Apr 10

Gravitational Wave Astronomy Will Be The Next Great Scientific Frontier


By Ethan Siegel

Imagine a world very different from our own: permanently shrouded in clouds, where the sky was never seen. Never had anyone see the Sun, the Moon, the stars or planets, until one night, a single bright object shone through. Imagine that you saw not only a bright point of light against a dark backdrop of sky, but that you could see a banded structure, a ringed system around it and perhaps even a bright satellite: a moon. That’s the magnitude of what LIGO (the Laser Interferometer Gravitational-wave Observatory) saw, when it directly detected gravitational waves for the first time.

An unavoidable prediction of Einstein’s General Relativity, gravitational waves emerge whenever a mass gets accelerated. For most systems — like Earth orbiting the Sun — the waves are so weak that it would take many times the age of the Universe to notice. But when very massive objects orbit at very short distances, the orbits decay noticeably and rapidly, producing potentially observable gravitational waves. Systems such as the binary pulsar PSR B1913+16 [the subtlety here is that binary pulsars may contain a single neutron star, so it’s best to be specific], where two neutron stars orbit one another at very short distances, had previously shown this phenomenon of orbital decay, but gravitational waves had never been directly detected until now.

When a gravitational wave passes through an objects, it simultaneously stretches and compresses space along mutually perpendicular directions: first horizontally, then vertically, in an oscillating fashion. The LIGO detectors work by splitting a laser beam into perpendicular “arms,” letting the beams reflect back and forth in each arm hundreds of times (for an effective path lengths of hundreds of km), and then recombining them at a photodetector. The interference pattern seen there will shift, predictably, if gravitational waves pass through and change the effective path lengths of the arms. Over a span of 20 milliseconds on September 14, 2015, both LIGO detectors (in Louisiana and Washington) saw identical stretching-and-compressing patterns. From that tiny amount of data, scientists were able to conclude that two black holes, of 36 and 29 solar masses apiece, merged together, emitting 5% of their total mass into gravitational wave energy, via Einstein’s E = mc2.

During that event, more energy was emitted in gravitational waves than by all the stars in the observable Universe combined. The entire Earth was compressed by less than the width of a proton during this event, yet thanks to LIGO’s incredible precision, we were able to detect it. At least a handful of these events are expected every year. In the future, different observatories, such as NANOGrav (which uses radiotelescopes to the delay caused by gravitational waves on pulsar radiation) and the space mission LISA will detect gravitational waves from supermassive black holes and many other sources. We’ve just seen our first event using a new type of astronomy, and can now test black holes and gravity like never before.


Image credit: Observation of Gravitational Waves from a Binary Black Hole Merger B. P. Abbott et al., (LIGO Scientific Collaboration and Virgo Collaboration), Physical Review Letters 116, 061102 (2016). This figure shows the data (top panels) at the Washington and Louisiana LIGO stations, the predicted signal from Einstein’s theory (middle panels), and the inferred signals (bottom panels). The signals matched perfectly in both detectors.


Mar 01

What’s Up in the Sky

What’s Up in the Sky – March, 2016

In the Middle Ages (before science) it was generally believed that heavier objects would fall to Earth faster than lighter objects. Some of my students still believed that twenty years ago. Galileo Galilei thought otherwise and just to demonstrate his theory (according to legend) he went to the top of the leaning tower of Pisa and dropped a 10-pound and a 5-pound cannon ball simultaneously. They both hit the ground at the same time.

His little demonstration probably did more to get him in trouble than to convince anyone their common sense was wrong. And, thanks to Edmund Halley’s prediction that a comet would return on a certain date (it did) it would take another hundred years or so before science was generally accepted.

Fast forward to now. One of modern science’s best known prediction makers was Albert Einstein. His list of hits is impressive: the bending of light by gravity, time dilation, anomalies in the orbit of Mercury, black holes. All of them have been verified many times. One prediction, however, was so extremely difficult to test that it took over a hundred years to verify. But it has been.

On February 11, scientists working with the Laser Interferometer Gravitational-wave Observatory (LIGO) announced that gravitational waves had been detected by the instrument on September 14, 2015. The time lag is how long it took them to verify the observation.

This is a monumental confirmation of general relativity, right up there with the 1919 test of the deflection of starlight by the eclipsed Sun, which made Einstein famous (he was right that time, also). Not only has this major prediction been verified, but also a new and unprecedented window into the cosmos has been opened.

