Previously Asked Questions (PAQ) for Ask A Particle Physicist – by the High Energy Physics Group at Syracuse University
Questions previously asked and answered are given here. See if your own question has been asked before.
Does light (photons) interact with protons? If so, at which frequency does light interact the most with protons. At which frequency does light gives the most energy to protons ?
Question by: Zack Hardwick
This is an interesting topic. Since a proton is charged it is effected by the electromagnetic force. The carrier of that force is a photon, so indeed light interacts with protons. Using low energy photons where the wavelength is much larger than the size of the proton (10-13 m) the interaction is the same as with an electron and a photon. When the photon energy gets large (i.e., it’s wavelength is small compared to the proton’s size), other interesting things happen. For example, the photon can directly interact with the constituents of the proton, the quarks. Furthermore, the photon may also act like a strongly interacting meson with the same quantum numbers as the photon, namely spin 1 and negative parity. Those mesons include the rho (r), omega(w), and phi (f). When these energies is reached the cross-sections grow and many particles are produced in the collision. This figure (see bottom plot) shows the measured total cross-section of photons on protons and deuterons. High energy photon beams have been made and used to study the structure of the proton and to produce other particles for study. Some of the best measurements of the properties of charm particles have recently been made with a photon beam at Fermilab by the FOCUS experiment (see http://www-focus.fnal.gov/)
Recently I became interested in solar power production. In looking into this type of energy production I became curious. If we can produce energy from a very small section of the electromagnetic spectrum aka visible light then why is it not possible to harness all or at least some of the parts of the electromagnetic spectrum? For example, UV panels, gamma panels, infared panels (which is probably the most abundant source) these panels could convert what is all around us to electricity. Have materials been identified that could absorb different parts of the spectrum and convert such to electricity? Perhaps focusing part of the spectrum is the way? If either of these were developed there could be free clean power day or night.
Question by: Robert Kneberg, UTAH
This is very insightful. In fact there are many scientists & companies devoting a great deal of time to improving our ability to convert solar power to electrical power. As you probably know, solar panels can be purchased to put on your roof. It turns out that visible light has the proper amount of energy to liberate electrons from the material in the solar cell (mostly silicon), which can be used to generate electrical current.
Here are some nice “friendly” references which may be of interest to you.
I understand that silicon is used for the visible light spectrum and that other parts of the spectra will not provide the desired results in silicon. I was just wondering if there were other materials that would produce an electric current from other parts of the electromagnetic spectrum or if a search for the desired materials was even in process?
Question by: Robert Kneberg, UTAH
The gist of it is that there is not very much UV energy from the sun to mine as compared to the visible part. So, I don’t think it is being heavily pursued. Here is a JPEG file showing the radiant intensity from the sun (and earth), and you can see it peaks in the visible, and not much in the UV.
How do lasers produce phase coherence?I would like to ask you a question related with laser theory. How could it be explained, physically speaking, that when radiation interacts with an excited atom, the emitted photon is in-phase with, has the polarization of, and propagates in the same direction as the stimulating radiation? Why not in any direction, with different phase?
Question by: Nilton Haramoni, Brazil, Faculty Respondent: Steve, Sheldon
Lasers operate as their names imply “Light Amplification by Stimulated Emission of Radiation”. Atoms can be put in what is known as a meta-stable state. Atoms Atoms can reside for a longer period of time in these meta-stable states. However, In this meta-stable state, atoms can be “tickled” or stimulated into dropping down to the ground state, and in doing so,
CP Symmetry and the Matter-Antimatter Asymmetry
Question by: Tim Kelley, Faculty Respondent: Artuso, Steve
I understand that there is evidence that CP-Symmetry is broken (such as in the decay of a kaon) and understand some about the concept of CP-symmetry. ( CP-Symmetry is when there is an inversion of charge and parity the system remains the same…) However, I do not fully understand how CP-Symmetry breaking could allow a particle to decay resulting in more matter than antimatter… Could you explain or point out an information source that might help me in my understanding?
