Friday, Feb. 7, 2014

Have a safe day!

Friday, Feb. 7

3:30 p.m.

4 p.m.
Joint Experimental-Theoretical Physics Seminar - One West
Speaker: Brandon Eberly, University of Pittsburgh
Title: Probing Nuclear Physics with Neutrino Pion Production at MINERvA

8 p.m.
Fermilab Lecture Series - Auditorium
Speaker: John Carlstrom, University of Chicago
Title: What Do Scientists Know About The Big Bang?
Tickets: $7

Sunday, Feb. 9

1-5 p.m.
Fermilab Family Open House

Monday, Feb. 10

2:30 p.m.
Particle Astrophysics Seminar - Curia II
Speaker: Stephan Meyer, University of Chicago
Title: The Fermilab Holometer: A Measurement of Planck Scale Quantum Geometry

3:30 p.m.

4 p.m.
All Experimenters' Meeting - Curia II
Special Topic: DES Update

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Wilson Hall Cafe

Friday, Feb. 7

- Breakfast: chorizo and egg burrito
- Breakfast: French bistro breakfast
- Beer-battered-fish sandwich
- Smart cuisine: Teriyaki pork stir-fry
- Vegetarian eggplant lasagna
- Cuban panino
- Breakfast-for-lunch omelet bar
- Manhattan clam chowder
- Texas-style chili

Wilson Hall Cafe menu
Chez Leon

Friday, Feb. 7
- Spinach salad with cranberries and pine nuts
- Flank steak with caramelized onions and balsamic glaze
- Walnut-crusted potato and blue cheese cakes
- Brussels sprouts
- Profiteroles au chocolat

Wednesday, Feb. 12
- Cuban black bean patties
- Pineapple rice
- Coconut tres leches cake

Chez Leon menu
Call x3524 to make your reservation.


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Physics in a Nutshell

By his bootstraps

"By His Bootstraps" (1941) was a science fiction story in which a man acquired a time machine from a future version of himself. This is an example of an acausal loop because the time machine need never be invented.

In my last Physics in a Nutshell, I started a series about quantum mechanics by addressing its first strange feature: the fact that quantities can be multivalued yet restricted to whole numbers, like a light switch that is both on and off but never halfway on.

The second weird thing about quantum mechanics is that it takes as much liberty with time and causality as is logically possible. Time travel, as it is usually presented in fiction, is full of logical paradoxes. Suppose you go back in time and prevent yourself from inventing a time machine. Without a time machine, you can't go back to make the change, and an infinite regress ensues. But there is another way of changing history that isn't impossible, merely contrived: Suppose you go back and teach yourself how to invent a time machine, retroactively making the trip possible. Heinlein's novella "By His Bootstraps" worked this way, and in a sense, so did Sophocles' "Oedipus Rex."

If quantum processes are taken literally as sequences, they resemble constructive time loops. The simplest example is the mutual repulsion of charged particles (the reason hair stands up on a dry day). Two charged particles repel each other because one emits a photon, recoiling from the photon's momentum, and the other catches it, recoiling the other way. However, they never miss — the pitcher doesn't throw the ball unless the catcher catches it. If viewed at relativistic speeds (charged particles in an accelerator, for instance), the catcher's catch can even precede the pitcher's throw, with the photon traveling backward between them. The same process, viewed by two different observers, happens in a different time order.

More complex examples demonstrate this more conclusively. The lesson physicists have drawn is that a quantum process is not a sequence of independent steps, but an undivided cloth that entirely happens or entirely does not happen. Oedipus would not have married his mother if he were not trying to avoid the prophecy that he would do so, and the prophecy would not have been uttered if he did not do so. The whole process can happen without inconsistency, and it can also not happen without inconsistency.

On a human scale, closed time loops would seem to imply a lack of free will, but the randomness of quantum events prevents us from drawing simple philosophical conclusions. Even though experiments on coupled processes have been scaled up such that measurements on one side of a workbench predict outcomes on the other, the sequence of messages is strictly random and cannot be influenced by the experimenter. Thus, we can't use this to communicate with or change the past. Quantum processes are as acausal as is logically possible, and no more.

In the next article, I will talk about waves, particles and the strange fact that matter at microscopic scales appears to be both.

