• Black Hole Bends Light Back on Itself (Caltech)

Do you like relativity? I hope you like relativity, because today I’ve got relativity. And relativity is awesome and weird.

Here is what you need to know: Light doesn’t have mass, but it has energy and thanks to good ol’ E=mc^2, gravity can affect light. Since light goes, by definition, the speed of light, gravity can’t speed up or slow down light, but it can change its energy – which is visible as a change in color – and gravity can totally change the path of light. This way light bends gravity is effectively identical to what we do with curved mirrors and lenses of different shapes, and chance alignments between nearby massive objects and distant light sources allows us to see magnified images that aren’t just magnified by our telescopes, but are rather also magnified by the intervening gravity. 

The thing is, just like fun house mirrors and some crazy and crazy lenses can warp light in really weird ways, gravity can also twist light in the extremes, and when I say extremes, I mean extremes. It was long ago realized that black holes should have the capacity to bend light effectively into orbit around them. Seeing this is nearly impossible however, because if the light is orbiting a black hole, it’s not exactly going to make it into our telescopes and onto our sensors. 

A multi-institutional team led by CalTech’s Riley Connors, however, found a way to see this kind of light nevertheless – they just needed the help of some dust. 

The stellar mass black hole XTE J1550-564 is actively feeding on a binary star companion, and the mass it pulls off forms a small accretion disk that emits light, and like all regular matter, is also capable of reflecting light. It’s this ability to reflect light that made the impossible possible. In this system, the super hot disk emits light that is bent by the black hole, and that new path can return light that left one side of the disk, back to the disk on the other side of the black hole. Now here is where this work got tricky. These researchers used archival data from the now retired Rossi X-ray Timing Explorer to try and discern which photons were coming directly from the disk, and which photons were reflected off the disk. This sounds like a crazy feat, but X-ray photons are so few in number that X-ray researchers will periodically name their photons… because they can. By looking carefully at the light, they could measure the tiny color differences between directly emitted photons, and reflected photons. Future work using this technique has the potential to be able to even measure the effect that rotation has on how black holes warp the space around them, but for now this new technique is enough to allow them to confirm relativity in one new way. 

Where that research left me comfortably thinking we do indeed understand the universe, the next story left me feeling that I need to go back to bed and start over, because nothing makes sense any more. You are warned. 

There are a few simple rules we use when we study our far flung universe. We assume the same physics here are at play everywhere. We assume that at a large enough scale the universe is the same everywhere; that it’s homogeneous. We assume that what we see happening here, at large scales is happening everywhere – essentially that it’s isotropic. Put together, this is our cosmological principle and it is at the heart of everything we do when we model our universe at the largest scales. We haven’t been able to directly measure these assumptions, until, maybe, now.