Since the first time gravitational waves were detected, people compared them to “hearing” the universe. Indeed, it’s a completely new way of observing the universe, one that does not need light. But it sure is an odd sort of “sound”.
First off, let’s make clear that LIGO is not really hearing anything. It’s just an analogy. Sound waves propagate by moving air (or other media) around. No air, no sound. That’s why in space nobody can hear you scream. Gravitational waves, instead, just need space to travel—no lack of that anywhere. That’s how scientists “heard” (again, not literally), through millions of light-years of cosmic void, the explosion of two dense neutron stars crashing into each other.
Another difference is that sound travels much slower than light, so we see the lightning a few seconds before the thunder that was born together with it. Gravitational waves, instead, travel at the speed of light. Flash and “bang” arrive together.
Turns out, though, that LIGO detected this latest batch of gravitational waves a couple of seconds before the burst of gamma rays arrived. That puzzled me: shouldn’t they arrive at the same time? Thing is, for a bit, gravitational waves traveled, quite literally, faster than the light.
Let me explain. The “speed of light”—that cosmic speed limit nothing can cross—is the speed at which light races in the vacuum. When light traverses a medium (air, water, glass,…), it slows down. As stars collide, they blasts off a whole lot of stuff, so the space around is very much not a vacuum. The flash of radiation travels through this cloud a teensy bit slower, whereas gravitational waves just plow through.
So not only we most definitely “hear” neutron stars scream, but we’ll do that before we see why.
If you want more
- I didn’t want to tell the story of the discovery because so many others did it, probably better. If you want a great recap, look at this post by Phil Plait
- Need a recap of what gravitational waves are and how LIGO works? Got you covered here!
PhysicalLove 