be at the center because there has to be something there? Or does there? The planets in revolution cannot be working against each other forcing each to stay in its own position?

According to our current theories of gravity, planets should orbit the center-of-mass. If a lot of planets are orbiting a single point in space, then that point should be the center of mass.

For example, you know the phrase "when the planets align"? Because our sun is the major mass in our solar system, the planets don't really care much that they're aligned. But if the sun wasn't there, they'd stop going in circles around the place where the sun used to be.

When you look at our galaxy, it has two arms that go from the center out:
.
These arms are curved, so you can see that the outer stars haven't rotated exactly as fast as the inner stars in terms of rotations per unit of time, but they're close enough that the arms haven't merged together into a big jumble. And that's weird.

What we expected was what we see in our own solar system: planets closer to the center have short rotation periods. Mercury, the closest planet, orbits the sun in about 88 days, while Pluto takes 248 Earth-years. By contrast, the stars in the galaxy's arms appear to have orbital periods that are a lot closer together despite their distance from the center. This discrepency between what we expected and what we see is the galaxy rotation problem.

So, why dark matter? One idea is that there might be a dark matter halo that's like an outer wheel around the galaxy. This outer wheel spins relatively quickly, pulling along the stars further from the center of our galaxy. This may explain what we're seeing.

PS - Actually, I tried to simplify the rotation thing by analogy to the solar system, but here's a video that's way better!
Natural ChemE
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

That is a far better illustration than the one I tried to follow a few weeks ago. It made me dizzy. This one shows the arms more clearly. Thanks.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

I concur with most of what Natural is saying. I might (well, actually would) quibble with the radius at which the dark matter halo becomes important. I'd also quibble with the statement that scientists are looking for more evidence for DM. There have been tons of additional evidence for pushing a century. After all, Fritz Zwicky's observations of the Coma cluster were a very long time ago.

The galaxy rotation curves are the easiest to understand and imagine, but they're not the only evidence.

But these are piddly things.

BTW...Faradave...sorry, but neutrinos are out for DM. Just can't be. Now, if sterile neutrinos exist, that's a different thing, but note that sterile neutrinos don't interact via the weak nuclear force. This is also true for black holes, either normal or exotic. Finally, we know that dark matter doesn't interact via the weak nuclear force. I know that they are called WIMPs and historically that the "W" meant "weak nuclear force," but that thought is like.....soooooooooo....five years ago. The field has moved on. The sensitivities of modern experiments have ruled this out. We are stuck with thus-far-unobserved forces of intermediate strength between the weak nuclear force and gravity, or gravity alone. If it is gravity alone, we're kinda screwed, because there is no conceivable experiment that will directly observe dark matter of that flavor.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Lincoln » Mon Jan 25, 2016 12:31 pm wrote:I concur with most of what Natural is saying. I might (well, actually would) quibble with the radius at which the dark matter halo becomes important. I'd also quibble with the statement that scientists are looking for more evidence for DM. There have been tons of additional evidence for pushing a century. After all, Fritz Zwicky's observations of the Coma cluster were a very long time ago.

The galaxy rotation curves are the easiest to understand and imagine, but they're not the only evidence.

But these are piddly things.

BTW...Faradave...sorry, but neutrinos are out for DM. Just can't be. Now, if sterile neutrinos exist, that's a different thing, but note that sterile neutrinos don't interact via the weak nuclear force. This is also true for black holes, either normal or exotic. Finally, we know that dark matter doesn't interact via the weak nuclear force. I know that they are called WIMPs and historically that the "W" meant "weak nuclear force," but that thought is like.....soooooooooo....five years ago. The field has moved on. The sensitivities of modern experiments have ruled this out. We are stuck with thus-far-unobserved forces of intermediate strength between the weak nuclear force and gravity, or gravity alone. If it is gravity alone, we're kinda screwed, because there is no conceivable experiment that will directly observe dark matter of that flavor.

Yes, I also concur.
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Re: Painting yourselves into a corner

Lincoln wrote: I know that they are called WIMPs and historically that the "W" meant "weak nuclear force," but that thought is like.....soooooooooo....five years ago.

As I'm not likely to be accepted among real physicists, I must content myself with sifting through the ashes of outcast theories (even five-year old ones).

Lincoln wrote:If it is gravity alone, we're kinda screwed...

It's fine to narrow down the specifics for WIMPs but I think it's premature to obscure the original meaning of "dark" in our hunt for DM. Even regular neutrinos are fundamentally dark, i.e. they do not participate in EM interaction. Thus, good reason to exclude them as ordinary matter and no apparent reason to boot them from the nascent DM category.

Black holes are somewhat a stretch. While they emit no EM, they could be construed as absorbers (of light), though not participating in the typical sense (via orbital transition).

Further, while it is quite easy to document the descent of ordinary matter into black hole event horizons, I think we must, for the time being, allow that 85% of what falls in is DM. Thus, black holes should be considered at least an important hybrid, with a strong bias toward DM. They might even be considered to decay via weak force if Hawking radiation is counted. (Now that's a WIMP! as Crocodile Dundee might say.)

It's not that I want to be argumentative, but if we don't keep open minds at this early stage in the search for DM, we're likely to end up with faces painted red.
Last edited by Faradave on January 25th, 2016, 4:03 pm, edited 1 time in total.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

You are, of course, free to your opinion, but not your facts.

