How can we see the light from a very distant galaxy?

If what we see when we observe a distant galaxy is the light that left that galaxy billions of years ago, does that mean it is impossible to know what is happening right now in that galaxy?

• Answer:

  On the surface, Yes. It is impossible to know anything about an event if it is impossible for a photon from that event to have reached you yet. Even changes in gravity can only propagate at the speed of light, which is actually better thought of as the maximum speed at which any "change" is able to propagate in the universe. A common example is to imagine if our sun were suddenly plucked out of the universe into nonexistence. From our perspective on Earth, not only would we continue to see the sun's light for the ~8 minutes it takes for light to travel the required distance, but during this time the Earth would continue on its orbit around the sun's former location. On a deeper level, there is no "right now" in the other galaxy. This question cannot even be asked because "now" is relative. Two events cannot be said to both be happening "now" in an absolute sense unless they are both occurring in the same place in space. This concept is called the relativity of simultaneity. You can read more about it here: http://en.wikipedia.org/wiki/Relativity_of_simultaneity