What Are Gravitational Waves?

In the red corner, weighing in at 36 solar masses, the reigning champion, Big brother black hole clone! In the blue corner, weighing in at 29 solar masses, the challenger, Little brother black hole clone! It’s going to be an epic fight. How will it end? Let’s find out with some physics! Hey Crazies. Some of you were wondering if I was ever going to do a video on Gravitational Waves. Well now’s the time! I realize the hype is over and there are already tons of videos about this, but there are sti

ll some connections that need to be made and some very misleading visuals that need to be addressed. Plus it doesn’t hurt to get your information from multiple sources. Especially with something this weird and crazy. So let’s get back to those black-hole clones! As the black-hole clones circle each other, they move closer and closer and speed up in the process. Eventually, there’s a connection, does one go down for the count? No, don’t be ridiculous! This is astrophysics. The two combine into on

e giant black-hole clone. They’re like a cosmic Voltron! Voltron. Defender of the Universe? Very successful cartoon? Someone has to know Voltron. I’m so old. Anyway, the combined giant black-hole clone is 62 solar masses, which might leave you thinking: Isn’t 36 + 29 = 65, not 62? And why weren’t their orbits stable? Both very good questions and both have the same answer: Gravitational waves! See, waves are a big deal because they move energy from one place to another without having to move matt

er from one place to another. That’s the definition of a wave. They’re so useful that not 1 but 2 of our senses have evolved to detect them. Mechanical Waves are waves in matter like strings, air, the Earth, or water. Those are the kind you think of when I say the word "wave." Light is also a wave, but in electric and magnetic fields instead of matter. We did a whole video on light if you’re interested! Since there isn’t a lot of matter in space, light was the only way we were able to find astro

nomical stuff. Stars like the Sun emit visible light, so we can see them, but planets like Jupiter don’t, so we have to wait for light from the Sun to get there, bounce off, and then come to Earth before we can see it. Unfortunately, not everything can emit or reflect enough light. Black Holes and Neutron Stars, for example. Sometimes we get lucky and those things pull in matter that we can see, but otherwise we’re left in the dark. That’s why gravitational waves are such a big deal. They’re a w

hole new type of wave. Wiggles in the fabric of space-time itself, but that means they act a little differently than other types of waves, so let’s be careful. For something one-dimensional like this string, you only need one number to represent its curvature. For curvature on a two-dimensional surface, you also only need one number, but the same cannot be said for higher dimensions. So this is a terrible graphic for space-time. For curvature within of a three-dimensional space, you need 6 numbe

rs. For curvature within a four-dimensional space-time like our universe, you need 20 numbers, which results in 10 independent Einstein equations. Those equations describe all of the curvature that results in gravitational motion. But not all curvature creates waves. You can’t have anything that’s spherically or cylindrically symmetric, so a lonely spinning black hole probably isn’t going to do it, but two black holes orbiting each other will, just like our black-hole clones from early. And that

’s exactly what LIGO detected on September 14th, 2015. What’s LIGO? The coolest thing to happen in experimental physics since the LHC! LIGO stands for Laser Interferometer Gravitational-wave Observatory. They detect gravitational waves using lasers. About 1.3 billion years ago, a 36 solar-mass black-hole and a 29 solar-mass black-hole collided to form a 62 solar-mass black-hole, releasing gravitational waves carrying 3 solar-masses worth of energy and then those waves traveled 1.3 billion light

years to get to Earth. How do we know all of that? OK, I guess that’s a fair question. The easiest information to get from a wave is frequency and energy. But that’s plenty if you can do some physics. The frequency of the wave is twice the orbital speed of black holes. Using general relativity, orbital speed gives us the masses of the black holes. and those masses tell us the energy they’re going to lose during the merger. That energy spreads thinner the farther it travels, until it finally arri

ves at LIGO. Comparing the predicted energy to the energy LIGO actually received gives us the distance to the black holes. Direction is a little harder. We don’t aim LIGO like a telescope. It just sits there on the surface of the Earth and monitors all directions at once, which is partly why we made two different observatories. One is in Louisiana and the other is in Washington state. Louisiana detected the event a whole 7 milliseconds before Washington. And since we know gravitational waves tra

vel at the speed of light, we can get a direction. Well, roughly anyway. LIGO could only narrow down the location of the merger to an area of the sky, measuring 600 square degrees, which is a little under 1% of the sky. But that’s pretty impressive considering it monitors out to 5 billion light years using 2.5 mi long vacuum tubes containing lasers that change in length by a fraction of the width of a proton. And since LIGO showed proof of concept, other countries are now building their own, whi

ch is going to make our measurements much more precise. It’s a great time to be alive! Thanks for liking and sharing this video. Don’t forget to subscribe if you’d like to see more science like this. And until next time, remember, it’s OK to be a little crazy.