A few days ago the following headline on the New York Times website caught my eye: Seeing What the Hubble Sees, in Imax and 3-D. There are two reasons this headline is worthy of note. First, it is amazing that an IMAX movie about the Hubble Space Telescope exists at all, and is worth mentioning on the front (web)page of the NYT. Hubble is a part of the popular imagination, and may be the object most closely tied with Science in the eyes of the general public (even more than the LHC). Furthermore, it is absolutely astounding that NASA launched hundreds of kilos of camera equipment and film into orbit, and spent valuable astronaut time (both on the ground and in space) to pull off the filming. I would claim one of the lasting legacies of the Apollo missions to the Moon are the photographs, and in particular Earthrise. That single photograph of our home as a small blue marble against the vastness of space put our planet into proper perspective for the very first time. NASA is well aware that part of its mission is to light up the public imagination, getting us to peer past our limited horizons, and out into the vast Universe beyond. This film is part of that tradition.
The second interesting aspect of the headline is that it’s nonsensical. Hubble has only one eye. It has one mirror. It can’t perceive depth, and therefore can’t see in 3-D. We see slightly different images in each of our eyes, and then a fairly impressive difference engine (called a “brain”) figures out the depth to everything we are looking at, and whether that rock is about to bonk us on the head and we need to duck NOW! 3D movies (such as Alice and Avatar) use circularly polarized light, and glasses with different filters in each lens, to produce the different images for each eye. (The light is circularly polarized so that, if you tilt your head, it all still works; the old linear polarization approach didn’t do this, and had a tendency to make one feel motion sick [at least, it did for me]).
In general astronomical sources are too far away for us to discern distance using parallax. That’s why the night sky looks “flat”, even though the planets and stars and galaxies are at a tremendous range of distances. If you wanted to be able to directly “see” the distance to the nearest star, in the same way that you ascertain the distance to an approaching lion, your eyes would need to be separated by roughly 10 billion km. (Eye separation = Distance*Angle. The human eye has an angular resolution of roughly 1 arcmin = 0.0003 radians, and the nearest star [Proxima Centauri] is 4 lightyears = 3.8e13 km.) The way we figure out distance in Hubble images is by using color information (and, in particular, the spectra) to discern recession velocity (redshift), and thereby distance (using Hubble’s law). This is not something we do with our eyes (although we do use color information to discern temperature; you’re unlikely to grab something that is so hot it’s glowing blue). Hubble sees a purely two-dimensional Universe. So “seeing what the Hubble sees …in 3-D” is a contradiction in terms. Was the headline carefully crafted to see if we were paying close attention?