Let me expand a bit on this Tim, at the risk of being too detailed. (Steve)
In SUSY, are neutralinos considered as force carriers?
Question by: Cyrus Bryant, Faculty Respondent: Artuso
I have read that a leading candidate for “cold dark matter” in supersymmetry theory is the neutralino, a superpartner of the photon, Zo, and Higgs bosons. That would make it a fermion, I suppose. My question is: should this particle exist (to be discovered by the LHC team?) would it also be a force transmitter n the realm of superpartner physics?
How are particles accelerated at Stanford Linear Accelerator (SLAC) and what happens?
Question by: Boris Milvich, Faculty Respondent: Steve
My textbook of physics states that the accelerator at SLAC accelerated electrons to a final speed of 0.999 999 999 948 of the speed of light, but does not provide any other information about it. Here is what I would like to know:
I am from the former Yugoslavia, living presently in Snowmass, Colorado. I am working on a writing project that touches upon particle collisions. I found you though a web-search. It is very nice to have the opportunity to submit questions directly to a particle physicist, as the answers are often difficult to find.
a) electron + positron —-> electron + positron (not that interesting, but useful for various calibrations)
Many other things can happen as well, but typically with smaller probability…
The last one (d) , the bottom, or simply “b” quark is the most interesting for physicists at the BaBar experiment for a number of reasons.
Is mass conserved in particle collisions?
Question by: Boris Milvich, Faculty Respondent: Steve
An electron that is moving at 0.999 999 999 948 c is supposed to have a mass of 8.93×10^–26 kg just before a collision, according to Einstein’s equation for a moving particle. This is the mass of about 98,000 electrons at rest. However, the result of the collisions that you gave in your answers of such an electron with an accelerated positron (98,000 positrons at rest) does not come even close to what it should be produced according to Einstein’s equation. The reaction that you listed, electron+positron yielding a tau and anti-tau particles, accounts only for the mass of about 7,200 electron masses at rest.
What is an electron’s mass and energy while being accelerated? What about radiation in circular accelerators?
Question by: Boris Milovich, Faculty Respondent: Steve
I read in the textbook by Ohanian (Principles of Physics) that at SLAC electrons are accelerated to a speed of 99.999 999 994 8% of the speed of light, or 0.999 999 999 948c. I would like to know the following:
1. The actual combined applied voltage used to attain this speed?
I read somewhere that in linacs, as opposed to circular accelerators, electrons do not lose gained energy due to acceleration. Was this the case in this example? If there were a loss, what would have been its magnitude in MeV or in percentage?
1) The applied voltage is not done in one step. Simply put, we can boost the kinetic energy (KE) of an electron as it passes across a voltage DV, by qDV. If you do his many times, you can eventually reach a very large KE. The KE is realted to the velocity, v, by KE=gmc2-mc2, where g=(1-v2/c2)-1/2 ., and c=speed of light=3×108 m/s. If you add up all the applied “voltage kicks”, it will add up to a very high voltage, of order several billion volts !
2) Their mass is increased dramatically, m=gm0, where m0 =9.11×10-31 kg is the rest mass of the electron and g was defined above.
3) Just use E=mc2, using m from (2) above.
4) When electrons are accelerated, they radiate Em radiation, and in this case, x-rays. In going around a circle, they radiate energy, and the power (Energy/time) ~ g4.
How do physicists know that particles observed during collisions are actual particles and not random pieces of the nucleons flying apart? (Shawn)
Question by: Shawn, Faculty Respondent: Sheldon and Steve
How do physicists know that particles observed during collisions are actual particles and not random pieces of the nucleons flying apart? What if smashing protons and neutrons together was analagous to smashing two glass jars together at high speed? The pieces would give almost no information about how the nucleus works! Alternatively, what if new particles were created during collisions that do not exist in normal nuclei? Wouldn’t particle physics then be way off course in it’s understanding of the atom?
