6 Things You Probably Didn’t Know About Gravitational Waves
Now that we have had some time to think about LIGO’s detection of gravitational waves, there are a few interesting comments I can make about it.
Gravitational waves don’t have to be useful
There is a common question that comes up in media whenever there is a new scientific discovery—“What can you do with it (gravitational waves)?” Can you build an anti-gravity machine? Could you use this to build a warp drive? These are all great ideas, but they miss the point. We don’t study gravitational waves so that we can make stuff. We study gravitational waves because we want to understand gravitational waves.
I think that Richard Feynman said it best:
“Physics is like sex: Sure, it might give some practical results, but that’s not why we do it.”
Anyway, we couldn’t really predict the kinds of new technologies that would come from some discovery. Take the laser for example. First created in 1960, many people thought it had no practical applications. Of course they were wrong. Lasers are everywhere now.
The LIGO detection didn’t prove the existence of gravitational waves
First there is the question of “proof.” Science never proves anything to be true—it just can’t do that. Instead, science is about building models. If these models agree with real data, that’s great—but it doesn’t prove the model is true. However, if you find data that doesn’t agree with your model then it could prove that the model is wrong. So really, “prove” should just be added to the list of scientific words we should stop using (the list already includes: hypothesis, theory, scientific law).
OK, fine. LIGO didn’t prove the existence of gravitational waves. But the project was the first to collect evidence to support the gravitational wave model. Is that better? No. There is still a problem. Flash back to 1993. Russell A. Hulse and Joseph H. Taylor, Jr received the Nobel Prize in physics for their discovery of a binary pulsar with a changing orbital period. According to Einstein’s theory of general relativity, these pulsars should radiate gravitational waves and decrease their orbital period in the exact same manner as detected by Hulse and Taylor. So, they were the first to give convincing evidence of gravitational waves.
But wait. Didn’t LIGO actually detect the waves instead of just gather evidence? Well, you could claim that—but I guess it’s all about you you define “direct measurement.” No one actually saw a gravitational wave. LIGO looked at the motion of mirrors and inferred ideas about gravitational waves. Don’t get me wrong. It’s still a big deal.
LIGO probably wouldn’t have detected this signal without Advanced LIGO
Advanced LIGO increased the sensitivity of the detectors. Since a gravitational wave signal decreases in strength with distance, a more sensitive detector lets you “see” farther into the universe. Much farther.
Without Advanced LIGO, you would need a gravitational event (like colliding neutron stars) much closer to Earth. If these events are rare, you might have to wait a long time for that to happen. By increasing the observational distance, LIGO has a much greater chance of detecting future events.
The National Science Foundation made a significant investment in LIGO
The NSF began funding the search for gravitational waves in the 1970s. Since then it has invested approximately $1.1 billion. That’s a lot of money over a very long time. I know everyone always wants an instant return on science investments, but that’s just not the way it works. Science can take a long time with seemingly little progress (although real progress is always made). Is this project worth a billion dollars? Absolutely. Just consider that in 2015 the US military spent $600 billion—LIGO doesn’t seem so expensive.
There are plans to put a gravitational wave detector in space
Yes. In space you could have a detector without much of that bothersome ground noise. Also, you already have a vacuum. A space-based gravitational observatory could also be quite large since you would just have to put mirrors at different locations. Of course there are still technical difficulties with a space-based observatory, but that doesn’t mean we aren’t trying.
This is the exact goal of the eLISA program. In fact, the program just launched two LISA Pathfinder test masses. This particular mission will test how accurate the two masses can be kept in position—a necessary step towards building a space-based gravitational observatory.
You can probably measure low frequency gravitational waves with a radio telescope.
Pulsars are like the universe’s clocks. The timing of the pulsar is measured with a radio telescope (that uses radio waves instead of visible light). How could this be used as a gravitational wave detector? The basic idea is to look at the signals from pulsars at different locations. As a low frequency gravitational wave passes through the pulsars, their individual timing clocks will change. Using the change in time with the location of the pulsars, you basically have a giant version of LIGO in space (really giant). The idea is called a pulsar timing array—it’s real.
LIGO is probably happy that they were able to report a detectable gravitational wave before the radio telescope people did.
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