Questions for Prisca Cushman

Learn more about my research In January, 2008, Prisca Cushman answered visitors questions about physics.

Your Comments, Thoughts, Questions, Ideas

Joe's picture
Joe says:

What inspired you to pursue a career in science?

posted on Thu, 01/17/2008 - 9:33pm
Prisca Cushman's picture

Hi Joe!

In one sense, I don't really know. I didn’t have a special mentor or scientist parents or some pivotal experience. In grade school I just read all the Astronomy books I could at my small public library and dreamt about space exploration. As a child growing up in the 60’s, I was sure that by 2008, we would be traveling back and forth to the moon on a regular basis. If I cannot explore the Universe by spaceship, at least I can “boldly go where no one has gone before”, by science discovery. Either way, there are mysteries to encounter and puzzles to solve.

One of the most remarkable things that has happened in the last couple decades is that new tools have been developed which also allow you to travel back in time, observing structures created in the very early Universe by looking at extremely distant objects whose light has taken billions of years to get here, and by studying small fluctuations in the microwave radiation still bathing us in the cooling afterglow of the Big Bang. That we can piece together the history of our Universe based on satellite and earth observations is indeed a remarkable voyage of our imagination.

posted on Fri, 01/18/2008 - 9:55pm
Anonymous's picture
Anonymous says:

Do you have to ride a scary elevator all the way down to your office deep under ground? How far under ground is it? Is it scary working down there?

posted on Fri, 01/18/2008 - 2:38pm
Prisca Cushman's picture

I work half a mile underground. I guess it would be scary if I thought about all that rock pushing down on top of me, but the lab itself is a clean and brightly lit place with offices and a lunch room and even a ping pong table. Now, it is true that the bats get in quite often and there you are writing something in a log book and, swoosh, one of them gets awfully close! But I like bats, actually.

The ride down is in the very same elevator the miners used in the last century when the Soudan Mine was a working iron mine. That is because the Mine is also an Historic State Park and they have to keep the part outside the lab just like it was in the past. The elevator ride IS a bit scary! It rattles and shakes and there is no light in the elevator. You can take a tour of the mine in the summer (and go down the very same elevator... but the tour guides always carry a flashlight, so don't worry). You can read more about it here.

posted on Fri, 01/18/2008 - 9:45pm
Anonymous's picture
Anonymous says:

What is dark matter? Seems to be a lot of talk about it lately.

posted on Fri, 01/18/2008 - 2:39pm
Prisca Cushman's picture

Dark Matter is what we call the extra mass we need to explain the motion of the stars and galaxies. There is just not enough mass if we count up all the stuff that glows (stars, nebula, etc) and all the non-luminous stuff we detect by other means (like planets and dust, etc). In fact, it looks like the dark matter makes up 80% of the mass of the Universe and the stuff we know about is only 20%. That is big "missing mass" piece! It would be fair to say that our Milky Way galaxy is really a dark matter galaxy with just a smattering of stars and planets.

Our best theory (from lots of indirect evidence and measurements) is that dark matter consists of elementary particles that were created in the Big Bang and have survived until today. Over the eons, the particles were attracted to each other (because they have mass and therefore gravity will affect them) and they actually created the galaxies, dragging the stars along with them. An elementary particle is just something that doesn't get subdivided any more. An electron is an elementary particle, for example. Each dark matter particle probably weighs about the same as a nucleus of silicon, but it is not made up of smaller parts, like a nucleus is.

The other thing about the particles is that they do not feel the electromagnetic or nuclear force. This means that they pass right through normal matter without being deflected by the charges in the atoms or the nucleus. Not, that is, unless they get really close to the nucleus. Only then do they feel what we call the "Weak Force". So this means that billions of them are raining down on our bodies every second, but they just pass harmlessly through us. Only 1 in 10 billion will ever interact with our detector, for example. It is for this reason that they are called WIMPs = Weakly Interacting Massive Particles.

posted on Fri, 01/18/2008 - 9:40pm
Ann and Miriam's picture
Ann and Miriam says:

Miriam, age 10, wants to know if there are colors involved with WIMP's.

Ann wants to know what made you look for these particles or what made you think there might be such a thing.

posted on Tue, 01/22/2008 - 8:14am
Prisca Cushman's picture

Hi Miriam – to answer your question first, WIMPs do not interact with light at all. They don’t shine and they don’t reflect light. Since color is the way we experience different wavelengths of light, the WIMPs cannot have any color at all. That is why they are “dark matter”. This makes it very hard to detect because most of our telescopes look for some kind of light coming from an object, even if that light has a color beyond what our eyes are sensitive to (like ultraviolet, or infrared, or even X-rays which are just a very short wavelength of light).

