Questions and Answers about Particle Physics

1. I have a few questions relating to dimensions: What is string theory? and how does it affect our perception of reality and our current 3rd dimension?  How would a 4th dimension function or be perceived? How could us earthlings possibly achieve connection with a higher dimension?

String theory says that everything in the universe can be broken down to a single type of fundamental particle, called a "string".  So imagine we're trying to break an atom into its smallest pieces.  We can take the atom and see that it is made up of protons, neutrons, and electrons. From what we can tell, electrons aren't made of anything smaller, but protons and neutrons can be broken down further into quarks.  Just like electrons, quarks can't be broken down either ... because they can't be broken down any further, quarks and electrons are referred to as "fundamental particles".  When we look at them experimentally, quarks and electrons look like very different particles, but string theory says that they are actually the SAME particle (a string) and that the differences we see between them, or any other unbreakable particle are the result of these strings vibrating at different frequencies ... kind of like the same guitar string can make different sounds depending on how you pluck it. This is really the underlying idea behind string theory.  Now to get into some of the more gory details.

All theories need to eventually explain what we see in everyday life.  In order for string theory to "make sense" in our everyday life, theorists say that we are living in an 11-dimensional space-time (10 space dimensions and 1 time dimension).  We currently are very familiar with our 4-dimensional space-time (3 space and 1 time dimension).  So what exactly does it mean to have 7 additional space dimensions?

To start, the three space dimensions that we are familiar with are "extended" dimensions, which means they extend in three directions infinitely.  In this case, every point in space can be uniquely defined by specifying an x, y, and z coordinate.  Now suppose at every point in our space, there is a tiny loop ... so tiny that we can't see it and no instrument on the earth can currently detect it.  So tiny that if we decide to ignore it for everyday life, it doesn't matter.  We have now introduced the idea of adding a 4th space dimension -- if we want to be extremely exact, it is not enough to specify x, y, and z for every point in space - now we have to define x, y, z, and where we are on the loop that is sitting at point (x,y,z)!  Our new 4th dimension (the loop) does not extend out to infinity the way x, y, and z do, but rather, it's "curled up"!  Now according to string theory, every point (x, y, z) doesn't have a hoop, but rather, it has a 6-dimensional object that is curled up and that we can't see.

So doing the math now, we have our familiar 3 extended space dimensions (x, y, z), we have 6 curled spaced dimensions, and we have 1 time dimension.  That gives a 10-dimensional space-time.  But we said that it's an 11-dimensional space time ... it turns out that there IS another extended dimension.  It is amazingly difficult to try to actually picture what it is like to have a 4th extended space dimension, so what we commonly do is assume we live in 2 dimensions and get a feel of what it is like to add the third dimension.  Try to imagine that you live on a completely flat sheet of paper that stretches out to infinity in two-dimensions.  You can't see anything that is not on this sheet ... no matter how close it is to this sheet, you just can't see it.  Now there is another sheet that is put really close to your sheet, but not touching it. The people on the other sheet can't see you, and you can't see them, even though they are unbelievably close ... the laws of physics just don't allow them.  Taking this and applying it from our 3-dimensional world is now to say that there is a 4th dimension that we cannot see or even picture extremely well since we are bound to 3-dimensions by our laws of physics.  This goes a bit towards Alicia's final question in that given our knowledge of physics now; it is not possible for us to see the additional extended dimension because we're bound to our traditional 3 extended ones.

On a final note, keep in mind that string theory is still just a theory. It is unbelievably elegant, to the point that people believe that it must be true.  However, we are still at least a decade or so away from testing even the simplest predictions that it makes, let alone the theory as a whole.  If it is still hazy, that's a bit expected since it is hazy for people who are even in the field  :)  but I will try to answer follow up questions if I have not covered everything.

Cole Miller

2. I have no background of studying this topic in school, so forgive me if my question seems elementary. How is the anti-matter created? What is the scientific definition of this strange "stuff" (or "unstuff", I suppose)? Any answer would be greatly appreciated, and I wouldn't mind even hearing about the very basics of what seems like a complicated subject.

