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Dark Matter: Is it right in front of us?

January 31, 2008 on 3:46 pm | In Astronomy, Dark Matter, Gravity, Physics, Scientific papers | 9 Comments

Last year, I had just finished my Ph.D. studies, and had moved to Madison, WI to teach introductory physics at the University of Wisconsin. I was working on this paper, and when I submitted it, I got a phone call from New Scientist magazine’s space division.

Fast-forward two weeks, and I find this article online, where I got to see my name in print:

Now, scientists led by Ethan Siegel of the University of Wisconsin, in Madison, US, have come up with a new way to potentially reveal blobs of dark matter drifting nearby and perhaps even pin down what it is once and for all.

And I thought to myself, wow, I actually did that! It’s still a neat idea, a year later, as I’ve kept working on it and just submitted another paper extending my work. Let me break it down for you, and tell you what the dark matter might be, and how we might be able to find it.

We know, pretty simply, how stars work. We know how much light they give off, and that’s very closely related to their mass. Brighter stars are more massive; dimmer stars are less so. So when we look at an entire galaxy, what we’re really measuring is all of the light coming from the stars in it. We know, therefore, how to tell how much mass is present in the stars in a galaxy, or in a cluster of galaxies, or in any astronomical object we look at.

But we have a different way of measuring mass as well: we know how gravity works. So we can watch how galaxies rotate,

or we can look at gravitational lensing, and we can measure how much mass is there from gravity. This way, we know how much total mass there is, and we can see how much of it is from stars.

The problem, as you might expect, is that the numbers don’t match up. In fact, what we find is that 98% of the mass is not in stars. Not only is it not in stars, but of that 98%, only about 13% is made up of protons, neutrons, and electrons; we have very little idea what the remaining 85% of the stuff in our galaxy is. So we give it a name, dark matter, since it has mass, and exerts a gravitational force, but doesn’t give off light or interact through any of the other forces. And figuring out what dark matter is will be a nobel-prize-worthy discovery, if we can detect it.

We know from observations that all galaxies have a dark matter halo surrounding them; what we don’t know is whether the halo is smooth, or whether there are clumps of dark matter in them. When we do numerical simulations, though, as well as calculations of different models, we find that they all wind up with clumps in them, like so:

Well, there’s a neat trick we can use to see whether there are these dark matter clumps in our galaxy. When you look out at anything, the light travels to your eye in a straight line. But if there’s something with mass in the way, the mass will bend the light around it according to Einstein’s theory of General Relativity. If the mass is close to the line-of-sight, the bending is more severe; if the mass is far away, the bending is very slight, and the actual position lines up very well with the apparent position, like so:

Well, these theoretical dark matter clumps are too light and too diffuse to cause a visible effect in starlight. But, there are special stars that emit radio light at such high precision that they could see the difference between a dark matter clump being close to it and one being farther away. These stars are called pulsars, and they are so well-measured that the best and fastest of them rotate nearly 1000 times per second (and remember, these are stars weighing as much as the Sun does), and have their periods known to within 10-18 seconds!

So, if we have dark matter clumps in our galaxy, watching pulsars could allow us to find them, since the pulse timing will change as the dark matter clumps move through the galaxy. Right now, the odds aren’t very good because we don’t have enough pulsars, and our radio telescopes aren’t good enough. But if they build the Square Kilometer Array, the odds improve tremendously, so much so that we should detect dark matter clumps if they exist using pulsar timing! If we don’t see them, it means that these halos aren’t nearly as clumpy as we think they should be — either way, this would be the first chance we have to learn anything about the dark matter in our own galaxy!

Also, FYI, every week (or so) many of the space-related blogs get together and share their best posts. It’s called the Carnival of Space, and this week my post on visualizing the Milky Way is on it!


Centripetal vs. Centrifugal force, & the Solar System

January 28, 2008 on 10:24 pm | In Astronomy, Gravity, Physics, Solar System | 34 Comments

One of the worst teaching tools physicists use (and they almost all do it) is to tell students,

There’s no such thing as centrifugal force.

What can you do when the top physics education website says, “It is important to note that the centrifugal force does not actually exist. We feel it, because we are in a non-inertial coordinate system.” There’s a very funny comic over at xkcd that goes as follows:
xkcd's James Bond's Doom!

Well, what’s the deal? What really goes on, physically, and what causes a centrifuge to work? Is your physics teacher right, or is there more to the story than “the centrifugal force does not actually exist?”

So, your physics teacher is partially right. When an object moves in a circle, it’s moving, at any instant, tangentially to the circle, and it’s the centripetal force, causing an acceleration towards-the-center, that keeps it moving in the circle.
uniform circular motion

But that’s not the end of the story, or I wouldn’t be posting this. Do you remember Newton’s third law? It says for every action, there’s an equal and opposite reaction. That is, for every *force*, there’s an equal and opposite force. Take the photo below:
Newton's 3rd law in action!