So then, Ann, you might ask, if you can’t see it, “what made you think there might be such a thing?” Oh, that’s right, you did ask that! Well part of it is this business about the stars in our galaxy moving so fast that they should have flown apart long ago, but something with mass is holding them together. It could be most anything – perhaps its just lots of big objects like asteroids or maybe dwarf stars that didn’t ignite. These are called Massive Compact Halo Objects or MACHOs. Yes, these names are all very bad science jokes. Anyway, there was a serious search for MACHOs by looking for the way their gravity would distort light that passed near them. The survey took more than a decade and by the end, it was clear that there were not anywhere enough MACHOs to make up this missing mass. Modeling the way galaxies form also gives a clue. Without the missing mass, the size of the galaxies and the distance between them would be different than it is today.

There are other clues as well, and they all point, not only to some sort of invisible mass, but to the same amount of invisible mass. Add to that the theory of Supersymmetry, which was not invented to solve the dark matter problem, but ends up predicting a whole class of new weakly-interacting particles with just the right amount of mass. It sure is suggestive. So if we could actually detect a few of these particles, then we could clinch the deal, so to speak.

posted on Tue, 01/22/2008 - 11:52pm
Anonymous's picture
Anonymous says:

What is the story behind the 200-year-old lead from a french shipwrek that shields your equipment from radioactivity? There has got to be a story there.

posted on Thu, 01/24/2008 - 6:41pm
Prisca Cushman's picture

All lead has trace amounts of naturally-occurring radioactive isotopes (uranium, thorium, bismuth, radium, polonium, cobalt, potassium) just like our air, water and soil. These very low levels are still not low enough for our experiment. We need to find lead where the contaminants introduced during the smelting have decayed away. Since the half-lives of these isotopes are measured in centuries, we look for lead that was smelted in historical times. The best lead comes from Roman pipes or ballast from ancient shipwrecks. However, this lead has archaeological significance and it is hard to justify using it to shield our experiment. We bought our lead from a French company, and have been told it was from a wreck in the Mediterranean. We cannot get further information because it is a somewhat sensitive issue.

posted on Sat, 01/26/2008 - 8:06pm
Anonymous's picture
Anonymous says:

So, are WIMPs real?

posted on Thu, 01/24/2008 - 6:43pm
Prisca Cushman's picture

Well, what would it take to prove they are real? Are electrons real? Have you ever seen one? We infer their existance because experiments detect electric currents when atoms are ionized - we say that the electrons are ejected. We see spectral lines (light of only one frequency) when atoms are heated - we say the electrons are jumping from one energy state to another. But no one can see an atom or the electrons and nucleus inside. They are real in the sense that every experiment is consistent with the model we have invented to describe them.

RIght now, we have a good model of how much stuff there is in the Universe, how much is mass and how much is energy and how much of the mass must not interact with light. So I am confident that "dark matter" is real and it was created at the beginning of the Universe. The next question is "what is the dark matter is made of?" Evidence points to weakly-interacting massive particles or WIMPs. However, no one has detected an individual WIMP and measured its mass. So I am open to the possibility that WIMPs are not the right answer, but something more exotic like "axions" or even extra dimensions. If we detect particles in our germanium crystals, then I would be prepared to say that WIMPs are real.

posted on Sat, 01/26/2008 - 8:31pm
Anonymous's picture
Anonymous says:

How do you look for WIMPS?

posted on Sat, 01/26/2008 - 5:05pm
Prisca Cushman's picture

We are looking for a direct interaction between the WIMP and a nucleus inside our detector. Our nuclei are inside a germanium crystal. Other people use different materials, like liquid xenon or even sapphire. The whole point is to have enough target material (enough nuclei) and wait long enough that one of the WIMPs bumps one nucleus. This is "exposure" and is measured in kg-days. There are many different ways to tell if a nucleus has been bumped. In our detector, when the nucleus gets displaced, it sends a shiver or vibration through the crystal and this heats it up slightly. This heat can be detected by thousands of tiny tungsten thermometers that have been deposited directly on one side of the crystal, Since this signal is so tiny, we have to keep our crystals at 0.04 K - very close to absolute zero - where there is no thermal motion at all. Only then can we detect the small heat signal of one displaced nucleus.