Think of Einstein's famous equation E equals M C squared and forget about the C squared since it doesn't matter for our purposes.  What the equation then says is that mass equals energy and you can think of that as meaning that mass is simply one form of energy.  In other words, you can convert mass into another form and still have a given amount of energy.  Matter and antimatter are really nothing more than two forms of mass that differ by a sign.  Matter has a positive mass and antimatter has a negative mass so that when they combine, the masses are completely transformed into a different form of energy called electromagnetic radiation.  Therefore, if you combine a certain mass of matter with an equal amount of antimatter, the mass content of this two-body system has been converted entirely from a mass form of energy to a radiation form of energy.  The matter vs. antimatter content of the universe is a fundamental question that early universe cosmologists think about.  I think these questions are of the following kind.  If the energy of the universe was all in the form of radiation at the big bang, and matter and antimatter are created in equal amounts, then why is our universe predominantly made of matter and not antimatter?  Did the antimatter become so separated from the matter that there are entire regions of the universe made of antimatter?

David Garofalo

3. I was wondering where the weird names for quarks come from?

First of all, the name "quark" was chosen in 1964 by Murray Gell-Mann, who proposed that all matter in the universe was made up of combinations of just three particles.  "Quark" is a nonsense word that appears in the book "Finnegan's Wake" by James Joyce.  It was later found that there are six quarks in three pairs, and their names are up and down, charm and strange, and top and bottom.  The names, for the most part, were not chosen to have specific meaning.  For example, the up and down quarks do not point up or down.  The strange quark did get its name for a reason - the particle that it was first found in, the lambda-particle, had a "strangely" long lifetime - it stayed around much longer than they thought it would before decaying into other particles.  This actually gave rise to a property named "strangeness" and a "law of conservation of strangeness," which sounds pretty funny to be an actual law of physics!

Another interesting piece of trivia is that the top and bottom quarks were once (and sometimes still are) called the truth and beauty quarks.  These names were changed for mostly unknown reasons, but it is thought that physicists thought they were too cheesy, even though physicists are no strangers to strange names!  Another reason might have been that the bottom (or beauty) quark was discovered years before its predicted partner, the top (or truth) quark.  During that time, people started to say that there was no "truth" to the quark theory.  They partially meant that the truth quark couldn't be found, and partially that the truth of the theory was based on finding it - but in any case, it didn't sound good.  So the names were changed to something a little less exciting, and they seem to have stuck.

Katie McGleam

4. I was wondering if you could answer a question I have about different dimensions.   If we live in only 3 dimensions do you believe there could be other dimensions with life similar to ours that we may not yet know about?

That is a tricky question you've asked me. There is a branch of theoretical physics called string theory which says that there are many more dimensions than just the three normal dimensions. Unfortunately, I don't know very much about this so I can't begin to explain it to you.

However, I will answer your question in a sneaky way. I believe we can describe the universe in 4 dimensions rather than traditional 3. These four dimensions are the usual 3 (x, y, and z), plus time. That is, you can describe anything that has occurred based on where it happened (x, y, z) and when it happened (time). Thus, two things that happened in the same place but at different times both need 4 dimensions to describe exactly when and where they occurred. Without considering time as the fourth dimension, I'd be likely to run into Christopher Columbus sailing over from Europe the next time I went swimming in the Atlantic Ocean!

So, when you consider time as another dimension, I believe that there could be life out there which we haven't encountered yet in that dimension. I believer that the universe is large enough and has been around for so long (about 15 billion years) that life MUST have developed elsewhere. The problem is that if it was far enough away or happened so long ago, we may not have had enough time to encounter it yet. As to whether or not that life is similar to ours, I'd guess that it is probably similar chemically (ie. it is carbon based), but probably looks completely different.

I'll leave you with a quote from one of my favorite movies, "Contact" starring Jodie Foster. Jodie Foster's character Ellie asks her dad, "Dad, do you think there's people on other planets?" and her dad replies "I don't know, Sparks. But I guess I'd say if it is just seems like an awful waste of space."

Matthew Knight

5. In our book, it said that scientists look at information that they get from these experiments to learn about the big bang. I find it fascinating that such a seemingly small occurrence, such as two atoms hitting each other, could tell us something about the big bang. My question is this: What have we learned about the big bang from these experiments so far? Is there more that we can learn from them? And, finally, how is it that something so small can help to explain such a big event in the history of our universe?

