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.