Explosions!

Today I attended a physics colloquium; every Tuesday they bring in someone to talk about physics research or a physics topic. Today it was "Recreating Astrophysical Explosions with Combustible Gases in the Laboratory."

Stars are giant fusion reactors, but they mostly convert hydrogen into helium. The heavier elements come from when a star 'dies' and explodes, in an x-ray burst, a nova, or a supernova. The particular system that today's talk was looking at was accretion driven explosions, which happen in binary star systems. Often, in a binary star system, one star will collapse into a neutron star or a white dwarf-- a highly massive, but very small star-- and the other will burn itself out and expand becoming a red giant. When this happens, material from the now much larger star will be attracted to the smaller one, and will be captured by this. As this happens, the small star, due to its compact state, will no longer act as an ideal gas-- when more matter is added to an ideal gas, temperature will rise and volume will increase, which will then cool it back down. In a degenerate star-- that is, the white dwarf or neutron star-- pressure will increase instead of volume. As more mass is added to the star, the pressure rises, increasing the temperature, which then increases pressure again, which then increases temperature; rather than the star cooling itself, it creates a runaway reaction. In this environment, temperatures are such that nuclear reactions compound, and heavier elements are created.

When a nuclear reaction like this happens, many, many transformations happen; an element is created, and before it can decay back down to a more stable state, it will be bombarded again and instead raised up to a new, heavier element.

It is impossible to create this sort of cascading reaction in a lab; the temperatures and pressures required are only present in these super-compact degenerate stars. Creating something like this on Earth would be catastrophic. What we can do, and what our laboratories are trying to do, is create the individual reactions in the cascade, one at a time.

Here is where the talk got really interesting; as much fun as the theory is, it is fascinating to see how these things are actually done.

The Oak Ridge National Laboratory, where our speaker was from, has an enormously complex setup, and I cannot speak about it in great detail because, to my dismay, I don't understand a lot of what it all does. I'll tell what I can, though. They have a cyclotron that produces radioactive ions, which they then send to a massive tower to accelerate them. The principle is fairly simple. They have a huge electromagnet, the top of which they set to positive. The negatively charged ions accelerate upwards towards it. At the top there is a carbon stripper foil; when the ions pass through it, the density and energy are increased, so they can no longer sustain as many electrons. Thus they are turned from negatively charged ions to positively charged ones. Now positive, the ions accelerate bock down the tower, to be directed into the appropriate mechanism from there-- depending on what sort of ions they are, the facility can direct them to a number of places. Today's talk focused on the Nitrogen 17 to Neon 18 reaction, so that is the only set up that we heard about.

In order to create the reaction they wished to measure, they needed three things: Nitrogen 17, Hydrogen, and a lot of energy. Above I discussed briefly the setup to create the high-energy beam of Nitrogen 17; now I'll talk a bit about the Hydrogen. The ORNL facility uses an extremely low pressure gas chamber, with multiple pumps set up to keep the pressure low. The pumps have to create a constant pressure for the gas window, as it is called, so that their measurements will be consistent. The benefit of having these pumps, the speaker told us, was that it doesn't take a whole lot of time to get the machines up and running; this is aided further by the fact that they are being used to lower pressure, rather than raise it.

So now we have a beam at high energy being focused into a target, of the right elements. This will produce the reactions desired-- though not very often. For every reaction produced, you need as many as 10^12 particles to pass through the window. Now we need to filter out the non-reactions so that we can measure the ones we want to look at. This particular reaction emits a gamma ray along with producing the Neon 18; unfortunately, gamma rays do not have a lot of momentum, so the reactions are barely deflected. However, they are at different enough velocities to be filtered that way. Directly after the gas window in the setup is a recoil separator. This device has two functions: it will re-focus the slightly deflected atoms back to where we want them, and it will only allow atoms traveling at a specific velocity to pass through it. The first is accomplished by magnetic fields that are vertical-- perpendicular to the path of the beam-- and the second by electric fields that are horizontal. The parts of the beam we don't want to look at can be deflected away, and only the one we do want passes through.

After that there is a dipole magnet that deflects the beam at an angle, measuring the momentum of the atoms. Now that we have both velocity and momentum, we can measure the mass, to be sure that we have what we wanted to create. There is also a detector that measures energy loss, which will yield the atomic number, after some calculations.