(Sheldon) Leon Lederman used to joke that smashing two protons together was similar to colliding two garbage cans and watching all the garbage spill out! (Leon won the Noble prize for his discoveries using proton-proton collisions.) Protons are composed of three quarks, as are neutrons. When these particles collide the quarks couple to strong force field through a kind of particle called a “colored gluon.” These gluons can interact with each other and produce other gluons and even more quarks. Thus we are producing many such particles depending on the energy of the collision. Now if the collisions are at low energy then we can examine the nature of the nucleus, i. e. as you say how it works. At higher energy the aim is foten to produce new states of matter that usually don’t exist in a nucleus but did exist in the early Universe. We need to understand all the states of matter in order to understand the underlying theory of how matter and forces are put together. This is a large topic. Perhaps you might want to look at
Followup (Shawn): What causes charge? I’ve looked around but haven’t really found any answer. It’s probably not known, but what is the actual cause of opposite charges attracting and like charges repelling? It would seem that some process is occurring that results in force being applied over a distance.
(Sheldon) The electromagnetic force acts through electrical charge. The strong force acts through “color” charge. Why there are such forces and charges is a question that science cannot address at this point. We are getting pretty good and figuring out HOW these forces work, however.
(Steve) Let me also chime in on this:
In physics, a quantity call “Potential Energy” is often useful. Between 2 charges, it is: U = k*q_1 q_2 / d, where, k is a constant, q_1 and q_2 are the charges of the two particles, and d their separation. Nature, if not perturbed, “likes” to be in a state of minimal, or lowest potential energy, namely the smallest value of U allowed.
If q_1*q_2 is negative ==> (i.e., opposite charges) ==> U is negative.
If q_1*q_2 > 0 (like charges) ==> U is positive.
What do Particle Physicists do?: I am an aspiring physicst who is currently attending high school. My English teacher has given us a term paper to do. Would you tell me what your job consists of
Question by: Robert Dean, Faculty Respondent: Steve
Well, I could go on for a long time on what we do. Let me be brief. Our goals as particle physicists are to understand nature at its most fundamental level. That it, we seek to identify and understand the most fundamental constituents of matter. Nowadays, these are the quarks and leptons, although it is certainly plausible that they are not fundamental. All w can say is that their size is less than 10-18 meters. Our second goal is to understand how the fundamental particles interact with one another. Today, we know of four forces (from weakest to strongest): Gravity, Weak force, Electromagnetic force, and the Strong Force. There is some hope that with a more complete theory, these four forces may be in fact different manifestations of the same fundamental force (so called “Unification of the Forces”). All of this is going to get veryexciting next year as a powerful accelerator, called the Large Hadron Collider (LHC) turns on and smashes protons into protons at 7 TeV.
The Cosmic Bathtub Drain: I realize this is outside your expertise, but I’m sure it’s general enough to be within the scope of your training. I just hope it’s not SO general as to be a waste of your time. If so, perhaps you can forward it to someone with copious free time to fritter away…. 🙂 Essentially, the question is this: How can it be determined that, instead of the universe expanding, it is actually being sucked into a very powerful singularity? It seems to me that if this were true, the galaxies “closer” to the event horizon of such a singularity would be accelerated so that they would be red-shifted relative to the Milky Way. Likewise, we would be accelerated beyond those that are “farther” from such an event than we, and thus they would be red-shifted too. I suppose that this hypothetical singularity would not have a specific position in current 3D space, just as the Big Bang’s hypothetical singularity did not. In fact, the more I think about this Cosmic Bathtub Drain, the more it seems to be just the other side of the Big Bang coin. In other words, it seems that since the two ideas are reciprocal, maybe both (or neither) are true….
Question by: Steve Barger: neurobiologist in Little Rock, Arkansas, Faculty Respondent: Sheldon
Is there any chance that mini black holes created by things such as the LHC could destroy the Earth?: I have been traveling so was not able to get to this before now. A good answer is “The black holes produced at CERN will be millions of times smaller than the nucleus of an atom; too small to swallow much of anything. And they’ll only live for a tiny fraction of a second, too short a time to swallow anything around them even if they wanted to.