Black holes have been ruled out for decades by experiments like MACHO and OGLE and their ilk. And while you emphasize the "no EM" thing, the data now demonstrates conclusively that dark matter is also not subject to ordinary weak nuclear reactions.

This, by the way, is what I mean when I say we know a ton about DM, even if we don't know what it is. We know lots of what it isn't.

If you wish to continue to support your position, but the only people who will listen are those who are not up to date. This is a vibrant research area and our understanding updates quite frequently.

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Re: Pyric Victory?

Lincoln wrote:You are, of course, free to your opinion, but not your facts

Fact: Neutrinos are dark (no EM interaction) and they are matter (they have rest mass accountable by energy, momentum, etc.). The incidentally participate in weak interaction.

Fact: Black holes derive from 85% DM. If not, DM sidesteps gravitation. Then we're really screwed.

Lincoln wrote:We know lots of what it [DM] isn't.

That's fine but be careful. They've said the same thing about quantum spin for many decades. Yet your colleagues are still clueless to explain it and will remain so until they broaden the range of what it possibly could be.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

Fact: neutrinos are dark. Fact: they have rest mass. Fact: they experience the weak nuclear interaction.

Fact: DM is dark. Fact: It has rest mass. Fact: DM >>DOES NOT<< experience the weak nuclear interaction.

Conclusion: DM aren't neutrinos.

Fact: dark matter does enter black holes. Fact: DM does not emit electromagnetic radiation and therefore does not lose energy as it orbits a black hole. Fact: DM is very rare inside a star and most of the mass of black holes comes from the stellar implosion and not from accretion (at least for stellar mass black holes, which is what I think you're talking about. Thus, fact: DM enters black holes less frequently than ordinary matter.

Conclusion: (Stellar size) lack holes are not constituted by 85% DM.

Regarding broadening the scope, have you not read the OP of this thread? The hypothesis of strongly self-interacting DM is a pretty new and broad idea.

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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

So sadly, the conversation will likely not advance as far as you might hope. Sorry about that, but the graduate students don't beat themselves.

I hear you Lincoln, no problem. You will be missed, but I understand.

The real problem is that you simply don't know enough about the science to say anything valuable.

Ok, let's test that assertion. Here are two paragraphs from another thread which I invite you to inspect. Here's the other thread...

viewtopic.php?f=51&t=29906&start=30

Here are the two paragraphs (which I won't discuss further here out of respect for this thread)

ASSERTION #1: As knowledge development accelerates, and the products of that process become ever more powerful, it's inevitable that sooner or later a power will emerge which humans are not mature and/or skilled enough to successfully manage. ("Successfully manage" is defined as avoiding civilization crushing events, such as nuclear war for example.)

ASSERTION #2: If the above is true, then scientific progress is not only dangerous, but largely pointless, given that all the many projects which are successful will be swept away in the civilization crushing event.

The science clergy guys are always trying to pull rank on we in the lowly general public peanut gallery, and NONE of you here or anywhere can effectively answer the above assertions which are central to your entire enterprise.

I say this to try to help you understand why I went all wacky (my bad, sorry) about dark matter above. I sincerely have great respect for the technical skills of scientists, but am losing confidence in scientists ability to see the bigger picture beyond the narrow little stovepipes of their chosen disciplines. I see the whole science machine racing blindly towards a cliff, while some scientists indulge the hobby of declaring the rest of us too dumb and ill informed to participate in the conversation.

Briefly touching on the religion thing, I'm sorry, but you're simply wrong. Most ideas of gods are easy to kill.

Sadly, we won't have time for this I guess. And I'm guessing you don't want your atheist certainty disturbed, and that's ok with me as I'm not an evangelist, just intensely interested in some subjects. Another day perhaps, would welcome it, as your certainty makes you a sitting duck (said with a friendly twinkle in the eye).

Ok, I mean no disrespect, and I don't think you do either, but to keep it calm here I um, apologies, just erased all the rest of the appeals to authority stuff.

When you can answer my assertions above (in the other thread if you decide to) then we can revisit the appeals to authority stuff perhaps.

And in any case, thanks for responding to me at all. I know I'm not always an easy read, for various reasons.
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Re: Is it good reading? Lisa Randall -Dark Matter and Dinos

I didn't say that I was an atheist. You inferred that. I simply said that most ideas of gods are easy to kill. That statement is simply true, irrespective of my religious position. You may think that this position makes me a sitting duck, but I actually think you didn't read my statement very closely if you think so. I've talked armies of very highly educated theologians into many a corner. I stopped when it no longer seemed sporting or an effective use of time. Faith is such a shaky foundation for any intellectual endeavor.

Your assertions #1 & #2 are really quite independent of (a) DM and (b) religion, but they do have the sniff of kneejerk dismissiveness of authority but have none of the scent of a comparably nuanced position. Dost thou have some overarching axe to grind? Is this nothing more than anti-intellectualism posing as wisdom?

Mind you, I'm not an academic elitist, but neither am I an intellectual populist. There are some really crappy arguments out there and the quality of those arguments are correlated with formal intellectual training. And correlated is the appropriate term, used in the accepted mathematical and scientific manner.

It's a bit sad, as your assertions #1 & #2 are mildly interesting, but the disjointed and scattered topics covered thus far don't bode well for a focused discussion, especially inasmuch as your assertions, religion and DM are unconnected topics.

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