Notice how the man’s face getting pummeled is not only experiencing a force from the fist, but the fist experiences an equal and opposite force! (And if you don’t believe me, go punch a brick wall.) Well, if there’s a centripetal force acting on an object, pulling it towards the center, then there’s a centrifugal force from the object reacting to it, pushing things away from the center. If it’s a ball attached to a string, the string pulls the ball towards the center, and the ball pulls the string away from the center. If it’s a wall pushing a person towards the center, the person pushes back against the wall, pushing it away from the center, like so (that’s why the walls need to be sturdy):
The Gravitron is a Human Centrifuge!

So, that’s an example of a real centrifugal force. But beyond that, how does a centrifuge actually work, with only centripetal forces acting on the stuff being centrifuged?
Laboratory Centrifuge

Centrifuges spin really fast, causing the stuff inside to separate out according to density, with the most dense pushed out the farthest, towards the bottom, and the least dense winding up at the top, closest to the center. Because we don’t know the difference between gravity and any other acceleration or force, centrifuging something is basically like turning the knob on gravity way up — by up to a factor of 15,000, depending on the centrifuge! In zero gravity, things don’t sort themselves by density, but in a high enough field of gravity, they do. Even solids. Like this thing:
layers of the Earth

The centripetal force of the sides of the centrifuge push back just like the seat of your chair pushes back against you when you’re in a gravitational field. The faster the centrifuge, the harder the sides push back; and the whole thing acts like an enhanced version of gravity. Well, what does this have to do with the Solar System? Sir Arthur Eddington once described all the life on Earth as follows:

We are bits of stellar matter that got cold by accident, bits of a star gone wrong.

While it’s true that all of the elements on Earth that we know and love (except for hydrogen and helium) were formed in stars, the Sun is almost all hydrogen and helium, and the planets are almost exclusively heavier elements! How did that happen? Was it an accident, as Eddington suggests? No; it was centrifugal force pushing the heavier elements (like carbon, oxygen, nitrogen, iron, phosphorus, silicon, etc.) away from the center relative to the light ones!

So the Earth, and for that matter, all other planets, are made out of denser elements than stars are. And the reason is all due to a force that your physics teacher probably told you doesn’t exist!

UPDATE (January 29, 2008): It occurred to me that some of you might like a way to *test* this. It’s well known that solar systems form from dusty disks, known as proto-planetary disks. If what I’ve just articulated is correct, the material closest to the center of the disk should be preferentially less dense than the material farther away. We don’t have a dusty protoplanetary disk around our sun, but we have an analogous, dusty disk around one of our larger planets:
Look at the dusty disk!

Any volunteers to test it out?


Brain-damaged arguments and Boltzmann Brains

January 28, 2008 on 8:21 pm | In Astronomy, Evolution, Physics, Q & A, Scientific papers | 45 Comments

Okay, so I got a question from my friend Tamara, who’s a high school teacher in my hometown of New York City. It concerns a recent article she read on the front page of the New York Times about something funny that us scientists are calling Boltzmann Brains.

I’ve read this article three times since it was featured on the front page of the science section in the NYT and I’m still confused about the Boltzmann brain problem, it’s (non?)validity, the reason it made it’s way onto the front page and whether Emerson’s philosophy about imagined worlds came from this…

There’s a lot of interesting stuff going on in cosmology and astrophysics, and so when I see something that’s, well, illogical (and that’s being nice) getting a lot of attention, I have to take issue with it.

The argument of it all is that the amount of entropy (disorder) in the universe is fairly large. Since there is a lot of entropy, and a lot of random fluctuations in the universe, there’s a low probability that a very unlikely fluctuation has occurred in our universe. That part is fine. The second part, where they say that brains are very unlikely random fluctuations, isn’t a problem either. That’s true as well. The third and final part is the problem, where they draw the conclusion that since conscious brains are unlikely random fluctuations, it’s more likely that we live in a universe with only one conscious brain (presumably, yours, mine, or the theoretical physicist proposing it) than a universe with billions and billions of them. This is wrong, but first let’s make sure you understand the Boltzmann Brain proposal (and for another, less scathing account of it, see Sean Carroll’s post over at cosmic variance):

  1. We live in a universe full of random fluctuations.
  2. We are the result of an unlikely, but not impossible fluctuation.
  3. It is more probable that we are the result of a more likely, rather than a less likely, fluctuation.
  4. A universe with one brain that results from random fluctuations is more likely than a universe with billions and billions of them.

Although there have been a number of scientific papers in the last couple of years on this topic, none of them were written, apparently, by anyone with a very deep understanding of biology. While the first three points in my list are undoubtedly true based on what we know about physics and probability, the last point is not. Why not? Because your brain is not a random fluctuation.