So! We have created a reaction. Now, what can we learn from it? After plotting the data, we can see the resonance strength-- the bell curve will either have a very steep slope, or a more gentle one, and that will show how sensitive the resonance is. (I'm not going to try to explain resonance at the moment; it has to do with quantum energy states, and I do not feel confident enough in my understanding to say much about it.) The resonance strength, it turns out, is directly proportional to the reaction rate-- and that is a very useful thing to know. This particular reaction, for example, has a half life of about 2 hours. This is long enough that we can measure it, with satellites, and based on those measurements, we can figure out what the star was like before it went nova.

If we can get accurate data for every one of the reactions we can produce in a lab, we can extrapolate the more high energy ones from the data. When we have a thorough understanding of every reaction that takes place in a star's explosion, we can create a more accurate model of a star's life. And as I have mentioned before, models are extremely useful things.

"I had no need of that hypothesis"

The above is what Pierre-Simon Laplace said to Napoleon when asked why his book on astronomy, unlike Newton's contained no mention of god. Newton had seen that his calculations were not entirely accurate, and had decided that what kept the planets from careening out of place was the hand of god. Laplace made some corrections, and was able to explain celestial motion without resorting to the supernatural.

I think that quote-- "I had no need of that hypothesis"-- is an important one. The more we learn about the universe, the less we need the supernatural to explain anything. There was a time when we did not understand much, and at that time, gods arose. What caused the lightning, or the rain, or the lack of it? An angry god, punishing the wicked. Likewise, favorable weather was caused by a happy god. Appease god, and things will go well. We know now about pressure systems and how weather works; we no longer have need of that hypothesis. Before Darwin, we had no idea how life came to be, in its many and varied forms. It was reasonable, then, that people believed god had made all the plants and animals, shaped them as an artist might. But Darwin saw a more reasonable way it could have happened; following the natural laws of the universe, life could have evolved to the state in which we now know it. And again, we now had a natural explanation, and have no more need of the god hypothesis. It is no longer reasonable to think that god did it, any more than it is reasonable to believe god causes the weather. We know how life came to be in its present form; we have no need of god.

As science advances, the need for god retreats. What once was seen as the vagaries of an all-powerful deity is now known to be simple cause and effect. It was, I think, simple cause and effect that led me to reject the idea of god. When you pray for something, and it happens, where, exactly, did god step in to give it to you? Say you prayed for a parking spot-- I know plenty of people that pray for that very thing, and other inanities. When you get to the store, there is a spot right up close-- your prayer was answered! But where did god come in? You left the house at such a time that when you arrived, someone else had just left. They left because they had an appointment to get to. That appointment had been set a week before, long before you prayed for that parking spot. Every effect has a cause, traceable by the laws of nature. Where is the need for god?

Some might say that god 'inspired' them to leave the house at the precise time they needed in order to get that parking spot, or 'inspired' the other person to leave at that time, but that falls apart too, if you think about it. Everything you did that day, including your thoughts, had a physical cause. We do not yet fully understand how the brain works, but we do know that thoughts are physical, chemical reactions in the brain. There is no need for a divine finger in your grey matter; you had that thought as a result of what came before.

So there is no need for a god in our day-to-day lives-- and, in fact, god cannot exist there. Everything follows physical laws. A miracle, were one to occur, would break those laws... but there is no evidence of anything breaking them, ever. There are no miracles that have been verified by science as having happened.

What about creation? Well, Darwin put paid to god creating life as it now exists. Abiogenesis-- that is, what sparked life in the first place-- is another matter... but given everything that we now know, it seems silly to simply say "god did it" when we could instead work on the problem and try to find out. And scientists are doing just that. There is no reason to believe that this one thing required a divine touch, when everything else can be explained by science.

And I do mean everything. We know how the planet formed, how stars and galaxies form. We've even got a pretty good handle on the Big Bang. We don't know exactly what sparked it... but there is reason to believe that we do not need a god, even there. I have just enough of a handle on the physics to put it very simply; I don't know it in great detail, but it is an explanation I got from Stephen Hawking, and I know just enough to find it reasonable. The universe, at the moment of the Big Bang, was infinitely small-- small enough, in fact, that it falls into the realm of quantum mechanics. In quantum mechanics, particles do not have a linear sort of existence; they pop in and out of existence, for lack of a better term, all the time. It is therefore not inconceivable that the universe itself, at the time a quantum particle, could have simply popped into existence. No need for that hypothesis, indeed!

