# Time and Space

1. Feb 10, 2010

### binbots

Why did Einstein combine time and space? Does this mean when we observe something we don't know where it is as opposed to when it is? Is a object 1 light year away or 1 light year ago. How do we seperate the 2 to know where something actually is?

2. Feb 10, 2010

### Kracatoan

You don't really need to bother with 4D until you start examining nasty things like black holes and the like, that is what spacetime is useful for. When talking about the examples you gave it is best to just stick with the simple physics - the object is 1 light year away, but we see what it looked like 1 year ago. The point in physics is to simplify reality, don't use concepts where they are not needed. Newtonian physics explains most stuff just fine, exotic theories like spacetime are there to accomodate for the bits which don't quite fit properly, and they are few and far between on our scale, when you get very small everything is messed up.

3. Feb 10, 2010

### binbots

Is space time useful when talking about distances between galaxies. When we see 2 galaxies how do we know how far apart are they as opposed to when they are. If what we observe takes place in a past time, that means we haev no idea where a object it now. So when we look at a distant galaxy we dont know where it is now, when it was, or where it use to be?

4. Feb 10, 2010

### Agent M27

I am in the beginning stages of relativity and space-time, but for distances between galaxies we determine this and cross check this with a few different measurement, i.e. Type 1a SN, Cepheids, Tully-Fisher, etc. so "space-time" isn't really needed. As for the question to "when" they are I don't think is quantifiable because when dealing in SR & GR there is no absolute time, it is relative to the observers refrence frame. It is true that when we observe say Andromeda, it's photons are from 2.5 MLY ago and since then it's location has indeed changed, but this does not mean we cannot know "where" it is in the present time. What we can do is we can infer it's "new" location first by determing its velocity via doppler shifts in its spectrum, and then combining that with time to solve for the distance. There are a few other aspects to it such as the radial & tangential components of the galaxy in question. Some one correct me if I am wrong. I hope I haven't sent the OP in the wrong direction.

Joe

5. Feb 10, 2010

### Kracatoan

Agent is totally correct, although the galaxy will have moved, we can work out its new position using simple maths.

6. Feb 10, 2010

### Frame Dragger

The light which reaches Earth from an object 1 ly (light year) distant is 1 year old light, and is like looking at a delayed movie. Thus, when we observe a 13.7 billion ly distant object, we are seeing it as it was that many years ago because local c is invariant. It is in fact, this very concept of the absolute speed of light that leads to a combination of time and space.

If you traveled FTL (Faster Than Light) 1 ly distant, you would have traveled in time from your perspective. Only if you traveled Classically (Slower than light in this case) would you avoid this. After all, if you see a supernova 500 ly from earth, and instantly teleported (magically) to that star, it would be as it was 500 years from your observation of its 'death'. In fact, there is the time travel and the paradox. What if there were an inhabited world around this star, and you warned inhabitants to prepare for the end of their world? You would have changed history from the perspective of different observers!

This is why you will often hear that one of the fundamental principles of relativity is that observers in all intertial frames (people anywhere at any time) will agree on the ordering of events. A causes B, (cups shatter, they don't repair themselves as far as we can all agree), and if at any time distant observes can observe B causing A, there is the potential for paradox and closed time-like curves.