There is not a person alive who knows how the mechanisms of biological evolution are derived from the laws of physics, but even the most elementary student of biology knows that it takes a population of organisms to evolve. While the life that has evolved on our planet is the result of a random fluctuation, the entire 14 billion years of evolution in the universe that has led to our existence at present isn’t a random process; we are the result of a random initial condition undergoing physical, chemical and biological evolution according to intricate natural laws. And since, once self-replicating life exists, sexual reproduction is a more successful template than asexual reproduction, it evolved. One can only assume since human brains evolved, those parts of us are successful templates as well, and are selected for, naturally.

In the absence of any organizing principles or selection laws, yes, the Boltzmann Brain argument is valid. But ignoring biology and biological evolution doesn’t make it go away, and it doesn’t make the Boltzmann Brain question any more interesting than asking how often a box full of deconstructed watch components will form an assembled watch when you shake it like a polaroid picture. Because we do have the laws of evolution in our universe, I’m not losing any sleep over the notion that I might be the only working brain it. Some days are long enough without speculating on the lack of brainpower in others.


Question: The Milky Way to an outsider…?

January 25, 2008 on 5:35 pm | In Astronomy, Q & A | 10 Comments

I got a great question earlier today from my buddy Zrinka, and decided to figure out the answer for her, and also for myself. She asks:

Ethan, is it possible to know, or better to say to imagine somehow how our galaxy looks from outside?

What a simple-sounding question! After all, we know what the Earth looks like from the outside: we just go outside of it and photograph it. But the galaxy is too big to do that to; it would take tens of thousands of years moving at the speed of light to get that far away! So we’re left with the option of looking at our galaxy from inside of it, and trying to figure out what it looks like.

It’s more difficult than it sounds. Consider this: what color are your eyes? Mine

Ethan's eyes

are the brown shown here; but how do I know? I either need a reflection, another person to view it and tell me the answer, or, I can take a picture and look at it. The problem is, we can try that for the milky way, but since we’re in it, all we see is this:

Milky Way

Pretty hard to tell anything with all that junk (we call it dust) in between us and the rest of the galaxy. We can launch a satellite and look with infrared light, where the dust isn’t so important, and that’s what the COBE satellite did with its infrared imager:

Well, that looks a lot like other edge-on spiral galaxies that we see, like NGC 4013:

Edge-on spiral

That’s basically what we’re stuck doing — looking in our own galaxy for the little bit of information we can find, then looking out at the millions of galaxies we know of and seeing which ones match the best. We’ve recently (in the last few years, with the Spitzer Space Telescope) discovered that in addition to the four major spiral arms, the lesser outer arms, and the central bulge, our galaxy also has a central bar about 3 kiloparsecs (10,000 light-years) across.

So now, we know a ton about our Milky Way. But what does it look like to an outside observer? Or, maybe a better question, is of all the galaxies we’ve seen, which one matches up best to the Milky Way? The answer is NGC 7331, which looks like this in the visible:

Milky Way in the Visible

and this in the infrared:
NGC 7331 in the IR

The other option is to go for the “artist’s rendition,” which just doesn’t do it for me. But in any case, hopefully this has given you a lot of help towards “visualizing” what we look like! If you ever get far enough away to take a picture, you can try to send it to me, but don’t bother; I’ll most likely be dead by then.


An amazing video on Evolution and Intelligent Design

January 24, 2008 on 10:58 pm | In Evolution, Video | 4 Comments

Evolution, creationism, and intelligent design are words that many people have extremely strong opinions about. Regardless of how you feel about why the laws of nature are what they are, which have evidently allowed us to exist, the evidence for the validity of the theory of evolution with one major mechanism being natural selection is absolutely overwhelming. That said, this is often very hard to communicate to people, especially those with strong biases against what they perceive as the implications of evolution, how evolution works, and why the case for it is so compelling.

Thankfully, there is a person calling him/herself cdk007 making youtube videos like this one to explain how evolution works, and why arguments against it are invalid, with dauntingly demonstrative examples:

The explanation basically boils down to, regardless of how life started, once you have even the simplest life in place that reproduces, mutates, and is subject to natural selection, you will get evolution. While reproduction and mutation are random (with far more variation occurring in sexual reproduction over asexual reproduction), natural selection is not random. The fact that certain traits are selected causes events that would be exceedingly improbable at random to occur all the time.

This video is excellent, and there are many others (the Bad Astronomer likes this one); I recommend them to anyone who is looking for simple, compelling examples of how this complicated biological process works. In fact, the only explanations that I’ve ever found easier to understand were watching Episode II of Carl Sagan’s Cosmos series and watching the off-Broadway play Trumpery, a fascinating play about the scientific and personal struggles of Charles Darwin (and I think Michael Cristofer deserves to be showered with accolades for his performance as Darwin).


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