Monday, June 20, 2016

A second gravitational wave event.

A few months ago, on February 11, there was an announcement from LIGO. It had detected a signal from a pair of a black holes - the first direct detection of gravitational waves. The signal was actually detected the previous September, but announced in February.

However there were other events. The process works this way: when an event occurs, computer algorithms very quickly sort events into “probably noise from a terrestrial event such as a truck passing nearby” and “probably a real event from black holes or neutron stars.” If the event is a real event, the algorithm estimates the mass of the black holes or neutron stars involved. For some background information on how these algorithms work see

When there are “probably real” events, additional analysis is done on the data and this analysis takes time. This is the reason for the delay between the event detection and the public announcement.

Shortly after the February announcement, there were already rumors of additional events. You'll have to forgive me, I was aware of these rumors, but did not report on it until now – nevertheless I had good reason to believe they were almost certainly true. We now have official confirmation they were true. A second event had been detected in December and was announced last week. See this article from New York Times science writer Dennis Overbye:

What happens now? Almost certainly there are and/or will be additional events beyond the two we know about, and we should see announcements of these events over time. Also in the near future, additional facilities similar to LIGO will come on line. These facilities will allow researchers to make more precise determinations of the direction the gravitational waves are coming from, possibly allowing the source of the waves to be located.

Unfortunately ground based gravitational wave detectors are limited in the frequencies they are able to detect. For this reason, there have been plans to place detectors in space. The first such plan was LISA. A set of spacecraft in space would perform measurements similar to what LIGO does, but because they are not attached to a solid object (namely the earth), they can respond to a wider range of frequencies, allowing a wider range of gravitational waves to be detected. LISA is now defunct, but the idea was resurrected in a new project called eLisa. For information see...

The first phase of this project called eLisa Pathfinder was launched this past December. It is a proof of concept which will not be able to detect gravitational waves, but will develop some of the technologies needed to detect gravitational waves. The current plan is to launch eLisa in the year 2034.

Sunday, May 8, 2016

Mercury Transit

Tomorrow May 9 is the Mercury Transit. A Mercury Transit occurs when Mercury passes directly between the Sun and the Earth. And that takes place approximately once every ten years or so.

To observe the transit you must use proper equipment (otherwise you might damage your equipment or your eyes).

For more information:

Monday, March 7, 2016

More information on LIGO and gravitational waves.

1. Before LIGO made their discovery of gravitational waves public, the LIGO team worked with SXS, a project with the goal of simulating black holes and other extreme phenomena.

In the process, SXS produced a number of animations. They are fun to watch, but are also based on the best available science and state of the art numerical simulations. The animations are each about a minute at most. To look at these animations, go to one of these links...

2. Keith Riles (One of the LIGO team members) gave two lectures on gravitational waves shortly after the LIGO announcement. Each one is about one hour in length. 

"Gravitational Waves - Einstein's Audacious Prediction" (February 13, 2016)

"The Hunt for Gravitational Waves - Was Einstein Right? (February 20, 2016)

Thursday, February 11, 2016

Gravitational Waves Detected!

An earlier blog post discusses rumors of the detection of gravitational wave. At the time we had no official confirmation. Now we do.

Early today, the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced that it had made a direct observation of a gravitational wave for the first time. LIGO consists of two instruments, and both had recently been upgraded to make them more sensitive (and thus increase the chance of discovery). Back in September 2015, after the upgrade, a signal had been detected by both instruments. After analysis, it was determined that these signals were consistent with two black holes in a close "death spiral." They spiraled closer and closer until they collided and merged to form a single black hole. One of the black holes was 29 times the mass of the sun, the other 36 times the mass of the sun. This occurred about 1 billion years ago and resulted in a gravitational wave that was detected by both of the LIGO instruments. This result was keep secret (except for occasional "leaks" of information) until earlier today (February 11), when LIGO gave a press conference releasing details of the discovery.

This is an important result, but why?

These videos from PBS Digital Studios will be of interest.

A video recorded after the announcement, explaining what the discovery is all about...

LIGO's First Detection of Gravitational Waves

As you may have seen in an earlier blog post, rumors of this result were known for a while, here is a video recorded a few months ago, asking if gravitational waves have already been discovered. The fact is they already had been, but only a few people directly involved with LIGO knew this for sure.

Have Gravitational Waves Been Discovered?

Some other videos discussing different aspects of General Relativity. General Relativity is Einstein's theory of gravity, and it predicts that gravitational waves exist.

"Are Space And Time An Illusion?":

"Is Gravity An Illusion?"

"Can A Circle Be A Straight Line?"

"Can You Trust Your Eyes In Spacetime?":

General Relativity and Curved Spacetime

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 that is not constant straight line motion and not simple rotation) 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…