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Science Breakthrough Explained: Gravitational Waves

The following is a guest post by our AP Physics and Advanced Pre-Calculus class instructor, Taryn. An advocate for science outreach and holder of a degree in Astrophysics, she is excited to share the following breakthrough with the TA community.

In the following post you’ll learn about what the heck gravitational waves are and how what scientists are doing now connect with what Einstein was doing over 100 years ago!

Almost exactly two years ago something huge happened in the scientific world. On February 11, 2016 news sites and science blogs exploded with exciting headlines: “Gravitational Waves Detected, Confirming Einstein’s Theory” (NY Times), “Gravitational waves: breakthrough discovery after a century of expectation” (The Guardian) and the physics world’s much more sober and to-the-point announcement: “LIGO detects first ever gravitational waves – from two merging black holes.” This February event even earned itself its own Wikipedia page: First observation of gravitational waves.

 

The first natural question to all this is, is this really such a big deal? Or is this just a shameless attempt to engage the public’s interest in science by making the drudgery labor of science research sound flashy and sci-fi-y?

 

Actually, it is a big deal. This is one of the most exciting things to happen in modern physics in our lifetime.

 

So the next question is, “Why?”

 

To answer that, we have to start a little ways back—or, specifically, about 100 years back.

 

In 1905, Albert Einstein published a paper called “The Electrodynamics of Moving Bodies” where he introduced an idea called “special relativity.” (These days, we often call this paper the “special relativity paper.”) In this paper, Einstein took on a problem that had been puzzling physicists for decades: light always travels at the same speed (about 186,000 miles per second). And he took this a step further: not only does light always travel at the same speed, but the speed of light is an upper limit and nothing can go faster than this, no matter what. This might not sound like a “puzzle” or a bold claim, but actually it flies in the face of what we know about speed. Isaac Newton, for one, would find these statements very alarming.

 

Take this thought experiment:

 

You’re standing on top of a moving train going 60 mph and you’re firing a very powerful nerf gun. About 50 yards away from you is a railroad intersection with a big yellow “Yield” sign. This is your target. Standing on the ground, about 5 yards from the intersection, your friend is also playing with a nerf gun, trying to hit the Yield sign target. You guys are using the same toy gun, which can fire nerf pellets at 35 mph. The train chugs along and when you pass your friend on the ground you both fire your nerf guns at the exact same time, dead on for the target.

 

So the question is, whose nerf pellet will reach the target first?

 

Answer? Yours. Even though your nerf pellets are both leaving the gun at 35 mph, since you’re moving on a train going 60 mph, the pellet is actually heading towards the target at 95 mph. So if you and your friend fire from 5 yards away, your nerf pellet will reach the target in 0.1 seconds, and hers will reach it in 0.3 seconds.

 

But now imagine you guys are shooting a laser beam at the target. Again, you pass each other 5 yards away from the sign and aim your laser beam at the target. Whose laser beam will reach the sign first?

 

Answer? Isaac Newton would say your laser beam will get there first. Einstein would say neither. The two laser beams will reach the sign at the same time even though you’re standing on a moving train and your friend is at rest.

 

Einstein is right.

 

Actually, it wasn’t Einstein who first postulated this wild conclusion. A very important physicist named James Clerk Maxwell made the claim in a paper in 1865 that light always travels at the same speed. So even if you’re standing on top of a train firing a laser and your friend is at rest, the laser beam is still going at the same speed from each gun.

 

Einstein was fascinated by this paradox since his teen years. The problem is that obviously Maxwell and Newton can’t both be right. So when Einstein dove into research himself he had to make a choice: either side with Maxwell, a physicist new to the scene in the last 50 years with a bunch of crazy new theories, or with Newton, the one whose Newtonian mechanics have accurately described our universe for the past 250 years. Amazingly, he chose Maxwell.

