Cafe Scientifique: "Listening" to the Universe

To celebrate the centennial of Albert Einstein's theory of general relativity, astrophysicist Daniel Holz from the University of Chicago delivered a fascinating lecture on gravitational waves for Saturday's Cafe Scientifique.

After an introduction by MindMover Pecier Decierdo, Dr. Holz began the lecture by sharing that we have learned much about the universe through light or electromagnetic radiation ("we have only seen the universe"), and we can learn more with gravitational waves. He also spoke briefly on Einstein, and about how the renowned scientist revolutionized physics and cosmology with the general theory of relativity. 

Published in 1915, the general theory of relativity elegantly described how gravity is a property of spacetime. Simply put, spacetime tells matter how to move, while matter tells spacetime how to curve. General relativity also predicted many things, which have been shown to be consistent with current experiments: black holes, the perihelion precession of Mercury, the dynamic nature of the universe, and gravitational waves, which have yet to be detected directly. 

But what exactly are gravitational waves? As objects move, they cause ripples in the curvature of spacetime; these are gravitational waves, which spread outward from the source (such as a massive object). 

These waves are analogous to sound (but are not the same as sound waves!), because gravitational wave detectors are omnidirectional (you can hear sound from all directions, unlike light), gravitational waves are not scattered or absorbed (it is harder to block sound than light), and they are not used to form images. 

Interestingly, when we gather the data from the detector of a gravitational wave and put the signal into a speaker to convert it to sound, it would sound like this sample, the signal from a 100 solar mass binary system of black holes. 

Secondly, objects that move and accelerate generate gravitational waves, with most being very weak sources. To emit strong waves, you would need a large mass or have to move very fast. Examples of strong sources include: a star falling into a big black hole, two black holes/neutron stars crashing into each other, supernovae explosions, and the Big Bang. 

Dr. Holz also briefly discussed black holes, and why nothing, even light, can escape them. First, it is useful to understand escape velocity. When you throw a ball into the air, it always falls back down. If the ball is thrown up with a greater speed, it reaches a greater height, but still goes back down. This is because of the gravitational pull of the Earth. To keep it from returning to Earth, the speed of the object has to be great enough that it escapes the gravitational pull and does not return. Similarly, if the Earth were more massive or dense, then the pull would be stronger, and the escape velocity would also be higher. Eventually, an object can be so dense that the escape velocity must be equal to or even exceed the speed of light - this is how black holes trap everything that falls into it.

But what happens to an object or information that falls into a black hole? Scientists believe that it will be compressed very quickly out of existence. Time is also said to stop at the event horizon: if we watch someone falling into a black hole, they will appear to freeze from our frame of reference. Time measured by us for them will stop, but from their perspective, they will keep falling into the black hole. 

He also shared that it is a common misconception that black holes suck - if the Sun were to become a black hole, the Earth will still revolve around it the way it did before. 

Next, Dr. Holz discussed that gravitational waves stretch and shrink the distance between two points, but the changes in distances are very small and therefore hard to measure. Because of this, scientists collaborated to build the LIGO: the Laser Interferometer Gravitational Wave Observatory. 

Built in three places (Louisiana, Washington, and Italy [the Virgo interferometer]), these detectors are very sensitive to gravitational waves, capable of measuring distances that range from over 4 km to 1/1000 the size of a proton, and able to hear black holes crashing into each other 200 million light-years away.

The LIGO detectors will be fully operational at high sensitivity in September, so we will be able to detect gravitational waves soon. Dr. Holz adds that we should be excited by this prospect because we can confirm Einstein's findings, learn more about neutron stars, measure the age of the universe, and also learn more about dark energy. 

The succeeding Q&A session with the audience was also very informative, as the audience asked many poignant questions. 

When asked how the scientists knew where to put the LIGO detectors, Dr. Holz shared that it had to be a very stable location, where there is no large source of noise or seismic activity. For example, even if you are far from the coast, the LIGO can still hear the waves.

Dr. Holz also mentioned that Interstellar's consultant physicist Kip Thorne's original idea for the movie was that the characters would discover a wormhole using gravitational waves. Theoretically, if there were another universe, we could detect the gravitational waves from it.

When asked about his opinion on sending people to Mars, Dr. Holz said it was more of a political question. We can send people to Mars and learn about how humans would fare living on another planet, but it would be much more useful and cost-efficient to send probes and telescopes and use the money for building more tools. 

Saturday's Cafe Scientifique was a great opportunity for the guests and space enthusiasts of the museum to learn about gravitational waves - a new way to explore the universe, and just as revolutionary for us as when the first telescopes were invented. 

If you enjoyed reading about the Cafe Scientifique and are a big fan of anything related to space and astronomy, you might be interested in joining our Science Sleepover this August 29-30! Check out the details in our poster below. To register, please email:  

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