How do we know gravitational waves really exist if we've never directly detected one?One question I often get while discussing LIGO science with others is, "How many gravitational waves has LIGO detected?" Well, the answer to that is none - yet. But, we also didn't expect to detect any yet. During our last science run, we were able to detect gravitational waves that change the length of LIGO's arms about 10,000x smaller than the diameter of a proton. Even though this is almost an unthinkably small distance, this is considered a big gravitational wave at Earth (where these gravitational waves are produced in the depths of the Universe, they are incredibly strong - strong enough to rip you apart) and therefore a rare one - so rare that we statistically didn't expect to see one in the amount of data we collected. (Of course, the Advanced LIGO upgrade will change that!)
So, if we have yet to make a direct (meaning measured with our own instruments) detection of gravitational waves, how do we know that they really exist? After all, this is a lot of effort and resources going into the search! Well, we have seen the effects of gravitational waves on astrophysical systems in the Universe.
In the early 1970's, a pulsar (a very dense star that has beams of radio waves coming out of the magnetic poles) was discovered in the constellation Aquila at the Arecibo radio telescope in Puerto Rico. The beam of radio waves passed over the Earth 17 times every second. After observing this star for a while, it was discovered that some of the radio pulses came a little late and others a little early. The periodicity of these arrival times indicated that the pulsar had a companion star and they orbited around each other (together, this system is known as PSR B1913+16 [referring to its sky coordinates]). After further observation, it was found that the orbit of these stars around each other was gaining speed indicating that the stars are getting closer together (this is just like how a figure skater starts spinning with their arms extended at their sides and then, as they pull their arms to their body, they spin faster). This can only happen if energy is being carried away from this system of stars.
The only energy loss that matched what the researchers, Taylor and Hulse, observed was the energy carried away by gravitational waves. After about 20 years of making observations on this system, their measurements consistently matched the energy loss caused by gravitational waves.
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| This plot shows the change in the periodic time of closest approach (periastron) of this pulsar system compared to when the first observations were made in the early 1970's. The red dots are observational measurements and the blue curve is the prediction from general relativity given the emission of gravitational waves. |
This provided evidence of the existence of gravitational waves and won them both the 1993 Nobel Prize in physics. Unfortunately, LIGO will not be sensitive to this particular pulsar system for about 300 million years even with upgrades to the detector.
Now that we know that gravitational waves really are out there, we want to detect them affecting our own instruments so that we can learn more about the sources that made them (after all, we know exactly what is going on for the source above). Gravitational waves have encoded in them information about what made them very much like how radio waves can have music encoded on them. Just like without a radio you can't hear the music, without detectors like LIGO, we can't learn more about what made these sources. Gravitational waves can be emitted by things that don't produce light, like black holes, so we will be able to see them in ways traditional astronomy (astronomy using light) never can. On top of all that, gravitational waves can travel through matter and emerge unchanged - basically, there is no such thing as a gravitational wave shadow! So we will be able to observe things in the Universe that will forever be obscured to traditional astronomy.
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