But it goes further than that; with what we know of the Big Bang, and of the universe, it seems there is no place for god, period. To begin with, you have to understand that time, like space, is a physical dimension. We exist in a universe of (at least) four dimensions, three spatial, one temporal. This is known as space-time. Space time, it has been proven, can be warped; the most stunning example is that of a black hole. If you were to fall into a black hole, and could somehow survive to observe what was happening around you, time would slow for you, and eventually come infinitely close to a stand-still; you could look out at the universe and see eons flying by. So, time is a physical dimension, contained within the universe, just like space. When the infant universe popped into existence, it contained time, as well as space. Therefore, before the Big Bang, there was no time. Asking what happened before the Big Bang is like asking what is north of the North pole; it is a meaningless question.

Without time, without a before in which to exist, there could not have been a god before the universe that could have created it.

Some people will say that god is eternal, existing outside of time and space; such a god seems pointless to me. In order to influence the universe, whether by creating it, or life, or by intervening in people's day to day lives, god must be part of the universe. If god is part of the universe, then he is bound by the laws of nature (having been created by the Big Bang himself), and cannot violate them, and he cannot have existed before the Big Bang to do any sort of cosmic on-switching. If god is outside the universe, he cannot influence it.

There is just barely room, if you wish to see it this way, for the deist god. It is possible that, in whatever existed before the universe, whatever currently exists outside the universe, there was a being who could have caused the particle-universe to pop into existence. Such a being is not known to have ever influenced anything inside the universe, and so in my mind, speculating about its existence is meaningless. We can only know the universe, at least so far in our understanding. Who knows? Maybe someday we'll reach outside that boundary, beyond what we now know as existence, and find something else there. Since we cannot assume that what lies outside the universe obeys the same natural laws, I suppose things found there could be called "supernatural." Somehow, I think that kind of supernatural would not be very satisfying to many theists.

The Old Boys' Club

When I was born, my dad tells me, he worried about the fact that I was a girl. Not that he thought I (and my sister, for that matter) would be any less capable of achieving our dreams, but he worried that we would not have as many opportunities. That was the eighties-- things were improving, but they were certainly not all the way there yet. They aren't today, either.

My parents did not press gender issues on my sister and I. I was allowed to wear all the frilly dresses I wanted, and my sister was encouraged to do all the sports she wanted. They shopped at "Play Fair Toys," a toy store that sadly no longer exists, where everything was non-discriminatory and Barbie was nowhere to be seen. And I was always, always told that I could do anything I wanted, when I grew up. I was brilliant and capable, they told me, and nothing would stop me if I didn't want it to. As a child, it never occurred to me that girls were not as smart as boys, or not as good at science and math-- to be honest, I always thought the opposite. Boys were strong, girls were smart. Boys were good at physical things, girls were good at school things.

When I showed an interest in science, my parents encouraged that as much as they had encouraged everything else, from the frilly dresses I loved as a kid to the sports Jess played to ballet classes. They-- well, Santa-- got me a telescope and the MASSIVE Smithsonian chemistry set that you probably can't buy anymore because it's unsafe. I was put into advanced science and math classes in school, and my mom took me to a summer chemistry class for kids up at CU Boulder. And when, many years later, I decided I wanted to study physics and attend the Colorado School of Mines, they were proud. No one-- not me, not my family, not my friends-- questions whether I could do it. Of course I could-- hadn't they said all along that I could do anything I wanted?

My dad worried, though. Those old, nagging doubts about my being a girl came back full force, as I prepared to go into a field that had always been and still was very much dominated by men. It's an old boys' club, he told me. Are you sure you are ready for this? he asked. He wasn't trying to dissuade me; Dad worries, it's just what he does. He was trying to warn me. I might never have met anyone who thought I couldn't do something because I was a girl, but they were out there, and choosing to study physics and go to Mines, I was increasing the chances that I would run into them.

