> Photons have no ticking time at all, which means, as far as they are concerned, they are absorbed the instant they are emitted, even if the distance traveled is across the universe itself.
Imagine the faces of Penzias and Wilson when they learned that the noise in their horn antenna wasn't bird poop but photons traveling 14 billion years without interacting.
See Cosmic Microwave Background.
Brighter, yes, but not very bright. The density of stars will not increase very much. Currently, to see the Milky Way, you have to be out in the boondocks, far from cities and other light polluters - the thing is just so faint.
After the merger, perhaps it will get brighter, but the increase will not be spectacular. The brightness of the combined galaxy as seen from Earth will be dictated by the stellar density, which will not change much.
according to QM idea of superposition collapse that photon has just decided to collapse in your retina while it had non-zero probability of collapsing into retina of an alien anywhere on that 2.5 million radius circle. That blows my imagination :)
1) When you look up in the sky at the Andromeda Galaxy, your retina is absorbing photons that have been screaming through space for 2.5 million years. If you were on the Earth then, you'd be hanging out with Smilodon, the saber-toothed cat. For 2.5 million years, those photons were freely zooming through space and time, and when you see them, those photons are gone forever, their energy powering a chemical reaction in the rods and cones lining your retina that enables you to see them. Those ancient photons were seen by nobody else but you, and they literally become a part of you.
2) In about 3.75 billion years, the Andromeda Galaxy, currently zipping through space toward the Milky Way Galaxy at 110 km/s (nearly 70 miles per second, or about 250,000 MPH), will collide with and merge with our galactic home. It's unlikely anything would happen to the majority of planetary systems in either galaxy (there's a lot of empty space in there) -- but the night sky (not from Earth, we'd probably be cooked by the Sun by then) sure would be beautiful. And bright.
Scientists even proposed the name for the new galaxy: Milkomeda.