Here is a questions to astronomy buffs :P?

Question

I'm reading a book right now about relativity / speed of light and I haven't read the rest as I'm taking a break from reading. My one question though is if one would peer at space with some high powered telescope and we see a galaxy; lets say galaxy X which is 4 billion light years away. My understanding is that we are seeing this galaxy as it was 4 billion years ago, does that mean that we don't really know exactly how it looks like presently?? SOrry if this has been answered before or if I'm asking it wrong, this book is amazing with all the facts I'm learning but since I won't be able to take up a class till next semester there is no professor ot ask :(

Answer 1

Yes, exactly.  Many of the stars we see in the sky might, in fact, no longer exist, especially the furthest ones.

Other stars may exist that we cannot see yet.

I agree, it's an interesting concept.

Answer 2

That sounds completely correct by my theories.  We would need light to hit us instantly to know what that galaxy looks like in this instant.  That also means that you could look through a telescope at a person as far away as the curvature of the Earth allows, and be seeing them fractions of a second later.  Weird huh?  Try this one...  If we could fly away from Earth faster than the speed of light and looked back, we would theoretically see light reflecting on the Earth from the past.  Maybe you could fly really fast and far away and watch WWII or see the Roman empire as it was built.  Have fun with that ;)

Answer 3

yes

Answer 4

The distance we measure using the length of the trip taken by light to reach us is called the proper distance.  It is the distance used by astronomers.

The comoving distance is the distance at which the galaxy would be today IF we could see it.  This is the distance used by cosmologists in describing the universe.

Light travels slowly (on the astronomical scale).  So, if the proper distance is 4 billion light-years, then we see it as it was 4 billion years ago.

We see different types of galaxies at 4 billion light-years.  We see a similar distribution of galaxies at 3 billion light-years (this allows us to guess how the galaxies have evolved in one billion years).

We see similar distributions at 2 and 1 billion l-y.  At 500 million, at 100 million...

This allows us to set a timeline in the life of different types of galaxies.  With that, we can guess at what the galaxy seen at 4 billion light-years might look like today.

A bit like being handed a series of photos of various types of humans at each age from 3 to 63.  Then someone hands you a photo of one particular person when she was 3 and asks you: can you tell what she might look like today at 63?

You look through your collections of pictures of 3-year-olds, find the one that looks closest.  Then take the 63-year-old picture of that same type.  You would then say:  she might look something like this.

Answer 5

Yes. When you see something, whether a distant star or the person across the room, you are looking back in time. In the case of the distant star this could be billions of light years; the person across the room, nanoseconds. The point is this, your perception of reality is at the mercy of the speed of light and it's relativity to time.

Answer 6

Yes'm

Light travels at the speed of light... go figure... so the age of everything we see is dependent on how far away it is. That is why we can see the first galaxies forming despite all galaxies today being fully formed... another interesting side effect caused by the limitation of light is that if for some unbelievable cataclysm the Sun went out we would not figure it out until 8 minutes after the fact.

And what is the title of the book? I'm thinking about reading a physics book before school starts.