 

One of the most brilliant things about Einstein is how he could take a complicated question and, rather than tearing through it with complex math, he would look at it like a philosopher and just turn it on its head. How is it possible that you can’t go faster than the speed of light? What if you’re on a rocket ship and you’re approaching the speed of light? What happens when you reach that magical speed limit?

 

Einstein looked at this question and thought – okay, we can’t change the speed. Speed is defined as distance per time (like miles per hour, meters per second, etc). So if the speed part can’t change, then maybe, as you approach the speed of light, it’s the distance part and the time part that change.

 

And he was right. As you approach the speed of light the space you’re travelling through gets contracted and time slows down. This isn’t an illusion or an effect – space literally contracts and time literally slows down.

 

This is one of the craziest and most astounding discoveries humankind has ever made. Time isn’t just an absolute thing that marches forward in the background of our reality. Time is just another dimension and it can slow down or speed up. Our universe, then, is made up of 4-dimensions: 3 dimensions of space and 1 dimension of time, and together they make up our “spacetime.”

 

But what does all this have to do with gravitational waves?

 

Well after Einstein’s incredible discovery about the speed of light, he started to think about gravity. Isaac Newton said that if two objects have mass (like, say, the Earth and you), they’ll attract each other. This force of attraction we call “gravity.” How strong this force of attraction is depends on two things: the objects’ masses and how far apart they are. But Newton doesn’t mention anything about time. Einstein saw this as a problem.

 

Imagine two stars (say, Rigel and Betelgeuse) way out in space, separated by a few hundred light years, attracting each other through this force of gravity. If Betelgeuse moves away from Rigel, Rigel should notice this change instantaneously in a decrease in its force of attraction. Somehow, the information that Betelgeuse moved reaches Rigel instantaneously — faster than the speed of light!

 

Einstein said this is impossible. So having debunked Newtonian mechanics with his special relativity paper, he decided to go on and disprove Newtonian gravity and deliver the finishing blow to poor Newton’s reputation.

 

Einstein proved that it’s not that two masses magically attract each other. Rather, when you put a massive object in our universe (again, imagine the Earth), this object will bend spacetime. The easiest way to picture this bending is to imagine placing a bowling ball on a trampoline. The trampoline sinks downwards. This is what happens to our universe’s spacetime due to the presence of the Earth. We humans on the Earth feel this spacetime bend and, as a result, we feel ourselves pushed “inwards” towards the Earth. That is what’s really happening when we talk about “gravity.”

 

Of course, the Earth is far from one of the most massive objects in our universe. Actually, compared to some objects it’s incredibly small and light. A black hole, for example, is an extremely dense and massive object. So a black hole creates a huge bend in the spacetime around it due to its mass. If there’s a sudden and massive change in the bending of spacetime (like, for example, when two massive black holes collide with one another), this event will send out ripples in our spacetime.

 

 

See this video from the New York Times for more on the LIGO experiment and black holes

 

These ripples are called “gravitational waves.” If the black holes are massive enough, then, as crazy as it might sound, with ridiculously advanced and sensitive technology, we can theoretically detect these waves here on Earth. The LIGO project in the US built an enormous and extremely sensitive pair of detectors in Washington state and Louisiana to detect these waves. After 40 years and $1.1 billion in funding, on February 11, 2016, they finally announced a confirmed detection of a gravitational wave.

 

A century later, Einstein’s beautiful, simplistic theory that revolutionized our understanding of nature received its best experimental validation. And the scientific community basically exploded in celebration.

 

If you want to learn more about Einstein’s theory of relativity, check out these articles!

http://iontrap.umd.edu/wp-content/uploads/2016/01/WudkaGR-7.pdf

http://physics.ucr.edu/~wudka/Physics7/Notes_www/Pdf_downloads/6.pdf

And be sure to check out PBS Space Time’s YouTube channel for some really crazy physics videos.

 

If you enjoyed this, stay tuned as we’ll be posting long form articles on science breakthroughs, historical figures, and more each month!

2018-12-11T14:19:46+09:00