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Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel Send a noteboard - 14/05/2011 05:35:56 AM
I'm not seeing why larger MACHOs have to emit light (I'm tempted to reference black holes again, but where they fit into the classification of normal/normal dark/exotic dark matter seems ambiguous). Probably just my ignorance showing again, but since stars produce all the heavy nuclei large aggregates of those nuclei, either as nebulae or non-luminous masses, seem viable, especially at the edge of the galaxy. As I understand it, stellar formation is thought to produce energy levels that preclude anything more complex than hydrogen and helium, but if heavier nuclei can exist within old stars it seems like they ought to be able to mass without the force of their compaction reducing them to hydrogen nuclei and ultimately igniting a star. Eliminating MACHOs from the Milky Ways perimeter through gravitational lensing eliminates any other matter as well, but I don't see why it eliminates MACHOs in other locations where gravitational lensing shows dark matter.

These are all questions that display a lack of education in astrophysics. Astrophysicists have put tons of work into investigating and describing star formation and related phenomena. A big enough collection of mass is going to emit some light because the gravitational force will cause nuclear fusion to occur. Note that fusion does not only occur with hydrogen nuclei, and amounts of heavier nuclei are limited by nucleosynthesis measurements from the cosmic microwave background.

Not a complete lack, but nothing like a professionals, no. A lot of it's outdated, too; a part of me died when I found out quasars are no longer considered possible white holes. :( I know fusion isn't restricted to hydrogen (else helium would be the heaviest element), but fuzing say, transuranides, seems energy prohibitive. I'm curious how cosmic background radiation has ruled out the possibility of heavy nuclei pockets surrounding galaxies; I'd have thought scattering over such a great distance would make detection of all but the most massive quantity of super heavy nuclei impossible, but presumably that's not the basis on which the exclusion was made.
You lost me here, sorry. If there's enough gravitational lensing (our main evidence of dark matter, right?) to account for the Milky Ways rotation how can a there be a LACK of such lensing that rules out non-luminous MACHOs? Wouldn't gravitational lensing that rules out one rule out the other?

Our initial evidence of dark matter were anomalous galactic rotation curves. Some of our best evidence for exotic dark matter is lensing from, e.g., the Bullet Cluster. Based on the former evidence, we know that dark matter is distributed fairly uniformly within galaxies. If dark matter is mostly MACHOs, we should still be able to see lensing from those concentrated bodies within our galaxy, and we don't. Exotic dark matter hypotheses tend to predict a more diffuse collection of particles, so lensing would not be easily visible from nearby sources. The far-away lensing that we observe is not consistent with MACHOs, as Sean Carroll explained in the blog post I linked previously.

That last statement doesn't follow from the others. Far away lensing is consistent with exotic dark matter, but no less consistent with MACHOs; MACHOs were ruled out because no nearby gravitational lensing was observed and galactic rotation says dark matter is uniformly distributed within galaxies. That works if there's enough dark matter to significantly affect galactic rotation but not enough to create nearby gravitational lensing, but those two propositions tend to be contradictory unless there's a BIG difference between the amount of dark matter necessary for the former and the latter. If that's the case I have no objection, I just want to be sure that's what you're saying.
It partly stems from that, unquestionably, but it mainly stems from knowing science has an impressive track record of doing that kind of thing. It's tempting to portray Galileos stand against Rome as science vs. religion, but the fact is there were plenty of well established members of the scientific community on the other side of that argument. It was not so much science vs. religion as revolutionary science vs. establishment science; establishment religion just had a vested interest in taking the latters side. Just recently I was reading about Max Planck being told when he started grad school that he was basically wasting his time because physics was a finished field with only a few known minor details left to flesh out fully, and that mindset made quantum mechanics very controversial for some time. Knowing that "quintessence" long flourished as an explanation for light behaving as both wave and particle in the days before duality was conceived makes it natural to worry it might be happpening again (especially, once again, when the same term has been revived with a new application). As you've suggested, I may guilty of my own charge in clinging to non-exotic dark matter, but I'm trying not to get married to either novel or established physics, but stay focused on accurate explanations regardless of source. There's ample precedent to fear such overcommitment; again, I accept your statement that it's not happening because you undeniably have more knowlege on which to judge. That said, I hope you don't mean it CAN'T happen, because expanding knowledge encourages rather than discouraging it.