The displaced nucleus also shakes some electrons free from nearby atoms and these get pulled to the other side of the crystal by an applied electric field. Thus, we see 2 signals: a vibration (or phonon) signal and an electrical current. The 2 signals help us distinguish background radiation from a WIMP signal because background radiation has a lot more electrical current than a WIMP produces. All this data is going to tape right now.

posted on Sun, 02/03/2008 - 12:42am
Jason W.'s picture
Jason W. says:

What is M - Theory? Is there a connection between it and your work?

posted on Mon, 01/28/2008 - 5:54pm
Prisca Cushman's picture

M-theory is the result of a mathematical relationship between various super string theories which allow physicists to relate the description of an object in one super string theory to the description of a different object in another super string theory. These relationships imply that each of the super string theories is a different aspect of a single underlying theory and realizing this gave new life to idea that a super strings might be a way to unify all the forces. It is only related to dark matter in the sense that coming up with a fundamental description of everything has to also include dark matter particles as one of the manifestations of cosmic vibrations on a string.

A less ambitious, but still unproven, means to unify the forces is Supersymmetry. Supersymmetry might be a subset of super string theory if properly formulated. Supersymmetry invents a whole set of partner particles for the ones we know. With these super-partners, the fundamental forces of nature can be unified at high energies, such as existed in the Big Bang. These predicted particles are just the right mass to be the dark matter particles and would be weakly interacting. Therefore, it is extremely tempting to think that the WIMPs might consist of these supersymmetric particles.

As an experimentalist, I am not particularly wedded to these theories, which are very hard to prove or disprove. I prefer to look for the WIMPs because I know dark matter must exist, rather than think I am looking for the supersymmetric particle. Finding a WIMP does not mean that supersymmetry is correct, but it does provide some more circumstantial evidence.

posted on Sun, 02/03/2008 - 4:20pm
Jason W.'s picture
Jason W. says:

What can you tell me about Parallel Universes?

posted on Mon, 01/28/2008 - 5:55pm
Prisca Cushman's picture

Hey Jason, you sure ask some hard questions! Except in "The Golden Compass", dark matter (dust?) is not directly related to parallel universes. The parallel universe idea comes from the many-world interpretation of quantum mechanics. At the simplest level, if subatomic particles impinge on a barrier with 2 holes, they go through one or the other, not both. However, when the pattern of hits on a distant screen is observed, there is an interference pattern as if the particles were waves going through both slits. This is a fundamental paradox in quantum mechanics. The many-world interpretation says that the universe bifurcates into two possibilities at that moment; in one world the particle went through the left hole and in another world it went through the right hole. Thus at every binary decision point, even on the atomic level, parallel universes are being born.

This is not necessarily any crazier than quantum mechanics itself, but it certainly doesn't lend itself to proof. Especially since it is forbidden to somehow break through to those parallel universes, since it is their very inaccessibility that makes the interpretation work for quantum mechanics. If they could be observed, then the laws of quantum mechanics would be different.

posted on Sun, 02/03/2008 - 4:40pm
Sierra's picture
Sierra says:

what do i need to do in high school to prepare for a career in science?

posted on Sun, 02/03/2008 - 3:26pm
Prisca Cushman's picture

Well, Sierra, the better your math skills, the easier it will be to speak the language of physics. These days even biology is depending more on mathematical modeling and basic calculus skills. Like any language, you want to get beyond worrying about parsing your verbs and into having real conversations. So - get that calculus under your belt as soon as you can!

It would be good to try for the AP science courses if they exist at your school, but many people go into science in college without a very good high school background, so don't let that stop you. You can make it up as you go along. Consider going to a summer school for science - I went to the Summer Science Program in Ojai, California between my junior and senior year and it was absolutely awesome! This type of experience combines doing fun science stuff with being on your own with your peers who are also excited about science. It is a lot of fun. Another possibility is to find a summer job with a lab or medical center. These all help you to figure out what you really want to do.

posted on Mon, 02/11/2008 - 7:45pm
Kelly Brant's picture
Kelly Brant says:

Do you have fun doing your job?

posted on Sun, 02/10/2008 - 3:11pm
Prisca Cushman's picture

Sometimes I have a blast and sometimes there are a heap of boring things that someone has to get done. That is like any job, I guess. Mostly I like the fact that there is such a variety - the boring stuff doesn't last that long and I can get on to fun stuff.

More explicity, let me tell you what my job really is.
20% teaching (preparing lectures & giving them, homework prep, seeing students)
15% writing grants and proposals for money to do my research
20% taking shifts, fixing equipment, making the experiment go
10% managing my research group - postdocs, grad student thesis prep
15% analyzing the data and talking with colleagues about what it means
10% Writing the papers and presenting results at conferences & seminars
5% Committee work for the University (curriculum, student recruitment, faculty hires)
5% Reviewing other people's grants, serving on goverment committees about research.

You can imagine which bits might be boring. Some are hard to generalize: e.g. teaching can be very rewarding if you have a small class of physics majors. But it can be extremely frustrating to teach 450 freshman who have to take 1st year physics and wish they were somewhere else.

posted on Mon, 02/11/2008 - 8:00pm