Sunday, February 7, 2016

Rumor of Gravitational Wave Detection.

One of the predictions of General Relativity (Einstein's theory of gravity) is the existence of gravitational waves. In theory, anytime a massive object accelerates (that is any motion other than constant straight line motion) it should generate gravitational waves. So in theory the universe should be filled with gravitational waves.

However despite years of trying, no one has succeeded in detecting them.

But in the past few days rumors have been circulating that LIGO, a project to detect gravitational waves, has finally succeeded.

Einstein thought gravitational waves would be impossible to detect, and in fact they have been difficult to detect. The tiny signals must be separated from other signals (such as earthquakes and passing trucks). Each time a potential signal is detected, a statistic analysis is performed. This in essence asks "What is the likelihood this a real signal, not something that only looks real." This is expressed in terms of "sigma". The higher the sigma, the more likely the signal is real.

According to the rumor, LIGO has in fact detected signals that exceed "five sigma." Normally results like this are not released until solid confirmation has been made, but one of the physicists spilled the beans.

This may prove to be false; but it seems to be real. We should know for sure on February 11th when an official report from LIGO is scheduled to be published.

Those of you living in or near Ann Arbor, might be interested in two upcoming lectures, which by a happy coincidence are on this very topic. Both are by Keith Riles, professor of physics at the University of Michigan.

Saturday, February 13 10:30am: "Gravitational Waves - Einstein's Audacious Prediction."

Saturday, February 20 10:30am: "The Hunt for Gravitational Waves - Was Einstein Right?"

Both events are held in rooms 170 & 182 Weiser Hall (formerly the Dennison Building), University of Michigan Central Campus, 500 Church Street, Ann Arbor, Michigan, 48109

See for more information about these lectures.

Tuesday, October 20, 2015

History repeats itself: Water on Mars...again?

Not to be a downer but my reaction to the recent Nasa announcement about liquid water on Mars was a bit different (although to be clear, it is an exciting discovery). A more suspicious individual might wonder about funding pressure and the timing of this announcement with the theatrical release of a spectacular movie, the Martian, but we won't go there.

I remember a lecture from a University of Michigan professor I attended 5 years ago about how one of the rover pictures almost certainly captured a briny flow on the surface of Mars. I also remember all kinds of announcements about water on Mars growing up. So, I turned to Wikipedia and was not disappointed:

Chronology of discoveries of water on Mars

Turns out someone has actually cataloged this! It is an interesting read for all of you space nerds out there.

Clear skies!

Sunday, August 2, 2015

The Pentaquark

Recently the discovery of a “pentaquark” has been in the news.

To explain what a pentaquark is, I first have to explain the standard model. The standard model is a theory of sub-atomic particles. It states that all the matter in the universe is composed of three types of particles: bosons, leptons and quarks. Bosons and leptons will not be discussed further in this article.

Quarks come in six different “flavors:” up, down, strange, charm, bottom and top. In turn each flavor has two versions: quark and anti-quark. This makes 12 different kinds of quarks. up, anti-up, down, anti-down, strange, anti-strange and so on.

Quarks group together to form larger particles; take a quark and an anti-quark (not necessary of the same flavor) and you get a meson. An up and anti-down OR a down and anti-up form the first mesons to be discovered: the positive pi meson and the negative pi meson respectively. Other combinations lead to other mesons. All mesons are unstable, and rapidly decay into other particles.

If you take three quarks or three anti-quarks, you get a baryon. The two lightest baryons, the first to be discovered, and the best known are the proton and the neutron. There are many other baryons. Except for the proton, all decay into other particles. The neutron takes about 10 minutes to decay, heavier particles decay much more quickly. (Note the electro-weak theory predicts that the proton should be unstable, but with a very long half-life. The decay of the proton has never been experimentally detected, in spite of four decades of attempts).

In turn you can combine protons and neutrons to form a larger structure: an atomic nucleus.

There are rules for how quarks combine. These rules come from quantum mechanics and the concept of asymptotic freedom. The later is a feature of  Quantum Chromodynamics (QCD). One of the rules can be summarized as follows: if you count each quark as +1 and each anti-quark as -1, and you add up all the quarks/anti-quarks in a quark structure, the sum must be zero, a positive multiple of 3 (such as 3, 6, 9 etc), or a negative multiple of 3 (such as -3, -6, -9 etc).

The sum will be 3 for all baryons, -3 for all anti-baryons and 0 for all mesons and all anti-mesons. Note that this rule prohibits a single quark or anti-quark from existing in isolation, but allows any combination of two more provided there is an appropriate mixture of quarks and anti-quarks.

In case you are wondering, forget about breaking a meson or baryon apart to obtain individual quarks. This is forbidden by asymptotic freedom and QCD.

Ignoring occasional claims for the discovery of tetraquarks, or pentaquarks (which I'll explain momentarily), these are the only quark structures known from experiments. Are other structures possible? There doesn't seem to be any theoretical reason why not.

The first possibilities to look for are the “tetraquark,” a combination of two quarks and two anti-quarks. Or a “pentaquark,” a combination of four quarks and one anti-quark. As I said a moment ago, there have been occasional claims for tetraquarks and/or pentaquarks, but these claims have typically been found be false.