Probably half of the physics professors I've known have, at one point or another, made fun of the people in the previous century who said that physics was almost "done." The increasing access to information today means that almost everyone in the field has heard those stories, so that sort of thinking simply doesn't occur as often as it once did in more parochial times.

Hopefully not, but that demonstrates that my concern that kind of thinking didn't end in the 1600s is valid. I don't doubt it's less likely, and significantly, but it's far from impossible, particularly if we allow ourselves the luxury of thinking it impossible; that's actually an EXAMPLE of the kind of erroneous and overconfident thinking that worries me, and further demonstrates it can still occur, because it manifestly has.
I'm not ignoring or marginalizing it, but it still seems inconclusive. If it rules out normal dark matter, which seems to be the case, I accept that correction and apologize for unfounded objections. As I understand the evidence you've presented, however, the case AGAINST normal dark matter is quite strong but the case FOR exotic dark matter much less so.

The latter is, to a large degree, reciprocal with the former. If we know that dark matter acts like matter in general in that it gravitates (which we do) and that evidence has just about ruled out normal matter (which it has), then the obvious conclusion is that dark matter is a new form of matter.

It could also be a form of normal matter acting in ways existing theory didn't anticipate, which is not so far fetched since existing theories obviously failed to anticipate SOMETHING or we wouldn't be seeking a way to reconcile them with recent anomalous observations. You've convinced me exotic dark matter is the best candidate, but I'd still like to be certain it's not prematurely designated the ONLY candidate.
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Exciting video about the universe Camilla - 28/04/2011 10:14:55 AM 1092 Views
Cool, and true *NM* Stephen - 28/04/2011 11:46:29 AM 332 Views
I still think dark matter's just non-luminous matter without a convenient light source to reflect. Joel - 28/04/2011 10:34:21 PM 817 Views
We've just about ruled out the idea that dark matter is just non-luminous "ordinary" matter. Dreaded Anomaly - 28/04/2011 11:44:34 PM 750 Views
I'm aware of the Bullet Cluster, though admittedly not much more than that. Joel - 29/04/2011 01:52:49 AM 682 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. Dreaded Anomaly - 29/04/2011 02:56:32 AM 793 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. Joel - 30/04/2011 05:02:49 PM 750 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. Dreaded Anomaly - 30/04/2011 08:56:35 PM 621 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. Joel - 02/05/2011 01:28:30 AM 654 Views
Re: I'm aware of the Bullet Cluster, though admittedly not much more than that. Dreaded Anomaly - 04/05/2011 04:18:18 AM 760 Views
There's such a thing as knowing when you're licked, and I believe I am. Joel - 07/05/2011 02:04:53 AM 831 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. Dreaded Anomaly - 09/05/2011 11:28:48 PM 674 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. Joel - 14/05/2011 05:36:45 AM 620 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. Dreaded Anomaly - 17/05/2011 02:09:40 AM 708 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. Joel - 19/05/2011 04:55:21 AM 629 Views
Re: There's such a thing as knowing when you're licked, and I believe I am. Dreaded Anomaly - 24/05/2011 09:32:27 PM 706 Views
The Pati-Salam model was the one I had in mind. Joel - 24/05/2011 10:34:04 PM 648 Views
Re: The Pati-Salam model was the one I had in mind. Dreaded Anomaly - 24/05/2011 11:08:01 PM 857 Views
Re: The Pati-Salam model was the one I had in mind. Joel - 25/05/2011 01:27:10 AM 669 Views
Re: The Pati-Salam model was the one I had in mind. Dreaded Anomaly - 31/05/2011 09:16:18 AM 738 Views
Apologies for the delay; internet's been spotty and I've been busy lately. Joel - 10/06/2011 12:09:04 AM 989 Views