Gravitational waves can be described as “ripples in the fabric of spacetime” and arrive at Earth after traveling for billions of years from the distant universe. Their existence was first demonstrated in the 1970s and 80s when scientists observing a pulsar and neutron star orbiting each other noticed that the orbit of the pulsar was slowly shrinking. They also showed that this was due to the release of energy in the form of gravitational waves and that measurements of gravitational waves would now be possible.

And that’s exactly what happened. The LIGO detectors are rather amazing pieces of technology. Each consists of two 4 km long, 4-ft diameter tubes kept at an almost perfect vacuum at right angles to each other. Two beams of laser light travel the lengths of the tubes to measure the distance between two precisely placed mirrors at the ends of the arms. According to Einstein, a gravitational wave passing the detector will cause the distance between the mirrors to change infinitesimally. The instrument is so sensitive that it can measure changes as small as one ten-thousandth the diameter of a proton!

Two such instruments are used, one in Washington and one in Louisiana, to determine the direction from which the waves originated and to rule out other possible sources. Interestingly, each had just undergone a major upgrade that increased its sensitivity and they were on their first observational run. Not bad for a first try.

Scientists are anxious to have more such devices at locations around the globe to give them an even better understanding of what’s up in the sky.

Feb 15

What’s Up in the Sky

What’s Up in the Sky – February, 2016

Planets and Stars in February

By now you have undoubtedly heard about the five planets visible in the morning sky so I will offer a few tips for successful viewing.

Go out about an hour before sunrise and look toward the southeast. There you will see the planet Venus, by far the brightest object in the sky, other than the Moon of course. Over in the southwest look for the second brightest non-lunar object, Jupiter. In between Venus and Jupiter are Mars and Saturn. Now, these guys have been visible for most of the winter, but what makes this alignment special is that they are now being joined by Mercury, just to the east (left) of Venus.

The show will continue through mid-February but you can use the Moon for the first week of the month to locate the dim planets in case you are unsure which is which. On February first, it will be close to Mars, on the third it will be just above Saturn, and on the sixth it will be near the eastern horizon very close to Mercury and Venus. In fact, that will probably your best chance to spot Mercury as it is normally faint and lost in the glare of the rising Sun. The best time will probably be 7:20 – 7:30 a.m. each morning.

You may have also heard about the “discovery” of a new planet, “Planet 9”. According to New Horizons project scientist and Holland resident, Harold Reitsema, “this is an interesting possibility but far from a “Discovery”. We don’t know much at all about (Kuiper Belt Objects or) their typical orbits. So this is really speculation. Educated speculation, but speculation none the less.” Clearly further study is needed.

If you are not a morning person, there is still plenty to see in the sky in the evening. In fact, February is one of the best month for stargazing, in spite of the cold weather. Facing south you will surely spot our old friend, Orion, the Hunter, with his distinctive group of four bright stars bisected by three stars that make up his belt and three fainter stars forming his sword.

You can use Orion as a guide to find lots of other cool stuff. Follow the line of belt stars down toward the horizon to find Sirius (as if you need guide stars . . . it’s the brightest star in the sky), and up toward the right to find the star Aldebaran and the V-shaped group of stars that form the head of the constellation Taurus, the Bull.

Finally, follow the line formed by the top two stars in Orion’s shoulders eastward to Procyon then continue straight up to Castor and Pollux. Complete your circle tour with Capella, a yellow/gold star almost directly overhead in the constellation Auriga. For added interest, bring along a pair of binoculars and just scan the area for nebulae and star clusters. Let the stars be your guide to what’s up in the sky.

Dec 05

What’s Up in the Sky

What’s Up in the Sky – December, 2015

A Variety of Offerings in This Month’s Sky

December is not known for clear skies and warm evenings, so it’s a good month to concentrate on some basic observing of easily found objects. Like the Sun.

The winter solstice, which occurs on the date when the Sun rises and sets at its most southern point on the horizon, occurs this year on Monday, December 21 and marks the beginning of winter. You may remember last September when the Sun was shining right in your face as you drove down an east-west road early or late in the day. It is interesting to see how much the location of sunset changes so try to spot one using a westbound road or other landmark as a guide. You may be amazed at the difference.

The Geminid meteor shower is active from December 4 – 17, reaching a maximum on December 14. This is an interesting shower to observe as some of the meteors come in at a very low angle and therefore appear as long, bright streaks.

