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home : columns : columns May 22, 2017

4/6/2016 3:51:00 PM
Riding the Gravitational Wave
By Meg Evans Smith

Gravitational waves – ripples in the “fabric” of space and time – have swept 1.3 billion light years across the universe to become astronomy’s exciting new discovery.  

Albert Einstein predicted their existence a century ago, but only recently did scientists have an instrument sensitive enough to detect them.

Last September, gravitational waves triggered by the powerful merger of two black holes passed through the Laser Interferometer Gravitational-Wave Observatory (LIGO), confirming Einstein’s prediction and generating a chirp heard ’round the world. Listen to it here: (http://tinyurl.comgwavechirp).

“This marks the beginning of a new era for physics and astronomy,” says Vicky Kalogera, Ph.D., an astrophysicist and data scientist at Northwestern University, “and launches a new field into existence.  We compare it to the time when Galileo turned a telescope toward the sky and was able to see things for the very first time.”  Now, she says, there is great potential to discover new things in the universe by listening for their gravitational wave signature.    

Dr. Kalogera and several Northwestern colleagues are part of the 1,000-member international LIGO Scientific Collaboration, who designed the L-shaped instrument that detected the waves as they passed through earth on Sept. 14, 2015.  

There are two LIGO detectors – in Louisiana and Washington state – each consisting of two 4-kilometer vacuum tubes through which laser beams circulate, creating a steady “interference pattern.” As the waves hit, the tubes stretched and compressed by a fraction of the diameter of an atom – enough to shift the interference patterns slightly. That shift signaled the presence of gravitational waves as well as their source:  two black holes orbiting each other and then merging into one.

Identifying Black Holes For the First Time
Until now, black holes – gravity-intense areas in space from which no objects or light can escape – could only be indirectly observed through their distorting effects on nearby stars and gases, explains Shane Larson, Ph.D., a gravitational wave astrophysicist at Northwestern University and member of the LIGO team. Now LIGO can directly identify black holes through their gravitational wave signals.

“Gravitational waves are great,” says Dr. Larson, “because we can see them when the objects aren’t emitting light at all (like black holes), or are too far away to see with light waves.” Light is the usual way to explore the universe: practically everything in space emits light in various forms of electromagnetic radiation including radio waves, microwaves, infrared radiation, ultraviolet radiation, X-rays, gamma rays and visible light – what humans can see with the naked eye.

It Started With Einstein’s Theories
Albert Einstein imagined the universe as four-dimensional “spacetime” – three-dimensional space plus the one dimension of time. Spacetime has a shape that is flexible and can change. Einstein explained gravity as the curving of spacetime, rather than a force that pulls things together.  

We feel the shape of spacetime as gravity when massive objects like our sun warp and curve spacetime, creating a gravitational “well” around which planets like Earth can orbit.   

Powerful events such as an exploding star (a “supernova”) or the merging of two black holes ripple spacetime as they release gravitational energy, changing the shape of spacetime, and LIGO measures that change.  

The merger detected by LIGO was powerful, indeed:  if it could have been seen as light (remember that black holes emit no light), it would have been 50 times brighter than all the light from all the stars in the universe. In spite of all that intensity, gravitational waves are surprisingly weak, so they require an exceptionally sensitive instrument like LIGO to detect them.

Now, About That Chirp  
Gravitational waves let scientists “hear” rather than see the universe, explains Dr. Kalogera, because they vibrate in the same frequency range as sound waves humans can hear.  

“You can take a gravitational wave signal and convert it into a sound,” she says, “which gives us a different way of perceiving and comprehending the signal.” And that’s where the “chirp” comes from – it is a sound interpretation of the gravitational wave.  

Fittingly, LIGO functions more like ears than eyes, says Dr. Kalogera. Standard astronomy telescopes are pointing instruments, helping us look in one direction.  LIGO detectors are sensitive to gravitational waves coming from all directions – just as our ears receive sounds from everywhere, no matter what direction we are facing.  

What Can We Do With Gravitational Waves?
The short answer, says Dr. Larson, is nothing – for now.  “It’s part of a continuing effort to understand how the universe is put together,” he says, “and for us to turn it into a gadget that is useful at home is not possible at the moment.”  

That may take decades, he says, just as it took decades to apply Einstein’s theories, which are now essential to the global positioning systems (GPS) in our phones and cars.  GPS determines location using time and the distance between the phone signal and three satellites. Satellite clocks run faster than clocks on Earth, and the time difference must be accounted for or the navigation will be off.  

Einstein predicted that effect in 1915 – when phone calls still required operator assistance – and almost a century later we figured out how to use it to find each other.

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