How does looking at distant galaxies allow us to look back in time?
Published: October 23, 2014
Strictly speaking, when telescopes look at the light from distant galaxies, they are not literally looking back in time. The past no longer exists, so no one can directly look at it. Instead, the telescopes are looking at the present-time pattern of a beam of light. Since the beam of light has been traveling through the mostly-empty vacuum of space for millions of years, it has been largely undisturbed. Therefore, the present-time pattern of this beam of light is the same as the pattern that it had when it was first created by the distant galaxy millions of years ago. By looking at the present-time state of a beam of light, we can thus infer what the galaxy that created the light looked like millions of years ago.
It's like taking and printing a photograph of your daughter as a baby, and then looking at the photo ten years later. When you look at the printed photo, you are figuratively "looking back in time" and seeing what your daughter looked like as a baby. But you are not literally looking into the past. Your daughter is no longer a baby and does not exist as a baby in any dimension or corner of the universe. Rather, you are looking at a present-time pattern of light that is being created by the reflection of the room's light from the inks in the printed photograph. But, because the ink in the photo has specifically been organized into a pattern resembling your daughter as a baby, and because the ink pattern has not changed over the last ten years, the present-time beam of light from the photograph has the same pattern as the beam of light that came from your baby daughter ten years ago. By looking at a present-time bundle of light, you are able to infer how people looked in the past. But implicit in this inference are three assumptions: 1. the camera accurately captured the pattern of light and converted it to a pattern of information, 2. the printer accurately transferred this pattern of information to a pattern of ink on the printed photo, and 3. the photo has not changed since it was printed. All of these assumptions must hold true in order for the pattern of light presently coming from the photo to represent the appearance of your daughter ten years ago.
For instance, suppose a bit of acid drops on the photo and causes a big white dot to appear above your daughter's head. If you were literally looking back in time when looking at the photo, you would have to conclude that there was a UFO or ball lightning hovering above your girl's head ten years ago. But you are likely smart enough to realize that you are not literally looking back in time, but are simply looking at a present-time pattern of light which no longer exactly represents the pattern of light ten years ago when you took the photo.
Similarly, the present-time beams of light hitting telescopes that are pointed at distant galaxies only give us information about the past insofar as the light has not changed over the years. More realistically, the light from distant galaxies can change as is travels, but it has to change in ways that we can understand and subtract out if we are to end up with an accurate representation of the past. One major change that happens to light traveling through intergalactic space is that the light is redshifted because of the expansion of the universe. As light travels through space, which is itself expanding, the light wave gets stretched and ends up with longer wavelength components. Longer wavelengths means that the entire light pattern is shifted towards the color red on the visible spectrum. Therefore, the present-time bundle of light hitting a telescope does not exactly match the appearance of the distant galaxy that first created the light millions of years ago; it is redder. Fortunately, scientists now understand the redshift and can shift the light pattern back by the appropriate amount in order to end up with a faithful representation of the distant galaxy when it emitted the light.
The light pattern from distant galaxies can travel undisturbed for millions of years for two reasons: 1. the distant galaxies are so far away that it simply takes millions of years for their light to travel this distance in a straight line, and 2. the universe is mostly empty so that light can travel a long way without hitting anything.