After doing a lab about galactic rotation curves, the following xkcd comic has gained more meaning for me;
Escalators
If this isn’t explicit enough for you, please compare it to the following;
Shock and awe.
Update: For those of you who are less astrophysically inclined, this may have been a little confusing. Also, some of us, like Charles, believe that the xkcd comic is coincidentally the exact same graph as the one for galactic rotation curves. Quote from Charles: “I’m sure the Ballmer peak looks exactly like another graph somewhere in the world.” That may be true about this comic as well, but I feel like the graph is too significant for this to be a coincidence (possibly because I spent about 12 hours staring at graphs like this a week ago for my astronomy class).
For the uninformed among you, the galactic rotation curve is sort of the reason why we believe in dark matter. Basically, in almost any galactic model, we know that the enclosed mass at any radius, Menc, relates the centripetal force and the gravitational force as
mv2/Renc = GMencm/R2enc, or v =√(GMenc/Renc). Depending on the form of Menc as a function of R, we should get several different possible velocity vs. distance from galactic center curves. With the distribution that we expect, with most of the mass in the center of the galaxy making a big bulge, we’d expect that M(R) = M, constant, since that’s what a galaxy looks like. For example; 
This is an artist’s impression of the Milky Way Galaxy, and as you can see, there’s a huge bulge in the center, which contains a supermassive black hole, Sagittarius A*, which has the mass of about a million sun-like stars. From what we can see, there’s tons and tons of matter at the center of the galaxy. So we expect v(R) ~ 1/√R, but if we look at the observed graph, we see that this is not the case, we see that the velocity stays roughly the same as the distance from the center increases. This means that there is way more mass in the galaxy than we see; what is it? We don’t know, but it isn’t luminous, so we call it dark matter. Essentially, the discrepancy between what we see and what we observe in this graph proves that we really don’t know what’s going on, and it’s these moments in science that push us forward, and force us to reevaluate our view of the universe; it’s graphs like these that make science exciting.
My point is basically that this graph is far too significant to accidentally pop up in a joke about escalators. Without the galactic rotation, it’s kind of cute, but when you compare them directly, you see that the analogy between the two is perfect. Please leave comments disputing or supporting this and Tom and I will attempt to address them.
#1 by Martin Ye on December 11, 2009 - 9:40 am
LIKE
#2 by rachel on December 12, 2009 - 3:58 pm
Conclusion: escalators are actually made of dark matter. Or maybe adulthood is. They tell me I’m supposed to feel like a grown-up, but it’s not happening yet.
This is seriously interesting, though–I didn’t know the reasoning behind the theory for dark matter. I’m not sure I know enough about the physics to make a strong case for or against this, but the general concept of it is fascinating.
#3 by duncan on December 12, 2009 - 4:12 pm
On Man Day, I ran up the escalator at Alewife and bloodied myself up pretty well. Good decision.
They often find too much mass not only inside galaxies but also between galaxies, when you look at galaxy clusters, and also sometimes there are dark objects that act as gravitational lenses when we look at distant objects. If you would like a lecture on dark matter, this would be a great week to ask, because I’ll be spending all week studying for my astronomy final.
#4 by rachel on December 12, 2009 - 10:29 pm
You’re giving me so many interesting things to read about when I should be reading about the biology at the bottom of the ocean. Dang it.
How do people know there’s too much mass–or dark matter, I guess, is what you’re implying (correct me if I’m wrong)–between galaxies when those areas aren’t rotating? Or are they? I was also looking at the wikipedia page on gravitational lenses. They look pretty neat.
#5 by tom on December 13, 2009 - 12:03 am
So it turns out that if you have some dynamic system of gravitationally interacting bodies (like a cluster of galaxies, or even a cluster of stars), the kinetic energy of the system (which is dependent on the velocities of stuff) is related to the gravitational potential energy of the system (which is dependent on the masses and distances between the stuff) via the “Virial theorem” (http://en.wikipedia.org/wiki/Virial_theorem). So, if you see a cluster of galaxies and get a sampling of their velocities, you can transform them into kinetic energies, use the Virial theorem to see how much gravitational pull there is, and get a mass of the system of it. Which might be a lot more than if you just added the masses of the individual galaxies together (even accounting for dark matter inside each galaxy)!
Also, sometimes this happens: http://en.wikipedia.org/wiki/Bullet_Cluster
Hey, you should tell us some cool things about ocean-floor life.
#6 by rachel on December 13, 2009 - 8:47 pm
One neat thing about ocean-floor life is that some animals near hydrothermal vents don’t have mouths. They don’t eat, but they have gills that take up nutrients from the volcanic vents (like hydrogen sulfide–yum yum). Instead, they’ve got these colonies of chemoautotrophic bacteria that live inside of them and use these really poisonous compounds to produce energy and the synthesis of carbohydrates for the animal. It totally redefined the way that we think about life existing in places without sunlight.
There’s also the fun feature of deep-sea gigantism, where normal-looking creatures grow to enormous sizes, when they’re much smaller in other places. For example, our TFs brought in an isopod (think roly-poly or pillbug) from the deep sea–except it was the size of my head. I got to hold it, dead and frozen of course, but still neat.
But I digress. And ought to be studying. Wooo–so close to being done!
#7 by Vicky Ge on February 11, 2010 - 1:11 pm
I will make this the viable alternative to HarvardFML.