Our Past Is In The Sky

Far-fetched as it might seem (and be!), we might be literally surrounded by information about the Earth’s, Sun’s, Galaxy’s past. By looking in the right direction with the right instruments, we could even be able to see how things were at different times, even billions of years ago.

By looking where? This idea is based on a little-known characteristics of black holes, namely the large amount of incoming light that is back-scattered, i.e. sent back more or less in the direction it came from. This phenomenon is visible as a halo around the black hole (see picture to the left).

Think then: by looking at a black hole 20 million light years away, we will be getting some light first emitted by our galaxy 40 million years ago, as the photons will have had to travel to the black hole and back. Correcting for the optical properties of the region around the black hole that we see as a halo, we would even be able to get a picture of our galactic surroundings.

Analogously for black holes nearer to us, eg 20,000 light years away, the halo will literally contain pictures of our neighborhood as of 40,000 years ago.

All of the above is unlikely to be easy, still any information in the back-scattered photons will be extremely valuable.