Fast forward to July of this year. Another claim of a pentaquark. This was not the result of a deliberate search, but rather of experiments conducted at the Large Hadron Collider (LHC) in Geneva. These experiments were designed to probe the properties of a particle known as Lambda (1405). This particle has generally been considered to be a baryon (with three quarks); however there was an idea (not universally accepted) that Lambda (1405) might be a pentaquark (with 5 quarks).

The end result of the LHC experiments strongly suggest that Lambda (1405) is in fact a pentaquark. These results await peer review and could turn out to be false, though these results are much stronger than earlier claims.

An unresolved question: are tetraquarks, pentaquarks and larger structures (assuming they really exist) just formless bags of quarks, or are they built up from combinations of mesons and/or baryons? There is strong reason to believe the latter is true. Computer simulations suggest not only that the Lambda (1405) is a pentaquark, but a pentaquark formed by combining one meson with one baryon. Also we know that the atomic nucleus is a combination of baryons, so there are already examples of quark structures composed of baryons. It is not a great leap to consider other quark structures.

This story is not finished; there will no doubt be additional developments over time.

For more information read the following…

Sunday, July 12, 2015

New Horizon approaches Pluto

The  New Horizons spacecraft is nearing its encounter with Pluto. New Horizon is a spacecraft with the primary mission to flyby Pluto and collect photographs and other data. The closest approach is expected to occur on Tuesday.

I've discussed the issue of whether Pluto is a planet elsewhere and I don't have much to add except that Alan Stern (the principle investigator of the New Horizons mission) argues that Pluto should be considered a planet. There are differences of opinion on the matter.

If you consider Pluto a planet, Pluto is the only planet which has never been visited by a planet (either landing or flyby). That is until New Horizons.

If you don't, Pluto is still an interesting place. We know very little about Pluto and New Horizons should give us a lot of new data. If fact it already has, and unless there is a major failure it will continue to do so. It will take time to analyze the data, but it is very likely we will have a new understanding of Pluto in the near future.

For more information...

Alan Stern talked about New Horizon and Pluto on NPR's Science Friday (July 10th), see

Dr Stern gave a talk at the University of Michigan in February 2009. This talk discussed Pluto and the solar system; for a description of this talk see .

See also see this page from NASA:

Wednesday, July 8, 2015

Save the date: a pulsar and a giant star are due to meet in 2018

This is going to be a highly anticipated meeting with plenty of fireworks, though be glad we are far enough away to enjoy the view safely.

Pulsars are rotating neutron stars, the leftover remnants of stars that once shone and were more massive than our sun. Pulsars "pulse" because they emit highly directional "beams" of electromagnetic energy from their poles, and we detect this every time they rotate and the beam crosses our path. This can happen thousands of times per second, making them excellent timekeepers!

Stars come in different sizes, and the one of interest in 2018 is about 15 times as massive as our sun. It is in the constellation Cygnus and is about 5,000 light years away from us. This star has a binary companion, a pulsar, that will actually move through its outer atmosphere in 2018. The orbit is fairly long at 25 years.

Check out the NASA press release for more information.

Friday, April 10, 2015

University of Michigan to help build next-generation dark energy probe

Update to blog post of Thursday, September 5, 2013 ( )

University of Michigan to help build next-generation dark energy probe
By Aimee Balfe (Department of Astronomy) and Carol Rabuck (Department of Physics)

University of Michigan scientists and students will build components of a giant camera that will map 30 million galaxies' worth of the universe in three dimensions.

The camera is officially known as the Dark Energy Spectroscopic Instrument (DESI) and it's designed to help answer one of the most puzzling scientific questions of our time: Why is the expansion of the universe accelerating?
To read the rest of the article, go to:

Wednesday, March 4, 2015

Help name an exoplanet

Last year, the International Astronomical Union (IAU for short) decided that exoplanets deserve names. The IAU is the organization responsible for assigning names to astronomical objects and for demoting Pluto from a planet to a dwarf-planet.

Exoplanets are planets in orbit around stars other than our own sun. Over the past decade, hundreds of exoplanets have been discovered. The hope: exoplanets similar to the earth will eventually be discovered, planets that might be capable of supporting life. In fact we've discovered a wide variety of exoplanets, but we haven't found life beyond our solar system.

Individuals like you can be part of the process of naming one of these exoplanets. For now everyone must go through an official IAU astronomical organization. At the time of this post, there are two such organizations in the Ann Arbor area, the Ann Arbor Hands Museum and the University Lowbrow Astronomers. If you want to participate now you should look at the rules for this process (see the following web site)...

If you want to work through the Lowbrows, please leave a suggested name along with a short reason why you think it is an good name for a planet as a comment on this post. We will go through the comments, we are allowed to submit one name, so we will pick what we consider to be the best name and submit it.

You can also go to to find other organizations to work with.

Over the next few months suggested names will be send to the IAU by the Lowbrows and other organizations. Sometime this summer, all interested parties will be allowed to vote on the different names that the IAU has received.