Re: Apologies for the delay; internet's been spotty and I've been busy lately. Dreaded Anomaly - 14/06/2011 03:38:18 AM 991 Views
Also, re: lensing from ordinary matter: Dreaded Anomaly - 29/04/2011 05:18:47 AM 681 Views
This seems like another example of what confuses the issue. Joel - 30/04/2011 05:25:04 PM 800 Views
Re: This seems like another example of what confuses the issue. Dreaded Anomaly - 30/04/2011 08:56:40 PM 773 Views
That discussion seems to reduce to "as little new and exotic physics as possible". Joel - 02/05/2011 01:29:03 AM 764 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly - 04/05/2011 04:18:24 AM 726 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel - 07/05/2011 02:05:02 AM 902 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly - 09/05/2011 11:29:36 PM 668 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel - 14/05/2011 05:35:56 AM 951 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly - 17/05/2011 02:09:55 AM 576 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel - 19/05/2011 02:47:25 AM 916 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly - 24/05/2011 09:46:30 PM 701 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel - 25/05/2011 12:20:10 AM 982 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly - 31/05/2011 09:16:22 AM 801 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Joel - 10/06/2011 12:04:06 AM 1040 Views
Re: That discussion seems to reduce to "as little new and exotic physics as possible". Dreaded Anomaly - 14/06/2011 03:38:12 AM 805 Views
Re: I still think... (apparently, there is a 100 character limit on subjects, and yours was 99) Tor - 28/04/2011 11:57:15 PM 999 Views
Seems to happen to me a lot; sorry. Joel - 29/04/2011 12:56:14 AM 699 Views
None of this reflects on the actual facts of dark matter. Dreaded Anomaly - 29/04/2011 01:32:52 AM 664 Views
I concede my grasp (or grope) is a somewhat superficial laymans, yes. Joel - 30/04/2011 04:30:28 PM 781 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. Dreaded Anomaly - 30/04/2011 08:56:44 PM 614 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. Joel - 02/05/2011 01:28:58 AM 1132 Views
Re: I concede my grasp (or grope) is a somewhat superficial laymans, yes. Dreaded Anomaly - 04/05/2011 04:18:27 AM 658 Views
I don't object to changing my mind, but can take more convincing than I really should. Joel - 07/05/2011 02:05:09 AM 850 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Dreaded Anomaly - 09/05/2011 11:32:17 PM 777 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Joel - 14/05/2011 05:36:24 AM 929 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Dreaded Anomaly - 17/05/2011 02:10:03 AM 686 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Joel - 19/05/2011 04:33:06 AM 930 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Dreaded Anomaly - 24/05/2011 09:59:38 PM 680 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Joel - 24/05/2011 11:19:43 PM 650 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Dreaded Anomaly - 24/05/2011 11:33:58 PM 603 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Joel - 25/05/2011 12:55:36 AM 732 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Dreaded Anomaly - 31/05/2011 09:16:24 AM 643 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Joel - 10/06/2011 12:09:13 AM 825 Views
Re: I don't object to changing my mind, but can take more convincing than I really should. Dreaded Anomaly - 14/06/2011 03:38:05 AM 803 Views
Might help if you clarified where your skepticism is at Isaac - 29/04/2011 02:32:07 AM 636 Views
Potentially either, or a combination of the two. Joel - 30/04/2011 02:36:50 PM 701 Views
It's hard to discuss without knowing your objections a bit more clearly Isaac - 30/04/2011 04:58:03 PM 615 Views
My primary objection is that alternatives to dark matter seem to have been ruled out prematurely. Joel - 02/05/2011 01:29:14 AM 754 Views
Well, they weren't, not really sure how else to put that Isaac - 10/05/2011 03:19:57 AM 789 Views
Alright then, objection withdrawn (cautious skepticism intact). Joel - 14/05/2011 05:34:17 AM 907 Views
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