Jim Pivarski

Photo of the Day


A bicyclist pedals his way through Wednesday morning's heavy snow and wind. Photo: Dennis Loppnow, FESS
In the News

Quarks know their left from their right

From Science, Feb. 5, 2014

How an electron interacts with other matter depends on which way it's spinning as it zips along — to the right like a football thrown by a right-handed quarterback or the left like a pigskin thrown by a lefty. Now, physicists have confirmed that quarks — the particles that join in trios to form the protons and neutrons in atomic nuclei — exhibit the same asymmetry.

The result could give physics a new weapon in the grand hunt for new particles and forces. Right now, scientists can try to blast massive new particles into existence, as they do at the world's biggest atom smasher, the Large Hadron Collider (LHC) in Switzerland. Or they can search for subtle hints of exotic new things beyond their tried-and-true standard model by studying familiar particles in great detail. In the latter approach, the new experiment gives physicists a way to probe for certain kinds of new forces, says Frank Maas, a nuclear physicist at Johannes Gutenberg University Mainz and the GSI Helmholtz Centre for Heavy Ion Research in Germany. "For a specific type of model, this type of experiment is much, much more sensitive than the experiments at the LHC," Maas says.

Read more

Frontier Science Result: MINERvA

What happens in hydrocarbon stays in hydrocarbon (sometimes)

This shows what an event in the MINERvA detector looks like when a neutrino comes in from the left and interacts with a proton in the detector, creating a pion that goes backwards, in addition to a proton and a muon.

Para una versión en español, haga clic aquí.

When a neutrino enters the nucleus of an atom, it can interact with the protons and neutrons inside and impart enough energy to create completely new particles. Often a pion (a particle made of a quark and an antiquark) is produced. However, the nucleus is such a dense place that sometimes the pions never make it out of the atom!

Figuring out how many pions are produced and how many exit the nucleus is very important in the field of neutrino physics because it determines how well the energy of the incoming neutrino can be measured. Experiments such as LBNE will measure how neutrinos oscillate as a function of neutrino energy, but they will need to understand what those pions are doing in order to get the neutrino energies right.

Particle physicists have been measuring pions and constructing models of how they interact for a long time, but the neutrino interactions that produce these pions and what happens to them as they exit the nucleus is not nearly as well modeled. The interactions felt by the pions on their way out of the nucleus are called final-state interactions, and they are difficult to calculate because there are so many moving parts — all the protons and neutrons in the nucleus. We do have a few models, but it is important to verify them with experimental data from neutrino experiments. When the MiniBooNE measurement of pion production was first released, it was clear that the most complete models of what happens inside the nucleus were not describing the data. MINERvA now has a sample of several thousand events where a pion, proton and muon are produced when a neutrino interacts with a neutron or proton in the detector's plastic scintillator, which is made of hydrocarbons (see top figure).

By studying the energy distribution of the pions that make it out of the nucleus, MINERvA can determine how big an effect the nucleus has on those pions. The better we understand (and then model) that effect, the better the whole field will be able to measure neutrino energies.

Carrie McGivern, University of Pittsburgh

This plot shows the cross section (probability) of producing a pion of a given energy for the neutrinos coming from the NuMI beamline. A few different models of that probability are shown, where the models have been normalized to the data.
Brandon Eberly (left) plays his trombone in front of a MINERvA detector prototype, taking advantage of the experimental cavern's acoustics. He will describe MINERvA's latest results in more detail in today's Wine and Cheese seminar at 4 p.m.

Artist reception for Jay Strommen - today

Strength Training registration due today

Barn Dance - Feb. 9

Budker Seminar - Feb. 10

Kyuki Do registration due Feb. 10

Family Science Days in Chicago - Feb. 15-16

Garden Club spring meeting - Feb. 20

URA Visiting Scholars Program deadline - Feb. 24

Interaction Management course: March 5, 12 and 19

Performance Review course: March 26 or 27

Interpersonal Communication Skills - Apr. 16

Fermi Singers invites new members

Yoga classes

Indoor soccer

Scottish country dancing meets Tuesday evenings at Kuhn Barn

International folk dancing meets Thursday evenings at Kuhn Barn

10 percent employee discount at North Aurora Dental Associates

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