A few weeks ago, awesome blogger and space advocate Zain Husain asked me to contribute to a roundup post he wrote. He contacted a bunch of people (most of them much more prestigious than me) and asked them one question:

What NASA or space project are you most excited about and why?

You can (and should) read everybody’s response to Zain’s question on his blog, here. However, I wanted to expand on part of my answer and tell you why I’m excited about multi-messenger astronomy.

Supernova Supernova

It all starts with the title image above. That’s an image of SN 1987A, the first supernova discovered in 1987. This is an example of a core-collapse supernova, where a star runs out of nuclear fuel and implodes on itself to form a black hole. (There are other types of supernovae, too. For a nicely accessible review, see this article.) After the implosion, the shockwave creates an explosion which we call the supernova.

SN 1987A is really special.  It is the only core-collapse supernova in the last century to have exploded close enough to Earth that it could be seen with the naked eye—only 168,000 light years away! But, amazingly, we didn’t detect SN 1987A from its light. We detected it because of the high-energy subatomic particles it gave off.

When a core-collapse supernova explodes, it gives off most of its energy in the form of tiny, extremely energetic particles called neutrinos. On Earth, we can only make neutrinos in supercolliders like the LHC. But in space, things are wild, and neutrinos are generated in all sorts of cataclysmic events.

For me, this is why SN 1987A is special.

Multi-Messenger Astronomy

When we talk about astronomy, we imagine using light. We think of an astronomer pointing a telescope into the sky and looking through its eyepiece. Things are more complicated nowadays, of course. We shoot telescopes into space. We take pictures in the infrared and the ultraviolet. But in general, we’re still looking at light to see space…right?

For now, this is still roughly true. But scientists are increasingly moving towards “multi-messenger astronomy,” where astronomers studying some object in space look at the light it emits, the subatomic particles it emits, and even the ripples in spacetime that it causes.

Here are two exciting projects that are just coming online (or came online recently) that move astronomy in this direction.

Advanced LIGO

We know from Einstein that space and time are not separate things, but intertwined to form a single spacetime. And this spacetime can be curved or warped, which we perceive as a distortion of the very notion of distance. The motion of large objects creates “ripples” in spacetime which we perceive as distance itself oscillating in time. We call these ripples “gravitational waves.”Check out this video of a simulation of two black holes colliding and the gravitational wave signature that can be extracted:

Since a gravitational wave is a warping of how we measure distance, we should be able to detect this warp with a big enough ruler. And we’re making a bunch of gigantic rulers to do just that. Meet Advanced LIGO, the gravitational wave observatory:

Actually, this is only one of the tow Advanced LIGO systems. This one is in Louisiana, but there’s another in Washington State. The coolest thing about LIGO is that it detects gravitational waves. The second coolest thing about LIGO is how it does it.

Light makes an extremely good ruler and LIGO takes advantage of this. It’s essentially a four-kilometer-long giant laser. If that’s not cool, I don’t know what is.

I know this has been a bit vague, so I promise I’ll write a more in-depth article on gravitational waves and LIGO in the future.

Ice Cube

As I mentioned above, astrophysical events like core-collapse supernovae emit subatomic particles called neutrinos. Neutrinos are tricky little things, though. Just as X-rays pass through walls, neutrinos pass through miles of solid rock and meters of dense lead. X-rays are stopped by bone and metal and can interact with our bodies, but neutrinos pass right through bone, so they’re completely harmless. Thus, if we make a neutrino detector, the probability that we’ll stop and detect a given neutrino passing through our detector is very small. To catch even one neutrino, then, we need to cast a very wide net.

And when I say wide, I mean WIDE.

Just recently, our widest net came online: the IceCube, a neutrino observatory at the South Pole. In the case of IceCube, our net is one cubic kilometer of detector dug into the ice of the South Pole. This should allow us to detect more energetic neutrinos than ever before. For a sense of scale, the IceCube collaborators helpfully diagrammed the under-ice structure next to the Eiffel Tower:

Applications

I mentioned supernovae at the top of the article for a reason: We still don’t completely understand the nature of core-collapse supernovae, and neutrino and gravitational wave observations would really help us understand these objects better.

But the applications are far wider than that. Neutron stars are some of the densest things known to man; they’re held apart simply by the Pauli exclusion principle. However, it’s very hard to make predictions about this exotic fluid they’re made of. Hopefully gravitational wave and neutrino observations would help us understand them better. The same goes for black holes, which emit gravitational waves and neutrinos.

Thanks, Zain!

A big thanks to Zain for putting together his post and prompting me to think about this. Zain puts a lot of work into his incredible blog, and it shows.

• His main site is here.
• And you can also find him on Google+, Facebook, and Twitter.

Related Reading

This is one of many articles I’ve written that involve general relativity. If you’d like to know more, check out these articles.

• In this article, I talk about special relativity and how it implies that space and time are unified in a single spacetime.
• In this article, I introduce general relativity as a way to travel faster than light.
• In this article, I discuss the geometry of spacetime.
• In a pair of articles here and here, I answer some questions about general and special relativity.