Question by: anonymous, Faculty Respondent: Sheldon
If it makes you feel any more comfortable, we’re pretty sure that if the LHC can produce black holes, then so can cosmic rays, high-energy particles that smash into our atmosphere every day. There are probably a few tiny black holes forming and dying far above you right now. So I think we should all relax, fire up the Large Hadron Collider, and get ready for a view of the universe that we’ve never seen before.”
Education for a particle physicist.
Question by: Joe Abrams
What schools to attend before becoming a physicist (What types of schools, ie: 4 year college, or specialty schools):
You can obtain some physics related jobs with a 4 year undergraduate degree, but to do research you will need a Ph.d. degree which takes an additional 5-6 years and would be done at a University, such as Syracuse.
Ph.d. degree with research activities.
This depends on the subfield in physics you choose to work in. Some subfields are more company oriented, while others are more academically oriented. For example, theoretical particle physics would be mostly at Universities.
Several questions on matter and antimatter differences
Question by: Theresa Mann, H.S. teacher, Faculty Respondent: sheldon
(Teresa) Thank you so much for your replies. I can’t wait to share them with the students tomorrow. These are all questions that they generated. They are thinking about this material far more deeply than any former class.
Q: How, in terms of fundamental particles, does beta decay occur? We are specifically wondering how a neutron appearantly is converted into a proton and a beta particle.
A:The quark content of the neutron is u d d. In beta decay one of the d quarks transforms into a u quark and massive particle called a W- and a u quark. The uud then form a proton while the W- transforms into an electron and a anti-neutrino (actually, precisely an anti-electron type neutrino.) The neutrino is usually not observable since it doesn’t interact much with matter. This kind of decay is called a “Weak” decay as it occurs by the Weak interaction. By the way the W- is a “virtual” particle in this decay; e. g. real W- particles have masses much much larger than the d quark, but since the transition occurs over a very very small time then it can happen actually as a consequence of the Heisenberg uncertainty principle. See http://particleadventure.org/frameless/npe.html for pictures
Q: Does anti- matter change simultaneously to the change in matter?
A:I am not sure I fully understand the question. When we now produce anti-matter in the laboratory, we change energy into matter + anti-matter. So we produce, for example, a proton and an anti-proton simultaneously. This was supposed to happy also in the early Universe, so there was, immediately after the big bang, an equal amount of matter and anti-matter. We now have found that there is difference between the decay rates of matter and anti-matter, so this contributes to an imbalance of matter and anti-matter but the effects we know about are not large enough to account for the fact that the Universe is mostly matter these days.
Q: Where is the anti-matter? Has it been isolated?
A:Its anti-baryons composed of three anti-quarks, or mesons with the opposite quark combinations than for matter. The anti-quarks have opposite charges to the quarks. Examples: a) Anti-proton: anti-u, anti-u, anti-d, so the charge is -1. b) Particle K- meson composed of s anti-u, while the anti-particle is a K+ composed of anti-s u. Yes anti-matter is easily produced in particle accelerators. It was first discovered in an experiment at Berkeley Ca. in 1955. The 1959 Nobel Prize was awarded to Emilio Segre and Owen Chamberlain who led the experiment. One of my friends Tom Ypsilantis was a graduate student who worked on the experiment. Even before that happened in 1932 Carl Anderson discovered the positron which is an electron with positive charge using cosmic rays. Electrons are fundamental particles just as we believe quarks are. See http://cerncourier.com/main/article/45/9/23 Recently (1995), atoms of anti-protons and positrons have been produced -anti-Hydrogen. See http://hussle.harvard.edu/~atrap/
Q: What are some examples of applications of particle physics in everyday life?
A: a) Pays my salary, or at least part of it.
Q: Can you explain the charge nature of the anti-neutron. We understand that it is made op of oppositely charged anti-quarks, but if the effect is the same what is the overall difference between the neutron and the anti-neutron?
A: The anti-neturon is formed of anti-quarks, so in a world dominated by anti-matter it would behave the same as a neutron in our matter dominated world. On the other hand if an anti-neutron ever met a neutron it would annihilate into energy and there would be no matter left.