That's an excellent question, because it is at the heart of why particle physicists have been so interested in the early universe. The early universe was extremely dense and hot.  The only way we can simulate something that dense and hot is by smashing atomic nuclei together.  If you do that with enough speed, then the protons and neutrons come together at nearly the speed of light, creating all sorts of weird particles (quarks, gluons, mesons, and many more). Therefore, looking at the state of matter in an accelerator can give us some insights into the early universe.

This has taught us a lot.  For example, knowing how subatomic particles behave at high energies is crucial to understanding how the very lightest elements formed in the early universe.  It is thought that two isotopes of hydrogen, and two isotopes of helium, were formed primarily in the first few minutes after the Big Bang.  To understand exactly how this happens requires knowledge of particle physics.  Using that input, physicists have figured out the total number of protons and neutrons in the universe. To their surprise, they found that this number is much less than the total amount of matter in the universe.  Therefore, "dark matter" must exist that is of an exotic type; in fact, you can show that it can't be any particle yet detected!

This, therefore, is a partial answer to the question of what more we can learn.  If new types of particles are detected in accelerators (or in some other way), we might learn what dark matter is, which would be pretty exciting!

Cole Miller

6. I am curious as to what the differences between the types of quarks are. What distinguishes one type of quark from another?

The main differences between types of quarks are mass and electric charge.  Here's a table of their properties.  For reference, the charge of a proton is 1 (and of an electron is -1).  The mass of an electron is 9.11x10^-31 kg, and the mass of a proton is about 1,800 times the mass of an electron.

Quark         Charge         Mass divided by electron mass

up             +2/3                     3 to 9

down           -1/3                    10 to 17

strange        -1/3                   160 to 300

charm          +2/3                 2,000 to 2,800

bottom         -1/3                 8,000 to 9,000

top            +2/3                 about 350,000

Now, a proton is composed of two up quarks and a down quark.  If you add up the masses of the quarks, it's much less than the mass of a proton.  How is this possible?  It is possible because there are also particles called "gluons" that interact between the quarks, and these have so much energy that it increases the mass.  Weird stuff!

Cole Miller

7. Could you go into detail about String Theory? What exactly happens?

The goal of string theory is to explain all subatomic particles and their interactions in a unified scheme.  A lot of people think that it is promising because of its mathematical properties, but it is so complicated that at this point no one has been able to use it to calculate something that can be compared to experiment.

The basic idea is as follows.  Do you play any musical instruments? Suppose you have a violin string.  You know that if you run a bow across the string you make the string vibrate.  In fact, if you looked very carefully you would see that the string isn't vibrating in just one way: instead, there are a whole set of vibrations.  Some of those cause the whole string to go back and forth together (while being fixed at the ends); some cause part of the string to go one way while another part goes the other way; and some have more complicated patterns, like the first third going left, the next third going right, and the final third going left again.

What string theorists ask us to imagine is that if we were to go to really tiny scales (and I mean *tiny*, on the order of 10^-34 centimeters; that's as much smaller than a nucleus as a nucleus is smaller than the United States!), we would see a lot of "strings".  Like violin strings, these vibrate in different ways.  Now (and we have to take a leap of imagination), we say that each one of these separate vibrations represents a particle!  Weird, I know, but that's what you get when you start thinking in a realm so far away from our everyday experience. To make it even stranger, it is thought that all this takes place in more dimensions than we're used to.  That is, rather than having three dimensions of space and one of time, you might have ten dimensions of space and one of time.  The extra dimensions don't extend very far (on the order of 10^-34 centimeters again), so it is the equivalent of living on a sheet of paper: it's three-dimensional, but only two dimensions are large.

This is a very mathematical idea that has had some of the world's best minds working on it for the last twenty years.  There has been mathematical progress made, but it might be a while before people can really calculate.  An example of a calculational success would be if researchers could use these ideas to predict correctly the masses of subatomic particles and how they interact with each other, from the underlying idea of strings.  If they could do that it would support the reality of the whole picture, but it might not be in our lifetimes that we see that!

If you are interested in more details, check out "The Elegant Universe" by Brian Greene.

Cole Miller

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