Hi everyone, ben here! I’m trying to experiment right now with a NEW MEDIUM:
SCIENCE INFOCOMICS.
we’re going to do our best, guys. I’m going to try to get better pens, and find some better paper as we go. So without much more rambling:
DARK MATTER: It’s not hocus pocus.
So. Dark matter…
Hey ben! Civilization has Collapsed! And we need to know how much the galaxy weighs! What should we do? (the answer is “stop using ballpoint pens they’re awful”)
Two ways to figure it out: directly (though counting) and indirectly (deduce it from evidence)
Ok, so that’s one way to tell how much our galaxy weighs. what about the other one? how does that work?
Due diligence requires that I outline the special trick* which lies at the heart of the physics of the next reasoning bit. THE NEXT PANEL IS OPTIONAL FOR PEOPLE WHO WANT TO UNDERSTAND WHAT IS GOING ON.
The moral of the story is that the gravity of a big system of orbiting planets is WAY SIMPLER than you would guess from the outset. all you care about is the TOTAL mass of all the stars lying inside your orbit. everything OUTSIDE your orbit doesn’t have an effect.
Great! we can use the velocities of stars going around the galaxy to figure out how much mass the galaxy has in it.
BUT HOW CAN WE TELL HOW FAST STARS ARE MOVING? the answer comes from The special theory of relativity: Redshift and blueshifting of the colour spectra of stars!
ok. well, that answers that question. but there’s a problem with what we’ve observed so far:
So that’s the origin of the DARK MATTER MYSTERY! What could it be?
I should probably do my due diligence and qualify the things i just said.
MaCHOs are probably not really candidates anymore due to observational evidence. but they’re important to add because, back in the 90’s dark matter was all about MaCHOs vs. WIMPs. which is hilarious.
Modified Non-newtonian theories of gravity are popular; but Einstein’s General Relativity (the modern theory of gravity) is very successful when it comes to satisfying experimental predictions, and relativity seems to agree with Newtonian gravity at the length scales we’re talking about. So the non-newtonian gravity game includes more than just changing the (1/r^2) force law; the theorist must also kind of address how GR will change. there are a lot of candidates, but none have really caught a foothold.
WIMPs are the most probable candidate theoretically, since their gravitational signature has been seen astronomically (see the Bullet Cluster), and they’re an important component in modeling the evolution of the universe. We’ve also measured the effect of their gravity in cosmological evolution models. In other words, since they were proposed, we’ve found that their inclusion is integral to modeling the cosmos. There are a variety of physicists who are currently concerned with detecting them, and there are a lot of different guesses as to what these shy particles might be.
So I only understood about half of that, and I was having to dig back into my memory of my Astronomy class like 4 years ago, but it was interesting. Now if only all my science classes (which I hope I will never-ever have to take again) came with a helpful little man with a top hat and a monocle. I think I might have actually stayed awake in Bio if he had popped up everyone once and a while.
(or the penguin. penguins are just awesome)
Cool! I definitely vote for more of this medium. Aside from the grainy-ness of being scanned or the pen or what not, it was very easy to read and understand. The doodles are awesome, too!
I’m a bit curious why massive bodies outside the orbit radius don’t have an effect on it, but I can probably look up the answer to that myself. :D Thanks Ben!
hey liesl,
the argument that “the things that lie outside your orbit don’t matter” comes as a result of a fairly true simplifying assumption: where you assume that the matter is distributed in a kind of circularly symmetric way: if the matter density only depends on the radius.
if this assumption is true, then the gravitational pull from everything outside your circular orbit will cancel each other out. while there is more stuff across your orbit from you, it’s also farther away that the stuff right adjacent to where you are. um. i can’t draw you a picture right now, but that’s okay because i never found the diagram for this argument to be all that convincing… but the mathematics work out in a very elegant way. … … if that’s any consolation. … … … maybe i should try harder.
So, I’m not a particle physicist by any means, but I did have a very convincing Modern Physics professor my sophomore year…
His argument was that most Dark Matter theories utilize very specific subsets of String Theory to guestimate the composition of Dark Matter/Dark Energy. (conveniently, we had a guest speaker come and give a lecture on exactly that: a string theory solution to DM/DE) However, in the professors opinion, String Theory is a poor base to build off of, because after 50 years of research, not a single experiment has been performed to “prove” any aspect of String Theory. And as a result, the advancement of String Theory relies not on the accuracy of ones predictions relative to observed phenomena, but rather on how “elegant” your mathematics are. It’s like Occam’s Razor taken to the extreme. “The simplest solution is the best solution, even if you haven’t clearly defined the problem”
So the logic goes like this:
– IF String Theory is accurate, AND
– IF this certain, specific sub-theory is accurate, AND
– IF Dark Energy/Dark Matter is explained by these particular theoretical particles/interactions, THEN
– Dark Matter/Dark Energy exists.
So this professor was a fan of modified theories of gravity, because he supposed that it is more likely that we have some aspect of gravity misunderstood (graviton, anyone?) than that we have made some REALLY AWESOME guesses at String Theory over the past 50 years.
So my question is this: Is String Theory still necessary to explain the theory (of WIMPs for example?)
oh, um. as i understand it there are both wimp models of particles from string theory, and also modified gravity models based on string theory. but. you don’t really need string theory to talk about dark matter (or dark energy, which is peripheral to this issue).
the logic goes:
-If our observations are to be believed, AND
-If general relativity, and its limit newtonian gravity are correct, THEN
-the galactic dynamics we observe (and cosmological observations that i didn’t get into) require the addition of some kind of dark matter to the bright shiny matter for the theoretical mechanics to match the observations.
so it’s not like the issue of dark matter was motivated by string theory… string theorists are just using it as an opportunity to apply their theory.
and i’m not even sure that all wimp candidates even require string theory… I think that there are some that don’t? (string theories, to me, are like the animals at the bottom of the ocean.)