Are there any good books at the elementary level for modern physics concepts?
Question by: Stan Jankowski, Faculty Respondent: Steve
I found you by simply typing “ask a physicist” in a search engine. I am a great fan of the late Dr. Isaac Asimov and have read his book “Atom” from cover to cover many times. The book was published by Penguin Group, N.Y., N.Y. in 1991. I would like to know if your are aware of a book that covers more recent developments and is written on a similar level. Although I have a healthy interest in the subject, I have no formal education. If your so inclined, pose my question to your colleagues.
A book which is quite readable is the book by Brian Greene, called “An Elegant Universe” http://www.amazon.com/Elegant-Universe-Superstrings-Dimensions-Ultimate/dp/0375708111
Tides & Cosmology questions
Question by: Martin Glover, Faculty Respondent: Steve
1. From earth the moon has the same apparent diameter as the sun. I assume that the sun is denser than the moon so why doesn’t the sun have at least as great an affect on earth’s tides as the moon?
The sun does have an affect on the tides, but it turns out to be less because of a a (r/R)^4 dependence of the forces. Here, r is the radius of the earth, and R is the distance from the moon or sun. Note also that it is only the horizontal forces which create tides, not the total gravitational force. If you want to understand this better, I found a nice web page: http://www.du.edu/~jcalvert/geol/tides.htm
2. How many light years across is the universe? I’ve seen estimates that are far greater than the estimated age of the universe. How is that possible? If inflation only brought the universe to grapefruit size that doesn’t seem to be much of a factor in bringing it to its present size.
Well this is a tricky one. The visible Universe is about 14.5 billion light years across. In a static Universe, this would mean that the furthest objects are 14.5 billion light years (BLY) away. Here, a light-year is the distance light travels in a year. However, the Universe id not static, in fact it’s expanding. That is General Relativity and Cosmology indicate that space itself is stretching. A result of this stretching is that we can “see” things, in principle, which are well beyond the 14.5 BLY distance. Unfortunately, the light from many of these objects are too faint to see them, but in principle, their light can reach us.
3. The universe is expanding and the farther away it is the faster it’s expanding. Are there parts of the universe that are so far away and moving away from us so fast that we never see their light?
Yes, absolutely, this should be the case. On the other hand, the only thing we can discuss is the “visible Universe”. We don’t know what’s beyond that. Is the Universe infinite or finte, open or closed, flat or curved? We think it’s flat, based on observations of the Cosmic Microwave background. Lots of great information is available at WMAP’s web site. http://map.gsfc.nasa.gov/
Development of a more fundamental theory of particles
Question by: Shantanu Thatte, India, Faculty Respondent: Steve
What I am developing is that why it is not possible that all the subatomic particles be made up of some very fundamental particles. Like in a picture tube only three colors make up all the colors why no in the universe.
Perhaps the most important issues to fundamental physics are:
(2) Uncover a deeper symmetry, which explains the spectrum of fundamental particles (quarks, leptons and gauge bosons)
These are ambitious goals, and many, many sceintists are devotingh their lives to answering these two questions. Many theories, such as string theory, hope to address these issues. String theory would describe all paricles (fermions and bosons) as the vibrational modes of a string. This theory has evolved over the years, and one can actually have 2D “branes”, as well as other objects. I encourage you to look at Brian Greene’s String Theory special on NOVA http://www.pbs.org/wgbh/nova/elegant/ for a pedagogocal presentation on string theory.
What are the smallest particles which exist, and what is their function?
Question by: Bob Smith, Lancaster, Pennsylvania, Faculty Respondent: Steve
How small of particles are know to exist? What are they called and what is their function? I found out about you from an internet search.
The smallest known particles to exist are quarks and leptons. The quarks are called: up, down, charm, strange, top, bottom, and the leptons are: electron, muon, tau, and electron-neutrino, muon-neutrino, and tau-neutrino. Each of these also has a corresponding antiparticle. (just add “anti-” in front of the name)