Lately I have been writing about spectacular observations of the planet Venus encountering the Moon or other planets and this month is no exception, except it’s another before sunrise event, this time on a Monday, no less.

But this one should be worth the effort because Venus will be a beautiful sight right below the crescent Moon. Since sunrise isn’t until about 8 a.m., it should be a spectacular sight as late as 7:30. For veteran observers, try tracking Venus until just past 12:30 p.m. when the planet will disappear behind the sunlit limb of the Moon. A challenging observation.

One of my favorite things to observe this time of year is the constellation Cygnus, the Swan. Also known as the Northern Cross, it’s bright star, Deneb, is part of the Summer Triangle that has been nearly overhead since late summer. This time of year, Cygnus sets a few hours after sunset and, if viewed as a swan, is diving into the western horizon. But if viewed as a cross, it stands straight up on the horizon and can be seen as one symbol of the Holiday Season. Regardless of your interpretation, it is a beautiful sight.

If you look toward the west, you see the constellations of summer and toward the east, the coming constellations of winter, but directly above you can find a constellation that points to another of my favorite objects.

Face south and then look straight up. A little beyond overhead you should see the “W” – shaped constellation, Cassiopeia. Using the right hand “point” of the W, follow it down to just about overhead and look for a fuzzy blob (binoculars help a lot). This is M31, the Andromeda galaxy. This is the only object in the sky visible to the naked eye that is not part of our own Milky Way galaxy and, at over two million light years, is the most distant object you can observe without the aid of a telescope.

All in all, it’s a good month for what’s up in the sky.

This month in history:
Dec. 3: Pioneer 10 spacecraft makes closest approach to Jupiter – 1973
Dec. 11: First auction of Soviet space hardware and artifacts – 1993
Dec. 14: Gene Cernan, Apollo 17 astronaut, is last human to walk on Moon – 1972
Dec. 14: Tycho Brahe born – 1546
Dec. 24: Apollo 8 astronauts give us inspirational moment from lunar orbit – 1968
Dec. 25: Isaac Newton born – 1642

Nov 08

What’s Up in the Sky

What’s Up in the Sky – November, 2015

The Morning Planet Show, Act 2

Normally, this time of year does not bode well for observing due to cloudy skies and generally bad weather. I don’t care. I always enjoy writing about it anyway.

Last month I recommended you observe the planets in the morning and this month the show continues. Unfortunately, the end of Daylight-Saving Time on November 1 means the Sun will rise one hour earlier so you will have to be out no later than 6:30 a.m. in order to catch the scene.

Starting on November 1, look toward the east one hour before sunrise and you will see the familiar sight of brilliant Venus with bright Jupiter above and to the right. With binoculars, look for much dimmer Mars slightly below and to the left of Venus, about one Moon diameter away. Watch as these two planets pass each other from day to day until the real show, which runs from Thursday, the 5th through Saturday, the 7th, when the main players are joined by a lovely crescent Moon. On Friday it will be right next to Jupiter and on Saturday very close to Venus, with Mars just above. Binoculars will add extra enjoyment.

If you’re really an astro-nut and are looking for a rare observing opportunity, you are in luck because this month you may have a chance to observe something I never saw in thirty years until last month – a full-Moon occultation. You may recall an occultation occurs when one object, such as the Moon or a planet, passes between Earth and a distant object, such as a planet or star. One of my first, and still most memorable, observations was of the Moon occulting Aldebaran in April of 1979. Readers with planetarium software should check out that date.

Anyway, full-Moon occultations occur when the full Moon passes in front of a star, which is fine except almost all stars are invisible when they are right next to a full Moon – like a candle next to a spotlight. I was fortunate enough to have witnessed such an event during September’s lunar eclipse. With the Moon embedded in Earth’s shadow, it no longer drowns out the nearby stars and I was able to observe it moving in front of a star – very exciting.

Your chance this month will not involve an eclipsed Moon, but rather a bright star. Each month the Moon follows a regular, rather narrow path across the sky which includes very few bright stars, but this month it occults Aldebaran, the brightest star it can ever pass across.

To witness this event, you will again have to be an early riser as the star disappears close to 5:40 a.m. Because of the Moon’s glare, you will need a telescope at high power, but because of the star’s brightness, you have a rare chance to make a difficult observation of something very cool up in the sky.

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