This is something I think about a lot; the fact that so many people have warned me about something that I have never felt the effect of. I wonder if I am merely oblivious to it, or perhaps if other people are more sensitive. I wonder if perhaps I was always so far ahead of everyone else that it didn't matter. (Yes, I have a healthy ego. It's also not an unreasonable thing for me to wonder.) If I had been less advanced, would people have discouraged me because of my gender? Or was I just incredibly lucky, to have led a life where such things never touched me?

Every year, the American Physical Society (a professional organization for physicists and people interested in physics) sponsors a Convention for Undergraduate Women in Physics, on about half a dozen campuses around the country. This year, in January, several of our students went, and they offered to have Mines be one of the host campuses next year. One of the really interesting things about this convention is that it is organized almost entirely by students; they were asking for volunteers to help out. So I signed up; I'm on four committees. Not only is this something that I think is a really good idea, something that I care a lot about, it's also something that will look really, really good on a resume.

Next year is going to be a really busy year...

LASERS

You know what is really cool? LASERS.

I put that in all caps for two reasons. One, not everyone knows (or remembers) that the word "laser" was originally an acronym. It stood for Light Amplification by Stimulated Emission of Radiation. Of course, it has become a word-- everyone knows what a laser is, and it's perfectly fine, grammatically, to write it just like that. I write it in all caps, mostly, because lasers are FREAKING COOL.

Everyone has seen, probably even used, a laser. Laser pointers are common technology (though when I was a kid and desperately wanted one, they were at least $50, I checked), CDs and DVDs are read by laser, barcodes are scanned by laser-- lasers are everywhere. The laser sight on a sniper rifle is such a well known piece of technology that a red dot appearing on someone in a movie is an instant source of tension-- that person is going to be shot!

The scientific uses for lasers are even cooler. The distance from the earth to the moon is measured by bouncing a laser off a reflector left by one of the Apollo missions. Lasers are used in microscopes to image extremely small things with great accuracy. Lasers can cut more precisely than any mechanical process. Lasers can perform surgery where no incision is necessary. They're used in spectroscopy, which is a really cool concept on its own.

So how do they work? Fairly simply, actually. You pump light into a medium that will amplify it, and then focus it out one end as a beam. Of course, it's not as easy as it sounds. You have to find the right medium so the wavelength that you want is amplified, you have to use an external power source, and so on. But really: create a tube, with mirrors on both ends, so light will reflect. Fill it with a gain medium-- it could be as simple as glass or neon, like a neon light, mixed with helium-- and then introduce a light source. For some, it could be an electrical pulse, like the helium-neon mix. For others it's a light, or a chemical reaction. It depends on what you're using as a gain medium. Make sure that one of the end mirrors is partially transparent, so the amplified light can go somewhere, and you've got a laser. Many will then put a cap on the end so that the light can only go out through a small hole, making the beam small and focused.

So, one of the really cool things about studying physics at this school is that there are a lot of opportunities to play with lasers. All of the labs on the top floor of the physics building have warnings posted on the doors about lasers. And we have some really, really cool lasers. We've got one setup that's doing microscopy-- that is using a laser as a microscope-- that literally counts photons, yielding a much clearer picture than other systems that amplify to that degree. They've got one that can give a 3 dimensional image of a fly's brain. But the coolest one is the one doing something unexpected.

They created a setup that focuses a beam in four dimensions-- that is, in the three spatial dimensions, and also in time. If you're not used to thinking of time as a dimension that's going to either sound nonsensical or extremely simple. In practice it is neither; this was quite an accomplishment. So they did what any good physicist would do with a brand new, very cool laser: they pointed it at stuff. One of the things they pointed it at was glass. Glass is an interesting thing-- it is structurally the same in all directions, because it doesn't have any sort of crystallization. It's kind of an amorphous blob, really. So, one would expect that, when carving things into it with lasers, the direction the laser was moving wouldn't matter-- the whole area touched by the laser is affected the same way. Generally, that's true. Not with this one. With this one, they get one kind of mark when moving to the left, and an entirely different kind of mark moving to the right. The conclusion they came to: their laser is somehow tiled in TIME. One side of the laser is hitting the glass slightly before the other. And they do not know why. So now there's a senior design project trying to take pictures of this phenomenon. And succeeding. At taking pictures of LASERS.

My school is awesome.